Browsing by Author "Zafaruddin, S.M."

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    Asymptotic performance analysis of zero forcing DSL systems
    (IEEE, 2016) Zafaruddin, S.M.
    In this paper, we present a novel approximation to the spectral efficiency at the output of a zero-forcing (ZF) receiver in a DSL upstream system. This approximation is extracted by a new asymptotic analysis, where we construct an alternative sequence of systems that includes the system of interest. Drawing on works in the field of large dimensional random matrices, we show that the user rate in this sequence converges to a non-zero rate. We also provide numerical simulations which show that this approximation is accurate even for relatively low dimensional systems.
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    Asymptotic performance of ZF and MMSE crosstalk cancelers for DSL systems
    (Elsevier, 2019-05) Zafaruddin, S.M.
    We present asymptotic expressions for user throughput in a multi-user digital subscriber line system (DSL) with a linear decoder, in increasingly large system sizes. This analysis can be seen as a generalization of results obtained for wireless communication. The features of the diagonal elements of the wireline DSL channel matrices make wireless asymptotic analyses inapplicable for wireline systems. Further, direct application of results from random matrix theory (RMT) yields a trivial lower bound. This paper presents a novel approach to asymptotic analysis, where an alternative sequence of systems is constructed that includes the system of interest in order to approximate the spectral efficiency of the linear zero-forcing (ZF) and minimum mean squared error (MMSE) crosstalk cancelers. Using works in the field of large dimensional random matrices, we show that the user rate in this sequence converges to a non-zero rate. The approximation of the user rate for both the ZF and MMSE cancelers are very simple to evaluate and does not need to take specific channel realizations into account. The analysis reveals the intricate behavior of the throughput as a function of the transmission power and the channel crosstalk. This unique behavior has not been observed for linear decoders in other systems. The approximation presented here is much more useful for the next generation G.fast wireline system than earlier DSL systems as previously computed performance bounds, which are strictly larger than zero only at low frequencies. We also provide a numerical performance analysis over measured and simulated DSL channels which show that the approximation is accurate even for relatively low dimensional systems and is useful for many scenarios in practical DSL systems.
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    Distributed Energy Efficient Channel Allocation
    (IEEE, 2019-08) Zafaruddin, S.M.
    Design of energy efficient protocols for modern wireless systems has become an important area of research. In this paper, we propose a distributed optimization algorithm for the channel assignment problem for multiple interfering transceiver pairs that cannot communicate with each other. We first modify the auction algorithm for maximal energy efficiency and show that the problem can be solved without explicit message passing using the carrier sense multiple access (CSMA) protocol. We then develop a novel scheme by converting the channel assignment problem into perfect matchings on bipartite graphs. The proposed scheme improves the energy efficiency and does not require any explicit message passing or a shared memory between the users. We derive bounds on the convergence rate and show that the proposed algorithm converges faster than the distributed auction algorithm and achieves near-optimal performance under Rayleigh fading channels. We also present an asymptotic performance analysis of the fast matching algorithm for energy efficient resource allocation and prove the optimality for a large enough number of users and number of channels. Finally, we provide numerical assessments that confirm the faster convergence of the proposed algorithm compared to the distributed auction algorithm.
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    Distributed Learning for Channel Allocation Over a Shared Spectrum
    (IEEE, 2019-08) Zafaruddin, S.M.
    Channel allocation is the task of assigning channels to users such that some objective (e.g., sum-rate) is maximized. In centralized networks such as cellular networks, this task is carried by the base station (BS) which gathers the channel state information (CSI) from the users and computes the optimal solution. In distributed networks such as ad-hoc and device-to-device (D2D) networks, no BS exists and conveying global CSI between users is costly or simply impractical. When the CSI is time varying and unknown to the users, the users face the challenge of both learning the channel statistics online and converging to a good channel allocation. This introduces a multi-armed bandit (MAB) scenario with multiple decision makers. If two or more users choose the same channel, a collision occurs and they all receive zero reward. We propose a distributed channel allocation algorithm that each user runs and converges to the optimal allocation while achieving an order optimal regret of O (log T ), where T denotes the length of time horizon. The algorithm is based on a carrier sensing multiple access (CSMA) implementation of the distributed auction algorithm. It does not require any exchange of information between users. Users need only to observe a single channel at a time and sense if there is a transmission on that channel, without decoding the transmissions or identifying the transmitting users. We demonstrate the performance of our algorithm using simulated LTE and 5G channels.
