ABSTRACT Accurate classification and segmentation of intracranial aneurysms from 3D point cloud data are critical for computer‐aided diagnosis and surgical planning. However, existing point‐based deep learning methods suffer from limited feature representation and poor segmentation performance on medical data due to insufficient training samples and complex geometric variations. M‐PointNet introduces a novel multi‐layer embedded deep learning architecture that significantly enhances the classification and segmentation of intracranial aneurysms through three key innovations: (1) an enhanced PointNet++ with an expanded hierarchical structure for better geometric feature extraction; (2) a multi‐layer embedding mechanism that integrates preprocessed and resampled point cloud data at multiple hierarchical levels to enrich feature representation; and (3) a deep supervision strategy with auxiliary output layers to accelerate convergence and improve performance. Experiments on the IntrA dataset demonstrate that M‐PointNet achieves 91.96% accuracy and a 0.923 F1‐score in classification, surpassing baseline by 5.27% and 3.0%, respectively. For segmentation, it attains 83.85% IoU and 90.25% DSC for aneurysm regions and 95.81% IoU and 97.82% DSC for vessel regions. Additionally, its generalization capability is validated by a 92.8% accuracy on the ModelNet40 dataset. M‐PointNet effectively addresses the challenges of medical point cloud analysis, achieving state‐of‐the‐art performance in intracranial aneurysms classification and segmentation while maintaining robust cross‐domain generalization.

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.

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 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.

Smart home technology has found wide-ranging applications in daily life, from enhancing energy efficiency to simplifying daily tasks and providing greater convenience. However, recent works have found that smart home devices are vulnerable to passive network observers (i.e., adversaries). While adversaries have demonstrated the ability to infer device events (e.g., whether a lamp is turned on) from the encrypted smart home traffic, we believe this only represents a less critical aspect of smart home privacy risks. Further analysis of demographic attributes presents greater risks to user privacy. Besides, from our deployment experience of real-world smart homes, we found that existing event inference methods can be greatly interfered with by event-unrelated traffic. Experiments show that this interference can result in up to a 10% drop in inference accuracy. Furthermore, it is challenging to infer finer-grained demographic attributes, due to the insufficient accuracy of event inference. Therefore, in this work, we propose a novel event inference model extracting multi-dimensional features that reduces the interference of event-unrelated traffic by analyzing packet length distribution and statistical properties. In addition, we design a dual-channel neural network to extract spatial and temporal relationships among triggered events to infer demographic attributes of smart home users, such as age group and career stage. Combining the above designs, we present TrafficDiary, the first user attribute inference approach based on smart home traffic traces. We prototype TrafficDiary and evaluate it in real-world smart homes. Experimental results show that TrafficDiary achieves 98.68% accuracy with a zero false positive rate in event inference and a high level of accuracy in user attribute inference, even when 16, 362 groups of event-unrelated traffic exist. TrafficDiary also performs well in terms of efficiency, with an inference latency of only 1.82 ms on a Raspberry Pi 4B device.

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.

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.

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.

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.

Mobile-centric AI applications have high requirements for the resource-efficiency of model inference. Input filtering is a promising approach to eliminate redundancy so as to reduce the cost of inference. Previous efforts have tailored effective solutions for many applications, but left two essential questions unanswered: (1)theoretical filterability of an inference workloadto guide the application of input filtering techniques, thereby avoiding the trial-and-error cost for resource-constrained mobile applications; (2)robust discriminability of feature embeddingto allow input filtering to be widely effective for diverse inference tasks and input content. To answer them, we first formulate the input filtering problem and theoretically compare the hypothesis complexity of inference models and input filters to understand the optimization potential. Then we propose the first end-to-end learnable input filtering framework that covers most state-of-the-art methods and surpasses them in feature embedding with robust discriminability. We design and implementInFithat supports different input modalities and mobile-centric deployments. Comprehensive evaluations confirm our theoretical results and show thatInFioutperforms strong baselines in applicability, accuracy, and efficiency.InFican achieve 8.5× throughput and save 95% bandwidth, while keeping over 90% accuracy, for a video analytics application on mobile platforms.

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.

Recent studies show that smart home devices are vulnerable to passive network observers, referred to as adversaries. An adversary can use mobile devices as a sniffer to infer device events (e.g., a door is opened/closed) even through encrypted WiFi traffic. However, to obtain high event identification accuracy, existing works heavily rely on either machine learning (ML) or large traffic feature sizes, which results in a costly retraining or pairing process. In this paper, we generate an event traffic fingerprint and propose an event identification method with good extensibility. To address the interference of network fluctuations and unrelated traffic, we filter all redundant and event-unrelated packets and extract event packet sequences based on time interval. After processing, we generate a representative packet-level fingerprint for each event and identify the event based on fingerprint matching. The precision and recall of event identification reach 96.77% and 92.31% on average. Our traffic processing method can work as an add-on to existing ML-based event inference methods, which leads to at least 12% increase on the accuracy.

Money laundering is a significant problem in the financial system and provides the conditions for financing various crimes. Previous methods apply many flexible algorithms, such as machine learning, graph mining, and anomaly detection. However, most of these contemporary methods do not adequately consider the dynamic characteristics of transactions, which may contain discriminative information for money laundering detection. To address this issue, in this paper, we propose a dynamic transaction pattern aggregation neural network (DTPAN) for money laundering detection. DTPAN utilizes two feature extractors to learn the dynamic features of transaction behaviors and the evolution of transfer relationships between accounts. Furthermore, it employs a feature enhancement module to enhance the behavior dynamic features, capturing the latent dependency between behavior dynamic and relationship evolution. Experimental results obtained with a real-world dataset demonstrate the effectiveness of DTPAN. The results also reveal that DTPAN can enhance the performance of ML detection by adequately exploring the dynamic information of transactions.

