Page 75 - ITU Journal Future and evolving technologies Volume 2 (2021), Issue 7 – Terahertz communications

ITU Journal on Future and Evolving Technologies, Volume 2 (2021), Issue 7

HIERARCHICAL BEAM ALIGNMENT IN SU‑MIMO TERAHERTZ COMMUNICATIONS

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Yifei Wu , Johannes Koch , Martin Vossiek , and Wolfgang Gerstacker 1
1 Institute for Digital Communications, Friedrich‑Alexander‑Universität Erlangen‑Nürnberg, Erlangen, Germany,
2 Institute of Microwaves and Photonics, Friedrich‑Alexander‑Universität Erlangen‑Nürnberg, Erlangen, Germany
NOTE: Corresponding author: Yifei Wu, yifei.wu@fau.de

Abstract – Single‑Carrier Frequency Division Multiple Access (SC‑FDMA) is a promising technique for high data rate indoor
Terahertz (THz) communications in future beyond 5G systems. In an indoor propagation scenario, the Line‑Of‑Sight (LOS)
component may be blocked by the obstacles. Thus, ef icient THz SC‑FDMA communications require a fast and reliable Beam
Alignment (BA) method for both LOS and Non‑Line‑Of‑Sight (NLOS) scenarios. In this paper, we irst adopt the hierarchical
discrete Fourier transform codebook for LOS BA, and introduce the hierarchical k‑means codebook for NLOS BA to improve the
beamforming gain. Simulation results illustrate that the hierarchical DFT codebook and the hierarchical k‑means codebook
can achieve the beamforming gain close to that of the maximum ratio transmission in LOS and NLOS cases, respectively. Based
on these two codebooks, we propose a Multi‑Armed Bandit (MAB) algorithm named Hierarchical Beam Alignment (HBA) for
single‑user SC‑FDMA THz systems to reduce the BA latency. HBA utilizes a hierarchical structure in the adopted codebook
and prior knowledge regarding the noise power to speed up the BA process. Both theoretical analysis and simulation results
indicate that the proposed BA method converges to the optimal beam with high probability for both the hierarchical DFT
codebook and the hierarchical k‑means codebook in the LOS and NLOS scenarios, respectively. The latency introduced by
HBA is signi icantly lower when compared to an exhaustive search method and other MAB‑based methods.
Keywords – Beam alignment, codebook design, SC‑FDMA, SU‑MIMO, terahertz communications

1. INTRODUCTION massive
Multiple-InputMultiple‑Output (MIMO) THz communi-
Terahertz (THz) communications are a key technology cations, the power ampli ier ef iciency in the THz band
for the future wireless communications (beyond 5G) is degraded significantly. Hence, the bene its of a low
owing to its ample frequency spectrum resource PAPR of SC‑FDMA are emphasized for THz
between 0.1 and 10 THz promising a much higher communications. Moreover, THz signals suffer from
capacity than mmWave communications [1], [2]. Major severe path loss caused by high carrier frequency
around hundreds of GHz, which limits THz
breakthroughs in hardware and theory have been
communications in an indoor transmission scenario [8],
achieved for the ef icient realization of THz range
[6]. To overcome the high path loss, directional beam‑
transmission [3], [1]. Besides the ultra‑high bandwidth,
forming with massive Transmitter (Tx) and Receiver (Rx)
THz wireless technology has the advantage that it could
antenna arrays is regarded as a reasonable solution. Be‑
be deployed much faster and more ef iciently than
cause of the short wavelength in THz bands, and the
optical iber systems, especially in a high‑density urban progress of antenna technology, the massive antenna ar‑
environment [4]. Moreover, THz communications are rays in principle can be packed into a small area, which
highly suited to the indoor environ‑ ment since THz enables a large beamforming gain at both Tx and Rx.
communications systems can utilize the Non‑Line‑ Besides the massive antenna array, the Tx and Rx
Of‑Sight (NLOS) Multipath Components (MPCs) to beams at the base station and user terminal must be
enhance the link quality in indoor application [5], [6]. formed accurately to achieve the beamforming gain.
However, the design of beamforming codes is usually
However, NLOS MPCs result in a highly frequency‑
based on perfect Channel State Information (CSI), which
selective channel, which requires the transmission sys‑
is dif icult to acquire in the THz case especially at the Tx
tem to deal with the Inter‑Symbol Interference (ISI)
side due to the large‑scale antenna array and the small
effect. The Single‑Carrier Frequency Division Multiple Signal‑to‑Noise Ratio (SNR) before beamforming.
Access (SC‑FDMA) transmission approach provides a
One way to circumvent the CSI requirement is to em‑
solution to conquer the high frequency selectivity of the
ploy a Beam Alignment (BA) scheme. BA is a process to
channel. Compared to Orthogonal Frequency Division
ind the optimal transmit‑receive beam pair from prede‑
Multiplexing (OFDM), SC‑FDMA utilizes a Discrete ined codebooks to maximize the receive signal strength.
Fourier Transform (DFT) pre‑coding to reduce Peak‑to‑
The beam alignment problem has been widely studied
Average Power Ratio (PAPR) [7]. Due to the high carrier in mmWave communications. The authors in [9] advo‑
cate decoupling the BA process in mmWave transmis‑
0 This work has been supported by the German Research Foundation sion into two steps to reduce the BA latency caused by
(DFG) under grant GE 861/9‑1.

© International Telecommunication Union, 2021 63