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.

Optical wireless communication (OWC) has been recognized as a promising solution for establishing high-speed, low-latency data connectivity between air and water. During the propagation of optical beams across dynamic water surfaces, random scattering and refraction can result in information leakage issues, significantly increasing the probability of an adversary listener (Willie) detecting the signal transmission. In this paper, we investigate the covert performance of bidirectional air-water cross-boundary OWC systems under the detection of arbitrary underwater/aerial Willie for the first time. To characterize the detection accuracy at Willie and communication quality between a legitimate transmitter (Alice) and a legitimate receiver (Bob), the covert performance metrics including the covertness outage probability and covert throughput are computed based on the theoretical derivations of channel gain under dynamic wave, surface refraction, and particle absorption and scattering. The numerical simulation reveals the variation patterns of covert performance metrics with respect to multiple environmental and system parameters, which shows that wave intensity and transmitted beam’s divergence angle have opposite effects on detection performance of Willie locating at different positions, and both the optical wavelength beyond blue-green band and strong ambient light can increase Willie’s detection uncertainty. Furthermore, it can be seen that the upper bound on covertness outage probability reaches 1 and the achievable covert throughput approaches 0 in some cases, while covert threat regions exist in both atmospheric and underwater areas within several meters, which indicates that covert transmission strategy and system parameters are required to be further designed and optimized to enhance communication security.

In this paper, we propose a novel computational method, termed Monte Carlo Integration with Common Volume Sampling (MCI-CVS), which enables direct photon sampling within the common scattering volume. This approach significantly accelerates the convergence of optical wireless scattering channel simulations. Although each sample in MCI-CVS incurs approximately 3.74 times the computational cost of conventional methods, numerical results for ultraviolet scattering channels demonstrate that it reduces the required number of samples by a factor of 1,000 compared with Monte Carlo Integration with Importance Sampling (MCI-IS). As a result, MCI-CVS achieves an overall efficiency gain of approximately 267.3 times faster than MCI-IS and about 1,206 times compared to the baseline Monte-Carlo Simulation (MCS) method. Furthermore, the proposed method effectively addresses inefficient photon path sampling in infrared scattering channels, where low scattering and absorption coefficients degrade sampling efficiency and thus result in slow convergence. By incorporating photon path reuse, MCI-CVS also enables fast generation of two-dimensional scattering fields on a standard personal computer.

Due to the high data rates, license-free operations and inherent security, visible light communication (VLC) is highly valued in Industrial Internet of Things (IIoT) applications. Short packet communication, which offers ultralow latency and efficient resource utilization, is suitable for the low-latency and massive connectivity demand of the IIoT system. It adopts finite blocklength codewords for data transmissions. In this article, we present a general framework of designing geometrically shaped constellations in short-packet VLC with peak and average intensity constraints for IIoT applications. By leveraging tools from large deviation theory, we first characterize the second-order asymptotics of the optimal constellation shaping region under aforementioned intensity constraints, which serves as a good performance metric for the best geometric shaping in finite blocklength. To further incorporate a sufficiently large coding gain and a nearly maximum shaping gain, we construct multidimensional constellations by the nested structure of Construction B lattices, where the constellation shaping is implemented by controlling the boundary of the embedded sublattice, i.e., a strategy called coarsely shaping and finely coding. Fast algorithms for constellation mapping and demodulation are presented as well. As an illustrative example, we present an energy-efficient 24-D constellation design based on the Leech lattice, whose superiority over existing constellation designs is verified by numerical results.

In this paper, we design and demonstrate a beam tracking system on the receiver side for ultraviolet (UV) communication using a single photomultiplier tube (PMT). Specifically, we investigate the properties of the angle-domain channel for UV communication. We characterize the effects of metallic housing and the spatial non-uniformity of the PMT on the angle-domain channel, both of which can significantly impact the channel’s performance. To adjust the orientation of the PMT and maintain satisfactory communication in mobile UV systems, we propose a beam alignment algorithm based on coordinate search and a beam tracking algorithm based on pattern search. A real-time system is developed to test the performance of these algorithms. Experimental results show that the proposed beam alignment algorithm can quickly search for the incident direction, while the beam tracking algorithm effectively tracks a moving transmitter at a rate of 5.5 degrees/sec, ensuring a reliable communication link. The proposed beam alignment and tracking scheme uses only a single PMT, making it lighter and more cost-effective compared to the PMT array scheme, and is thus well-suited for deployment in compact UV communication terminals.

To construct time slot synchronization for further resource management of ultraviolet (UV) networks, we propose a slot synchronization protocol based on flooding time and power optimization. Beacon sequence is applied to broadcast slot information without exchanging messages. Firstly, we propose slot synchronization schemes based on beacon sequence for two cases, with both multiple initialization nodes and a single initialization node. Then, we investigate the network synchronization accuracy with non-overlapping and overlapping beacons, as well as the synchronization accuracy of the entire network. Based on the network synchronization accuracy results, we propose power optimization to expand the synchronization range and enhance the synchronization probability. Numerical results show that power optimization can approximately achieve 18% higher synchronization success probability and 15% larger network synchronization range. Moreover, the proposed synchronization scheme is approximately 10 time periods shorter than meshed emergent firefly synchronization (MEMFIS), with approximately 40% lower network synchronization error.

Considering the scattering characteristics of wireless ultraviolet (UV) channels, the double-scatter assumption is revealed to well characterize the scattering-enhanced channel over a range of less than 100 m. Based on the cuboid scatterers with representative shapes, we present a ray-tracing algorithm under scattering enhancement conditions and give the probability density function of the free path in a non-uniform atmosphere. Then, we propose a semi-analytical (SA) link gain model with scattering enhancement, via generating a set of coordinates for a series of conical integral paths using a rotation matrix. On this basis, a one-dimensional integral expression for the single-scatter of the proposed SA link gain model is obtained, and the expression for the double-scatter is further derived. The link gain evaluation results show that the presence of scattering region can improve the received power of the non-line-of-sight UV system at a receiver. For all considered transceiver geometry, the results of the proposed SA model match well with the counterparts of Monte-Carlo (MC) based numerical calculation, with a gap of around 2 dB for both narrow and wide beam. In terms of computational efficiency, the execution time of the proposed SA model for the single-scatter and double-scatter is significantly reduced to 0.065% and 9.69% of those of the MC simulations, respectively. We further establish experiments using a fog-filled quartz cubic cavity in an outdoor environment to verify the correctness of the proposed model. The link gain from the numerical evaluation assuming double-scatter can improve the agreement with the experimental results by 1 dB compared with that of the single-scatter assumption.

