《OFDM技术开发系列》（英文版） Beyond 3G Wideband wireless data access based on OFDM and dynamic packet assignment

MULTIPLE ACCESS FOR BROADBAND WIRELESS NETWORKS Beyond 3G: Wideband wireless Data Access based on OFDM and dynamic Packet assignment Justin Chuang and Nelson Sollenberger, AT&T Labs-Research ABSTRACT sion capabilities, increasingly demanding Internet applications and user expectations have emerg The rapid growth of wireless voice sub- Experience with laptop computers and personal scribers, the growth of the Internet, and the digital assistants(PDAs) has shown that many Icreasing use of portable computing devices end users desire their portable equipment to pro- rapidly over the next few years. Rapid progress cations they enjoy at their desks with fer in digital and RF technology is making possible compromises. Experience with wireless access has highly compact and integrated terminal devices, demonstrated the singular importance of ere data software is making wireless Internet access access. Wireless packet data access in macrocell- more user-friendly and providing more value. lar environments at peak rates beyond 2 Mb/s is Transmission rates are currently only about 10 likely to be needed in the near future to provide kb/s for large cell systems. Third-generation users with an application environment with few wireless access such as WCDMA and the evolu- compromises from fixed environments. Chal ion of second-generation systems such as lenges for the high-speed wireless data access TDMA IS-136+, EDGE, and CDMA IS-95 will future are transmission speeds at 100-1000 times provide nominal bit rates of 50-384 kb/s in existing rates; costs of a few cents per minute for macrocellular systems. [1 This article discusses access; RF power transmission efficiency that is packet data transmission rates of 2-5 Mb/s in 10-20 dB better than existing systems; and sub- macrocellular environments and up to 10 Mb/s stantially increased spectral efficiency in microcellular and indoor environments as a Two important business drivers for comple complementary service to evolving second-and mentary packet data access at speeds above 2 hird-g tion wireless D Mb packet assignment for high-efficiency resource Integration of wireless data services across management and packet admission; OFDM macrocellular, microcellular, and private the physical layer with interference suppression, indoor systems, and with other services d fre diversity;as·High efficiency well as smart antennas to obtain good power Wireless service providers pay dearly to acqui and spectral efficiency are discussed in this pro- spectrum. Efficiency of spectrum usage is always posal. Flexible allocation of both large and a strong factor in a decision on wireless technol small resources also permits provisioning of ogy Spectrum efficiency becomes crucial for services for different delay and throughput very high-speed data services(e. g,>2 Mb/s) requirements. By taking advantage of improvements in digital signal processing(DSP)and radio frequency INTRODUCTION (RF) technologies, orthogonal frequency-divi- sion multiplexing(OFDM) provides the possibil- wireless Internet access is expected to grow rapid- ity to provide 2 Mb/s packet data at a cost an ly, because of the maturing of digital cellular, with a spectrum efficiency that allow wireles portable computing, and fixed Internet technolo- providers to compete with wireline carriers for gies Data transmission rates are growing rapidly data services Integrated services also provid in fixed networks with the use of wavelength-divi- significant billing advantages for both customers sion multiplexing(WDM) in backbone fiber net- and service providers. Based on customers' pref works and the introduction of cable modems and erences, telecommunications companies such high-speed digital subscriber line(HDSL) technol- AT&T are moving in the direction of delivering ogy in the fixed access networks In parallel with tegrated services which cover local residential the expanding availability of high-speed transmis- and business, long distance, and both wireline 0163-68040010002000IEEE IEEE Communications Magazine. July 2000

78 IEEE Communications Magazine • July 2000 Beyond 3G: Wideband Wireless Data Access Based on OFDM and Dynamic Packet Assignment 0163-6804/00/$10.00 © 2000 IEEE ABSTRACT The rapid growth of wireless voice sub￾scribers, the growth of the Internet, and the increasing use of portable computing devices suggest that wireless Internet access will rise rapidly over the next few years. Rapid progress in digital and RF technology is making possible highly compact and integrated terminal devices, and the introduction of sophisticated wireless data software is making wireless Internet access more user-friendly and providing more value. Transmission rates are currently only about 10 kb/s for large cell systems. Third-generation wireless access such as WCDMA and the evolu￾tion of second-generation systems such as TDMA IS-136+, EDGE, and CDMA IS-95 will provide nominal bit rates of 50–384 kb/s in macrocellular systems. [1] This article discusses packet data transmission rates of 2–5 Mb/s in macrocellular environments and up to 10 Mb/s in microcellular and indoor environments as a complementary service to evolving second- and third-generation wireless systems. Dynamic packet assignment for high-efficiency resource management and packet admission; OFDM at the physical layer with interference suppression, space-time coding, and frequency diversity; as well as smart antennas to obtain good power and spectral efficiency are discussed in this pro￾posal. Flexible allocation of both large and small resources also permits provisioning of services for different delay and throughput requirements. INTRODUCTION Wireless Internet access is expected to grow rapid￾ly, because of the maturing of digital cellular, portable computing, and fixed Internet technolo￾gies. Data transmission rates are growing rapidly in fixed networks with the use of wavelength-divi￾sion multiplexing (WDM) in backbone fiber net￾works and the introduction of cable modems and high-speed digital subscriber line (HDSL) technol￾ogy in the fixed access networks. In parallel with the expanding availability of high-speed transmis￾sion capabilities, increasingly demanding Internet applications and user expectations have emerged. Experience with laptop computers and personal digital assistants (PDAs) has shown that many end users desire their portable equipment to pro￾vide essentially the same environment and appli￾cations they enjoy at their desks with few compromises. Experience with wireless access has demonstrated the singular importance of widespread coverage and anywhere/anytime access. Wireless packet data access in macrocellu￾lar environments at peak rates beyond 2 Mb/s is likely to be needed in the near future to provide users with an application environment with few compromises from fixed environments. Chal￾lenges for the high-speed wireless data access future are transmission speeds at 100–1000 times existing rates; costs of a few cents per minute for access; RF power transmission efficiency that is 10–20 dB better than existing systems; and sub￾stantially increased spectral efficiency. Two important business drivers for comple￾mentary packet data access at speeds above 2 Mb/s are: • Integration of wireless data services across macrocellular, microcellular, and private indoor systems, and with other services • High spectrum efficiency Wireless service providers pay dearly to acquire spectrum. Efficiency of spectrum usage is always a strong factor in a decision on wireless technol￾ogy. Spectrum efficiency becomes crucial for very high-speed data services (e.g., > 2 Mb/s). By taking advantage of improvements in digital signal processing (DSP) and radio frequency (RF) technologies, orthogonal frequency-divi￾sion multiplexing (OFDM) provides the possibil￾ity to provide > 2 Mb/s packet data at a cost and with a spectrum efficiency that allow wireless providers to compete with wireline carriers for data services. Integrated services also provide significant billing advantages for both customers and service providers. Based on customers’ pref￾erences, telecommunications companies such as AT&T are moving in the direction of delivering integrated services which cover local residential and business, long distance, and both wireline Justin Chuang and Nelson Sollenberger, AT&T Labs-Research MULTIPLE ACCESS FOR BROADBAND WIRELESS NETWORKS

include voice services, circuit data, and packer Pos amic packet assignment(DPA) has been pro and wireless services Integrated services also d sed, with the potential to provide 384 kb/s DM can largely data with transmission rates from 30 kb/s to a data services in macrocellular environments using few hundred megabits per second. Providing only I MHz of spectrum 3]. It is possible to eliminate the nomadic customers in areas such as airports, and this acis concept into a wideband con hotels, and other public areas with the same user text in 5 MHz while providing a complementary effects of experience they have in their office is the key service to third generation systems such as driver to deploy such high-rate complementary EDGE and WCDMA. This wideband OFDM packet data services. system would support an order of magnitude interference for Wideband code-division multiple access higher peak data transmission rate in macrocells (WCDMA)will use 5 MHz channels, and it is a at 2 to 5 Mb/s and up to 10 Mb/s in microcells leading candidate for third-generation wireless IS-136, GSM or WCDMA would provide circuit transmission rates access[1]. However, it will be limited to about voice and other circuit-based services and basic 384 kb/s(nominal)peak data rates for macro- data services. A complementary high-speed cellular wireless access(up to 2 Mb/s rates are packet data mode would provide fast wireless ed for indoor environments). Global S, acket data access to meet the demand for wire tem for Mobile Communications (GSM less data in the future that provides access pe and it readily enhancements based on enhanced data rates formance similar to wideband fixed access. Since for GSM Evolution(EDGE)using adaptive portable equipment is power-limited, strongly modulation will provide bit rates up to 384 kb/s net al traffic should be supported, and interference in the near future. [1] Second-generation wire link transmission rates should be allowed to less systems will evolve with complementary adapt downward as necessary to support the suppression and packet data solutions that generally use frequen ed link budgets. Wideband oFDM wire cy channels separated from circuit voice and cir less access might also be configured to introduce cuit data access. Time-division multiple access new broadband capabilities using OFDM only coding to (TDMA)and CDMA systems are being consid- on the downlink, which is then integrated wit ered in which circuit and packet access share a emerging wireless packet data systems such as common frequency channel and access modes General Packet Radio Service(GPRS), EDgE, are separated by time slots or spreading codes. or WCDMA to provide two-way access. An However, the expected demands for high peak- example of such a system with the EDGE uplink rate Internet access are motivating increasing is discussed in[4].2 consideration of complementary access based on There are a number of reasons to consider separate frequency channels to provide maxi- such a high-rate complementary packet data mum peak rates and to allow optimization for capability for downlinks. Wireless Internet packet data transmission alone OFDM was proposed for digital cellular sys- more, for data services, peak bit rate is very tems in the mid-1980s [2]. OFDM has also beer portant in determining overall system perfor shown to be effective for digital audio and digi- mance, because of the highly bursty nature of tal video broadcasting at multimegabit rates in Internet traffic. GPRS, EDGE, and WCDMA Europe, and it has been incorporated into stan- solutions will support transmission rates of dards by the European Telecommunications 144-384 kb/s in macrocellular environments. To Standards Institute(ETSI). The IEEE 802.11 achieve rates in the megabits-per-second range tandards group recently chose OFDM modula- for all environments using -5 MHz spectrum is ion for wireless LANs operating at bit rates up challenging for both the physical layer and radio to 30 Mb/s at 5 GHz. In this article, OFDM resource management design Single-carrier modulation combined with dynamic packet TDMA solutions are limited in supportable assignment with wideband 5 MHz channels is transmission bit rate by equalizer complexity proposed for high-speed packet data wireless Even though new techniques such as interfer access in macrocellular and microcellular envi- ence suppression and space-time processing are ronments, supporting a family of peak bit rates promising, the interactions of these techniques Peak rates exceeding I sive environments, and it readily supports inter- intercode interference at high bit rates limits considered for some ference suppression and space-time coding CDMA solutions. The use of oFDM with suffi- tems enhance efficiency. Dynamic packet assignment ciently long symbol periods of 100-200 us for can support excellent spectrum efficiency and packet data transmission addresses these issues. 2 In/4 we focused on the high pea It supports a high bit rate in time delay spread architecture of such a mm in a macrocullar WIDEBAND OFDM environments with performance that improves with increasing delay spread up to a point of system. This article pro- WCDMA is now recognized as one of the lead- extreme dispersion. Another reason to consider vides a detailed discussion ng candidates for third-generation wireless a complementary packet data solution is to use of the design considera- access. Based on direct-sequence spread-spec- optimized admission procedures for packet data tions under different con- trum with a chip rate of 3. 84 Chips/s, it occu- access that is fairly aggressive in order to achieve ditions. However, the pies a bandwidth of about 5 MHz. It will support high spectral efficiency. An aggressive admission numerical results shown ircuit and packet data access at nominal rates policy will result in high word error rates in /4 were based up to 384 kb/s in macrocellular environments, (WERs) that can generally be managed for improved radio link and provide simultaneous voice and data ser- Internet services using automatic repeat reques design using convolution (ACIS) concept based on OFDM signaling and (ARQ)techniques but are problematic for al codes to achieve even vices. An advanced cellular internet delay-sensitive voice services. Therefore, a com- better performance Magazine·Ju

