Power and resource allocation for orthogonal RDF and NDF relay systems with QoS and peak-power constraints

The availability of a relay to assist a source in communicating with its intended destination has the potential to provide a significant reduction in the power required to achieve a specified level of quality-of-service (QoS). In this paper we consider the case in which the relay operates in a channel that is orthogonal in time to that used by the source, and employs regenerative decode-and-forward (RDF) or non-regenerative decode-and-forward (NDF) relaying. We assign a price to the power of the relay relative to that of the source, and we consider the problem of jointly optimizing the source power, the relay power and, in the case of NDF relaying, the fractions of the time block allocated to the source and relay, so as to minimize the total cost of the power required to achieve a specified target rate, subject to constraints on the peak power levels employed by the source and the relay. In the RDF case, we obtain a closed-form solution to the problem. This solution clearly identifies when the peak power constraints are too tight to enable the target rate to be achieved. In the NDF case, the natural formulation of the problem is not convex, but we show that it can be precisely transformed into a convex problem that can be efficiently solved. Our numerical examples show that in a Rician fading environment the flexibility in channel resource allocation inherent in the NDF relaying strategy enables the target rate to be achieved on a significantly larger proportion of the channels.