 Transmit Diversity The Mobile Transmit Diversity (MTD) scheme adds a transmit antenna in the handset for better com- munications with the base stations. Currently, MTD research is under way and the improvements have not got a sign-off from many players, though it expects to provide similar gains like MRD. MTD has not been incorporated in devices and its development is still proprietary. MTD as such has not been endorsed by any of the standard bodies. Some details of MRD benefits can be found in [30]. 10.9.3 MIMO MIMO (Multiple Input and Multiple Output) is one of the key technologies that have been strongly considered for next generation mobile networks. MIMO is explored to increase capacity of cellular and broadband systems. MIMO technology exploits multiple antennas both at the base station transmitter and user device receivers in the downlink. In the base station transmitter, the data channel intended for the user is encoded in time and space across multiple transmit antennas. The process is conducted with the same carrier at each transmit antenna. Signal processing is then used to decode composite signals at the user's end where the spatial antenna processing is able to resolve multipath scattering. In this way, MIMO provides multiple parallel pipes between the base station and the user as compared to the existing scenario where only a single data pipe is available between the two entities. Thus, MIMO is able to capitalize on the presence of multiple pipes improving both the data performance and system capacity. MIMO comes in various forms including 2x2 MIMO, 2x4 MIMO, and 4x4 MIMO. Figure 10.12 shows a conceptual view of a 4x4 MIMO scheme which consists of the RF front end, baseband front end, and MIMO decoder. As the number of transmit paths increases, the data performance also improves considerably with 4x4 MIMO for next generation systems [31]. An example of WiMAX/LTE MIMO Baseband Frontend RF-Frontend RF-Frontend Figure 10.12 4x4 MIMO block diagram. Reproduced with permission from Adjoudani et al. C 200