IEEE Communications Magazine • July 2000 79 and wireless services. Integrated services also include voice services, circuit data, and packet data with transmission rates from 30 kb/s to a few hundred megabits per second. Providing nomadic customers in areas such as airports, hotels, and other public areas with the same user experience they have in their office is the key driver to deploy such high-rate complementary packet data services. Wideband code-division multiple access (WCDMA) will use 5 MHz channels, and it is a leading candidate for third-generation wireless access [1]. However, it will be limited to about 384 kb/s (nominal) peak data rates1 for macro￾cellular wireless access (up to 2 Mb/s rates are proposed for indoor environments). Global Sys￾tem for Mobile Communications (GSM) enhancements based on Enhanced Data Rates for GSM Evolution (EDGE) using adaptive modulation will provide bit rates up to 384 kb/s in the near future. [1] Second-generation wire￾less systems will evolve with complementary packet data solutions that generally use frequen￾cy channels separated from circuit voice and cir￾cuit data access. Time-division multiple access (TDMA) and CDMA systems are being consid￾ered in which circuit and packet access share a common frequency channel and access modes are separated by time slots or spreading codes. However, the expected demands for high peak￾rate Internet access are motivating increasing consideration of complementary access based on separate frequency channels to provide maxi￾mum peak rates and to allow optimization for packet data transmission alone. OFDM was proposed for digital cellular sys￾tems in the mid-1980s [2]. OFDM has also been shown to be effective for digital audio and digi￾tal video broadcasting at multimegabit rates in Europe, and it has been incorporated into stan￾dards by the European Telecommunications Standards Institute (ETSI). The IEEE 802.11 standards group recently chose OFDM modula￾tion for wireless LANs operating at bit rates up to 30 Mb/s at 5 GHz. In this article, OFDM modulation combined with dynamic packet assignment with wideband 5 MHz channels is proposed for high-speed packet data wireless access in macrocellular and microcellular envi￾ronments, supporting a family of peak bit rates ranging from 2 to 10 Mb/s. OFDM can largely eliminate the effects of intersymbol interference for high-speed transmission rates in very disper￾sive environments, and it readily supports inter￾ference suppression and space-time coding to enhance efficiency. Dynamic packet assignment can support excellent spectrum efficiency and high peak-rate data access. WIDEBAND OFDM WCDMA is now recognized as one of the lead￾ing candidates for third-generation wireless access. Based on direct-sequence spread-spec￾trum with a chip rate of 3.84 Mchips/s, it occu￾pies a bandwidth of about 5 MHz. It will support circuit and packet data access at nominal rates up to 384 kb/s in macrocellular environments, and provide simultaneous voice and data ser￾vices. An advanced cellular Internet service (ACIS) concept based on OFDM signaling and dynamic packet assignment (DPA) has been pro￾posed, with the potential to provide 384 kb/s data services in macrocellular environments using only 1 MHz of spectrum [3]. It is possible to expand this ACIS concept into a wideband con￾text in 5 MHz while providing a complementary service to third generation systems such as EDGE and WCDMA. This wideband OFDM system would support an order of magnitude higher peak data transmission rate in macrocells at 2 to 5 Mb/s and up to 10 Mb/s in microcells. IS-136, GSM or WCDMA would provide circuit voice and other circuit-based services and basic data services. A complementary high-speed packet data mode would provide fast wireless packet data access to meet the demand for wire￾less data in the future that provides access per￾formance similar to wideband fixed access. Since portable equipment is power-limited, strongly asymmetrical traffic should be supported, and uplink transmission rates should be allowed to adapt downward as necessary to support the required link budgets. Wideband OFDM wire￾less access might also be configured to introduce new broadband capabilities using OFDM only on the downlink, which is then integrated with emerging wireless packet data systems such as General Packet Radio Service (GPRS), EDGE, or WCDMA to provide two-way access. An example of such a system with the EDGE uplink is discussed in [4].2 There are a number of reasons to consider such a high-rate complementary packet data capability for downlinks. Wireless Internet usage is likely to be downlink-limited. Further￾more, for data services, peak bit rate is very important in determining overall system perfor￾mance, because of the highly bursty nature of Internet traffic. GPRS, EDGE, and WCDMA solutions will support transmission rates of 144–384 kb/s in macrocellular environments. To achieve rates in the megabits-per-second range for all environments using ~5 MHz spectrum is challenging for both the physical layer and radio resource management design. Single-carrier TDMA solutions are limited in supportable transmission bit rate by equalizer complexity. Even though new techniques such as interfer￾ence suppression and space-time processing are promising, the interactions of these techniques with equalization significantly lower achievable bit rates in hostile operating environments for single-carrier solutions. Low spreading gain or intercode interference at high bit rates limits CDMA solutions. The use of OFDM with suffi￾ciently long symbol periods of 100–200 ms for packet data transmission addresses these issues. It supports a high bit rate in time delay spread environments with performance that improves with increasing delay spread up to a point of extreme dispersion. Another reason to consider a complementary packet data solution is to use optimized admission procedures for packet data access that is fairly aggressive in order to achieve high spectral efficiency. An aggressive admission policy will result in high word error rates (WERs) that can generally be managed for Internet services using automatic repeat request (ARQ) techniques but are problematic for delay-sensitive voice services. Therefore, a com￾OFDM can largely eliminate the effects of intersymbol interference for high-speed transmission rates in very dispersive environments, and it readily supports interference suppression and space-time coding to enhance efficiency. 1 Peak rates exceeding 1 Mb/s under limited condi￾tions for very few simulta￾neous users are also considered for some sys￾tems. 2 In [4] we focused on the architecture of such a system in a macrocullar system. This article pro￾vides a detailed discussion of the design considera￾tions under different con￾ditions. However, the numerical results shown in [4] were based on an improved radio link design using convolution￾al codes to achieve even better performance

lementary high-peak-rate packet dat ili- the application of multiple transmit antennas for With the wider ty designed with non-delay-sensitive sending adjacent subchannel signals to achieve a priority is attractive. In this article lurIn bandwidth OFDM to overcome physical layer for hopping or interleaving in the time domain 9 requency diversity without red iscussed in this attaining high bit rates, and we consider DPA to which introduces delay. More advanced trans- enable aggressive packet access with high s nitter diversity based on space-time odin trum efficiency. In addition, we will also discuss can further enhance spectrum efficiency provid subchannels are a frame structure which allows flexibility to ed accurate channel estimation is available. Sim accommodate low-delay services with small fied transmitter diversity can be achieved by available which resources, so potential benefits of multimedia transmitting the same OFDM symbols on multi- services can be realized ple antennas with delayed transmission times The re manae of this article is organized as With the wider bandwidth discussed in this arti follows. We discuss OFDM-based physical layer cle, many subchannels are available, which pro- techniques and DPA-based medium access con- vides a possibility to achieve good performance achieve goo trol(MAC)techniques for realizing the proposed by exploiting time and frequency diversity with- wideband oFDM system. Through a combination out multiple transmit antennas. of OFDM, DPA, adaptive modulation and cod Assume a bandwidth of 5 MHz is divided into exploiting time ing, smart antennas, and space-time coding, dif- about 20 radio resources of 200 kHz each with 1 erent bit rates can be provided with varying MHz reserved for guard bands. Every 200-kHz and frequency efficiency and robustness We describe a possible radio resource can be constructed by grouping a diversity without frame structure in which all these techniques can cluster of (25)8-kHz subchannels. Frequency be implemented for both large-resource high-rate diversity can be achieved by hopping over differ data services and small-resource low-delay ser- ent clusters on different time slots. The same transmit vices. Simulation results based on the large hopping pattern is repeated once every frame of resource assignment procedure are shown to 8 slots. Up to 20 users can be simultaneously antennas demonstrate the potential performance achiev- assigned, one resource each, using different hop- ble in macrocellular environments. We conclude ping patterns that are free from collisions. High this article by outlining important attributes of rate users can be assigned multiple or al this proposal and areas for further study resources. Date rates equivalent to a fraction of nominal radio resource can also be assigned by cheduling transmission in the time domain. w PHYSICAL AND MAC LAYER will discuss assignment of large and small TECHNIQUES AND DEPLOYMENT esources for different applications. a key fea- ture of a 5 MHz bandwidth is the availability of ScENARIOS diversity and interleaving in both time and fre quency domains, which enables high coding gain This section discusses how wideband OFDM can to achieve performance enhancement using a be implemented in both macrocells and micro- single transmit antenna. cells to provide ubiquitous broadband services OFDM has been proposed for the physical Most of the techniques discussed next for macro- layer for ACIS in macrocells with 1-2 b/s/Hz cells are also applicable to enable wideband channel coding using mode adaptation with OFDM in microcells with potential for even quadrature phase shift keying(QPSK)and 8- Igher rates. PSK modulation to support peak bit rates up to 1 Mb/s channe WIDEBAND OFDM IN MACROCELLS ls[3」.This allows for various overheads to account for up to Physical Layer Techniques- In typical wire- 50 percent of the total available bandwidth. With line applications, communication channels are a 4 MHz bandwidth, similar to WCDMA, generally static over the connection period. In 5 Mb/s can be achieved. OFDM provides this case, OFDM subchannel power and bit allo- support for interference suppression and s cation can be optimized through measurement antennas [7] because the effects of dispersion and feedback in the initial link setup process. can be removed at a receiver easily by first pro- Measurement errors and feedback delay signifi- cessing each antennas signal with a discrete cantly reduce the performance of this technique Fourier transform(DFT) before combining with in time-varying wireless fading channels. In wire- an interference suppression algorithm Packet less channels, good link performance can be data wireless access tends to be dominant-inter achieved by OFDM when combined with diversi- ference-limited, so linear interference suppre ty, interleaving, and coding [2]. OFDM inherent- sion techniques are effective to increase capacity ly provides frequency diversity over subchannels, with a two-branch receiver. These technique which introduces an opportunity for interleaving support operation near 0 dB signal-to-interfc in the frequency domain. However, adjacent ence(S/T)and at about 5 dB signal-to-noise ratio subchannels may still be highly correlated. Sony (SNR) for 1 b/s/Hz coding 7] has proposed an OFDM-based scheme [5]using One of the strong challenges of providing up time-domain interleaving combined with fre- to 5 Mb/s transmission rates on downlinks for quency hopping to enhance performance. This packet data in macrocells is the link budget. RF ystem also uses frequency hopping to achieve power amplifier cost is a major factor in base station cost, and it is a major contributor to when high peak rate is desired power supply requirements, heat management, while bandwidth is limited, there may generally and equipment size. An IS-136 channel deliver not be enough"clusters"of subchannels to use about 24 kb/s of coded user data with acceptable for frequency hopping Reference [ 3] proposed quality on a fading channel at about 17 dB SNR IEEE Communications Magazine. July 2000

