OFDM Baseband Receiver Design for Wireless Communications

7

MIMO Techniques

Multiple antenna configuration at the transmitter and/or the receiver can greatly enhance the capacity and performance of wireless communication systems. The multiple-input multiple-output (MIMO) technology, with multiple antennas at the transmitter and/or the receiver, has been integrated into many OFDM systems.

7.1 Introduction

According to the number of transmit (TX) antennas and the number of receive (RX) antennas, wireless systems can be classified as single-input single-output (SISO), single-input multiple-output (SIMO), multiple-input single-output (MISO) and multiple-input multiple-output (MIMO) systems, in which the input and output are with respect to the channel between the transmitter and the receiver, as shown in Figure 7.1. The advantages of employing multiple antennas and related signal processing include [1,2]:

• Array gain As multiple copies of the signals are received at a receiver with more than one antenna, the signals can be combined coherently to achieve gain in effective SNR. Such gain is usually called array gain. Combining methods such as equal-gain combining (EGC) and maximal ratio combining (MRC) are very popular [3]. In an SIMO system, the average SNR increase is proportional to the number of receive antennas. In the case of multiple transmit antennas, however, array gain can also be obtained, provided that spatial pre-coding based on the channel information is implemented. With this pre-coding, the multiple copies of transmitted signals ...