Joint optimization of radio parameters—A top–down approach

We consider the optimization of physical layer (PHY) and medium access control (MAC) layer radio parameters in a third generation multi-user mobile communication system like high speed downlink packet access (HSDPA). Following the need for efficient QoS compliant services the problem is formulated as a minimization of the average transmit power subject to constraints on the resulting quality of service, i.e. data throughput and delay. To this end we present analytical models for all regarded sublayers, i.e. spread spectrum signal processing, forward error correction coding, protocols and the MAC scheduling. We demonstrate how the resulting mapping from transmit powers to QoS parameters depends upon the regarded optimization parameters and how it can be inverted through a fast converging iterative computation of the systems outage probability and the offline inversion of an equivalent channel independent single-user problem. The obtained solution gives means to answer a service request with minimum resources. Numerical evaluations quantify the large potential power savings in a cellular setting with random traffic and demonstrate that the optimization of radio parameters has significant influence on the system capacity through the limited resource transmit power. An optimum mode selection as presented in this work thus allows for drastic load gains in HSDPA settings.