On the Uplink Capacity of High Speed Railway Communications with Massive MIMO Systems

This paper studies the performance and design of high speed railway communications that employ massive MIMO systems. The users onboard the train communicate with the BS by using antennas mounted on top of the train as gateways. Two antenna layouts on the train are considered, one has co-located antennas (CA) at the center of the train, and the other has distributed antennas (DA) uniformly placed along the train. The analytical uplink ergodic capacities for both layouts are derived by averaging over all the locations in a cell. For the CA layout, the average uplink cell capacity is concave in the number of antennas on the train, and the optimum number of antennas on the train is identified by maximizing the average uplink capacity. For the DA layout, the average cell capacity is upper and lower bounded by two concave functions, which are then used to find a sub-optimum value of the number of antennas on the train. Both analytical and numerical results demonstrate that the CA layout has a higher average cell capacity while the DA layout has less variations in terms of uplink capacity at different locations inside a cell.