Soft Information Relaying for Wireless Networks with Error-Prone Source-Relay Link

Relays in wireless networks can be used to decrease transmit power while additionally increasing diversity. Distributed turbo coding as a special case of decode-and-forward is very powerful in relay networks when assuming error free decoding in the relay. In practical wireless networks, however, this assumption is only justifiable if an ARQ protocol is applied which leads to lower throughput. Soft-reencoding and transmission of the reliability of reencoded bits helps the destination to decode the message. Reencoding in the relay with a recursive convolutional code as used for turbo-codes, can lead to error propagation. In this paper the effects of error propagation in relay networks are investigated and more suitable distributed coding schemes are presented for soft-reencoding. As the often used assumption of Gaussian distributed disturbance at the destination is not valid for the considered system setup, the calculation of Log-Likelihood-Ratios (LLR) for the received noisy reliability information is derived analytically.