Relaying in fading channels using quantize and encode forwarding through optical wireless links

A relaying scheme that uses both radio frequency (RF) and free space optical (FSO) links is proposed. The RF signal sent by the mobile source is received by both the relay and the destination. In our proposed quantize and encode forwarding (QEF), the relay quantizes each received log-likelihood ratio (LLR) value and transmits the corresponding bits to the destination through a high speed FSO link. The bit error rate (BER) performance of the proposed system is analyzed and approximate expressions avoiding numerical integrations of several integrals are given. A 3-bit quantizer is found to provide BER performance close to a genie-aided maximal ratio combiner that directly combines information available at the relay and the destination. Stronger atmospheric turbulence in the FSO link is found to reduce the benefit of using a larger number of quantization levels at the relay.