Fountain coding is a valuable tool for multicast transmissions in half-duplex packet radio networks if it is supported by an adaptive transmission protocol that compensates for fluctuations in the propagation losses on the radio links. Our goal is to devise low-complexity packet-by-packet adaptive modulation and channel coding protocols that provide high throughput for a broad range of multicast channels and operate with limited feedback and no knowledge of the fade levels or even the probability distribution for the fading on the radio links. The only control information for the protocols is derived from simple receiver statistics that are obtained by the demodulators and channel decoders in the receiving radios. Performance results are presented for practical fountain coding with low-complexity adaptive modulation and channel coding. Results on ideal fountain coding with adaptive transmission based on hypothetical perfect channel-state information are employed as performance benchmarks.

We examine the integration of adaptive modulation and channel coding protocols with fountain coding in a packet radio system. We present three modes of operation for the adaptive transmission system, each of which adapts the channel code in a way that accommodates the fixed-length packets produced by the fountain encoder. Each mode works with the fountain coding system to mitigate the effects of time-varying fading on the radio links and provide high throughput.

For time-varying fading, adaptation of the physical-layer channel code greatly enhances the throughput of a multicast transmission system, even if the system employs an application-layer fountain code for packet erasure correction. Performance evaluations for adaptive channel-coding protocols typically require extensive simulations of the fading channel, the iterative decoding process, and the generation of the control information that is used for the adaptation of the channel code. We present a method for the evaluation of such protocols that avoids a substantial amount of the computation required by traditional simulations, and we demonstrate that some important performance measures can be evaluated entirely by analysis. By the use of capacity-achieving channel codes in our investigation, we also decouple the performance of the adaptive coding protocol from the performance of a particular type of channel code and decoding method, which facilitates comparisons among different protocols. The application of our methods is demonstrated for two adaptive channel-coding protocols that are candidates for fountain-coded multicast transmission in packet radio systems.