Space–Time Coding

4 Space–time coding for wireless

communications: principles and

applications

4.1

Introduction

The essential feature of wireless transmission is the randomness of the communication

channel which leads to random fluctuations in the received signal commonly known

as fading. This randomness can be exploited to enhance performance through diversity.

We broadly define diversity as the method of conveying information through multiple

independent instantiations of these random fades. There are several forms of diversity;

our focus in this chapter will be on spatial diversity through multiple independent

transmit/receive antennas. Information theory has been used to show that multiple

antennas have the potential to dramatically increase achievable bit rates [76], thus

converting wireless channels from narrow to wide data pipes.

The earliest form of spatial transmit diversity is the delay diversity scheme proposed

in [81, 84] where a signal is transmitted from one antenna, then delayed one time slot,

and transmitted from the other antenna. Signal processing is used at the receiver to

decode the superposition of the original and time-delayed signals. By viewing multipleantenna diversity as independent information streams, more sophisticated transmission

(coding) schemes can be designed to get closer to theoretical performance limits. Using

this approach, we focus on space–time coding (STC) schemes defined by Tarokh et al.

[74] and Alamouti [5], which introduce temporal and spatial correlation into the signals

transmitted from different antennas without increasing the total transmitted power or the

transmission bandwidth. There is, in fact, a diversity gain that results from multiple paths

between the base-station and the user terminal, and a coding gain that results from how

symbols are correlated across transmit antennas. Significant performance improvements

are possible with only two antennas at the base-station and one or two antennas at the...