Transceiver hardware impairments (THI) significantly affect the performance of communication systems, necessitating their analysis for realistic performance evaluation and robust system design. Furthermore, integrating channel coding is imperative for enhancing error resilience under diverse fading environments. However, the joint impact of THI and channel coding on error performance in downlink rate-splitting multiple access (RSMA) systems remains unexplored. This paper investigates the impact of THI on the symbol error rate (SER) and explores the effectiveness of block codes in mitigating these impacts in RSMA systems. More precisely, a detailed analysis of the constellation-based SER is conducted for a multi-user RSMA setup, considering both transmitter and receiver hardware impairments with perfect and imperfect detection events of the common stream. Moreover, closed-form expressions for the coded average SER (ASER) are derived under both zero and non-zero THI for multi-user RSMA systems over Nakagami-mfading with varying shape parameters in single-input single-output (SISO) scenarios. This study also presents asymptotic ASER expressions with diversity order analysis under both zero and non-zero THI. The accuracy of these derived ASER expressions is verified through Monte Carlo simulations. Furthermore, the SISO analytical framework is extended to the multiple-input multiple-output scenario, with comparative evaluations against the SISO scenario to demonstrate the practical applicability of the proposed coded RSMA system.

This paper presents two asymmetric one-way full-duplex (FD) cooperative communication systems that incorporate advanced multiple access schemes. Both systems are formed using a mixed free-space optical (FSO)/millimeter wave (mmWave)-radio-frequency (RF) configuration, where the first hop employs a hybrid FSO/mmWave and the second hop utilizes RF links to support mobile connectivity for multiple users. The first system employs rate-splitting multiple access (RSMA) to minimize successive interference cancellation (SIC) operations, termed the FD asymmetric cooperative-RSMA (C-RSMA) system. In contrast, the second system adopts quadrature RSMA (QRSMA) to eliminate SIC, termed the FD asymmetric C-QRSMA system. For comprehensive analysis, the FSO, mmWave, and RF links are modelled using Málaga-Mdistribution with pointing error, Nakagami-m, and κ − μ fading, respectively. Unlike existing works that focus on sum rate or signal-to-interference-plus-noise ratio, this study derives outage probability, coverage probability, average sum rate, and energy efficiency. These metrics, derived in multi-user environments with imperfect SIC and signal space diversity, are validated through Monte Carlo simulations. Finally, the proposed systems are compared with state-of-the-art schemes, revealing that the systems not only enhance spectral efficiency but also significantly reduce system complexity.

This paper proposes a novel quantum key distribution (QKD) set-up designed to enhance the security of a free-space optical (FSO) quantum communication system. By incorporating channel ordering information, the proposed QPSK-QKD (P-QPSK-QKD) system leverages the inherent randomness of FSO channels, providing dual protection against eavesdroppers (Eve). The first layer of security is achieved through the QPSK-QKD mechanism, while the second arises from the randomness of the FSO channels. To ensure a comprehensive investigation, the performance of P-QPSK-QKD and the existing QPSK-QKD (E-QPSK-QKD) system is analyzed using a Málaga distributed atmospheric turbulence channel model incorporating pointing errors. Novel closed-form expressions are derived for different performance metrics, including probability of sifted keys, quantum bit error rate, probability of Eve (Peve), and ergodic secret key rate (ESKR), under both perfect and imperfect feedback conditions. Intensive examination indicates that, unlike E-QPSK-QKD, wherePeveshows a water-falling trend with increasing signal-to-noise ratios (SNR) under dual thresholds, P-QPSK-QKD achieves a saturatedPeve, enhancing security. Moreover, under the single threshold,Peveof P-QPSK-QKD is twice that of E-QPSK-QKD. Additionally, due to channel randomness, the mutual information between Alice and Eve remains zero (for both single and dual thresholds), even at infinite SNR for P-QPSK-QKD, providing consistently higher ESKR than E-QPSK-QKD system under both perfect and imperfect feedback conditions.

