Synchronization is a fundamental problem in communication system. In this paper, we consider a scenario where an unmanned aerial vehicle broadcasts information to several underwater unmanned vehicles. Beacon signals are adopted to achieve global time synchronization. We conduct experiments in a laboratory water tank and an outdoor deep water pool under different background radiation intensities. The experimental results demonstrate that spatial optical filter can mitigate the impact of background radiation. Additionally, via analyzing the experimental data, it can be concluded that the number of consecutive unsynchronized frames in air-to-water visible light communication system follows a geometric distribution. Then, we analyze the probability of data slot overlaps due to offset accumulation. We calculate the optimal guard interval for two-user scenario to maximize throughput via semi-analytical method, which is extended to n-user scenario via numerical approaches. Finally, the practical feasibility of the transmission model is validated via comparing the results of semi-analytical model and experiments.

This paper demonstrates an automatic beam alignment system for a laser diode (LD)-photomultiplier tube (PMT) based underwater optical wireless communication (UOWC) system. Initially, the imaging characteristics of the underwater laser spot are described. Subsequently, a laser outlet estimation algorithm based on principal component analysis (PCA) is proposed. The direction of the emitted light beam is rapidly determined by reflecting it with a micro-electro-mechanical system (MEMS) mirror to align with the receiver. Experimental results validate the feasibility of the proposed beam alignment method, indicating an acceptable alignment error. Moreover, the proposed system effectively enhances the bit error rate (BER) performance, meeting the BER requirements of the forward error correction (FEC) limit when the receiver moving speed is below 0.45 m/s.

In this work, we develop a theoretical transmission model for optical wireless communication systems with receiver nonlinearity. To provide a framework for analyzing transmission rates, we derive lower and upper bounds on channel capacity under average and peak optical power constraints in both single-input single-output and multiple-input multiple-output systems using entropy power inequalities and dual expression. Numerical results verify the tightness of our derived capacity bounds and highlight the significant relationship between channel capacity and receiver nonlinearity.

Considering existing lighting infrastructure, the indoor VLC network can be modeled as a multi-tier network containing different types of access points with different band-widths. In this paper, we investigate the cooperative resource allocation (RA) problem in multi-tier VLC networks based on direct current biased orthogonal frequency division multiplexing (DCO-OFDM). The sub carriers are reorganized to avoid the interference caused by redundant-information-bearing subcar-riers of DCO-OFDM. To maximize the quality of experience (QoE) satisfaction of user equipments (UEs), a bipartite graph greedy algorithm (BGGA) is proposed by taking into account the quadrature amplitude modulation (QAM) and the clipping dis-tortion. Numerical results show that under non-ideal conditions, the proposed RA algorithm outperforms existing graph-based RA algorithms in terms of QoE satisfaction.

In this paper, we propose a joint communication and sensing (JCAS) waveform design method for pulse amplitude modulation (PAM) signal based on visible light. Common communication and sensing performance metrics, including symbol error rate, achievable transmission rate, miss detection probability and Kullback-Leibler (KL) divergence are adopted. We provide an optimization criterion with respect to parameter of the integrated waveform under peak power constraint to balance the communication performance and the sensing performance. We conduct the JCAS experiments for the objects under short distance of 1 meter and long distance of 7 meters. Both numerical and experimental results show that, for PAM signals, there exists a fundamental tradeoff between the communication performance and the sensing performance. Given the same sampling rate and the same number of samples for target detection, high-order PAM signals provide better sensing performance due to the lower tail probability. Moreover, with longer distances and stronger diffuse reflections, the detection duration needs to be extended to obtain better sensing performance.

