Integrated sensing and communication (ISAC) systems improve unmanned aerial vehicles (UAVs) efficiency and safety for low-altitude economy, but struggle with target identification due to reliance on primary radar. Thus, inspired by secondary surveillance radar (SSR) systems that can recognize target identities, this paper proposes an target identity recognition (TIR)-enabled ISAC technology to further improve UAV identification and tracking capabilities, optimizing airspace management and collision avoidance. Firstly, a novel integrated waveform design for TIR-enabled ISAC systems is proposed, where the waveform carries the interrogation/reply information of TIR through pulse modulation, and maintains a low-level amplitude (LLA) during pulse interval, allowing communication data to be modulated onto the carrier phase. Then, the proposed integrated waveform shows a trade-off between TIR bit error rate (BER) and communication symbol error rate (SER) due to the LLA, with an algorithm developed to optimize LLA for minimizing TIR BER while maintaining low communication SER.

This letter focuses on the optimal sum secure degree of freedom (SDoF) in a two-way wiretap channel (TW-WC), wherein two legitimate full-duplex multiple-antenna nodes cooperate with each other and are wiretapped by a multiple antenna eavesdropper simultaneously. It aims to find the optimal sum SDoF pertaining to secret-key capacity for the TW-WC. First, we analyze the upper bound and lower bounds of the optimal sum SDoF by establishing their equivalence to the expression of the optimal SDoF corresponding to the secrecy rate for the TW-WC. Subsequently, in scenarios where the legitimate nodes are configured with an equal number of transmit and receive antennas, it is elucidated that the upper and lower bounds of the optimal SDoF converge. Furthermore, the findings suggest that a higher SDoF can be achieved than the existing works, thereby heralding an enhancement in secure spectral efficiency.

Integrated sensing and communication (ISAC) holds significant commercial potential in future 6G. The 3GPP technical report indicates that it is crucial to prioritize the sensing services of the ISAC system to ensure the efficient execution of critical services due to resource-constrained environments. Thus, this paper investigates priority-based resource allocation in ISAC systems, where an ISAC base station (BS) simultaneously serves a communication user (CU) and detects both a primary sensing target (PST) and a secondary sensing target (SST). First, A priority criteria for this ISAC system is proposed: 1) Above all, to ensure that the Cramer-Rao bound (CRB) for PST is maintained below a threshold; 2) Following this, the CRB of the SST should be optimized, while ensuring the communication spectral efficiency is preserved. Then, based on this criterion, three cases are derived, and for each case, a distinct resource allocation problem is formulated, yielding closed-form or semi-closed-form expressions of the optimal resource allocation for each problem. Furthermore, An algorithm is also proposed to determine the occurrence of each case and provide the corresponding optimal resource allocation. In addition, system performance under each case is analyzed based on these expressions. Finally, the simulation results demonstrate the impact of the priority strategy on performance, which is consistent with the previous performance analysis.

The advent of reconfigurable intelligent surfaces (RIS) introduces a novel paradigm to address mutual interference issues within radar-communication coexistence (RCC) systems. However, the predominantly discrete reflection coefficients (DRC) of most real-world RISs pose new challenges for the design of interference-mitigating reflection coefficients. This paper investigates a RIS-assisted RCC system with DRC. First, a radar SINR maximization problem under the constraints of communication signal-to-interference plus noise ratio (SINR) requirements is formulated to obtain feasible designs for the DRC and radar beamforming vectors. Then, an alternating optimization approach is applied, decomposing the original problem into two sub-problems. For the DRC optimization sub-problem, a design approach based on semidefinite relaxation (SDR) and Gaussian randomization is proposed. For the radar beamforming sub-problem, the closed-form expressions for the optimal radar beamforming vectors are obtained by the generalized Rayleigh quotient and the Karush-Kuhn-Tucker (KKT) conditions. Finally, it is demonstrated that within a communication-centric scenario, the RIS needs to maintain orthogonality between the interference channel and the radar-target channel in order to achieve the upper bound of the radar SINR. Based on this conclusion, a low-complexity approach is proposed to optimize the radar SINR in a communication-centric scenario.