For an underwater optical wireless communication (UOWC) system, alignment between transmitter and receiver is indispensable, which raises high requirements on the positioning technique. In the practical marine environment, light travels in a bending trajectory rather than in a straight line due to the non-uniformity of the refractive index of seawater, which leads to the position error. In this work, the position error under light beam bending due to refraction index variation is investigated concerning a UOWC system, and a simplified layered model is introduced to approximate the continuous refractive index variation in the practical ocean scenario. Besides, an approximated bending light path function is derived as well. Experimental results showed that a great consistency exists between the predicted positioning error and the measured one, and the potential causes of derivation between these results are further analyzed.

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.

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.

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.

We propose a vectorized color modulation (VCM) method for a covert camera-screen communication (CSC) system under the human visual constraint. Instead of using light intensity of each image pixel block for data modulation in the existing work, we explore the color mixing characteristics of different colors while maintaining the chromaticity perception quality. Thus a color vector in the chromatic diagram can carry information in its angle, revealing a new signal dimension and mitigating the flickering issue. The best color vector pair is searched and selected for data modulation to yield the lowest bit error rate (BER). Experimental test based on representative images from an open database shows that the proposed method yields much better communication performance than the existing intensity modulation method.

With the development of autonomous driving technology, inter-vehicle ranging has attracted a lot of attention. In this paper, we propose a simple and low cost visible light ranging system. It exploits a visible light emitting diode (LED) as the transmitter and an avalanche photodiode (APD) array as the receiver in each vehicle. The module measures the round-trip distance of the signal from the rear vehicle LED to the front vehicle which receives and sends back the signal. The position of the front vehicle is then estimated by least-squares and Levenberg-Marquardt algorithms. Upon a complete channel and noise model, simulation results are compared against experimental results, with distance from 1 m to 19 m at a step of 1 m. The system is able to measure the absolute distance up to 14 m and 19 m with an error less than 5 cm and 20 cm respectively.

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.

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 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.

We are at the dawn of an era in information and communication technology with unprecedented demand for connected and automated everything. Both Shannon theories and industrial advances have clearly evidenced that more densely packed networks and a much wider operating bandwidth are key drivers for meeting the escalating wireless network demands. Recently, there have been substantial research efforts on the exploitation of higher frequency bands, in particular the millimetre wave and optical wireless bands. After a decade of active research and development, and along with the maturity of device technology, Visible Light Communications (VLC) has emerged as a very promising technology to enable next generation digital innovations and support wide range of applications. This special issue on VLC focuses on three core thrusts of the discipline:Localisation, CommunicationandNetworking. The overall aim of the special issue is to inspire multi-disciplinary international communities to work together in order to achieve further research advances. Indeed, a total of 96 high quality papers were received from both academia and industry. After a careful peer-reviewing process, 17 papers were selected based on their combined novelty, rigour, and impact. Owing to the highly selective nature of JSAC, many other interesting papers were not selected for the special issue, but we hope that these papers might appear elsewhere.

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.

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.

Experimental demonstration of a low-complexity on-off keying (OOK) transceiver for indoor visible light communication (VLC) system using a commercial white illumination light-emitting diodes (LED) module is undertaken based on advanced RISC machines (ARMs). In the transmitter, a boost circuit and a microprocessor (STM32) are used for LED driving and OOK signal generation, whilst in the receiver a high-sensitivity photoelectric circuit and ARM are applied for signal detection and recovery. The influence of the interference from the sunshine and fluorescent light on the VLC transmission performance is discussed. It is shown that real-time 200kbps OOK transmission is successfully demonstrated over 7 meters VLC links with strong ambient light interference from the fluorescent light in the room and the sun light through the window. In addition, the transmission performance is shown to be robust to the rotation angle of the receiver in the range of less than 80° at a distance of 6 meters for a bit-error ratio of below 10-3.

Sampling clock offset (SCO) is a key factor affecting the transmission performance of a vehicular visible light communication (VLC) system. For the traditional on-off keying (OOK) scheme, timing recovery is required for mitigation of the SCO effect in a vehicular light-emitting diode (LED) based VLC system, which suffers nonlinearity. To investigate the impact of the LED nonlinearity on the performance of the timing recovery, a piecewise-parabolic interpolator-based timing recovery method with three different timing error detectors (TEDs) is used to evaluate the transmission performance of 25Mb/s OOK signal in a real-time vehicular VLC system with SCO. The impact of ambient light on the transmission performance is also investigated. It is shown that a SCO of up to ±1000ppm can be effectively compensated with the timing recovery with these TED schemes. A real-time 25Mb/s OOK system is successfully demonstrated over a VLC link up to 60m at a bit error ratio below 3 × 10-3.

We design and demonstrate a covert camera-screen communication (CSC) system. It embeds data in display images and retrieves information using a commercial camera. Pixels or blocks of pixels are modulated by a spatial visual modulation scheme. To ensure reliable data transmission while incurring little effect on the screen-viewing experience, two-dimensional Gaussian distribution of the intensity is applied. Also the synchronization anchors are added to each data frame to tackle the unstable frame rate issue of the camera. The system throughput and bit error rate (BER) performance under different imperfect factors are experimentally tested. Results show that the proposed system achieves an average throughput of 16.67 kbps at a regular screen refresh rate 60Hz and remains imperceivable to an observer. The throughput is comparable to that by existing works relying on a rate 120Hz.