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    Diversity Analysis of Multi-Aperture UWOC System over EGG Channel with Pointing Errors
    (ARXIV, 2022-12) Zafaruddin, S.M.
    Single aperture reception for underwater wireless optical communication (UWOC) is insufficient to deal with oceanic turbulence caused by the combined effect of temperature gradient and air bubbles. This paper analyzes the performance of multi-aperture reception for UWOC under channel irradiance fluctuations characterized by the mixture exponential generalized gamma (EGG) distribution. We analyze the system performance by employing both selection combining (SC) and maximum ratio combining (MRC) receivers. In particular, we derive the exact outage probability expression for the SC-based multi-aperture UWOC receiver and obtain an upper bound on the outage probability for the MRC-based multi-aperture UWOC receiver. With the help of the derived results, we analytically obtain the diversity order of the considered multi-aperture UWOC system.
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    Dual Sensor Impulse Noise Cancellation for Downstream DSL Systems
    (IEEE, 2021-05) Zafaruddin, S.M.
    Impulse noise presents a severe performance bottleneck in multicarrier systems. For digital subscriber lines (DSL), the use of a common-mode (CM) sensor is a viable technique to mitigate the impulse noise that couples into the useful differential-mode (DM) signal. In this paper, we use time and frequency domain approaches to develop algorithms using the joint processing of CM and DM signals and analyze the performance of dual-sensor based interference cancellation schemes for downstream DSL systems. First, we consider the frequency domain approach and analyze the performance of a per-tone impulse noise canceler without requiring an impulse detector. We derive closed-form expressions on the performance of canceler in various interference scenarios in terms of system parameters. Next, we develop a novel per-symbol time-domain impulse noise canceler by simultaneous estimation of the CM2DM transfer function and the CM impulse noise using a limited number of null carriers. The proposed algorithm eliminates the issues of convergence of the per-tone canceler and stationarity of the impulse noise over multiple symbols. Using derived analytical expressions, we show that the proposed canceler effectively mitigates the impulse noise on a per-symbol basis. We also demonstrate the performance of the considered canceler schemes using measurement and simulation results.
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    Energy Consumption Performance of Opportunistic Device-to-Device Relaying Under Log-Normal Shadowing
    (IEEE, 2021-12) Zafaruddin, S.M.
    Efficient transmission protocols are required to minimize the energy consumption of mobile devices for ubiquitous connectivity in the next-generation of wireless networks. In this article, we analyze the energy consumption performance of a two-hop opportunistic device-select relaying (ODSR) scheme, where a device can either transmit data directly to a base station (BS) or relay the data to a nearby device, which forwards the data to the BS. We select a single device opportunistically from a device-to-device (D2D) network based on the energy required for transmission, including the energy consumed in the circuitry of the devices. By considering the log-normal shadowing as the dominant factor between devices and the BS, and Rayleigh fading in D2D links, we derive analytical bounds and scaling laws on average energy consumption. The derived analytical expressions show that the energy consumption of the ODSR decreases logarithmically with an increase in the number of devices, and achieves near-optimal performance only with a few nearby devices. This is an important design criterion to reduce latency and overhead energy consumption in a relay-assisted large-scale network. We also demonstrate the performance of the ODSR using simulations in realistic scenarios of a wireless network.
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    Exact Analysis of RIS-Aided THz Wireless Systems Over α-μ Fading With Pointing Errors
    (IEEE, 2021-11) Zafaruddin, S.M.