Rydberg atomic sensors have been seen as novel radio frequency (RF) measurements and the high sensitivity to a large range of frequencies makes it attractive for communications reception. In high-speed symbol transmission scenarios, the effect of rising and falling edges as symbol switching should not be ignored. In this work, we adopt a mixed Gaussian distribution to characterize the output distribution under symbol switching, especially under higher signal power and shorter symbol duration according to experimental measurements. Under high symbol rate, unequal rising and falling edges on signal detection make the system nonlinear. Based on the experimental measurement, we characterize such nonlinear effects via a state transition model, and adopt Hidden Markov Model (HMM) to characterize the received signal. We propose a Monte-Carlo method to compute the achievable transmission rate and bit error rate (BER) performance. In real experiments, a lower BER can be achieved by the Viterbi algorithm compared with single symbol detection (SSD). Moreover, the performance of Viterbi decoding is better with higher symbol rate compared with SSD.

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.

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.

This work aims to optimize the synchronization sequence for optical wireless communication with a photoncounting receiver. We propose a “Sandwich” structure of the synchronization sequence, including two equilong RANDOM parts on both ends and a 1 − 0 alternate part between them. We investigate the synchronization error event and analyze its statistical characteristics up to the second order. Based on the second-order properties, we propose an upper bound on the synchronization mean-squared error (MSE) via Gaussian approximation. Furthermore, we optimize the synchronization sequence based on the upper bound, subject to the constraint that the probability of deviating even symbols is lower than a certain threshold. Numerical results on the correlation show the accuracy of statistical properties up to the second order, as well as the accuracy of Gaussian approximation. Moreover, it is numerically demonstrated that the optimized sequence obtained from the upper bound is close to that obtained from the real synchronization MSE. Compared with pure random 0 − 1 sequence, m-sequence, Gold sequence, and Barker-like sequence, the synchronization MSE can be significantly reduced.

We design a receiver assembling several photomultipliers (PMTs) as an array to increase the field of view (FOV) of the receiver and adapt to multiuser situation over non-line-of-sight (NLOS) ultraviolet (UV) channels. Channel estimation and signal detection have been investigated according to the space division characteristics of the structure. Firstly, we adopt balanced structure on the pilot matrix, analyze the channel estimation mean square error (MSE), and optimize the structure parameters. Then, with the estimated parameters, an analytical threshold detection rule is proposed as a preliminary work of multiuser detection. The detection rule can be optimized by analyzing the separability of two channel states based on Gaussian approximation of Poisson weighted sum. To assess the effect of imperfect estimation, the sensitivity analysis of channel estimation error on threshold detection performance is carried out. Based on the threshold detection rule, we propose a successive elimination method for on-off keying (OOK) modulated multiuser symbol detection. A tractable closed-form upper bound on the detection error rate is calculated, which turns out to be a good approximation of that of multiuser maximum-likelihood (ML) detection. The proposed successive elimination method has a much faster execution speed than the ML multiuser detection with negligible detection error rate degradation.

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.

Avalanche photodiodes (APD) are key devices in optical communication, and its nonlinear characteristics have a significant impact on the communication performance of the system. Based on the detection principle of photodetector, we characterize the nonlinearity of avalanche photodiodes, and demonstrate the theoretical prediction results in experiments. For the APD module integrated with trans-impedance amplifer (TIA), the nonlinear AC signal response caused by amplifier saturation is observed and explained. Based on the nonlinear characteristics of the photodetector, we can reasonably set the DC operation point and ensure the signal extraction in nonlinear region.

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.

The communication systems for Poisson channels require long pilot for channel estimation and may result in large percentage of overhead due to the signal-dependent noise of Poisson-distributed signal, i.e., both signal part and noise part experience random processes. In this paper, both blind and semi-blind channel estimation methods are studied to shorten the overhead and increase the transmission efficiency for Poisson channels. First, fractionally spaced equalizers are proposed based on modified constant modulus algorithm (CMA) and subspace (SS). Second, a data-aided iterative channel estimation (ICE) is designed and analyzed in terms of its asymptotic unbiasedness and convergence. The proposed methods are evaluated based on both a constant channel scenario and a varying channel scenario. Numerical simulation results show that the modified CMA has the worst bit-error rate performance but requires the lowest computational complexity. Besides, both SS based channel estimation and ICE have negligible overhead and comparable bit-error rate performances with respect to the conventional periodic pilot based channel estimation having 50% overhead.

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 propose a novel radio-frequency (RF) receiving architecture based on the microelectromechanical system (MEMS) and optical coherent detection module. The architecture converts the received electrical signal into mechanical vibration through the piezoelectric effect and adopts an optical detection module to detect the mechanical vibration. We analyze the response function of the piezoelectric film to an RF signal, the noise-limited sensitivity of the optical detection module, and the system transfer function in the frequency domain. A simple on–off keying (OOK) modulation with carrier frequency 1 GHz is used to numerically evaluate the detection sensitivity. The result shows that considering the main noise sources in system, the signal detection sensitivity can reach around −116.6 dBm at bandwidth 5 MHz. Such sensitivity significantly outperforms that of the currently deployed long-term evolution (LTE) system. Finally, we conduct preliminary experiments based on an off-the-shelf aluminium nitride piezoelectric film, which shows that the vibration amplitude based on the theoretical prediction can be achieved.

We investigate the achievable rate and capacity of a non-perfect photon-counting receiver assuming that the shot and thermal noise is negligible, and that the sampling interval is not shorter than the dead time. For long and fixed symbol duration, the achievable rate under on-off keying modulation is investigated based on Kullback-Leibler divergence and Chernoff divergence. We prove the tightness of the derived bounds for large peak power with zero background radiation at an exponential convergence rates, and for low peak power at an order-two convergence rates. Moreover, we propose an approximation on the achievable rate, which is more accurate compared with the derived bounds in the medium signal to noise ratio (SNR) regime. We also verify the accuracy of the proposed model on the achievable rate analysis via the comparison with the model with non-negligible shot and thermal noise. As for the capacity analysis, we assume that the symbol duration can be arbitrarily short, and demonstrate that the capacity approaches that of the continuous-time Poisson channel as both the sampling interval and the symbol duration approach zero, and the sampling interval equals the symbol duration. For large peak power, the capacity with a non-perfect receiver converges, while that of continuous Poisson capacity channel linearly increases.

We adopt artificial$\Lambda $-type three-level system with superconducting devices for microwave signal detection, where the signal intensity reaches the level of discrete photons instead of continuous waveform. Based on the state transition principles of the three-level system, we propose a statistical model for microwave signal detection. In addition, achievable transmission rates and signal detection based on the proposed statistical models are investigated for low temperature conditions in deep space communication scenario. It is predicted that high sensitivity can be achieved by the proposed system. We further characterize the received signal considering the saturation phenomenon of three-level system, which reveals negligible performance degradation caused by saturation under weak received power regime.