In this paper, we study linear transceiver designs for indoor visible light communications (VLCs) with multiple light emitting diodes (LEDs). Specifically, we investigate VLCs including white emitting diodes and VLCs including red/green/blue (RGB) LEDs. The transmitter precoding and the offset are jointly designed by considering certain key practical lighting constraints, such as optical power, non-negativeness, and color illumination. Various non-convex transceiver design problems are formulated aiming to minimize total mean-square-error to improve transmission reliability. We show that for multi-input single-output white VLCs, the optimal precoding reduces to a simple LED selection strategy. For multi-input multi-output (MIMO) white VLCs, we prove that the optimization problem with multiple constraints can be equivalently simplified to a problem with single constraint, which enables us to propose efficient algorithms to search local optimal solutions. For MIMO RGB VLCs, by using certain useful transformations, we show that the precoding design is equivalent to covariance matrix design of transmit signals, which can be further transformed to a convex optimization problem. To develop an algorithm to find the optimal solution, we derive the optimal structure of the covariance matrix and show that the optimal solution can be obtained via a water-filling approach. Extensive simulation results are provided to verify the performance of the proposed designs.

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.

Visible light communication (VLC) combined with advanced illumination may be expected to become an integral part of next-generation heterogeneous networks. In order to mitigate the performance degradation imposed by the intercell-interference (ICI), a user-centric (UC) cluster formation technique employing vectored transmission (VT) is proposed for the VLC down-link system, where multiple users may be simultaneously supported by multiple access points (APs). In contrast to the traditional network-centric (NC) design, the UC-VT cluster formation is dynamically constructed and adjusted, rather than remaining static. Furthermore, we consider the critical issue of multiuser scheduling (MUS) relying on maximizing the “sum utility” of this system, which leads to a joint cluster formation and MUS problem. In order to find a practical solution, the original problem is reformulated as a maximum weighted matching (MWM) problem relying on a user-AP distance-based weight and then a low-complexity greedy algorithm is proposed, which offers a suboptimal yet compelling solution operating close to the optimal value found by the potentially excessive-complexity exhaustive search. Our simulation results demonstrate that the proposed greedy MUS algorithm combined with the UC-VT cluster formation is capable of providing an average user throughput of about 90% of the optimal throughput, which is about three times the throughput provided by the traditional cellular design in some of the scenarios considered.

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.

Optical wireless communication (OWC) systems provide many advantages over radio frequency (RF) wireless technologies in some scenarios, including significantly higher data rates and a large amount of available license-free frequency spectrum. Recently, OWC has also been proposed in fifth generation 5G standard as a tool to augment capacity due to RF spectrum crunching challenges. Various forms of OWC can be used to augment RF capacity at both access and backhaul/fronthaul levels. Despite the major advantages and various application areas, the widespread deployment of OWC is delayed by several challenges, such as the demand to maintain strict line-of-sight alignment between transmitter and receiver in long range outdoor applications; the need to combat attenuation due to adverse weather conditions such as fog, cloud, and turbulence; to modulate light emitting diodes at high frequencies without distortion at indoor visible light communication (VLC) applications and retaining power levels within the eye safety limits for laser transmitters and comfortable illumination levels for LED transmitters.

New data services and applications are emerging continuously and enhancing the mobile broadband experience. The ability to cope with these varied and sophisticated services and applications will be a key success factor for the highly demanding future network infrastructure. One such technology that could help address the problem would be optical wireless communications (OWC), which presents a growing research interest in the last few years for indoor and outdoor applications. This paper is an overview of the OWC systems focusing on visible light communications, free space optics, transcutaneous OWC, underwater OWC, and optical scattering communications.

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.

Orthogonal frequency-division multiplexing (OFDM) has been widely used in fiber-wireless (FiWi) networks, but it suffers from reduced spectral efficiency, high peak-to-average power ratio (PAPR), and severe sensitivity to carrier frequency offset (CFO). In this paper, we propose a flexible multi-block OFDM (MB-OFDM) transmission scheme to simultaneously solve those problems. First, one guard interval is shared by multiple blocks by exploiting the slow time-varying property of the fiber-wireless channel, so the spectral efficiency could be typically improved by about 10%. Second, the proposed scheme further divides every data block into multiple small sub-blocks, whereby each sub-block is generated by an inverse fast Fourier transform (IFFT) of smaller size accordingly. It thus provides a flexible compromise between the multi-carrier and single-carrier transmissions, and reduces the PAPR and the sensitivity to CFO. In addition, a hybrid-domain channel equalization method is proposed to detect the MB-OFDM signal by utilizing the single-carrier and multi-carrier transmission formats successively. Simulation results are provided to demonstrate the enhanced performance of the proposed scheme.

Orthogonal frequency division multiplexing (OFDM) is a promising technology for the next-generation optical transmission systems beyond 100 Gb/s. To further improve the spectral efficiency and system reliability, we propose a flexible coherent zero padding OFDM (CO-ZP-OFDM) scheme with signaling-embedded preamble and polarization-time-frequency (PTF) coded pilots for high-speed optical transport networks. Our judicious design embeds signaling in the specially designed preamble whose Delta-like correlation function helps to simultaneously achieve very accurate timing and frequency synchronization. Unlike the periodically inserted training symbols, the PTF-coded pilots are properly distributed within the time-frequency grid of the ZP-OFDM payload symbols and used to realize low-complexity multiple-input multiple-output (MIMO) channel estimation at high accuracy. Compared with the conventional optical OFDM systems, CO-ZP-OFDM increases payload by about 6.68%, and the low-density parity-check (LDPC) coded bit error rate only suffers from no more than 0.3 dB compared with the back-to-back case even when the channel dispersion impairments are severe.

Visible light communications (VLC) are gaining popularity and may provide an alternative means of communications in indoor settings. However, to date, there is very little research on the deployment or higher layer protocol design for VLC. In this paper, we first perform channel measurements using a physical layer testbed in the visible light band to understand its physical layer characteristics. Our measurements suggest that in order to increase data rates with VLC (1) the beam width of a communicating link can be shrunk, and (2) the transmission beam can be tuned to point towards the target recipient. We then perform Matlab simulations to verify that the human eye is able to accommodate the changes brought by shrinking a beam or by tuning the beam direction appropriately. As our main contribution, we then design a configuration framework for a VLC indoor local area network, which we call VICO; we leverage the above features towards achieving the highest throughput while maintaining fairness. VICO first tunes the beamwidths and pointing angles of the transmitters to configurations that provide the highest throughput for each client. It then tries to schedule transmissions while accounting for conflicts and the VLC PHY characteristics. Finally, it opportunistically tunes the idle LEDs to reinforce existing transmissions to increase throughput to the extent possible. We perform extensive simulations to demonstrate the effectiveness of VICO. We find that VICO provides as much as 5-fold increase in throughput compared to a simple scheduler that does not exploit the possible variations in beamwidth or beam-angle.

