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Abstract
Spectral Efficiency, measured in bits/s/Hz, was the main performance criterion for design of wireless communication systems till about the last decade. However, with the advent of 'internet of things' as well as with the increase in system bandwidth and transceiver complexities, cost and energy efficiency have also become important design metrics today. Thus, in the interest of making systems practically viable, a significant amount of focus has been laid on the design of 'reduced complexity' transceivers that can reap the benefits of state-of-the-art communication technologies, like massive Multiple Input Multiple Output (MIMO) and Ultra-Wide Band (UWB) communication, while being cost and energy efficient. A common feature of these reduced complexity transceivers, for e.g., antenna selection, hybrid beamforming, selective Rake receivers and transmit reference receivers, is reduction of the signal dimension via analog pre-processing/beamforming, thus facilitating the use of a lower dimensional digital processor. Such 'hybrid' signal processing techniques usually require some knowledge of the wireless propagation channel which is conventionally obtained using digital channel estimation (CE) techniques. However, due to the lack of full signal digitization in these transceivers, a significant fraction of transmission resources may have to be expended to obtain this channel knowledge. Thus, conventional CE techniques contribute to a large CE overhead.
In the first part of this thesis, a novel class of such reduced complexity transceivers, namely Hybrid Beamforming with Selection (HBwS), is presented, that utilize switches to aid in the analog beamforming. The use of switches enables the analog beamforming to adapt to the instantaneous channel variations, providing better user separability, beamforming gain, and/or simpler hardware than some conventional reduced complexity transceivers. In this part, the capacity of a system with a HBwS transmitter is analyzed and good designs for the analog beamforming stage are proposed. The trade-off between the system capacity and the CE overhead is also characterized and incorporated into the proposed transceiver designs.
In the second part, it shall be shown that in sparse multi-path channels, estimation of the amplitude and phase of a single transmitted sinusoidal tone encompasses 'sufficient channel knowledge' for enabling hybrid signal processing at the receiver. Since amplitude and phase estimation of a single tone is significantly simpler than conventional CE, it can be performed in the analog domain. Thus, by avoiding digital CE, the estimation overhead of reduced complexity receivers can be lowered significantly. To illustrate this, three novel receiver architectures that can perform analog CE shall be presented. These new receivers admit both coherent and non-coherent implementations and are in fact inspired by a class of non-coherent receivers for UWB systems. The part concludes with a discussion about the advantages and limitations of such analog CE techniques and some approaches to resolve them.
