means for increasing the channel capacity. Matrix A outperforms Matrix B when SNR is weak whereas when SNR is higher Matrix B is well suited. STTD improves the SNR for cell edge users while SM provides higher capacity when users are in good RF conditions and are closer to the radio tower. An ideal WiMAX system employing MIMO techniques will support both Matrix A and Matrix B. The system will calculate an optimal switching point and dynamically shift between the two approaches to offer the necessary coverage or capacity gains demanded from the network at any given time or location [8]. IEEE WiMAX MIMO Matrix A (STTD) Multiple Antennas at Base Station Data Stream A Data Stream A Multiple Antennas at Base Station MIMO Matrix B (Spatial Multiplexing) Data Stream A Data Stream B Figure 4.8 Matrix A and Matrix B MIMO Collaborative Uplink MIMO is an additional open-loop MIMO technique considered by WiMAX vendors to increase the spectral efficiency and capacity of the uplink communications path. A practical realization of this technique would allow for two separate end-users' WiMAX devices, each having a single transmit lineup, to utilize the same frequency allocation to communicate with the dual-antenna WiMAX base station. With this technique two devices (having only transmit antenna each) can collabo- ratively transmit on the same sub-channel which can increase the uplink capacity [8]. Next Generation Mobile Communications Ecosystem 4.3.1.3.2 Closed Loop MIMO Closed Loop MIMO techniques, also known as Transmitter Adaptive Antenna (TX-AA) techniques, are simply referred to by the industry as "beamforming." Closed Loop MIMO typically utilizes Maximum Ratio Transmission (MRT) or Statistical Eigen Beamforming (EBF) techniques leading to the shorthand name for this approach - beamforming. In MRT, an antenna beam is formed on each OFDM sub-carrier whereas in EBF one only antenna beam is formed across multiple allocated sub-carriers. Beamforming uses antenna array and signal processing techniques to maximize signal strength f