Coding Versus ARQ in Fading Channels: How reliable should the PHY be?

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This paper studies the tradeoff between channel coding and ARQ (Automatic Repeat reQuest) in Rayleigh block-fading channels. A heavily coded system corresponds to a low transmission rate with few ARQ re-transmissions, whereas lighter coding corresponds to a higher transmitted rate but more retransmissions. The optimum error probability, where optimum refers to the maximization of the average successful throughput, is derived and is shown to be a decreasing function of the average signal-to-noise ratio and of the channel diversity order.