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Interference has been a key roadblock against the effectively deployment of applications for end-users in wireless networks including fifth-generation (5G) and beyond fifth-generation (B5G) networks. Protocols and standards for various communication types have been established and utilised by the community in the last few years. However, interference remains a key challenge, preventing end-users from receiving the quality of service (QoS) expected for many 5G applications. The increased need for better data rates and more exposure to multimedia information lead to a non-orthogonal multiple access (NOMA) scheme that aims to enhance spectral efficiency and link additional applications employing successive interference cancellation and superposition coding mechanisms. Recent work suggests that the NOMA scheme performs better when combined with suitable wireless technologies specifically by incorporating antenna diversity including massive multiple-input multiple-output architecture, data rate fairness, energy efficiency, cooperative relaying, beamforming and equalization, network coding, and space–time coding. In this paper, we discuss several cooperative NOMA systems operating under the decode-and-forward and amplify-and-forward protocols. The paper provides an overview of power-domain NOMA-based cooperative communication, and also provides an outlook of future research directions of this area.

Precoding is a technique employed to enhance transmission rates in various communication systems, including massive multiple-input multiple-output (MIMO) and optical wireless communication (OWC). In this study, we focus on network massive MIMO OWC (NM-MIMO-OWC) systems and investigate the precoder design to enhance the sum rate and improve the system performance. We present the network’s massive MIMO OWC framework. By utilizing this framework, we are able to calculate the achievable sum rate. Subsequently, we consider the precoding design for maximizing the sum rate while adhering to the total power constraint. To solve this optimization problem, we provide a necessary condition of the optimal solution based on the Karush–Kuhn–Tucker (KKT) conditions, and propose a low-complexity algorithm to further enhance the efficiency of the proposed precoding technique. The numerical results demonstrate that the proposed precoder design significantly improves the transmission rate and effectively maximizes the sum rate.

Hybrid combiner and precoder architectures, radio frequency (RF) chain, analog phase shifters, digital-to-analog converter (DAC), and analog-to-digital converter (ADC) are components of a millimeter wave cellular system. Prior works in the area of millimeter wave cellular system design employ receiver with infinite bit and large amount of RF chain that scales linearly with the quantity of transmitting and receiving antennas. This mode of design no doubt increases power demand or requirement of a typical millimeter wave system. In this work, hybrid architecture with few RF chains and small number of ADC bits are proposed and are used as candidate for millimeter wave channel estimation and cellular communication. In that connection, least square (LS), orthogonal matching pursuit (OMP), compressed sampling matching pursuit (CoSAMP), and deep learning (DL) techniques are utilized for analytical investigation. Indeed, computational results reveal that, when ADC consisting of uniform mid- rise quantizer is employed, the performance of 4 and 6 bits at signal-to-noise ratio (SNR) values of − 10 dB and 20 dB is at par with infinite bit (unquantized case). As a validation, DL compares favorably well with adaptive compressed sensing (ACS) technique previously used in the literature for channel estimation, while OMP and CoSAMP show better performance than ACS.

To achieve the best performance in terms of accuracy and complexity of massive multiple-input multiple-output (Massive MIMO) in wireless communication systems, hybrid beamforming (HBF) is a promising technique that provides high data rate multiplexing gains and enhances the spectral efficiency (SE) of the system. In this paper, a novel received signal strength indicator (RSSI) method is proposed to design an HBF for Massive MIMO BF via multitasking deep learning (DL) that minimizes the reliance on the channel state information (CSI) feedback. The trade-off between the enhancement SE of the system and the deep neural networks (DNNs) performance is optimized, and the results reveal that the proposed novel DL techniques achieve predicted spectral efficiencies with accuracy of 99.23% and 95.64% for Deep-HBF and Deep-AFP, respectively. The processing times for Deep-HBF and Deep-AFP are 709.2914 sec and 1425.864 sec, respectively. Notably, Deep-AFP exhibits a higher range of computational complexity compared to Deep-HBF. It is worth mentioning that the proposed techniques utilize the same DNN architecture.

Beam Division Multiple Access (BDMA) with hybrid precoding has recently been proposed for multi-user multiple-input multiple-output (MU-MIMO) systems by simultaneously transmitting multiple digitally precoded users’ data-streams via different beams. In contrast to most existing works that assume the number of radio frequency (RF) chains must be greater than or equal to that of data-streams, this work proposes a novel BDMA downlink system by first grouping transmitting data-streams before digitally precoding data group by group. To fully harvest the benefits of this new architecture, a greedy user grouping algorithm is devised to minimize the inter-group interference while two digital precoding approaches are developed to suppress the intra-group interference by maximizing the signal-to-interference-and-noise ratio (SINR) and the signal-to-leakage-and-noise ratio (SLNR), respectively. As a result, the proposed BDMA system requires less RF chains than the total number of transmit data-streams. Furthermore, we optimize the power allocation to satisfy each user’s quality of service (QoS) requirement using the D.C. (difference of convex functions) programming technique. Simulation results confirm the effectiveness of the proposed scheme.

