10.3.1 Multi-Antenna Transmission One of the fundamental technologies introduced with the adoption of OFDM or OFDMA is support for different multi-antenna transmission techniques. The performance of wireless communications is degraded by poor propagation-events such as signal fluctuations, interference, dispersion, fad- ing, path loss, and SO on. Multi-antenna systems are used to improve the efficiency and quality of the received signal. Consider a radio where a data stream is transmitted from a single transmit antenna to a single re- ceive antenna (known as single in, single out). This data stream may get lost or corrupted before the receiver can recover the data-due to any number of anomalies caused by poor conditions in the propagation of the signal. However, if the same data stream can be transmitted to multiple receive antennas over different paths, there is a better chance for the receiver to properly recover the signal. The introduction of additional paths on a single frequency to improve the reliability of the overall system is known as spatial or antenna diversity. For a system with NT transmitters and NR re- ceivers, the maximum number of diversity paths is NT . NR-although the improvement dimin- ishes with an increasing number of paths. Diversity can be introduced at either the receiver end (single input, multiple outputs), the transmitter end (multiple inputs, single output), or both ends (multiple input, multiple output-often called MIMO). When this technique is used to divide the given data stream into multiple parallel sub-streams and transmit each on a different antenna, it is known as spatial multiplexing. With this technique, a high-rate signal is split into low-rate streams, and each stream is transmitted from different transmit antenna in the same frequency channel (see Figure 10-14). Receivers can separate these streams if received with sufficiently different spatial signatures. Note that with MIMO, the number of interfer- ing signals also increases. Tx antennas Tx antennas Rx antennas Rx ant