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·JuIEEE 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