Cross-layer optimization of rateless coding over wireless fading channels

Rateless codes are recently-proposed erasure correction codes. To apply rateless codes over wireless communication channels, a physical-layer forward error correction (FEC) code, such as a convolutional code, is usually used to correct errors within each packet while Raptor codes are used in the application layer to correct erased packets. Traditionally, the physical-layer modulation and coding rate are chosen to guarantee an overall packet error rate to be below a certain level. However, such a choice does not always provide the best overall system performance. This paper proposes a cross-layer scheme to optimize physical layer modulation and coding rate to maximize system throughput. Both slow and fast fading channels are considered. For slow fading channels, cross-layer adaptive modulation and coding schemes are also proposed. Numerical results show that the proposed cross-layer schemes outperform traditional schemes significantly in terms of system throughput. The results also indicate that in many situations, allowing for more packet error correction in the application-layer through erasure codes can be more efficient than ensuring a low packet error rate using a low-rate physical-layer code.