Xiaolin Zhou

The Internet of Things is constrained by energy consumption and battery limitations, leading to high network maintenance costs and environmental impact. Battery-free ambient backscatter communication (AmBC) provides a sustainable alternative by enabling passive tags to harvest RF energy for continuous data transmission. The inherently low power of AmBC signals increases signal outage probability and bit error rates (BER), requiring effective adaptation mechanisms. This paper proposes a new reflection coefficient adjustment approach that enables both energy harvesting and reliable data transmission at each passive tag. In our decentralized multi-agent framework, tags compete to achieve higher transmit powers for reliable communication. To mitigate interference between tags, we model the interaction among tags as a non-cooperative Cournot game, where each tag acts as a rational agent adjusting its reflection coefficient to maximize its own utility—defined as a trade-off between transmission reliability and interference mitigation. Our game-theoretic approach eliminates the need for explicit coordination or information exchange, promoting scalability and robustness. Numerical results show that our approach significantly outperforms existing methods by reducing BER and improving energy efficiency, making it a viable solution for sustainable, battery-free IoT communications.

Photon counting (PhC) is an effective detection technology for underwater optical wireless communication (OWC) systems. The presence of signal-dependent Poisson shot noise and asynchronous multi-user interference (MUI) complicates the processing of received data signals, hindering the effective signal detection of PhC OWC systems. This paper proposes a novel iterative signal detection method in grant-free, multi-user, underwater PhC OWC systems with signal-dependent Poisson shot noise. We first introduce a new synchronization algorithm with a unique frame structure design. The algorithm performs active user identification and transmission delay estimation. Specifically, the estimation is performed first on a user group basis and then at the individual user level with reduced complexity and latency.We also develop a nonlinear iterative multi-user detection (MUD) algorithm that utilizes a detection window for each user to identify interfering symbols and estimate MUI on a slot-by-slot basis, followed by maximum a-posteriori probability detection of user signals. Simulations demonstrate that our scheme achieves bit error rates comparable to scenarios with transmission delays known and signal detection perfectly synchronized.

Coherent states have been increasingly considered in optical quantum communications (OQCs).With the inherent non-orthogonality of coherent states, non-orthogonal multiple-access (NOMA) naturally lends itself to the implementation of multi-user OQC. However, this remains unexplored in the literature. This paper proposes a novel successive interference cancellation (SIC)-based photon-number-resolving detection (PNRD)-Kennedy receiver for uplink NOMA-OQC systems, along with a new approach for power allocation of the coherent states emitted by users. The key idea is to rigorously derive the asymptotic sum-rate of the considered systems, taking into account the impact of atmospheric turbulence, background noise, and lossy photon channel. With the asymptotic sum-rate, we optimize the average number of photons (or powers) of the coherent states emitted by the users. Variable substitution and successive convex approximation (SCA) are employed to convexify and maximize the asymptotic sum-rate iteratively. A new coherent-state power allocation algorithm is developed for a small-to-medium number of users. We further develop its low-complexity variant using adaptive importance sampling, which is suitable for scenarios with a medium-to-large number of users. Simulations demonstrate that our algorithms significantly enhance the sum-rate of uplink NOMA-OQC systems using coherent states by over 20%, compared to their alternatives.

Photon counting (PhC) is an effective detection technology for underwater optical wireless communication (OWC) systems. The presence of signal-dependent Poisson shot noise and asynchronous multi-user interference (MUI) complicates the processing of received data signals, hindering the effective signal detection of PhC OWC systems. This paper proposes a novel iterative signal detection method in grant-free, multi-user, underwater PhC OWC systems with signal-dependent Poisson shot noise. We first introduce a new synchronization algorithm with a unique frame structure design.The algorithm performs active user identification and transmission delay estimation. Specifically, the estimation is performed first on a user group basis and then at the individual user level with reduced complexity and latency.We also develop a nonlinear iterative multi-user detection (MUD) algorithm that utilizes a detection window for each user to identify interfering symbols and estimate MUI on a slot-by-slot basis, followed by maximum \textit{a-posteriori} probability detection of user signals.Simulations demonstrate that our scheme achieves bit error rates comparable to scenarios with transmission delays known and signal detection perfectly synchronized.

