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Combining powerful error-correction codes (ECCs) with multipulse pulse-position modulation (MPPM) has great potential to realize high-throughput and high-reliability transmissions in free-space optical (FSO) communication systems. This paper conducts an insightful investigation on protograph low-density parity-check (PLDPC)-coded MPPM systems over Poisson channels. To be specific, we first put forward a novel type of constellations, referred to as irregular pulse-distribution (IPD) constellations, for MPPM. We also conceive a bit-level rate-adaptive (RA) scheme, which can exploit the structural property of the IPD constellations to significantly improve the convergence performance at the price of transmission-rate loss. Furthermore, with the aid of a modified protograph extrinsic information transfer (PEXIT) algorithm, we construct three enhanced PLDPC codes with code rates $1/2, 2/3$ and $3/4$ that exhibit excellent decoding thresholds over Poisson channels. The proposed PLDPC-coded MPPM systems with the use of the IPD constellations, RA scheme, and enhanced PLDPC codes are superior to the state-of-the-art counterparts. [ABSTRACT FROM AUTHOR]

This correspondence considers the performance evaluation of precoded orthogonal frequency division multiplexing (P-OFDM) based on Walsh-Hadamard Transform (WHT) in the presence of symbol timing offset (STO). The signal-to-interference plus noise ratio (SINR) is evaluated for flat and frequency-selective fading channels (FSC), where closed-form analytical expressions are derived for the flat fading case. In FSC, the SINR is obtained using Monte Carlo simulation due to the analysis intractability. The bit error rate (BER) performance of OFDM and P-OFDM based on WHT, Haar, and Zadoff-Chu matrix transforms are considered to demonstrate the STO impact on the BER using various precoding transforms. The obtained results show that the impact of STO on P-OFDM is substantially different from conventional OFDM. In particular, it is found that STO affects the SINR of each subcarrier differently, where the SINR of certain subcarriers is severely degraded while others are immune. Furthermore, the diversity gain advantage of the precoding process may vanish for large STOs. [ABSTRACT FROM AUTHOR]

Chaos-based communications can be applied to high-speed vehicular information transmissions thanks to the anti-jamming and anti-interference capabilities of chaotic transmissions. In traditional practical chaotic systems, reference chaotic signals are required to be delivered to remove complex chaotic synchronization circuits. However, the direct transmission of reference signals will degrade the security performances, while interferences and noises imposed on the reference signals due to imperfect channel conditions will deteriorate the reliability performances. In order to enhance the reliability and security performances over vehicular channels such as the railway channel and the channels undergoing fast fadings, in this paper, we propose a deep learning (DL) aided intelligent OFDM-DCSK transceiver. In this design, no reference chaotic signals are delivered, and we propose to utilize the time-delay neural network (TDNN) to learn the chaotic maps, followed by the long short-term memory (LSTM) units to extract and exploit the correlations between chaotic modulated signals, and multiple fully connected layers (FCLs) to estimate the user bit data. With the aid of the constructed deep neural network (DNN), after the offline neural network training, the receiver can recover the transmitted information with lower bit error rate (BER) and enhance security performances. Theoretical performance is then analyzed for the proposed intelligent transceiver. Simulation results validate the proposed design, and demonstrate that the intelligent DL-based OFDM-DCSK system can achieve better BER and security performances over fast fading and railway channels compared with the benchmark systems. [ABSTRACT FROM AUTHOR]

A quadrature spatial modulation with the fourth order transmit diversity (FO-QSM) scheme is presented in this paper. In this scheme, two dispersion-matrix (DM) sets are assigned at the transmitter, and $P$ out of $Q$ DMs are activated in each set to extend the real and imaginary parts of $P$ QSM signals respectively, to large number of transmit antennas. The two DM sets are constructed by performing Kronecker product between two simple extension matrices and the DMs of Sezginer-Sari-Biglieri (SSB) code. The framework designs of FO-QSM and the two DM sets make FO-QSM possess the properties: 1) it guarantees the fourth order transmit diversity without requiring any parameter optimization; 2) it has higher spectral efficiency than the newly proposed generalized space-time block coded spatial modulation (GSTBC-SM) scheme. Furthermore, a repeated computation reduced sphere decoding (RCR-SD) is proposed. RCR-SD is very suitable for the schemes constructed by a large number of DMs, including FO-QSM. By sorting the index vectors, the RCR-SD detector can efficiently reduce the repeated computations in the SD searching, which significantly reduces the decoding complexity without sacrificing optimal decoding performance. Simulation results show that FO-QSM has obvious better bit error rate (BER) performance than GSTBC-SM. [ABSTRACT FROM AUTHOR]

Although LoRa modulation can achieve low-power and long-range transmission, its performance is significantly affected by fading channels. On the other hand, its low data rate limits the capacity. To mitigate these disadvantages, a new reconfigurable intelligent surface (RIS) assisted LoRa uplink system is proposed in this paper, where the reflection coefficients of the RIS are generated by utilizing an $M$ -ary differential phase shift keying modulator. Moreover, a low-complexity non-coherent receiver is designed for the proposed system. The new system is able to inherit the non-coherent detection of the conventional LoRa system and also transmit additional information of $\log _{2}{M}$ bits while reducing the impact of the fading channels. Furthermore, the bit-error-rate, symbol-error-rate and throughput expressions of the system are derived in closed form. Theoretical and simulated results show that the system has better performance than the conventional LoRa system and single-relay LoRa system. [ABSTRACT FROM AUTHOR]

This paper presents techniques that are expected to improve the bit error rate (BER) performance of Unmanned Aerial Vehicles (UAVs) communication systems. The decode-and-forward (DF)-based cooperative multi-hop Rician network using space shift keying (SSK) modulation is employed for this purpose. The SSK modulation and cooperative communication are merged and used together. This enhances the BER performance for multiple-input multiple-output (MIMO) channels, thus accentuating its significance as an important technique for consistently reliable communication methodology while, at the same time, eliminating wastage of energy. The new technique is named as high performance (HP) due to not only BER performance improvement but also increase of the diversity of the communication system. This new model is further used for multi-hop networks and the resulting BER characteristics are assessed for possible enhanced performance in Rician channels. The paper also showcases the generalized-n-hop BER equation for various multi-hop DF-based cooperative communication systems in diverse UAV models. The Monte Carlo simulations further confirm the theoretical results that already exist for transmit-receive antennas. This work also underlines the fact that the number of hops in a communication system has no bearing whatsoever on the BER performance diversity value. [ABSTRACT FROM AUTHOR]

We investigate user activity detection (UAD) and symbol detection (SD) for grant-free communication in the presence of massive users that are actively connected to base station (BS), where a small portion of to-be-connected users wake up in a burst. The number of effective interfering connected users is reduced to only one, by applying a preconditioning matrix to the received signals from multiple antennas at BS. Subsequently, an iterative UAD and SD scheme is applied, where the priori information about the remaining interfering user is exploited and the symbols of the burst users as well as the signature connected user are simultaneously detected by iterative exchanging of the soft information of user activity and symbols. The proposed system outperforms the existing work in terms of the success rate of UAD and bit error rate. [ABSTRACT FROM AUTHOR]

