TEL AVIV UNIVERSITY Gaddi Blumrosen

3.3 ST coding.
3.3.1 Introduction
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Figure 10: All eigenvector spatial response for 4 antenna elements array.
ST diversity techniques, in particular Tx diversity techniques as assumed in our
system model, exploit space selectivity and thus gain spatial diversity. In particular,
we will focus on this section on exploiting Space-time diversity- Exploit space and
time selective fading by multiplexing and or replicating transmitted signal in time and
in space. It can be done, by decreasing the correlation between spatial diversity
branches and thus increase the diversity order of the system.
One way for increasing diversity order is by forcing a different delay to each antenna
element, a technique called also delay diversity. A second way can be achieved by
introducing different phase (random) to each element [18]. A third way, combines
time shifting and altering the shifted symbol phase (conjugate symbol) is called Space
Time Codes (STC). When the coding is coded in blocks, it is called Space Time
Block Codes (STBC). When the blocks are orthogonal it is called Orthogonal Space
Time block Codes (OSTBC). Other space-time coding techniques are layered space–
time architecture spatial multiplexing (e.g. BLAST) which enhance capacity by spacetime
multiplexing, ST trellis codes (STTC) and ST Turbo codes. Common to all the
space–time coding schemes mentioned above is that they do not exploit CSI
knowledge inherently at the transmitter. We will focus mainly in OSBTC and unless
mentioned otherwise by the term STC or STBC we mean OSBTC.
We start this chapter with STBC Design Principles, based on minimization of SER.
Simple orthogonal ST coding was first derived in [20], for two transmit antennas,
later extended to four and eight transmit antennas by [22], with low-complexity
receiver. It was shown that these OSTBC provide the system its maximum diversity
order. We will end this chapter with examination of OSTBC performance as a
function of the number of antennas and with antenna patterns induced by OSTBC.
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