Codebook Design and Performance Analysis for Wideband Beamforming in Terahertz Communications

Future 6G and beyond wireless systems aim at ultra-high data rates from very large bandwidths in millimeter-wave and terahertz bands. Large bandwidth signals in antenna arrays lead to beam squint or frequency-selective array response causing losses in the beamforming gain. Recently, several hybrid analog-digital beamforming (HBF) schemes, such as the delayphase precoding employing time delay networks or wider-beam codebook design, have been proposed to tackle the beam squint. However, the severity of beam squint has not been thoroughly investigated in the literature. In this paper, by analyzing the impacts of the beam squint on the uniform planar array (UPA) and uniform linear array (ULA) structures, we analytically and numerically show that the effect of beam squint is less severe in the former. Based on this fact, we propose a simplified HBF design in which the analog beamformer is designed for the center frequency, while digital beamformers for all subcarrier frequencies are obtained based on water-filling and minimum mean square error solutions. Simulation results show that the proposed scheme for UPAs performs very close to the optimal digital beamforming scheme.

For millimeter wave (mmWave) or Terahertz (THz) communications, by leveraging the high spatial resolution offered by large antenna arrays and the multi-path sparsity of mmWave/THz channels, a novel inter-symbol interference (ISI) mitigation technique called delay alignment modulation (DAM) has been recently proposed. The key ideas of DAM are <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">delay pre-compensation</i> and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">path-based beamforming</i>. However, existing research on DAM is mainly based on fully digital beamforming, which requires the number of radio frequency (RF) chains to be equal to the number of antennas. This paper proposes the hybrid analog/digital beamforming based DAM, including both fully and partially connected structures. The analog and digital beamforming matrices are designed to achieve performance close to DAM based on fully digital beamforming. While DAM was considered for the path-based channel model with integer delays in the previous work, this paper extends DAM to a more general tap-based model that accounts for fractional path delays. To further reduce the cost of channel estimation and improve the performance for wireless channels with fractional delays, DAM with codebook-based beam alignment and DAM-orthogonal frequency division multiplexing (DAM-OFDM) with hybrid beamforming are proposed. The effectiveness of the proposed techniques is verified by extensive simulation results.

Reconfigurable intelligent surface (RIS) has shown its potential in terahertz (THz) communications, due to its capability to expand coverage and compensate for the severe attenuation of THz signals. This paper investigates a large-scale RIS-assisted THz communications system in the near-field. Nevertheless, the beam squint effect of RIS, caused by the frequency-independent phase shifting circuit, results in severe array gain loss across the wide bandwidth. While true time delays (TTDs) that generate frequency-dependent phase shifts can mitigate beam squint, they often suffer from high power consumption. To address the drawback, we introduce a set of fixed true time delays (FTTDs) with low power consumption and low insertion loss for the RIS. These FTTDs, shared by the elements of RIS, can generate frequency-dependent phase shifts, thereby addressing the beam squint effect. To overcome the limitation of FTTDs being unable to change delays, we propose a dynamic architecture that consists of a switch network and two-layer phase shifters, allowing the elements of RIS to select the FTTDs. Subsequently, we analyze the theoretical array gain of the proposed FTTD-equipped RIS and determine the minimum number of FTTDs required for effective mitigation. Then, we formulate a problem of maximizing the achievable rate and propose a two-stage algorithm. Specifically, in the first stage, we obtain the optimal wideband beamforming design for both the BS and the RIS. In the second stage, we approximate this optimal design with the beamforming design from our proposed architecture. Finally, simulation results demonstrate that our proposed RIS design, with a small number of FTTDs, can achieve a near-optimal achievable rate and higher energy efficiency.

To aggregate or not to aggregate, that is the question. Analog beamforming with phased arrays is a promising technique for 5G wireless communication in millimeter wave bands. However, beam squint degrades the performance of analog beamforming for wideband systems with a large number of antennas, because the array response varies with frequency. In this paper, we show that carrier aggregation for phased-array analog beamforming should take beam squint into consideration. Specifically, we study the optimal beam alignment to maximize channel capacity, and demonstrate that, with sufficient band separation, focusing on only one band outperforms carrier aggregation. Approximations are developed for a system with two bands to determine the critical values of system parameters like band separation, angle of arrival, and signal-to-noise ratio beyond which it is preferable not to aggregate.

The hybrid beamforming structure is widely used in terahertz (THz) communications due to the low hardware complexity. However, its performance is severely degraded by beam squint effects in wideband THz communications resulting from the non-frequency-specific analog beamforming. To overcome this issue, in this paper, we investigate THz multiuser communications and propose a joint subcarrier allocation and hybrid beamforming (JSH) algorithm to compensate for the beam squint effects and maximize the sum-rate of users. The JSH algorithm alternates between subcarrier allocation and hybrid beamforming. Given the hybrid beamforming, the subcarrier allocation is formulated as an assignment problem and solved using the classic Hungarian algorithm. Given the subcarrier allocation, the hybrid beamforming is solved by developing a penalty-based two-loop hybrid beamforming algorithm. Simulation results show that the proposed JSH algorithm can effectively compensate for the beam squint effects and outperforms the existing ones in terms of the multiuser sum-rate.