This paper explores the impact of signal-dependent noise (SDN) on the band-limited Nyquist pulse design in visible light communication (VLC) systems. We provide a bandlimited signal transmission model with SDN, and also extend the transmission scheme with time-varying bias in the existing work. We analyze the noise features of the band-limited VLC systems with SDN, and show that the SDN strength after receive filtering is determined by both transmitter and receiver pulses. For the inter-symbol interference (ISI)-free transmission with sampling receiver and time-varying bias, we provide a bandlimited Nyquist pulse design method to maximize the low bound of output signal-to-noise ratio (SNR) after receiver filtering, and the designed pulses are unrelated to channel condition and SDN strength. For the ISI-free transmission with matched filter, we prove that there is no theoretical optimal band-limited Nyquist pulse, and provide the Nyquist pulse design methods for the two transmission schemes without bias and with time-varying bias, respectively. Numerical results demonstrate that the designed pulses outperform the existing pulses for different excess bandwidth factors and modulation formats in VLC systems with SDN.

In this paper, downlink multi-tier visible light communication (VLC) networks in the Industrial Internet of Things (IIoT) are studied in terms of outage probability and achievable rate. The industrial VLC network is modeled as a multi-tier network in a finite space under limited receiver field-of-view (FOV), in which access points (APs) of each tier at their own height follow a Poisson point process (PPP) distribution. Random interference between different tiers in the multi-tier network and the boundary effects in a finite space lead to difficulties in analysis, so we propose a new stochastic geometric based analytical framework to tackle this problem. Our analytical expressions precisely capture the impact of typical IIoT device’s FOV and position in a finite space on the outage probability and average achievable rate. For a typical IIoT device which is far away from the boundary, the boundary effect is found to be negligible. Finally, the analytical results are verified by simulations. We observe that the changes in the FOV and the number of APs mainly affect interference and cell coverage, and further influence the network performance of multi-tier VLC networks. We also investigate how the other network parameters influence performance metrics, emphasizing that reasonable parameter settings can yield better VLC network performance.

Covert communications hide the information transmission from a watchful adversary (Willie) while ensuring a satisfactory decoding performance at a legitimate receiver (Bob). In this paper, we propose a covert transmission scheme based on narrow signal waveband and nonlinear suppression effect of avalanche photodiodes (APDs) for water-to-air (W2A) optical wireless communication (OWC) systems. Narrow signal waveband is selected based on a given wavelength selection pattern which is negotiated by the transmitter and receiver. Willie’s detection performance is investigated under APD characteristics and ambient radiation. Under the covertness constraints on Willie’s detection performance, the covert throughput between Alice and Bob is maximized by optimizing the power and blocklength of transmitted signals. Except for the case of Alice with knowledge of Willie’s position and optical filter passband-width, we explore a more practical scenario where the knowledge is not available to Alice. The optimal passband-width of Willie’s filter is calculated by taking into account both overlapped waveband and detection performance, and a covert region is established to limit Willie’s detection range. Numerical and experimental results indicate that strong ambient radiation and narrow signal waveband can enhance the signal covertness significantly.

In this paper, a novel multiple-camera-based visible light positioning (MC-VLP) algorithm is proposed for indoor three-dimensional (3D) positioning. The basic idea of MC-VLP is to utilize multiple cameras to simultaneously capture the beacon signal transmitted by a light-emitting diode (LED) mounted on the mobile terminal. Since the pinhole camera can achieve high imaging accuracy and the fish-eye camera has a wide field of view (FOV), both of them can be adopted as the receiver to improve the positioning accuracy and coverage area. Meanwhile, the positioning errors along$X$-axis,$Y$-axis, and$Z$-axis are derived to evaluate the performance of the MC-VLP algorithm. Based on the derived expression of positioning error, we optimize the camera layout and tilt angle to minimize the positioning error in the coverage area. Simulation results show that higher positioning accuracy can be achieved via optimizing the camera layout and tilt angle. Experimental results show that the mean 3D positioning accuracy is 1.7 mm with 3 pinhole cameras at height 2.5 m.

