Random Beamforming Research Articles

A novel EO-based optimum random beamforming method in mmWave-NOMA systems with sparse antenna array

Millimeter-wave (mmWave) Non-Orthogonal Multiple Access (NOMA) with random beamforming is a promising technology to guarantee massive connectivity and low latency transmissions of future generations of mobile networks. In this paper, we introduce a cost-effective and energy-efficient mmWave-NOMA system that exploits sparse antenna arrays in the transmitter. Our analysis shows that utilizing low-weight and small-sized sparse antennas in the Base Station (BS) leads to better outage probability performance. We also introduce an optimum low complexity Equilibrium Optimization (EO)-based algorithm to further improve the outage probability. The simulation and analysis results show that the systems equipped with sparse antenna arrays making use of optimum beamforming vectors outperform the conventional systems with uniform linear arrays in terms of outage probability and sum rates.

Performance Analysis of sub-6 GHz/mmWave NOMA Hybrid-HetNets Using Partial CSI

The ever-increasing number of wireless users (or devices) and their varied demand require the need for an advanced architecture for the future wireless network. To support massive connectivity, non-orthogonal multiple access (NOMA) has been recognized as a promising solution. NOMA increases the number of simultaneous connections using available resources for users with varying demands. Furthermore, recent measurements and experiments suggest that wide underutilized bandwidth available at millimeter-wave (mmWave) frequencies provide high data rate and therefore are capable of addressing the issue of spectrum scarcity at sub-6 GHz bands utilized by the 4 G network. Consequently, co-existence of multi-radio access technologies (RATs) for 5 G and beyond networks has been of interest to both industries and academia. In this context, this work studies the co-existence of the two RATs, namely, sub-6 GHz and mmWave communication using NOMA-enabled hybrid heterogeneous network (NOMA-HHN) for massive connectivity. The application of NOMA requires ordering users, which in turn requires the knowledge of users' channel state information (CSI). However, gathering and processing CSI of such a large number of users is difficult to implement in practice. Thus, a solution based on partial CSI is proposed. Additionally, a feedback scheme for user scheduling and RAT selection using dual association is proposed to reduce the initial access delay in beam-training at the mmWave network. Moreover, utilizing directional nature of the mmWave communication, random beamforming is used to reduce system overhead in a network with massive users. The analytical results are confirmed using Monte-Carlo simulation, and various significant advantages are noted for the proposed NOMA-HHN over existing architectures.

Reconfigurable Intelligent Surface-Aided Cell-Free Massive MIMO with Low-Resolution ADCs: Secrecy Performance Analysis and Optimization

The secure communication in reconfigurable intelligent surface-aided cell-free massive MIMO system is investigated with low-resolution ADCs and with the existence of an active eavesdropper. Specifically, an aggregated channel estimation approach is applied to decrease the overhead required to estimate the channels. Using the available imperfect channel state information (CSI), the conjugate beamforming and random beamforming are applied at the APs and the RIS for downlink data transmission, respectively. The closed-form expression of the achievable secrecy rate is acquired to appraise the achievable secrecy performance using only the channel statistics. With the achievable analytical results, the impacts of the quantization bit of ADCs, channel estimation error, the number of RIS elements, and the number of the APs can be unveiled. Aiming to maximize the minimum achievable rate of all legitimate users subject to security constraints, the power control optimization scheme is first formulated. To tackle this nonconvex property of the proposed optimization problem, a path-following algorithm is then utilized to solve the initial problem with continuous approximations and iterative optimization. Numerical results are presented to verify the achieved results along with availability of the presented power allocation approach.

Joint Optimal Power Allocation and Beamforming for MIMO-NOMA in mmWave Communications

In this letter, we propose an optimal joint power allocation and beamforming for MIMO-NOMA in mmWave communications. In this method, to reduce the system overhead, we consider a random near random far (RNRF) pairing scheme in our NOMA data transmission. We formulate an optimization problem to find the optimal power allocation and random beamforming (RBF) coefficients to improve the system performance. We prove the convexity of the underlying optimization problem and propose an iterative algorithm to jointly update the power allocation and RBF coefficients to maximize the communication reliability. Our proposed technique significantly reduces the outage probability (OP) of the system.

