One of the key enablers of future wireless communications is constituted by massive multiple-input multiple-output (MIMO) systems, which can improve the spectral efficiency by orders of magnitude. In ...existing massive MIMO systems, however, conventional phased arrays are used for beamforming. This method results in excessive power consumption and high hardware costs. Recently, reconfigurable intelligent surface (RIS) has been considered as one of the revolutionary technologies to enable energy-efficient and smart wireless communications, which is a two-dimensional structure with a large number of passive elements. In this paper, we develop a new type of high-gain yet low-cost RIS that bears 256 elements. The proposed RIS combines the functions of phase shift and radiation together on an electromagnetic surface, where positive intrinsic-negative (PIN) diodes are used to realize 2-bit phase shifting for beamforming. This radical design forms the basis for the world's first wireless communication prototype using RIS having 256 two-bit elements. The prototype consists of modular hardware and flexible software that encompass the following: the hosts for parameter setting and data exchange, the universal software radio peripherals (USRPs) for baseband and radio frequency (RF) signal processing, as well as the RIS for signal transmission and reception. Our performance evaluation confirms the feasibility and efficiency of RISs in wireless communications. We show that, at 2.3 GHz, the proposed RIS can achieve a 21.7 dBi antenna gain. At the millimeter wave (mmWave) frequency, that is, 28.5 GHz, it attains a 19.1 dBi antenna gain. Furthermore, it has been shown that the RIS-based wireless communication prototype developed is capable of significantly reducing the power consumption.
Terahertz (THz) communication is considered to be a promising technology for future 6G network. To overcome the severe attenuation and relieve the high power consumption, massive multiple-input ...multiple-output (MIMO) with hybrid precoding has been widely considered for THz communication. However, accurate wideband channel estimation, which is essential for hybrid precoding, is challenging in THz massive MIMO systems. The existing wideband channel estimation schemes based on the ideal assumption of common sparse channel support will suffer from a severe performance loss due to the beam split effect. In this paper, we propose a beam split pattern detection based channel estimation scheme to realize reliable wideband channel estimation in THz massive MIMO systems. Specifically, a comprehensive analysis on the angle-domain sparse structure of the wideband channel is provided by considering the beam split effect. Based on the analysis, we define a series of index sets called as beam split patterns, which are proved to have a one-to-one match to different physical channel directions. Inspired by this one-to-one match, we propose to estimate the physical channel direction by exploiting beam split patterns at first. Then, the sparse channel supports at different subcarriers can be obtained by utilizing a support detection window. This support detection window is generated by expanding the beam split pattern which is determined by the obtained physical channel direction. The above estimation procedure will be repeated path by path until all path components are estimated. Finally, the wideband channel can be recovered by calculating the elements on the total sparse channel support at all subcarriers. The proposed scheme exploits the wideband channel property implied by the beam split effect, i.e., beam split pattern, which can significantly improve the channel estimation accuracy. Simulation results show that the proposed scheme is able to achieve higher accuracy than existing schemes.
Terahertz (THz) massive multiple-input multiple-output (MIMO) has been considered as one of the promising technologies for future 6G wireless communications. It is essential to obtain channel ...information by beam tracking scheme to track mobile users in THz massive MIMO systems. However, the existing beam tracking schemes designed for narrowband systems with the traditional hybrid precoding structure suffer from a severe performance loss caused by the beam split effect, and thus cannot be directly applied to wideband THz massive MIMO systems. To solve this problem, in this paper we propose a beam zooming based beam tracking scheme by considering the recently proposed delay-phase precoding structure for THz massive MIMO. Specifically, we firstly prove the beam zooming mechanism to flexibly control the angular coverage of frequency-dependent beams over the whole bandwidth, i.e., the degree of the beam split effect, which can be realized by the elaborate design of time delays in the delay-phase precoding structure. Then, based on this beam zooming mechanism, we propose to track multiple user physical directions simultaneously in each time slot by generating multiple beams. The angular coverage of these beams is flexibly zoomed to adapt to the potential variation range of the user physical direction. After several time slots, the base station is able to obtain the exact user physical direction by finding out the beam with the largest user received power. Unlike traditional schemes where only one frequency-independent beam can be usually generated by one radio-frequency chain, the proposed beam zooming based beam tracking scheme can simultaneously track multiple user physical directions by using multiple frequency-dependent beams generated by one radio-frequency chain. Theoretical analysis shows that the proposed scheme can achieve the near-optimal achievable sum-rate performance with low beam training overhead, which is also verified by extensive simulation results.
