Dimension Expansion Relaying for Slow Fading Channels based on Hybrid Digital-Analog Source-Channel Coding

Hybrid digital-analog coding schemes for dimension compression/expansion have been proposed in source- channel coding to increase the robustness toward channel mismatch, in the absence of transmitter channel state information (CSIT). Recognizing that the same kind of robustness is needed at the relay in a three-node relay network, we propose novel relaying protocols based on hybrid digital-analog transmission. Since we focus on transmission over slow fading channels in the absence of CSIT, we use the notion of expected rate as performance measure. In the scenario where the incoming dimension is expanded by the relay, we present new expressions for expected rates achieved by the proposed hybrid digital-analog protocols. We also compare the performance of the new schemes to traditional digital-only (decode-and-forward or compress-and-forward) or analog-only (amplify-and-forward) relaying, as well as to performance bounds corresponding to genie-aided compress-and-forward relaying. Our new relaying protocols achieve significant gains in terms of achievable expected rates, and they are able to close in on the performance bounds. In particular we conclude that the best overall performance is obtained by an adaptive combination of decode-and-forward and hybrid digital-analog relaying.