Channel correlation impact on spatial diversity optical key generation

Privacy in form of anonymous communication could be comparably both faster and harder to break in optical routers than in today's anonymous IP networks based on The Onion Routing (Tor). Implementing the practical privacy all-optically, however, is not straightforward, as it requires key generation in each anonymization node to avoid distribution of long keys, and layered encryption, both at the optical line rate. Due to the unavailability of cryptographically strong optical key generation and encryption components, not only a layered encryption is a challenge, but an optical encryption in general. In this paper, we address the challenges of optical anonymous networking for the first time from the system's perspective, and discuss options for practical implementation of all-optical layered encryption. To this end, we propose an optical anonymization component realized with the state-of-the-art optical XOR logic and optical Linear Feedback Shift Registers (oLFSRs). Given that LFSR alone is known for its weak cryptographic security due to its linear properties, we propose an implementation with parallel oLFSRs and analyze the resulting computational security. The results show that proposed optical anonymization component is promising as it can be practically realized to provide a high computational security against deanonymization (privacy) attack.

This work presents an 80 Gbps photonics-aided millimeter-wave (mm Wave) wireless communication system employing 16-Quadrature Amplitude Modulation (16-QAM) and a 1 × 2 single-input multiple-output (SIMO) architecture with maximum ratio combining (MRC) to achieve robust 87.5 GHz transmission over 4.6 km. By utilizing polarization-diverse optical heterodyne generation and spatial diversity reception, the system enhances spectral efficiency while addressing the low signal-to-noise ratio (SNR) and channel distortions inherent in long-haul links. A blind equalization scheme combining the constant modulus algorithm (CMA) and decision-directed least mean squares (DD-LMS) filtering enables rapid convergence and suppresses residual inter-symbol interference, effectively mitigating polarization drift and phase noise. The experimental results demonstrate an SNR gain of approximately 3 dB and a significant bit error rate (BER) reduction with MRC compared to single-antenna reception, along with improved SNR performance in multi-antenna configurations. The synergy of photonic mm Wave generation, adaptive spatial diversity, and pilot-free digital signal processing (DSP) establishes a robust framework for high-capacity wireless fronthaul, overcoming atmospheric attenuation and dynamic impairments. This approach highlights the viability of 16-QAM in next-generation ultra-high-speed networks (6G/7G), balancing high data rates with resilient performance under channel degradation.

We describe 24-port and 36-port antennas based on a cube structure. The antennas are formed by densely packing individual antennas onto a cube. The 24-port antenna cube has a volume of 0.72 lambda <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> while the 36-port antenna has a volume of 1.13 lambda <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> . Even though the individual antennas are densely packed, most combinations of mutual couplings between ports exhibit better than -20 dB isolation. The basic 24-port cube antenna is formed by utilizing cross polarized quarter-wave slots at the cube edges. This provides polarization diversity on every edge of the cube, and both spatial and polarization diversities on the entire structure, leading to good isolation. Additionally 6 pairs of half-wave slot antennas are placed on each surface of the cube to provide 12 extra ports and therefore form our 36-port antenna. The cube antennas are made from FR-4 printed-circuit-boards (PCB), which is low-cost and allows ease of prototyping. The proposed cubes are investigated by simulation and measurement. One potential application for the cube antennas is in multiple-input-multiple-output (MIMO) wireless communication systems. The expected channel capacity of the 36-port cube with both mutual coupling and channel correlation considered is between 131-159 b/s/Hz at an SNR of 20 dB per receiver branch (compared to an ideal channel capacity of 197 b/s/Hz) when MIMO channels in a rich scattering environment with sufficient multipath are available.

This letter investigates the security of physical layer secret key generation in highly reconfigurable scattering environments. Based on our derived secret key capacity expression, we show that the eavesdropper’s accurate knowledge of surrounding environments and propagation laws can significantly decrease the secret key capacity, i.e., the security of keys does not only rely on the channel correlation. We term this new physical layer attack as Environment Reconstruction based Attack for SEcret keys (ERASE) and provide a practical application of this attack based on a generalized 3D channel model. Finally, the impact of crucial parameters related to the channel scattering conditions is analyzed through simulation results, and useful insights for the secure and practical design of physical layer key generation are highlighted.

