The value of cooperation between relays in the multiple-access channel with multiple relays

We study the discrete memoryless multiple-access channel with two independent sources, two relays and a single destination. We refer to this configuration as the multiple-access channel with multiple relays (MACMRs), which is a generalisation of the multiple-access relay channel model obtained by adding a relay node. We present inner and outer bounds on the capacity region of the MACMR. The inner bound is based on a hierarchical decode-and-forward scheme, in which each relay decodes the messages of the lower hierarchy. We extend the regular encoding, sliding-window decoding and backward decoding techniques, previously applied to multiple-access relay channels and multiple-relay channels, to MACMRs. The outer bounds are obtained using the cut-set bound. For Gaussian MACMRs, the bounds are evaluated and compared with those obtained for the multiple-access channel with parallel relays. We conclude that a significant improvement in performance can be obtained by letting the relays interact with each other.