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 200082 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