arXiv subjects

Ishtiaque Ahmed

Publications and source records attributed to Ishtiaque Ahmed.

6 recordsLinked to original sources

arXiv2026

Waveguide to Meaning: Semantic-Aware NOMA for Pinching-Antenna Systems

We investigate the performance of the pinching-antenna systems (PASS) for semantic communication (SC) in both single-waveguide and multi-waveguide scenarios, under the constraints of bit-user quality of service (QoS) and bit-to-semantic decoding order in a heterogeneous users downlink non-orthogonal multiple access (NOMA). Multiple pinching antennas in the single-waveguide scenario are at a minimum adjacent spacing required to prevent mutual coupling. An alternating optimization (AO)-based algorithm optimizes users power allocation coefficients and position of pinching antennas in the single-waveguide NOMA framework. For the multi-waveguide scenario, assuming adjacent waveguides at a sufficient lateral distance apart, the waveguides power allocation subproblem is solved using monotonic optimization and minorization-maximization (MM) approach. Specifically, a lower bound surrogate is iteratively maximized under the feasibility constraints such that a non-decreasing sequence of objective is obtained. Numerical results demonstrate that the NOMA based PASS exploiting SC offers higher semantic spectral efficiency (SE) while fulfilling the bit-user QoS requirement when compared to the considered conventional fixed antenna system. Notably, the multi-waveguide scenario becomes more beneficial for creating adjustable wireless channels in stringentconditions with higher bit-user QoS and wider coverage area requirements.

eess.SP↗

arXiv2026

Coupling-Aware Pinching-Antenna Radiation for Heterogeneous NOMA Users

Pinching-antennas systems (PASS) offer reconfigurable wireless channels via low-cost dielectric mediums by creating line-of-sight (LoS) communication links. Most of the existing PASS cover mechanisms of equal power pinching antennas for conventional bit-based communication, whereas flexible radiation control remains largely unexplored, particularly for heterogeneous semantic and bit users. In this paper, we investigate the performance of semantic communication (SC) using an adjustable radiation model over PASS, where the coupling strength between the dielectric waveguide and each pinching antenna is determined by the antenna-waveguide spacing. Specifically, the non-orthogonal multiple access (NOMA)-assisted heterogeneous users are served by multiple pinching antennas using spacing-controlled adjustable radiation ratios. Uunder this setting, we maximize the semantic spectral efficiency (SE) subject to the bit-user quality of service (QoS) requirement, successive interference cancellation (SIC) feasibility, and the minimum adjacent antennas spacing constraint. An alternating optimization (AO) approach optimizes users power allocation and positions of pinching antennas. Simulations demonstrate the effectiveness of the proportional power PASS model in providing higher semantic SE in different geometrical and numerical settings compared to conventional benchmark schemes.

eess.SP↗

arXiv2025

Semantic Communications in 6G: Coexistence, Multiple Access, and Satellite Networks

The exponential growth of wireless users and bandwidth constraints necessitates innovative communication paradigms for next-generation networks. Semantic Communication (SemCom) emerges as a promising solution by transmitting extracted meaning rather than raw bits, enhancing spectral efficiency and enabling intelligent resource allocation. This paper explores the integration of SemCom with conventional Bit-based Communication (BitCom) in heterogeneous networks, highlighting key challenges and opportunities. We analyze multiple access techniques, including Non-Orthogonal Multiple Access (NOMA), to support coexisting SemCom and BitCom users. Furthermore, we examine multi-modal SemCom frameworks for handling diverse data types and discuss their applications in satellite networks, where semantic techniques mitigate bandwidth limitations and harsh channel conditions. Finally, we identify future directions for deploying semantic-aware systems in 6G and beyond.

eess.SP↗

arXiv2025

Hybrid NOMA Assisted Heterogeneous Semantic and Bit Users Communication

In this paper, we utilize a downlink hybrid Non-Orthogonal Multiple Access (NOMA) framework to support multiple semantic and bit users within the communication network. The hybrid NOMA setup exploits both NOMA and Orthogonal Multiple Access (OMA) which has the benefit of enhancing Spectral Efficiency (SE) by allowing users to dynamically access the resources in multiple heterogeneous slots. This enables integrating semantic and bit users based on their channel gains, while adopting bit-to-semantic decoding order in slots including heterogeneous users. An optimization problem for the power allocation is formulated with the aim of maximizing the equivalent ergodic semantic SE with a constraint on the total available power of the Access Point (AP). The proposed algorithm uses NOMA in shared slots and OMA in bit-user-only slots. Simulation results validate the benefits of heterogeneous users hybrid NOMA setup in comparison to OMA-only for heterogeneous users.

eess.SP↗

arXiv2022

Anisotropic subwavelength grating perturbation enables zero crosstalk in a leaky mode

Electromagnetic coupling via either exponentially decaying evanescent field or radiative wave is a primary characteristic of light, allowing optical signal/power transfer but limiting integration density in a photonic circuit. A leaky mode combines both evanescent field and radiative wave, causing stronger crosstalk and thus not ideal for dense integration. Here we show that a leaky mode with anisotropic perturbation rather can achieve completely zero crosstalk realized by subwavelength grating (SWG) metamaterials. The oscillating fields in the SWGs enable coupling coefficients in each direction to counteract each other, resulting in completely zero crosstalk. We experimentally demonstrate such an extraordinarily low coupling between closely spaced identical leaky SWG waveguides, suppressing the crosstalk by $\approx$40 dB compared to conventional strip waveguides, corresponding to $\approx$100 times longer coupling length. This leaky-SWG suppresses the crosstalk of transverse-magnetic (TM) mode, which is challenging due to its low confinement, and marks a novel approach in electromagnetic coupling applicable to other spectral regimes and generic devices.

physics.optics↗

arXiv2020

Exceptional coupling in extreme skin-depth waveguides for extremely low waveguide crosstalk

Photonic chips can miniaturize complicate optical systems very tiny and portable, providing versatile functionalities for many optical applications. Increasing the photonic chip integration density is highly desired as it provides more functionalities, low cost, and lower power consumption. However, photonic chip integration density is limited by the waveguide crosstalk, which is caused by the evanescent waves in the cladding. Here we show that the waveguide crosstalk can be suppressed completely with the exceptional coupling in extreme skin-depth (eskid) waveguides. The anisotropic dielectric perturbations in the coupled eskid waveguides cause such an exceptional coupling, resulting in infinitely long coupling length. We demonstrate the extreme suppression of waveguide crosstalk via exceptional coupling on a silicon-on-insulator (SOI) platform, which is compatible with a complementary metal-oxide-semiconductor (CMOS) process. The idea of exceptional coupling in eskid waveguides can be applied to many other photonic devices as well, significantly reducing entire chip footprints.

physics.optics↗