We design a centralized and a decentralized variational Bayesian learning (C- and D-VBL) algorithms for the base station (BS) of a frequency division duplex massive multiple input multiple output (mMIMO) cellular system, wherein users send compressed information for it to estimate their downlink channels. The BS in the decentralized algorithm consists of multiple processing units (PUs), and each PU separately estimates the channels of a group of users, by employing the proposed D-VBL algorithm. To reduce channel estimation error, the PUs exploit the structured sparsity inherent in multi-user mMIMO channels by exchanging information among themselves. We investigate the proposed C-VBL and low-complexity D-VBL algorithms and show that i) they substantially outperform the state-of-the-art centralized and decentralized algorithms in terms of the normalized mean squared error and the bit error rate. This is because they beneficially exploit the inherent channel sparsity, while the existing state-of-the-art solutions fail to do so. The proposed D-VBL is also robust to PU failures, and provides a similar performance as its centralized counterpart (C-VBL), but with a much reduced complexity. [ABSTRACT FROM AUTHOR]

The joint transmission scheme of polar codes and sparse code multiple access (SCMA) has been regarded as a promising technology for future wireless communication systems. However, most of the existing polar-coded SCMA (PC-SCMA) systems suffer from high latency caused by the feedback iteration and list decoding. In addition, the error performance of PC-SCMA systems is unsatisfactory for ultra-reliable transmission. Inspired by the compelling benefits of non-binary polar codes, in this paper, we design a non-binary polar-coded SCMA (NB-PC-SCMA) system with a free order matching strategy to address the issues of delay and reliability. Specifically, we first formulate a joint factor graph for NB-PC-SCMA and propose a non-binary successive cancellation list (NB-SCL) and damping based joint iterative detection and decoding (NSD-JIDD) multiuser receiver to improve the BER and latency performance. Then, a lazy-search based NB-SCL (L-NB-SCL) decoding is proposed to reduce the computational complexity by simplifying the path search pattern of the list decoder. After that, we modify the update of user nodes for SCMA detection to improve the convergence error and finally propose the improved NSD-JIDD (ISD-JIDD) algorithm, which can avoid redundant operations by exploiting L-NB-SCL decoding. Simulation results show that the proposed NB-PC-SCMA system achieves better bit error rate (BER) performance and considerable latency gain when compared to its counterparts. In particular, the proposed ISD-JIDD can achieve similar BER performance of NSD-JIDD with less complexity. [ABSTRACT FROM AUTHOR]

From adapting to the propagation environment to reconstructing the propagation environment, reconfigurable intelligent surface (RIS) changes the design paradigm of wireless communications. To reconstruct the propagation environment, joint beamforming of RIS and multi-input multi-output (MIMO) is crucial. Unfortunately, due to the coupling effect of the active beamforming of MIMO and passive beamforming of RIS, it is difficult to find the optimal solution to the joint beamforming problem, so a serious performance loss will be caused. In this paper, inspired by the end-to-end (E2E) learning of communication system, we propose the E2E learning based RIS-aided communication system to mitigate the performance loss via deep learning techniques. The key idea is to simultaneously optimize the signal processing functions at base station (BS), RIS, and user, including active beamforming for BS and user, passive beamforming for RIS. This way is able to avoid the performance loss caused by alternately optimizing each function of the RIS-aided system. Specifically, we firstly utilize a deep neural network (DNN) to realize the modulation and beamforming for BS and utilize another DNN to realize the demodulation and combining for user. Then, the RIS passive beamforming is also represented by trainable parameters, which could be simultaneously optimized with the DNNs at BS and user. Simulation results show that the proposed E2E learning based RIS-aided communication system could achieve the better bit error rate (BER) performance than traditional RIS-aided communication systems. [ABSTRACT FROM AUTHOR]

Reconfigurable intelligent surfaces (RIS) have been actively researched as a potential technique for future wireless communications, which intelligently ameliorate the signal propagation environment. In the conventional design, each RIS element configures and reflects its received signal independently of all other RIS elements, which results in a diagonal phase shift matrix. By contrast, we propose a novel RIS architecture, where the incident signal impinging on one element can be reflected from another element after an appropriate phase shift adjustment, which increases the flexibility in the design of RIS phase shifts, hence, potentially improving the system performance. The resultant RIS phase shift matrix also has off-diagonal elements, as opposed to the pure diagonal structure of the conventional design. Compared to the state-of-art fully-connected/group-connected RIS structures, our proposed RIS architecture has lower complexity, while attaining a higher channel gain than the group-connected RIS structure, and approaching that of the fully-connected RIS structure. We formulate and solve the problem of maximizing the achievable rate of our proposed RIS architecture by jointly optimizing the transmit beamforming and the non-diagonal phase shift matrix based on alternating optimization and semi-define relaxation (SDR) methods. Moreover, the closed-form expressions of the channel gain, the outage probability and bit error ratio (BER) are derived. Simulation results demonstrate that our proposed RIS architecture results in an improved performance in terms of the achievable rate compared to the conventional architecture, both in single-user as well as in multi-user scenarios. [ABSTRACT FROM AUTHOR]

In this correspondence, we propose a novel orthogonal frequency division multiplexing based pre-coded chaos shift keying (OFDM-PC-CSK) transceiver for reliable vehicle-to-vehicle (V2V) communications. In this design, different from traditional OFDM-aided differential chaos shift keying (OFDM-DCSK) systems, we remove the delivery of reference chaotic signals and utilize the diagonalization principle for information transmissions to achieve better reliability than the differential demodulation in OFDM-DCSK. At the transmitter, we construct the chaotic phase vector with chaotic sequences having different initial values, which are then combined with the discrete Fourier transform (DFT) matrix and a conjugate phase matrix of an eigenvalue matrix to compose a precoder. After precoding, the user data are modulated by the chaotic circulant matrix and transmitted over multiple subcarriers. At the receiver, reverse operations are conducted. Subsequently, we derive the theoretical bit error rate (BER) over V2V channels, the spectrum efficiency and the computational complexity. Simulation results validate the theoretical analysis, and demonstrate that the OFDM-PC-CSK system outperforms the counterpart schemes with better reliability and security performances over V2V channels. [ABSTRACT FROM AUTHOR]

