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Achieving high data rates and low error rates are vital in wireless communications. High data rates can be achieved using higher-order modulations (e.g., M-ary QAM, M-ary PSK). Low error rates can be achieved using packet retransmissions (automatic repeat request-ARQ) in case of uncorrectable errors in a packet. Various approaches have been proposed for packet combining and improving diversity among these retransmissions; Chase combining being one of the well known combining methods.
In this talk, we will focus on a packet combining method known as the bit-to-symbol mapping diversity, where the bit-to-symbol mapping in M-ary modulation is varied for each packet (re)transmission. Such a system has been shown to enhance the diversity among the multiple (re)transmissions and provide improved error performance. A key question in the symbol diversity scheme is how to choose the optimum mappings for multiple transmissions of a packet. We present two methods to choose the mappings; one based on an optimization that minimizes an upper bound on the bit error rate, and another based on maximizing a bit log-likelihood ratio (LLR) sum. We show that the mappings obtained by maximizing the LLR sum offers better bit error performance. Possible extensions to this work will be highlighted.
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