For the scenario of large-scale users in millimeter wave (mmWave) multi-user multiple-input multiple-output (MU-MIMO)-orthogonal frequency division multiplexing (OFDM) systems, power allocation on each subcarrier (SC), association between SCs and users (SC-UE association) and precoder design are mutually dependent due to the hybrid MIMO-OFDM structure. This interdependence suggests that these factors should be jointly considered and optimized to make full use of both spatial and frequency diversity. In this paper, an iterative optimization framework is proposed for optimizing both the sum rate and the minimum user rate considering the beam squint effect. The optimization framework first performs a coarse SC-UE association based on an SC-level channel clustering, and then alternatively optimizes the hybrid analog and digital (HAD) precoder and SC-UE association in an iterative manner. The SC-UE association problems are solved by the greedy algorithm and heuristic method for sum rate and minimum rate optimization problems, respectively. The computational complexities of the proposed methods are evaluated and compared with several existing benchmarks. Numerical simulations show that the proposed methods outperform the corresponding benchmarks, while hold comparable running times by contrast due to its fast convergence rate.

In contrast to the intelligent reflecting surface (IRS) as a pure reflection device in the past, in this letter, from a more micro perspective, we propose to employ pseudo-random constellation rotation to secure microwave quick response (QR) code communication. Specifically, confidential information is encoded into a QR code, which is displayed on an IRS. Pseudo-random constellation rotation is applied to each element of the IRS to encrypt the QR code. This ensures that only the authenticated user (Bob) can decode and access the confidential information, while an eavesdropper (Eve) is unable to correctly interpret the QR code. We derive a closed-form expression for the bit error rate (BER) and analyze the impact of various factors on the BER for both Bob and Eve, including the distance between Bob and the IRS, Bob’s transmit power, and the Rician factor. Simulation results demonstrate the proposed scheme’s effectiveness in achieving secure communication.

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.

Dual-functional radar and communication (DFRC) system is capable of sensing potential eavesdroppers close to the DFRC base station (BS) and further ensuring secure transmission using physical layer security technologies based on the obtained location information of eavesdroppers. However, such security can be threatened by a malicious intelligent reflecting surface (IRS) that simultaneously changes both radar and communication channels. To reveal this threat, an IRS-based active attack scheme is proposed in this paper under the assumption of a prevalent DFRC framework. In this scheme, the attacker, equipped with a malicious IRS, carefully controls and optimizes the IRS phase shifts in each stage of the DFRC framework to reduce its reflected radar echo power to the BS and/or strength the wiretap link gain for pilot spoofing attack according to the statistical channel state information. Numerical results show that our proposed attack scheme can significantly reduce the secrecy rate of the legitimate user, while avoid being detected by the DFRC BS.

Sybil attack is one of the most serious attacks in vehicular ad hoc network (VANET), where a malicious node forges many illegal identities to interfere with or even control the network, which brings severe security threats to the intelligent transportation system. Received signal strength indicator (RSSI) is highly relative to the positions of communication nodes, and can be applied for sybil attack detection according to the fact that two real nodes cannot be in the same position. However, the high mobility of nodes in VANET makes it more difficult to be applied directly. In this paper, an improved RSSI-based sybil attack detection method is proposed against the fading channel and the high mobility of nodes in VANET. This method first estimates the distance between each communication node through maximum likelihood estimation, and then detects sybil nodes through mean-shift clustering algorithm. The numerical simulation results show the propose sybil attack detection has high accuracy in VANET by appropriately setting the detection time and the searching radius of the mean-shift algorithm.

Millimeter wave (MmWave) dual-functional radar-communication (DFRC) technology is believed to hold the ability to alleviate spectrum congestion and inter-radar interference in 5G vehicle-to-everything (V2X) systems. The radar target sizes in V2X system may not be ignored in views of the demands of short-range sensing and ultra-narrow beams supported by massive MIMO mmWave beamforming. Under such scenario, a novel single-target-multi-beams (STMB) radar beam alignment scheme is proposed to acquire more accurate information on estimated ranges and velocities by allocating multiple radar beams to a certain target. For instance, the relative velocity direction can be accurately estimated based on STMB scheme by using a weighted linear estimation methods. Then, the hybrid analog-digital beamforming under STMB scheme is formulated and optimized by maximizing transmission rate subject to radar signal-to-interference-and-noise (SINR) constraints, where a radar beam cancellation algorithm is proposed to adjust adaptively the radar beam number pointing to a certain target, which can guarantee strict radar SINR constraint under different transmission power levels. The numerical results verify the effectiveness and reliability of STMB scheme and show that the proposed beamformer outperforms the benchmark in both spectral efficiency and minimum radar SINR.

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.

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.

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.

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.

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.

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.

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.