Aspects of Massive MIMO

Abstract: Next generation cellular wireless technology faces tough demands: increasing the throughput and reliability without consuming more resources, be it spectrum or energy. Massive mimo (Multiple-Input Multiple-Output) has proven, both in theory and practice, that it is up for the challenge. Massive mimo can offer uniformly good service to many users using low-end hardware, simultaneously, without increasing the radiated power compared to contemporary system. In Massive mimo, the base stations are equipped with hundreds of antennas. This abundance of antennas brings many new, interesting aspects compared to single-user mimo and multi-user mimo. Some issues of older technologies are nonexistent in massive mimo, while new issues in need of solutions arise. This thesis considers two aspects, and how these aspects differ in a massive mimo context: physical layer security and transmission of system information. First, it is shown that a jammer with a large number of antennas can outperform a traditional, single-antenna jammer in degrading the legitimate link. The excess of antennas gives the jammer opportunity to find and exploit structure in signals to improve its jamming capability. Second, for transmission of system information, the vast number of antennas prove useful even when the base station does not have any channel state information, because of the increased availability of space-time coding. We show how transmission without channel state information can be done in massive mimo by using a fixed precoding matrix to reduce the pilot overhead and simultaneously apply space-time block coding to use the excess of antennas for spatial diversity.