Rate-splitting multiple access (RSMA) has emerged as a promising solution for managing multi-user interference in wireless communication systems. However, a significant challenge persists in mitigating high interference during private stream detection in conventional RSMA and quadrature-RSMA (Q-RSMA), particularly in multi-user scenarios. To address this issue, this work proposes a rate-splitting technique called private stream interference-management RSMA (P-RSMA), which leverages coordinate interleaving among combinations of private streams to reduce the number of interfering symbols. To analyze and compare P-RSMA with RSMA and Q-RSMA, a constellation-based analytical framework is presented to derive symbol error rate (SER) expressions for multi-user P-RSMA, RSMA, and Q-RSMA schemes under the single-input single-output (SISO) setting, considering both perfect and imperfect detection events of the common stream. A detailed power-bound analysis is also performed to resolve the constellation ambiguity problem in all considered schemes for arbitrary modulation orders of the common and private streams, emphasizing the strategic importance of adapting modulation orders for different symbols. Closed-form uncoded and coded average SER (ASER) expressions are then derived under Rayleigh fading channels with channel ordering. This work further incorporates a diversity order evaluation for the constellation-based framework to characterize asymptotic performance and benchmark against SINR- based methods. Additionally, the analytical framework is extended to the multiple-input multiple-output (MIMO) setting, and results are compared with those from the SISO scenario to demonstrate the performance gains and practical relevance of the proposed P-RSMA scheme. Numerical results demonstrate the superiority of P-RSMA over RSMA and Q-RSMA, which are corroborated through Monte Carlo simulations.

This paper explores the limitations of a conventional reconfigurable intelligent surface-assisted spatial modulation (RIS-SM) system by analyzing the signal-spatial constellation diagram. These limitations include closely packed signalspatial points and a lack of flexibility in adjusting their positions, negatively affecting system performance. To address these challenges, this work proposes a unified RIS-SM (U-RIS-SM) system that utilizes optimal RIS reflector phases and a modified modulation scheme with a tuning parameter. The proposed U-RIS- SM system facilitates the constellation redesigning of signalspatial points by adjusting the tuning parameter. Additionally, the optimal tuning parameter value is determined to achieve the best possible performance of U-RIS-SM. Closed-form expressions for the symbol error rate (SER), average SER (ASER), and average mutual information (MI) of the proposed U-RIS-SM system under the Nakagami-m fading channel are derived and validated through Monte-Carlo simulations. Moreover, since the proposed U-RIS-SM system can be transformed into existing RIS-SM and RIS-assisted space shift keying (RIS-SSK) systems by varying the tuning parameters and RIS reflector phases, the performance metrics for existing RIS-SM/SSK systems are derived from expressions for U-RIS-SM. The U-RIS-SM system is compared with existing RIS-SSK and RIS-SM systems across various parameters, demonstrating its superior performance. Furthermore, to assess its effectiveness under practical conditions, its performance is examined using a low-complexity two-stage maximum likelihood detector, as well as under RIS phase errors and discrete RIS phase shifts.

Multi-carrier power-domain non-orthogonal multiple access (MC-PD-NOMA) has been considered a key technology for next-generation wireless networks. In MC-PD-NOMA, the available channel bandwidth is divided into several sub-carriers in order to serve the multiple users in each sub-carrier. In this work, to reduce the number of successive interference cancellation (SIC) operations, detection delay, SIC complexity and to enhance the SIC stability of the MC-PD-NOMA, a system, namely, MC-quadrature-NOMA (MC-Q-NOMA), is proposed. Furthermore, the proposed MC-Q-NOMA follows the essential upconversion architecture to provide orthogonality in the pass-band. Moreover, a novel analytical framework is introduced to evaluate analytical expressions of average sum-rate (ASR), outage probability (OP), and diversity order for the proposed system under the generalized$\alpha -\mu $channel for perfect and imperfect SICs. In addition, the proposed framework is utilized to determine the analytical expression of ASR, OP, and diversity order for MC-PD-NOMA. Comparative analysis indicates that under imperfect SIC, MC-Q-NOMA outperforms the MC-PD-NOMA due to the lesser number of SICs. However, under perfect SIC, MC-Q-NOMA demonstrates comparable performance with the MC-PD-NOMA. Furthermore, the analytical expression of the average bit-error-rate is evaluated for the MC-Q-NOMA. Numerical results demonstrate that the error performance of MC-Q-NOMA is superior to MC-PD-NOMA for all signal-to-noise ratio values.

Rate splitting multiple access (RSMA) is a novel multiple access scheme that reduces interference by partially decoding the interference and partially treating the interference as noise. This paper presents a constellation-based analytical framework to derive novel symbol error rate (SER) expressions for a two-user RSMA system by considering both perfect and imperfect decoding of the common stream (CS). The SER expressions are evaluated by considering$M$-ary pulse amplitude modulation as users' private stream (PS) and quadrature phase shift keying as a composite CS. Moreover, the power bound analysis is performed to derive power constraints, which solves the constellation ambiguity problem for arbitrary modulation orders of PS. The closed-form average SER (ASER) is then derived under the Rayleigh channel with order statistics. Numerical result demonstrates proper water-falling ASER curves with an increased signal-to-noise ratio and verifies the derived ASER expressions by Monte-carlo simulation. Furthermore, the optimal power assignment is determined that minimizes the overall system's ASER.

