Real-Time Maximum Spectral Efficiency for Massive MIMO and its Limits

This paper evaluates the impact of spatially multiplexing an increasing number of users within a single-cell massive multiple-input, multiple-output (MIMO) system. The highest spectral efficiency (SE) of 145.6 bits/s/Hz achieved for any wireless system to date and its limitation factors are presented. Recent works on massive MIMO show that there is a peak value for sum 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 from the maximum before the sum SE reached its peak value, through an optimization of the modulation scheme. This is done by calculating the sum SE based on the error vector magnitude (EVM) performance and extrapolating this to match the EVM requirements of candidate modulation formats. The impact of uplink (UL) CSI accuracy on the downlink (DL) data transmission is also introduced, showing the heightened sensitivity that it has to inaccurate CSI mapping due to hardware imbalance between UL and DL transmissions. It is also shown that hardware with high quality could be better than increasing the number of antennas at the BS. All the aforementioned points are validated with experimental results obtained from a massive MIMO testbed.