Effective and timely prediction of bit error rate (BER) disturbances is an important means to improve the safety of balise uplink transmission. However, the joint influence of multiple non-stationary factors makes it difficult to predict the abrupt changes of disturbances. To solve this problem, an adaptive attention-based bidirectional stateful encoder-decoder model is proposed for BER disturbances prediction. By persistently transferring the forward and backward states in a batch-to-batch mapping manner, the bidirectional stateful encoder can enhance the capture ability for disturbances. Additionally, an adaptive stateful decoder is used to dynamically synthesize the temporal correlation and contextual summarization to reduce the error accumulation in long-term forecasting. Through effective disturbance discrimination and accumulative error elimination, the proposed model can improve the overall prediction accuracy for BER disturbances. Experiment results on real-world high-speed railway datasets show that the proposed model can achieve superior performance than the state-of-the-art methods. [ABSTRACT FROM AUTHOR]

Sparse code multiple access (SCMA) and multiple input multiple output (MIMO) are considered as two efficient techniques to provide both massive connectivity and high spectrum efficiency for future machine-type wireless networks. However, the computational complexity of the conventional message passing algorithm (MPA) for the downlink MIMO-SCMA detection increases exponentially with the degree of variable nodes (VNs). To address this issue, two novel fixed low complexity MPA detectors are proposed by utilizing the sparse feature of codewords, which are based on the serial schedule strategy in order to accelerate their convergence. First, a maximum distance MPA (MDMPA) detector is introduced to reduce the number of VNs involved in the message updating procedure in the first $T_0$ iterations and fix $TJ_0$ codewords at the $L_0$ iteration, which subsequently drops the redundant codeword combinations in the remaining iterations. To further improve the bit error ratio (BER) performance, an improved MDMPA (IMDMPA) is proposed, in which, the VNs associated with the target user are always allowed to participate in the iterative propagation of message. An efficient linear squares (LS) channel estimation scheme is also derived based on block pilots by stacking all the received signals at the same subcarrier together. Simulation results illustrate that the proposed MDMPA and IMDMPA detectors significantly reduce the computational complexity of the original MPA, but with comparable decoding capabilities, and the proposed LS channel estimator can closely approach the corresponding lower bound over frequency selective channels. [ABSTRACT FROM AUTHOR]

A forward error correction (FEC) and unity-rate coded (URC) autoencoder (AE)-assisted communication system is proposed for the first time, which relies on soft iterative decoding for attaining a vanishingly low error probability. The AE-demapper is specifically designed for directly calculating the extrinsic logarithmic likelihood ratios (LLRs), which can be directly entered into the URC decoder for soft iterative decoding. This avoids the potential degradation due to the conversion of symbol probabilities to bit LLRs. A comprehensive capacity analysis of the AE is performed, which demonstrates the capacity advantage of the AE-aided constellation design over its conventional quadrature amplitude modulation (QAM)/ phase shift keying (PSK) counterpart. Furthermore, we carry out its EXtrinsic Information Transfer (EXIT) chart analysis, which indicates that as a benefit of our URC, the EXIT curve always reaches the [1,1] point of perfect convergence, leading to a vanishingly low error probability. More explicitly, our bit error ratio (BER) and block error ratio (BLER) results demonstrate that the proposed FEC-URC-AE system achieves significant iterative gains both in additive white Gaussian noise (AWGN) and Rayleigh channels, outperforming both its model-based FEC-AE and its conventional coded QAM/QPSK counterparts. [ABSTRACT FROM AUTHOR]

This work tackles the joint optimization of the modulation order $L$ and transmit power $P$ of pulse-position modulation (PPM) in covert communications. Specifically, we consider two scenarios where $L$ used by the PPM at a transmitter Alice for communicating with a receiver Bob is known and unknown to the warden Willie, respectively. In the former scenario, our numerical examination obtained based on our analysis indicates that the optimal $L$ is generally 2, which is mainly due to the fact that Willie’s detection performance increases with $L$ when he knows it. Interestingly, in the latter scenario, we show that the optimal $L$ is 2 when the covertness constraint is strict, while it is 4 when the covertness constraint is relaxed. The main reason is that Willie’s detection performance does not depend on $L$ when he does not know it, and the bit error rate for $L=2$ is close to that for $L=4$ when the signal-to-noise ratio is extremely low caused by the strict covertness constraint, while $L=2$ requires less bandwidth and may achieve a higher bandwidth efficiency. Furthermore, we theoretically prove that the covert communication performance is better when Willie does not know $L$ relative to when he knows $L$. [ABSTRACT FROM AUTHOR]

A zero forcing (ZF) assisted single layer beamforming is developed for spatial modulation (SM) and space shift keying (SSK) modulation multiple input multiple output (MIMO) systems, termed as SZF-SM/SZF-SSK, to achieve both transmit beamforming and receive diversity gain in this paper. Specifically, the proposed SZF-SM/SZF-SSK maps part of the bits to spatial domain, simultaneously forming a ZF precoding assisted single layer data both in multiple input single output (MISO) and MIMO systems, that differs greatly from conventional ZF beamforming assisted MIMO systems. With MISO configuration, the ZF precoder of SZF-SM/SZF-SSK converges to the maximum ratio transmission (MRT), achieving only the transmit beamforming gain. With MIMO configuration, SZF-SM/SZF-SSK tranceiver could be further modified along with the MIMO channel to achieve single layer transmission and joint detection, attaining both the transmit beamforming and receive diversity gain in SM-MIMO system. The theoretical analysis and the simulation results of the proposed SZF-SM/SZF-SSK with MISO and MIMO configurations are presented in this paper respectively, which show an attractive performance gain over the conventional SM/SSK. Compared with the conventional ZF beamforming assisted MIMO systems and some other CSI assisted SM-MIMO schemes, the proposed scheme exhibits better performance, especially under highly correlated MIMO channels, rendering a new and practical way to achieve low rank MIMO transmission with good performance and high data rate. [ABSTRACT FROM AUTHOR]

Wireless communication in vehicular networks is highly vulnerable to electromagnetic interference, which seriously undermines the communication effectiveness. Thus, interference estimation and elimination are essential in vehicular communications. To solve the multitasking problem of interference estimation and cancelation within the transformation domain of vehicular communications where they are divided into the consistency and inconsistency based on the different conditions of spectrum sensing, the present study proposes an iterative Bayesian sparse learning approach for simultaneous interference recovery and elimination which applies multiple Bayesian learning for measured signals, and the structured sparse learning method for simultaneous interference mitigation with signal recovery. Furthermore, the updating optimization based on Bayesian estimation framework is performed to facilitate the learning process and improve the performance of interference estimation and signal reconstruction. Simulation results show that the proposed algorithms achieved more rapid convergence and more accurate recovery (31.84% and 27.88% higher in spectrum sensing with consistency and inconsistency, respectively) than conventional algorithms. During all iterations, the proposed algorithms, because of updating optimization, were more robust than conventional ones, but the computational complexity remained largely the same. To explore the correlation between the parameters and the interference recovery performance, we performed experiments, which revealed linear, logarithmic, and exponential regression correlations for particular forms of interference. We also compared the demodulation performance of the proposed algorithm with existing methods, and it turned out the proposed algorithm also obtained higher gains on average in the bit error rate. [ABSTRACT FROM AUTHOR]