Millimetre‐wave (mmWave) massive multiple‐input multiple‐output (MIMO) is one of the promising techniques for 5G wireless communications and beyond. Low‐resolution hybrid precoding using ...low‐resolution phase shifters (PSs) is considered to be promising for mmWave massive MIMO, since it can realize an acceptable performance with significantly reduced energy consumption. However, to realize accurate channel state information acquisition, the traditional channel feedback codebooks that quantize the channel with high resolution are not suitable for low‐resolution hybrid precoding. To solve this problem, angle‐based codebook is proposed here. In the proposed codebook, the analog codebook is designed based on the channel angle‐of‐departures (AoDs) and the digital codebook is generated by the random vector quantization. Specifically, the analog codewords are optimized by a neighbour search algorithm under the constraint of low‐resolution PSs. These analog codewords are designed to be aligned with channel AoDs. In this way, they can remain unchanged in a much larger time scale, since the angle‐coherence time is much longer than the channel‐coherence time. Therefore, the channel feedback overhead can be significantly reduced. Both theoretical analyses and simulation results illustrate that the proposed codebook can achieve the acceptable achievable rate performance with low channel feedback overhead.
Benefiting from the ultra-wide bandwidth, terahertz (THz) communication is becoming a promising technology for future 6G networks. In order to support the desired coverage, precoding by using a ...large-scale antenna array is an essential technique for THz communication to overcome the severe path loss of THz signals. In this article, we systematically investigate the dominant THz precoding techniques for future 6G networks, with the highlight of its key challenges and opportunities. Specifically, we first clarify the major differences between milli-meter-wave and THz channels, based on which we reveal the key challenges of THz precoding, such as the distance-dependent path loss, the beam split effect, the high power consumption, and so on. To address these challenges, five representative THz precoding techniques: analog beamforming, hybrid precoding, delay-phase precoding, dynamic RF chains based precoding, and reconfigurable intelligent surface based precoding, are extensively investigated in terms of their different structures, designs, most recent results, and pros and cons. We also provide simulation results of spectrum and energy efficiencies to compare these typical THz precoding schemes to draw some insights for their applications in future 6G networks. Finally, several important open issues and potential research opportunities, such as the integrated sensing and communication, are pointed out and discussed.
To explore the influence of circumferential grooved casing treatment on subsonic cascade performance, a numerical simulation of subsonic cascade was conducted. In contrast to traditional research on ...variable single parameters for casing treatment, this paper used the Latin hypercube sampling method to randomly sample multiple geometric parameters of casing treatment and compared many sample data with the total pressure loss of the cascade as a measurement standard. After selecting several typical cases of high and low total pressure loss cases for in-depth flow field analysis, it was found that casing treatment affects the strength and structure of the leakage vortex, thereby reducing the blockage of fluid in the passage of the cascade. Changes in the total pressure loss and in the margin of the cascade meant that casing treatment affected cascade performance. This prompted analysis of the correlation between the casing treatment parameters and total pressure loss of the cascade. The clearance height and groove depth had the greatest influence on the total pressure loss of the cascade.
It is widely believed that the channel reciprocity holds in time division duplexing (TDD) massive multiple-input multiple-output (MIMO) systems, so the uplink channel feedback is unnecessary. ...However, due to the constraint of transmission power and hardware complexity, users may use more receiving antennas than transmitting antennas in practical wireless communication systems. In this practical yet important scenario, only partial channel reciprocity holds, and thus channel feedback is still required in TDD massive MIMO systems. Unfortunately, this important topic has not been investigated in the literature. To fill in this gap, we propose an angle-domain support (ADS) based channel feedback scheme for TDD massive MIMO systems with unequal number of receiving and transmitting antennas. Firstly, by utilizing the angle-domain channel sparsity, we illustrate that a small number of elements in angle-domain channel, which are indexed by ADS, can capture the most power of the channel. Then, we propose the base station (BS) can obtain full downlink channel by decomposing the channel into two parts: the ADS and the low-dimensional angle-domain representation indexed by the ADS. At first, due to the partial channel reciprocity, the ADS is obtained by an angle-domain windows based support detection method at the BS without channel feedback. Then, the low-dimensional angle-domain representation indexed by the ADS is quantized and fed back to the BS. Theoretical analysis and simulation results demonstrate that the proposed scheme can achieve the near-optimal achievable sum-rate performance with low channel feedback overhead.