This paper advocates the use of reconfigurable intelligent surface (RIS) in an active manner to suppress the over-the-air co-channel interference. Different from the commonly used passive RIS, the active RIS reflects and amplifies the electromagnetic wave with phase shifts, enabling more flexible configuration of wireless propagation environment with respect to e.g., interference mitigation. In this paper, we consider a single-cell multiple-input single-output (MISO) channel with the presence of an unknown interferer, where an active RIS is placed to suppress such co-channel interference. Under such setting, we first propose an algorithm to minimize the normalized maximal interference, by which we identify the condition under which the co-channel interference can be perfectly mitigated. Then, we formulate an achievable rate maximization problem to design reflection coefficients, for which an iterative algorithm built upon fractional programming and the convex-concave procedure is proposed.

This paper focuses on interference suppression in Reconfigurable Intelligent Surface (RIS)-assisted multicast communication, where a multi-antennas base station (BS) transmits identical messages to a group of single-antenna users, while a multi-antennas jammer simultaneously sends interference signals to the users. A RIS consisting of many reflectors is deployed near BS to enhance the multicast channel and suppress the interference channel by carefully designing reflection coefficients. Under such settings, a signal to interference plus noise ratio maximization problem is formulated to maximize the achievable rate by designing feasible reflection coefficients at the RIS and a beamforming vector at the BS. Then, an alternating optimization combined with semidefinite relaxation and bisection search over a series of the solutions of feasibility problems is adopted to iteratively optimize the reflection coefficients and beamforming vector separately. At last, simulation results show the RIS can significantly improve the multicast rate with interference.

Providing quality communications under adversarial electronic attacks, e.g., broadband jamming attacks, is a challenging task. Unlike state-of-the-art approaches which treat jamming signals as destructive interference, this paper presents a novel active anti-jamming (AAJ) scheme for a jammed channel to enhance the communication quality between a transmitter node (TN) and receiver node (RN), where the TN actively exploits the jamming signal as a carrier to send messages. Specifically, the TN is equipped with a programmable-gain amplifier, which is capable of re-modulating the jamming signals for jamming modulation. Considering four typical jamming types, we derive both the bit error rates (BER) and the corresponding optimal detection thresholds of the AAJ scheme. The asymptotic performances of the AAJ scheme are discussed under the high jamming-to-noise ratio (JNR) and sampling rate cases. Our analysis shows that there exists a BER floor for sufficiently large JNR. Simulation results indicate that the proposed AAJ scheme allows the TN to communicate with the RN reliably even under extremely strong and/or broadband jamming. Additionally, we investigate the channel capacity of the proposed AAJ scheme and show that the channel capacity of the AAJ scheme outperforms that of the direct transmission when the JNR is relatively high.

The reconfigurable intelligent surface (RIS), which consists of many low-cost reflecting elements, can enhance the reception of the desired signal and suppress the interference. Considering the above advantages, this paper investigates the RIS-assisted wireless communication in the presence of an interferer, where a receiver receives the desired signal and interference from a transmitter and an interferer, respectively, and the RIS is deployed to assist this system. First, an interference-limited channel capacity maximization problem is formulated, and a numerical algorithm based on the fraction programming and optimality conditions is proposed to obtain the optimal solution. Then, this paper shows the upper boundary of the maximal capacity with interference and discusses what kind of channel conditions can achieve the upper boundary and the corresponding optimal phase shifts. Next, this paper analyzes the asymptotic behaviors of the maximal capacity in the scenarios that some or all of the number of reflecting elements, antennas at the transmitter, and interferer go to infinity. Finally, this paper generalizes the above results to the multiple interferers scenario.

