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Tharm Ratnarajah

Publications and source records attributed to Tharm Ratnarajah.

13 recordsLinked to original sources

Audio-Visual Speech Enhancement: Architectural Design and Deployment Strategies

Real-time audio-visual speech enhancement (AVSE) is a key enabler for immersive and interactive multimedia services, yet its performance is tightly constrained by network latency, uplink capacity, and computational delay. This paper presents the design, deployment, and evaluation of a complete cloud-edge-assisted AVSE system operating over a public 5G edge network. The system integrates CNN-based acoustic enhancement and OpenCV-based facial feature extraction with an LSTM fusion network to preserve temporal coherence, and is deployed on a Vodafone-compatible AWS Wavelength edge cloud. Through extensive stress testing, we analyze end-to-end performance under varying network load and adaptive multimedia profiles. Results show that compute placement at the network edge is critical for meeting real-time coherence constraints, and that uplink capacity is often the dominant bottleneck for interactive AVSE services. Only 5G and wired Ethernet consistently satisfied the required communication delay bound for uncompressed audio-video chunks, while aggressive compression reduced payload sizes by up to 80% with negligible perceptual degradation, enabling robust operation under constrained conditions. We further demonstrate a fundamental trade-off between processing latency and enhancement quality, where reduced model complexity lowers delay but degrades reconstruction performance in low-SNR scenarios. Our findings indicate that public 5G edge environments can sustain real-time, interactive AVSE workloads when network and compute resources are carefully orchestrated, although performance margins remain tighter than in dedicated infrastructures. The architectural insights derived from this study provide practical guidelines for the design of delay-sensitive multimedia and perceptual enhancement services on emerging 5G edge-cloud platforms.

cs.SD

Antenna Health-Aware Selective Beamforming for Hardware-Constrained DFRC Systems I

This paper addresses the optimization challenges in dual-functional radar-communication (DFRC) systems with a focus on array selection and beamforming in dynamic and heterogeneous operational contexts. We propose a novel array selection criterion that integrates antenna health information into the optimization process, distinguishing our approach from traditional methods. Our methodology employs gradient dual ascent and dual proximal-gradient ascent for tackling the constrained non-convex and non-smooth nature of sparse array selection problems. A key feature of our strategy is the implementation of proportional fairness among communication users, which aligns with system resource limitations while meeting the minimum rate requirements for all users. This facet of our method not only enhances system efficiency and responsiveness but also ensures a fair distribution of resources. Through extensive simulations, the efficacy of the proposed solutions in optimizing DFRC system performance is validated, illustrating their applicability in integrated sensing and communication (ISAC) scenarios. Our findings contribute to the evolving field of DFRC systems, offering new perspectives and solutions for the challenges in array selection and beamforming optimisation.

eess.SP

Antenna Health-Aware Selective Beamforming for Hardware-Constrained DFRC Systems II

This study introduces an innovative beamforming design approach that incorporates the reliability of antenna array elements into the optimization process, termed "antenna health-aware selective beamforming". This method strategically focuses transmission power on more reliable antenna elements, thus enhancing system resilience and operational integrity. By integrating antenna health information and individual power constraints, our research leverages advanced optimization techniques such as the Group Proximal-Gradient Dual Ascent (GPGDA) to efficiently address nonconvex challenges in sparse array selection. Applying the proposed technique to a Dual-Functional Radar-Communication (DFRC) system, our findings highlight that increasing the sparsity promotion weight ($ρ_s$) generally boosts spectral efficiency and communication data rate, achieving perfect system reliability at higher $ρ_s$ values but also revealing a performance threshold beyond which further sparsity is detrimental. This underscores the importance of balanced sparsity in beamforming for optimizing performance, particularly in critical communication and defense applications where uninterrupted operation is crucial. Additionally, our analysis of the time complexity and power consumption associated with GPGDA underscores the need for optimizing computational resources in practical implementations.

eess.SP

A Novel Chaos-based Light-weight Image Encryption Scheme for Multi-modal Hearing Aids

Multimodal hearing aids (HAs) aim to deliver more intelligible audio in noisy environments by contextually sensing and processing data in the form of not only audio but also visual information (e.g. lip reading). Machine learning techniques can play a pivotal role for the contextually processing of multimodal data. However, since the computational power of HA devices is low, therefore this data must be processed either on the edge or cloud which, in turn, poses privacy concerns for sensitive user data. Existing literature proposes several techniques for data encryption but their computational complexity is a major bottleneck to meet strict latency requirements for development of future multi-modal hearing aids. To overcome this problem, this paper proposes a novel real-time audio/visual data encryption scheme based on chaos-based encryption using the Tangent-Delay Ellipse Reflecting Cavity-Map System (TD-ERCS) map and Non-linear Chaotic (NCA) Algorithm. The results achieved against different security parameters, including Correlation Coefficient, Unified Averaged Changed Intensity (UACI), Key Sensitivity Analysis, Number of Changing Pixel Rate (NPCR), Mean-Square Error (MSE), Peak Signal to Noise Ratio (PSNR), Entropy test, and Chi-test, indicate that the newly proposed scheme is more lightweight due to its lower execution time as compared to existing schemes and more secure due to increased key-space against modern brute-force attacks.