In mm-wave MIMO systems, hybrid precoder and combiner designs enhance antenna gain for improved transmission efficiency. However, beam-squint conditions during transmission impact throughput, affecting codebook and increasing beam focus and angle of arrival difference, degrading channel performance. Hence, a novel the Pylon ∂ PSO Method has been proposed to minimize codebook size and array gain, reducing the difference between beam focus and angle of arrival. A Grassmannian codebook is created without compromising throughput. For channel state estimation, existing techniques using the Kronecker product which face convergence errors due to improper hyperparameter matrix selection. Hence, an innovative Lagrange Dual technique and Separable K-Singular Value DE polymerization (K-SVDEp) have been used in dictionary learning that results in the Pt3 product to find the best dictionaries in which block sparse values are estimated using a Deep Ridge Regression Neural Network-based estimator that gives an optimum hyperparameter matrix and eliminates convergence error. Furthermore, designing a combiner from the hyperparameter matrix faces mathematical challenges. Hence, a novel Glasgow technique is utilized which converges the design parameter value with a local optimum obtained using the GEO algorithm. The proposed design has been implemented on the MATLAB platform and outperforms existing techniques with a high spectral efficiency of 45 bits/Hz, SNR of 13.4 dB, and low SER of 1 0 - 4 .

Nowadays the communication of massive multi-user multiple-input multiple-output (MU-MIMO) takes an important role in wireless systems, as they facilitate the ultra-reliable transmission of data and high performance. In order to sustain massive user equipment (UE) with tremendous reliability and spectral efficiency, more antennas are deployed per base station (BS) in the MU-MIMO system. To overcome such problems, the recurrent neural network (RNN) with crossover-gradient based optimizer (GBO) model called RNN-crossover GBO is proposed for precoding in the MU-MIMO system. However, it is essential to diminish the complexity to attain the maximum sum rate for obtaining the optimal solution. Moreover, the kernel linear discriminant analysis (KLDA) dimensionality reduction technique is employed for mapping high dimensional data into a low dimension by considering a linear combination matrix. In order to obtain the best feature the GBO is employed that predict the optimal solution and restrict falling from the local solution. Furthermore, the crossover-GBO algorithm is applied with the RNN to estimate the output precoding matrix with considerable features to obtain the best search space. The experimental results revealed that the proposed method achieves higher performance with a higher sum rate and shows significant improvement in spectral efficiency (SE) values than the existing methods. SE rises due to the selection of closely associated features. This indicates the robustness and stability of the proposed model.

Modern day millimeter wave communication systems prefer hybrid precoding architecture over digital architecture due to higher energy efficiency, lower power consumption and comparable spectral efficiency. Both energy efficiency and spectral efficiency defines the system performance of a hybrid precoder and are dependent on the number of available active RF chains. The aim to maximize energy efficiency without any obvious performance degradation in terms of spectral efficiency has created a tradeoff due to dependency of energy and spectral efficiency on RF chains. This tradeoff is being investigated in this paper by performing RF chain selection using evolutionary algorithms. We present a hybrid heuristic approach comprising of low computationally complex evolutionary algorithms for RF chain selection and successive interference cancellation for precoding. Furthermore, we have shown that for low SNR regime the analog percoding is optimal in terms of energy efficiency and for high SNR regime we can adopt the RF chain selection procedure to maximize the energy efficiency. Moreover, the channel irregularities do not effect our proposed scheme.

At hybrid analog-digital (HAD) transceiver, an improved HAD estimation of signal parameters via rotational invariance techniques (ESPRIT), called I-HAD-ESPRIT, is proposed to measure the direction of arrival (DOA) of a desired user, where the phase ambiguity due to HAD structure is dealt with successfully. Subsequently, a machine-learning (ML) framework is proposed to improve the precision of measuring DOA. Meanwhile, we find that the probability density function (PDF) of DOA measurement error (DOAME) can be approximated as a Gaussian distribution by the histogram method in ML. Then, a slightly large training data set (TDS) and a relatively small real-time set (RTS) of DOA are formed to predict the mean and variance of DOA/DOAME in the training stage and real-time stage, respectively. To improve the precisions of DOA/DOAME, three weight combiners are proposed to combine the-maximum-likelihood-learning outputs of TDS and RTS. Using the mean and variance of DOA/DOAME, their PDFs can be given directly, and we propose a robust beamformer for directional modulation (DM) transmitter with HAD by fully exploiting the PDF of DOA/DOAME, especially a robust analog beamformer on RF chain. Simulation results show that: (1) the proposed I-HAD-ESPRIT can achieve the HAD Cramer-Rao lower bound (CRLB); (2) the proposed ML framework performs much better than the corresponding real-time one without training stage; (3) the proposed robust DM transmitter can perform better than the corresponding non-robust ones in terms of secrecy rate.

A low complexity precoding method is proposed for practical multiple-input multiple-output (MIMO) orthogonal frequency-division multiplexing (OFDM) systems. Based on the two-step optimal precoder design algorithm that maximises the lower bound of the mutual information with finite-alphabet inputs, the proposed method simplifies the precoder design by fixing the right singular vectors of the precoder matrix, eliminating the iterative optimisation between the two steps, and discretising the search space of the power allocation vector. For a 4 × 4 channel, the computational complexity of the proposed precoder design is reduced to 3 and 6% of that required by the original two-step algorithm with quadrature phase shift keying (QPSK) and 8 phase-shift keying (8PSK), respectively. The proposed method achieves nearly the same mutual information as the two-step iterative algorithm for a large range of signal-to-noise ratio (SNR) region, especially for large MIMO size and/or high constellation systems. The proposed precoding design method is applied to a 2 × 2 MIMO-OFDM system with 2048 subcarriers by designing 1024 precoders for extended channel matrices of size 4 × 4. A transceiver test bed implements these precoding matrices in comparison with other existing precoding schemes. Indoor experiments are conducted for fixed-platform non-line-of-sight channels, and the data processing results show that the proposed precoding method achieves the lowest bit error rate compared with maximum diversity, classic water-filling and channel diagonalisation methods.