In an orthogonal frequency division multiplexing (OFDM) system, cyclic prefix (CP) has to be added to eliminate multipath-induced inter-symbol interference (ISI) with a severely sacrificed bandwidth efficiency. Peak-to-average ratio (PAPR) may impair its performance further if a large number of carriers are used. On the other hand, traditional multi-carrier modulation suffers a prohibitive complexity, which can not be implemented with colorred current available silicon technologies. This work proposes a cost-effective way to implement multi-carrier modulation, namely multi-layer quadrature (MLQ) modulation, which requires only two oscillators. As an application example, we apply MLQ modulation to a complementary coded code division multiple access (MLQ-CC-CDMA) system, whose multi-carrier transmission is based on MLQ modulation to mitigate ISI and multi-user interference (MUI) with ideal auto- and cross-correlations of complementary codes. Compared to a frequency domain (FD) CC-CDMA system, MLQ-CC-CDMA has a lower RF complexity. Compared to OFDM direct sequence (DS) CDMA and single carrier frequency division multiple access (SC-FDMA) CDMA schemes, MLQ-CC-CDMA achieves a higher spectrum efficiency (SE) and a lower bit error rate with Rake and minimum mean square error (MMSE) detection due to its powerful multipath and multi-carrier diversity gains. [ABSTRACT FROM AUTHOR]

Visible light communication (VLC) has emerged as a potential wireless connectivity solution for infrastructure-to-vehicle (I2V) communications. In this paper, we investigate the performance of I2V VLC systems with access points in the form of streetlights. Particularly, we consider a typical two-lane highway road where the light poles are located at both roadsides and uniformly separated from each other. Based on non-sequential ray tracing simulations, we first propose a closed-form path loss expression as a function of transceiver and infrastructure parameters. Then, we statistically analyze the path loss and derive a closed-form expression for its probability distribution function (PDF). Utilizing the derived PDF, we derive an approximate closed-form bit error rate (BER) expression. We confirm the accuracy of derived BER expression through comparison with Monte Carlo simulation results and demonstrate the effect of transceiver and infrastructure parameters such as receiver aperture, pole height, car height, lateral shift, and spacing between light poles on the BER performance. [ABSTRACT FROM AUTHOR]

The performance of the recently proposed orientational beamforming (OBF) system for ultra-wideband (UWB) signals degrades seriously in multiuser situation. Although various techniques have been proposed in the literature to suppress multiuser interference in the time-hopping UWB system, there has been no research on interference suppression in the OBF UWB system. In this paper, we propose a nonlinear OBF system via a modified radial basis function (RBF) neural network, named the RBF-OBF system, to reject orientational UWB interferences caused by multiuser transmission in the OBF system. The RBF neural network is selected mainly because of its simple structure, convenient training process, and fast convergence speed. However, the conventional Euclidean-distance-based RBF neural network is not suitable for the training process used in this work, as the training samples are noiseless orientational steering vectors in frequency domain. Therefore, a modified RBF neural network is proposed in this paper, which evaluates the similarity between an input vector and the center vector of a hidden layer neuron by their signed complex correlation coefficient. Numerous simulations demonstrate that compared to the conventional OBF system, the proposed RBF-OBF system can significantly reduce the bit error rates (BERs) under different noise and interference situations. The BER reduction rate of the proposed RBF-OBF system is higher than 92% in additive white Gaussian noise channel and higher than 89% in line-of-sight multipath channel. [ABSTRACT FROM AUTHOR]

Link adaptation (LA) is an adaptive transmission technique that determines the modulation and coding scheme (MCS) based on channel-state information. In LA, an accurate estimation of the link performance is required to optimally determine the MCS level. In this correspondence, a high-accuracy machine-learning (ML)-aided link-level performance-prediction method for coded multiple-input–multiple-output (MIMO) systems is proposed. The basic concept of this scheme is to apply the ML model to train the relation between the inputs, such as the channel matrix and signal-to-noise ratio, and the output of the block-error rate (BLER). Specifically, we predict the index of the quantized BLER value using a random forest classifier. The simulation results show that the proposed scheme is able to accurately predict the link performance of MIMO systems and outperforms the conventional link performance-prediction schemes. [ABSTRACT FROM AUTHOR]

Multi-carrier faster-than-Nyquist (MFTN) signaling constitutes a promising spectrally efficient non-orthogonal physical layer waveform. In this correspondence, we propose a pair of low-complexity generalized approximate message passing (GAMP)-based frequency-domain equalization (FDE) algorithms for MFTN systems operating in multipath channels. To mitigate the ill-condition of the resultant equivalent channel matrix, we construct block circulant interference matrices by inserting a few cyclic postfixes, followed by truncating the duration of the inherent two-dimensional interferences. Based on the decomposition of the block circulant matrices, we develop a novel frequency-domain received signal model using the two-dimensional fast Fourier transform for mitigating the colored noise imposed by the non-orthogonal matched filter. Moreover, we derive a GAMP-based FDE algorithm and its refined version, where the latter relies on approximations for circumventing the emergence of the ill-conditioned matrices. Our simulation results demonstrate that, for a fixed spectral efficiency, MFTN signaling can significantly improve the bit error rate (BER) performance by jointly optimizing the time- and frequency-domain packing factors. Compared to its Nyquist-signaling counterpart, our proposed MFTN systems employing the refined GAMP equalizer can achieve about 39% higher transmission rates at a negligible BER performance degradation. [ABSTRACT FROM AUTHOR]

Highmobility is an important scenario for the 5G wireless communications. However, the high mobility results in serious time-varying fading, bringing challenges to the reliabilities of wireless transmission. Diversity technique is an essential method to overcome the effect of fading. It is noted that an LDPC (low-density parity-check) code is a concatenation of single parity-check codes (SPC) and repetition codes (REP) by a random interleaver, and a REP can be viewed as a typical example for achieving time/frequency diversity. Thus, it is appealing to jointly consider LDPC codes and diversity techniques. This paper presents parity-check coding transmit diversity (PCTD) schemes based on half-rate invertible LDPC codes, which are capable of achieving both diversity and coding gains from the time-varying fading channels. Besides, we investigate two types of PCTD schemes, and take into consideration two transmission modes, which are single-carrier (SC) and orthogonal frequency division multiplexing (OFDM) transmission modes. A chase combining based message passing algorithm (CC-MPA) is proposed for the proposed schemes, which can take full advantages of signal processing and coding gains. Furthermore, the Shannon limits of time-varying fading channels with BPSK signaling are deduced. Simulation results show that the proposed PCTD with CC-MPA algorithm can provide better BER performances than the classical time-domain diversity scheme in time-varying fading channels, which is appealing for the high mobility communications. [ABSTRACT FROM AUTHOR]

