Degree Name

Doctor of Philosophy


School of Electrical, Computer and Telecommunications Engineering


Orthogonal frequency division multiplexing (OFDM) and multi-input multi-output (MIMO) are key techniques for high-speed wireless communications. Besides, there are raising energy costs and carbon footprint associated with the operation of wireless networks. Consequently, it is important to design MIMO-OFDM systems with high energy-efficiency for the next generation of wireless systems.

This thesis studies antenna selection MIMO-OFDM systems from an energyefficiency perspective. The aim of the thesis is to propose and analyse novel antenna selection methods to improve the energy efficiency of the systems. The proposed methods include: i) adaptive antenna selection that jointly selects the number of active radio frequency (RF) chains and antenna indices; ii) power-amplifier aware antenna selection; and iii) jointly optimising transmit power allocation and antenna selection under quality-of-service (QoS) constraints.

Firstly, this thesis analyses energy efficiency in MIMO-OFDM systems that deploy conventional antenna selection approaches. The results show that these antenna systems are not effective from an energy-efficiency viewpoint. Thus, an adaptive selection method is proposed to improve energy efficiency. In the adaptive scheme, the number of active RF chains and the antenna indices are jointly selected to attain maximum energy efficiency. This proposed scheme is shown to achieve a better energy efficiencyspectral efficiency (EE-SE) trade-off compared to the existing selection schemes. In addition, the efficacy of power loading across subcarriers for improved energyefficiency in antenna selection MIMO-OFDM systems is investigated.

Secondly, this thesis considers energy efficiency of antenna selection MIMO-OFDM systems from a power amplifier (PA) perspective. The PA aware antenna selection approach exploits the fact that antenna selection schemes that involve selecting antennas independently for each subcarrier result in power unbalance across transmit antennas, which affects power amplifier. A constrained selection scheme that can equally allocate data subcarriers among antennas by means of linear optimisation is proposed for the systems with an arbitrary number of multiplexed data streams. Moreover, the effectiveness of this scheme is analysed directly in the nonlinear fading channels. Additionally, to overcome the issue of significant fluctuations of both the average power and peak power across transmit antennas, this thesis proposes and analyses a two-step strategy for data allocation in a space-frequency domain. This strategy is based on the aforementioned equal allocation of data subcarriers and the proposed peak-power reduction using cross-antenna permutations. The results demonstrate that a significant improvement in terms of energy efficiency could be achieved in the proposed systems in comparison with the conventional systems.

Lastly, this thesis investigates energy efficiency in antenna selection MIMO systems under QoS constraints. Both antenna selection MIMO and antenna selection MIMO automatic repeat request (ARQ) schemes are considered. Analytical expressions of the achieved energy efficiency in these systems over quasi-static Nakagami-m fading channels are derived. The energy-efficiency metrics take into account several important system parameters, such as channel codes, modulation schemes and detection methods, which is of great significance to practical system designs. Based on a convexity analysis of the energy-efficiency expressions, the optimal average energy per transmitted symbol is determined such that the energy efficiency of the systems is maximised.