 Next Generation Mobile Communications Ecosystem Multiple Antenna Technologies in WiMAX Receive Transmit Diversity Diversity Open Loop Closed Loop Collaborative Matrix A (STTD) Matrix B (SM) Beamforming Uplink MIMO Figure 4.7 Smart antennas in mobile WiMAX 4.3.1.2 Transmit Diversity Transmit Diversity unlike Receive Diversity is not as such common in 2G and 3G systems. However, base station transmit diversity can be utilized to enhance the downlink coverage and capacity. There are hardly any 2G/3G terminals that employ transmit diversity. 4.3.1.3 MIMO (Multiple input and Multiple Output) MIMO use multiple transmitters and receivers to achieve several advantages over simpler wireless systems. MIMO has been under development since 1985 (based on original works at Bell Labs) but it wasn't introduced in the networks until the late 1990s. It can have multiple configurations; for example, a 4x4 MIMO configuration means four transmit antennas at the base station and four receive antennas at the customer premise equipment. MIMO uses multiple antennas to send multiple parallel signals; the receiving end uses signal processing algorithms to sort out the multiple signals to produce one signal that contains the originally transmitted data. Various forms of MIMO are shown in Figure 4.7. 4.3.1.3.1 Open Loop MIMO WiMAX includes two versions of Open Loop MIMO techniques referred to as Matrix A and Matrix B. With Open Loop MIMO, the communications channel does not utilize explicit information regarding the propagation channel. Common Open Loop MIMO techniques include Space Time Block Coding based Transmit Diversity (STTD) and Spatial Multiplexing (SM-MIMO), and Collaborative Uplink MIMO. In WiMAX systems MIMO Matrix A refers to the STTD technique and MIMO Matrix B refers to the SM-MIMO technique (Figure 4.8). STTD utilizes Space-Time Block Code (STBC) in order to exploit re- dundancy in multiple transmitted versions of a signal. MIMO Matrix A is fundamentally used to enhance system coverage while Matrix B provides a very capable 