Electromagnetic interference causes considerable performance degradation on wireless communication systems. The performance degradation is largely affected by the characteristics of the interference. Impulse interference commonly occurs in industrial and automotive environments, where many closely located systems and electronic equipment coexist. In this work, we propose two novel algorithms for adaptive demodulation in impulse noise. The proposed methods compute appropriate log-likelihood ratios based on interference classification and estimation of Middleton’s Class A noise model as well as the symmetric $\alpha$ -Stable (S $\alpha$ S) model. The analytical complexity and intractability of the S $\alpha$ S probability density function (PDF) makes real-time applications unfeasible. Therefore, the proposed algorithms are developed using two different techniques for approximating the S $\alpha$ S PDF. The approximations are achieved through interpolation via i) approximate closed-form expressions of the PDF for a specific parameter set and ii) via numerically pre-calculated look-up tables of the PDF for arbitrary parameter choices. The proposed adaptive techniques are shown to provide a performance gain in terms of bit error rate (BER) of up to 20 dB energy per bit to noise power ratio (Eb/N0), in comparison to traditional, non-adaptive, methods for varying levels of impulse interference. The performance loss of approximating the PDF in comparison to exact calculations amount to less than 8 dB Eb/N0, which is small in relation to the performance gain achieved in comparison to traditional methods. However, the complexity in terms of simulation execution time of the proposed approximate algorithms is reduced by up to a factor 250, compared to methods based on exact computation. The algorithms are validated using regular low density parity check (LDPC) codes, in addition to quasi-cyclic (QC)-LDPC and turbo codes. The proposed adaptive demodulation techniques can be used to improve decoding and demodulation performance in many real-life situations where non-Gaussian interference commonly occurs. [ABSTRACT FROM AUTHOR]

This paper aims to improve the error rate performance of the enhanced spatial modulation based orthogonal frequency division multiplexing (ESM-OFDM) system by jointly optimizing the signal constellation and the antenna power allocation (PA). We derive a closed-form expression for the optimal PA of the ESM-OFDM scheme for some practical antenna setups and then implement and verify the system by using an Universal Software Radio Peripheral (USRP) platform. Monte Carlo simulations and experimental results show the superiority of the derived optimal PA over the equal-gain case. [ABSTRACT FROM AUTHOR]

Sparse code multiple access (SCMA) is a promising multiuser communication technique for the enabling of future massive machine-type networks. Unlike existing codebook design schemes assuming uniform power allocation, we present a novel class of SCMA codebooks which display power imbalance among different users for downlink transmission. Based on the Star-QAM mother constellation structure and with the aid of genetic algorithm, we optimize the minimum Euclidean distance (MED) and the minimum product distance (MPD) of the proposed codebooks. Numerical simulation results show that our proposed codebooks lead to significantly improved error rate performances over Gaussian channels and Rayleigh fading channels. [ABSTRACT FROM AUTHOR]

Multiple-input multiple-output (MIMO) technology with high order signal modulation method has been proposed recently as a scheme to achieve high throughput requirements of next-generation communication applications. However, the use of high-order modulation signals degrades the error rate performance. Generalized frequency division multiplexing (GFDM) has attracted enormous consideration for deploying in new communication scenarios due to its advantages in terms of out-of-band (OOB) emission, spectral efficiency (SE), and latency. Index modulation (IM) methods convey digital information by utilizing transmission entities with adjustable index parameters to provide flexible system design and offer attractive advantages such as energy and SE without increasing the computational complexity. The Multiple-Mode (MM) method permits multiple subcarriers to transmit multiple modes and employs the whole permutation of these modes to convey additional information bits. The proposed MIMO MM-GFDM-IM scheme improves the bit error rate and the spectral efficiency compared to the existing schemes. The proposed method can select the modulation order and the number of modes to achieve the design goals. Furthermore, a system with high modulation order can improve the spectral efficiency when the constellation point is fixed. This paper also proposes a practical and effective MM selection strategy and two low-complexity detectors. [ABSTRACT FROM AUTHOR]

A plethora of new applications will be enabled by next generation wireless networks. These demand sophisticated medium access control and physical layer protocols in order to fulfill the requirements in terms of spectral efficiency, bit error rate (BER), energy efficiency, reliability, and security, among others. This paper develops an advanced space-time multiple-input, multiple-output (MIMO) solution that builds on the concept of space-time shift keying (STSK) for single-carrier frequency domain equalization (SC-FDE). STSK-aided SC-FDE, or SSF, ensures space-time and frequency diversity in dispersive channels, but can only support suboptimal linear receiver schemes. We therefore introduce a turbo equalizer for SSF in frequency-selective multipath channels. It enables inter-symbol-interference cancellation by means of iterative space-time processing. We coin it the iterative space-time block equalizer (SSF-ISTBE). We provide the mathematical formulation for the proposed SSF-ISTBE framework in non-linear channel conditions. We demonstrate that the SSF-ISTBE achieves a substantial 4 dB gain in terms of BER over the linear minimum mean square error equalizer while significantly reducing the computational complexity of the optimal maximum likelihood SSF. When considering practical transceivers, in particular non-linear power amplifiers, the gain of the SSF-ISTBE becomes more prominent over state-of-the-art approaches, demonstrating its robustness against non-linear distortion. [ABSTRACT FROM AUTHOR]

This paper proposes a novel bit loading algorithm to multicarrier non-orthogonal multiple access (NOMA) systems to maximize the system total throughput while satisfying the users’ individual quality of service (QoS) constraints. Although bit loading is generally a non-deterministic polynomial-time (NP)-hard problem, even for orthogonal multiple access (OMA), the mutual interference between the users, and the dependence of power coefficients and modulation orders are additional challenges that add substantial complexity to the optimization problem. Therefore, in this paper, we propose an efficient bit loading algorithm for multicarrier NOMA systems and compare the complexity and throughput with a corresponding OMA system. The obtained results show that for the case of NOMA, bit loading provides an additional degree of freedom that allows non-uniform spectrum sharing among the users. In particular scenarios, the system may experience hybrid modes of operation, where the system switches between NOMA and OMA. The obtained numerical results show that NOMA can outperform OMA by 100% in terms of spectral efficiency, for a two-user scenario. The complexity of the loading process for NOMA is noticeably higher than OMA, which is due to the high complexity associated with the bit error rate (BER) computation in the case of NOMA. [ABSTRACT FROM AUTHOR]

