Optimum channel coding rate and spreading factor values achieving one-cell frequency reuse in downlink VSF-spread OFDM packet radio access

Abstract

This paper investigates the optimum minimum coding rate of a very-low-rate Turbo coding and spreading factor value that achieves one-cell frequency reuse in Variable Spreading Factor (VSF)-Spread Orthogonal Frequency Division Multiplex- ing (OFDM) radio access in the downlink. Simulation results show that the optimum minimum coding rate is approximately R = 1/5 to 1/9 to obtain a clear additional coding gain from random code spreading. Moreover, by applying hybrid ARQ with packet combining, the required spreading (repetition) factor value can be decreased by the time diversity gain associated with soft-com- bining, compared to the case without ARQ. Accordingly, we show that the spreading factor of 8, 4, and 2 is necessary in addition to Turbo coding with R = 1/8 when the maximum number of retrans- missions in hybrid ARQ with packet combining is 0, 1, and 3 times, respectively, in order to achieve one-cell frequency reuse at the required residual packet error rate of 10 -3 for a cell radius of 1 km with 46-dBm transmission power in a 10-MHz channel bandwidth. Furthermore, for a large cell radius such as greater than 2 km, a large spreading factor of greater than 16 is required to mitigate the influence of background noise under such noise limited conditions. I. INTRODUCTION