Since the passband of an optical filter will shift as the optical incidence angle changes, random receiver orientation will lead to variation of normalized channel matrix in multi-color visible light communication (VLC) systems. This work investigates the constellation design for color shift keying (CSK) system under random receiver orientation. We consider a multi-color visible light sensing and communication system. Taking the chromaticity and color rendering index constraints into account, we propose two CSK constellation design methods, one to generate constellations for each optical incidence angle and the other to generate constellations with the optical incidence angle regarded as a random variable. Then, taking the sensing information as a prior knowledge, we construct the CSK constellation maps to reduce the real-time computational complexity, and propose a sensing-aided communication mechanism. Moreover, to obtain the sensing information, we propose a 5D user equipment (UE) location and pose estimation method for the considered systems. Simulation results demonstrate the performance gain of the designed CSK constellations, the accuracy of the proposed UE location and pose estimation method, and the potential with CSK constellation map.

Typically, a photomultiplier tube (PMT) can be leveraged as the photon detector in the underwater optical wireless communication (UOWC) system to further extend the transmission distance, whose output electric signal can be generally classified into three regimes named discrete-pulse regime, transition regime, and continuous waveform regime according to various received optical power (ROP). In this paper, we experimentally demonstrate a low-power UOWC system that adopted a PMT as the receiver, and a detection method by taking the cumulative sum of the sampled values in a symbol duration is proposed and experimentally evaluated under different ROP and symbol rates. Experimental results show that the proposed detection method outperforms various statistics-based comparison benchmarks. Furthermore, the effectiveness of the proposed method in alleviating strong ambient radiation is experimentally verified.

Visible-light communication (VLC) is promising for the future application in Industrial Internet of Things (IIoT) due to limited interuser interference from directional transmission, which can provide a good guarantee for the delay and reliability of communication. In this work, we consider the model of a downlink indoor VLC network covered by multiple lighting-emitting diodes (LEDs). We reduce the delay violation probability via minimizing the superframe duration and analyze the delay violation probability based on the optimized superframe duration. We analyze the delay violation probability using stochastic network calculus (SNC). More specifically, we propose a low-complexity resource slicing scheme and exploit LED layout characteristics to minimize the superframe duration. Then, assuming that the users are uniformly distributed in each served area, we obtain the achievable rate distribution for each served area. Our results show that the proposed resource slicing scheme yields a shorter superframe duration than random scheme and the delay violation probability increases with traffic arrival rate, LED spacing, and the coverage radius.

We consider performance enhancement of asymmetrically-clipped optical orthogonal frequency division multiplexing (ACO-OFDM) and related optical OFDM schemes, which are variations of OFDM in intensity-modulated optical wireless communications. Unlike most existing studies on specific designs of improved receivers, this paper investigates information theoretic limits of all possible receivers. For independent and identically distributed (IID) complex Gaussian inputs, we obtain an exact characterization of information rate of ACO-OFDM with improved receivers for all SNRs. It is proved that the high-SNR gain of improved receivers asymptotically achieve 1/4 bits per channel use, which is equivalent to 3 dB in electrical SNR or 1.5 dB in optical SNR; as the SNR decreases, the maximum achievable SNR gain of improved receivers decreases monotonically to a non-zero low-SNR limit, corresponding to an information rate gain of 36.3%. For practically used constellations, we derive an upper bound on the gain of improved receivers. Numerical results demonstrate that the upper bound can be approached to within 1 dB in optical SNR by combining existing improved receivers and coded modulation. We also show that our information theoretic analyses can be extended to Flip-OFDM and PAM-DMT. Our results imply that, for the considered schemes, improved receivers may reduce the gap to channel capacity significantly at low-to-moderate SNR.

