Distribution of L-values in gray-mapped M2-QAM: closed-form approximations and applications
IEEE Transactions on Communications
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In this paper, we propose a method for evaluation of the bit error rate (BER) for packet combining based on constellation rearrangement (CoRe). Such mapping diversity scheme, adopted in the high speed downlink packet access (HSDPA), uses Gray-mapped constellations and is based on suboptimal accumulation of the reliability metrics generated in each of the transmissions. We present an exact model for the logarithmic likelihood ratios (LLR) obtained by means of the so-called max-log approximation, and we show that their conditional probability density functions (pdf) are piecewise Gaussian. We then present the derivation of the uncoded BER and illustrate it with simulation results that confirm our formulation. Finally, we propose simplifications which significantly reduce the complexity of the evaluation method and provide results with a very good accuracy; an extension to transmissions over faded channel is also presented. Copyright © 2007 John Wiley & Sons, Ltd. This work was presented in part at the IEEE Global Telecommunications Conference (GLOBECOM) 2006, San Francisco, U.S.A., and at the 23rd Biennial Symposium on Communications (QBSC), Queen's University, Kingston, Canada, May 2005.