Capacity Analysis of Terahertz Line-of-Sight UCA-MIMO Channels

Abstract

Terahertz (THz) line-of-sight (LOS) massive multiple-input multiple-output (MIMO) communication is promising to achieve a few terabit rates for beyond 5G. The tiny wavelength in THz allows us to pack massive antenna elements onto a small aperture size. As the number of antenna elements scales, the computational complexity of the capacity-achieving singular value decomposition (SVD) precoding method increases cubically. Thus, it is critical to improving the energy efficiency in the baseband processing while maintaining spectral efficiency performance. In this thesis, I present a two-stage precoding method that achieves the MIMO channel capacity with significantly reduced computational complexity. The principal idea is to decompose a uniform planner array (UPA) into a superposition of multi-layered uniform circular subarrays (UCAs) at both ends. Thanks to the structure of the circulant channel matrix between the subarrays, the proposed precoding exploits the discrete Fourier transform (DFT)-based inner precoding per subarray conjunction with SVD outer precoding per eigen-mode. I show that this two-stage precoding method provides a multiplicative gain in the computational complexity reduction compared to the conventional SVD precoding. On the other hand, as both the signal bandwidth and the number of antenna elements increase, the power efficiency of transmit and receive quantizers degrades severely. A one-bit wireless transceiver is power- and cost-efficient system architecture for THz MIMO communication systems using a few gigahertz bandwidths and more. Understanding the channel capacity has been limited because of the lack of explicit mutual information expressions when using one-bit transceivers. In this thesis, I derive a tight lower bound of the capacity of the THz LOS MIMO channel constructed by UCAs at both ends. The proposed strategy harnesses a discrete Fourier transformbased analog precoder and combiner in conjunction with a novel waterfilling policy under the one-bit transceiver constraint. Using simulation, I verify that the proposed waterfilling solution provides a substantial gain in spectral efficiency compared to the uniform allocation policy.