Energy and Delay Guaranteed Joint Beam and User Scheduling Policy in 5G CoMP Networks

Massive Multi-Input Multi-Output (MIMO) and Coordinated MultiPoint (CoMP) technologies in Cloud-RAN (C-RAN) architecture become inevitable trend due to the advent of next-generation mobile applications, which are traffic-intensive, such as ultra high definition (UHD) video. In this paper, we study a joint beam activation and user scheduling problem in a 5G cellular network with massive MIMO and CoMP utilizing orthogonal random beamforming technique. This paper aims to minimize total Remote Radio Heads’ (RRHs’) energy expenditure in a dynamic C-RAN architecture while ensuring finite service time for all user traffic arrivals in the communication coverage. We leverage Lyapunov drift-plus-penalty framework to transform an original long-term average problem into a series of per-slot modified problems. Since the provided per-slot problem is combinatorial and nonlinear optimization problem, we are inspired by a greedy algorithm to design energy and delay guaranteed joint beam activation and user scheduling policy, namely BEANS. We prove that the proposed BEANS ensures finite upper bounds of average RRH energy consumption and average queue backlogs for all traffic arrival rates within constant ratio of capacity region and all energy-delay tradeoff parameters. These proofs are the first attempt to theoretically demonstrate guarantees of energy and queue bounds in a framework consisting of possibly negative submodular objective function and non-matriod constraints. Finally, via extensive simulations, we compare the capacity region and energy-queue backlog tradeoff of BEANS with optimal and existing algorithms, and show that BEANS attains up to 65% of energy saving for the same average queue backlog compared to the algorithms which do not take traffic dynamics and energy consumption into considerations.

Achieving Covert Wireless Communication With a Multi-Antenna Relay

We investigate covert wireless communication in a multi-antenna relay network, where the relay transmits its own covert message to the destination when assisting the source’s information delivery, and the source acts as a warden to detect this covert transmission. Based on whether the channel state information of the relay-destination link is available at the source or not, we propose two relay beamforming schemes, namely random beamforming and maximum-ratio transmission (MRT) beamforming schemes, to guarantee the reception reliability at the destination while deliberately introducing uncertainty to the source to degrade its detection. Under the worst-case covert communication scenario where the source is capable of optimizing its detection threshold, analytical expressions for the minimum detection error probability achieved by each of the proposed schemes are derived to evaluate the detection limits of the source. By utilizing the above analytical results as the covertness constraint, an optimization problem of transmit power allocation for each scheme is formulated and solved to maximize the covert rate. The impact of imperfect channel state information on the covert communication performance is also examined. Simulation results are performed to confirm the accuracy of the derived analytical results and quantify the communication covertness enhancement of the proposed schemes. Our results also show that the MRT beamforming scheme offers a higher covert rate than that of the random beamforming scheme, especially when the covertness constraint becomes loose and/or the number of antennas at the relay increases.

Outage analysis of mmWave-NOMA transmission in the presence of LOS and NLOS paths

Non-orthogonal multiple access (NOMA) as an enabling technology can be integrated with millimeter-Wave (mmWave) communication to achieve massive connectivity and better spectral efficiency in future wireless cellular networks. In this work, to reduce the system overhead, random beamforming is employed in the mmWave-NOMA networks where we optimized the power allocation factors to improve the random beamforming performance. Also, the impact of the non-line-of-sight (NLOS) path alongside the line-of-sight (LOS) path is analyzed under the outage probability term. Based on the distance between the base station (BS) and machine type communication (MTC) device, novel clustering schemes involving the near, mid-cell and far device are proposed. The proposed clustering schemes are named as (1) RRR consisting of one random device from each section, (2) NNN involving the nearest device from each region, and (3) NFF containing the nearest device to the BS from the near section, and the farthest device to the BS from both middle and far parts. The clustering schemes can be considered as a trade-off between the system overhead and outage performance. Finally, computer simulation results are presented to show the accuracy of the developed analytical and approximation results.

Joint Beam Training and Positioning for Intelligent Reflecting Surfaces Assisted Millimeter Wave Communications

Intelligent reflecting surface (IRS) offers a cost-effective solution to link blockage problem in mmWave communications, and the prerequisite of which is the accurate estimation of (1) the optimal beams for base station/access point (BS/AP) and mobile terminal (MT), (2) the optimal reflection patterns for IRSs, and (3) link blockage. In this paper, we carry out beam training designs for IRSs assisted mmWave communications to estimate the aforementioned parameters. To acquire the optimal beams and reflection patterns, we firstly perform random beamforming and maximum likelihood estimation to estimate angle of arrival (AoA) and angle of departure (AoD) of the line of sight (LoS) path between BS/AP (or IRSs) and MT. Then, with the estimated AoDs, we propose an iterative positioning algorithm that achieves centimeter-level positioning accuracy. The obtained location information is not only a fringe benefit but also enables us to cross verify and enhance the estimation of AoA and AoD, and it also facilitates the estimation of blockage indicator. Numerical results show the superiority of our proposed beam training scheme and verify the performance gain brought by location information.