Synthetic aperture radar (SAR) automatic target recognition (ATR) plays an important role in SAR image interpretation. However, at least hundreds of training samples are usually required for each target type in the existing SAR ATR algorithms. In this article, a novel few-shot learning framework named hybrid inference network (HIN) is proposed to tackle the problem of SAR target recognition with only a few training samples. The recognition procedure of HIN consists of two main stages. In the first stage, an embedding network is utilized to map the SAR images into an embedding space. In the second stage, a hybrid inference strategy that combines the inductive inference and the transductive inference is adopted to classify the samples in the embedding space. In the inductive inference section, each sample is recognized independently according to a metric based on Euclidean distance. In the transductive inference section, all samples are recognized as a whole according to their manifold structures by label propagation. Finally, in the hybrid inference section, the classification result is obtained by combining the above two inference methods. To train the framework more effectively, a novel loss function named enhanced hybrid loss is proposed to constrain samples to gain better interclass separability in the embedding space. Experimental results on the moving and stationary target acquisition and recognition (MSTAR) benchmark data set illustrate that HIN performs well in few-shot SAR image classification.

It has been shown that long pilots should be employed in a Poisson channel with inter-symbol interference to obtain accurate channel estimate, which may reduce the transmission efficiency. In this paper, an iterative data-aided channel estimation and signal detection algorithm is proposed with significant shortened overhead but can approach the bit-error rate performance under perfect channel state information. In this algorithm, periodic pilots are not required and only one short pilot sequence is utilized to obtain a rough initial channel estimate, which will be strengthened and updated by iteratively employing the blockwise data symbols as additional pilots. It is proved that treating the detected data symbols as pilots is a biased estimation, where the bias is proportional to the bit-error rate. Numerical results show that the proposed approach employing only 20 initial pilots can approach the optimal bound. The same bound needs to be achieved by assigning at least 500 pilots using periodic pilot based channel estimation.

Consider directional antenna array in an ultraviolet (UV) scattering communication Ad-Hoc network. To efficiently obtain the link gain coverage of single antenna, we propose an algorithm based on one-dimensional (1D) numerical integration and an off-line data library. We also analyze the two-dimensional (2D) scattering intensity distribution, where numerical results show that the path loss profile can be well fitted by elliptic models. In addition, assume that the node distribution in Ad-Hoc network obeys Poisson point process (PPP). For both directional antenna array and omnidirectional antenna, adopting ellipses and circles to represent the coverage of nodes, we investigate the minimum power such that the network still keeps connected. Simulation results show that employing directional antenna array can effectively reduce the minimum power that guarantees coverage.

Strong correlation of link gains over multiple wavelengths has been demonstrated in non-line-of-sight optical wireless scattering communication. In this paper, we further demonstrate the strong correlation of the channel impulse responses for different wavelengths. A novel channel estimation method is proposed based on such correlation, where only certain reference wavelengths are assigned with pilot signals. On each channel assigned by the pilot signal, we adopt an improved least-squares method to estimate the channel tap coefficients and the tap length. The channel tap coefficients for other wavelengths can be estimated based on the inter-wavelength correlation. Under the proposed channel estimation, the channel estimation accuracy can be improved as shown by the simulations. We further propose an optimization method for the correlation-based channel estimation pilot pattern based on the receiver channel state feedback. Simulation results show that optimized pilot pattern can achieve significant performance gain over the fixed pilot pattern. Moreover, the channel estimation performance is insensitive to the channel state feedback frequency.

Compressive sensing (CS)-based image coding scheme has been enthusiastically studied, but it still has a poor rate-distortion performance compared with the traditional image coding techniques. In this paper, we propose a CS multi-layer residual coding scheme to rectify this problem to a certain extent. By dividing CS measurements into multi-layers and predicting a particular layer's measurements with all its preceding layers' measurements, we can transform CS measurements into multi-layer residual coefficients, which are easier to compress. By calculating the residual between the quantized ground-truth CS measurements and their corresponding quantized inference measurements and using Huffman coding to associate each residual quantization index with a binary code, we can reduce the redundancies among CS measurements efficiently. Besides, the prediction and quantization process is designed to be layer-independent, which can save much of the encoding time. The proposed approach introduces a novel framework for using CS in the compression domain. The experimental results show that the proposed scheme can significantly outperform JPEG2000 and approach or reach the performance of HEVC-Intra on some test images.

None-line-of-sight (NLOS) optical wireless scattering communication over broad spectra can further increase the transmission rate beyond single wavelength scattering communication. A fundamental question is to investigate the link gain correlation over multiple wavelengths, which may lead to more efficient communication design and optimization approaches. We investigate the link gain correlation over multiple wavelengths under varying transmission media environments and transceiver geometries, and show that the link gain vector can be represented in a reduced dimensional space using principle components analysis (PCA). We also justify such correlation properties theoretically through a simplified single-scattering model. The link gain prediction is formulated as a joint continuous and discrete optimization problem, and is transformed into tractable forms. Linear prediction approaches are proposed based on zero-forcing (ZF) and linear minimum mean square error (LMMSE) criteria. It is seen that the LMMSE-based approach generally leads to lower prediction error compared with the ZF-based one, especially for the scenario with large residual eigenvalues of the link gain covariance matrix. We also investigate the link rates on multiple wavelengths, which also exhibit strong correlation for negligible background radiation intensity.

We propose content compression coding for massive users, where correlated contents of grouped users can be utilized to achieve high compression efficiency. We divide the users into several groups. For each group, we select the group head, encode the information of group head independently, and encode other users referring to the group head. We formulate the grouping optimization problem, transform it to a zero-one discrete optimization problem as well as a subset selection problem, and provide upper and lower bounds on the compression ratio. We investigate the sufficient condition and necessary condition on the optimality of one user group. We also investigate two specific type of sources, with star-type and chain-type user statistics, and provide the optimal user grouping. The compression ratio is also studied from both theoretical and experimental perspectives.

We characterize practical optical signal receiver in a wide range of signal intensity for optical wireless communication, from discrete pulse regime to continuous waveform regime. We first propose a statistical non-linear model based on photomultiplier tube (PMT) multi-stage amplification and Poisson channel, and then derive the optimal and tractable suboptimal duty cycle with peak-power constraint and average-power constraint for on-off key (OOK) modulation in the linear regime. Subsequently, a threshold-based classifier is proposed to distinguish three PMT working regimes based on the non-linear model. The performance of photon counting detection with optimal threshold for different sampling rates is evaluated from both theoretical and numerical perspectives. The proposed model can be adopted to select the optimal duty cycle and analyze the detection performance of PMT output signal in real applications.

Strong correlation of link gains over multiple wavelengths has been demonstrated in non-line-of-sight optical wireless scattering communication. In this paper, we further demonstrate that the channel impulse responses for different wavelengths are also strongly correlated. Thus, a novel channel estimation method based on such correlation over multiple wavelengths is proposed, where only certain reference wavelengths are assigned pilot signal. On each channel with assigned pilot signal, we adopt an improved least-squares channel estimation method to estimate the tap parameters and the tap length (or the number of taps). The channel tap coefficients on other wavelengths can be estimated based on the correlation. Under the proposed channel estimation, the channel estimation accuracy can be improved as shown by the simulations. Moreover, under the same total channel estimation overhead, the accuracy of the proposed channel estimation first increases and then decreases with the number of reference wavelengths.