Smart grids, which have the ability to detect and monitor necessary system parameters and user behaviors, have gradually become the development trend for future power networks. With increasing scale ...and access of new energy source, SGs become more unstable and vulnerable when changes of network topology occur abruptly. Among different kinds of abrupt changes, unexpected line outages may give rise to significant potential damages to SGs and lead to the very bad user experience. Thus, accurate and rapid detection of line outages turn into one of the important tasks and challenges in SGs. By utilizing the sparse property of line outages, compressive sensing (CS)-based line outage detection schemes have been proposed recently, in which the phase information collected from phasor measurement units is also fully utilized. In this paper, a novel block-wise CS (BW-CS)-based multiple line outage detection scheme is proposed for SGs. Firstly, by exploiting the sparsity of line outages and topology structure of the power grid, a novel reactance model is introduced, which makes the block-wise CS algorithm can be adopted. Then, by modifying one step of the conventional CS algorithm with the redefined element selection, criterion is modified to improve the recovery accuracy. Finally, the proposed scheme is extended into a three-phase power system. The simulation results show that our proposed BW-CS method can achieve superior precision with the similar complexity compared to the traditional methods.
Benefiting from tens of GHz of bandwidth, terahertz (THz) communication has become a promising technology for future 6G network. To deal with the serious propagation loss of THz signals, massive ...multiple-input multiple-output (MIMO) with hybrid precoding is utilized to generate directional beams with high array gains. However, the standard hybrid precoding architecture based on frequency-independent phase-shifters cannot cope with the beam split effect in THz massive MIMO caused by the large bandwidth and the large number of antennas, where the beams split into different physical directions at different frequencies. The beam split effect will result in a serious array gain loss across the entire bandwidth, which has not been well investigated in THz massive MIMO. In this paper, we first quantify the seriousness of the beam split effect in THz massive MIMO by analyzing the array gain loss it causes. Then, we propose a new precoding architecture called delay-phase precoding (DPP) to mitigate this effect. Specifically, the proposed DPP introduces a time delay network composed of a small number of time delay elements between radio-frequency chains and phase-shifters in the standard hybrid precoding architecture. Unlike frequency-independent phase shifts, the time delay network introduced in the DPP can realize frequency-dependent phase shifts, which can be designed to generate frequency-dependent beams towards the target physical direction across the entire bandwidth. Due to the joint control of delay and phase, the proposed DPP can alleviate the array gain loss caused by the beam split effect. Furthermore, we propose a hardware structure by using true-time-delayers to realize frequency-dependent phase shifts for realizing the concept of DPP. A corresponding precoding algorithm is proposed to realize the precoding design. Theoretical analysis and simulations show that the proposed DPP can mitigate the beam split effect and achieve near-optimal rate with higher energy efficiency.
Non-Orthogonal Multiple Access (NOMA) has been considered to be one of the promising key technologies for future wireless communications and broadcasting return channels due to its high spectral ...efficiency under massive connectivity. The major challenge of NOMA is to realize interference cancellation and detect the optimal transmitted signals among multiple users. The message passing algorithm (MPA) based multi-user detection (MUD) has been developed to approximate optimal signals from multiple users. However, the conventional MPA always suppose that the user-activity is exactly known at the receiver, which is impractical in real systems. Thus, precise user-activity detection is significant in realizing MPA based NOMA system. In this paper, we propose a compressive sensing based time-frequency joint NOMA scheme in the uplink grant-free low density signature orthogonal frequency division multiplexing (LDS-OFDM) systems, where the priori information obtained from the time-domain m-sequence and the frequency-domain training sequence are utilized for user-activity detection under the framework of CS, while the MPA is performed for the successive user-data detection. The proposed method has a superior performance and less complexity compared to the conventional MPA detector in numerical stimulation.
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