Impact of User Number on Massive MIMO with a Practical Number of Antennas

This paper evaluates the impact of spatially multiplexing more users within a massive multiple-input, multiple output (MIMO) system. Recent works on massive MIMO show that there is a peak value for sum spectrum efficiency (SE) achieved by serving a certain number of users. It was shown that until the sum SE reached its peak value, the maximum sum SE is achieved by serving all users simultaneously. These results were based on perfect channel state information (CSI), Shannon capacity calculations or using a very large number of antennas at the base station (BS). As opposed to the aforementioned results, we show that the maximum sum SE with practical number of antennas could be achieved by decreasing the number of users before the sum SE reached its peak value. This is shown by calculating the sum SE based on the Error Vector Magnitude (EVM) performance. Sum SE with zero-forcing (ZF) and matched filtering (MF) is presented for uplink (UL) data transmission in a single-cell. Channel matrices formed from both independent and identically distributed (IID) Rayleigh samples and measured data from a real massive MIMO system were used for performance evaluation. The impact of adding more users is also demonstrated by real constellations captured from an indoor massive MIMO trial.