Cooperative Communication Using Bit-Selective Adaptive Demodulation and Raptor Codes: The Gaussian Relay Channel Case

In this paper, we propose a cooperative communication system where every link adapts to the channel condition without any feedback, by using a recently proposed adaptive demodulation (ADM) scheme. In ADM, the receiver demodulates only a subset of the bits from a Raptor coded Gray mapped M-QAM received symbol, where the number of demodulated bits depends on the channel condition. The ADM scheme, which was originally proposed for point to point communication, determines the most reliable bits using approximate decision regions obtained through a binary composite hypothesis test in the AWGN environment. In contrast, we prove a simple theorem that identifies the most reliable bits in a Gray mapped M-QAM received symbol in a straight forward and exact manner, and use it in every receiver of our cooperative communication system to adaptively extract the appropriate bits. Performance results demonstrate that our ADM based Raptor coded cooperative communication system is robust enough to realize diversity order of two, even under adverse channel conditions in various links.