cs.CR

Towards a Realistic Assessment of Multiple Antenna HCNs: Residual Additive Transceiver Hardware Impairments and Channel Aging

Given the critical dependence of broadcast channels by the accuracy of channel state information at the transmitter (CSIT), we develop a general downlink model with zero-forcing (ZF) precoding, applied in realistic heterogeneous cellular systems with multiple antenna base stations (BSs). Specifically, we take into consideration imperfect CSIT due to pilot contamination, channel aging due to users relative movement, and unavoidable residual additive transceiver hardware impairments (RATHIs). Assuming that the BSs are Poisson distributed, the main contributions focus on the derivations of the upper bound of the coverage probability and the achievable user rate for this general model. We show that both the coverage probability and the user rate are dependent on the imperfect CSIT and RATHIs. More concretely, we quantify the resultant performance loss of the network due to these effects. We depict that the uplink RATHIs have equal impact, but the downlink transmit BS distortion has a greater impact than the receive hardware impairment of the user. Thus, the transmit BS hardware should be of better quality than user's receive hardware. Furthermore, we characterise both the coverage probability and user rate in terms of the time variation of the channel. It is shown that both of them decrease with increasing user mobility, but after a specific value of the normalised Doppler shift, they increase again. Actually, the time variation, following the Jakes autocorrelation function, mirrors this effect on coverage probability and user rate. Finally, we consider space division multiple access (SDMA), single user beamforming (SU-BF), and baseline single-input single-output (SISO) transmission. A comparison among these schemes reveals that the coverage by means of SU-BF outperforms SDMA in terms of coverage.

cs.IT

Mitigation of Phase Noise in Massive MIMO Systems: A Rate-Splitting Approach

This work encompasses Rate-Splitting (RS), providing significant benefits in multi-user settings in the context of huge degrees of freedom promised by massive Multiple-Input Multiple-Output (MIMO). However, the requirement of massive MIMO for cost-efficient implementation makes them more prone to hardware imperfections such as phase noise (PN). As a result, we focus on a realistic broadcast channel with a large number of antennas and hampered by the unavoidable PN. Moreover, we employ the RS transmission strategy, and we show its robustness against PN, since the sum-rate does not saturate at high signal-to-noise ratio (SNR). Although, the analytical results are obtained by means of the deterministic equivalent analysis, they coincide with simulation results even for finite system dimensions.

cs.IT

Performance of Massive MIMO Uplink with Zero-Forcing receivers under Delayed Channels

In this paper, we analyze the performance of the uplink communication of massive multi-cell multiple-input multiple-output (MIMO) systems under the effects of pilot contamination and delayed channels because of terminal mobility. The base stations (BSs) estimate the channels through the uplink training, and then use zero-forcing processing to decode the transmit signals from the users. The probability density function (PDF) of the signal-to-interference-plus-noise ratio is derived for any finite number of antennas. From this PDF, we derive an achievable ergodic rate with a finite number of BS antennas in closed form. Insights of the impact of the Doppler shift (due to terminal mobility) at the low signal-to-noise ratio regimes are exposed. In addition, the effects on the outage probability are investigated. Furthermore, the power scaling law and the asymptotic performance result by infinitely increasing the numbers of antennas and terminals (while their ratio is fixed) are provided. The numerical results demonstrate the performance loss for various Doppler shifts. Among the interesting observations revealed is that massive MIMO is favorable even in channel aging conditions.

cs.IT

Downlink MIMO HCNs with Residual Transceiver Hardware Impairments

A major limitation of heterogeneous cellular networks (HCNs) is the neglect of the additive residual transceiver hardware impairments (ARTHIs). The assumption of perfect hardware is quite strong and results in misleading conclusions. This paper models a general multiple-input multiple-output (MIMO) HCN with cell association by incorporating the RTHIs. We derive the coverage probability and shed light on the impact of the ARTHIs, when various transmission methods are applied. As the hardware quality decreases, the coverage probability worsens. Especially, this effect is more severe as the transmit power increases. Furthermore, we verify that in an HCN, it is better to employ at each base station as few transmit antennas as possible.

cs.IT

Uplink Performance of Conventional and Massive MIMO Cellular Systems with Delayed CSIT

