Throughput-optimal cross-layer resource allocation in DS-CDMA systems with Nakagami multipath fading

Joint optimization of outer loop power control (OLPC) signal to noise ratio-target (SNR-target) and variable spreading factor (VSF), at the physical(PHY)-layer, with truncated automatic repeat request (ARQ), at data link layer (DLL), in accordance to the number of active users in the cell is considered. This investigation is on a single cell conventional cellular DS-CDMA communication system with frequency-selective fading channels, where the number of active users is modelled through a one-dimensional discrete Markov chain. The optimally is in the sense of maximizing the sum-throughput, given a coherent RAKE receiver is employed with maximum ratio combining (MRC). We determine the optimal spreading factor using OLPC SNR-target at the PHY-layer, which satisfies the QoS imposed by packet error rate-target (PER-target) as a function of maximum number of allowed ARQ retransmissions at the DLL. The channel model considers independent paths with Nakagami fading characteristics. Total and truncated channel inversion techniques are used within the inner loop power control (ILPC) to adapt the transmission power to short time channel variations. Sum-throughput performance of the optimized system and non optimized system, where the SNR-target is assumed to be constant, under various multipath fading conditions are studied. A considerable gain in sum-throughput is achieved through coupling of PHY-layer and DLL parameters.