In this work, we consider "decision" variants of a well-known monogamy-of-entanglement game by Tomamichel, Fehr, Kaniewski, and Wehner [New Journal of Physics '13]. In its original "search" variant, Alice prepares a (possibly entangled) state on registers ABC; register 𝖠, consisting of n qubits, is sent to a Referee, while 𝖡 and 𝖢 are sent to Bob and Charlie; the Referee then measures each qubit in the standard or Hadamard basis (chosen uniformly at random). The basis choices are sent to Bob and Charlie, whose goal is to simultaneously guess the Referee’s n-bit measurement outcome string x. Tomamichel et al. show that the optimal winning probability is cos^{2n}(π/8), following a perfect parallel repetition theorem. We consider the following "decision" variants of this game: - Variant 1, "XOR repetition": Bob and Charlie’s goal is to guess the XOR of all the bits of x. Ananth et al. [Asiacrypt '24] conjectured that the optimal advantage over random guessing decays exponentially in n. Surprisingly, we show that this conjecture is false, and, in fact, there is no decay at all: there exists a strategy that wins with probability cos²(π/8) ≈ 0.85 for any n. Moreover, this strategy does not involve any entanglement between Alice, Bob, and Charlie! - Variant 2, "Goldreich-Levin": The Referee additionally samples a uniformly random n-bit string r that is sent to Bob and Charlie along with the basis choices. Their goal is to guess the parity of r⋅ x. We show that the optimal advantage over random guessing decays exponentially in n for the restricted class of adversaries that do not share entanglement. A similar result was already shown by Champion et al. and Çakan et al.; we give a more direct proof. Showing that Variant 2 is "secure" (i.e., that the optimal winning probability is exponentially close to 1/2) against general adversaries would imply the existence of an information-theoretically "unclonable bit". We put forward a reasonably concrete conjecture that is equivalent to the general security of Variant 2.

Security analysis of continuous variable quantum key distribution (CV-QKD) remains a topic of extensive academic investigation. In this work, we propose a coherent-state CV-QKD protocol integrating a displacement-based quantum receiver and post-selection strategies. We establish a framework of the protocol under both individual and collective attacks, incorporating practical constraints such as noises and device imperfections. To enhance performance of the protocol, the postselection strategy and the transmitted signal power are optimized for different channel attenuation levels. Numerical results offer insights into the strategy of employing displacement receivers in CV-QKD protocols, revealing that larger transmitted signal power is required under low channel attenuation or severe receiver-side imperfections and noises to achieve higher the secret key rate.

Data in real-world scenarios often exhibit a long-tailed distribution. Deep models trained on these long-tailed data usually perform poorly on tail classes. Existing methods like re-sampling and re-weighting strategies, cannot address the long-tailed problem well. This is because a single-expert network is unable to learn sufficient knowledge from the original imbalanced data due to its limited learning ability. To tackle this issue, we propose a novel Dual-Branch Knowledge Distillation (DBKD) to enrich the knowledge of a single expert. DBKD consists of two main components: Auxiliary Knowledge Distillation(AKD) and Balanced Focal Loss(BFL). We apply the proposed DBKD to train a network with two branches, which are the teacher and student branches in knowledge distillation respectively. AKD complements the knowledge of the student branch by transferring knowledge from the teacher branch during the training process. To enrich the knowledge of the student branch further, BFL strengthens the learning of hard samples by re-weighting the classification loss. In the inference phase, we only use the student branch to make predictions. To verify the effectiveness of our DBKD, we conduct extensive experiments on four popular long-tailed datasets: CIAFR10-LT, CIAFR100-LT, ImageNet-LT, and Places-LT. The results consistently manifest the effectiveness and superiority of the proposed method.

In this paper, a 4-weighted-type fractional Fourier transform (4-WFRFT) technology is presented to enhance the physical layer security (PLS) performance in a system of multiple eavesdroppers. The average signal-to-noise ratio (SNR) of eavesdroppers is first derived. Based on the derived average SNR, closed-form expressions of average security capacity (ASC) and secrecy outage probability (SOP) are obtained by employing the 4-WFRFT technology. The numerical results demonstrate that: 1) when the number of eavesdroppers increases, the 4 -WFRFT technology can also obtain a higher secrecy capacity. 2) the ASC decreases first but finally keeps at a constant value as the number of eavesdroppers increases. Simulations verify our analysis and demonstrate the 4 -WFRFT technology can validly improve the PLS performance in the multiple eavesdroppers system.

Non-orthogonal multiple access (NOMA) is capable of serving different numbers of users in the same time-frequency resource element, and this feature can be leveraged to carry additional information. In the orthogonal frequency division multiplexing (OFDM) system, a novel enhanced NOMA scheme called NOMA with informative envelope (NOMA-IE) is proposed to explore extra flexibility from the envelope of NOMA signals. In this scheme, data bits are conveyed by the quantified signal envelope in addition to classic signal constellations. The subcarrier activation patterns of different users are jointly decided by the envelope former at the transmitter of NOMA-IE. At the receiver, successive interference cancellation (SIC) is employed, and the envelope detection coefficient is introduced to eliminate the error floor. Theoretical expressions of spectral efficiency, energy efficiency, and detection complexity are provided first. Then, considering the binary phase shift keying modulation, the block error rate and bit error rate are derived based on the two-subcarrier element. The analytical results reveal that the SIC error and the index error are the main factors degrading the error performance. The numerical results demonstrate the superiority of the NOMA-IE over the OFDM and OFDM-NOMA in terms of the error rate performance when all the schemes have the same spectral efficiency and energy efficiency.

