Multi-envelope precoding for massive mimo systems
Duman, Tolga Mete
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Wireless communications is an important part of information and communication technologies. Particularly, with the introduction of 5G wireless systems, higher data rates, ultra-low latencies and improved power efficiencies are demanded. It is understood that multiple-input multiple-output (MIMO) systems constitute some of the promising technologies to meet these demands, however, currently used number of antennas at the base stations (BS) is not sufficient to reveal the full potential. As a result, massive MIMO systems which use a very large number of antennas at the BSs have recently been proposed as enabling solutions. While massive MIMO promises much for 5G and beyond wireless technologies, there are many problems to be solved including lowering of high built-in and operating costs of BSs to make this technology practical. Constant envelope (CE) precoding has recently been proposed as a way to reduce the hardware complexity of massive MIMO systems. CE precoding technique for downlink enables a BS structure with one (nonlinear) power amplifier (PA) coupled with continuous or discrete phase shifters in front of each antenna instead of separate highly linear PAs driving each. While CE precoding offers significant reductions in hardware costs, it results in some performance loss in terms of achievable data rates and power efficiencies compared to conventional zero forcing precoding based approaches.In this thesis, we build on the CE precoding idea and propose the use of a multienvelope precoding technique for massive MIMO systems which utilizes more than one (but only a few, e.g., 2 or 3) PAs with the objective of recovering some of the performance loss due to the use of CE precoding. The proposed multi-envelope precoding method relies on the standard zero forcing algorithm to group the antennas, and then it utilizes an envelope with a higher level on the antenna group(s) requiring higher power. In other words, the number of power levels used equals to the number of antenna groups. We explore the use of both continuous and discrete phase shifters, and via extensive simulations, we demonstrate that the newly proposed approaches provide significant performance improvements over the CE solutions closing some of the performance gap with average power constraint precoding.