Designing and Evaluating of MIMO Systems for Millimeter-Wave Communications
Authors: Ahmadi Almasi Mojtaba 
Published: February 2021

The fifth generation (5G) of wireless communications will integrate all existing technologies while bringing its own to the system. Amongst these technologies, millimeter-wave (mmWave) is emerging as a promising solution for 5G systems. However, to fully harness the potential of mmWave communications, obstacles such as severe path loss, channel sparsity, and hardware complexity should be overcome. The existing cost-effective systems can considerably reduce the hardware complexity and partially severe path loss, while channel sparsity still remains a main problem. Other factors such as transmission reliability and coverage area should be considered in 5G mmWave communications. Non-orthogonal multiple access (NOMA) is another enabling technology for 5G systems to improve spectral efficiency through serving more than one user at the same time/frequency/code resources. In particular, users' signals are superimposed in power domains at the transmitter which allows for simultaneously exploiting the available resources. Therefore, mmWave bands along with NOMA plays a crucial role in 5G wireless communications.

Aiming to overcome the mentioned obstacles, in the first part of this dissertation, we design a new lens-based reconfigurable antenna multiple-input multiple-output (RA-MIMO) architecture that takes advantage of multi-beam antennas for point-to-point communications. The considered antennas can generate multiple independent beams simultaneously using a single RF chain. This property, together with RA-MIMO architecture, is used to combat small-scale fading and shadowing in mmWave bands. We use well-known space-time block codes (STBCs), together with phase-shifters at the receiver, in the RA-MIMO to suppress the effect of small-scale fading and shadowing. We also study the impact of practical quantized phase-shifters on the performance of the proposed RA-MIMO.

On the other hand, to make the most of these multi-beam antennas, a novel multiple access technique is developed for multi-user scenarios named reconfigurable antenna multiple access (RAMA). This technique transmits only each user's intended signal at the same time/frequency/code. This property makes RAMA an inter-user interference-free technique. Further, we integrate the well-known non-orthogonal multiple access (NOMA) technique in the proposed and other available mmWave systems. Moreover, to support a huge number of groups of users, we integrate RAMA into NOMA named reconfigurable antenna NOMA (RA-NOMA). This new technique divides the users with respect to their angle of departures (AoDs) and channel gains. Users with different AoDs and comparable channel gains are served via RAMA while users with the same AoDs but different channel gains are served via NOMA.

In the second part of this dissertation, we investigate NOMA in mmWave MIMO systems with phased array antennas. Two major obstacles in implementing NOMA are beam misalignment and limited channel coherence time due to the directional transmission. First, the effect of beam misalignment on rate performance in downlink of hybrid beamforming-based NOMA (HB-NOMA) systems is studied. To this end, an HB-NOMA framework is designed and a sum-rate maximization problem is formulated. An algorithm is introduced to design digital and analog precoders and efficient power allocation. Then, regarding perfectly aligned line-of-sight (LoS) channels, a lower bound for the achievable rate is derived. When the users experience misaligned LoS or non-LoS (NLoS) channels, the impact of beam misalignment is evaluated.

We take the limited channel coherence time into account for non-orthogonal multiple access (NOMA) in mmWave hybrid beamforming systems. Due to the limited coherence time, the beamwidth of the hybrid beamformer affects the beam-training time, which in turn directly impacts the data transmission rate. To investigate this trade-off, we utilize a combined beam-training algorithm. Then, we formulate a sum-rate expression which considers the channel coherence time and beam-training time as well as users' power and other system parameters.