Random Beam-Based Non-Orthogonal Multiple Access for Low Latency K-User MISO Broadcast Channels.

In this paper, we propose random beam-based non-orthogonal multiple access (NOMA) for low latency multiple-input single-output (MISO) broadcast channels, where there is a target signal-to-interference-plus-noise power ratio (SINR) for each user. In our system model, there is a multi-antenna transmitter with its own single antenna users, and the transmitter selects and serves some of them. For low latency, the transmitter exploits random beams, which can reduce the feedback overhead for the channel acquisition, and each beam can support more than a single user with NOMA. In our proposed random beam-based NOMA, each user feeds a selected beam index, the corresponding SINR, and the channel gain, so it feeds one more scalar value compared to the conventional random beamforming. By allocating the same powers across the beams, the transmitter independently selects NOMA users for each beam, so it can also reduce the computational complexity. We optimize our proposed scheme finding the optimal user grouping and the optimal power allocation. The numerical results show that our proposed scheme outperforms the conventional random beamforming by supporting more users for each beam.

Ohmic Dissipation‐Assisted Complex Amplitude Hologram with High Quality

AbstractTailoring the phase and amplitude of electromagnetic waves has drawn great attention in the microwave, terahertz, and even optics domains. However, the existing method for simultaneous control of these two essential properties suffers from inadequate efficiency and narrow bandwidth, especially for microwave devices. Here, a strategy of overcoming this difficulty is proposed by introducing the ohmic sheet into a Fabry–Pérot‐like cavity. The arbitrary phase modulation can be realized by changing the geometric parameters and the orientation of elliptical split resonance ring; via changing the intensity of ohmic dissipation, the amplitude can be continuously adjusted from 0 to 1. Its most significant advantage is arbitrary wide‐band complex‐amplitude modulation without introducing cross‐polarization crosstalk and backward scattering field interference. To verify its feasibility, three phase‐amplitude holographic imaging ​prototypes at 12, 14, and 16 GHz are designed. Both the simulated and measured results manifest the excellent performances of the proposed metasurface, demonstrating the excellent performances with a remarkable signal‐to‐noise ratio and low root‐mean‐square error. This proposed strategy provides an alternative method to control the phase and amplitude arbitrarily, which not only realizes high‐quality holography but also paves a way for random beamforming and so on.

Intelligent Reflecting Surface Enabled Random Rotations Scheme for the MISO Broadcast Channel

The current literature on intelligent reflecting surface (IRS) focuses on optimizing the IRS phase shifts to yield coherent beamforming gains, under the assumption of perfect channel state information (CSI) of individual IRS-assisted links, which is highly impractical. This work, instead, considers the random rotations scheme at the IRS in which the reflecting elements only employ random phase rotations without requiring any CSI. The only CSI then needed is at the base station (BS) of the overall channel to implement the beamforming transmission scheme. Under this framework, we derive the sum-rate scaling laws in the large number of users regime for the IRS-assisted multiple-input single-output (MISO) broadcast channel, with optimal dirty paper coding (DPC) scheme and the lower-complexity random beamforming (RBF) and deterministic beamforming (DBF) schemes at the BS. The random rotations scheme increases the sum-rate by exploiting multi-user diversity, but also compromises the gain to some extent due to correlation. Finally, energy efficiency maximization problems in terms of the number of BS antennas, IRS elements and transmit power are solved using the derived scaling laws. Simulation results show the proposed scheme to improve the sum-rate, with performance becoming close to that under coherent beamforming for a large number of users.

On the Performance Analysis of mmWave MIMO-NOMA Transmission Scheme

In this paper, we propose an optimal random beamforming technique in mmWave MIMO-NOMA transmission systems. In our model, we consider a NOMA system with a user pairing scheme that works based on the distance between the cellular base station (BS) and the active mobile users. The aim of this is to reduce the system overall overhead for massive connectivity and achieve the desired Quality of Service (QoS). We present an optimization problem with the aims to find the optimal random beamforming coefficients by minimizing the outage probability. We analyze the convexity of the outage probability for three user pairing schemes and prove that the proposed optimal random beamforming is convex in high signal-to-noise ratio (SNR). We utilize the gradient-based algorithm namely the gradient descent algorithm to find optimal random beamforming coefficients. Our simulation results demonstrate that the proposed transmission schemes reduce the outage probability and improve network performance compared to previous works.