In this article, the signal detection problem in indoor visible light communication (VLC) system aided by generalized space shift keying (GSSK) is modeled as a sparse signal reconstruction problem, which has lower computational complexity by exploiting the sparse reconstruction algorithms in compressed sensing (CS). In order to satisfy the measurement matrix property to perform sparse signal reconstruction, a preprocessing approach of measurement matrix is proposed based on singular value decomposition (SVD), which theoretically guarantees the feasibility of utilizing CS based sparse signal detection method in indoor GSSK-VLC system. Then, by adopting classical orthogonal matching pursuit (OMP) algorithm and compressed sampling matching pursuit (CoSaMP) algorithm, the GSSK signals are efficiently detected in the considered indoor GSSK-VLC system. Furthermore, a more efficient detection algorithm combined with OMP and maximum likelihood (ML) is also presented especially for SSK scenario. Finally, the effectiveness of the proposed sparsity aided detection algorithms in indoor GSSK-VLC system are verified by computer simulations. The results show that the proposed algorithms can achieve better bit error rate (BER) and lower computation complexity than ML based detection method. Specifically, a signal-to-noise ratio (SNR) gain as high as 12 dB is observed in the SSK scenario and about 5 dB in case of a GSSK scenario upon employing our proposed detection methods. [ABSTRACT FROM AUTHOR]

Spectrum efficiency is a critical performance metric in wireless communications due to an increasing demand for radio spectrum to support high data rate applications. Direct sequence spread spectrum technology enables a code division multiple access (CDMA) system a strong anti-interference capability but with a relatively low spectrum efficiency. To address this issue, complementary coded scrambling multiple access (CCSMA) (X. Liu et al., in 2018) was proposed to use three-dimensional (space-time-frequency) resources for performance enhancement. However, in a CCSMA system, if the number of multipath returns is larger than the number of transmit (Tx) antennas, a code reuse problem occurs to degrade bit error rate performance significantly. This work designs a new scrambling code, namely generalized complementary coded scrambling multiple access (GCCSMA) codes, to overcome the code reuse problem in CCSMA. GCCSMA uses complementary codes, scrambling modulation, and multiple-input-multiple-output (MIMO) techniques jointly, which works well even in multipath channels with more paths than the number of Tx antennas. The analysis and simulations are conducted to validate the effectiveness of the proposed GCCSMA system. [ABSTRACT FROM AUTHOR]

Low-complexity detectors are conceivedfor the spatial modulation-aided sparse code-division multiple-access (SM-SCDMA). Firstly, the approximate message passing (AMP) algorithm is adapted for the detection of SM-SCDMA, which has less than 1% detection complexity of the state-of-the-art message passing-aided (MPA) detection albeit at a bit error ratio (BER) degradation. Therefore, inspired by the benefits of interference cancellation (IC) and the unique characteristics of SM, an enhanced AMP (EAMP) detector is proposed for mitigating BER degradation without increasing the detection complexity. Finally, the classic expectation propagation (EP) is intrinsically amalgamated with our EAMP detector, leading to a novel EAMP-EP detector, which further reduces the detection complexity of the EAMP algorithm. The three detectors are compared both in terms of their coded and uncoded BER, as well as complexity. [ABSTRACT FROM AUTHOR]

Optical wireless networks, particularly free-space optical (FSO) in outdoor environments and visible light communication (VLC) in indoor environments, suffer from severe deterioration in signal quality due to various factors such as the availability of line-of-sight link, the turbulence of the atmosphere, or pointing errors. To enable high-quality service point-to-point transmission through optical wireless communication, an efficient and reliable backhaul link is essential. Towards this objective, this paper proposes a novel optical backhauled non-orthogonal multiple access (NOMA) in VLC systems. Unlike earlier works, our model considers the effects of various transmission parameters in a mixed cooperative FSO/radio over a fiber backhaul link with NOMA-VLC to serve two users. The closed-form expressions of outage probability, sum throughput, ergodic sum capacity, and average bit error rate are delivered for a variety of channel conditions using Meijer's G and Gaussian hypergeometric functions. More specifically, both the theoretical and simulation results demonstrate that the performance gain of the proposed optical-based NOMA systems can improve the performance of the users in comparison with the conventional OMA systems at medium to high SNR ranges. Furthermore, a deep learning-based framework is proposed to accurately estimate the ergodic capacity of the proposed system and significantly reduce the complexity cost of the theoretical analysis. [ABSTRACT FROM AUTHOR]

A novel index modulation with spherical code and its constellation mapping are proposed. We derive the closed form of the approximation bound of the bit error rate (BER) of the OFDM-IM system based on four-dimensional spherical code. Our derived BER is very close to the simulation results. And different constellation mapping method have different BER performance and the gap is less than 0.2 dB. To reduce the detection complexity which is caused by the high-dimensional modulation, a DNN-based receiver with partial-connected layer is proposed. Simulation results show that our proposed DNN-based receiver can greatly reduce the complexity while its BER performance is close to the classical maximum likelihood detection method. [ABSTRACT FROM AUTHOR]

This paper presents and investigates a dual-hop mixed radio frequency (RF) and underwater optical communication (UWOC) system. In the proposed system, an unmanned aerial vehicle (UAV) such as drone (S) located at low altitude is transmitting data signal towards the destination (D) which is located under water such as a submarine through a decode and forward (DF) relay (R) mounted on a ship over the sea surface. The data signal is transmitted from the UAV towards the relay located on the ship via RF channel modelled by the Nakagami- $ m$ distributed fading statistics. The relay decodes the received RF data signal and re-transmits it towards the destination via visible light channel modelled by Exponential Generalized Gamma (EGG) distribution. Using the DF relaying protocol, we derive closed form analytical expressions for the outage probability and average bit error rate of the proposed mixed RF-UWOC system. Additionally, UAV optimal altitude analysis has been carried out to find the optimal elevation angle corresponding to the optimal altitude of the UAV where the system performance is maximized. The numerical simulation is carried out to support the performance analysis of the mixed RF-UWOC system and shows the influence of the various channel parameters such as air bubbles, UAV altitude, water salinity variations and scintillation on the end to end performance of the mixed RF-UWOC system. [ABSTRACT FROM AUTHOR]

In this paper, an improved Quadrature Multi-carrier Differential Chaos Shift Keying (QMC-DCSK) based on Chaotic Shape-forming Filter (CSF) and Cross Correction (CC) is proposed. The chaotic signal generated by CSF is used as reference spreading sequence in QMC-DCSK. Quadrature multi-carriers are used to carry the reference and multiple information bearing signals simultaneously in order to improve the Bit Transmission Rate (BTR). By this way, the proposed system is able to transmit $2M-1$ bits Traditional Data Stream (TDS) during one traditional DCSK bit duration, where $M$ is the number of carrier frequencies. Meanwhile, the reference signal transmits Additional Data Stream (ADS) encoded by CSF together with $2M-1$ identical or inverse data stream carried by the rest $2M-1$ channels (information bearing signals). By this system configuration, the channel redundance can be used to improved bit error rate (BER) performance of ADS, after decoding TDS. In turn, the TDS reliability can also be improved by checking most ADS polarity of the corresponding carrier. This process is referred to as CC. The contribution of this work lies in the CC operation, by which the reliability of both TDS and ADS is improved significantly. It is the CSF and multi-carrier that make CC possible. The analytical BER is derived for the proposed method. The merits of the proposed system include, first, it improves the BTR as compared to that of QMC-DCSK, because ADS conveys more information bits; second, the ADS reliability is improved dramatically because the $\text{2}\,M$ duplication can be used to improve the signal to noise ratio; third, the TDS reliability is improved as well because the information bits conveyed by each carrier frequency is enhanced and corrected by the $\text{2}\,M$ repetitions. By this way, not only TDS and ADS are transmitted (i.e., the higher BTR is achieved), but also both reliability are improved (i.e. the lower BER is achieved). In order to testify the feasibility and superiority of the proposed method, simulation and wireless open-access research platform (WARP) experiment are conducted. The simulation and experiment results show that the proposed method is superior to other comparison methods. [ABSTRACT FROM AUTHOR]