In this paper, interference cancellation with multilayer coding and constrained partial group decoder (CPGD) is adopted for visible light communication (VLC) with a light emitting diode (LED) array. To reduce the online computational complexity, a decoding map on the decoding orders and the associated rate allocations is constructed via solving the max-min fairness problem based on the derived achievable rate of a VLC multiple access channel. A classification-based algorithm is proposed to reduce the decoding map size. Finally, the system throughput is further enhanced via solving the transmitter-user association problem followed by an iterative update of rate allocation and decoding order. From the numerical results, symmetric decoding map is observed for the squared transmitting LED array.

Power allocation for optical wireless scattering communication over broad spectra is studied based on link gain correlation. We formulate two types of power allocation problems that aim to maximize the transmission rate and to minimize the linear minimum mean square error of detection. Two scenarios of power allocation problems are considered where the parallel channels are aggregated for single-user transmission and separated for different users. We solve the power optimization problems and show that the power allocation remains fixed if the link gain ratios are uniform over the broad spectra. Furthermore investigations show that for single-user transmission optimization, under imperfectly uniform link gain ratios, the optimal power allocation for one link gain realization performs close to optimality for other link gain realizations. Numerical results show that for the single-user transmission with parallel channels, fixed power allocation incurs negligible losses in terms of transmission rate, and signal distortion. Such a finding can significantly reduce the amount of channel feedback for single-user transmission optimization. However, for the multi-user transmission, these performance losses are noticeable.

We consider a centralized caching network, where a server serves several groups of users, each having a common shared homogeneous fixed-size cache and requesting arbitrary multiple files. An existing coded prefetching scheme is employed where each file is broken into multiple fragments and each cache stores multiple coded packets, each formed by XORing fragments from different files. For such a system, we propose an efficient file delivery scheme by the server to meet the arbitrary multi-requests of all user-groups. Specifically, the stored coded packets of each cache are classified into four types based on the composition of the file fragments encoded. A delivery strategy is developed, which separately delivers a part of each packet type first, and then combinatorially delivers the remaining different packet types in the last stage. The rate, as well as the worst rate of the proposed delivery scheme, are analyzed. We show that our caching model and delivery scheme can incorporate some existing coded caching schemes as special cases. Moreover, for the special case of uniform requests and uncoded prefetching, we make a comparison with existing results, and show that our approach can achieve a lower delivery rate. We also provide numerical results on the delivery rate for the proposed scheme.

Consider a fixed orientation multi-user visible light communication (VLC) system with multi-color light emitting diodes (LEDs) where each transmitter serves a user with limited coordination impeding transmitter-side beamforming. In this paper, a multi-layer coding and constrained partial group decoding (CPGD) method is proposed to tackle strong color interference and increase the system throughput. After channel model formulation, user information rates are allocated and decoding order for all the received data layers is obtained by solving a max-min fairness problem using a greedy algorithm. An achievable rate is derived under the truncated Gaussian input distribution. To reduce the decoding complexity, a map on the decoding order and rate allocation is constructed for all positions of interest on the receiver plane parallel to the transmitter plane, and its size is reduced by a clustering algorithm. Meanwhile, the symmetrical geometry of LED arrays is exploited. Finally, the transmitter-user association problem is formulated and solved by a genetic algorithm. It is observed that the system throughput increases as the receivers are slightly misaligned with corresponding LED arrays due to the reduced interference level, but decreases afterwards due to the weakened link gain.

We characterize the practical photon-counting receiver in optical scattering communication with a finite-sampling rate and electrical noise. Finite-small pulse width incurs dead time effect that may lead to sub-Poisson distribution on the recorded pulses. We analyze the first-order and second-order moments on the number of recorded pulses with a finite-sampling rate at the receiver side under two cases, where the sampling period is shorter than or equal to the pulse width as well as longer than the pulse width. Moreover, we adopt the maximum likelihood (ML) detection. In order to simplify the analysis, we adopt binomial distribution approximation on the number of recorded pulses in each slot. A tractable holding time and decision threshold selection rule is provided to maximize the minimal Kullback-Leibler (KL) distance between the two distributions. The performances of proposed sub-Poisson distribution and the binomial approximation are verified by the experimental results. The equivalent photon arrival rate and pulse holding time predicted by the sub-Poisson model and the associated proposed binomial distribution are well validated by the simulation results, where the finite-sampling rate and electrical noise are considered. The proposed holding time and decision threshold selection rule performs close to the optimal ML receiver.

We investigate the upper and lower bounds on the quantization distortions for independent and identically distributed continuous sources in the finite block-length regime. We derive a lower bound on the quantization distortion, as well as an upper bound on the quantization distortion based on random quantization codebooks. Moreover, we apply the obtained bounds to the continuous Gaussian source. For the Gaussian source, we propose a computationally tractable method to numerically compute the upper and lower bounds, for both bounded and unbounded quantization codebooks. Numerical results show that the gap between the upper and lower bounds is small for the block length of several hundreds, and the upper and lower bounds are tighter than those proposed in the existing works.

We characterize practical optical signal receiver in a wide range of signal intensity for optical wireless communication, from discrete pulse regime to continuous waveform regime. We first propose a statistical non-linear model based on the photomultiplier tube (PMT) multi-stage amplification and Poisson channel, and then derive the optimal and tractable suboptimal duty cycle with peak-power and average-power constraints for on-off key (OOK) modulation in the linear regime. Subsequently, a threshold-based classifier is proposed to distinguish the PMT working regimes based on the non-linear model. Moreover, the performance of mean power detection and photon counting detection under maximum likelihood (ML) criterion for the sampling interval shorter than dead time is evaluated from both theoretical and numerical perspectives. The proposed model can be adopted to select the optimal duty cycle and analyze the detection performance of PMT output signal in real applications.

None-line-of-sight (NLOS) optical wireless scattering communication over broad spectra can further increase the transmission rate beyond single wavelength scattering communication. A fundamental question is to investigate the link gain correlation among the multiple wavelengths, which may lead to more efficient communication design and optimization approaches. We investigate the correlation between path losses over the wavelengths under varying atmospheric and transmission geometry parameters, and show that the random link gain vector can be represented in a reduced dimensional space using principle components analysis (PCA). We also justify such correlation properties theoretically through a simplified single-scattering model. We predict path losses for all wavelengths using samples of only some wavelengths. It is seen that for the scenarios with 41 wavelengths under consideration, three reference wavelengths suffice to minimize the prediction distortion while four reference wavelengths may lead to an over-fitting.

