Submitted to the IEEE Transactions on Information Theory , July 2005 Cooperative Relay Broadcast Channels

The capacity regions are investigated for two relay broadcast channels (RBCs), where relay links are incorporated into standard two-user broadcast channels to support user cooperation. In the first channel, the Partially Cooperative Relay Broadcast Channel, only one user in the system can act as a relay and transmit to the other user through a relay link. An achievable rate region is derived based on the relay using the decode-and-forward scheme. An outer bound on the capacity region is derived and is shown to be tighter than the cut-set bound. For the special case where the Partially Cooperative RBC is degraded, the achievable rate region is shown to be tight and provides the capacity region. Two Gaussian cases of the Partially Cooperative RBC are studied. For the system where the additive Gaussian noise term at one receiver is a degraded version of the other, which we refer to as the D-AWGN Partially Cooperative RBC, the capacity region is established. For the system where the additive Gaussian noise term at one receiver is independent of the other, which we refer to as the AWGN Partially Cooperative RBC, inner and outer bounds on the capacity region are derived and are shown to be close. Furthermore, it is shown that feedback does not increase the capacity region for the degraded Partially Cooperative RBC, but that it may improve the capacity region for the non-degraded Partially Cooperative RBC. In particular, feedback improves the capacity region for the AWGN Partially Cooperative RBC. In the second channel model being studied in the paper, the Fully Cooperative Relay Broadcast Channel, both users can act as relay nodes and transmit to each other through relay links. This is a more general model than the Partially Cooperative RBC. All the results for Partially Cooperative RBCs are correspondingly generalized to the Fully Cooperative RBCs. In particular, the capacity regions are established for the degraded Fully Cooperative RBC and its Gaussian example of the D-AWGN Fully Cooperative RBC. The capacity region is also established for the Fully Cooperative RBC with feedback. It is further shown that the AWGN Fully Cooperative RBC has a larger achievable rate region than the AWGN Partially Cooperative RBC. The results illustrate that relaying and user cooperation are powerful techniques in improving the capacity of broadcast channels.