In this paper, we focus on the design and analysis of the Analog Fountain Code (AFC) for short packet communications. We first propose a density evolution (DE) based framework, which tracks the evolution of the probability density function of the messages exchanged between variable and check nodes of AFC in the belief propagation decoder. Using the proposed DE framework, we formulate an optimisation problem to find the optimal AFC code parameters, including the weight-set, which minimises the bit error rate at a given signal-to-noise ratio (SNR). Our results show the superiority of our AFC code design compared to existing designs of AFC in the literature and thus the validity of the proposed DE framework in the asymptotically long block length regime. We then focus on selecting the precoder to improve the performance of AFC at short block lengths. Simulation results show that lower precode rates obtain better realised rates over a wide SNR range for short information block lengths. We also discuss the complexity of the AFC decoder and propose a threshold-based decoder to reduce the complexity. [ABSTRACT FROM AUTHOR]

Differential modulation schemes are very relevant in receivers having power and processing limitations, as these schemes dispense with the need for knowledge of channel coefficients for symbol detection. Spatial modulation (SM) is a scheme used in multi-antenna transmission scenarios where the data is transmitted in the amplitude, phase and spatial domains through selected antennas. In the coherent domain, generalized SM (GSM) employs multiple antennas in combination during every time slot to enhance the spectral efficiency (SE). In this paper, we propose two differential schemes which activate two or more antennas at a time to transmit the modulated symbol. These schemes achieve higher SE using a lesser number of antennas and lower order modulation schemes instead of the higher number of antennas required for conventional SM schemes based on differential modulation. Simulation studies reveal that the proposed schemes have better bit error rate performance than traditional differential SM schemes. We also derive the analytical union bound for the proposed schemes and is satisfied from medium to high signal-to-noise ratio (SNR) ranges. [ABSTRACT FROM AUTHOR]

In this paper, the problem of timing error for free-space optical links with high mobility is investigated. In particular, we consider a real scenario in which the sampling clock offset is randomly changing and, in addition, the receiver has no information about the instantaneous channel coefficient as well as instantaneous timing error. Moreover, we consider a general case with signal dependent noise. For the considered scenario, for a conventional receiver with one sampler, we propose an iterative detection method based on the expectation-maximization (EM) algorithm. In the presence of large values of sampling clock offset, shifted error is the dominant error. Hence, in order to reduce the impact of shifted error probability, we propose an iterative data detection method for a receiver which utilizes two shifted samplers. The proposed iterative detection method only requires a series of simple addition and multiplication operation and does not require to solve any integral equations. A set of simulation results are provided to study the system performance for the proposed detection methods. Simulation results demonstrate that for a receiver with two samplers, by using the proposed EM-based detector, we can achieve bit error rate performance very close to the ideal receiver that perfectly knows the instantaneous channel coefficient and sampling clock offset. [ABSTRACT FROM AUTHOR]

Analysis on the exact error rate of jointly optimal (JO) maximum likelihood (ML) multiuser detection is limited in the existing literature owing to its high computational complexity. In this paper, the JO ML detection of two quadrature phase-shift keying (QPSK) signals with an arbitrary relative phase offset is investigated. Exact, analytical symbol error rate (SER) and bit error rate (BER) formulas are derived for two users with no less than 3 dB power difference between them. Closed-form union bounds on the user SER and BER with an arbitrary phase offset and an arbitrary user power ratio are also given as a simpler approach to acquire the asymptotic error rate performance. Furthermore, insights on the potentials and properties of the JO ML detector regarding its error rate behaviors for each user are revealed through comparisons with the successive interference cancellation receiver. The accuracies of the obtained formulas are demonstrated through simulation. The results show the error rates for the two-user QPSK case no longer follow the classic knowledge on the error rate behaviors in the single signal detection. In addition, the performance and dB penalty employing different modulation types are also compared and discussed. [ABSTRACT FROM AUTHOR]

Non-orthogonal multiple access based cooperative relaying system (NOMA-CRS) has been proposed to alleviate the decay in spectral efficiency of the conventional CRS. However, existing NOMA-CRS studies assume perfect successive interference canceler at the relay and mostly investigate sum rate whereas the error performance has not been taken into consideration. In this paper, we analyze error performance of the NOMA-CRS and the closed-form bit error probability (BEP) expression is derived over Nakagami-m fading channels. Then, thanks to the high performance of machine learning (ML) in challenging optimization problems, a joint power sharing-power allocation (PS-PA) scheme is proposed to minimize the bit error rate (BER) of the NOMA-CRS. The proposed ML-assisted optimization has a very low online implementation complexity. Based on provided extensive simulations, theoretical BEP analysis is validated. Besides, the proposed ML-aided PS-PA provides minimum BER (MBER) and outperforms previous PA strategies for the NOMA-CRS notably. [ABSTRACT FROM AUTHOR]

This paper focuses on the issue of achieving transmit diversity for the multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) system working with quadrature index modulation (QIM). Inspired by the ideas of quadrature spatial modulation (QSM) and quadrature space-frequency index modulation (QSF-IM), parts of the space-frequency resource units in each resource block (RB) of the MIMO-OFDM system are activated twice independently to send the in-phase and quadrature components of a space-frequency code, respectively, to achieve the second order transmit diversity. Thus the proposed scheme is called space-frequency coded QIM (SFC-QIM). The SFC-QIM scheme allows for a linear-complexity maximum likelihood (ML) receiver, which benefits from the orthogonality between the real and imaginary parts of the symbols in the SFC codewords. Owing to obtaining higher diversity order, simulation results of bit error rate (BER) performance demonstrate that the new SFC-QIM scheme outperforms some existing IM schemes in both independent and identically distributed (i.i.d.) and correlated Rayleigh fading channels. [ABSTRACT FROM AUTHOR]