    Reconfigurable intelligent surfaces (RIS) can have an excellent use case for terahertz (THz) wireless transmissions. In this letter, we analyze the performance of a RIS-empowered THz system over the combined effect of α - μ fading and pointing errors. We derive exact closed-form expressions of density and distribution functions of the resultant signal-to-noise ratio (SNR) considering an independent and not identically distributed (i.ni.d.) channel model. Using the derived statistical results, we present an exact analysis on outage probability, ergodic capacity, and average bit-error-rate (BER) of the considered system. We also develop asymptotic analysis on the outage probability and average BER to derive diversity order in terms of system parameters. The proposed analysis provides insights to mitigate the effect of pointing errors and shows that the RIS can significantly improve the performance of THz communications. We validate the analytical results using numerical and Monte Carlo simulations and demonstrate the scaling of system performance with an increase in the number of RIS elements.
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    Exact Performance Analysis for Multi-Aperture UWOC Systems Over EGG Oceanic Turbulence
    (IEEE, 2024) Zafaruddin, S.M.
    A research gap exists in analyzing the exact performance of wireless systems that employ selection combining diversity technique with Meijer-GlFox-H function representation for signal fading. In this paper, we develop a general framework to analyze the exact performance of wireless systems that employ the Meijer-G function with integer exponents for statistical characterization. We derive exact analytical expressions for the average bit-error rate (BER) for multi-aperture underwater wireless optical communication (UWOC) system operating over exponential-generalized gamma (EGG) oceanic turbulence combined with pointing errors. We also develop asymptotic expressions at a high signal-to-noise (SNR) to capture insights into the system's performance. Our simulation findings confirm the accuracy of our analytical expressions and illustrate that the exact expression may provide a better estimate for the efficient deployment of UWOC.
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    Exact Performance Analysis of THz Link Under Transceiver Hardware Impairments
    (ARXIV, 2023-02) Zafaruddin, S.M.
    Transceiver hardware impairment (THI) is inevitable for high-date rate terahertz (THz) communication. Existing statistical analysis either neglects THI's effect or provides approximate results when analyzing the performance of the THz system combined with channel fading and antenna misalignment. In this paper, we develop exact analytical expressions for the average signal-to-noise ratio (SNR), ergodic capacity, and average bit-error-rate (BER) performance of a THz wireless link under the combined effect of α-μ fading channel, zero-boresight pointing errors, and the Gaussian distributed THI. We also derive asymptotic expressions for the outage probability and average BER, which shows that the diversity order of the THz link is independent of THI's parameters. Simulations validate the derived analytical results and demonstrate the impact of the THI parameters on the THz performance.
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    An Exact Statistical Representation of α−η−κ−μ Fading Model for THz Wireless Communication
    (IEEE, 2023-12) Zafaruddin, S.M.
    The α−η−κ−μ is one of the most generalized and flexible channel models having an excellent fit to experimental data from diverse propagation environments. The existing statistical results on the envelope of α−η−κ−μ model contain an infinite series, prohibiting its widespread application in the performance analysis of wireless systems. This paper employs a novel approach to derive density and distribution functions of the envelope of the α−η−κ−μ fading channel without an infinite series approximation. The derived statistical results are presented using a single Fox's H-function for tractable performance analysis, especially for high-frequency mmWave and terahertz wireless transmissions. We also develop an asymptotic analysis using the Gamma function, which converges to the exact values within a reasonable range of channel parameters. To further substantiate the proposed analysis, we present the exact outage probability and average bit-error-rate (BER) performance of a wireless link subjected to the α−η−κ−μ fading model using a single trivariate Fox's H-function. We obtain the diversity order of the system by analyzing the outage probability at a high signal-to-noise (SNR) ratio. We use simulations to demonstrate the significance of the developed statistical results compared with the existing infinite series representation of the α−η−κ−μ model.