80 IEEE Communications Magazine • July 2000 plementary high-peak-rate packet data capabili￾ty designed with non-delay-sensitive services as a priority is attractive. In this article we consider OFDM to overcome physical layer barriers for attaining high bit rates, and we consider DPA to enable aggressive packet access with high spec￾trum efficiency. In addition, we will also discuss a frame structure which allows flexibility to accommodate low-delay services with small resources, so potential benefits of multimedia services can be realized. The remainder of this article is organized as follows. We discuss OFDM-based physical layer techniques and DPA-based medium access con￾trol (MAC) techniques for realizing the proposed wideband OFDM system. Through a combination of OFDM, DPA, adaptive modulation and cod￾ing, smart antennas, and space-time coding, dif￾ferent bit rates can be provided with varying efficiency and robustness. We describe a possible frame structure in which all these techniques can be implemented for both large-resource high-rate data services and small-resource low-delay ser￾vices. Simulation results based on the large resource assignment procedure are shown to demonstrate the potential performance achiev￾able in macrocellular environments. We conclude this article by outlining important attributes of this proposal and areas for further study. PHYSICAL AND MAC LAYER TECHNIQUES AND DEPLOYMENT SCENARIOS This section discusses how wideband OFDM can be implemented in both macrocells and micro￾cells to provide ubiquitous broadband services. Most of the techniques discussed next for macro￾cells are also applicable to enable wideband OFDM in microcells with potential for even higher rates. WIDEBAND OFDM IN MACROCELLS Physical Layer Techniques — In typical wire￾line applications, communication channels are generally static over the connection period. In this case, OFDM subchannel power and bit allo￾cation can be optimized through measurement and feedback in the initial link setup process. Measurement errors and feedback delay signifi￾cantly reduce the performance of this technique in time-varying wireless fading channels. In wire￾less channels, good link performance can be achieved by OFDM when combined with diversi￾ty, interleaving, and coding [2]. OFDM inherent￾ly provides frequency diversity over subchannels, which introduces an opportunity for interleaving in the frequency domain. However, adjacent subchannels may still be highly correlated. Sony has proposed an OFDM-based scheme [5] using time-domain interleaving combined with fre￾quency hopping to enhance performance. This system also uses frequency hopping to achieve interference averaging. However, when high peak rate is desired while bandwidth is limited, there may generally not be enough “clusters” of subchannels to use for frequency hopping. Reference [3] proposed the application of multiple transmit antennas for sending adjacent subchannel signals to achieve frequency diversity without requiring frequency hopping or interleaving in the time domain, which introduces delay. More advanced trans￾mitter diversity based on space-time coding [6] can further enhance spectrum efficiency provid￾ed accurate channel estimation is available. Sim￾plified transmitter diversity can be achieved by transmitting the same OFDM symbols on multi￾ple antennas with delayed transmission times. With the wider bandwidth discussed in this arti￾cle, many subchannels are available, which pro￾vides a possibility to achieve good performance by exploiting time and frequency diversity with￾out using multiple transmit antennas. Assume a bandwidth of 5 MHz is divided into about 20 radio resources of 200 kHz each with 1 MHz reserved for guard bands. Every 200-kHz radio resource can be constructed by grouping a cluster of (25) 8-kHz subchannels. Frequency diversity can be achieved by hopping over differ￾ent clusters on different time slots. The same hopping pattern is repeated once every frame of 8 slots. Up to 20 users can be simultaneously assigned, one resource each, using different hop￾ping patterns that are free from collisions. High￾rate users can be assigned multiple or all resources. Date rates equivalent to a fraction of a nominal radio resource can also be assigned by scheduling transmission in the time domain. We will discuss assignment of large and small resources for different applications. A key fea￾ture of a 5 MHz bandwidth is the availability of diversity and interleaving in both time and fre￾quency domains, which enables high coding gain to achieve performance enhancement using a single transmit antenna. OFDM has been proposed for the physical layer for ACIS in macrocells with 1–2 b/s/Hz channel coding using mode adaptation with quadrature phase shift keying (QPSK) and 8- PSK modulation to support peak bit rates up to 1 Mb/s in about 800 kHz channels [3]. This allows for various overheads to account for up to 50 percent of the total available bandwidth. With a 4 MHz bandwidth, similar to WCDMA, up to 5 Mb/s can be achieved. OFDM provides good support for interference suppression and smart antennas [7] because the effects of dispersion can be removed at a receiver easily by first pro￾cessing each antenna’s signal with a discrete Fourier transform (DFT) before combining with an interference suppression algorithm. Packet data wireless access tends to be dominant-inter￾ference-limited, so linear interference suppres￾sion techniques are effective to increase capacity with a two-branch receiver. These techniques support operation near 0 dB signal-to-interfer￾ence (S/I) and at about 5 dB signal-to-noise ratio (SNR) for 1 b/s/Hz coding [7]. One of the strong challenges of providing up to 5 Mb/s transmission rates on downlinks for packet data in macrocells is the link budget. RF power amplifier cost is a major factor in base station cost, and it is a major contributor to power supply requirements, heat management, and equipment size. An IS-136 channel delivers about 24 kb/s of coded user data with acceptable quality on a fading channel at about 17 dB SNR. With the wider bandwidth discussed in this article, many subchannels are available, which provides a possibility to achieve good performance by exploiting time and frequency diversity without using multiple transmit antennas

Therefore, 2.5 Mb/s would require 100 times as cuit services. As a result, the dCa gain is limit much transmit power(20 dB)unless additional ed to somewhat better traffic resource utiliza One of the echniques are introduced. Smart antenna tech- tion, which may be achieved at the cost of nology using four switched 30" beams in a 120 nonoptimal interference management. To benefits of dpa sector is now a well-developed technology with achieve the potential of DCA gain, channel reas- some early deployment. This technology pro- signments must take place at high speed to avoid vides up to 6 dB in link budget improvement rapidly changing interference. DPA, based on and also improves capacity. Terminal two-branch properties of an OFDM physical layer, is pre receiver diversity combined with concatenated posed, which reassigns transmission resources on onvolutional/Reed-Solomon coding supports a packet-by-packet basis using high-speed receiv it is relatively receiver sensitivities of less than 5 dB SNR with er measurements to overcome these problems 1 b/s/Hz coding Space-time coding can provide [9 Having orthogonal subchannels well defined Insensitive to SNR gain based on transmit diversity By com- in time-frequency grids, OFDM has a key advan bining smart antenna technology at base stations e here with the ability to rapidly measure errors In power with terminal receiver sensitivities of less than 5 interference or path loss parameters in parallel dB SNR, the downlink for wideband OFDM can on all candidate channels, either directly or upport peak transmission rates of 2-5 Mb/s with based on pilot tones. One of the benefits of about the same transmit power and coverage as Dpa based on interference avoidance is that it performance even a single transceiver for IS-136 TDMA or analog is relatively insensitive to errors in power con ellular technolog rol, and provides good performance even with without power out power control. Reference 8 shows that control MAC-Layer Techniques- Very high spec Ca without power control decreases capacity trum efficiency will be required for wideband to a factor of 2. however. even without OFDM, particularly for macrocellular opera- control, interference avoidance can outperform tion First-generation cellular systems used interference averaging with power control. This fixed channel assignment. Second-generation is particularly advantageous for packet transmis- cellular systems generally use fixed channel sion where effective power control is problemat- assignment or interference averaging with due to the rapid arrival and departure of pread spectrum. WCDMA will also use inter- interfering packets. ference averaging. Interference avoidance ol The basic protocol for a downlink comprises dynamic channel assignment(DCA)has been four basic steps used in some systems, generally as a means of . A packet automatic channel assignment or local capacity minal page from a base station to a ter- enhancement, but not as a means of large sys-. Rapid measurements of resource usage by a temwide capacity enhancement. Some of the tial capacity gain of DCA are the difficulties .A short report from the terminal to the base introduced by rapid ch station of the potential transmission quality intensive receiver measurements required by a (a unit of high-performance DCA or interference avoid width that is separately assignable) ance algorithm. OFDM pre mises to overcome Selection of resources by the base and trans- these challenging implementation issues. It was mission of the data shown by Pottie 8] that interference averaging This protocol could be modified to move some techniques can perform better than fixed chan- of the over-the-air functions into fixed network nel assignment techniques, whereas interfer- transmission functions to reduce wireless trans ence avoidance techniques can outperform mission overhead at the cost of more demand- interference averaging techniques by a factor of ing fixed network transmission requirements. 2-3 in spectrum efficiency The frame structures of adjacent base stations For existing second-generation systems, the are staggered in time(i.e. neighboring ba ntially b/s/Hz/sector(assuming 3 sectors/cell) is much DPA functions outlined above with a predeter lower than that shown in [8 which was obtained mined rotation schedule). This avoids collisions under idealized conditions. IS-136 TDMA today of channel assignments(i.e, the possibility for provides a spectrum efficiency of about 4 per- adjacent base stations to independently select cent(3 x 8 kb/30 kHz x 1/21 reuse). GSM also the same channel, thus causing interference provides a spectrum efficiency of about 4 per- when transmissions occur). In addition to cent(8 x 13 kb/200 kHz x 1/12 reuse) IS-95 achieving much of the potential gain of a rapid CDMA provides a spectrum efficiency of 4 per- interference avoidance protocol, this protocol cent to 7 percent(12 to 20 x 8 kb/1250 kHz x 1 provides a good basis for admission control and reuse x 1/2 voice activity). DCA combined with mode(bit rate)adaptation based on measured circuit-based technology(which is the approach generally taken to date)can provide some bene Figure l shows the performance of this algo- fits. However, it cannot provide large capacity rithm with several modulation/coding schemes gains, because of the dynamics of interference in and with either two-branch maximal-ratio-com a mobile system as well as the difficulty in imple- bining or two-branch receiver interference sup menting rapid channel reassignments In circuit- pression using packet traffic models based on based systems channel variations, especially Internet statistics [9]. Results with interferenc those caused by the change of shadow fading, suppression for space-time coding are not includ are frequently faster than what can be adapted ed because each transmitted signal appears as by the slow assignment cycle possible in the cir- multiple signals, which significantly limits the

