Two MS form factors have been considered in these simulations: a handheld device with omni- directional antennas and a desktop device with low-gain directional antennas. The handheld device is representative of a mobile or a portable network; the desktop device, of a fixed net- work. In the case of a desktop device, the MS is equipped with multiple low-gain antennas; at any given instant, the receiver chooses the antenna(s) with the strongest signal. Such a feature in the MS gives the benefit of having a directional antenna without the need for the antenna to be Table 12.3 System Configurations
Parameter Basic Enhanced 1 Enhanced 2 Enhanced 3
FEC type Turbo code Turbo code Turbo code Turbo code
H-ARQ type Type I Type I Type I Type I
Channel bandwidth 10MHz 10MHz 10MHz 10MHz
Number of subcarriers 1,024 1,024 1,024 1,024
MIMO mode (DL) Open loop 2×2 Open loop 2×4 Open loop 4 ×2 Closed loop 4×2 MIMO mode (UL) Open loop 1×2 Open loop 1×2 Open loop 1×4 Open loop 2×4 UL collaborative
MIMO Yes Yes Yes Yes
manually oriented in order to get a strong signal. Most WiMAX desktop devices are expected to be equipped with six to eight such antennas, each with a gain of 3dBi to 6dBi.
Figure 12.3 and Figure 12.4 show the average throughputs per sector for the basic configu- ration in Ped B and Ped A environments, respectively. The average throughput per sector is slightly better in the case of a Ped A channel than in a Ped B channel because the Ped A channel provides better multiuser diversity due to larger variations in channel amplitude, which is exploited by the proportional fairness scheduler.
The overall per sector throughput in the case of (1,1,3) reuse is better when a directional antenna is used at the MS, since the amount of cochannel interference is reduced by the direc- tional nature of the channel. However, in the case of (1,3,3) reuse, the additional directionality of the antenna at the MS in an interference-limited environment does not provide any significant benefit, since (1,3,3) frequency reuse provides a sufficient geographical separation of cochannel BSs. It should be noted that in the case of noise-limited design—a design with larger cell radii—
the gain of the directional antenna at the MS would provide an improvement in the sector throughput even with (1,3,3) frequency reuse.
Figure 12.5 and Figure 12.6 show the probability distributions of per subchannel user DL data rate for the Ped B and Ped A environments, respectively. One can conclude that in the case of (1,3,3) reuse, the fifth and tenth percentile data rates are much higher than the case of (1,1,3) reuse. This happens because in the case of (1,1,3) reuse, a large percentage of MSs that are present toward the cell edge experience a low SINR, due to cochannel interference and thus a low data rate. Based on the per user data rate distribution it should be noted that although (1,1,3) reuse is more spectrally efficient, it is achieved at the price of poor performance at the cell edge.
In order to achieve an acceptable cell-edge performance, (1,3,3) reuse or (1,1,3) reuse with seg- mentation is required. When segmentation is used, all the subchannel are divided into three groups, and each of the three sectors is allocated one group of subchannels. Segmentation thus achieves an effective (1,3,3) reuse.
The fifth and tenth percentile data rates can also be improved in the case of (1,1,3) reuse by using directional antennas at the MS, as shown in Figure 12.6. This controlled trade-off between network reliability and spectral efficiency allows a system designer to choose the appropriate network parameters, such as cell radius, frequency reuse, and antenna pattern, that will meet the design goal. From here on, we limit our discussion to the handheld-device scenario with (1,1,3) frequency reuse.
Table 12.4 and Table 12.5 summarize the throughput per BS and the fifth and tenth percen- tile data rates for the various scenarios. The throughput of all the sectors is combined to get the throughput of the BS. Since a total of 30MHz of spectrum is assumed, as per Table 12.2, in the case of (1,1,3) frequency reuse, we assume that each sector is allocated three 10MHz TDD chan- nels. Although the average throughput channel is less in the case of (1,1,3) frequency reuse than for (1,3,3) reuse, the overall capacity is higher with (1,1,3) reuse, since each sector is allocated three channels as opposed to one channel in the case of (1,3,3) reuse. On the other hand, network reliability is significantly improved by going from (1,1,3) reuse to (1,3,3) reuse.
Figure 12.3 Downlink and uplink average throughput per sector for band AMC in Ped B
Figure 12.4 Downlink and uplink average throughput per sector for band AMC in Ped A
0 2 4 6 8 10 12 14 16 18 20
(1,1,3) Ped B (handheld)
(1,3,3) Ped B (handheld)
(1,1,3) Ped B (desktop)
(1,3,3) Ped B (desktop)
Throughput per 10MHz TDD Channel (Mbps)
Downlink Uplink
0 2 4 6 8 10 12 14 16 18 20
(1,1,3) Ped A (handheld)
(1,3,3) Ped A (handheld)
(1,1,3) Ped A (desktop)
(1,3,3) Ped A (desktop)
Throughput per 10MHz TDD Channel (Mbps)
Downlink Uplink
Figure 12.5 User DL data rate per band AMC subchannel for handheld and desktop devices in Ped B
Figure 12.6 User DL data rate per band AMC subchannel for handheld and desktop devices in Ped A
0.0 0.2 0.4 0.6 0.8 1.0 1.2
0.00 0.50 1.00 1.50 2.00 2.50
User Datarate per Suhchannel (Mbps)
Cumulative Distribution Function
(1,1,3) (Handheld) (1,3,3) (Handheld) (1,1,3) (Desktop) (1,3,3) (Desktop) (1,1,3) Reuse
(1,3,3) Reuse
10th Percentile Data Point
0.0 0.2 0.4 0.6 0.8 1.0 1.2
0.00 0.50 1.00 1.50 2.00 2.50
User Data Rate per Subchannel (Mbps)
Cumulative Distribution Function
(1,1,3) (Handheld) (1,3,3) (Handheld) (1,1,3) (Desktop) (1,3,3) (Desktop) (1,1,3) Reuse
(1,3,3) Reuse
10th Percentile Data Point
The DL throughput shown in this section is significantly higher than the UL throughout, since the number of OFDM symbols (28) allocated for the DL subframe is larger than the number of OFDM symbols (9) allocated for the UL subframe. Varying the number of symbols to be used for DL and UL subframes makes it possible to control the ratio of the DL and UL throughputs.
Figure 12.7 shows the throughput performance for the PUSC and the band AMC subcarrier permutations. Since no precoding or beamforming is used, depending on the multipath channel band AMC provides an improvement of only 14 percent to 18 percent in the overall sector throughput compared to PUSC.
Figure 12.8 shows the throughput performance of the round-robin and proportional-fairness (PF) scheduling algorithms. The sector throughout improves by approximately 25 percent by using a proportional fairness (PF) scheduler compared to a round-robin (RR) scheduler, due to the ability of the PF scheduler to exploit multiuser diversity to a certain extent. Table 12.6 sum- marizes the DL throughput for the PF and RR schedulers in various multipath environments.