We consider the design of coding schemes for a channel affected by flat, sl
ow fading and additive noise. Specifically, by using the "block-fading" cha
nnel model, we allow delay constraints to be taken into account. Optimum co
ding schemes for this channel model lead to the development of new criteria
for code design, differing markedly from the Euclidean-distance criterion
which is commonplace over the additive white Gaussian noise (AWGN) channel.
In fact, the code performance depends strongly, rather than on the minimum
Euclidean distance of the code, on its minimum Hamming distance (the "code
diversity"). If the channel model is not stationary, as it happens for exa
mple in a mobile-radio communication system where it may fluctuate in time
between the extremes of Rayleigh and AWGN, then a code designed to be optim
um for a fixed channel model might perform poorly when the channel varies.
Therefore, a code optimal for the AWGN channel may be actually suboptimum f
or a substantial fraction of time. In these conditions, antenna diversity w
ith maximum-gain combining may prove useful: in fact, under fairly general
conditions, a channel affected by fading can be turned into an AWGN channel
by increasing the number of diversity branches. Another robust solution is
based on bit interleaving, which yields a large diversity gain thanks to t
he choice of powerful convolutional codes coupled with a bit interleaver an
d the use of a suitable bit metric. An important feature of bit-interleaved
coded modulation is that it lends itself quite naturally to "pragmatic" de
signs, i.e., to coding schemes that keep as their basic engine an off-the-s
helf Viterbi decoder. Yet another solution is based on controlling the tran
smitted power so as to compensate for the attenuations due to fading. (C) 2
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