IEEE Communications Magazine • July 2000 81 Therefore, 2.5 Mb/s would require 100 times as much transmit power (20 dB) unless additional techniques are introduced. Smart antenna tech￾nology using four switched 30˚ beams in a 120˚ sector is now a well-developed technology with some early deployment. This technology pro￾vides up to 6 dB in link budget improvement and also improves capacity. Terminal two-branch receiver diversity combined with concatenated convolutional/Reed-Solomon coding supports receiver sensitivities of less than 5 dB SNR with 1 b/s/Hz coding. Space-time coding can provide SNR gain based on transmit diversity. By com￾bining smart antenna technology at base stations with terminal receiver sensitivities of less than 5 dB SNR, the downlink for wideband OFDM can support peak transmission rates of 2–5 Mb/s with about the same transmit power and coverage as a single transceiver for IS-136 TDMA or analog cellular technologies. MAC-Layer Techniques — Very high spec￾trum efficiency will be required for wideband OFDM, particularly for macrocellular opera￾tion. First-generation cellular systems used fixed channel assignment. Second-generation cellular systems generally use fixed channel assignment or interference averaging with spread spectrum. WCDMA will also use inter￾ference averaging. Interference avoidance or dynamic channel assignment (DCA) has been used in some systems, generally as a means of automatic channel assignment or local capacity enhancement, but not as a means of large sys￾temwide capacity enhancement. Some of the reasons for not fully exploiting the large poten￾tial capacity gain of DCA are the difficulties introduced by rapid channel reassignment and intensive receiver measurements required by a high-performance DCA or interference avoid￾ance algorithm. OFDM promises to overcome these challenging implementation issues. It was shown by Pottie [8] that interference averaging techniques can perform better than fixed chan￾nel assignment techniques, whereas interfer￾ence avoidance techniques can outperform interference averaging techniques by a factor of 2–3 in spectrum efficiency. For existing second-generation systems, the achieved spectrum efficiency measured in b/s/Hz/sector (assuming 3 sectors/cell) is much lower than that shown in [8], which was obtained under idealized conditions. IS-136 TDMA today provides a spectrum efficiency of about 4 per￾cent (3 x 8 kb/30 kHz x 1/21 reuse). GSM also provides a spectrum efficiency of about 4 per￾cent (8 x 13 kb/200 kHz x 1/12 reuse). IS-95 CDMA provides a spectrum efficiency of 4 per￾cent to 7 percent (12 to 20 x 8 kb/1250 kHz x 1 reuse x 1/2 voice activity). DCA combined with circuit-based technology (which is the approach generally taken to date) can provide some bene￾fits. However, it cannot provide large capacity gains, because of the dynamics of interference in a mobile system as well as the difficulty in imple￾menting rapid channel reassignments. In circuit￾based systems channel variations, especially those caused by the change of shadow fading, are frequently faster than what can be adapted by the slow assignment cycle possible in the cir￾cuit services. As a result, the DCA gain is limit￾ed to somewhat better traffic resource utiliza￾tion, which may be achieved at the cost of nonoptimal interference management. To achieve the potential of DCA gain, channel reas￾signments must take place at high speed to avoid rapidly changing interference. DPA, based on properties of an OFDM physical layer, is pro￾posed, which reassigns transmission resources on a packet-by-packet basis using high-speed receiv￾er measurements to overcome these problems [9]. Having orthogonal subchannels well defined in time-frequency grids, OFDM has a key advan￾tage here with the ability to rapidly measure interference or path loss parameters in parallel on all candidate channels, either directly or based on pilot tones. One of the benefits of DPA based on interference avoidance is that it is relatively insensitive to errors in power con￾trol, and provides good performance even with￾out power control. Reference [8] shows that DCA without power control decreases capacity up to a factor of 2. However, even without power control, interference avoidance can outperform interference averaging with power control. This is particularly advantageous for packet transmis￾sion where effective power control is problemat￾ic due to the rapid arrival and departure of interfering packets. The basic protocol for a downlink comprises four basic steps: • A packet page from a base station to a ter￾minal • Rapid measurements of resource usage by a terminal using the parallelism of an OFDM receiver • A short report from the terminal to the base station of the potential transmission quality associated with each resource (a unit of band￾width that is separately assignable) • Selection of resources by the base and trans￾mission of the data This protocol could be modified to move some of the over-the-air functions into fixed network transmission functions to reduce wireless trans￾mission overhead at the cost of more demand￾ing fixed network transmission requirements. The frame structures of adjacent base stations are staggered in time (i.e., neighboring base sta￾tions sequentially perform the four different DPA functions outlined above with a predeter￾mined rotation schedule). This avoids collisions of channel assignments (i.e., the possibility for adjacent base stations to independently select the same channel, thus causing interference when transmissions occur). In addition to achieving much of the potential gain of a rapid interference avoidance protocol, this protocol provides a good basis for admission control and mode (bit rate) adaptation based on measured signal quality. Figure 1 shows the performance of this algo￾rithm with several modulation/coding schemes and with either two-branch maximal-ratio-com￾bining or two-branch receiver interference sup￾pression using packet traffic models based on Internet statistics [9]. Results with interference suppression for space-time coding are not includ￾ed because each transmitted signal appears as multiple signals, which significantly limits the One of the benefits of DPA based on interference avoidance is that it is relatively insensitive to errors in power control, and it provides good performance even without power control

oles and building walls. In addition, high bit rates are desirable to provide a capability as near to that of wired access as possible. For 8 indoor and private system access, unlicensed pectrum at 5 GHz or higher may be desirable, delay div, int sup where large bandwidths are available. For ≥ these environments. small antennas are equired. Because of the large angular spread 2 b/s/Hz experienced at radio ports located in the clut ter of buildings and trees, simple omnidirec tional or low-gain antennas are appropriate. In 920 that environment. antenna beam switchi provides limited gains in performance, but 15 adaptive antenna arrays and/or space-time coding can be very effective. For example, in a 5 MHz channel, peak rates of 10 Mb/s could 1 b/s/Hz 5 be supported using two transmit and two eceive antennas for the radio link with space ding of 16-quadrature 0 lation(QAM) to achieve a 4 b/s/Hz codis 0 Occupancy (% rate while allowing for about 50 percent over head Mode adaptation to 5 or 2 Mb/s would a Figure 1. Performance as a function of occupancy for different modulation support appropriate link budgets for robust and diversity schemes Microcell radio ports could be implemented that provide little more than radio modem func- tions to allow for very small radio ports. One suppression of interference. These results are possible approach is to use a combination of based on an ofdm radio link with a bandwidt dual antennas at each port and multiport pro- of about 800 kHz, and the bit rates in the follow- cessing per user at a centralized headend For ng discussion are scaled up for an occupied example, if a user delivers, on average, a strong bandwidth of 4 MHz. A system is considered signal to M ports, the dual-branch signals ba with three sectors per base station, each having a hauled from the M "best" ports can be transceiver. All base stations share one wideband cessed at the central site using selection or FDM RF channel by using DPA to avoid co- combining techniques. Simulation studies have annel interference. DPA enables frequency shown that grouping of microcell ports in this use in the time domain among all radio esults transceivers Occupancy is defined to be the frac- ty and capacity due to macroscopic diversity tion of slots being used. As traffic intensity Moreover, this approach requires a minimal increases, occupancy increases, which results in amount of processing at the ports, thus keeping higher interference and more retransmissions. them simple. The processing at the central site Power control was not used to obtain these can also be fairly simple if the signals being esults. Simulation results based on the wideband combined are not dispersed by significant multi- set of parameters will be presented following a path propagation. The grouping approach i description of a possible frame structure. These therefore compatible with the use of OFDM, results show that good performance is obtained herein each frequency (or subgroup of fre with 1 b/s/Hz coding even at an average occupan-qu s)can be processed with paramete cy per base station of 100 percent(33 percent per optimized for that frequency. This kind of pro sector). With two-branch interference suppression cessing works best with time-division duplexing and 1 b/s/Hz coding, the average retransmission (TDD), which requires using the same carrier probability is only about 3 percent throughout the frequency for transmission and reception. This system with the average delivered bit rate of onsistent with the planning for very high about 2.5 Mb/s per base station Using ARQ at speed micro- and picocellular services in third the radio link layer will permit Internet service at generation systems. this retransmission probability with good quality Backhaul could be a significant cost of service (QoS). Higher retransmission probabili- microcellular systems. Various innovative way ty may be acceptable at the expense of longer to use fiber, coax, microwave radio, and millime packet delay. Peak rates up to 5 Mb/s are possible ter-wave radio can be envisioned to make this with lower occupancies using 2 b/s/Hz coding. part of the system reliable. The key require Finally, in addition to interference suppression at ments are to deploy microcells only in areas the receiver, beam switching smart antenna tech- where there is a strong expectation of high niques, performed by the transmitter, can also be demand and provide wide-area applied to reduce interference, thus achieving coverage with a compatible technology. y per bas at 5 Mb/s good performane DPA requires low delay between the air inter- face and resource assignment function, so any architecture that minimizes radio port function- WIDEBAND OFDM IN MICROCELLS ality would need to consider that constraint. This For microcell deployment, very compact radio also means that DPA should allow able to permit convenient siting on existing equipmen, delay in microcellular ports with low power requirements are desir timing for IEEE Communications Magazine. July 2000

