Large Diversity Gain Research Articles

Coding for the fading channel: a survey

We consider the design of coding schemes for a channel affected by flat, slow fading and additive noise. Specifically, by using the “block-fading” channel model, we allow delay constraints to be taken into account. Optimum coding 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 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.

Channel coding strategies for cellular radio

To improve re-use of time/frequency slots in a cellular radio system, it is desirable for the average interference levels seen by all users to be made approximately equal. The authors provide constructions based on orthogonal latin squares that guarantee different sets of users will interfere in successive slots. When signal to interference ratios are independent from successive symbols, channel codes can provide a large diversity gain which is far in excess of the gain against additive noise. Consequently, coding with interleaving fits together very naturally with this construction. The authors illustrate how to achieve large performance improvement using convolutional codes with low decoding delay in a slow hopped system. >

Experimental evaluation of postdetection diversity reception of narrow-band digital FM signals in Rayleigh fading

Postdetection diversity is attractive for narrowband digital FM signal reception because the cophasing function, which may be difficult to realize in a fast Rayleigh fading environment, is not required. The combining scheme evaluated here is to weight each frequency demodulator (FD) output in proportion to the nu th power of the received signal envelope of that branch. Maximum diversity improvement can be obtained with nu =2 (this combiner is referred to as a postdetection maximal ratio chamber (MRC)). Experimental results are presented on postdetection diversity reception in the Gaussian minimum shift keying (GMSK) signal transmission system. Diversity combining and FD-decision algorithms (decision feedback equalizer (DFE) and maximum-likelihood sequence estimator (MLSE)) are performed by software on a computer using the data of the sampled FD output and received signal envelope obtained from a laboratory transmission system. It is shown that the MRC can attain about a 1-dB larger diversity gain than the selection combiner (SC) when two-branch diversity is used. The degradations of two-branch diversity improvement caused by the differences between demodulator sensitivities and between received signal envelope detector gains are evaluated.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>