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2.7.1 Throughput and Spectral Efficiency
Table 2.7 shows a small sampling of some the results of a simulation-based system performance study we performed. It shows the per sector average throughput achievable in a WiMAX system using a variety of antenna configurations: from an open-loop MIMO antenna system with two transmit antennas and two receiver antennas to a closed-loop MIMO system with linear precoding using four transmit antennas and two receive antennas.
The results shown are for a 1,024 FFT OFDMA-PHY using a 10MHz TDD channel and band AMC subcarrier permutation with a 1:3 uplink-to-downlink ratio. The results assume a multicellular deployment with three sectored base stations using a (1,1)[13] frequency reuse. This is an interference-limited design, with adjacent base stations assumed to be 2 km apart. A multipath environment modeled using the International Telecommunications Union (ITU) pedestrian B channel[14] is assumed. Results for both the fixed case where an indoor desktop CPE is assumed and the mobile case where a portable handset is assumed are shown in Table 2.7.
The average per sector downlink throughput for the baseline case—assuming a fixed desktop CPE deployment—is 16.3Mbps and can be increased to over 35Mbps by using a 4 x 2 closed-loop MIMO scheme with linear precoding. The mobile-handset case also shows comparable performance, albeit slightly less. The combination of OFDM, OFDMA, and MIMO provides WiMAX with a tremendous throughput performance advantage. It should be noted that early mobile WiMAX systems will use mostly open-loop 2 x 2 MIMO, with higher-order MIMO systems likely to follow within a few years. Also note that there may be fixed WiMAX systems deployed that do not use MIMO, although we have not provided simulated performance results for those systems.
Table 2.7 also shows the performance in terms of spectral efficiency, one of the key metrics used to quantify the performance of a wireless network. The results indicate that WiMAX, especially with MIMO implementations, can achieve significantly higher spectral efficiencies than what is offered by current 3G systems, such as HSDPA and 1xEV-DO.
It should be noted, however, that the high spectral efficiency obtained through the use of (1,1,) frequency reuse does entail an increased outage probability. As discussed in Chapter 12, the outage can be higher than 10 percent in many cases unless a 4 x 2 closed-loop MIMO scheme is used.
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