In this paper, two transmission strategies for multiple relay-based full-duplex (FD) mixed radio-frequency/free space optical (RF/FSO) system are proposed and analyzed while considering the effect of self-interference at the relay nodes and RF source node. Unlike existing relay selection (RS)-based techniques, the proposed strategies do not require relay selection phase, as all relays are always active. Therefore, the proposed strategies preserve the time required to select the best relay, leading to improved throughput of the systems. The first strategy transmits the information from all relays to the RF source node with uniform power and is therefore termed the uniform transmission (UT) strategy. On the other hand, to examine the absolute limit of FD mixed RF/FSO system, the second strategy utilizes transmit beamforming (TB) instead of UT strategy. The analytical expression for outage probability (OP) is derived for both strategies considering Rayleigh channel for RF links and Gamma-Gamma channel with pointing error for FSO links. Furthermore, the upper bound-achievable sum-rate (UB-ASR) is examined after self-interference cancellation. Numerical results show that the proposed strategies outperform existing RS strategies in terms of the OP and UB-ASR.

This work analyzes a terrestrial-based free-space quantum key distribution system employing quadrature-phase shift keying signaling (QKD-QPSK). An analytical framework is proposed to determine various joint probabilities under the gamma-gamma turbulence channel model without utilizing the Gaussian–Laguerre (GL) method. Although GL-based method provides ease of computation of integral forms, but it is not always stable. Using derived joint probabilities, novel analytical expressions for the probability of sifted keys (Psift), quantum bit error rate (QBER), error probability for the eavesdropper (Peve), and the ergodic secret-key rate (ESKR) are determined for QKD-QPSK system. This work also reveals that the QKD-QPSK system can be transformed into QKD-based binary phase-shift keying (QKD-BPSK) system. Furthermore, using the proposed analytical framework, all the considered performance metrics are also evaluated for the QKD-BPSK system. Finally, QKD-QPSK and QKD-BPSK systems are compared under moderate and strong turbulence regimes. Comparative results indicate that QKD-QPSK outperforms QKD-BPSK in terms of Peve. However, QKD-BPSK is superior to QKD-QPSK for Psiftand QBER. Furthermore, a positive ESKR can be achieved for both systems with the proper selection of threshold coefficient (ρ) and transmitted power (Pt).

Reconfigurable intelligent surface (RIS)-assisted power-domain non-orthogonal multiple access (PD-NOMA) system has emerged as a revolutionary technology to enhance the spectrum efficiency for future wireless networks. This work introduces two novel RIS systems, namely, RIS partition-assisted (RISP) PD-NOMA (RISP-PD-NOMA) and RISP-Quadrature-NOMA (RISP-Q-NOMA), to maximize the diversity order and to improve the signal quality of all users by dedicating fixed RIS units to each user. The closed-form expressions of average sum-rate, outage probability, and diversity order are evaluated for both systems under the Rician fading channel for perfect and imperfect successive interference cancellation (SIC) operations. Further, the performance of both RISP-PD-NOMA and RISP-Q-NOMA systems is compared with RIS partitioned PD-NOMA, RIS-assisted PD-NOMA, and PD-NOMA systems. It is noticed from the analysis that under perfect SIC, RISP-PD-NOMA outperforms all other systems. However, under imperfect SIC, RISP-Q-NOMA demonstrates superior performance to other systems due to the lesser number of SIC operations that reduce detection delay and decoding complexity. Furthermore, the analytical expression for bit error rate (BER) of RISP-PD-NOMA and RISP-Q-NOMA is derived, and then the closed-form expression of average-BER is evaluated. Numerical results demonstrate that RISP-Q-NOMA is superior to RISP-PD-NOMA.

In this paper, a novel constellation domain non-orthogonal multiple access (Con-NOMA) system using signal-space diversity technique is proposed for the downlink network. More specifically, users will be multiplexed in the in-phase and quadrature part of the rotated constellation points of M-ary phase-shift keying/quadrature amplitude modulation symbol. The proposed Con-NOMA is capable of bypassing the requirement of successive interference cancellation, joint detection, uneven power assignment, etc., which are required in the power domain NOMA (PD-NOMA). Further investigation shows that the Con-NOMA is superior to PD-NOMA in terms of the achievable data rate for each user as well as the sum-rate of the overall system. The proposed Con-NOMA is then examined under Nakagami-m fading channel by evaluating the closed-form expression of the average symbol error rate (ASER) for each user. After comparison, it is concluded that the error performance of each user in the proposed Con-NOMA system is much better than in the PD-NOMA system.