Ensuring reliable and secure communications in the increasingly pervasive beyond <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$5{^{\text {th}}}$ </tex-math></inline-formula> generation (B5G) wireless networks is one of the key research challenges. Within the physical layer security (PLS) paradigm, the secret key generation (SKG) technique exploits the wireless channel’s randomness to generate symmetric secret keys for message encryption/decryption by legitimate communication nodes. The SKG procedure involves various steps such as wireless channel probing, quantization, information reconciliation, and privacy amplification to render effective symmetric secret key bits. This work proposes a novel multi-level channel quantization scheme, which for the given distribution of the channel envelopes’ gain, ensures an identical likelihood of the envelope samples falling into each quantization interval. Inspired by the classical companding transform (CT), the proposed probability integral transform (PIT)–based quantization scheme works in three sequential steps. First, average contiguous duration (ACD), a second-order fading statistics metric, is considered to select and bias the channel samples that are more likely to fall in the same quantization interval. Subsequently, by using an invertible transform the envelope values are transformed into samples drawn from a uniform distribution, which are then processed through a uniform quantizer. The proposed PIT-based transformation and quantization scheme aims to enhance the trade-offs between the SKG performance metrics namely the key generation rate (KGR), key disagreement rate (KDR), and the key randomness properties. A comprehensive performance analysis of the proposed PIT-based quantization scheme for generalized gamma (GG) fading channels is conducted and its performance is evaluated in relation to several channel and system parameters. Moreover, a comparative analysis of the proposed scheme with its counterparts in the literature is also provided to demonstrate its relative performance gains, where our proposed scheme is observed to outperform the existing schemes. Notably, when the correlation of the reciprocal channel is set to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\rho =0.8$ </tex-math></inline-formula>, the samples excursion qualification threshold is set to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$L=3$ </tex-math></inline-formula>, and GG fading conditions set as defined by <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\alpha =2$ </tex-math></inline-formula>, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\xi =1$ </tex-math></inline-formula>, and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\upsilon =1$ </tex-math></inline-formula>, the proposed algorithm, which employs sample biasing and transformation, offers an average improvement of 0.02% in KGR and 0.15% in KDR performance compared to the conventional UQ scheme. Additionally, the algorithm demonstrates superior performance in all considered eight the National Institute of Standards and Technology (NIST) randomness tests, exemplified by an average P-value of 0.78 in the frequency monobit test. Furthermore, for the considered system and channel parameters the proposed algorithm provides an average improvement of 0.06% in KDR compared to cumulative distribution function (CDF)-based non-uniform quantization (CDF-NUQ).

In free space optical (FSO) communication systems, atmospheric turbulence is the major cause of bit error rate (BER) performance degradation. The error performance of the system can be significantly improved with the help of spatial diversity by employing multiple transmit or receive apertures. However, correlation of channels under turbulence in the case of small separation between transmit apertures can substantially reduce the benefits of spatial diversity. Still, a thorough analysis of the system performance in terms of diversity order and coding gain for the FSO multiple-input single-output (MISO) system under turbulence influenced correlated channels has not been studied when the repetition coding is used to achieve improved BER performance. In this paper, we provide unique results of the BER performance analysis of the considered correlated FSO-MISO system over Gamma–Gamma distributed turbulence channel. Using the joint moment generating function of received signal-to-noise ratio (SNR), a novel generalized approximate BER expression is derived, followed by convergence test of the power series based BER expression using the Cauchy's ratio test. Then, an asymptotic analysis at high SNR is performed to obtain a novel closed-form expression for BER of FSO-MISO system. We also derive expressions for coding gain, diversity gain, and coding gain loss due to correlation in channels. Although the effect of correlation in channels on the BER performance of the system is analyzed under different scenarios, while it is observed that it does not affect the diversity order; it significantly degrades the coding gain of the considered system.

Ultraviolet communication (UVC) is becoming increasingly popular due to its ability to effectively communicate in non-line-of-sight (NLOS) mode, which offers alternative communication system advantages over visible light communication and infrared links. However, the performance of UVC is affected by atmospheric turbulence, which is generally ignored under the assumption of short-distance links. In this paper, we consider a NLOS UVC system experiencing turbulence due to variation in the refractive index of the atmosphere. The atmospheric turbulence results in fading due to fluctuations in the received signal strength, thus deteriorating the quality of communication. Spatial diversity is proven to mitigate the effect of fading by introducing multiple parallel communication paths between the transmitter and receiver. We present a spatial diversity reception in the form of ${N_r}$-branches selection combining at the receiver. The channel coefficients are assumed to be exponentially correlated, and turbulence is modeled using log-normal distribution under weak turbulence conditions. Closed-form expressions for the outage probability and average symbol error rate for general order rectangular quadrature amplitude modulation (RQAM), cross-QAM, and hexagonal QAM schemes are derived. Furthermore, the ergodic capacity of the system is computed as a function of the channel correlation coefficient. The numerical values are compared with computer simulations to validate the accuracy of the theoretical analysis.

A novel cooperative automatic repeat request (ARQ) method with network coding is proposed for broadcast networks where single source node broadcasts multiple packets to multiple destinations. Upon a failed transmission, multiple relays cooperate with the source node at the retransmission phase, where network coding is used to combine multiple lost packets. A random access mechanism is employed where retransmission is done according to back-off counters. The proposed method employs the back-off mechanism to achieve space diversity by punishing the nodes which fail at previous retransmissions, and to reduce the number of retransmissions by giving priority to nodes having opportunity to employ network coding. The proposed method is compared with network coded ARQ without cooperation and traditional ARQ using Monte Carlo simulations for different channel settings. The effect of channel correlation is investigated where correlation is modeled as a finite-state Markov model. Simulation results show that the proposed use of cooperation and network coding for ARQ improves throughput performance when relay channels' conditions are better than the channels between source and destinations. It is also observed that, the proposed strategy exploits cooperation and space diversity to reduce the performance degradation due to correlation.