We demonstrate indoor parallel visible light communication (VLC) achieving 3 Gb/s date rate under distance 5 meters with transmission power 15.9 mW, 189.1 mW and 63.4 mW for red, green and blue LEDs, respectively. The original bandwidths for the adopted LEDs are approximately 10 MHz. A lens is employed at the transmitter side to concentrate the light. Due to the 5-meter distance, different single-color LEDs exhibit separated light spots at the receiver side, which creates parallel transmission paths with negligible inter-color interference. Analogue pre-equalization is adopted to extend the 3-dB bandwidth to approximately 300 MHz, and Volterra series-based digital post-equalizer is adopted to mitigate the nonlinearity and residue inter-symbol interference. In the experiments, on-off keying modulation is adopted due to low reception power at 5-meter distance, and the symbol rate of 1 G per second is adopted for each single-color LED. It is demonstrated that inter-color signal-interference ratio is above 20 dB, and the residue bit error rates of 1.82 × 10-4, 1.4 ×10-3and 1.3 × 10-3are achieved by red, green and blue LEDs, respectively, all below threshold 3.8 × 10-3for standard forward error correcting code with 7% redundancy.

This paper proposes a visible light positioning (VLP) scheme based on dual cameras. The mobile terminal adopts a single light emitting diode (LED) as a beacon, and the dual cameras are mounted on the ceiling with known three-dimensional (3D) coordinates and postures. After the LED is captured by the dual cameras, a vector positioning method is adopted to calculate the 3D coordinate of the LED. Moreover, a 3D positioning experiment is conducted in a space of 2×2×2 m3. The experimental results demonstrate that the proposed VLP scheme can achieve mean 3D positioning accuracy of 2.9 mm, and 90% of the positioning errors are within 4 mm.

In this paper, we establish an air-to-water (A2W) visible light communication (VLC) link adopting one green light emitting diode (LED) as the transmitter and one white LED as the ambient light source. The effect of the ambient light on the A2W-VLC link gain is investigated under calm and wavy water surface conditions. By transmitting a high-frequency sinusoid signal, we extract the variation of overall A2W-VLC link gain from the received signal in the presence of ambient light. The experimental results show that the strong ambient light can cause the saturation of photodiode (PD) and yield no communication possibility under calm water surface condition. Under wavy water surface condition, the effect of strong ambient light can be alleviated to some extent, yielding some communication possibilities.

We consider performance enhancement of asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM), which is a variation of OFDM in intensity modulated optical wireless communications. To improve the conventional ACO-OFDM receiver that utilizes only odd-indexed subcarriers, several specific designs have been proposed in the literature. Unlike these studies, this paper investigates information theoretic limits of all possible improved receivers. More specifically, we prove that the maximum achievable information rate of ACO-OFDM is the sum of (i) the information rate of the conventional receiver, and (ii) a single-letter conditional mutual information expression corresponding to the maximum achievable gain of improved receivers. Asymptotic analysis shows that the high-signal-to-noise-ratio (high-SNR) limit of the maximum achievable gain of arbitrary improved receivers is 3 dB in (electrical) SNR, corresponding to an information rate gain of exactly 1/4 bits per channel use. We also show that the low-SNR limit of the maximum achievable gain of improved receivers is approximately 1.35 dB in SNR. Numerical evaluation of finite-SNR gains demonstrate that improved ACO-OFDM receivers may reduce the gap to capacity significantly, especially at low-to-moderate SNR.

With the rapid development of cloud computing, the user-centric networks with the baseband unit pool have attracted a great deal of attentions in academic and industrial fields. However, limited fronthaul capacity and computation resource have become the bottlenecks inevitably. Thus, this paper investigates the power-efficient transmission in user-centric networks by considering both fronthaul capacity and computation resource constraints, where multiple access points (APs) and user equipments (UEs) are distributed. Specifically, a joint optimization of the beamforming vectors, AP-UE association strategy and transmission time is proposed to minimize the total power consumption (TPC). The formulated mixed integer non-linear problem (MINLP) is NP-hard. To address this problem, the MINLP is first transformed into a convex one via the successive convex approximation and semidefinite relaxation methods. Then, an iterative but effective algorithm is designed by using the property of the solution and applying the Lagrangian dual method. Simulation results show that the proposed algorithm converges rapidly and outperforms benchmark algorithms in terms of TPC.

