Spatial Sigma-Delta Modulation in a Massive MIMO Cellular System

Abstract

In cellular networks, frequency channels are used to separate the users. However frequency spectrum is an ever-decreasing resource as wireless units and applications increase. Thus, other means to separate the users are of interest. By using hundreds of antennas and beamforming in the system, the users can be efficiently multiplexed in the spatial domain, while using the same time-frequency resource. The large amount of antennas however requires extensive data processing and transport which is difficult and expensive to handle. Thus, an investigation of how to reduce the data bandwidth required in such a system is of interest. This master thesis project evaluates the use of the spatial dimension in a modulator as part of the analog-to-digital conversion. It is put into the context of beamforming and evaluated. A simulator developed in MATLAB is utilized to show the properties and qualities of different modulator structures in a beamforming application. In particular, performance at low converter speeds is evaluated. The results show that by using the spatial dimension in a modulator it is possible to reduce the bandwidth requirement, through reduction of sampling rate while still retaining the signal quality. However, this is applicable only in one direction at a time, and there are multiple implementation issues regarding the proposed design. The findings suggest that with the spatial dimension in a modulator there are possibilities to reduce the bandwidth requirement in antenna array systems but that there are considerable challenges to overcome.