Sparse code multiple access (SCMA) is attracting significant research interests currently, which is considered as a promising multiple access technique for 5G systems. It serves as a good candidate for the future communication network with massive nodes due to its capability of handling user overloading. Introducing SCMA to visible light communication (VLC) can provide another opportunity on design of transmission protocols for the communication network with massive nodes due to the limited communication range of VLC, which reduces the interference intensity. However, when applying SCMA in VLC systems, we need to modify the SCMA codebook to accommodate the real and positive signal requirement for VLC.We apply multi-dimensional constellation design methods to SCMA codebook. To reduce the design complexity, we also propose a symmetric codebook design. For all the proposed design approaches, the minimum Euclidean distance aims to be maximized. Our symmetric codebook design can reduce design and detection complexity simultaneously. Simulation results show that our design implies fast convergence with respect to the number of iterations, and outperforms the design that simply modifies the existing approaches to VLC signal requirements.

Due to the extremely large path loss of non-line of sight optical wireless communication, the received signal exhibits the characteristics of discrete photoelectrons. In this paper, we investigate the achievable rates and signal detection of on-off keying modulation for discrete-time Poisson multiple access channel . Both non-orthogonal multiple access and code division multiple access are considered, for both single-rate and multi-rate multiple access channels. Maximum likelihood (ML) detections are adopted as the optimal criterion for signal detection, whose complexity grows exponentially with the number of users. Low computational complexity detections are proposed with M-order correction that can perform closely to the ML detection. Numerical and simulation results are presented to show the achievable rate regions, optimal power allocations, the performance of ML, and proposed low-complexity detections. Finally, the experimental results of the proposed detection approaches agree with the simulation results.

We consider the secrecy rate for the multiple-input single-output optical wireless scattering communication, where the detected signal can be characterized by discrete photoelectrons. Assuming a legitimate receiver and an eavesdropper, we propose two secure communication protocols, the non-jamming protocol, and the cooperative jamming protocol, where for the former the transmitter does not send jamming signals but for the latter the transmitter sends jamming signals. We formulate the corresponding power and duty cycle optimization problems for both protocols. We also analyze the properties on the optimal solution, and adopt the alternating optimization to obtain a tractable solution. Moreover, we analyze the condition on the positive secrecy rate for the case where all the sub-channels to the receiver are weaker than their counterparts to the eavesdropper, where the secrecy transmission cannot be realized via a non-jamming protocol. The performance of the proposed two protocols is evaluated by numerical results. It is seen that for the iterative optimization solution, two to three iterations suffice for the convergence of the secrecy rate.

Image transmission in a wireless visual sensor network (WVSN) with limited resources over an unreliable and bandwidth-limited wireless channel is challenging. This paper presents a highly efficient and robust image coding and transmission scheme with a simple encoder based on compressive sensing (CS) for WVSNs. First, an image measurement based on scrambled block compressive sampling with a separable sensing operator is proposed to simplify the encoder. Second, a progressive nonuniform quantization, which exploits the measurement distribution at the encoder side and the measurement dependencies at the decoder side, is designed to improve the rate-distortion (R-D) performance while maintaining low complexity at the encoder. Third, to further improve the R-D performance, a progressive non-local low-rank reconstruction is designed at the decoder. The experimental results show that the proposed scheme can achieve higher R-D performance compared with the benchmark CS-based image coding and transmission schemes. Higher robustness can be achieved compared with the traditional source-channel coding, such as Consultative Committee for Space Data Systems$-$Image Data Compression (CCSDS-IDC) with Raptor codes under a time-varying packet loss channel, and the encoding time can be significantly reduced compared with the traditional image coding schemes. The experimental results also show that the proposed scheme achieves state-of-the-art coding efficiency with lower computational complexity at the encoder while still supporting error resilience.

Due to the scattering events in the ultra-violet (UV) spectrum, UV optical wireless communication enables transmission from a single transmitter to multiple receivers, which forms a broadcast channel. In this paper, we address the transmission protocols for such a broadcast channel. Based on the degradeness of such a broadcast channel, we propose time-dilation and time-shrinking superposition transmission. For the former, the duration of the base-layer symbols is shorter than that of the enhancement-layer symbols; while for the latter, the duration of the base-layer symbols is longer. We propose the signal detection of the two types of superposition transmission protocols, analyze the detection error probability and the associated upper bounds, and formulate the problems of power allocation among the signal layers. The numerical results on the sum power obtained from the exact detection error probability and the upper bounds are provided. It is shown that for the superposition with two signal layers, the sum power based on the upper bounds can be close to that based on the exact detection error probability.

We address full-duplex relaying for optical wireless communication with stochastic and non-negligible self-interference at the relay node with on-off keying modulation. Assume a relay communication system with a source node, a relaying node, and a destination node, where there is no interference at the source and destination nodes, since they merely transmit or receive. We propose two full-duplex relay protocols, namely, the detect-and-forward relaying protocol and the decode-and-forward relay protocol. For the former, the relay node performs the symbol detection and forwards the detected symbols to the destination node, and for the latter, the relay node performs block decoding and forwards the re-encoded block to the destination node. We prove that for sufficiently high self-interference intensity, the achievable transmission rate of the full-duplex relay system can be lower than that of the half-duplex relay system without self-interference for the detect-and-forward relay protocol. We also prove that for the decode-and-forward relay protocol, the full-duplex relay system always has a larger transmission rate compared with the half-duplex relay system. This can be justified by the fact that, for the decode-and-forward protocol, the source-relay link adopts two codebooks in the scenarios with and without self-interference. The performance of the two proposed protocols is evaluated by the numerical results.

We address the matching-based communication system optimization over parallel channels in wide spectrum. The dimension of the logarithm channel link gain vector can be reduced significantly under certain line-of-sight millimeter wave and optical wireless communication scenarios. We design the quantization points in the reduced dimensional space and store the optimal scheduling policy corresponding to each quantization point. We first investigate the covariance of the achievable rates of the parallel channels and the system outage probability. Then, we formulate a system optimization framework via matching the link gains of parallel channels to the quantization points, and obtain the sufficient conditions such that the utility loss of employing the quantization-based solution can be sufficiently small under sufficiently small quantization distortion in the reduced dimensional space. Such sufficient conditions can be satisfied for certain common optimization examples. We also address the matching-based communication system design under noisy channel estimation via providing a probabilistic upper bound on the utility loss in terms of the quantization design and channel estimation pilot. It can be shown that with sufficiently large probability the utility loss due to the quantization can be sufficiently small, if the quantization distortion is sufficiently small and the pilot energy is sufficiently large.

In optical wireless scattering communication, the received signal in each symbol interval is captured by a photomultiplier tube and then sampled through very short but finite interval sampling. The resulting samples form a signal vector for symbol detection. The upper and lower bounds on transmission rate of such a processing system are studied. It is shown that the gap between two bounds approaches zero as the thermal noise and shot noise variances approach zero. In order to reduce the computational cost in maximum a posteriori (MAP) receiver, the threshold-based signal detection is also studied, where two threshold selection rules are proposed based on the detection error probability and the Kullback-Leibler (KL) distance. For the latter, it is shown that the KL distance is not sensitive to the threshold selection for small shot and thermal noise variances, and thus, the threshold can be selected among a wide range without significant loss from the optimal KL distance. The performances of the transmission rate bounds, the signal detection, and the threshold selection approaches are evaluated by the numerical results.