The solar-blind ultraviolet (UV) spectrum has useful properties for wireless communication and sensing. Strong atmospheric scattering in the UV spectrum enables non-line-of-sight (NLOS) communication. The authors present recent experimental and analytical results in NLOS UV channel modelling, including impulse response and path loss. The authors further study the NLOS UV link performance for short-range communication scenarios based on our theoretical modelling results. Relations between power limitation and channel bandwidth limitation are examined. Some link budget results are analysed for long-range communication links up to 5 km.

Ultraviolet (UV) communication is an attractive option for tactical networks or environmental monitoring. The underlying UV PHY layer has unique characteristics that render previously proposed higher layer protocols for RF communications inappropriate or inefficient. Neighbor discovery is an important functional component of a UV ad hoc wireless network. While there has been some work in UV PHY layers, there is very limited work in network study. In this paper, we propose a new neighbor discovery protocol for this setting; unlike prior protocols, our approach alleviates the negative effects of random access based collisions by choosing a leader that arbitrates the discovery process. Without prior knowledge of the number of nodes in the network, the approach facilitates neighbor discovery in a fast, fair and efficient manner. We perform extensive simulations with a realistic UV PHY layer and demonstrate that the approach reduces the required neighbor discovery time by as much as 90%. We also examine the impact of various system parameters that can be especially useful to UV network and system designers.

This paper presents visible light positioning (VLP) techniques based on a traffic light and photodiodes. The position of a vehicle can be determined using both the traffic light position information obtained through a visible light communication (VLC) link and the time difference of arrival (TDOA) of the traffic light signal to two photodiodes mounted in the front of the vehicle. Methods for VLP with one traffic light and two traffic lights are both proposed, and the error due to the non-coplanar effect is studied and the related coplanar rotation methods for both cases are developed. The numerical result shows bias performance of the positioning methods with coplanar rotation is much better than the ones without coplanar rotation, and the effects of the vehicle distance to traffic light and vehicle speed are also discussed.

In the above titled paper (ibid., vol. 27, no. 9, pp. 1535-1544, Dec. 09), two figures were inaccurate due to a software programming error when implementing the path loss prediction algorithm. The corrected Figs. 4 and 6 are presented here. The correction does not otherwise affect results in the above paper.

The solar blind ultraviolet (UV) scattering channel makes non-line-of-sight UV communications very attractive for military applications, particularly for communication on-the-move with low probability of detection and low probability of interception. Despite significant research effort on the UV physical layer, work on protocol design at the upper layers is quite limited. We consider a mobile ad hoc UV network, with each node equipped with a transceiver capable of transmitting in multiple directions and performing omni-directional receptions. Full-duplexing is enabled. We develop efficient neighbor discovery protocols by accounting for the unique UV physical (PHY) layer characteristics, namely varying channel qualities along different scattering directions. In addition to a list of neighbor nodes' identities, our protocols also construct and maintain a table which contains a ranked list of node pointing directions between each pair of nodes in terms of channel qualities. Our approach does not need support from the global positioning system (GPS) or temporal synchronization across nodes like many radio frequency (RF) protocols. Specifically, two algorithms are proposed with and without the need for direction synchronization. We further improve the latter by more efficiently utilizing neighbor feedback. We perform extensive simulations to evaluate our algorithms.

We develop Ziv-Zakai bounds (ZZBs) on time delay estimation (TDE) for known frequency hopping or multicarrier waveforms over random wideband (frequency selective) Gaussian channels, with a uniform prior on the delay. The channel model incorporates correlation, for example between subcarriers. A unified signal model is applicable to several cases of interest. The receiver does not have channel state information to estimate the time delay, but does have knowledge of the channel statistics. The ZZB provides a tight bound on the mean square error for Bayesian estimation in wideband random channels, revealing SNR threshold behavior indicating estimator breakdown.

As an alternative to radio-frequency (RF) communications, optical wireless communications (OWC) can support high data rates and low power operations while providing good jamming resistance. Our focus in this paper is on deep ultraviolet (UV) outdoor communications (UVOC) where solar blind and non-line-of-sight operations are attractive. Light beams from UV LED arrays serve as information carriers. In an abstract sense, this is similar to directional transmissions in RF; however, the PHY layer characteristics significantly differ due to atmospheric scattering. First, we perform extensive experiments on a UV testbed towards understanding signal propagation and the impact of the PHY on medium access. We find that UV propagation supports (a) fully duplex communications and (b) multiple data rate transmissions. Next, we propose a novel contention-based media access control (UVOC-MAC) protocol that inherently accounts for the UV PHY layer and fully exploits multi-fold spatial reuse opportunities. Evaluations via both simulations and analysis show that UVOC-MAC effectively mitigates collisions and achieves high throughput. In particular, up to a 4-fold increase in throughput and 50% reduction in collision are possible compared to a MAC protocol agnostic to the UV PHY properties.

A stochastic non-line-of-sight (NLOS) ultraviolet (UV) communication channel model is developed using a Monte Carlo simulation method based on photon tracing. The expected channel impulse response is obtained by computing photon arrival probabilities and associated propagation delay at the receiver. This method captures the multiple scattering effects of UV signal propagation in the atmosphere, and relaxes the assumptions of single scattering theory. The proposed model has a clear advantage in reliable prediction of NLOS path loss, as validated by outdoor experiments at small to medium elevation angles. A Gamma function is shown to agree well with the predicted impulse response, and this provides a simple means to determine the channel bandwidth. The developed model is employed to study the characteristics of NLOS UV scattering channels, including path loss and channel bandwidth, for a variety of scattering conditions, source wavelength, transmitter and receiver optical pointing geometries, and range.

