Cochannel interference reduction in dynamic-TDD fixed wireless applications, using time slot allocation algorithms

Abstract

In this paper, we consider a fixed wireless cellular network that uses dynamic time division duplex (D-TDD). We analyze the signal-to-interference ratio (SIR) outage performance of a D-TDD fixed cellular system, and propose a scheme to improve the outage probability performance. First, outage probability is evaluated using an analytical model, when omnidirectional antennas are deployed at a base-station site and a subscriber site. Our model is verified, using Monte Carlo simulations. According to our investigation, the outage performance of the D-TDD system is severely limited by a strong interference from the cochannel cell on the downlink, while the reference cell is in the uplink cycle. To improve the outage performance during uplink receptions, we introduce two time-slot allocation methods, combined with sector antennas: max min{SIR} and max{SIR}. Max min{SIR} is an exhaustive search algorithm for assigning subscribers to a few extra uplink time slots, so as to maximize the minimum SIR expectation value over the extra uplink time-slots region. It is used as a performance benchmark in our analysis. Meanwhile, the max{SIR} is a simpler and efficient algorithm for improving the outage performance. It is established that the performance difference between the two algorithms is not noticeable. Especially, the difference is negligible, when the dynamic range of the traffic pattern between uplink and downlink is small. Also, the outage performance of a system that employs the max{SIR} algorithm combined with sectored antennas is compared to that of a system employing adaptive-array antennas. The proposed system shows promise, and offers a compromise between system complexity and network guaranteed availability.