Due to the extremely large path loss of non-line of sight optical wireless communication, the received signal exhibits the characteristics of discrete photons. In this work we investigate the signal detection and achievable rates of OOK modulation for discrete Poisson multiple access channel (MAC), for both code division multiple access (CDMA) and non-orthogonal multiple access (NOMA). For the signal detection, we adopt the maximum likelihood (ML) criterion for the optimal detection, whose complexity grows exponentially with the number of users. Low computational complexity detection is proposed with ℳ-order correction algorithm that can perform closely to ML detection. Moreover, we address the achievable rates and rationalize the power allocation for discrete Poisson MAC for both NOMA and CDMA. Numerical and simulation results are presented to show the performance of the proposed detection, the achievable rate region as well as the optimal power allocation of two-user discrete Poisson NOMA and CDMA.

We characterize the practical photon-counting receiver in optical scattering communication with finite sampling rate and electrical noise. In the receiver side, the detected signal can be characterized as a series of pulses generated by photon-multiplier tube (PMT) detector and held by the pulse-holding circuit. Those pulses are then sampled by the analog-to-digital convertor (ADC) with finite sampling rate and counted by a rising-edge pulse detector. However, finite small pulse width incurs dead time effect that may lead to sub-Poisson distribution on the recorded pulses. We analyze the first-order and second-order moments on the number of recorded pulses with finite sampling rate and existing shot noise at the receiver side. In order to simplify the further analysis on the error probability, we adopt binomial distribution approximation on the number of recorded pulses in each slot. A tractable holding time and decision threshold selection rule is provided aiming to maximize the minimal Kullback-Leibler (KL) distance between the two distributions. The performances of the proposed sub-Poisson distribution and the binomial approximation are verified by the experimental results. The equivalent arrival rate and holding time predicted by the sub-Poisson model are validated by the simulation results. Besides, the numerical results show that the proposed holding time and decision threshold selection rule performs close to the optimal one.

Organic light emitting diodes (OLEDs) show promising applications due to their unique advantages compared with conventional commercial LEDs. In OLED-base visible light communication (VLC), the nonlinearity may severely degrade the system performance, especially for modulation format with high peak to average power ratio (PAPR) such as orthogonal frequency-division multiplexing (OFDM). We experimentally analyze the OLED nonlinearity by Volterra series and apply the Volterra-based nonlinear equalizer to demodulate the signal. It is shown that the proposed system achieves transmission distance of 3 meters, which exceeds the longest reported in the organic VLC literature. It can mitigate both inter-symbol interference (ISI) and nonlinearity more effectively than other equalizers. Experimental results at different distance and driving voltage are also reported.

In this paper, we investigate the problem of the joint transceiver and offset design for point-to-point multiple-input- multiple-output (MIMO) and multiple user multiple-input- single-output visible light communication (VLC) systems. Both uplink and downlink multi-user scenarios are considered. The shot noise induced by the incoming signals is considered, leading to a more realistic MIMO VLC channel model. Under key lighting constraints, we formulate non-convex optimization problems aimed at minimizing the sum mean squared error. To optimize the transceiver and the offset jointly, a gradient projection-based procedure is resorted to. When only imperfect channel state information is available, a semidefinite programming-based scheme is proposed to obtain robust transceiver and offset. The proposed method is shown to non-trivially outperform the conventional scaled zero forcing and singular. value decomposition-based equalization methods. The robust scheme works particularly well when the signal is much stronger than the noise.

In this paper, we investigate the problem of the joint transceiver and offset design (JTOD) for the multiple-input-multiple-output (MIMO) multi-color visible light communication (VLC) systems. The shot noise induced by the incoming signals is considered, leading to a new MIMO VLC channel model. Under key lighting constraints, we formulate non-convex optimization problems aiming at minimizing the sum mean squared error by optimizing the precoder, the post-equalizer and the offset jointly. To tackle the associated non-convex optimization problems, an alternative way is resorted to decouple the original problem into convex optimization procedures. Numerical results show that our methods significantly outperform the conventional scaled zero forcing (ZF) and singular value decomposition (SVD) based transceiver design methods for all cases where channel blockage, or channel correlation or/and color cross-talks is/are taken into consideration.

This paper addresses the analysis and design of amplitude modulation in optical wireless scattering communication with Poisson channel and shot noise. We show that the detection error probability of the receiver with photoelectric conversion and post-processing circuits cannot be lower than that of the photon-counting receiver. We also provide an upper bound on the error detection probability, based on the Chebyshev's inequality. According to the proposed upper bound, we provide the modulation signal constellation, with equal space between the squared roots of the signal power levels, instead of between the signal power levels. We prove that the upper bound can approach zero as the transmission power approaches infinity. In numerical evaluations, under the same peak power constraint, the proposed signal constellation shows smaller detection error probability and larger capacity compared with the signal constellation with equal space between the signal power levels. Numerical results also show that the modulation with 4 power levels suffice except for high transmission power and small path loss in the optical wireless scattering communication.

Channel estimation is conceived for optical wireless scattering channels associated with Laser Diode transmitters and photon-counting/photomultiplier tube receivers. The proposed channel estimation approach consists of two stages, namely the estimation of the channel tap second-order moments followed by the estimation of the channel taps based on the estimate of second-order moments. In the first stage, we provide the general framework of the moment estimation complemented by the conception of an estimation approach based on a sparse pilot structure, as well as by the analysis of the estimation error. In the second stage, we conceive the channel tap estimation based on the eigenvalue decomposition of the matrix of estimated second-order moments, and analyze the associated performance. It is shown that as the length of the pilot sequence approaches infinity, the probability of having an estimation distortion above a threshold can be arbitrarily small. Simulation results show that a sparse pilot sequence can lead to a smaller estimation error than its counterpart using random 0-1 bits.

We consider the power allocation of optical wireless scattering communication over multi-wavelength parallel channels. We formulate the power allocation problems that aim to maximize the transmission rate or minimize the linear minimum mean square error (LMMSE) distortion. We consider two scenarios, where the parallel channels are aggregated for single-user transmission or separated for different users. For the latter, we resort to a fairness criterion to maximize the minimum transmission rate or minimize the maximum estimation distortion. The power allocation for both photon-counting and photomultiplier tubes (PMT) receivers are taken into consideration. Numerical and simulation results show the significant performance gain of the proposed optimal power allocation over equal power allocation among the parallel channels, which is typically more than 1dB and can reach up to 2dB.

