Unmanned aerial vehicles (UAVs) have been widely deployed as aerial relays in wireless networks. Moreover, a UAV swarm can form a virtual array and improve the transmission distance and energy efficiency of relay signals through collaborative beamforming (CB). However, due to the different propagation channels between the source node and each relay UAV, there are time delays and phase offsets among the amplify-and-forward (AF) processed signals of each UAV, resulting in CB performance deterioration. Hence, a time-phase-robust collaborative beamforming algorithm (TPR-CB) for UAV swarm-enabled AF relay systems is proposed in this paper. Firstly, a TPR-CB architecture based on the tapped delay line (TDL) is designed. Subsequently, the iterative calculation formula for the TDL weight vector is deduced by minimizing mean square error (MSE) through stochastic gradient descent (SGD). The MSE expression and feasible step size range of SGD are provided. Finally, the expressions for the average far-field beampattern and coherent gain are derived to characterize the performance of TPR-CB. The simulation results show that compared with the existing CB method, TPR-CB can improve the coherent gain and beampattern in the presence of time delay and phase offset.

In the Internet of Things (IoT), distributed idle devices can transmit cooperative jamming to confuse eaves-dropping devices, while authorized receivers utilize distributed cooperative jamming (DCJ) cancellation techniques to achieve signal-to-noise ratio gain, thereby enabling physical layer secure connectivity. However, constrained by the cost and size of IoT terminals, the crystal oscillators they employ typically exhibit low frequency stability, leading to challenges in achieving precise time-frequency alignment during DCJ cancellation. This significantly degrades the DCJ cancellation capability. To address this, the effect of time-frequency mismatch on DCJ cancellation and system sensitivity loss are analysed, and a closed-form expression for the jamming cancellation ratio with respect to the normalized time-frequency error is derived. Simulation results confirm the theoretical analysis that time-frequency mismatch causes a dramatic degradation in the performance of DCJ cancellation and results in a significant increase in the system sensitivity loss, particularly under scenarios involving large-scale jamming nodes.

The space-air-ground integrated networks (SAGNs) are promising components of the next generation communication system, and unmanned aerial vehicle (UAV) swarms are the primary infrastructures of the SAGNs aerial layer, used for relaying information. Furthermore, UAV swarms form virtual antenna arrays (VAAs) and implement collaborative beamforming (CB), which can improve the deep-space communication distance and expand the ground coverage. However, in practical engineering, due to the doppler effect and local oscillator random frequency drift, there is a frequency offset between UAVs, even after frequency synchronization, resulting in CB performance degradation. Therefore, in the presence of frequency offset, this paper investigates and presents the derivation of the average far-field beampattern, 3-dB beamwidth, and complementary cumulative distribution function (CCDF) for UAV swarm executing CB in SAGNs. The results show that the average far-field beam-pattern, peak power, 3-dB beamwidth, and CCDF, deteriorate as frequency offset intensifies. Furthermore, as the UAV swarm scale increases, the peak power attenuation is aggravated, and the decrease in CCDF is steeper. With the increase in the UAV swarm distribution range, the 3-dB beamwidth expansion is alleviated.

While 6G provides communication services for massive wireless devices, it faces huge security risks due to its open feature. Cooperative jamming techniques in physical layer security provide a viable way to ensure 6G security. By actively transmitting jamming signals to deteriorate the eavesdropping channel, while authorized receivers implement cooperative jamming cancellation (CJC) using the prior knowledge of cooperative jamming, thus securing the communication at the physical layer. In practice, incomplete matching of oscillator device characteristics will lead to the in-phase and quadrature (IQ) imbalance problem, where the quadrature branches of the up and down converted carrier signals are not perfectly orthogonal to the in-phase branches, which may degrade the CJC performance. For this reason, this paper specifically analyzes the impact of the IQ imbalance problem on the CJC performance, where a closed-form expression of the jamming cancellation ratio (JCR) with respect to amplitude imbalance and phase imbalance is derived. The simulation results show that when jamming-to-noise ratio (JNR) is 35 dB, the system can tolerate up to 1% amplitude imbalance and 1° phase imbalance with 3 dB JCR degradation. Moreover, the IQ imbalance has a noticeable effect on the reachable security rate of the communication system.

Cooperative jamming is one of the promising technologies for securing wireless communications. The cooperative jammer transmits jamming to deteriorate the eavesdropping channel, while the authorized receiver implements cooperative jamming cancellation (CJC) using the pre-stored prior information, thus achieving confidential communication at the physical layer. However, the signal transmission will introduce time delay and frequency offset, as well as zero-intermediate-frequency structures that will cause in-phase and quadrature (IQ) imbalances and local oscillator (LO) leakage, which lead to degradation of the CJC performance. In this paper, we specifically analyze the combined effect of time-frequency mismatch, IQ imbalance and LO leakage on the CJC performance by deducing the closed-form expression for the jamming cancellation ratio. A CJC architecture considering time-frequency mismatch, IQ imbalance and LO leakage is proposed, and the corresponding recursive algorithm for CJC is presented. Simulation results show that the proposed algorithm can effectively improve the CJC performance with the presence of time-frequency mismatch, IQ imbalance and LO leakage, and thus improves the security of the communication system.

