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 theIEEE 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