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2.5 Advanced Features for Performance Enhancements
WiMAX defines a number of optional advanced features for improving the performance. Among the more important of these advanced features are support for multiple-antenna techniques, hybrid-ARQ, and enhanced frequency reuse.
2.5.1 Advanced Antenna Systems
The WiMAX standard provides extensive support for implementing advanced multiantenna solutions to improve system performance. Significant gains in overall system capacity and spectral efficiency can be achieved by deploying the optional advanced antenna systems (AAS) defined in WiMAX. AAS includes support for a variety of multiantenna solutions, including transmit diversity, beamforming, and spatial multiplexing.
Transmit diversity: WiMAX defines a number of space-time block coding schemes that can be used to provide transmit diversity in the downlink. For transmit diversity, there could be two or more transmit antennas and one or more receive antennas. The space-time block code (STBC) used for the 2 x1 antenna case is the Alamouti codes, which are orthogonal and amenable to maximum likelihood detection. The Alamouti STBC is quite easy to implement and offers the same diversity gain as a 1 x 2 receiver diversity with maximum ratio combining, albeit with a 3 dB penalty owing to redundant transmissions. But transmit diversity offers the advantage that the complexity is shifted to the base station, which helps to keep the MS cost low. In addition to the 2 x 1 case, WiMAX also defines STBCs for the three- and four-antenna cases.
Beamforming: Multiple antennas in WiMAX may also be used to transmit the same signal appropriately weighted for each antenna element such that the effect is to focus the transmitted beam in the direction of the receiver and away from interference, thereby improving the received SINR. Beamforming can provide significant improvement in the coverage range, capacity, and reliability. To perform transmit beamforming, the transmitter needs to have accurate knowledge of the channel, which in the case of TDD is easily available owing to channel reciprocity but for FDD requires a feedback channel to learn the channel characteristics. WiMAX supports beamforming in both the uplink and the downlink. For the uplink, this often takes the form of receive beamforming.
Spatial multiplexing: WiMAX also supports spatial multiplexing, where multiple independent streams are transmitted across multiple antennas. If the receiver also has multiple antennas, the streams can be separated out using space-time processing. Instead of increasing diversity, multiple antennas in this case are used to increase the data rate or capacity of the system. Assuming a rich multipath environment, the capacity of the system can be increased linearly with the number of antennas when performing spatial multiplexing. A 2 x 2 MIMO system therefore doubles the peak throughput capability of WiMAX. If the mobile station has only one antenna, WiMAX can still support spatial multiplexing by coding across multiple users in the uplink. This is called multiuser collaborative spatial multiplexing. Unlike transmit diversity and beamforming, spatial multiplexing works only under good SINR conditions.
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