In this paper, a novel differential space-time block coded spatial modulation (differential STBC-SM) is proposed for uplink multi-user massive multiple-input multiple-output (MIMO) communications, which combines the concept of differential coding and STBC-SM to enhance the diversity benefits in the absence of the channel state information (CSI). The transmission structure of the proposed system is on a block basis, where each block contains two sub-blocks. More specifically, the first sub-block only conveys amplitude and phase modulation (APM) symbol bits, since its transmit antennas (TAs) obey a pre-designed activation pattern, which do not carry any information bit. For the second sub-block, the input bits are modulated to STBC-SM matrices, which are then differentially coded between two adjacent sub-blocks. Moreover, a novel block-by-block based non-coherent detector is presented. Finally, we derive an upper bound on the average bit error probability (ABEP) by using the moment generating function (MGF). Our simulation results show that the proposed differential STBC-SM transmission structure is able to acquire considerable bit error rate (BER) performance improvements compared to both the conventional differential spatial modulation (DSM) and differential Alamouti schemes. [ABSTRACT FROM AUTHOR]

Efficient and reliable underwater sonar image transmission is increasingly desired for underwater investigation by autonomous underwater vehicles (AUV). However, it is extremely challenging due to the severe interference nature of underwater acoustic channels and the large data size of sonar images. In this paper, we first combine multicarrier modulation techniques with characteristics of compressed sonar image data to improve the transmission performance, where in previous research, image compression and multicarrier modulation were investigated separately. The compressed bitstream of sonar images is regarded as a new type of image with unique frequency characteristic. A non-uniform non-orthogonal multicarrier modulation scheme based on frequency characteristics of compressed sonar image data (F-NNMC) is proposed to meet the requirements of efficient and reliable sonar image transmission. The non-orthogonality between subcarriers improves the spectrum efficiency, and the non-uniform subcarrier structure can carry more effective information of the transmitted compressed data to improve the reliability. The simulation and experiment show that the proposed F-NNMC needs less frequency resource and has better peak signal-to-noise ratio (PSNR), structural similarity (SSIM), as well as lower bit error rate (BER) compared to orthogonal and non-orthogonal modulations. [ABSTRACT FROM AUTHOR]

Terahertz (THz) frequency bands can be promising for data transmissions between the core network and access points (AP) for next-generation wireless systems. In this paper, we analyze the performance of a dual-hop THz-RF wireless system where an AP facilitates data transmission between a core network and user equipment (UE). We consider a generalized model for the end-to-end channel with independent and not identically distributed (i.ni.d.) fading model for THz and RF links using the $\alpha$ - $\mu$ distribution, the THz link with pointing errors, and asymmetrical relay position. We derive a closed-form expression of the cumulative distribution function (CDF) of the end-to-end signal to noise ratio (SNR) for the THz-RF link, which is valid for real-valued $\mu$ for a generalized performance analysis over THz fading channels. Using the derived CDF, we analyze the performance of the THz-RF relayed system using decode-and-forward (DF) protocol by deriving closed-form expressions of outage probability, moments of SNR, average BER, and a tight approximation of ergodic capacity in terms of system parameters. We develop asymptotic analysis on the ergodic capacity, outage probability, and average BER in the high SNR region and derive the diversity order of the system. We also analyze the considered system with an i.i.d. model and develop simplified performance to provide insight on the system behavior analytically under various practically relevant scenarios. Simulation and numerical analysis show a significant effect of fading parameters of the THz link and a nominal effect of normalized beam-width on the performance of the relay-assisted THz-RF system. [ABSTRACT FROM AUTHOR]

In order to mitigate the error propagation (EP) effect on the bit error rate (BER) performance due to the imperfect successive interference cancellation for the non-orthogonal multiple access (NOMA)-based visible light communication (VLC) downlinks, adjustable bit rate superposition coding (ABRSC) and sample-and-sum successive interference cancellation (SSSIC) decoding schemes are proposed in this paper. Based on the modified fixed-power allocation (MFPA) strategy, the closed-form expressions of BER are derived for the two-user case with on-off keying, and further extended to the multi-user scenario. The residual probability of EP is introduced and derived to theoretically assess the effectiveness of the proposed ABRSC and SSSIC schemes. Monte Carlo simulation is also conducted to validate the analytical BER results. The BER performance of the NOMA-based VLC system with multiple users is further analyzed and discussed in detail. It is found that under the total average optical transmission power constraint, the allocated average optical transmission power to current user should be higher than that to all rest of users, i.e., the power allocation factor (PAF) is supposed to be greater than 1/2, in order to avoid the error floor issue of current user, hence facilitating the successful implementation of SSSIC. Besides, increasing the bit-rate factor (BRF) could mitigate the effect of EP, and larger BRF leads to more efficient outcomes in alleviating the effect of EP. [ABSTRACT FROM AUTHOR]

This paper evaluates the practically achievable best error performance obtained from a dimmable visible light communication (VLC) system that conveys information using binary modulations, such as on-off keying (OOK). As compared to previous studies that address the ideal performance of dimmable VLC systems only over reliable channels with arbitrarily small error probability, this paper presents the best achievable error probability of various configurations with the dimming targets, signal quality, and data rate. The numerical results establish the ultimate decoding error performance targeted for dimmable VLC systems and are expected to provide a practical guideline for the enhanced VLC system design. [ABSTRACT FROM AUTHOR]

Adaptive optics (AO) has the potential to mitigate the effect of atmospheric turbulence and improve the performance of orbital angular momentum (OAM)-based optical wireless communication (OAM-OWC) links. Here, we propose a single-intensity-measurement phase retrieval algorithm (SPRA)-based AO technique of compensating for the distortion of the OAM beam. The only parameter required by the SPRA wave-front sensor is the intensity of the probe beam in the Fourier domain, which substantially simplifies the AO system. We first derive an analytical expression to characterize the expansion of probe beam in OAM-OWC links and then determine the diameter constraints as the apriori information of the SPRA required for guaranteeing a certain compensation performance. The simulation results illustrate that the SPRA-AO approach can indeed correct a distorted OAM beam both in a single-channel scenario and in multiplexed OAM-OWC systems. The bit error rate can be improved by orders of magnitude with the aid of SPRA-AO compensation. Furthermore, we establish noise models of AO-based OAM-OWC systems and analyze the robustness of the SPRA-AO technique. In a nutshell, this paper provides new insights for the applications of AO and forms the theoretical basis of employing probe beams in OAM-OWC systems. [ABSTRACT FROM AUTHOR]

This paper proposes a new transmitter precoding approach for co-existence between multiple-input multiple-output (MIMO) radar and multi-user MIMO communication systems employing phase shift keying (PSK) modulation. To enable the satisfactions of radar communication functionalities, the common practice is to suppress the mutual interferences, while our proposed scheme is to use the known interferences existing already in radar and communications systems and make the most of them: selectively reverse the destructive interferences to the negative direction while retaining the interferences which are beneficial to the desired signal. The resulting interferences are always kept constructive and can contribute to the power of the useful signal. Numerical results validate that the effective signal to noise ratio (SNR) can be enhanced by 7 dB with no additional power-peruser investment. [ABSTRACT FROM AUTHOR]