Adaptive Load Balancing MDP-based Approach of Two-Dimension Spreading for VSF-OFCDM in Next Generation Cellular Communications

• Main Authors: Chih-hsien Wu, 吳致憲
• Other Authors: Ben-Jye Cheng
• Format: Others
• Language: en_US
• Published: 2007
• Online Access: http://ndltd.ncl.edu.tw/handle/14631427024750335633

碩士 === 朝陽科技大學 === 資訊工程系碩士班 === 95 === The VSF-OFCDM system has been proposed as the forward link interface for achieving high data rate in 4G mobile communications. VSF-OFCDM allocates orthogonal channelization codes of an OVSF code tree in two-dimension (2D) spreading in the time and frequency domains, but suffers from two disadvantages: moderate utilization and low transmission quality. The moderate utilization is caused by code blocking and the low transmission quality is due to the multicode interference from high channel loading in the time domain of the two-dimension spreading. A trade-off exists between code blocking and multicode interference. Specifically, lower the code blocking the system has, higher the interference it yields. To achieve high utilization while providing high transmission quality becomes the critical issue that should be addressed in 4G VSF-OFCDM. Therefore, in this paper, we propose the Adaptive Load-balancing with Markov decision process based 2D spreading approach (i.e., denoted by ALM) for the purpose. The ALM approach consists of three phases. First, for balancing channel load, the adaptive 2D spreading phase is proposed to select that the 2D spreading combinations are with low channel load and high frequency diversity as the candidates. Second, a dynamic re-combination of a 2D spreading is proposed to decrease channel load while supporting transmission quality from high frequency diversity if the channel load of a time domain code exceeds the defined channel load threshold. Third, for minimizing the code blocking, the cost-based Markov decision process code selection approach is adopted. The MDP phase selects the least cost channelization code as the optimal solution if there are several 2D spreading combinations that satisfy the channel load limitation. In addition, the time complexity of ALM is analyzed. Numerical results indicate that the proposed approach outperforms other approaches in transmission quality ratio, fractional reward loss and the total number of reassignments.