82 IEEE Communications Magazine • July 2000 suppression of interference. These results are based on an OFDM radio link with a bandwidth of about 800 kHz, and the bit rates in the follow￾ing discussion are scaled up for an occupied bandwidth of 4 MHz. A system is considered with three sectors per base station, each having a transceiver. All base stations share one wideband OFDM RF channel by using DPA to avoid co￾channel interference. DPA enables frequency reuse in the time domain among all radio transceivers. Occupancy is defined to be the frac￾tion of slots being used. As traffic intensity increases, occupancy increases, which results in higher interference and more retransmissions. Power control was not used to obtain these results. Simulation results based on the wideband set of parameters will be presented following a description of a possible frame structure. These results show that good performance is obtained with 1 b/s/Hz coding even at an average occupan￾cy per base station of 100 percent (33 percent per sector). With two-branch interference suppression and 1 b/s/Hz coding, the average retransmission probability is only about 3 percent throughout the system with the average delivered bit rate of about 2.5 Mb/s per base station. Using ARQ at the radio link layer will permit Internet service at this retransmission probability with good quality of service (QoS). Higher retransmission probabili￾ty may be acceptable at the expense of longer packet delay. Peak rates up to 5 Mb/s are possible with lower occupancies using 2 b/s/Hz coding. Finally, in addition to interference suppression at the receiver, beam switching smart antenna tech￾niques, performed by the transmitter, can also be applied to reduce interference, thus achieving good performance at 5 Mb/s even at 100 percent occupancy per base station. WIDEBAND OFDM IN MICROCELLS For microcell deployment, very compact radio ports with low power requirements are desir￾able to permit convenient siting on existing poles and building walls. In addition, high bit rates are desirable to provide a capability as near to that of wired access as possible. For indoor and private system access, unlicensed spectrum at 5 GHz or higher may be desirable, where large bandwidths are available. For these environments, small antennas are required. Because of the large angular spread experienced at radio ports located in the clut￾ter of buildings and trees, simple omnidirec￾tional or low-gain antennas are appropriate. In that environment, antenna beam switching provides limited gains in performance, but adaptive antenna arrays and/or space-time coding can be very effective. For example, in a 5 MHz channel, peak rates of 10 Mb/s could be supported using two transmit and two receive antennas for the radio link with space￾time coding of 16-quadrature amplitude modu￾lation (QAM) to achieve a 4 b/s/Hz coding rate while allowing for about 50 percent over￾head. Mode adaptation to 5 or 2 Mb/s would support appropriate link budgets for robust coverage. Microcell radio ports could be implemented that provide little more than radio modem func￾tions to allow for very small radio ports. One possible approach is to use a combination of dual antennas at each port and multiport pro￾cessing per user at a centralized headend. For example, if a user delivers, on average, a strong signal to M ports, the dual-branch signals back￾hauled from the M “best” ports can be pro￾cessed at the central site using selection or combining techniques. Simulation studies have shown that grouping of microcell ports in this way can yield impressive results in link reliabili￾ty and capacity due to macroscopic diversity. Moreover, this approach requires a minimal amount of processing at the ports, thus keeping them simple. The processing at the central site can also be fairly simple if the signals being combined are not dispersed by significant multi￾path propagation. The grouping approach is therefore compatible with the use of OFDM, wherein each frequency (or subgroup of fre￾quencies) can be processed with parameters optimized for that frequency. This kind of pro￾cessing works best with time-division duplexing (TDD), which requires using the same carrier frequency for transmission and reception. This is consistent with the planning for very high￾speed micro- and picocellular services in third￾generation systems. Backhaul could be a significant cost issue in microcellular systems. Various innovative ways to use fiber, coax, microwave radio, and millime￾ter-wave radio can be envisioned to make this part of the system reliable. The key require￾ments are to deploy microcells only in areas where there is a strong expectation of high￾speed service demand and to provide wide-area coverage with a compatible technology. DPA requires low delay between the air inter￾face and resource assignment function, so any architecture that minimizes radio port function￾ality would need to consider that constraint. This also means that DPA should allow some margin in timing for delay in microcellular transmission equipment. ■ Figure 1. Performance as a function of occupancy for different modulation and diversity schemes. 0 Retransmission probability (%) Occupancy (%) 1 b/s/Hz 2 b/s/Hz 0 5 10 15 20 25 30 35 40 45 10 20 30 40 QPSK, space-time coding QPSK, delay diversity QPSK, delay div, int sup 8PSK, delay div, int sup

SYSTEM PARAMETERS AND Three control FRAME STRUCTURE channels 22 packet data channels The frame structure described in this section supports both control information, which is needed to perform the dpa procedure, as well as the bearer traffic. a frame is 20 ms. The con trol part uses a staggered schedule, in which only one base station at a time, from a group of four adjacent base stations, transmits informa tion for DPA.3 The bearer traffic. on the other EE5 hand, is transmitted on the assigned radio resources 4(“ channels”) without a staggered schedule. To implement a staggered schedule four frames(80 ms) are grouped as a"super frame. Effectively this achieves a reuse factor 24 OFDM blocks I 104 OFDM blocks in 8 slots of 4 for control information while allowing a reuse factor of I for bearer traffic by using DPA Figure 2. Division of radio resources in time and frequency domains to allow i.e. all traffic channels can be used ever DPA for high-peak-rate data services; small radio resources, needed for low- where). A reuse factor of 4 and three sectorized delay services, occupy only one slot, which is further divided into four mini lots to allow coding across different frequency clusters error protection for the control channels, where as interference avoidance based on dpa with admission control provides good quality for the achieve this frequency diversity for small resources traffic channels a slot is divided into mini-slots at the cost of The total bandwidth is divided into 8-kHz sub- reduced efficiency due to TDMA overhead channels ("tones"). In the time domain, this can be constructed by grouping OFDM blocks(blocks HIGH-PEAK-RATE DATA SERVICES: of OFDM subchannels) with a 125 us signaling LARGE RADIO RESOURCES interval and a 31.25 us guard time to accommo date significant delay spread in macrocells. In the 528 subchannels(4.224 MHz) are organized into following discussion, the duration of an OFDI 2 clusters of 24 subchannels(192 kHz)each block (or simply"block"), 156.25 us, is used as the and 8 time slots of 13 OFDM blocks each within basic time unit in the discussion of the frame a 20-ms frame of 128 blocks, figure 2 shows this structure. A frame of 20 ms is equivalent to 128 resource allocation scheme. The control channel blocks. This corresponds to a 6. 4 kbaud block rate. functions are defined in[3]. This allows flexibili- Also a total of 528 subchannels are considered in channel resulting in a 4. 224 MHz bandwidth. The discus- blocks of control overheard to perform the dPA sion below focuses on the case of oPSK modula- tion and 1/2-rate coding(1 b/s/Hz), resulting in a This arrangement of tone clusters is simila total rate of 3.3792 Mb/s without considering to the arrangements in the band-division multi- other overheads Coding and modulation schemes ple access(BDMA) proposal by Sony. Figure 3 with higher efficiency could provide higher rates, depicts this operation. Each tone cluster would specially for microcellular environment contain 22 individual modulation tones plus 2 Considerations for organization of resources guard tones, and an OFDM block would have a are the resolution of resource size, the overhead time duration of 156.25 us with 31.25 us for required to allocate individual resources, and the guard time and ramp time to minimize the 3 The grouping can be expected size of objects to be transmitted over a effects of delay spread up to about a 20-us span. configured similar to con- resource. Minimization of overhead can be Of the 13 OFDM blocks in each traffic slot, two ventional frequency plan into large resources, but if many objects are rela- leading block for synchronization(phase/fre- using a regular and repet- tively small in size or higher-layer protocols gener- quency/timing acquisition and channel estima- tive pattern, for example, ate small objects that the lower layers must carry, on)and a trailing block as guard time for with timing groups I and there will be a need to allocate small resources to separating consecutive time slots. A single radio 2 altemating in the odd equire resources that are small locally in time pattern, by which the packets are transmitted in there. d groups 3 and 4 achieve good efficiency. Also, streaming data may resource is associated with a frequency-hoppi rows an avoid the need for buffe ource bits before using eight different tone clusters in each of the transmission, which causes delay. A 2 Mb/s system eight traffic slots. Coding across eight traffic 4 The word"resource"is with 20-25 resources would support about 80-100 ts for user data, as shown in Fig. 3, exploits used to emphasize that th kb/s rates locally in time. This rate would be suit- frequency diversity which gives sufficient coding assignment of radio chan- able for high-bit-rate data services. If supporting gain for performance enhancement in the fading nels for traffic bearers can about 10 kb/s locally in time were desirable(e. g, channel. This arrangement supports 22 resources be a general combination voice or audio services of 8 kb/s with additional in frequency that can be assigned by DPA. Tak- of time slots, frequen coding for error correction in wireless channels), ing into account overhead for OFDM block sub-carriers and user this would be equivalent to about 200 resources. In guard time, synchronization, slot separation, and codes. The user code con- the following, small resource assignment is consid- DPA control, a peak data rate of 2. 1296(3.3792 trol the sequence by which ered using only one of the 8 slots in a 20-ms frame. x 22/24 x 11/13 x 104/128)Mb/s is available for a given user access differ- Frequency hopping over different slots is employed packet date services using all 22 radio resources, ent frequency sub-carmier to gain frequency diversity for large resources. To each 96.8 kb at different time slots. Magazine·Ju

IEEE Communications Magazine • July 2000 83 SYSTEM PARAMETERS AND FRAME STRUCTURE The frame structure described in this section supports both control information, which is needed to perform the DPA procedure, as well as the bearer traffic. A frame is 20 ms. The con￾trol part uses a staggered schedule, in which only one base station at a time, from a group of four adjacent base stations, transmits informa￾tion for DPA.3 The bearer traffic, on the other hand, is transmitted on the assigned radio resources4 (“channels”) without a staggered schedule. To implement a staggered schedule, four frames (80 ms) are grouped as a “super￾frame.” Effectively, this achieves a reuse factor of 4 for control information while allowing a reuse factor of 1 for bearer traffic by using DPA (i.e., all traffic channels can be used every￾where). A reuse factor of 4 and three sectorized antennas in each base station provide extra error protection for the control channels, where￾as interference avoidance based on DPA with admission control provides good quality for the traffic channels. The total bandwidth is divided into 8-kHz sub￾channels (“tones”). In the time domain, this can be constructed by grouping OFDM blocks (blocks of OFDM subchannels) with a 125 ms signaling interval and a 31.25 ms guard time to accommo￾date significant delay spread in macrocells. In the following discussion, the duration of an OFDM block (or simply “block”), 156.25 ms, is used as the basic time unit in the discussion of the frame structure. A frame of 20 ms is equivalent to 128 blocks. This corresponds to a 6.4 kbaud block rate. Also, a total of 528 subchannels are considered, resulting in a 4.224 MHz bandwidth. The discus￾sion below focuses on the case of QPSK modula￾tion and 1/2-rate coding (1 b/s/Hz), resulting in a total rate of 3.3792 Mb/s without considering other overheads. Coding and modulation schemes with higher efficiency could provide higher rates, especially for microcellular environments. Considerations for organization of resources are the resolution of resource size, the overhead required to allocate individual resources, and the expected size of objects to be transmitted over a resource. Minimization of overhead can be achieved by organizing the available bandwidth into large resources, but if many objects are rela￾tively small in size or higher-layer protocols gener￾ate small objects that the lower layers must carry, there will be a need to allocate small resources to achieve good efficiency. Also, streaming data may require resources that are small locally in time to avoid the need for buffering source bits before transmission, which causes delay. A 2 Mb/s system with 20–25 resources would support about 80–100 kb/s rates locally in time. This rate would be suit￾able for high-bit-rate data services. If supporting about 10 kb/s locally in time were desirable (e.g., voice or audio services of 8 kb/s with additional coding for error correction in wireless channels), this would be equivalent to about 200 resources. In the following, small resource assignment is consid￾ered using only one of the 8 slots in a 20-ms frame. Frequency hopping over different slots is employed to gain frequency diversity for large resources. To achieve this frequency diversity for small resources a slot is divided into mini-slots, at the cost of reduced efficiency due to TDMA overhead. HIGH-PEAK-RATE DATA SERVICES: LARGE RADIO RESOURCES 528 subchannels (4.224 MHz) are organized into 22 clusters of 24 subchannels (192 kHz) each and 8 time slots of 13 OFDM blocks each within a 20-ms frame of 128 blocks. Figure 2 shows this resource allocation scheme. The control channel functions are defined in [3]. This allows flexibili￾ty in channel assignment while providing 24 blocks of control overheard to perform the DPA procedures. This arrangement of tone clusters is similar to the arrangements in the band-division multi￾ple access (BDMA) proposal by Sony. Figure 3 depicts this operation. Each tone cluster would contain 22 individual modulation tones plus 2 guard tones, and an OFDM block would have a time duration of 156.25 ms with 31.25 ms for guard time and ramp time to minimize the effects of delay spread up to about a 20-ms span. Of the 13 OFDM blocks in each traffic slot, two blocks are used as overhead, which includes a leading block for synchronization (phase/fre￾quency/timing acquisition and channel estima￾tion) and a trailing block as guard time for separating consecutive time slots. A single radio resource is associated with a frequency-hopping pattern, by which the packets are transmitted using eight different tone clusters in each of the eight traffic slots. Coding across eight traffic slots for user data, as shown in Fig. 3, exploits frequency diversity which gives sufficient coding gain for performance enhancement in the fading channel. This arrangement supports 22 resources in frequency that can be assigned by DPA. Tak￾ing into account overhead for OFDM block guard time, synchronization, slot separation, and DPA control, a peak data rate of 2.1296 (3.3792 x 22/24 x 11/13 x 104/128) Mb/s is available for packet date services using all 22 radio resources, each 96.8 kb/s. 3 The grouping can be configured similar to con￾ventional frequency plan￾ning for reuse factor 4 using a regular and repeti￾tive pattern, for example, with timing groups 1 and 2 alternating in the odd rows and groups 3 and 4 in the even rows. 4 The word “resource” is used to emphasize that the assignment of radio chan￾nels for traffic bearers can be a general combination of time slots, frequency sub-carriers and user codes. The user code con￾trol the sequence by which a given user access differ￾ent frequency sub-carriers at different time slots. ■ Figure 2. Division of radio resources in time and frequency domains to allow DPA for high-peak-rate data services; small radio resources, needed for low￾delay services, occupy only one slot, which is further divided into four mini￾slots to allow coding across different frequency clusters. x x x 528 tones divided into 22 24-tone clusters Frequency Assignment channel Paging channel Pilot channel 24 OFDM blocks 104 OFDM blocks in 8 slots Three control channels 22 packet data channels