We develop linear equations for low-complexity source localization, based on time difference of arrival (TDOA) measurements at a linear sensor array. The derivation comes from hyperbolic geometry in Cartesian coordinates, and the algorithm is valid for (possibly wideband) sources in the near or far field. An error analysis is used to predict bias and mean-square error, as well as provide optimal weighting for a low-complexity noniterative weighted least squares solution. A connection is made with an algorithm for arbitrary array geometry. Simulation results show that the proposed algorithm achieves maximum-likelihood performance and the CramrRao bound at medium to low TDOA measurement noise level.

The Ziv-Zakai bound (ZZB) provides a general mean-square error analytical baseline to evaluate time delay estimation (TDE) techniques for a wide range of time-bandwidth products and signal-to-noise ratios, but generally can only be numerically evaluated. The Weiss-Weinstein bound (WWB) further improves characterization of the attainable system performance, for narrowband and wide-band signals with small to moderate fractional bandwidth. Similar to the WWB, here we find a simplified closed-form ZZB for TDE with ultra-wideband (UWB) signals. The resulting simplified bound is found over disjoint segments, separated by thresholds that characterize different regions of ambiguity. The closed-form simplified bound can be analytically studied, and approaches both the ZZB and the TDE performance of a maximum likelihood estimator.

The wireless ultraviolet (UV) communication technology has received great attention for either short-range high data-rate services or medium-to-large range low-rate sensing applications. However, performance of a UV link is limited by multiple constraints from the current UV laser source and detector technology, and atmospheric link conditions. This paper analyzes performance of both line of sight (LOS) and non-line of sight (NLOS) UV links in terms of signal to noise ratio and bit error rate of the detector. Roles of different parameters, such as data rate, communication range, transmitter power, beam and focal angle, receiving angle and field of view, are studied. Receiving powers for two typical links demonstrate square and linear decaying laws with respect to the communication range respectively. Compared with an LOS link, an NLOS link significantly degrades detection performance from one up to three orders of magnitude for the range from hundreds of meters down to meters.

In this paper, we develop contention-based medium access control (MAC) schemes for both best-effort data transmissions and delay-sensitive multimedia transmissions over WLANs. A user detection module and a multi-rate adaptation module are proposed in the physical layer to assist in link differentiation. With these two modules, for best-effort data transmissions, a new distributed queuing MAC protocol (PALD-DQMP) is proposed. Based on different users' channel states, PALD-DQMP makes use of a distributed queuing system to schedule the transmissions. To support delay-sensitive multimedia transmissions, an enhanced PALD-DQMP (E-PALD-DQMP) is designed by providing two-level optimized transmission scheduling for four access categories, thus eliminating both external and internal collisions among mobile stations. Simulation results show that our proposed protocols outperform the standard MAC protocols for both delay-sensitive and best-effort traffics. All these improvements are mainly contributed by the availability of cross-layer channel state information, and the consequent multi-rate adaptation scheme.

This paper develops a multipolling MAC protocol which exploits cross-layer information to support delay-sensitive multimedia services over WLANs. A user detection module and a multi-rate adaptation module are proposed in the physical layer to assist in link differentiation. With these two modules, our new multi-polling MAC protocol, named PALD-MPMP not only reduces the polling overhead, but also provides an effective polling scheduler by allocating transmission priorities to users according to their delay requirements or levels and channel gains. Simulation results show that our proposed protocol outperforms the standard point coordination function (PCF) for delay-sensitive services. All the performance improvements are mainly contributed by the awareness of cross-layer channel state information, and the consequent multi-rate adaptation schemes.

An ultra-wideband (UWB) transmitted reference (TR) system transmits an un-modulated pulse and a delayed modulated pulse pair. Then, a correlation receiver uses the former to demodulate the latter. Because of the long spread of a typical UWB channel, time delay between the two pulses is preferable to be larger than the channel delay spread for reduced noise at the receiver. However, for bandwidth efficiency, that delay should be made small, resulting in inter-pulse interference at the receiver. In this paper, digital receivers are constructed for TR-UWB systems including inter-pulse interference. A typical mean matching technique, appropriate for both PPM and PAM schemes, is implemented digitally to obtain a good template for symbol detection. Joint estimation and detection performance of this family of digital receivers, using finite number of bits in analog-to-digital conversion and finite noisy observations, is analyzed. Closed form results are derived and verified by computer simulations. In addition, the effect of time offset between the reference pulse and information carrying pulse is studied. Overlap of the two pulses does not incur noticeable performance degradation. The proposed analytical framework can be applied to study detection performance of other related digital receivers not covered in this paper

On-off keying (OOK) and pulse position modulation (PPM) are widely used modulation formats in wireless optical communications. Performance of OOK depends on the photon count threshold that should be adapted to channel fading, while performance of PPM degrades with pulse distortion caused by channel. Transmitted reference scheme, widely adopted in radio frequency communications, can help acquire channel information used for demodulation. This scheme is introduced to both OOK and PPM optical systems in this paper. The average signal and noise photon counts are estimated from reference sequences for OOK from which the optimal threshold is obtained. Since PPM symbols carry information in the pulse position in a non-linear form, Fourier transform translates time shift into a linear phase shift. Based on the linear regression technique, the parameter in the phase equation that is related to the information of pulse position is estimated. A noise model in the frequency domain is also presented.

