Channel estimation error and beamforming performance in repeater-enhanced massive MIMO systems

The demand for data service is increasing dramatically and wireless systems with high throughput and the capability to serve a large number of User Equipments (UEs) are desired. The massive MIMO system is considered as one of the most promising systems for the fifth generation of mobile telecommunication technology. Compared to conventional wireless systems, the spectrum efficiency of massive MIMO can be increased by an order of magnitude as tens of UEs can be served on the same time-frequency resource by exploring spatial multiplexing. Though extensively studied, massive MIMO still faces tough practical issues, such as excessive cost for fronthaul, channel estimation, beamforming matrix calculation within the latency requirement, signal quality at the deep fading zones or cell edge, etc. This paper presents a repeater-enhanced massive MIMO system with improved channel quality and reduced fronthaul cost, where Amplify-and-Forward Repeaters (AFRs) are employed without additional baseband processing at the base station or UEs. The disadvantages of AFRs, such as extra delay, increased delay spread and compromised channel reciprocity, are discussed and the corresponding solutions are suggested. Performances of different beamforming methods are analyzed and verified by simulations.