In full-duplex multipath self-interference (SI), each of the resolvable multipath SI components consists of a group of non-resolvable (NR) components with similar delays. Since the transceiver cannot distinguish the NR components in reality, the canceller can only generate the approximate constructed SI to imperfectly cancel the received SI. This leads to residual SI remaining. This paper analyzes the effect of the NR components on the SI cancellation, and proposes a practical scheme to eliminate the residual SI caused by the NR components. First, a closed-form expression of the residual SI is obtained. Next, the average power of the residual SI and the SI cancellation ratio are derived to characterize the effect of NR components on SI cancellation in the multipath SI channel. Last, a switch scheme is proposed to eliminate the residual SI.

The reconfigurable intelligent surface (RIS), which consists of a large number of passive and low-cost reflecting elements, has been recognized as a revolutionary technology to enhance the performance of future wireless networks. This paper considers an RIS assisted multicast transmission, where a base station (BS) with multiple-antenna multicasts common message to multiple single-antenna mobile users (MUs) under the assistance of an RIS. An equivalent channel model for the considered multicast transmission is analyzed, and then an optimization problem for the corresponding channel capacity is formulated to obtain the optimal covariance matrix and phase shifts. In order to solve the above non-convex and non-differentiable problem, this paper first exploits the gradient descent method and alternating optimization, to approach the locally optimal solution for any number of MUs. Then, this paper considers a special case, which can obtain the global optimal solution, and shows the sufficient and necessary condition for this special case. Finally, the order growth of the maximal capacity is obtained when the numbers of the reflecting elements, the BS antennas, and the MUs go to infinity.

This paper considers a point-to-point communication system which is protected by a cooperative jammer from being eavesdropped. Previous works in this field always assume that the cooperative jamming is cancelled perfectly at the receiver, without considering any imperfections in the jamming cancellation of practical systems. In this paper, by taking into account the residual jamming caused by frequency offset at the receiver, a power allocation scheme is proposed to maximize the secrecy capacity over the additive white Gaussian noise (AWGN) channel, which is derived as an exact closed-form solution. In addition, the relationship between the optimal jamming power and the relative channel quality is analyzed under imperfect jamming cancellation. Simulation results confirm that the proposed power allocation scheme outperforms the power allocation scheme without considering any imperfections in the jamming cancellation, and illustrate that the frequency offset significantly degrade the security performance. Specially, for small frequency offset, the total power consumption increases with the frequency offset, which means the frequency offset consumes extra power to achieve the secrecy capacity target. However, for large frequency offset, the secrecy capacity may fail to meet its preset goal no matter how sufficient the power budget is. More seriously, when the frequency offset is remarkable, the secrecy capacity may drop to 0, which implies that the eavesdropper can always overhear the confidential information.

In order to improve the multicast transmissions, a full-duplex (FD) receiver cooperation scheme is proposed in this paper. The transmitter sends one common message to two FD receivers, and each receiver forwards its received signals to its counterpart by the amplify-and-forward (AF) scheme. Due to the imperfect SI cancellation at the receivers and the AF scheme, the residual self-interference and the additive noise (RIAN) will be accumulated over time. First, this paper analyzes the equivalent channel model of the considered system as well as the the statistics of the accumulated RIAN. Then, with the forward decoding scheme, the corresponding achievable rates are derived, and the optimal power allocation is obtained via solving a max-min problem. In particular, one-side cooperation scheme (i.e., only one receiver forwards its received signal to its counterpart) is shown to be optimal to achieve the best system performance.

In this paper, a point-to-point fading channel with the transmitter powered by energy harvester is considered. On account of the battery half-duplex constraint, this paper adopts a save-then-transmit scheme: the transmitter first harvests energy, and then stops EH to transmit information with all accumulated energy. Obviously, for the save-then-transmit scheme, a proper stopping time needs to be determined, since larger EH duration provides more the accumulated energy while decreases the average throughput. First, this paper proves the existence of the optimal stopping rule and shows that this rule has a state-dependent “threshold-based” structure for the fading channel scenario. Then, the corresponding threshold and the maximum throughput are efficiently computed by a proposed algorithm.