Joint users selection and beamforming in downlink millimetre‐wave NOMA based on users positioning

In this study, joint users selection and beamforming based on users positioning are proposed for enabling non-orthogonal multiple access (NOMA) in the millimetre-wave (mmWave) transmissions. In the proposed scheme, the mmWave users are arranged in descending order based on their estimated distances from the mmWave base station (BS), which guarantees the users ordering based on their line-of-sight availability, as well. Out of the ordered users, the two users having the minimum pairing angle and expected to lie within the specified mmWave beamwidth are selected as the two NOMA users. In this study, the users' pairing angle is defined as the maximum expected difference angle between two users considering their location-uncertainty areas. Consequently, the BS antenna beam is formed towards the two selected NOMA users by adjusting its boresight angle to lie in between their estimated location angles. Then, the fixed power allocation scheme is used to adjust the power allocation coefficients of the selected NOMA users. Mathematical analysis is performed to bound the performance of the proposed mmWave NOMA scheme. Also, numerical simulations are conducted to prove the mathematical findings and to show the superior performance of the proposed scheme over the well-known technique of using random beamforming.

Robust Beamforming Design for OTFS-NOMA

This paper considers the design of beamforming for orthogonal time frequency space modulation assisted non-orthogonal multiple access (OTFS-NOMA) networks, in which a high-mobility user is sharing the spectrum with multiple low-mobility NOMA users. In particular, the beamforming design is formulated as an optimization problem whose objective is to maximize the low-mobility NOMA users' data rates while guaranteeing that the high-mobility user's targeted data rate can be met. Both the cases with and without channel state information errors are considered, where low-complexity solutions are developed by applying successive convex approximation and semidefinite relaxation. Simulation results are also provided to show that the use of the proposed beamforming schemes can yield a significant performance gain over random beamforming.

Random Beam-Based Random Access for Low-Latency Device-to-Device Communication Systems

In this paper, we consider a device-to-device communication system, where a multi-antenna mobile user directly communicates with nearby multiple devices, and propose a low-latency random access protocol utilizing random beams at the user. In our system model, each device tries to access the user with randomly selected one among multiple orthogonal (near-orthogonal) preambles, so the devices with different preambles can be discerned at the user. Meanwhile, the mobile user has multiple antennas and tries to fully utilize multiplexing gains. In our proposed protocol, the user adopts the orthogonal random beams both for reception and transmission regardless of channel conditions. Thus, in some cases, the multiple devices with the same preamble can be decoded at the user. Moreover, the user of random beamforming can reduce the computational complexity and time delay for beamforming. We analyze the access probability with our proposed random access protocol and find each device's one-shot access probability with the approximated one. Our simulation results show that our proposed protocol increases the random access probability compared to the conventional protocol that does not utilize the multiplexing gains, and our analysis well matches in various scenarios.

Secrecy Outage Analysis Over Random Beamforming Transmission With User Scheduling: Collusion vs. Exclusion

In this letter, we study the interacting effects of the exclusion zone and eavesdropper colluding strategy on the secrecy performance of multiple-antenna transmission system. In particular, a base station (BS) applies random beamforming and user scheduling to enhance the legitimate user channel while multiple eavesdroppers try, either individually or jointly, to overhear the transmitted signal. We derive the closed-form expressions of secrecy outage probability (SOP) of the system with and without exclusion zone under different colluding strategies. Through selected numerical examples, we develop the design guideline on the size of the exclusion zone for a target security level subject to eavesdroppers’ density and colluding strategy.

Ergodic Secrecy Rate of $K$-User MISO Broadcast Channel With Improved Random Beamforming

In this paper, we study the secrecy performance of a $K$ -user multi-input single-output (MISO) broadcast channel with random beamforming (RB), where the transmitter is equipped with $N$ antennas, and the eavesdropper has $N_e$ antennas. We first propose a signal-splitting random beamforming (SSRB) scheme for a single-user MISO wiretap channel, and then improve the SSRB scheme with power-minimizing (PM), which is called a PM-SSRB scheme. To improve the spectral efficiency of a multiuser network, we combine PM with non-orthogonal multiple access (NOMA), and propose a hybrid NOMA (H-NOMA) scheme and a signal-splitting NOMA (SS-NOMA) scheme for the $K$ -user MISO wiretap channel, respectively. The sum ergodic secrecy rate of the $K$ -user scenario is analyzed comprehensively, and a closed-form expression for the lower bound on the ergodic secrecy rate is also derived for the single-user case. Simulation results show that, compared with the traditional hybrid artificial fast-fading scheme (AFF) and artificial noise (AN) scheme, the proposed SSRB scheme and PM-SSRB scheme perform much better in terms of the ergodic secrecy rate in all power regimes. More importantly, when the eavesdropper has more antennas than the transmitter, our schemes always outperform the AFF and AN schemes. The PM-SSRB scheme is also shown to be superior to the secret-key AFF scheme. For the multiuser case, the SS-NOMA scheme can achieve higher ergodic secrecy rate than the H-NOMA scheme.