divided into four mini-slots for frequency hop- Code work order Radio resource mappin ing,and one slot is assigned per frame as a I 104 OFDM blocks in 8 slots I basic radio resource. Therefore the frame struc ture is the same as shown in Fig. 2 except that there are 176( 8x 22)small resources, and each resource bit rate is reduced by additional TDMA overhead needed at the be mini-slot The same control channel can be used to assign both large and small resources using taggered frame dPa § Figure 4 depicts the coding scheme for small esources. Each tone cluster would contain 22 Of the 13 ofdm blocks in each traffic slot. three blocks are used as overhead. This includes a t of two leading blocks(duration of one-half block lord index 13 Time for each of the four mini-slots: this can be real- I Figure 3. Coding of a large radio resource with clustered OFDM and fre ized by using one block in every other tone) for ynchronization and a trailing block (one-fourth quency hopping in a frame; radio resource mapping onto OFDMs time/fre- block for each of the four mini-slots)as guard quency structure provides interleaving, which is required for effective error-correction coding time for separating consecutive mini-slots. A sin- gle radio resource is associated with a f hopping pattern, by which the packets are transmitted using four different tone clusters in For the base station, where uplink transmis- each of the four mini-slots Coding across four ion for all radio resources is asynchronous, a lini-slots for user data, as shown in Fig. 4 receiver may separate 192-kHz clusters with filters exploits frequency diversity. However, it shoul followed by independently synchronized demodu- be noted that when large and small resources are lators. For the mobile terminal, where downlink simultaneously assigned in different clusters of ransmission for base station radio resources is given slot, the frequency range over which small typically synchronous, a receiver may use a single resources can hop to achieve frequency diversity demodulator with receiver windowing to result in ght be limited. Mixed assignment of large and strong attenuation of undesired clusters. Howev- small resources is a topic for further study. Tak er, adjacent clusters may be asynchronous if ing into account overhead for OFDM block guard transmitted by different base stations. The time, synchronization, slot separation, and DPA receiver structure requires further study control, a peak data rate of 1.936(3.3792 x 22/24 x 10/13 x 104/128) Mb/s is available using all 176 LOW-DELAY SERVICES: dio resources, each of 11 kb, SMALL RADIO RESOURCES A FRAME STRUCTURE FOR Similar to the previous section, 528 suchan DYNAMIC PACKET ASSIGNMENT (4. 224 MHz) are organized into 22 clusters of subchannels(192 kHz)each and 8 time slots of The downlink structure is shown in Fig. 5. The 13 blocks each within a 128-block(20 ms)frame. uplink structure is similar, but the control func a difference is that these time slots are further tions are slightly different. At the beginning of each frame. the control channels for both the DPA procedures described earlier sequentially 13 OFDM blocks with a predetermined staggered schedule among adjacent base stations. Some control channe overhead is included to allow three sectors to per form DPA at different time periods, thus obtain BB G ing additional signal-to-interference ratio(SIR) enhancement for the control information for traffic channels, spectrum reuse is achieved by interference avoidance using DPa to avoid slots BBB G hat can cause potential interference; a reuse of 1 旨 is achieved with this intelligent"partial loading. This frame structure permits SiR estimation on all unused traffic slots The desired signal mated by the received signal strength from the two OFDM blocks used for paging, while the 1 mini-slot I interference can be estimated by measuring three 1 slot blocks of received pilot signals. The pilot channels are generated by mapping all the radio resources B: 1 OFDM block currently in use onto corresponding pilot sub- G: Guard equivalent to 0.25 OFDM block in duration channels, thus providing an"interference map I Figure 4. Assignment of a small radio resource with clustered OFDM and [3]. The OFDM the actual traffic subchannels scheme can process many sub- frequency hopping in four mini-slots within a slot. hannels in parallel, which provides a mechanism IEEE Communications Magazine. July 2000

84 IEEE Communications Magazine • July 2000 For the base station, where uplink transmis￾sion for all radio resources is asynchronous, a receiver may separate 192-kHz clusters with filters followed by independently synchronized demodu￾lators. For the mobile terminal, where downlink transmission for base station radio resources is typically synchronous, a receiver may use a single demodulator with receiver windowing to result in strong attenuation of undesired clusters. Howev￾er, adjacent clusters may be asynchronous if transmitted by different base stations. The receiver structure requires further study. LOW-DELAY SERVICES: SMALL RADIO RESOURCES Similar to the previous section, 528 subchannels (4.224 MHz) are organized into 22 clusters of 24 subchannels (192 kHz) each and 8 time slots of 13 blocks each within a 128-block (20 ms) frame. A difference is that these time slots are further divided into four mini-slots for frequency hop￾ping, and one slot is assigned per frame as a basic radio resource. Therefore, the frame struc￾ture is the same as shown in Fig. 2 except that there are 176 (8 x 22) small resources, and each resource bit rate is reduced by additional TDMA overhead needed at the beginning and end of a mini-slot. The same control channel can be used to assign both large and small resources using staggered frame DPA. Figure 4 depicts the coding scheme for small resources. Each tone cluster would contain 22 individual modulation tones plus 2 guard tones. Of the 13 OFDM blocks in each traffic slot, three blocks are used as overhead. This includes a total of two leading blocks (duration of one-half block for each of the four mini-slots; this can be real￾ized by using one block in every other tone) for synchronization and a trailing block (one-fourth block for each of the four mini-slots) as guard time for separating consecutive mini-slots. A sin￾gle radio resource is associated with a frequency￾hopping pattern, by which the packets are transmitted using four different tone clusters in each of the four mini-slots. Coding across four mini-slots for user data, as shown in Fig. 4, exploits frequency diversity. However, it should be noted that when large and small resources are simultaneously assigned in different clusters of a given slot, the frequency range over which small resources can hop to achieve frequency diversity might be limited. Mixed assignment of large and small resources is a topic for further study. Tak￾ing into account overhead for OFDM block guard time, synchronization, slot separation, and DPA control, a peak data rate of 1.936 (3.3792 x 22/24 x 10/13 x 104/128) Mb/s is available using all 176 radio resources, each of 11 kb/s. A FRAME STRUCTURE FOR DYNAMIC PACKET ASSIGNMENT The downlink structure is shown in Fig. 5. The uplink structure is similar, but the control func￾tions are slightly different. At the beginning of each frame, the control channels for both the uplink and downlink jointly perform the four DPA procedures described earlier sequentially with a predetermined staggered schedule among adjacent base stations. Some control channel overhead is included to allow three sectors to per￾form DPA at different time periods, thus obtain￾ing additional signal-to-interference ratio (SIR) enhancement for the control information. For traffic channels, spectrum reuse is achieved by interference avoidance using DPA to avoid slots that can cause potential interference; a reuse of 1 is achieved with this intelligent “partial loading.” This frame structure permits SIR estimation on all unused traffic slots. The desired signal is esti￾mated by the received signal strength from the two OFDM blocks used for paging, while the interference can be estimated by measuring three blocks of received pilot signals. The pilot channels are generated by mapping all the radio resources currently in use onto corresponding pilot sub￾channels, thus providing an “interference map” without monitoring the actual traffic subchannels [3]. The OFDM scheme can process many sub￾channels in parallel, which provides a mechanism ■ Figure 3. Coding of a large radio resource with clustered OFDM and fre￾quency hopping in a frame; radio resource mapping onto OFDM's time/fre￾quency structure provides interleaving, which is required for effective error-correction coding. χ x x x x x x x x Word index Time 528 1 1 13 Bit index (within a word) 528 tones divided into 22 24-tone clusters Frequency Code work ordering Radio resource mapping 104 OFDM blocks in 8 slots ■ Figure 4. Assignment of a small radio resource with clustered OFDM and frequency hopping in four mini-slots within a slot. B: 1 OFDM block G: Guard equivalent to 0.25 OFDM block in duration 1 slot Time 1 mini-slot B B B BB B G BB B G BB B G Frequency 528 tones divided into 22 24-tone clusters 13 OFDM blocks G

Superframe Superframe 4 2 4 Control slots 8 traffic slots Control slots 1.BS4 Traffic slots transmits 1,2,3and4 ansmit based 1.5625 15625ms0.625 on dPa hannes/ACK 10 OFDI O OFDM paging informat BS 2 broadcasts 3.BS2,3,4 Bs1.3.4 3 blocks 3 blocks 3 block 1B‖13 blocks s 3 blocks 3 blocks 1B3 block Sector #1 \Sector #2 Sector #3 Guard Sector #1/ Sector#2 Secto井3 Guard Pilots Guard L8 28 Base station □1B28um a Figure 5. A staggered frame structure for downlink dPA for very fast SIR estimation. In addition, since a one radio provides eight traffic slots to deliver total of 528 subchannels are available to map 2 downlink traffic packets. The same channel can large resources and 176 small sources over three be used in different sectors of the same base sta OFDM blocks, significant diversity effects are tion as long as the SiR at the DPA admission achieved to reduce measurement errors. The esti- process exceeds 10 dB. Based on the downlink mated sir is compared to an admission threshold frame structure shown in Fig. 5, four base sta (e. g, 10 dB in our example), so channel occupan- tions in each reuse area take turns performing cy can be controlled to achieve good Qos for the the DPA procedure, and the assignment cycle is admitted users. QoS provisioning for different reused in a fixed pattern. The co-channel-inter services is an area for further study. To reduce ference -limited case is considered; that is, noise time delay for small resource assignment, this ignored in the simulation. In the propagation frame structure can be modified to allow assig model. the ived decre ment of 1/4 resources per frame with distance d as d-4 and the large-scale shad ow-fading distribution is log-normal with a stan- dard deviation of 10 dB. Rayleigh fading is DOWNLINK PERFORMANCE FOR d el assignment, which HIGH-PEAK-RATE DATA SERVICES approximates the case where antenna diversity employed and sufficient averaging in both time In the following, downlink performance is studied and frequency domains is achieved in signal and by large-scale computer simulations. Only the interference estimations downlink simulation results are shown here since Uniformly distributed mobile stations(MSs) downlink transmission requires a higher RF band- receive packets, which are generated from the width and its information bandwidth demand network and arrive at different base stations popular applications (e.g, Web browsing) is also data service traffic model, described in [9]. based higher. Although uplink efficiency could be on wide-area network traffic statistics, which reduced by collisions, downlink spectrum efficien- exhibit a"self-similar" property when aggrega cy is the crucial factor in system deployer ing multiple sources, was used to generate pack ets. A radio resource ("channel")is statistically THE SIMULATION MODEL Itiplexed to deliver packets for different MS To characterize DPA performance, a system of MSs are fairly allocated as many unused radio 36 base stations arranged in a hexagonal pattern channels as possible provided the SIr exceed is assumed, each having three sectors using ide- 10 dB for resources. When the number of pend alized antennas with 120% beamwidths and a 20- ing packets exceeds the number of channel dB front-to-back ratio. The mobile antennas are assigned, they are queued for later delivery. The assumed to be omnidirectional. In each sector, assigned channels are reserved for the MS Magazine·Ju