Channel estimation is conceived for optical wireless scattering channels associated with laser diode transmitters and photon-counting/photomultiplier tube receivers. The proposed channel estimation approach consists of two stages, namely, of the estimation of the channel tap second-order moments followed by the estimation of the channel taps based on the estimate of second-order moments. In the first stage, we provide the general framework of the moment estimation complemented by the conception of an estimation approach based on a sparse pilot structure, as well as by the analysis of the estimation error. We also propose a novel sparse pilot design as well as the associated low-complexity channel estimation, and prove the optimality of the proposed channel estimation. In the second stage, we conceive the associated channel tap estimation based on the eigenvalue decomposition of the matrix of estimated second-order moments, and analyze the associated performance. It is shown that as the length of the pilot sequence tends to infinity, the probability of having an estimation distortion above a certain threshold can be reduced arbitrarily small. Simulation results show that the proposed sparse pilot sequence can lead to a smaller estimation error than the pilot design using random 0-1 bits.

Effective interference management in the multiuser interference channel necessitates that the users form their transmission and interference management decisions in coordination, and adapt them to the state of the channel. Establishing such coordination, often facilitated through information exchange, is prohibitive in fast-varying channels, especially when the network size grows. This paper focuses on the multiuser Gaussian interference channel and offers a receiver-centric approach to interference management. In this approach, the transmitters deploy rate-splitting and superposition coding to generate their messages according to independent Gaussian codebooks. The receivers can freely decode any arbitrary set of interfering messages along with their designated messages in any desired joint or ordered fashion, and treat the rest of the interferers as Gaussian noise. The proposed receiver-centric interference management approach is applied to two class of problems (outage optimization and fairness-constrained rate allocation), and constructive proofs are provided to establish the following properties for the proposed approach: 1) the optimal set of codebooks to be decoded by each receiver is a local decision made by each receiver based on its local channel state information (CSI); 2) the globally optimal transmission rates are related to locally optimal rates computed by the receivers based on their local information, which implies that the transmitters do not require explicit knowledge of the CSI and can determine their rates via limited feedback from the receivers; and 3) obtaining the optimal interference management strategy at each receiver has controlled complexity.

Visible-light communication (VLC) will be deployed in various indoor environments, including those requiring different lighting colors, which imposes the needs for transmission optimization under certain lighting constraints. This work establishes a framework for the transmission power and rate optimization, for light emitting diode (LED)-based VLC systems. We describe the system model of LED-based color-division VLC system, which incorporates the transmission power spectral, the receiving optical filters, the human eye perception, and the lighting constraints. Then we formulate the corresponding power and rate optimization problems, for both point-to-point communication system and broadcast communication system. More specifically, for a point-to-point communication system, it maximizes the transmission rate subject to the transmission power constraint and the lighting constraint; and for a broadcast system, it minimizes the total transmission power subject to some quality of service (QoS) constraint across all receivers. It is observed that the optimal symbol modulation power allocation scheme significantly outperforms the equal power allocation scheme.

Due to the unlicensed spectrum nature of the optical wireless communication, for a transceiver pair, the receiver needs to periodically sense the communication media, aiming to better schedule its transmission. This work proposes two detection rules on whether there are optical signals falling into the receiver field of view within certain time slot(s), based on the sum number of received photons over all sensing slots as well as the number of received photons in each slot. Considering that the data symbols are unknown, we adopt generalized likelihood ratio test (GLRT), which is known to be optimal for detection with unknown parameters based on finite samples. We also provide asymptotic results on the miss detection probability.

This work introduces the wavelet packet division multiplexing (WPDM) to visible-light communication (VLC) system. We provide the transceiver system model, as well as the associated optimal waveform design in conjunction with light emitting diodes (LEDs). The proposed joint design can be completed in an off-line manner. Numerical results show the performance gain of the WPDM over the OFDM, in terms of more robustness to LED nonlinearity and channel dispersion. Moreover, the optimized waveform shows performance gain over the conventional wavelet function, e.g., Daubechies wavelet functions. Such an advantage makes WPDM attractive for future VLC systems.

We propose a count-and-forward relay framework for non-line of sight optical wireless relay communications. Both the relay and the destination employ an ideal photon counting receiver. The relay forwards on-off keying (OOK) modulated optical signals using discrete power levels corresponding to the number of received photons, and the destination employs maximum-likelihood detection. We formulate the forwarding power optimization problem that aims to minimize the destination detection error probability given the relay forwarding power budget. We pursue exact optimal solutions to two special cases. To solve the non-convex forwarding power optimization problem in general, we transform to a piecewise exponential approximation problem. Numerical results show that the proposed scheme outperforms the relay hard-decision forwarding scheme, and can approach the optimal power allocation performance based on simulated annealing.

A statistical channel model for the non-line of sight (NLOS) optical wireless scattering communication link is fundamental for developing further receiver-side digital signal processing techniques. We adopt a linear time-invariant (LTI) Poisson channel modeling method, which was originally proposed for molecular communication, to model the NLOS optical wireless scattering communication. In such a model, we characterize stochastic inter-symbol interference (ISI) based on a probabilistic delay profile. We derive the first- and second-order statistics for the received signals. Moreover, we propose channel estimation schemes to estimate the delay profile, based on the least-squares and correlation criteria, and analyze the performance of the proposed channel estimation schemes. Finally, we propose and compare two signal detection schemes, namely the simple linear minimum mean square error (LMMSE) receiver treating ISI as noise and the maximum-likelihood sequence detection (MLSD) scheme that takes ISI into account. It is shown that MLSD significantly outperforms LMMSE under channel ISI. The proposed ISI channel model and the associated channel parameter estimation schemes serve as a theoretical foundation for high symbol rate with high transmission power in optical wireless scattering communication.

We propose linear minimum mean square error (LMMSE) receivers for optical wireless scattering communications with single transmitter and multiple receivers, namely the single-input multiple-output (SIMO) system, where the received signals exhibit the characteristics of discrete photons. We first propose an LMMSE receiver scheme based on photon-counting detection, and then generalize to other type of receivers with amplification gains such as photomultiplier tube (PMT) and avalanche photodetector (APD). The proposed LMMSE receiver is designed for both on-off key (OOK) modulation and pulse-position modulation (PPM), and estimated bit mean square error performance is analyzed in detail. Simulation results are provided to show the bit estimation performance of the proposed LMMSE receivers. It is concluded that OOK incurs larger estimation error at smaller required bandwidth, while M-PPM with M > 4 leads to smaller estimation error with more bandwidth requirements.

A statistical channel model for the non-line of sight (NLOS) scattering optical wireless communication link, is fundamental for developing further receiver-side digital signal processing techniques. Assume an optical source emits energy units, which produces transmitted particles experiencing independent random delays. This is termed as particle stream channel (PSC) in this work, characterizing stochastic inter-symbol interference based on a delay profile. For signal detection, we derive the first- and second-order moment statistics for the received signals. We propose channel estimation schemes to estimate the channel parameters, based on the least-squares and correlation criteria. Numerical results show that the channel estimation work well for the particle stream channel.