A Pseudo-Random Beamforming Technique for Improving Physical-Layer Security of MIMO Cellular Networks

In this paper, we propose a pseudo-random beamforming (PRBF) technique for improving physical-layer security (PLS) in multiple input multiple output (MIMO) downlink cellular networks consisting of a legitimate base station (BS), multiple legitimate mobile stations (MSs) and potential eavesdroppers. The legitimate BS can obtain available potential eavesdroppers’ channel state information (CSI), which is registered in an adjacent cell. In the proposed PRBF technique, the legitimate BS pseudo-randomly generates multiple candidates of the transmit beamforming (BF) matrix, in which each transmit BF matrix consists of multiple orthonormal BF vectors and shares BF information with legitimate MSs before data transmission. Each legitimate MS generates receive BF vectors to maximize the receive signal-to-interference-plus-noise (SINR) for all pseudo-randomly generated transmit beams and calculates the corresponding SINR. Then, each legitimate MS sends a single beam index and the corresponding SINR value of the BF vector that maximizes the received SINR for each BF matrix since a single spatial stream is sent to each legitimate MS. Based on the feedback information from legitimate MSs and the CSI from the legitimate BS to eavesdroppers, the legitimate BS selects the optimal transmit BF matrix and the legitimate MSs that maximizes secrecy sum-rate. We also propose a codebook-based opportunistic feedback (CO-FB) strategy to reduce feedback overhead at legitimate MSs. Based on extensive computer simulations, the proposed PRBF with the proposed CO-FB significantly outperforms the conventional random beamforming (RBF) with the conventional opportunistic feedback (O-FB) strategies in terms of secrecy sum-rate and required feedback bits.

Multi-Stream Opportunistic Network Decoupling: Relay Selection and Interference Management

We study multi-stream transmission in the $K \times N \times K$ channel with interfering relay nodes, consisting of $K$ multi-antenna source--destination (S--D) pairs and $N$ single-antenna half-duplex relay nodes between the S--D pairs. We propose a new achievable scheme operating with partial effective channel gain, termed multi-stream opportunistic network decoupling (MS-OND), which achieves the optimal degrees of freedom (DoF) under a certain relay scaling law. Our protocol is built upon the conventional OND that leads to virtual full-duplex mode with one data stream transmission per S--D pair, generalizing the idea of OND to multi-stream scenarios by leveraging relay selection and interference management. Specifically, two subsets of relay nodes are opportunistically selected using alternate relaying in terms of producing or receiving the minimum total interference level. For interference management, each source node sends $S \,(1 \le S \le M)$ data streams to selected relay nodes with random beamforming for the first hop, while each destination node receives its desired $S$ streams from the selected relay nodes via opportunistic interference alignment for the second hop, where $M$ is the number of antennas at each source or destination node. Our analytical results are validated by numerical evaluation.

Clustered Millimeter-Wave Networks With Non-Orthogonal Multiple Access

We introduce clustered millimeter wave networks with invoking non-orthogonal multiple access~(NOMA) techniques, where the NOMA users are modeled as Poisson cluster processes and each cluster contains a base station (BS) located at the center. To provide realistic directional beamforming, an actual antenna array pattern is deployed at all BSs. We propose three distance-dependent user selection strategies to appraise the path loss impact on the performance of our considered networks. With the aid of such strategies, we derive tractable analytical expressions for the coverage probability and system throughput. Specifically, closed-form expressions are deduced under a sparse network assumption to improve the calculation efficiency. It theoretically demonstrates that the large antenna scale benefits the near user, while such influence for the far user is fluctuant due to the randomness of the beamforming. Moreover, the numerical results illustrate that: 1) the proposed system outperforms traditional orthogonal multiple access techniques and the commonly considered NOMA-mmWave scenarios with the random beamforming; 2) the coverage probability has a negative correlation with the variance of intra-cluster receivers; 3) 73 GHz is the best carrier frequency for near user and 28 GHz is the best choice for far user; 4) an optimal number of the antenna elements exists for maximizing the system throughput.