In this paper, the effect of jamming in free space optical (FSO) link, is evaluated by deriving closed-form expressions of the bit error rate (BER) and outage probability (OP) for single-input single-output (SISO) and multiple-input single-output (MISO) FSO systems. The effects of partial-band jamming and broadband jamming over the error performance of the considered FSO systems are also analyzed. The jammer behaves as a random noise source, following the negative exponential distribution-due to the atmospheric turbulence. Therefore, in the presence of jammer, the error performance of FSO systems is governed by the additive negative exponential noise. It is shown by a rigorous analysis that a MISO FSO system can significantly mitigate the effect of jamming. Specifically, we consider a $2\times 1$ FSO system for analysis, and demonstrate the improvement in BER and OP performance an FSO system can gain by using an additional spatial dimension, in the presence of a random jammer. The mitigation of jamming, due to implementation of arbitrary transmit apertures, is also verified through simulation results; which lies in complete agreement with the analytical results. Also, the error performance under the jamming effect is studied numerically over the Gamma-Gamma fading channel incorporated with pointing error effect.

In this paper, we study the impact of correlation on the bit error rate (BER) and the channel capacity of a free-space optical (FSO) multiple-input-multiple-output (MIMO) system employing optical space shift keying (OSSK) over a fading channel. In order to study a practical correlated channel, we consider the effect of channel correlation due to both small-and large-scale eddies and show that the use of OSSK over correlated FSO channel can lead to an improved system performance with increasing correlation level of upto 0.9. In this work, we first develop an analytical framework for different performance metrics of the OSSK multiple-input single-output system with correlation and then extend our investigation by proposing an asymptotically accurate mathematical framework for MIMO. We also validate all the analytical results using MATLAB simulations. Finally, we develop an experimental setup of FSO with two correlated links to study the throughput and latency of the links at different turbulence levels.

In free space optical (FSO) communication, jamming can cause inaccuracy in the received information. In this paper, we investigate the impact of jamming over atmospheric turbulence (AT) fading channels. To describe the AT, negative exponential (NE) and Gamma-Gamma (ΓΓ) fading channel models are considered. The effect of jamming is studied over a single-input single-output (SISO) FSO link. We introduce a theoretical framework to acquire a closed-form expression of the average bit error rate (ABER) under jamming-leading to an additive non-Gaussian noise channel. It is observed by thorough analysis that a 2 x 1 FSO system performs better than SISO FSO system in presence of jamming signal. It is realized that under jamming the performance of FSO link is dependent on different AT conditions. Furthermore, increasing number of transmit apertures allows FSO system to significantly overcome the effect of jamming, in order to obtain an improved ABER.

Due to the rapid growth of free-space optical (FSO) communication over the last few years, there is an exigency to develop diversity-multiplexing tradeoff (DMT) metric in order to compare and design new multiple-input multiple-output (MIMO) FSO schemes. This paper introduces the DMT for MIMO-FSO systems that utilize the intensity modulation and direct detection technique for which the channel gains are real and positive, contrary to MIMO radio-frequency (MIMO-RF) systems where the channel gains are complex. Log-normal, gamma-gamma, and negative exponential channel models are used in this paper to cover weak to saturated turbulence environments. It is shown that at zero multiplexing gain (r), the log-normal channel offers maximum but finite diversity order that depends on the block length within which the channel matrix is constant. On the other hand, negative exponential channel offers minimum diversity order. However, for r ≥ 1, DMT curves under all channel models are of the same nature; and for a given r, the diversity is half as that observed in the MIMO-RF channels. To gain additional insights into the DMT of an MIMO-FSO system, we also develop upper and lower bound expressions of DMT and determine the minimum block length at which these bounds are identical.