We consider a point-to-point delay-stringent video communication system. To reduce the fluctuation of the coded data packet sizes for efficient wireless transmission, we adopt the intra-refreshment and adaptive slice partitioning for video encoding. We model the delay-constrained video transmission with the proposed video coding and slicing scheme using an MDP model, and compute the optimal coding and modulation scheme corresponding to each channel state and the encoding and transmission delay using the value iteration algorithm. Extensive simulations based on real videos show that the MDP-based transmission scheme significantly reduces the transmission cost and enhances the quality of the reconstructed video sequences compared with the non-adaptive transmission scheme, which transmits constant number of transmitted symbols for all slices. Simulation results also show that the intra-refreshment and adaptive slice partitioning significantly enhances the quality of the reconstructed video sequences compared with the video coding without intra-refreshment.

This paper proposes group decoding and analyzes the associated rate allocation schemes for the relay interference channel where multiple relays assist the transmissions from the sources to destinations. All the relays and destinations employ an advanced decoding strategy called constrained group decoding, where the desired messages are decoded jointly with some interferers' messages when doing so is beneficial. This paper considers two types of relay systems, the hopping relay system with no direct source-destination links, and the inband relay system with direct source-destination links. For each relay type, the objective is to design the relay assignment and the group decoding strategies at the relays and destinations, in order to maximize the minimum information rate among all source-destination pairs. For hopping relays with pre-specified relay assignments, we provide the optimal distributed algorithm for solving the above max-min rate allocation problem. Moreover, for hopping relays with dynamic relay assignments, and for inband relays, the problem becomes intractable and we offer heuristic schemes that perform close to the optimum solutions. Numerical results demonstrate the significant performance improvement provided by the proposed group decoder over the traditional systems that employ the linear minimum mean-square error (MMSE) decoders at both the relays and the destinations, where all interference is treated as noise, as well as the effectiveness of the proposed dynamic relay assignment strategies.

We propose novel coding and decoding methods for a fully connected K-user Gaussian interference channel via assigning multiple codebooks (layers) to each transmitter, such that at each receiver decoding interferers partially becomes feasible. Each receiver first identifies which interferers it should decode and which layers of them should be decoded, and then successively decodes a group of layers with a constraint on its group size treating the remaining layers as Gaussian noise. We provide a distributed algorithm that determines the transmission rate at each transmitter also finds the order of the layers to be successively decoded at each receiver. We also consider practical design of a system that employs the quadrature amplitude modulations (QAM) and rateless codes. Numerical results are provided on the achievable sum-rate under the ideal case of Gaussian signaling with random codes as well as on the system throughput under practical modulations and channel codes. The results show that the proposed multi-layer coding scheme with CPGD offers significant performance gain over the traditional un-layered transmission with single-user decoding.

We propose novel coding and decoding methods for a fully connected K-user Gaussian interference channel. Each transmitter encodes its information into multiple layers and transmits the superposition of those layers. Each receiver employs a constrained partial group decoder (CPGD) that decodes its designated message along with a part of the interference. In particular, each receiver performs a twofold task by first identifying which interferers it should decode and then determining which layers of them should be decoded. Determining the layers to be decoded and decoding them are carried out in a successive manner, where in each step a group of layers with a constraint on its group size is identified and jointly decoded while the remaining layers are treated as Gaussian noise. The decoded layers are then subtracted from the received signal and the same procedure is repeated for the remaining layers. We provide a distributed algorithm, tailored to the nature of the interference channels, that determines the transmission rate at each transmitter based on some optimality measure and also finds the order of the layers to be successively decoded at each receiver. We also consider practical design of a system that employs the quadrature amplitude modulations (QAM) and rateless codes. Numerical results are provided on the achievable sum-rate under the ideal case of Gaussian signaling with random codes as well as on the system throughput under practical modulations and channel codes. The results show that the proposed multi-layer coding scheme with CPGD offers significant performance gain over the traditional un-layered transmission with single-user decoding.

We propose a practical message-wise unequal error protection (UEP) scheme using low-density parity-check (LDPC) codes, where one or more special messages are more protected than other ordinary messages. In contrast to the information theoretic cavity coding scheme, which discards the codewords of ordinary messages near those of special messages, the proposed coding scheme performs codeword flipping to separate the codewords of special and ordinary messages without discarding any codewords. To better distinguish the original and flipped codewords, the LDPC codes with all-odd degree check nodes are employed. The decoder performs message type detection and codeword flipping detection based on the unsatisfied check nodes in iterative decoding. We provide performance analysis for both the message type detection and the codeword flipping detection. Moreover, we provide an asymptotic analysis on the detection error exponent to reveal the relationship between the proposed practical coding scheme and the information theoretically optimal cavity coding. Simulation results are provided to show that the proposed practical message-wise UEP schemes offer capacity-approaching protections to both types of messages as if only one type is transmitted.

We consider the joint channel and network code design for a half-duplex 4-node multiple-access relay system with two sources, one relay, and one destination. The relay combines the information from both sources and transmits it to the destination together with both sources. We consider two network coding schemes for information combining at the relay, namely, the superposition coding (SC) and the Raptor coding (RC). For both SC and RC, the profiles for joint channel and network coding are optimized based on the extrinsic information transfer (EXIT) function analysis. For both additive white Gaussian noise (AWGN) and block fading channels, the multiple-access relay system with optimized profiles exhibits significant performance gains over that employing the code profile optimized for either the single-user AWGN channel or the 2-user multiple-access channel.

We propose a practical message-wise unequal error protection (UEP) scheme using low-density parity-check (LDPC) codes, where one or more special messages are more protected than other ordinary messages, which performs codeword flipping to separate the codewords of special and ordinary messages. To better distinguish the original and flipped codewords, the LDPC codes with all-odd degree check nodes are employed. The decoder performs message type detection and codeword flipping detection by tracking the number of unsatisfied check nodes in iterative decoding. We provide both finite-length and asymptotic performance analysis for the proposed coding scheme. Simulation results are provided to show that the proposed practical message-wise UEP schemes offer capacity-approaching protections to both types of messages as if only one type of message is transmitted.

We consider the code design for a half-duplex 4-node joint relay system with two sources, one relay, and one destination. The relay combines the information from both sources and transmits it to the destination together with both sources. We consider two coding schemes for information combining at the relay, namely, the superposition coding (SC) and the Raptor coding (RC). The Raptor codes are employed at the sources as well as the relay. The relay and the destination perform iterative a posteriori probability (APP) detection and soft Raptor decoding. For both SC and RC, the profiles for Raptor codes are optimized based on the extrinsic information transfer (EXIT) function analysis. For both the additive white Gaussian noise (AWGN) and block fading channels, the joint relay system with optimized profiles exhibits significant performance gains over that employing the code profile optimized for either the singleuser AWGN channel or the 2-user multiple-access channel.