A multimode CDMA with reduced intercell interference for broadband wireless networks

This paper investigates the application of CDMA to broadband wireless access systems commonly known as LMDS networks. After a review of the intercell interference in the emerging LMDS networks based on TDMA, we analyze the interference in CDMA-based networks and show that, while conventional CDMA is superior to TDMA in terms of the worst-case interference on the upstream channel, it turns out to be inferior to TDMA in terms of interference on the downstream channel. Next, based on the observation that strong interference occurs only for a small range of user locations, we introduce a multimode CDMA concept that efficiently handles this interference by assigning orthogonal spreading sequences whose number and length are a function of the user position with respect to the base station. We show that in terms of worst-case SIR, the proposed technique leads to a significant improvement with respect to conventional CDMA and TDMA.