  This paper addresses robust communication on a fading relay channel in which
the relay is connected to the decoder via an out-of-band digital link of
limited capacity. Both the source-to-relay and the source-to-destination links
are subject to fading gains, which are generally unknown to the encoder prior
to transmission. To overcome this impairment, a hybrid automatic retransmission
request (HARQ) protocol is combined with multi-layer broadcast transmission,
thus allowing for variable-rate decoding. Moreover, motivated by cloud radio
access network applications, the relay operation is limited to
compress-and-forward. The aim is maximizing the throughput performance as
measured by the average number of successfully received bits per channel use,
under either long-term static channel (LTSC) or short-term static channel
(STSC) models. In order to opportunistically leverage better channel states
based on the HARQ feedback from the decoder, an adaptive compression strategy
at the relay is also proposed. Numerical results confirm the effectiveness of
the proposed strategies.
