OFDM-Based Broadband Wireless Networks: Design and Optimization

Chapter 4

PHY Layer Issues — Spatial Processing

A wireless modem equipped with multiple antennas is capable of exploiting the spatial diversity of wireless channels. The space domain enabled by the antenna array introduces an additional dimension to the time-frequency radio resource. Through space-time processing, an antenna array can improve key operational parameters such as the SINR, the data rate, and the outage probability over a single antenna wireless system. Other major benefits of spatial processing include

• better interference/jammer rejection
• higher power efficiency
• lower cost, distributed hardware

Antenna array technology encompasses a wide variety of techniques that can be used at both the base-station and in the user terminal. There are two main classes of spatial processing techniques, namely, beamforming and space-time coding. Early beamforming techniques are SINR-oriented, involving linear combining of transmit and received signals from multiple antennas. The main objective is to maximize the instantaneous signal-to-interfernece-and-noise ratio (SINR). More recent techniques, such as space-time coding, are mostly diversity-and-efficiency-oriented, focusing on lowering the BER of a high-speed link over fading channels. As will be shown in the ensuing discussion, these two types of techniques can be synergistically combined for broadband OFDM systems.

4.1 Antenna array fundamentals

To better understand the principles of array processing, consider the scenario depicted ...