Joint power control and beamforming codebook design for MISO channels under the outage criterion

Spatial diversity advantages such as improved signal-to-noise ratio and in-band blocker filtering can be achieved through beamforming in the digital and/or analog domain. Digital beamforming benefits from the precision and efficient parallelization of digital signal processing. On the other hand, analog beamforming allows the filtering of in-band but out-of-beam blockers before the ADC which can improve the dynamic range performance of the receiver. A delay method based on resampling has recently emerged as a viable solution for enabling true-time-delay analog beamforming receivers, which overcome the fractional bandwidth limitation of phase-shift beamforming due to beam squint. This paper presents a 22-nm CMOS receiver prototype that enables reconfiguration between true-time-delay analog and digital beamforming to allow choosing the more suitable operation mode in different signal environments. The reconfigurability is achieved by exploiting the time-interleaved nature of both the resampling delay setup and high speed ADCs. In addition to the beamforming mode reconfigurability, the receiver achieves state-of-the-art 2 GHz instantaneous beamformed bandwidth in the analog mode. The receiver reaches a 100% fractional bandwidth at the low end of the 1–6 GHz frequency range.

Communication systems with low-resolution analog-to-digital-converters (ADCs) can exploit channel state information at the transmitter and receiver. This paper presents codebook designs and performance analyses for limited feedback MIMO systems with finite-bit ADCs. A point-to-point single-user channel is firstly considered. When the received signal is sliced by 1-bit ADCs, the absolute phase at the receiver is important to align the phase of the received signals. A new codebook design for beamforming, which separately quantizes the channel direction and the residual phase, is therefore proposed. For the multi-bit case where the optimal transmission method is unknown, suboptimal Gaussian signaling and eigenvector beamforming is assumed to obtain a lower bound of the achievable rate. It is found that to limit the rate loss, more feedback bits are needed in the medium SNR regime than the low and high SNR regimes, which is quite different from the conventional infinite-bit ADC case. Second, a multi-user system where a multiple-antenna transmitter sends signals to multiple single-antenna receivers with finite-bit ADCs is considered. Based on the derived performance loss due to finite-bit ADCs and finite-bit CSI feedback, the number of bits per feedback should increase linearly with the ADC resolution in order to restrict the rate loss.

This paper presents a novel approach to designing beam codebooks for downlink multiuser hybrid multiple-input–multiple-output (MIMO) wireless communication systems, leveraging multi-agent reinforcement learning (MARL). The primary objective is to develop an environment-specific beam codebook composed of non-interfering beams, learned by cooperative agents within the MARL framework. Machine learning (ML)-based beam codebook design for downlink communications have been based on channel state information (CSI) feedback or only reference signal received power (RSRP), consisting of an offline training and user clustering phase. In massive MIMO, the full CSI feedback data is of large size and is resource-intensive to process, making it challenging to implement efficiently. RSRP alone for a stand-alone base station is not a good marker of the position of a receiver. Hence, in this work, uplink CSI estimated at the base station along with feedback of RSRP and binary acknowledgment of the accuracy of received data is utilized to design the beamforming codebook at the base station. Simulations using sub-array antenna and ray-tracing channel demonstrate the proposed system’s ability to learn topography-aware beam codebook for arbitrary beams serving multiple user groups simultaneously. The proposed method extends beyond mono-lobe and fixed beam architectures by dynamically adapting arbitrary shaped beams to avoid inter-beam interference, enhancing the overall system performance. This work leverages MARL’s potential in creating efficient beam codebooks for hybrid MIMO systems, paving the way for enhanced multiuser communication in future wireless networks.

Terahertz (THz) communication has been regarded as one promising technology to enhance the transmission capacity of future Internet-of-Things (IoT) users due to its ultrawide bandwidth. Nonetheless, one major obstacle that prevents the actual deployment of THz lies in its inherent huge attenuation. Intelligent reflecting surface (IRS) and multiple-input-multiple-output (MIMO) represent two effective solutions for compensating the large path loss in THz systems. In this article, we consider an IRS-aided multiuser THz MIMO system with orthogonal frequency-division multiple (OFDM) access, where the sparse radio frequency chain antenna structure is adopted for reducing the power consumption. The objective is to maximize the weighted sum rate via jointly optimizing the hybrid analog/digital beamforming at the base station (BS) and reflection matrix at the IRS. Since the analog beamforming and reflection matrix need to cater all users and subcarriers, it is difficult to directly solve the formulated problem, and thus, an alternatively iterative optimization algorithm is proposed. Specifically, the analog beamforming is designed by solving a MIMO capacity maximization problem, while the digital beamforming and reflection matrix optimization are both tackled using semidefinite relaxation (SDR) technique. Considering that obtaining perfect channel state information (CSI) is a challenging task in IRS-based systems, we further explore the case with the imperfect CSI for the channels from the IRS to users. Under this setup, we propose a robust beamforming and reflection matrix design scheme for the originally formulated nonconvex optimization problem. Finally, simulation results are presented to demonstrate the effectiveness of the proposed algorithms.