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    Exact sum distribution of alpha-eta-kappa-mu fading channels for statistical performance analysis of rrs-based wireless transmission
    (2025-02) Zafaruddin, S.M.
    Reconfigurable refractive surface (RRS) is an efficient alternative to holographic multiple input multiple outputs (HMIMO) systems that can serve as a transmission unit operating in the signal refraction mode. RRS transmissions experience near-field propagation due to the proximity of the transmission feed and far-field propagation for the user located farther from the transmitting unit. There is limited research on the effect of channel fading on far-field users in RRS-based transmissions. In this paper, we conduct an exact statistical analysis of RRS-based transmission considering alpha-eta-kappa-mu fading model for the far-field user and the near-field effect of transmission feed. First, we show the exact statistical analysis for the RRS transmission over alpha-eta-kappa-mu fading model consisting of multiple infinite-series representations with multivariate Fox-H function. Next, we develop a novel approach to derive the density and distribution functions for the resultant fading channel of the RRS system in terms of multivariate Fox-H functions without involving infinite series approximations for tractable performance analysis. We present the exact outage probability and average bit-error-rate (BER) performance of single-element and multiple-element RRS systems to validate the proposed analysis further. We also obtain the diversity order of the system by analyzing the outage probability at a high signal-to-noise ratio (SNR). Computer simulations demonstrate the relevance of the developed statistical results for RRS-based wireless systems over the generalized fading model for a comprehensive performance evaluation.
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    Exploiting Common Mode signals for FEXT cancellation in upstream VDSL
    (IEEE, 2008) Zafaruddin, S.M.
    In this paper, we study the utility of the Common Mode (CM) signal for Far End Crosstalk (FEXT) cancellation in upstream Very high-speed Digital Subscriber Line (VDSL). We begin by extending the stochastic channel model developed for Differential Mode (DM) signals to model the channels of both CM and DM signals. The characteristics of the resulting CM-DM joint channel are first studied. This is expected to help in the development of computationally efficient algorithms for FEXT cancellation. In, the capacity improvement with use of CM-DM was studied for the overall channel. The rate gain with CM-DM joint processing as compared to DM only processing is shown to be a strong function of frequency and the loop length, an observation that is of critical importance for development of FEXT cancellation algorithms. It is shown that such joint processing is beneficial for longer loops and for high frequency subcarriers.
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    Fixed-Gain AF Relaying for RF-THz Wireless System Over α-κ-μ Shadowed and α-μ Channels
    (IEEE, 2022-05) Zafaruddin, S.M.
    Recent research investigates the decode-and-forward (DF) relaying for mixed radio frequency (RF) and terahertz (THz) wireless links with zero-boresight pointing errors. We analyze the performance of a fixed-gain amplify-and-forward (AF) relaying for the RF-THz link to interface the access network on the RF technology with wireless THz transmissions. We develop probability density function (PDF) and cumulative distribution function (CDF) of the end-to-end SNR for the relay-assisted system in terms of bivariate Fox’s H function considering α - μ fading for the THz system with non-zero boresight pointing errors and α - κ - μ shadowed ( α -KMS) fading model for the RF link. Using the derived PDF and CDF, we present exact analytical expressions of the outage probability, average bit-error-rate (BER), and ergodic capacity of the considered system. We also analyze the outage probability and average BER asymptotically for a better insight into the system behavior at high SNR. We use simulations to compare the performance of the AF relaying having a semi-blind gain factor with the recently proposed DF relaying for THz-RF transmissions.
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    A Generalized Statistical Model for THz Wireless Channel with Random Atmospheric Absorption
    (IEEE, 2024) Zafaruddin, S.M.
