Coverage Analysis and Cooperative Hybrid Precoding for 5G Cellular Networks

Licentiatavhandling, 2019

5G innovations have been made in both the network deployment and the transceiver architectures in order to increase coverage, energy- and spectrum-efficiency. Future base stations (BSs) are expected to be densely deployed in places such as walls and lamp posts and cover a smaller area compared to current macro BS systems. Using large spectrum at millimeter-wave (mmWave) frequency bands and highly directional beamforming with large antenna arrays, 5G will bring gigabit-per-second data rate and low-latency communications and enable many novel services such as high-speed mmWave wireless interconnections between devices, vehicular communications, etc.. Moreover, mmWave communication systems will be based on novel hybrid beamforming architectures which have reduced hardware power consumption and cost. Thus, for better understanding of 5G performance and limitations, one of the main goals in this thesis is to analyze new models that give tractable performance metrics for dense small BS networks. Another goal in this thesis is to study mmWave hybrid beamforming schemes which enable joint transmissions in multi-cell multi-user systems. In the thesis, we show the advantages of small cells in increasing the coverage probability and reducing the path loss and shadowing, and we show the value of cooperation in terms of power consumption and outage.

In [Paper A] we derive analytical expressions for the successful reception probability of the equal gain combining receiver in a network where interfering transmitters are distributed according to a Poisson point process and interfering signals are spatially correlated. The results show that the spatial correlation reduces the successful reception probability and the effect of the spatial correlation increases with the number of antennas. [Paper B] follows to study the performance of a partial zero forcing receiver. The results are simulated in an environment with blockages and are analyzed under both Rayleigh and Rician channels. The coverage probability is shown to be maximized when using a subset of antennas' degree-of-freedom for useful signal enhancement and using the remaining degrees of freedom for canceling the interference from strongest interferers. Finally, in [Paper C], we propose a hybrid beamforming scheme which minimizes the total power consumption of a multi-cell multi-user network, subject to per-user quality-of-service constraints. The proposed scheme is based on decoupling the analog precoding and digital precoding. The analog precoders are only dependent on the local channel state information at each BS. Then, the digital precoders are obtained by solving a relaxed convex optimization for given analog precoders. Simulation results show that the proposed algorithm leads to almost the same RF transmit power as that of fully digital precoding, while saving considerable hardware power due to the reduced number of RF chains and digital-to-analog converters.