MDP-based CAC for two-dimension spreading VSF-OFCDM in 4G cellular communications

The VSF-OFCDM system has been proposed as the forward link interface for achieving high data rate in 4G mobile communications, which allocates orthogonal channelization codes of an OVSF code tree in two-dimension (2D) spreading in the time and frequency domains. However, it suffers from two disadvantages namely moderate utilization and low transmission quality. Moderate utilization is caused by code blocking, and low transmission quality is due to the multicode interference from high channel loading in the time domain of the 2D spreading. A trade-off thus exists between code blocking and multicode interference. Lower code blocking means higher interference. For achieving high utilization while also providing high transmission quality is a critical issue that should be addressed in 4G VSF-OFCDM. Therefore, this paper proposes a 2D spreading approach based on Adaptive Load-balancing with Markov decision process (denoted by ALM). The ALM approach consists of three phases: 1) the adaptive 2D spreading phase to select the 2D spreading combinations, 2) a dynamic re-combination of a 2D spreading to decrease channel load while supporting transmission quality, and 3) the cost-based Markov decision process (MDP) code selection approach to minimize code blocking and to select the least cost channelization code as the optimal solution. Numerical results indicate that the proposed approach outperforms other approaches in transmission quality ratio and fractional reward loss.