To meet the demand of supreme data rates in terabits-per-second, the next-generation mobile system needs to exploit the abundant spectrum in the millimeter-wave and terahertz bands. However, high-frequency transmission heavily relies on large-scale antenna arrays to reap high beamforming gain, used to compensate for severe propagation loss. It raises a problem of omni-directional beamforming during the phase of initial access, where a base station is required to broadcast synchronization signals and system information to all users within its coverage. This paper proposes a novel initial beamforming scheme, which provides instantaneous gain equally in all directions by forming a pair of complementary beams. Numerical results verify that it can achieve omni-directional coverage with the optimal performance that remarkably outperforms the previous scheme called random beamforming. It is applicable for any form of large-scale arrays, and all three architecture, i.e., digital, analog, and hybrid beamforming.

The combination of large bandwidth at terahertz (THz) and the large number of antennas in massive MIMO results in the non-negligible spatial wideband effect in time domain or the corresponding beam squint issue in frequency domain, which will cause severe performance degradation if not properly treated. In particular, for a phased array based hybrid transceiver, there exists a contradiction between the requirement of mitigating the beam squint issue and the hardware implementation of the analog beamformer/combiner, which makes the accurate beamforming an enormous challenge. In this paper, we propose two wideband hybrid beamforming approaches, based on the virtual sub-array and the true-time-delay (TTD) lines, respectively, to eliminate the impact of beam squint. The former one divides the whole array into several virtual sub-arrays to generate a wider beam and provides an evenly distributed array gain across the whole operating frequency band. To further enhance the beamforming performance and thoroughly address the aforementioned contradiction, the latter one introduces the TTD lines and propose a new hardware implementation of analog beamformer/combiner. This TTD-aided hybrid implementation enables the wideband beamforming and achieves the nearoptimal performance close to full-digital transceivers. Analytical and numerical results demonstrate the effectiveness of two proposed wideband beamforming approaches.

The fast fading mobile radio channels place fundamental limitations on the performance of wireless communication systems, such that the prediction of the changing channel behaviors is of interest. For the frequency-selective fading channels in wideband systems, the knowledge of the channel transfer function over the entire frequency band is required. We propose an ESPRIT-type algorithm to model and further predict the wideband time-varying channel at different frequencies jointly, assuming that the scatterers remain constant. Simulation results show that the joint-frequency prediction scheme has superior performance over conducting the channel prediction on a single frequency.

In this article, we discuss the potential of radio frequency (RF) lens for ultra-wideband millimeter-wave (mmWave) systems. In terms of the beam squint, we compare the proposed RF lens antenna with the phase shifter-based array for hybrid beamforming. To reduce the complexities for fully digital beamforming, researchers have come up with RF lens-based hybrid beamforming. The use of mmWave systems, however, causes an increase in bandwidth, which gives rise to the beam squint phenomenon. We first find the causative factors for beam squint in the dielectric RF lens antenna. Based on the beamforming gain at each frequency, we verify that, in a specific situation, RF lens can be free of the beam squint effect. We use 3D electromagnetic analysis software to numerically interpret the beam squint of each antenna type. Based on the results, we present the degraded spectral efficiency by system-level simulations with 3D indoor ray tracing. Finally, to verify our analysis, we fabricate an actual RF lens antenna and demonstrate the real performance using a mmWave, NI PXIe, software-defined radio system.

Wireless Virtual Reality (VR) is a new-arising technology to enable the untethered connection between VR server and VR client, whose traffic characteristics include both ultra-high data rate in downlink and periodic small-size data packets in uplink. The downlink requires ultra-wide bandwidth and hence leads to the usage of millimeter-wave (mmWave) spectrum with analog beamforming; however, this causes difficulty for the uplink: the efficient multiple access of small-packet users in the same mmWave carrier by time-division duplex (TDD). Therefore, this paper proposes a multi-beam multi-access scheme for the uplink traffic of multiple VR users with mmWave analog beamforming. Specifically, this scheme contains an analog beamforming weight determination function and a frequency-division user scheduling function, which are executed iteratively with the increasing number of selected users. Therein, the proposed multi-beam analog beamforming pattern generation method is able to generate arbitrary number of beams with flexible beam directions, beam widths and beamforming gain ratios, by the constant-modulus weights with one radio-frequency chain. It is demonstrated by the simulation results that the proposed multi-beam multi-access scheme increases 200% for the maximum number of simultaneously served VR uplink users than the traditional single-beam multi-access scheme in 5G systems with mmWave analog beamforming.