The binary exponential backoff (BEB) algorithm used in IEEE 802.11 DCF suffers from the unfairness problem and yields low throughput under heavy load. With physical layer assisted loss differentiation, this paper proposes a new distributed queuing medium access control (MAC) protocol (PALD-DQMP). In this protocol, utilizing the user detection module in the physical layer, losses due to collisions are distinguished from those due to link errors, and such information is made available to the MAC layer. Based on different users' channel states, PALD-DQMP schedules their transmissions. Simulation results show that the proposed scheme outperforms the standard MAC protocol in terms of network throughput and fairness. This improvement is mainly due to the availability of cross-layer channel information, and the elimination of collisions and backoff periods.

A conventional transmitted reference (TR) modulation scheme is an effective means to combat severe multipath distortion in an ultra-wideband (UWB) system, significantly relaxing the equalization requirements. However, it suffers from multiple-access interference and a data rate loss. In this paper, we propose a multiuser TR scheme to extend TR modulation to the multiuser case, while almost doubling the data rate by allowing arbitrarily small spacing of pulse pairs. The proposed scheme assigns a pair of pseudorandom sequences to each user to enable multiple access, modulating the amplitude of data and reference pulses, respectively. A waveform template is first estimated, followed by data demodulation. A time-hopping code can be employed to further minimize the effect of signal collisions. The method is appropriate for both pulse amplitude modulation and pulse position modulation. Waveform estimation and bit-error rate analysis are provided, and confirmed with simulations. Substantial detection improvements over conventional TR detectors are observed.

A power of R (POR) technique has been proposed to blindly estimate multipath parameters of the desired user in a multiple access ultra wideband (UWB) system. In this paper, we first analyze the performance of the POR method in terms of both channel estimation mean-square-error (MSE) and receiver's output signal to interference plus noise ratio (SINR) and bit error rate (BER). Then we compare this method with both subspace and maximum likelihood methods based on practical UWB channels. Simulation results verify our analysis. They also show that the proposed POR method outperforms the subspace method for heavily loaded systems, and is superior to the maximum likelihood approach in all examined examples.

In this paper, a novel modulation diversity assisted (MDA) ultra wideband (UWB) communication system is proposed. Pulse position modulation and pulse amplitude modulation are applied to successive pulses alternately to create modulation diversity and increase system capacity. The proposed system is similar to a transmitted-reference system in such that two pulses are always transmitted together in a doublet. One of the multipath distorted pulses can serve as a reference signal without explicit channel estimation. However, a MDA-UWB system possesses more advantages, such as smoother spectrum and higher bit rate. Coupled with an advanced low complexity receiver, detection performance is shown to be significantly better.

An ultra wideband (UWB) transmitted reference (TR) scheme transmits an un-modulated pulse and a delayed modulated pulse each time. Then a correlation receiver uses the former to demodulate the latter. However, to guarantee satisfactory detection performance in severe multipath distortion, two pulses have to be well separated by at least the channel spread, resulting in reduced data rate. In this paper, the restrictive assumption is relaxed which consequently permits interference from neighboring pulses (termed as inter-pulse interference). Instead of using instantaneous signal from the former pulse as a template, improved estimates by a mean matching technique under different modulation schemes are proposed and used for better detection performance. Besides low complexity approaches, joint maximum likelihood (ML) template estimators and detectors are also proposed. Their statistical performance is analyzed and compared.

In an ultrawideband (UWB) system, typically, a Rake receiver is employed to detect information symbols. To fully capture signal energy spread over multiple paths, the desired user's channel parameters are necessary. In this paper, we first convert a time-hopping (TH) UWB signal model using pulse position modulation (PPM) to a linear form similar to that of a code division multiple access (CDMA) system. Then, we apply a power-of-R (POR) technique to blindly estimate multipath parameters of the desired user in a multiple-access (MA) UWB system, where "R" represents the data covariance matrix. The POR technique is related to the subspace technique in that it directly estimates the unknown noise-subspace component in the subspace method but avoids rank estimation where errors may be incurred by perturbations in practical conditions. Performance of the proposed POR channel estimator is analyzed and shown to converge to that of the ideal subspace approach. Detection performance of the Rake receiver constructed from the estimated channel is also studied in terms of output signal-to-interference-plus-noise ratio (SINR) and bit error rate (BER). The proposed method demonstrates superiority to existing maximum-likelihood (ML)-based approaches in terms of either channel estimation or symbol detection. It also outperforms the subspace method for heavily loaded systems.

In a time hopping ultra-wideband (UWB) impulse radio system, each user is assigned a unique time-hopping sequence. If aperiodic codes are used for time-hopping sequences, some benefits, such as smoothed spectrum and better performance, can be achieved. Recently, a correlation matching method has been proposed to estimate channels in aperiodic time-hopping systems. Performance of corresponding multiuser and single user channel estimators is studied in This work. Closed-form analytical results are provided and verified by computer simulation.

In a CDMA downlink, subspace channel estimation can not be applied directly since long spreading codes destroy the rank condition of the received data covariance matrix. However, after introducing temporal diversity in the transmitted chip sequence for all users, subspace techniques can be applied to estimate channels. Detection of the desired user's signal is then performed by a channel equalizer, cascaded with the desired user's code despreader. The analytical channel estimation mean-square-error is derived, based on perturbation theory. Experimental comparison with our previously proposed spatial diversity method shows superior performance of the proposed scheme.

In a traditional time-hopping multiple access ultra-wideband communication system, the orthogonal time-hopping sequence can be designed to improve system performance. However, system capacity is restrictive. We propose a new multiple access system model for a UWB downlink and deploy orthogonal design in three dimensions: time-hopping code, multiuser code, and pulse waveform, or equivalently the position, polarity, and shape of a basic pulse. Such a system provides more freedoms than a traditional time-hopping system. Thus, it is more robust to multiple access interference and can support more users. Performance of the system is analyzed and verified by simulations.