Massive multiple-input multiple-output (MIMO) networks, where the base stations (BSs) are equipped with large number of antennas and serve a number of users simultaneously, are very promising, but suffer from pilot contamination. Despite its importance, delayed channel state information (CSI) due to user mobility, being another degrading factor, lacks investigation in the literature. Hence, we consider an uplink model, where each BS applies zero-forcing decoder, accounting for both effects, but with the focal point on the relative users' movement with regard to the BS antennas. In this setting, analytical closed-form expressions for the sum-rate with finite number of BS antennas, and the asymptotic limits with infinite number of BS antennas epitomize the main contributions. In particular, the probability density function of the signal-to-interference-plus-noise ratio and the ergodic sum-rate are derived for any finite number of antennas. Insights of the impact of the arising Doppler shift due to user mobility into the low signal-to-noise ratio regime as well as the outage probability are obtained. Moreover, asymptotic analysis performance results in terms of infinitely increasing number of antennas, power, and both numbers of antennas and users (while their ratio is fixed) are provided. The numerical results demonstrate the performance loss in various Doppler shifts. An interesting observation is that massive MIMO is favorable even in time-varying channel conditions.

cs.IT

On Achievable Schemes of Interference Alignment in Constant Channels via Finite Amplify-and-Forward Relays

This paper elaborates on the achievable schemes of interference alignment in constant channels via finite amplify-and-forward (AF) relays. Consider $K$ sources communicating with $K$ destinations without direct links besides the relay connections. The total number of relays is finite. The objective is to achieve interference alignment for all user pairs to obtain half of their interference-free degrees of freedom. In general, two strategies are employed: coding at the edge and coding in the middle, in which relays show different roles. The contributions are that two fundamental and critical elements are captured to enable interference alignment in this network: channel randomness or relativity; subspace dimension suppression.

cs.IT

Close-Form Design of Antenna-Constrained Multi-Cell Multi-User Downlink Interference Alignment

This paper investigates the downlink channels in multi-cell multi-user interfering networks. The goal is to propose close-form designs to obtain degrees of freedom (DoF) in high SNR region for the network composed of base stations (BS) as transmitters and mobile stations (MS) as receivers. Consider the realistic system, both BS and MS have finite antennas, so that the design of interference alignment is highly constrained by the feasibility conditions. The focus of design is to explore potential opportunities of alignment in the subspace both from the BS transmit side and from the MS receive side. The new IA schemes for cellular downlink channels are in the form of causal dynamic processes in contrary to conventional static IA schemes. For different implementations, system conditions are compared from all aspects, which include antenna usage, CSI overhead and computational complexity. This research scope covers a wide range of typical multi-cell multi-user network models. The first one is a $K$-cell fully connected cellular network; the second one is a Wyner cyclic cellular network with two adjacent interfering links; the third one is a Wyner cyclic cellular network with single adjacent interfering link considering cell-edge and cell-interior users respectively.

cs.IT

On Achievable Schemes of Interference Alignment with Double-Layered Symbol Extensions in Interference Channel

This paper looks into the $K$-user interference channel. Interference Alignment is much likely to be applied with double-layered symbol extensions, either for constant channels in the H$ø$st-Madsen-Nosratinia conjecture or slowly changing channels. In our work, the core idea relies on double-layered symbol extensions to artificially construct equivalent time-variant channels to provide crucial \textit{channel randomness or relativity} required by conventional Cadambe-Jafar scheme in time-variant channels \cite{IA-DOF-Kuser-Interference}.

cs.IT

Performance Analysis of Optimal Single Stream Beamforming in MIMO Dual-Hop AF Systems

This paper investigates the performance of optimal single stream beamforming schemes in multiple-input multiple-output (MIMO) dual-hop amplify-and-forward (AF) systems. Assuming channel state information is not available at the source and relay, the optimal transmit and receive beamforming vectors are computed at the destination, and the transmit beamforming vector is sent to the transmitter via a dedicated feedback link. Then, a set of new closed-form expressions for the statistical properties of the maximum eigenvalue of the resultant channel is derived, i.e., the cumulative density function (cdf), probability density function (pdf) and general moments, as well as the first order asymptotic expansion and asymptotic large dimension approximations. These analytical expressions are then applied to study three important performance metrics of the system, i.e., outage probability, average symbol error rate and ergodic capacity. In addition, more detailed treatments are provided for some important special cases, e.g., when the number of antennas at one of the nodes is one or large, simple and insightful expressions for the key parameters such as diversity order and array gain of the system are derived. With the analytical results, the joint impact of source, relay and destination antenna numbers on the system performance is addressed, and the performance of optimal beamforming schemes and orthogonal space-time block-coding (OSTBC) schemes are compared. Results reveal that the number of antennas at the relay has a great impact on how the numbers of antennas at the source and destination contribute to the system performance, and optimal beamforming not only achieves the same maximum diversity order as OSTBC, but also provides significant power gains over OSTBC.

cs.IT