Interference analysis and resource allocation for TDD-CDMA systems to support asymmetric services by using directional antennas

This paper explores the advantages of using directional antennas in time division duplex (TDD) code division multiple access (CDMA) systems to support asymmetric traffic services. In the TDD-CDMA system, the transmission for asymmetric traffic from neighboring cells may cause the cross-slot interference, which can seriously degrade system capacity. To avoid the cross-slot interference for TDD-CDMA systems, many current resource allocation algorithms typically require a global control on the transmission direction (either downlink or uplink) in each time slot. Apparently, this requirement substantially constrains the flexibility of TDD-CDMA systems to deliver asymmetric traffic services in a more practical situation where every cell may have different uplink and downlink bandwidth requirements. We analyze the interference of the TDD-CDMA system with a trisector cellular architecture, where three directional antennas are employed at each base station. We find that the directivity of directional antennas can provide an additional degree of freedom for allocating radio resource. Taking advantage of this property, we introduce the concept of virtual cell, defined as the same converge area of a cell but is composed of three sectors from the three adjacent base stations. Furthermore, we propose a new virtual cell-based distributed code/time slot allocation algorithm to enable a TDD-CDMA system to provide asymmetric services with different rates of asymmetry in every cell coverage area. We demonstrate that the proposed algorithm does not only offer more flexibility in handling nonuniform traffic patterns, but maintain good radio link performance and call blocking performance.