Abstract
The demand for high data rate is increasing day by day due to dramatic growth in wireless-connected devices. The shortage of bandwidth at sub-6GHz frequency bands cannot fulfil this demand. Therefore, future wireless systems will use millimetre wave (mmWave) frequency bands due to the availability of large bandwidth at these frequencies. The mmWave-enabled systems are supposed to have large antenna arrays to compensate for the high pathloss of mmWave frequency bands, and beamforming is one of the key enabling technologies to achieve it by forming highly directive beams. Digital beamforming (DBF) is considered optimal since each antenna signal is independently preserved from the digital baseband until the antenna aperture. However, a conventional DBF system requires a separate radio-frequency (RF) chain per antenna which creates a bottleneck for the DBF implementation of large antenna arrays due to high power consumption, hardware form factor, complexity, and cost. Therefore, sub-optimal analog beamforming and hybrid beamforming have gained more popularity because they can operate with a small number of RF chains while maintaining the same beamforming gain.
However, the RF-precoder in analog and hybrid beamforming becomes power-hungry when the number of transmit antennas or data streams is increased. In this dissertation, we have three major contributions.
Contribution 1: A low RF-complexity DBF architecture is proposed for future large-scale multi-antenna systems. This architecture ensures that the baseband unit maintains full control of each physical antenna at the aperture. Antennas at the aperture domain are divided into groups where each group transmits an independent data stream. All antennas in the same group share the same RF chain in a time-multiplexed manner to preserve the digitally weighted signal from baseband till the antenna aperture to enable a DBF system with reduced RF-complexity. Under ideal hardware conditions, the results presented in this dissertation show that the proposed architecture can achieve nearly the same performance as that of the conventional full RF-complexity DBF in terms of bandwidth and spectral efficiency.
Contribution 2: The performance of the proposed architecture has further been evaluated under the non-ideal hardware conditions.
For this purpose, mathematical models have been developed for non-linearities such as phase-noise, inphase-quadrature (IQ) imbalance, and inter-RF-chain cross-talk. The presented results show that, in terms of phase noise and IQ imbalance, the proposed and full RF-complexity DBF architectures demonstrate equivalent performance. However, due to reduced inter-RF-chain cross-talk, the proposal significantly outperforms full RF-complexity DBF in terms of error-vector-magnitude (EVM), channel capacity, and energy efficiency. The comparison of the proposed low RF-complexity DBF with the analog and hybrid beamforming systems shows that the proposal is more robust because it does not require a power-hungry RF beamformer.
Contribution 3: In the final part of this work, a multiuser extension of the proposed DBF architecture is presented. A dynamic antenna grouping algorithm is proposed such that it adapts itself according to the varying channel conditions. Notice that in contrast to a single-user beamforming architecture, here one antenna group is used to serve only a single user. Different antenna groups at the antenna aperture are used to serve different users. The mutual orthogonality among the multiuser channels is confirmed by the channel block-diagonalisation technique. The contribution also focuses on some simplified fixed antenna grouping proposals. Results show that the proposed low RF-complexity multiuser beamformer performs nearly the same, in terms of sum-rate performance, as that of state-of-the-art hybrid beamforming and full RF-complexity DBF techniques. Notice that in terms of robustness, the proposed architecture remains superior than hybrid and analog beamforming due to digital control of the antenna aperture.