In this paper, a partially informed transmitter-based optical space shift keying (PIT-OSSK) scheme is proposed, wherein the transmitter only possesses the knowledge of channel gain ordering. This partial information is used to adapt the OSSK constellation and power allocation (PA). The adaptation of OSSK constellation reduces the number of bits in error between two consecutive constellation points, which in turn lowers the overall bit error rate (BER) of the proposed scheme. On the other hand, it is demonstrated by a mathematical analysis that the PA achieves improved diversity order as compared to the conventional OSSK scheme. These modifications make the PIT-OSSK scheme significantly better than the conventional OSSK scheme in terms of the BER under both indoor and outdoor conditions. For a single receiver aperture, accurate closed-form expressions of average BER (ABER) for two and four transmitter aperture-based PIT-OSSK schemes are derived under negative-exponential channel model. Moreover, a very tight approximate closed-form expression of the ABER for an arbitrary number of transmitter apertures is also developed. All the derived expressions are verified via Monte-Carlo simulations. Furthermore, the proposed strategy is extended and examined for an arbitrary number of receiver apertures by the simulation-based results under different channel models.

Stream processing applications reduce the latency of batch data pipelines and enable engineers to quickly identify production issues. Many times, a service can log data to distinct streams, even if they relate to the same real-world event (e.g., a search on Facebook's search bar). Furthermore, the logging of related events can appear on the server side with different delay, causing one stream to be significantly behind the other in terms of logged event times for a given log entry. To be able to stitch this information together with low latency , we need to be able to join two different streams where each stream may have its own characteristics regarding the degree in which its data is out-of-order . Doing so in a streaming fashion is challenging as a join operator consumes lots of memory, especially with significant data volumes. This paper describes an end-to-end streaming join service that addresses the challenges above through a streaming join operator that uses an adaptive stream synchronization algorithm that is able to handle the different distributions we observe in real-world streams regarding their event times. This synchronization scheme paces the parsing of new data and reduces overall operator memory footprint while still providing high accuracy. We have integrated this into a streaming SQL system and have successfully reduced the latency of several batch pipelines using this approach.

In this paper, the bit error rate (BER) and the channel capacity of a multiple-input multiple-output (MIMO) system employing optical space shift keying (OSSK) scheme (termed as MIMO-OSSK) are investigated under combined effect of atmospheric turbulence (AT) and pointing errors (PEs). The analysis reveals that the diversity order of MIMO-OSSK remains intact over all AT regimes and under PE severity. This behavior is contrary to traditional pulse-based modulation schemes, where diversity order depends upon AT conditions and PE severity. In addition, the analysis also demonstrates that the MIMO-OSSK utilizes receiver diversity efficiently as compared to existing pulse-based modulations. In order to examine the effect of all ranges of AT along with PE severity on MIMO-OSSK, three different channel models are considered, i.e., log normal, gamma-gamma, and negative exponential. The probability density functions of the sum of squares of difference between two channel coefficients under these channel models are evaluated, to develop analytical expressions of average BER and ergodic channel capacity for the considered setup. On comparing MIMO-OSSK with the same rate MIMO-pulse amplitude modulation (MIMO-PAM) scheme employing repetition coding, it is observed that the MIMO-OSSK scheme outperforms the MIMO-PAM for a spectral efficiency of more than 2 b/s/Hz, provided that the number of receivers is sufficiently large.

In this paper, the ergodic capacity of optical space shift keying (OSSK) based multiple-input multiple-output (MIMO) system under atmospheric turbulence (AT) is investigated. Negative exponential (NE) and Gamma-Gamma (G-G) channel models are used to characterize the AT. At first, a novel expression of the distribution of a sum of square of the difference between two NE/G-G random variables is evaluated. Later, a closed-form expression of the ergodic capacity for OSSK-MIMO system under different AT conditions is obtained with the help of derived distributions. It is shown by the analysis that the value of signal-to-noise ratio (γ̅th) to reach 90% of the maximum achievable capacity (Cmax) is same under moderate, strong, and saturation AT regimes for a particular OSSK-MIMO system. The required value of γ̅thremains constant irrespective of number of transmitter (Txs); however, Cmaxis enhanced with increasing number of Txs. On the other hand, a reduction in requirement of 7th is observed when the number of receivers (Rxs) is increased, while Cmaxremains constant irrespective of number of Rxs. Additionally, a new series based average bit error rate (ABER) expression, which has better accuracy and less mathematical complexity than a previously known ABER expression for OSSK-MIMO system, is obtained using the proposed distributions.

Realtime data processing powers many use cases at Facebook, including realtime reporting of the aggregated, anonymized voice of Facebook users, analytics for mobile applications, and insights for Facebook page administrators. Many companies have developed their own systems; we have a realtime data processing ecosystem at Facebook that handles hundreds of Gigabytes per second across hundreds of data pipelines.