Relay Channels with Unreliable Helpers

The relay channel with unreliable helper is introduced and studied. The model is that of a classical relay channel where the input from the relay to the channel has an extra primitive link whose presence is not assured a priori. The extra link represents a helper who may decide not to cooperate in transmission. The goal is to devise robust coding schemes that exploit all the relay links when they are present, but can also operate, possibly at reduced rates, when the extra primitive link (helper) is absent. The capacity region of this class of problems is defined, and fully characterized for degraded relay channels. The degraded Gaussian relay channel with unreliable relay link is solved.