In a CDMA system, imperfect channel estimation causes performance degradation of a minimum mean-square-error (MMSE) receiver. This paper studies the effects of channel estimation error on the performance of a direct matrix inversion (DMI) MMSE receiver and subspace MMSE receiver when channel parameters are estimated by a covariance-matching technique based on finite data samples. The receivers' output signal-to-interference-plus-noise ratios (SINRs) and bit-error-rates (BERs) are adopted for performance measures. Those performance indicators under such an imperfect condition are derived from a perturbation perspective and verified by simulation examples.

Blind multiuser detection is studied in the presence of multipath distortion. The kurtosis of the receiver's output is minimized with respect to the receiver's parameters subject to multiple linear constraints. The constraints are dependent on the spreading codes of the user of interest. In order to combine multipath signals optimally, the kurtosis is parameterized by the constraint vector as well and is further maximized. It is shown that under some conditions, the optimal constraint vector converges to the channel vector of the desired user irrespective of noise, and the proposed receiver ensures perfect cancellation of both intersymbol interference and multiuser interference. Meanwhile, a minimum mean-square-error receiver can be constructed from the constraint vector.

In blind and group-blind multiuser detection, different detectors can be designed using either the sample data covariance matrix directly or its eigencomponents. Due to finite-sample effect in practice, their performance deviates from the corresponding optimum. A perturbation technique is developed rigorously and systematically to analyze those detectors in this work. Subject to the assumption that the first-order perturbation dominates, corresponding results can be applied to a practical system of a given sample size. In particular, performance of the following typical detectors is studied for either flat or estimated multipath channels: direct-matrix-inversion (DMI) blind minimum mean-square error (MMSE) detector, subspace blind MMSE detector, direct zero-forcing (ZF) detector, subspace ZF detector, and group-blind hybrid detector. Simulation examples further verify various analytical results.

Recently, a blind downlink channel estimator was proposed for long-code CDMA systems based on a covariance matching idea. By modeling long spreading codes as random codes and pre-computing some code-dependent quantities, the algorithm shows extremely low complexity. Due to inaccuracy of estimated data covariance from finite number of received data samples, its asymptotic performance depends on the sample size. In this paper, it is further analyzed in terms of the covariance and mean-square-error of the estimated channel vector.

In the uplink of a long-code CDMA system, the base station knows the spreading codes of all serviced users. Given propagation delays, code-decorrelation can be performed to separate users' inputs in each symbol interval even in a multipath environment. To detect information symbols, we first apply the subspace technique to estimate multipath parameters for the user of interest based on the correlation matrix of whitened data. Then we construct different linear receivers. The statistics of the estimated channel vectors are analyzed and verified by our numerical examples. Performance of the receivers is also studied in simulations.

In a long-code uplink CDMA system, subspace method is recently proposed to estimate all users' multipath parameters and noise power based on the correlation matrix of whitened data. In this paper, we focus on performance of three typical linear receivers, known as zero-forcing (ZF), minimum mean-square-error (MMSE), and RAKE receivers, constructed from the estimated channels and noise power. All analyses are performed from a perturbation perspective and verified by our numerical examples.

With availability of spatial/temporal diversity in the downlink of long code CDMA systems, subspace technique has been shown to be able to estimate channel parameters. Based on the estimated channel, we construct three typical linear receivers as ZF, MMSE and RAKE receivers. Due to channel estimation error, those receivers show perturbed performance compared with their optimal counterparts. Their statistical performance is thus analyzed by applying perturbation theory. Those analyses are well supported by numerical results.

The second-order statistics of the sample covariance are encountered in many covariance based processing algorithms such as subspace based and covariance-matching based channel estimation methods. This paper provides closed-form expressions for the covariance of either the weighted sample covariance matrix or vectored sample covariance. Given a system model, the results explicitly rely on the second-order and fourth-order statistics of the channel noise and inputs. They can be directly applied to study the statistical properties of covariance based channel estimators and detectors. Various numerical examples are provided to further verify derived results.

Channel estimation is studied for a multicode (MC) or multiple processing gain (MPG) multirate CDMA system based on correlation matching. All users are partitioned into groups in terms of their raw data rates. Since a communication channel may span several symbol intervals especially for a high rate user, severe intersymbol interference (ISI) exists in the received data. However, signature waveforms of various symbols from a user are convolutions of spreading codes with a common channel. This structure is beneficial to channel estimation due to multiple contributions to the output correlation. By properly selecting a weighting matrix in the matching cost function, significant improvement is observed. The optimal weighting matrix and covariance of the channel estimate are further derived in closed forms.

We have proposed a blind channel estimation method for a precoded multirate multiuser communication system. The method requires subspace decomposition of a data covariance matrix. In this paper we analyze the method in the presence of imperfect estimation of the covariance matrix due to finite data sample effect. Based on perturbation theory, the first order perturbation of the noise subspace is derived. Then the covariance and the mean square error of the channel estimates are obtained in closed forms. They explicitly depend on the statistical properties of the input and the additive noise up to the fourth order. Therefore, they can be evaluated for different scenarios.

In this paper, we study blind equalization for a DS/CDMA system in multipath. The kurtosis of the equalizer's output is maximized with respect to the equalizer's parameters subject to multiple linear constraints. The constraints are dependent on the spreading codes of the user of interest. In order to optimally combine signals from multiple propagation paths, we parameterize the kurtosis by the constraint vector as well and further minimize the maxima of kurtosis to obtain the optimal constraints. It is shown that the constraint vector converges to the channel vector of the desired user irrespective of noise. The equalizer guarantees cancellation of both intersymbol interference and multiuser interference.

In this paper, a constant modulus algorithm (CMA) based multiuser detector is proposed under linear constraints. Different from previous work, we explicitly consider the multipath effect. Therefore multiple constraints are incorporated into the optimization problem to mitigate multipath distortions. The algorithm will work well if constraint parameters are properly pre-selected. However, the detector's performance highly depends on the constraints. Thus we jointly update the detector and the constraint vector by minimizing the Godard's cost function with respect to both the detector and the constraint vector. It is shown that under certain conditions in the absence of noise, the algorithm guarantees global convergence. Numerical examples are presented to demonstrate the results.