Network coding is widely adopted in ad hoc networks to enhance the network throughput while few attention has been paid on applying it in device- to-device (D2D) networks. In this paper, we consider a relay-assisted D2D network with inter- session network coding, where multiple D2D users coexist in an underlaying manner. We aim to maximize the sum throughput via joint power and resource group allocation, which is formulated as a mixed integer programming problem. To solve this problem, an iterative algorithm with low complexity is accordingly proposed. Simulation results verify the effectiveness and superiority of the proposed algorithm compared with existing algorithms.

In this paper, we investigate the transceiver design for multiple-input multiple-output visible light communication (VLC) systems that employ the integer-forcing (IF) lattice decoding technique. To facilitate the joint design of the transmitter precoder, the integer matrix and the receiver equalizer, we first give a necessary and sufficient condition for the integer matrix to be invertible over 1-D lattice. Based on this condition, we then propose a new method for choosing the integer matrix which achieves better performance than the existing lattice reduction method. Moreover, taking into account two typical constraints in VLC, we optimize transmit and receive matrices using the conditional gradient method or the projected gradient method. Finally, the integer matrix and the transmit and receive matrices are jointly optimized in an iterative manner. Simulation results are provided to demonstrate the performance improvement achieved by the proposed framework.

Due to the interchannel interference and the varying active antennas in generalized spatial modulation (GSM), it is challenging to detect the transmitted data effectively while imposing low computational burden to the receiver. In this paper, we propose a novel algorithm based on the K-best sphere decoding for the soft detection of the GSM signal, which performs the breadth-first tree search in two stages sequentially. By exploiting the null space of the GSM channel, the inactive antenna searching is carried out antennawise in the first stage, in which a recursive algorithm for updating the QL decomposition (a decomposition of a matrix into a product of an orthogonal matrix and a lower triangular matrix) is proposed to calculate the branch metrics and an effective examination scheme is developed to prune those illegal or repetitive child nodes. Based on the available QL decomposition structure that has been updated recursively during the first stage, the second stage of the proposed detector is then readily concatenated to search for the modulated symbols carried on the active antennas in a one-by-one manner. Moreover, owing to the soft-input soft-output nature, the proposed detector can be applied in a turbo decoder for a coded GSM system. Both complexity analysis and simulation results corroborate the superiority of the proposed soft detector for the GSM system.

This paper presents an enhanced hybrid asymmetrically clipped optical orthogonal frequency division multiplexing (OFDM) scheme referred to as EHACO-OFDM. The scheme uses asymmetrically clipped optical OFDM (ACO-OFDM) on the odd subcarriers, pulse-amplitude-modulated discrete multitone modulation (PAM-DMT) on the imaginary parts of even subcarriers, and direct-current-biased optical OFDM (DCO-OFDM) on the real parts of even ones. Since the entire available bandwidth is utilized for data modulation, this scheme can achieve higher spectral efficiency than HACO-OFDM and ACO-OFDM. Moreover, as a smaller DC bias is introduced in our scheme, it is more power efficient than asymmetrically clipped DC-biased optical OFDM (ADO-OFDM) and DCO-OFDM. Simulation results show that EHACO-OFDM performs much better than ADO-OFDM and HACO-OFDM at high bit rates.

In this paper, we investigate energy-efficient hybrid precoding which consists of baseband precoding and radio frequency (RF) precoding for Millimeter Wave (mmWave) systems over multi-input multi-output (MIMO) interference channels. The considered optimization problem is intractable because both the object function and the constraint are non-convex. Instead of solving the multivariate optimization problem directly, we propose a near-optimal algorithm which includes two steps. First, the original problem is reformulated into an equivalent univariate optimization problem. Second, a near-optimal hybrid precoder is obtained via an orthogonal matching pursuit (OMP) based algorithm. Numerical results verify that the proposed hybrid precoding algorithm achieves a near-optimal EE performance. Moreover, the energy efficiency (EE) maximization algorithm outperforms the sum rate maximization algorithm in terms of the EE performance, especially at high transmit power.

