Downlink Power Distribution in a Wireless CDMA Network with Cooperative Relaying

This paper studies power distribution in the downlink of a wireless CDMA network, where mobile stations (MSs) cooperatively relay traffic for their peer stations, and the destination station (DS) combines signals received from both the base station (BS) and the relay station (RS). Both the DS and the RSs transmit at the same frequency band. We consider both decode-and-forward (DF) and amplify-and- forward (AF). An optimization problem is first formulated for distributing the transmission power of the BS and the RSs. The objective is to minimize the transmission power of the BS and the total transmission power of the RSs, subject to the average transmission rate and signal-to-interference-plus-noise ratio (SINR) requirement of the user traffic. The optimum power distribution requires link gains among different MSs, which are usually not available at the BS. We then propose a practical power distribution scheme based on link gains of the RS and DS to the BS. Our results show that i) the proposed link-gain based power distribution scheme achieves close-to-optimum performance, ii) by appropriately selecting the RS and forwarding techniques, cooperative relaying in the downlink of a CDMA network can reduce the communication outage probability and significantly save the BS transmission power in the downlink transmissions, and iii) cooperatively relaying requires very low transmission power from the RSs.