A joint code/time assignment strategy with minimal fragmentations for CDMA systems

This paper handles the internal and external fragmentation problems of CDMA systems using orthogonal variable spreading factor (OVSF) codes. Internal fragmentation occurs when the allocated data rate is larger than what is requested while external fragmentation occurs when the OVSF code tree is too fragmented to support a call even if there are sufficient capacity remaining in the code tree. The key factor in solving these two problems is the OVSF code assignment strategy. Most works in the literature do not consider the time-varying and location-dependent channel conditions. In this paper, we formulate the fragmentation problem as a multiple knapsack problem where each OVSF code is considered as a knapsack. We propose single-code, time-shared strategies that consider channel conditions while solving these fragmentation problems. We show that internal fragmentation can be significantly reduced if we can properly select users to share OVSF codes. Simulation results verify that our strategies do use the precious wireless bandwidth efficiently