In this paper, we consider the problem of maximizing the weighted sum energy efficiency (WS-EE) for multi-input single-output (MISO) interference channels (ICs), which are well acknowledged as general models of heterogeneous networks (HetNets), multicell networks, etc. To address this problem, we develop an efficient distributed beamforming algorithm based on a pricing mechanism. Specifically, we carefully introduce a price metric for distributed beamforming design, which fortunately allows efficient closed-form solutions to the per-user beam-vector optimization problem. The convergence of the distributed pricing-based beamforming design is theoretically proven. Furthermore, we present an implementation strategy of the proposed distributed algorithm with limited information exchange. Numerical results show that our algorithm converges much faster than existing algorithms, while yielding comparable, sometimes even better performance in terms of the WS-EE. Finally, by taking the backhaul power consumption into account, it is interesting to show that the proposed algorithm with limited information exchange achieves better WS-EE than the full information exchange-based algorithm in some special cases.

In this paper, we consider the problem of maximizing the weighted sum energy efficiency (WS-EE) of a multi-input single-output (MISO) ad hoc network. To solve this problem, we develop one low-complexity distributed beamforming algorithm based on pricing mechanism. Specifically, each node updates its price information and broadcasts it to all the other nodes. Having collected all the information, each node selects its beam-vector in closed-form with low computational complexity. The convergence of this algorithm is strictly proved. Compared with the existing two-layer optimization algorithm, our algorithm has lower computational complexity without performance loss. Simulation results show that the proposed algorithm performs slightly worse than the centralized algorithm, but requires much less information exchange overhead.

This paper exploits the noncentral communication architecture for increasing system throughput with popular video files cached in user equipments, and transmitted through D2D links under control of base station. A cell is divided into equilateral hexagon clusters among which frequency resources are reused. In order to reduce inter-cluster interference, we propose to limit the transmit power of each user and adopt relay-assisted D2D communication when direct D2D link can not be established in a cluster. Simulation results show that the proposed scheme can achieve higher spectral efficiency without much degradation in system throughput when compared with the scenario where different frequency resources are used by different clusters.

In this paper, we consider the problem of maximizing the energy efficiency (EE) for multiple-input-multiple-output (MIMO) interference channels (ICs), subject to the per-link power constraint. To avoid extensive information exchange among all links, the optimization problem is formulated as a noncooperative game, where each link maximizes its own EE. We show that this game always admits a Nash equilibrium (NE) and the sufficient condition for the uniqueness of the NE is derived for the case of large enough maximum transmit power constraint. To reach the NE of this game, we develop a totally distributed EE algorithm, in which each link updates its own transmit covariance matrix in a completely distributed and asynchronous way. Some players may update their solutions more frequently than others or even use the outdated interference information. The sufficient conditions that guarantee the global convergence of the proposed algorithm to the NE of the game have been given as well. We also study the impact of the circuit power consumption on the sum EE performance of the proposed algorithm in the case when the links are separated sufficiently far away. Moreover, the tradeoff between the sum EE and the sum spectral efficiency (SE) is investigated with the proposed algorithm under two special cases: 1) low transmit power constraint regime; and 2) high transmit power constraint regime. Finally, extensive simulations are conducted to evaluate the impact of various system parameters on the system performance.

We consider the problem of maximizing the weighted sum energy efficiency (WS-EE) in ad hoc networks. To solve this problem in a distributed manner, one novel distributed adaptive-pricing algorithm is developed based on limited information exchange among the nodes. Specifically, each node updates its current interference information and broadcasts it to the other nodes. Having collected all this information, each node can adjust its transmit power accordingly with simple arithmetical operations. Then iterate these two steps. This algorithm is strictly proven to be convergent and can attain the KKT optimality conditions of the problem. Moreover, an alternative centralized algorithm based on gradient projection method is proposed to serve as the performance benchmark. Simulation results show that the proposed distributed algorithm converges rapidly. Furthermore, this distributed algorithm performs as well as the centralized one and significantly outperforms the existing algorithm in terms of the WS-EE.