    Current statistical channel models for Terahertz (THz) wireless communication primarily concentrate on the sub-THz band, mostly with α−μ and Gaussian mixture fading distributions for short-term fading and deterministic modeling for atmospheric absorption. In this paper, we develop a generalized statistical model for signal propagation at THz frequencies considering random path-loss employing Gamma distribution for the molecular absorption coefficient, short-term fading charac-terized by the α−η−κ−μ distribution, antenna misalignment errors, and transceiver hardware impairments. The proposed model can handle various propagation scenarios, including indoor and outdoor environments, backhauVfronthaul situations, and complex urban settings. Using Fox's H-functions, we present the probability density function (PDF) and cumulative distribution function (CDF) that capture the combined statistical effects of channel impairments. We analyze the outage probability of a THz link to demonstrate the analytical tractability of the proposed generalized model. We present computer simulations to demonstrate the efficacy of the proposed model for performance assessment with the statistical effect of atmospheric absorption.
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    GMRES Algorithm for Large-Scale Vectoring in DSL Systems
    (IEEE, 2018-08) Zafaruddin, S.M.
    We propose an iterative crosstalk cancellation scheme based on the generalized minimal residual (GMRES) algorithm for large-scale digital subscriber line (DSL) systems. The proposed scheme does not require channel inversion and stores fewer vectors for crosstalk cancellation. We analyze the convergence of the GMRES algorithm and derive computable bounds on the residual error and signal-to-noise ratio in terms of system parameters at each iteration for upstream DSL systems. We show that the GMRES algorithm typically requires a single iteration for very large vectored systems to achieve crosstalk-free performance for the very high-speed DSL (VDSL) frequencies and only a few more in the highest frequency bands of the G.fast spectrum. This yields significant complexity savings and reduction in memory storage, compared to the zero forcing scheme under certain conditions.
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    Intelligent Connectivity Through RIS-Assisted Wireless Communication: Exact Performance Analysis With Phase Errors and Mobility
    (IEEE, 2023-06) Zafaruddin, S.M.
    The current research offers only approximations for evaluating the performance of wireless systems that use reconfigurable intelligent surfaces (RIS) over generalized fading channels with phase error caused by imperfect phase compensation at the RIS. This article presents an exact analysis of RIS-assisted vehicular communication considering uniformly distributed phase error and generalized fading channels with a coherent combining of received signals reflected by RIS elements and direct transmissions from the source terminal. We use a generalized- K shadowed distribution for the direct link and asymmetrical channels for the RIS-assisted transmission with κ - μ distribution for the first link and double generalized Gamma (dGG) distribution for the second link, combined with a statistical random waypoint (RWP) model for the moving vehicle. We employ a novel approach to represent the probability density function (PDF) and cumulative distribution function (CDF) of the resultant channel in terms of a single univariate Fox-H function and use the multivariate Fox-H approach to develop an exact statistical analysis of the end-to-end signal-to-noise ratio (SNR) for the RIS-assisted system. We also use the inequality between the arithmetic and geometric means to simplify the statistical results of the considered system in terms of the univariate Fox-H function. Our analysis also provides exact, upper bound, and asymptotic expressions of the outage probability and average bit-error-rate (BER) performance using the derived density and distribution functions. We conduct computer simulations in various practically relevant scenarios to assert that mitigating phase errors is achievable by augmenting the number of elements in the RIS module and employing a higher quantization level.
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    Interference Cancellation in Wireless Communications: Past, Present, and Future
    (Taylor & Francis, 2022) Zafaruddin, S.M.
    The advent of electronic communication has revolutionized the exchange of information with a plethora of services such as military, security, entertainment, and of course, telecommunications. Over the several decades, wireless communication has evolved from enabling voice calls to sending text messages, images, videos, and high-definition video conferencing to high-end applications of wireless cognition, augmented reality, and autonomous driving. However, the coexistence of many users and services in a limited spectrum may cause interference among each other. The problem of signal interference has been associated with communication systems since the beginning and has evolved with the evolution of wireless communications. In this tutorial article, we present the evolution of communication systems for the interference problem considering the cancelation methods developed in the past, an overview of the current scenario, and anticipation of future technologies