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Date Submitted: 04/23/2012 05:05 AM
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...