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 200080 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