Abstract
This paper considers a hybrid relay network consisting of the source, the amplify-and-forward (AF) relay, the decode-and-forward (DF) relay, and the destination. In hybrid three-hop relay systems, the transmitted signal from source can be received at the destination after processing the signals through two relays. If the first relay amplifies and forwards the received signal, and the second relay decodes and forwards the received signal, the system model is considered to be an AF-DF relay system. The reverse case is considered for the DF-AF relay system. The AF-DF and DF-AF relay systems have different error rates and achievable throughput with respect to the channel conditions between two nodes. We propose optimal power allocation schemes for two different relays in order to maximize the achievable rate under a sum relay power constraint for given channel gains and transmit power from the source. By solving the optimization problem to maximize the achievable rate for each relay network, the transmit power values in closed form are derived. When the channel gains are the same, the optimal power allocation scheme for the AF-DF relay network proves that greater power should be allocated at the first relay to maximize the achievable rate. In the case of the DF-AF relay network, we derive an optimal power allocation scheme for the four possible cases. Under the same signal-to-noise ratio (SNR) condition, at the first hop we show that the achievable rate of the AF-DF relay network is greater than that of the DF-AF relay network when the channel gain between two relays is greater than that between the second relay and destination. Simulation results show that the proposed power allocation schemes provide a higher achievable rate than the equal power allocation scheme and the grid search schemes.
Highlights
Cooperative communication has recently received much attention as a method to improve network performance [1,2,3,4,5,6,7,8,9,10,11]
The grid search scheme with step size ∆ = 0.01 shows the performance close to the optimal power allocation scheme because this scheme can precisely find α ad in 0.01 unit which maximizes the achievable rate
Under a sum relay power constraint, this paper proposed the optimal power allocation schemes to maximize the achievable rates for hybrid three-hop relay networks when the channel gains and the transmit power from the source are given
Summary
Cooperative communication has recently received much attention as a method to improve network performance [1,2,3,4,5,6,7,8,9,10,11]. Optimal power allocation schemes for hybrid networks are analyzed for a two-hop AF and DF cooperative relay system, employing the outage probability as the optimization criterion in [16]. After applying the proposed power allocation schemes, the achievable rates for two hybrid relay networks, i.e., AF-DF and DF-AF, are distinct in accordance with the differences in channel conditions from one relay to another and from the relay to a destination. The proposed schemes improve the channel capacity through optimal power allocation and a hybrid relaying network in accordance with channel conditions without a sophisticated scheduler and additional overhead. For adaptive power allocation of each relay node, we derive the transmit power values in a closed form for each relay network according to the channel condition.
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