Sounding resource management for QoS support in massive MIMO systems

In massive multiple-input multiple-out (M-MIMO) systems, the base station estimates each forward-link channel by using the reverse-link sounding pilot and the channel reciprocity property of time division duplex (TDD) operation. However, conventional non-cooperative sounding schemes, like a sounding sequence assignment scheme with reuse factor-1, cause cell edge users to suffer from pilot contamination in multi-cell scenarios. The pilot contamination problem becomes even worse in an environment where mobile users are travelling through cell edge areas frequently. To alleviate this problem, a cooperative sounding resource reuse method that assigns each cell a set of resources different from other neighboring cells by using a reuse factor-3 scheme, can be considered, but it significantly lowers the number of served users. In this paper, we propose a partial sounding resource reuse (PSRR) method which aims to seamlessly support the quality of service (QoS) of each mobile user by mitigating the pilot contamination and to minimize the reduction in the number of served users. To this end, the PSRR divides each cell area into center and edge areas, and applies a reuse factor-1 scheme for center users while a reuse factor-3 scheme for edge users. We use a Markov chain model to analyze the performance of the PSRR, and evaluate the accuracy of our analysis through simulations. Then we confirm that the PSRR considerably improves QoS performance over the conventional competitive schemes.