We address the multi-user quantum key distribution (QKD) problem under malicious quantum attacks, which is critical for realizing a large-scale quantum Internet. This paper maximizes the sum secret key rate (SKR) of a novel uplink non-orthogonal multiple access based continuous-variable QKD (NOMA-CVQKD) system under collective attacks. The proposed system uses Gaussian-modulated coherent states and a quantum successive interference cancellation based heterodyne receiver. We derive closed-form asymptotic bounds for the legitimate users' achievable key rates via the entropy power inequality and maximum entropy principle, as well as for the eavesdropper's intercepted information based on Holevo information. A successive convex approximation based power allocation algorithm is developed to maximize the asymptotic sum SKR of the NOMA-CVQKD system under collective attacks, with guaranteed convergence to a locally optimal Karush-Kuhn-Tucker solution. Simulation results show that the proposed NOMA-CVQKD system with the power allocation algorithm achieves up to 23% higher sum SKR than quantum-orthogonal multiple access, supports 16 users at excess noise variance 0.1, and remains robust under varying turbulence intensities and transmission distances.

Coherent states have been increasingly considered in optical quantum communications (OQCs). With the inherent non-orthogonality of coherent states, non-orthogonal multiple-access (NOMA) naturally lends itself to the implementation of multi-user OQC. However, this remains unexplored in the literature. This paper proposes a novel successive interference cancellation (SIC)-based Kennedy receiver for uplink NOMA-OQC systems, along with a new approach for power allocation of the coherent states emitted by users. The key idea is to rigorously derive the asymptotic sum-rate of the considered systems, taking into account the impact of atmospheric turbulence, background noise, and lossy photon channel. With the asymptotic sum-rate, we optimize the average number of photons (or powers) of the coherent states emitted by the users. Variable substitution and successive convex approximation (SCA) are employed to convexify and maximize the asymptotic sum-rate iteratively. A new coherent-state power allocation algorithm is developed for a small-to-medium number of users. We further develop its low-complexity variant using adaptive importance sampling, which is suitable for scenarios with a medium-to-large number of users. Simulations demonstrate that our algorithms significantly enhance the sum-rate of uplink NOMA-OQC systems using coherent states by over 20\%, compared to their alternatives.

We investigate detection for practical photon-counting receivers in underwater optical wireless communication (UOWC) systems, where dead-time effects and multi-user interference (MUI) pose significant challenges. In this paper, we propose an iterative detection method for multi-user uplink UOWC with signal-dependent shot noise, finite sampling rates, and thermal noise. We design a sparse interleaver and construct a sparse factor graph dedicated to the low-photon regime of non-perfect photon-counting systems, improving noise resilience, mitigating MUI, and enhance detection performance. We also employ iterative multi-user detection (MUD) based on a robust maximum a posteriori probability framework to address the photomultiplier tube (PMT) dead-time effects. Simulation results show that the proposed method achieves up to 5 dB bit error rate (BER) gains over conventional photon-counting schemes and maintains robust performance under diverse channel and noise conditions.

Coherent states are increasingly being adopted for transmitting information in underwater quantum communications (UQCs). This paper presents what we believe to be a novel quantum multiple-symbol detection (QMSD) scheme designed to alleviate the performance degradation present in classical quantum symbol-by-symbol detection schemes, while operating without the need for channel state information (CSI) at the receiver. We model the communication system under the influence of an underwater channel and thermal noise at the receiver, and propose a novel QMSD metric based on positive operator-valued measure (POVM) quantum measurement results. Furthermore, we derive a closed-form expression for the QMSD metric for facilitating easy evaluation under marine turbulence conditions and thereby reducing implementation complexity. Using the simplified QMSD metric, we employ a sliding window algorithm to efficiently process the received quantum symbol sequence. Simulation results demonstrate that our proposed QMSD scheme enhances the quality of UQCs and exhibits robustness under marine turbulence conditions. Specifically, as the length of the detection sequence increases, the error probability of our scheme decreases. Moreover, as the average number of transmitted photons increases, the proposed QMSD scheme demonstrates superior performance, reducing error probability by 3 orders of magnitude compared to the quantum maximum likelihood detection (QMLD) scheme and by 5 orders of magnitude relative to the traditional homodyne detection (HD) scheme.