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We consider the rationales behind the selection of coding
schemes for wireless channels.
Optimum coding schemes for this
channel 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. For example, for a flat, slow-fading Rayleigh channel
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 example
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 optimum for a fixed channel model
might perform poorly when the channel varies. Therefore,
a code optimal for the AWGN channel
may be actually suboptimum for a substantial fraction of time.
In these conditions, antenna diversity with 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 the choice of powerful convolutional codes
coupled with a bit interleaver and the use of a suitable
bit metric. An important feature of bit-interleaved coded
modulation is that it lends itself quite naturally
to ``pragmatic'' designs, i.e.,
to coding schemes that keep as their basic engine an
off-the-shelf Viterbi decoder. Yet another solution is based on
controlling the transmitted power so as to compensate for the
attenuations due to fading.
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Advanced Study Institute