arXiv subjects

Bokai Lai

Publications and source records attributed to Bokai Lai.

5 recordsLinked to original sources

arXiv2025

Metasurface-Enabled Superheterodyne Transmitter With Decoupled Harmonic-Free Signal Generation and Precoding

The evolution of programmable metasurfaces (PM) from passive beamforming to active information transmission marks a paradigm shift for next-generation wireless systems. However, this transition is hindered by fundamental limitations in conventional metasurface transmitter architectures, including restricted modulation orders, symbol-level spatial inconsistency, and significant harmonic interference. These issues stem from the intrinsic coupling between baseband signal processing and radio-frequency beamforming in monolithic designs reliant on simplistic switching mechanisms. This paper proposes a novel metasurface-enabled superheterodyne architecture (MSA) that fundamentally decouples these functionalities. The MSA introduces a dual-stage up-conversion process, comprising a digital up-conversion module for in-phase/quadrature modulation and baseband-to-intermediate frequency conversion, a precoder module for precoding, and a custom-designed magnitude-phase-decoupled metasurface that acts as a reconfigurable reflective mixer array. This decoupling of harmonic-free waveform generation from spatial precoding overcomes the critical drawbacks of existing approaches. Experimental results from a 5.8 GHz proof-of-concept prototype system validate the MSA's superior performance. The system generates spatially isotropic constellations for arbitrary-order QAM modulations, ensures consistent time-frequency signatures for applications like Doppler-spoofing, and achieves data rates up to 20 Mbps within a linear operating region that minimizes nonlinear distortion. The capability of employing spatial diversity and multi-stream interference cancellation has been demonstrated for the first time in a PM-based transmitter.

cs.IT↗

arXiv2025

Metasurface-Enabled Superheterodyne Transmitter for Arbitrary-Order Modulation with Spatially Isotropic Symbol Distribution

Electromagnetically programmable information metasurfaces, as dynamically controllable 2D metamaterials, hold significant promise as low-profile hardware enabling passive wave control and signal generation for backscatter systems. However, current metasurface-based transmitters architecture fundamentally suffer from hardware non-modularization, forcing all transmitter functions onto nonlinear switch-based unit cells, which introduces symbol mapping inconsistency via phase coupling. Moreover, both temporal coding (limited by unit cell diodes) and space-time coding (impaired by symbol anisotropy) exhibit irreducible harmonic interference and entangled control of amplitude, phase, and beam direction. This paper proposes a metasurface-enabled superheterodyne architecture (MSA), comprising a digital up-conversion (DUC) module performing baseband-to-intermediate frequency (IF) conversion, filtering, and digital-to-analog conversion (DAC), and a reconfigurable metasurface featuring programmable unit cells that independently control both the magnitude and phase of the reflection coefficient. Systematically, the architecture leverages a dual-stage up-conversion process, typical of superheterodyne systems, but uniquely employs the metasurface for the final RF conversion stage. Building upon this framework, a proof-of-concept prototype featuring a 5.8 GHz magnitude-phase decoupled (MPD) metasurface (<15 degree phase deviation per state) and a DAC-based DUC module is presented. Extensive validation confirms the metasurface's capability for distortion-free mixing with arbitrary IF signals while maintaining consistent radiation patterns. The prototype successfully implements diverse QAM modulation schemes (4QAM to 256QAM) in mono-static and bi-static configurations, demonstrating symbol isotropy for spatially separated receivers and achieving a data rate of approximately 20 Mbps (at 5 MHz IF)...

eess.SY↗

arXiv2025

Arbitrary Waveform Generated Metasurface: A New Paradigm for Direct Modulation and Beamforming Decoupling

Information Metasurface, also known as reconfigurable intelligent surface (RIS) has gained significant attention owing to its impressive abilities in electromagnetic (EM) wave manipulation with simple structures. Numerous studies focus on achieving efficient and versatile information transmission using RIS across various fields like wireless communication, radar detection, integrated sensing, and communications, among others. Previous studies demonstrate diverse approaches to achieve reflection modulation by utilizing the superposition of the quantified reflection coefficient (RC) of each unit but suffer from the computing complexity of codebook sequence, the safety of communication, and the flexibility of modulation. To address these challenges, we introduce a novel concept of information metasurface, namely AWG-RIS, which is capable of independently producing arbitrary baseband waveforms and beam patterns through a design that decouples magnitude and phase, without changing the beam pattern. The AWG-RIS functions as a reflection mixer, directly embedding the intended signal into the incoming EM waves. Subsequently, we developed an analysis framework and introduced the waveform factor and beamforming factor into the new model, offering theoretical support for the transition from the control signal to the outgoing electromagnetic wave. Additionally, we unveil the world's first prototype showcasing passive arbitrary waveform generation while maintaining the beam pattern unaltered. Leveraging the decoupling of direct modulation and beamforming, we explore additional applications in several domains relative to traditional RISs. Finally, we present experiments that confirm the generation of arbitrary waveforms and particular spectrograms.

eess.SY↗

arXiv2024

FlashSloth: Lightning Multimodal Large Language Models via Embedded Visual Compression

Despite a big leap forward in capability, multimodal large language models (MLLMs) tend to behave like a sloth in practical use, i.e., slow response and large latency. Recent efforts are devoted to building tiny MLLMs for better efficiency, but the plethora of visual tokens still used limit their actual speedup. In this paper, we propose a powerful and fast tiny MLLM called FlashSloth. Different from previous efforts, FlashSloth focuses on improving the descriptive power of visual tokens in the process of compressing their redundant semantics. In particular, FlashSloth introduces embedded visual compression designs to capture both visually salient and instruction-related image information, so as to achieving superior multimodal performance with fewer visual tokens. Extensive experiments are conducted to validate the proposed FlashSloth, and a bunch of tiny but strong MLLMs are also comprehensively compared, e.g., InternVL2, MiniCPM-V2 and Qwen2-VL. The experimental results show that compared with these advanced tiny MLLMs, our FlashSloth can greatly reduce the number of visual tokens, training memory and computation complexity while retaining high performance on various VL tasks.

cs.CV↗

arXiv2023

Wireless Communications in Cavity: A Reconfigurable Boundary Modulation based Approach

This paper explores the potential wireless communication applications of Reconfigurable Intelligent Surfaces (RIS) in reverberant wave propagation environments. Unlike in free space, we utilize the sensitivity to boundaries of the enclosed electromagnetic (EM) field and the equivalent perturbation of RISs. For the first time, we introduce the framework of reconfigurable boundary modulation in the cavities . We have proposed a robust boundary modulation scheme that exploits the continuity of object motion and the mutation of the codebook switch, which achieves pulse position modulation (PPM) by RIS-generated equivalent pulses for wireless communication in cavities. This approach achieves around 2 Mbps bit rate in the prototype and demonstrates strong resistance to channel's frequency selectivity resulting in an extremely low bit error rate (BER).

eess.SY↗