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This paper presents a simple, yet effective method of enhancing the inherent diversity among packet retransmissions in digital communication systems that employ high-order modulations such as M-PSK and M-QAM through frequency-selective channels. Diversity is enhanced by uniquely redefining the bit-to-symbol mapping for each retransmission. Finding the optimal mappings leads to iterative solutions of the quadratic assignment problem. More importantly, jointly equalizing transmission with different mappings only requires a marginal increase in complexity, while providing substantial gains in bit error rates (BER) and frame error rates (FER).