Research On Optimal Power Allocation For The Cooperative Wireless Communication System

In wireless networks, multipath fading severely degrades the system performance. The MIMO system is an efficient way to combat multipath fading by exploiting space diversity and doesn't lose spectral efficiency. However, many mobile terminals, e.g., mobile phones and sensor nodes, can not practically adopt multiple-antenna techniques, due to their size, power, cost limitations. Therefore, cooperative diversity has been proposed, which enables single-antenna users to exploit space diversity in a distributed way by sharing their antennas and creating the virtual antenna array (VAA) for relayed transmission.In cooperative communication, relayed transmission is completed by the cooperation between source and relay. Both source and relay consume power while transmitting the signals. So diversity gains are obtained at the expense of the total power of source and relay. As a result, optimal power allocation between source and relays becomes an important issue of resource allocation in wireless cooperative diversity networks.With statistical channel knowledge at the transmitters and perfect channel state information at the receivers, we investigate the optimal power allocation of amplify- and- forward (AF), decode-and-forward (DF) and distributed space-time coded (DSTC) relaying protocols in cooperative communication system with multiple relay nodes operating over Rayleigh fading channels. We put forward the approximated expressions of the outage probability in high signal-to-noise ratio (SNR) regime. On the assumption of constant total transmit power, we propose an optimal power allocation scheme to minimize the outage probability of the mutual information (MI) at the destination, using the approximated expressions in high SNR as objective function.. We demonstrate that, at high SNR, the outage probability expressions for various protocols are convex functions of the transmit power vector, and the nature of the optimal power allocation depends on whether or not a direct link exists between the source and the destination. Additionally, for AF and DF protocols, this allocation depends only on the ratio of the mean channel power gains (i.e., the ratio of the source-to-relay channel power gain to the relay-to-destination channel power gain), whereas with a DSTC protocol with a direct link this allocation also depends on the transmission rate. Our results without a direct link show that both the DF and DSTC protocols have identical optimal power vectors and identical asymptotic coding gain ratios (CGR, i.e., the ratio of the ACG with optimal power allocation to the ACG with equal power allocation). Our analysis reveals that, in addition to the outage probability improvements, optimal power allocation also brings impressive coding gains over equal power allocation. While our optimization is performed with a sum power constraint, our results can be modified to account for a per-node maximum power constraint by simply clipping the excess power of a given node, and reallocating the remaining power to the nodes satisfying the constraints in an optimal manner.