In this letter, a constant-modulus-algorithm-based multiuser detection scheme is proposed for a communication system under multipath propagation. To mitigate channel distortion and multiuser interference, we integrate multiple constraints into the optimization criterion. According to our analysis, the ability of the detector to remove all interference is ensured in the absence of noise when the constraints are properly preselected. However, in the presence of noise, the constraints highly affect the performance of the receiver. In order to optimally combine signals from different paths to achieve performance gains, those constraints can also be treated as variables and jointly optimized with the receiver, as verified by numerical examples.

In our previously proposed equalization criterion with MPSK modulated channel inputs (see Proc. of 10th IEEE Workshop on Statistical Signal and Array Processing p.692-6, Pocono Manor, PA, USA, August 2000), perfect equalization was guaranteed in principle under constrained optimization. The algorithm was implemented based on stochastic gradient method. In this paper, we investigate the convergence property of that algorithm. By carefully examining all stationary points of the objective function, we prove that all other points are unstable except the desired solution. The effects of the ary number M and the background noise on the performance of the proposed equalizer are studied in detail in our simulations. Satisfactory bit error rate performance and output constellation are observed.

Multiuser CDMA systems with aperiodic spreading codes have received considerable attention. One major difficulty in multiuser detection is the user's time-varying signature. In a multipath communication environment, this signature is not a priori known, making direct design of the detector more intractable. Multiuser detection is performed by a two-step approach: estimate the unknown channel coefficients and construct the detector based on the MMSE criterion. We model the interference as colored Gaussian noise with unknown correlations and estimate the channel parameters based on the correlation matching idea. The spreading codes are assumed random with known up to the fourth order statistics, resulting in a computationally efficient method. In the case of unknown statistics however, the method is still applicable by estimating those statistics from given codes. To guarantee identifiability, both the channel's output and the output of the matched filter for the desired user are considered in one cost function. Comparisons with a previously proposed method and other existing methods are made by simulations.

In a code-division multiple access (CDMA) system, if all users are either bit-synchronous or bit-asynchronous, the number of active users can be easily estimated from the dimension of the signal subspace, and users can be correspondingly identified. However, identification of co-existing synchronous and asynchronous users has not been addressed in the literature. In this letter, a subspace-based blind identification approach is presented by exploring correlations of relevant data vectors and evaluating projection of possible code vectors onto the noise subspace.

Constrained optimization methods have received considerable attention as a means to derive blind multiuser receivers with low complexity. The receiver's output variance is minimized subject to appropriate constraints which depend on the multipath structure of the signal of interest. When multipath is present, the constraint equations can be written in parametric form, and the constraint parameters jointly optimized with the linear receiver's parameters. We develop adaptive solutions for this joint, constrained optimization problem. Both stochastic gradient and recursive least-square-type algorithms are developed. The performance of the proposed methods is compared with other blind and trained methods and turns out to be close to the trained minimum mean-square-error receiver.

Multiple access interference (MAI) as well as multipath distortions are two major obstacles in multiuser communication. The MAI in the received data vectors can be eliminated by projecting the received data onto the subspace orthogonal to the code space of interfering users. After this operation, an equivalent single user model with AWGN is obtained. Therefore various detection techniques, e.g., maximum SNR, subspace, constrained optimization, iterative least squares with projection (ILSP), etc. can be successfully applied to estimate transmitted symbols and the multipath channel of the desired user, but with reduced complexity and asymptotically optimal performance. The proposed receiver is simulated by ILSP and constrained optimization method and its performance compared with the RAKE receiver and the decorrelating receiver. Much lower bit error probability is achieved for a wide range of SNRs.

The constant modulus algorithm (CMA) is very efficient in equalizing a signal with the constant modulus property. It seeks to minimize the dispersion of the amplitude of the equalizer output about a dispersion constant. However, a modulated signal is usually represented by not only its amplitude, but also its phase. Both of them may exhibit certain characteristics. We carefully examine the M-PSK modulated signal constellation and explore its properties in the equalization context. By combining the dispersion of both the amplitude and phase value in one cost function, new criteria for blind equalization are developed. Simulation results are presented to compare the proposed method with the CMA algorithm in terms of the intersymbol interference and bit error probability.

In this paper, correlation-matching techniques are employed to estimate multipath channel parameters for a multiuser CDMA system with long spreading codes. For a given code sequences, the output correlation matrix (parametrized by the unknown channel coefficients) is compared with its instantaneous approximation. By minimizing the Frobenious norm of the resulting error matrix the channel parameters can be estimated up to a scalar ambiguity. Under the assumption of i.i.d. code sequences, identifiability for each channel is guaranteed and the asymptotic convergence of the proposed algorithm is established. Simulation results confirm our claims. Comparisons with other methods are also provided.

Constrained optimization techniques are studied for direct design of linear multichannel equalizers. Novel blind algorithms are derived by minimizing the equalizer's output variance subject to appropriate constraints. The constraints are chosen to guarantee no desired signal cancellation, and their parameters are jointly optimized to maximize the signal component at the output. The resulting blind algorithm was observed to have near optimal performance at high signal-to-noise ratio, i.e., close to the performance of the trained minimum mean-square-error receiver. Also, the proposed method is not sensitive to the color of the transmitted sequence. Analytical expressions are derived to quantify the algorithm's performance.

Constrained adaptive optimization techniques are employed to design direct blind equalizers. The method is based on minimizing the equalizer's output variance subject to appropriate constraints. The constraints are chosen to guarantee no desired signal cancellation and are also jointly and recursively optimized to improve the performance. Our method provides adaptive solutions which directly optimize the equalizer's parameters, while its performance compares favorably to that of the linear prediction based approaches. Global convergence is established and comparisons with other blind and trained methods are presented.