Opportunistic Communications in Fading Multiaccess Relay Channels

The problem of optimal resource allocation is studied for ergodic fading orthogonal multiaccess relay channels (MARCs) in which the users (sources) communicate with a destination with the aid of a half-duplex relay that transmits on a channel orthogonal to that used by the transmitting sources. Under the assumption that the instantaneous fading state information is available at all nodes, the maximum sum-rate and the optimal user and relay power allocations (policies) are developed for a decode-andforward (DF) relay. With the observation that a DF relay results in two multiaccess channels, one at the relay and the other at the destination, a single known lemma on the sum-rate of two intersecting polymatroids is used to determine the DF sum-rate and the optimal user and relay policies. The lemma also enables a broad topological classification of fading MARCs into one of three types. The first type is the set of partially clustered MARCs where a user is clustered either with the relay or with the destination such that the users waterfill on their bottle-neck links to the distant receiver. The second type is the set of clustered MARCs where all users are either proximal to the relay or to the destination such that opportunistic multiuser scheduling to one of the receivers is optimal. The third type consists of arbitrarily clustered MARCs which are a combination of the first two types, and for this type it is L. Sankar and H. V. Poor are with the Department of Electrical Engineering, Princeton University, Princeton, NJ 08544, USA. Y. Liang is with the University of Hawaii, Honolulu, HI 96822, USA. N. B. Mandayam is with the WINLAB, Rutgers University, North Brunswick, NJ 08902, USA. A part of this work was done when L. Sankar was with the WINLAB, Rutgers University and Y. Liang was with Princeton University. The work of L. Sankar, (previously Sankaranarayanan), Y. Liang, and H. V. Poor was supported by the National Science Foundation under Grants ANI-03-38807 and CNS-06-25637. The work of N. B. Mandayam was supported in part by the National Science Foundation under Grant No. ITR-0205362. The material in this paper was presented in part at the IEEE International Symposium on Information Theory, Nice, France, Jun. 2007. February 10, 2009 DRAFT 2 shown that the optimal policies are opportunistic non-waterfilling solutions. The analysis is extended to develop the rate region of a K-user orthogonal half-duplex MARC. Finally, cutset outer bounds are used to show that DF achieves the capacity region for a class of clustered orthogonal half-duplex MARCs. Index Terms Multiple-access relay channel (MARC), decode-and-forward, ergodic capacity.