IEEE Communications Magazine • July 2000 85 for very fast SIR estimation. In addition, since a total of 528 subchannels are available to map 22 large resources and 176 small sources over three OFDM blocks, significant diversity effects are achieved to reduce measurement errors. The esti￾mated SIR is compared to an admission threshold (e.g., 10 dB in our example), so channel occupan￾cy can be controlled to achieve good QoS for the admitted users. QoS provisioning for different services is an area for further study. To reduce time delay for small resource assignment, this frame structure can be modified to allow assign￾ment of 1/4 resources per frame. DOWNLINK PERFORMANCE FOR HIGH-PEAK-RATE DATA SERVICES In the following, downlink performance is studied by large-scale computer simulations. Only the downlink simulation results are shown here since downlink transmission requires a higher RF band￾width, and its information bandwidth demand in popular applications (e.g., Web browsing) is also higher. Although uplink efficiency could be reduced by collisions, downlink spectrum efficien￾cy is the crucial factor in system deployment. THE SIMULATION MODEL To characterize DPA performance, a system of 36 base stations arranged in a hexagonal pattern is assumed, each having three sectors using ide￾alized antennas with 120∞ beamwidths and a 20- dB front-to-back ratio. The mobile antennas are assumed to be omnidirectional. In each sector, one radio provides eight traffic slots to deliver downlink traffic packets. The same channel can be used in different sectors of the same base sta￾tion as long as the SIR at the DPA admission process exceeds 10 dB. Based on the downlink frame structure shown in Fig. 5, four base sta￾tions in each reuse area take turns performing the DPA procedure, and the assignment cycle is reused in a fixed pattern. The co-channel-inter￾ference-limited case is considered; that is, noise is ignored in the simulation. In the propagation model, the average received power decreases with distance d as d–4 and the large-scale shad￾ow-fading distribution is log-normal with a stan￾dard deviation of 10 dB. Rayleigh fading is ignored in the channel assignment, which approximates the case where antenna diversity is employed and sufficient averaging in both time and frequency domains is achieved in signal and interference estimations. Uniformly distributed mobile stations (MSs) receive packets, which are generated from the network and arrive at different base stations. A data service traffic model, described in [9], based on wide-area network traffic statistics, which exhibit a “self-similar” property when aggregat￾ing multiple sources, was used to generate pack￾ets. A radio resource (“channel”) is statistically multiplexed to deliver packets for different MSs. MSs are fairly allocated as many unused radio channels as possible provided the SIR exceeds 10 dB for resources. When the number of pend￾ing packets exceeds the number of channels assigned, they are queued for later delivery. The assigned channels are reserved for the same MS ■ Figure 5. A staggered frame structure for downlink DPA. Frame 20 ms 1 Control slots Control slots 1.5625 ms 1.5625 ms 0.625 ms 8 traffic slots ..... BS: Base station 2 3 4 1 2 3 4 ..... Superframe 80 ms Superframe 80 ms Sector #1 Sector #1 Sector #2 Sector #2 Sector #3 Sector #3 Guard Guard Guard Pilots 1. BS 4 transmits a list of assigned channels/ACK 2. BS 1 broadcasts paging information 3. BS 2,3,4 transmit pilots 10 OFDM blocks 10 OFDM blocks BS 2 broadcasts paging information BS 1,3,4 transmit pilots Unused channel 4 OFDM blocks BS1 transmits a list of assigned channels/ACK Traffic slots BS 1, 2, 3 and 4 transmit based on DPA 1B Sync 2B 3 blocks 3 blocks 3 blocks 1 B 3 blocks 3 blocks 3 blocks 1 B 3 blocks 1 B 1B Sync 2B

· Beamformer Interference suppression Both beamforming and interference suppression employ two receive antennas for signal process- 820 ing to improve sIR. downlink beamforming is erformed at the base station using four trans mit antennas to form four narrow beams By different beams to deliver packets for MSs inside the desired beamwidth. sir is enhanced Interference suppression, on the other hand, relies on two MS receive antennas to suppress interference. For beamforming, each 120 secto is simply divided into four 30 beams(with the same 20-dB front-to-back ratio and idealized 喜5 antenna pattern), and the assumption is made covers the desired ms. It is important to note that the case of beamforming shown in the fol 0 lowing requires implementation of four narrow Occupancy( %) eam transmit antennas at the bs, but each active link still uses one transmit and two receive anten Figure 6. Average retransmission probability as a function of occupancy nas. as discussed above. PERFORMANCE RESULTS until all packets are delivered or the DPA reas- Figure 6 shows the overall average probability of signs radio channels in the next superframe. packet retransmission as a function of occupancy ARQ is employed, assuming perfect feedback, to With a 3-6 percent target retransmission proba request base stations for retransmission when a bility, about 15-50 percent occupancy per radio in packet ("word")is received in error, which is each sector is possible with this DPA scheme simulated based on the WER curve obtained in This result is significantly superior to the efficien- [3]using differential demodulation with four cy provided by current cellular systems. The cor transmit-diversity and two receive-diversity responding average packet dropping probability is ntennas. Recent simulations of the clustered lown in Fig. 7. Notice that both interference FDM scheme described earlier found that suppression and downlink beamforming are effec- almost the same WER can be obtained using tive in improving retransmission probability. How- coherent demodulation with one transmit and ever, the improvement in packet dropping two receive antennas. Therefore. the results ence suppression is some shown can be achieved by using one transmit and what limited because interference suppression is not wo receive antennas. If a packet cannot be suc- employed in SIR estimation, which is used for cessfully delivered in 3 s, which may be a result admission control. Specifically, some of the pack of traffic overload or excessive interference, it is ets are delayed if the Sir estimated during dropped from the queue. The control messages resource assignment does not exceed 10 dB, are assumed to be error-free in the designated although SiR may be acceptable with interference control slots suppression performed in the demodulation pro- We consider two radio link enhancement cess after admission is granted. Based on the techniques to study dPA performance results of Fig. 7, it appears that the reasonable operating region of occupancy is about 20-25 and 30-35 percent occupancy per radio for cases with 15 out and with beamforming, respectively. Under this condition, interference suppression and/or beamforming can achieve acceptable retransmis- 8 sion probability, providing good QoS. If neither oppression enhancement is employed, the traffic capacity 10 must be lowered to ensure good performance When both technic ployed, three interference suppression radios in three sectors can utilize 100 percent of radio resources in every base station. Finally, Fig 8 shows that 2-3 Mb/s can be successfully delivered by each base station with an average delay on the order of 60-120 ms. This indicates that OFDM and DPA combined enable a spec ally efficient air interface for broadband ser es,even for macrocell environments, providing beyond what third-generation systems can offer Based on the performance shown here and he coding/modulation alternatives discussed Occupancy (%) earlier, it is reasonable to expect that an 8-PSK based modulation can deliver 5 Mb/s in peak a Figure 7. Average packet dropping probability as a function of occupancy rate packet data access. The wideband oFdm IEEE Communications Magazine. July 2000

86 IEEE Communications Magazine • July 2000 until all packets are delivered or the DPA reas￾signs radio channels in the next superframe. ARQ is employed, assuming perfect feedback, to request base stations for retransmission when a packet (“word”) is received in error, which is simulated based on the WER curve obtained in [3] using differential demodulation with four transmit-diversity and two receive-diversity antennas. Recent simulations of the clustered OFDM scheme described earlier found that almost the same WER can be obtained using coherent demodulation with one transmit and two receive antennas. Therefore, the results shown can be achieved by using one transmit and two receive antennas. If a packet cannot be suc￾cessfully delivered in 3 s, which may be a result of traffic overload or excessive interference, it is dropped from the queue. The control messages are assumed to be error-free in the designated control slots. We consider two radio link enhancement techniques to study DPA performance: • Beamforming • Interference suppression Both beamforming and interference suppression employ two receive antennas for signal process￾ing to improve SIR. Downlink beamforming is performed at the base station using four trans￾mit antennas to form four narrow beams. By using different beams to deliver packets for MSs inside the desired beamwidth, SIR is enhanced. Interference suppression, on the other hand, relies on two MS receive antennas to suppress interference. For beamforming, each 120∞ sector is simply divided into four 30∞ beams (with the same 20-dB front-to-back ratio and idealized antenna pattern), and the assumption is made that a packet is delivered using the beam that covers the desired MS. It is important to note that the case of beamforming shown in the fol￾lowing requires implementation of four narrow￾beam transmit antennas at the BS, but each active link still uses one transmit and two receive anten￾nas, as discussed above. PERFORMANCE RESULTS Figure 6 shows the overall average probability of packet retransmission as a function of occupancy. With a 3–6 percent target retransmission proba￾bility, about 15–50 percent occupancy per radio in each sector is possible with this DPA scheme. This result is significantly superior to the efficien￾cy provided by current cellular systems. The cor￾responding average packet dropping probability is shown in Fig. 7. Notice that both interference suppression and downlink beamforming are effec￾tive in improving retransmission probability. How￾ever, the improvement in packet dropping probability for interference suppression is some￾what limited because interference suppression is not employed in SIR estimation, which is used for admission control. Specifically, some of the pack￾ets are delayed if the SIR estimated during resource assignment does not exceed 10 dB, although SIR may be acceptable with interference suppression performed in the demodulation pro￾cess after admission is granted. Based on the results of Fig. 7, it appears that the reasonable operating region of occupancy is about 20–25 and 30–35 percent occupancy per radio for cases with￾out and with beamforming, respectively. Under this condition, interference suppression and/or beamforming can achieve acceptable retransmis￾sion probability, providing good QoS. If neither enhancement is employed, the traffic capacity must be lowered to ensure good performance. When both techniques are employed, three radios in three sectors can utilize 100 percent of radio resources in every base station. Finally, Fig. 8 shows that 2–3 Mb/s can be successfully delivered by each base station with an average delay on the order of 60–120 ms. This indicates that OFDM and DPA combined enable a spec￾trally efficient air interface for broadband ser￾vices, even for macrocell environments, providing complementary high-bit-rate data services beyond what third-generation systems can offer. Based on the performance shown here and the coding/modulation alternatives discussed earlier, it is reasonable to expect that an 8-PSK￾based modulation can deliver 5 Mb/s in peak￾rate packet data access. The wideband OFDM ■ Figure 6. Average retransmission probability as a function of occupancy. Retransmission probability (%) Occupancy (%) 5 10 15 20 25 0 0 10 20 30 40 50 60 No beamforming, no interference suppression Beamforming, no interference suppression No beamforming, interference suppression Beamforming, interference suppression ■ Figure 7. Average packet dropping probability as a function of occupancy. Packet dropping probability (%) Occupancy (%) 5 10 15 0 0 10 20 30 40 50 60 No beamforming, no interference suppression Beamforming, no interference suppression No beamforming, interference suppression Beamforming, interference suppression

