5G, the new generation of mobile communications, is expected to provide huge improvements in spectral efficiency and energy efficiency. Specifically, it has been proven that the adoption of large antenna arrays is an efficient means to improve the system performance in both of these efficiency measures. For these reasons, the deployment of base stations with large amount of antennas has attracted a substantial amount of research interest over the recent years. However, when pure digital beamforming is pursued in large array system context, a large number of transmitter and receiver chains must also be implemented, increasing the complexity and costs of the deployment.
In general, power consumption of the cellular network is recognized as a major concern. Radio transmitters tend to be really power hungry, especially because of the potential energy inefficiency of their power amplifiers. Due to the characteristics of the current and future waveforms utilized in wireless communications, power amplifiers need to work in a relatively linear regime in order not to distort the signal, making the energy efficiency of such highly linear amplifiers to be rather low. If power amplifiers were capable of working in the nonlinear regime without degrading system performance, their energy efficiency could be notably increased, resulting in considerable savings in energy, costs and system complexity.
In this Thesis, the development and evaluation of a constant envelope spatial precoder is being addressed. The precoder is capable of generating a symbol-rate constant envelope signal, which despite pulse-shape filtering yields substantial robustness against the nonlinearities of power amplifiers. This facilitates pushing power amplifiers into heavily nonlinear regime, with the consequent increase in their energy efficiency. At the same time, the precoder is able to perform spatial beamforming processing in order to mitigate the multi-user interference due to spatial multiplexing. It is assumed that the number of antennas in the base station is much larger than the number of simultaneously scheduled users, implying that large-scale MU-MIMO scenarios are considered, which allows us to exploit the additional degrees of freedom to perform waveform shaping. For the sake of evaluating the proposed precoder performance, different metrics such as PAPR, BER, multi-user interference and beamforming gain are compared to those of currently used precoding techniques.
The obtained results indicate that the studied constant-envelope precoder can facilitate running the PA units of the large-array system in heavily nonlinear region, without inducing substantial nonlinear distortion, while also simultaneously providing good spatial multiplexing and beamforming characteristics. These, in turn, then facilitate larger received SINRs for the scheduled users, and therefore larger system throughputs and a more efficient utilization of the power amplifiers
