We present and evaluate two low-complexity protocols for adaptive transmission in tactical packet radio systems that employ higher layer fountain codes. The adaptive-rate coding protocol is permitted to adjust the rate of the channel code between each pair of consecutive packets. The adaptive modulation and coding protocol can change the modulation between each pair of consecutive packets, but it can adjust the code rate only between consecutive frames of packets. Each protocol responds to dynamic fading and other time-varying propagation losses. For control of the adaptation, the protocols rely solely on a simple statistic derived by the receiver. They require no channel measurements, parameter estimates, pilot symbols, or training. The throughput performance of each protocol is evaluated for a Rayleigh fading channel modeled by a finite-state Markov chain. We show that our adaptive-rate coding protocol in tandem with higher layer fountain coding outperforms fountain coding with fixed-rate channel coding. We also compare the performance of our adaptive-rate coding protocol with the performance of a hypothetical ideal adaptive-rate coding protocol for which the transmitter is given perfect channel state information for the previous packet. We demonstrate that our protocol performs nearly as well as the ideal protocol.
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