Bit-to-Symbol Mapping Diversity in Wireless Channels

A. Chockalingam

Indian Institute of Science, Bangalore


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.

[A. Chockalingam]
[Indian Institute of Science, Bangalore]