This paper investigates the performance analysis of dual selection (i.e., combining user selection and transmit antenna selection) with maximal ratio combining (MRC) over nonidentical imperfect channel estimation in a multiple-input multiple-output system. This paper derives the exact and approximate cumulative density function and probability density function (PDF) for the dual selection with MRC over nonidentical imperfect channel estimation. Using the distribution, the exact and approximate closed-form expressions of the outage probability are derived with quantification of diversity order. Further, this paper derived the exact ergodic capacity and approximate outage capacity of the the dual selection via Gaussian and Gamma approximations. The numerical results show that the capacity of the dual selection with MRC is enhanced by the number of receive antennas as well as the number of user terminals and sum of channel correlation coefficients, while the order of the receive spatial diversity for outage probability is eliminated by the nonidentical channel estimation error.

Dynamic resource allocation combined with multiple antennas (MIMO) can farther increase the system capacity and the quality of service (QoS) of conventional OFDM systems. Selection diversity is the simplest way to realize spatial diversity, but the performance improvement of this form of diversity is limited. Adaptive antenna arrays promise to achieve significant increases in system capacity and performance in wireless communications, but they are characterized by a relatively higher implementation complexity than that of selection diversity. In this paper, we propose a dynamic resource allocation scheme with selective beamforming for MIMO/OFDM systems. By applying eigenvalue decomposition on the subset of all available channel correlation matrices, the best beamforming, which corresponds to the largest eigenvalue in the subset, is adaptively selected. Then the proposed dynamic resource allocation algorithm adaptively assigns the bit and power distribution according to the channel gain corresponding to the selected best beamforming, under the constraint of fixed power and overall bit rate. By doing so, the maximal SNR on each sub-carrier obtained by selective beamforming is further maximized. Numerical results show that under frequency-selective multipath fading, the proposed system yields significant performance improvements over the conventional OFDM transmission.

In this paper we present a local channel correlation based zero-forcing (ZF) beamforming framework for the cell edge users. Without the instantaneous CSI (ICSI) assisting, we derive the exact expression of the averaged pairwise error probability (PEP) upper bounds at both of the receivers. According to the derived PEP bounds, an multi-objective optimization problem is formulated to get the elaborate power allocation and ZF beamforming design with the knowledge of local statistical channel state information (SCSI) at the transmitter side. By factorizing each beamforming matrix as the multiplication of two matrices, we can avoid inter-user interference through column orthogonal matrix design. Moreover, the original complicated optimization problem can be further decoupled into four simple sub-optimization problems. Then utilizing the Lagrange multiplying method, we finally achieve the feasible distributed statistical ZF beamforming design, which only involves the distributed computations based on the local channel correlations. Both theoretical analysis and simulation results confirm that the multi-cell spatial diversity gain can be fully exploited with only local SCSI. For comparison, the traditional perfect ICSI based ZF beamforming is analyzed and discussed. It is shown that in the high regime of signal-to-noise ratio (SNR), the diversity gain of the proposed statistical ZF beamforming is even better than that achieved by the conventional ICSI-aided ZF beamforming.

The stringent reliability and latency requirements of industrial networks are challenged by harsh propagation environments. To design effective communication systems, a thorough understanding of the underlying channel characteristics is essential. Deploying multiple, spatially distributed Access Points (APs) is a promising strategy to overcome these challenges by creating resilient communication links through macro-diversity. This paper presents an experimental characterization of a multi-AP radio channel in a representative industrial laboratory environment, focusing on the 6 GHz band (upper FR1), enabling the investigation of frequency-dependent propagation mechanisms beyond traditional sub-6 GHz cellular bands. We analyze key statistical properties, including inter-link correlation, coherence distance, and coherence bandwidth, to assess the potential for both macro- and micro-diversity. The results demonstrate that the channels from distributed APs are largely uncorrelated, highlighting a significant opportunity for macro-diversity. Conversely, the measured coherence distances far exceed typical device dimensions, suggesting limited returns from single-device spatial diversity. An evaluation of selection combining, equal-gain combining, and maximal-ratio combining techniques confirms that distributed APs can provide substantial diversity gains, ranging from 2.6 dB to 23.1 dB. These findings validate the deployment of coordinated, distributed AP architectures as a crucial strategy for achieving robust, low-latency connectivity in future industrial settings.

Y MIMO leads to dramatic improvement in channel capacity and/or link reliability of wireless systems. However, a MIMO channel has only one degree of freedom in a keyhole environment. As a result, this environment reduces achievable channel capacity and link quality. This paper proposes a MIMO repeater system, which can realize a multi-stream transmission. Although the averaged channel capacity in the MIMO repeater system is discussed in several published papers, the probability density functions of eigenvalues of correlation matrix are not analyzed. MIMO transmission performance can basically be estimated from eigenvalues of the channel correlation matrix. We derive an approximated formula for the probability density function of all eigenvalues linked to the space diversity. It is shown that the calculated values based on the proposed method agrees very well with the simulated values.