technology discussed here can provide rates with robust performance that is not ach ble in second-or third- generation technologies r,it is a less mature technology th requires more research and development effort 160 邮业 CoNCLUSIONS erference suppressio Peak bit rates of 2-5 Mb/s are likely to be desir- interference suppression able for future packet wireless data service for Internet applications with widespread macrocell- Adaptive modulation will where/anytime access maximum efficiency and allow for the more limit ed transmit levels of portable terminals The 5 MHz channelization discussed can cket data bit rates of 2-5 Mb/s in macrocellular 1000 environments in a complementary ket data Delivered bit rate per cell(kb/s) mode Bit rates up to 10 Mb/s can be supported in microcellular and indoor environments using Figure 8. Average delay of the delivered packets as a function of the through ace-time coding with two transmit and two put per base stati receive antennas Space-time coding may also be applicable in macrocellular environments. Private indoor systems should probably use unlicensed 16V. Tarokh, N Seshadri, and A.R. Calderbank,"Space- spectrum for hig igh-speed wireless data access, because of the need for large amounts of spec- Performance cr vol.44,Mar.1998 trum and emerging wireless LAN standards, [7Y(Geoffrey)Li and N R Sollenberger, "Adaptive based on OFDM. Dynamic packet assignment, erference, " IEEE Trans. Commun., vol. 47, no. 2, Feb 999,pp.217-2 OFDM physical layer, adaptive modulation and [8]GJ Pottie, "System Design oding, space-time coding and interference sup- techniques to provide wideband OFDM packet 9]J.C.-. and N.R. Sollen be wireless data access in macrocellular and micro Resource Allocation for wireless pack pplication to Advanced Cellular Intern cellular environments. The target bit rates are JSAC, voL. 16, no. 6, Aug. 1998, pp. 82 ubstantially higher than what third-generation stems can achieve in macrocellular environ BIOGRAPHIES nents, and can reduce the gap between wireline andwirelessdataratesandapplications.AreasJusTInC-L.HUANGFustin@research.att.com)receiveda for further study include receiver structures and implementations, resource assignment, and Qos provisioning for mixed services, as well as many he was with GE Corporate Research and Development, Sch other issues not discussed here obile dy, New York, where he studied personal and ACKNOWLEDGMENTS Bellcore(now Telcordia Technologies), Red Bank, New Jer- The concepts in this article are based on the work 993 to 1996 he was with the electrical and Electron and ideas of a number of colleagues within AT&T Engineering Department of the Hong Kong University of Greenstein contributed to concepts on microcells. tions ing June 1996 he returned to the United States and Len Cimini and Ye li contributed to the oFdm joined AT&T Labs-Research in New Jersey, where he is now techniques that were discussed. Vahid Tarokh, a technology leader in the Wireless Systems Research Nambi Seshadri, and Rob Calderbank contribut- Department, involved in creating technologies to provide ed to space-time coding concepts. Hong Zhao reliable services on wireless platt contributed to concepts for applications and tor for Wireless Communications for IEEE Transactions requirements for high-speed data services Communications. He is a member of phi Kappa phi REFERENCES NELSONSoLLENBERGER(FI(nelson@research.att.com)heads t at at&t. h [1]T. oj [2]L. J. Cimini, Jr, "Analysis and Simulation of a Digital rocessing, system architectures, and radio link techniques 7.Juy1985pp.665-75 [3] L.J. Cimini, Jr, J. C-L. Chuang, and N. R Sollenberger (ae, boh ind electrica en ineerin t from 1979 threse Advanced Cellular Internet Service, "IEEE Commun. 1986 he was a member of the cellular radio developme Mag,oct.1998,pp.1509 where he investigated EDGE with Wideban I5I roy so ora tioe MTM e Thes Mal ale A ce s scheme of thait department tfom iss to 1995. At ellcore he. (UTRA), ETSI SMG2, London, U.K., June 23-27, 1997 munications System. In 1995 he joined AT&T IEEE Communications Magazine. Jul 87

IEEE Communications Magazine • July 2000 87 technology discussed here can provide high peak rates with robust performance that is not achiev￾able in second- or third-generation technologies. However, it is a less mature technology that requires more research and development effort. CONCLUSIONS Peak bit rates of 2–5 Mb/s are likely to be desir￾able for future packet wireless data service for Internet applications with widespread macrocel￾lular coverage to enable anywhere/anytime access. Adaptive modulation will be important to achieve maximum efficiency and allow for the more limit￾ed transmit power levels of portable terminals. The 5 MHz channelization discussed can support packet data bit rates of 2–5 Mb/s in macrocellular environments in a complementary packet data mode. Bit rates up to 10 Mb/s can be supported in microcellular and indoor environments using space-time coding with two transmit and two receive antennas. Space-time coding may also be applicable in macrocellular environments. Private indoor systems should probably use unlicensed spectrum for high-speed wireless data access, because of the need for large amounts of spec￾trum and emerging wireless LAN standards, including the IEEE 802.11 standard at 5 GHz based on OFDM. Dynamic packet assignment, an OFDM physical layer, adaptive modulation and coding, space-time coding and interference sup￾pression, and smart antennas are proposed as techniques to provide wideband OFDM packet wireless data access in macrocellular and micro￾cellular environments. The target bit rates are substantially higher than what third-generation systems can achieve in macrocellular environ￾ments, and can reduce the gap between wireline and wireless data rates and applications. Areas for further study include receiver structures and implementations, resource assignment, and QoS provisioning for mixed services, as well as many other issues not discussed here. ACKNOWLEDGMENTS The concepts in this article are based on the work and ideas of a number of colleagues within AT&T as well as others. Lek Ariyavisitakul and Larry Greenstein contributed to concepts on microcells. Len Cimini and Ye Li contributed to the OFDM techniques that were discussed. Vahid Tarokh, Nambi Seshadri, and Rob Calderbank contribut￾ed to space-time coding concepts. Hong Zhao contributed to concepts for applications and requirements for high-speed data services. REFERENCES [1] T. Ojanpera and R. Prasad, “An Overview of Air Inter￾face Multiple Access for IMT-2000/UMTS,” IEEE Com￾mun. Mag., vol. 36, no. 9, Sept. 1998, pp. 82–95. [2] L. J. Cimini, Jr., “Analysis and Simulation of a Digital Mobile Channel Using Orthogonal Frequency Division Multiplexing,” IEEE Trans. Commun., vol. COM-33, no. 7, July 1985, pp. 665–75. [3] L. J. Cimini, Jr., J. C.-I. Chuang, and N. R. Sollenberger, “Advanced Cellular Internet Service,” IEEE Commun. Mag., Oct. 1998, pp. 150–9. [4] J. C.-I. Chuang et al., “High-Speed Wireless Data Access based on Combining EDGE with Wideband OFDM,” IEEE Commun. Mag., Nov. 1999, pp. 92–8. [5] Sony Corporation, “BDMA, The Multiple Access Scheme Proposal for the UMTS Terrestrial Radio Air Interface (UTRA),” ETSI SMG2, London, U.K., June 23–27, 1997. [6] V. Tarokh, N. Seshadri, and A. R. Calderbank, “Space￾Time Codes for High Data Rate Wireless Communica￾tions: Performance Criterion and Code Construction,” IEEE Trans. Info. Theory, vol. 44, Mar. 1998, pp. 744–65. [7] Y. (Geoffrey) Li and N. R. Sollenberger, “Adaptive Antenna Arrays for OFDM Systems with Co-Channel Interference,” IEEE Trans. Commun., vol. 47, no.2, Feb. 1999, pp. 217–29. [8] G. J. Pottie, “System Design Choices in Personal Com￾munications,” IEEE Pers. Commun., vol. 2, no. 5, Oct. 1995, pp. 50–67. [9] J. C.-I. Chuang and N. R. Sollenberger, “Spectrum Resource Allocation for Wireless Packet Access with Application to Advanced Cellular Internet Service,” IEEE JSAC, vol. 16, no. 6, Aug. 1998, pp. 820–29. BIOGRAPHIES JUSTIN C.-I. CHUANG [F] (justin@research.att.com) received a B.S. degree (1977) from National Taiwan University, and M.S. (1980) and Ph.D. (1983) degrees from Michigan State University, all in electrical engineering. From 1982 to 1984 he was with GE Corporate Research and Development, Sch￾enectady, New York, where he studied personal and mobile communications. From 1984 to 1993 he was with Bellcore (now Telcordia Technologies), Red Bank, New Jer￾sey, as a member of the Radio Research Department. From 1993 to 1996 he was with the Electrical and Electronic Engineering Department of the Hong Kong University of Science and Technology (HKUST), where he established the teaching and research program in wireless communica￾tions. In June 1996 he returned to the United States and joined AT&T Labs-Research in New Jersey, where he is now a technology leader in the Wireless Systems Research Department, involved in creating technologies to provide reliable services on wireless platforms. He continues to serve as an adjunct professor of HKUST. He is the Area Edi￾tor for Wireless Communications for IEEE Transactions on Communications. He is a member of Phi Kappa Phi. NELSON SOLLENBERGER [F] (nelson@research.att.com) heads the Wireless Systems Research Department at AT&T. His department performs research on next-generation wireless systems concepts and technologies, including high-speed transmission methods, smart antennas and adaptive signal processing, system architectures, and radio link techniques to support wireless multimedia and advanced voice ser￾vices. He received his Bachelor’s degree from Messiah Col￾lege (’79) and his Master’s degree from Cornell University (81), both in electrical engineering. From 1979 through 1986 he was a member of the cellular radio development organization at Bell Laboratories, where he investigated spectrally efficient analog and digital technologies for sec￾ond-generation cellular radio systems. In 1987 he joined the radio research department at Bellcore, and was head of that department from 1993 to 1995. At Bellcore he investigated concepts for PACS, the Personal Access Com￾munications System. In 1995 he joined AT&T. ■ Figure 8. Average delay of the delivered packets as a function of the through￾put per base station. Average delay of delivered packets (ms) Delivered bit rate per cell (kb/s) 80 40 160 120 200 0 0 1000 2000 3000 No beamforming, no interference suppression Beamforming, no interference suppression No beamforming, interference suppression Beamforming, interference suppression