In this paper, a stochastic geometry based analytical framework is proposed for secure simultaneous transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) assisted non-orthogonal multiple access (NOMA) transmissions, where legitimate users (LUs) and eavesdroppers are randomly distributed. Both the time-switching protocol (TS) and energy splitting (ES) protocol are considered for the STAR-RIS. To characterize system performance, the channel statistics are first provided, and the Gamma approximation is adopted for general cascaded κ-μ fading. Afterward, the closed-form expressions for both the secrecy outage probability (SOP) and average secrecy capacity (ASC) are derived. To obtain further insights, the asymptotic performance for the secrecy diversity order and the secrecy slope are deduced. The theoretical results show that 1) the secrecy diversity orders of the strong LU and the weak LU depend on the path loss exponent and the distribution of the received signal-to-noise ratio, respectively; 2) the secrecy slope of the ES protocol achieves the value of one, higher than the slope of the TS protocol which is the mode operation parameter of TS. The numerical results demonstrate that: 1) there is an optimal STAR-RIS mode operation parameter to maximize the secrecy performance; 2) the STAR-RIS-NOMA significantly outperforms the STAR-RIS-orthogonal multiple access.

An analytical framework for physical layer security in simultaneous transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) assisted non-orthogonal multiple access (NOMA) transmissions is proposed, where legitimate users and eavesdroppers are randomly deployed. To characterize system performance, the channel statistics are first provided, and the Gamma approximation is adopted for general cascaded$\kappa-\mu$fading. Afterwards, the energy splitting (ES) protocol is considered and closed-form expressions of average secrecy capacity are derived. To obtain further insights, the asymptotic secrecy slope is deduced. The theoretical results show that the secrecy slope of the ES protocol is one. The numerical results demonstrate that: 1) there is an optimal resource allocation ratio of STAR-RIS to maximize the system performance; 2) the STAR-RIS-aided NOMA significantly outperforms the STAR-RIS-aided orthogonal multiple access.

In the orthogonal frequency division multiplexing with index modulation (OFDM-IM) framework, we propose an enhanced non-orthogonal multiple access (NOMA) scheme called NOMA with informative envelope (NOMA-IE) to explore the flexibility of signal envelope. In this scheme, data bits are conveyed by the quantified signal envelope in addition to classic signal constellations. Subcarrier activation patterns of different users are jointly decided by the envelope former at the transmitter. At the receiver, successive interference cancellation (SIC) is employed, and we introduce the envelope detection coefficient to eliminate the detection error floor. Considering binary phase shift keying, we derive the asymptotic bit error rate (BER). Analytical results reveal that the imperfect SIC is the main factor degrading the error performance. Numerical results demonstrate the superiority of the NOMA-IE over the OFDM and OFDM-NOMA.

The simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) is capable of providing full-space coverage of smart radio environments. This work investigates STAR-RIS aided downlink non-orthogonal multiple access (NOMA) multi-cell networks, where the energy of incident signals at STAR-RISs is split into two portions for transmitting and reflecting. We first propose a fitting method to model the distribution of composite small-scale fading power as the tractable Gamma distribution. Then, a unified analytical framework based on stochastic geometry is provided to capture the random locations of RIS-RISs, base stations (BSs), and user equipments (UEs). Based on this framework, we derive the coverage probability and ergodic rate of both the typical UE and the connected UE. In particular, we obtain closed-form expressions of the coverage probability in interference-limited scenarios. We also deduce theoretical expressions in conventional RIS aided networks for comparison. The analytical results show that optimal energy splitting coefficients of STAR-RISs exist to simultaneously maximize the system coverage and ergodic rate. The numerical results demonstrate that: 1) STAR-RISs are able to meet different demands of UEs located on different sides; 2) STAR-RISs with appropriate energy splitting coefficients outperform conventional RISs in the coverage and the rate performance.

Ultra dense network (UDN) is one of the key technologies to improve the network capacity by densely de-ployed small cells. As for the asymmetry and variations of future traffic, dynamic time division duplex (D-TDD) has been proposed as a promising solution to dynamically adjust the uplink (UL)/downlink (DL) configuration. In this paper, we propose a general and tractable analytical framework for the theoretical performance under D-TDD two-tier heterogeneous networks. We firstly model user equipment (UE) and base station (BS) as homogenous Poisson point process (HPPP). Then, by leveraging stochastic geometry and geometry relationship, we propose an analysis method based on geometry approximation and derive analytical expressions for the UL and DL coverage probability. To increase the accuracy of analysis, we consider the coupling between UEs and BSs, so that more accurate interference can be obtained. Our results show that the derived coverage probabilities match the simulation results quite well and suitable for heterogeneous UDN scenarios. In addition, a comparison of the coverage probabilities between D-TDD system and static TDD (S-TDD) system is also provided to show that compared with S-TDD system, D-TDD system sacrifices uplink coverage performance to obtain better downlink performance.

Due to the explosive growth of mobile data traffic, the Ultra-Dense Networks (UDN) becomes one of the hot research technologies in the 5thGeneration mobile communication system (5G). With the multi-layer network structure, UDN brings spectrum multiplexing gain and system capacity improvement by densely deployed small cells in hot spot area. In this paper, in order to improve system Energy Efficiency (EE) and Spectrum Efficiency (SE), we propose a joint optimization algorithm considering both EE and SE for small cells in UDN with the constraints of interference threshold and transmission data rate requirement. We first formulate a compromised objective function considering both EE and SE, and then the optimization problem is transformed from the fractional form to subtractive form to simplify the solution procedure, and finally the results of resource allocation and power allocation are obtained by the iteration of Lagrangian dual decomposition method. Simulation results show that both EE and SE performance of the proposed algorithm is much better than the algorithms used in conventional communication systems, and compared with the algorithm to maximize SE in UDN, the EE can be significantly improved with very slightly reduced SE performance.