On the value of MIMO rank coordination for interference cancellation-based 5G flexible TDD systems

5G systems can use time division duplex (TDD) small cells employing interference cancellation (IC) receivers. IC-aware scheduling can be used to exploit strong interference cancellation with coordination. The novelty of the approach is that a scheduler purposely schedules strong interference situations in TDD instead of avoiding the interference. In this paper, we propose centralized multiple input multiple output (MIMO) joint rank adaptation for IC-aware coordinated scheduling. This is also called IC-aware joint rank allocation. The performance upper bound is evaluated in an indoor small-cell bursty traffic scenario assuming imperfect channel estimation. This is compared to a baseline of centralized scheduling with standalone rank adaptation and without interference cancellation. Numerical results show that IC-aware joint rank allocation achieves approximately 42% packet delay reduction as compared to the baseline. We further propose a low complexity approach called successively paired rank allocation (SPARK) for joint rank allocation. This heuristic is also seen to achieve around 42% packet delay reduction for the same scenario. We have also compared the benefit of IC-aware joint rank allocation to joint rank allocation without any interference cancellation and with and without downlink power reduction. Results show that IC-aware joint rank allocation outperforms joint rank allocation without interference cancellation in median packet delay by around 24%.