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 × 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.
The most prevalent form of spatial diversity is receive diversity, often with only two antennas. This type of diversity is nearly ubiquitous on cellular base stations and wireless LAN access points. Receive diversity places no particular requirements on the transmitter but requires a receiver that processes the Nr received streams and combines them in some fashion .
Widely used combining algorithms: selection combining (SC) and maximal ratio combining (MRC). Although receive diversity is highly effective in both flat fading and frequency-selective fading channels, now focus on the flat-fading scenario, in which the signal received by each of the antennas is uncorrelated and has the same average power.