In recent years, cooperative jamming (CJ) is introduced as a promising method to improve the security performance in the presence of eavesdroppers. By masking the confidential signal with the help of the cooperative jammer, the signal-to-noise ratio of the eavesdropper can be selectively reduced without prior information, whereas the authorized receiver is unaffected since the CJ can be suppressed by some means. Unfortunately, with the impacts of the clock jitter caused by non-ideal crystals, the received CJ may suffer sampling clock offset, carrier frequency offset and phase noise, which deteriorates the performance of the CJ cancellation at the authorized receiver. In this paper, the impacts of clock jitter on CJ cancellation are investigated. First, the deterioration of CJ caused by sampling clock offset is modeled as inter-symbol interference (ISI) in the time domain. Then, the impacts of the sampling clock offset, carrier frequency offset and phase noise on the spectral purity of CJ are modeled as inter-frequency interference (IFI). Furthermore, the expression of the power of residual CJ (PRCJ) is derived to analyze the impacts of clock jitter on CJ cancellation. Finally, numerical results prove the theoretical analysis, indicating that the PRCJ increases as the carrier frequency offset and the 3 dB coherence bandwidth of phase noise increases, and is more sensitive to phase noise.

Cooperative jamming (CJ) can mask signal-of-interest (SOI) by additional jamming signal in the presence of eavesdroppers, and thus improves the security performance from the perspective of physical layer. In exchange, an additional CJ canceller should be designed for the destination node to restore the SOI, whose performance is possibly degraded by the nonideal hardware features. Typically, the inherent nonlinearity of power amplifier is unavoidably distorted the CJ as the transmit power increases, leading to the limit of the CJ cancellation performance. Therefore, nonlinear CJ cancellation is suggested to alleviate the impact of nonlinear CJ components on the SOI. Besides, the short-term instability of the oscillator is characterized by phase noise in the time domain, resulting in a random phase mismatch between the received CJ and the reference signal, which further degrades the nonlinear CJ cancellation performance. Inspired by the above problems, this paper explicitly analyzes the impact of phase noise on nonlinear CJ cancellation, where the expression of CJ cancellation ratio (CJCR) in the presence of phase noise and nonlinear distortion is derived, where the coefficients of the nonlinear canceller are estimated by LS estimator, and the deterioration of the spectral purity of CJ caused by phase noise is modeled as inter frequency interference (IFI) additional noise. Theoretical analyses and simulation results show that the CJCR decreases as the normalized 3 dB coherence bandwidth of phase noise increases.

Nonlinear self-interference (SI) cancellation is crucial for eliminating the impact of the transmitter-side nonlinearity on the comprehensive SI cancellation performance in full-duplex radios. However, in the propagation environment with rich reflection paths of signals, the number of the delayed taps required for time-domain nonlinear SI cancellation booms exponentially with the increase of the multi-path number, leading to unacceptable complexity. In this paper, a novel frequency-domain nonlinear SI canceller based on successive interference cancellation (SIC) technique is proposed for orthogonal frequency division multiplexing (OFDM) modulated full-duplex systems subjected to both transmitter nonlinearity and receiver nonlinearity. The proposed frequency-domain nonlinear SIC (FD-NSIC) cancels nonlinear components one by one via separately estimating the channel frequency response suffered by each order nonlinear component, which significantly relaxes the computational cost. Besides, to avoid the influence of the strong correlation between nonlinear components, orthogonalization is operated before SIC to white basis functions, which accelerates the convergence speed of the proposed FD-NSIC. Simulations are explicitly performed, showing that compared with the conventional frequency-domain nonlinear canceller based on recursive least squares (RLS) channel estimation, the proposed FD-NSIC is capable of canceling the SI to the noise floor with only 52% computational cost under 50 dB interference-to-noise ratio.

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 full-duplex (FD) systems, time alignment error between the received self-interference (SI) and the reconstructed one results in performance degradation of the SI cancellation. As a crucial part of SI cancellation, nonlinear interference (NI) cancellation is usually invested in the ideal situation, and the impact of the time alignment error on NI cancellation performance is rarely considered. In this paper, a robust NI cancellation algorithm is proposed to tackle the time alignment error in FD systems. At first, the impact of the time alignment error on NI cancellation is analyzed. The approximate expression of the residual NI power with time alignment error is derived to predict the NI cancellation performance. Then, an improved memory polynomial (IMP) model is derived to describe both time alignment error and nonlinearity. Finally, based on the IMP model, a novel NI cancellation algorithm with robustness to the time alignment error is proposed. Simulations present the impact of the time alignment error on NI cancellation capability, in which the theoretical value is coincident with the actual value. Besides, the proposed algorithm significantly enhances the NI cancellation performance. Specifically, when the time alignment error is 10% of the sampling interval and the interference to noise ratio is 60 dB, the proposed algorithm outperforms the conventional algorithm by 33 dB.

To further enhance the security of the wireless communication systems, an architecture of the cooperative jamming (CJ) assisted slow frequency-hopping (FH) systems is proposed in this paper. Then, after the CJ cancellation operation, the residual CJ raised from synchronization error is analyzed, where the inter-symbol interference (ISI) is employed to depict the time misalignment between the received signal and the local reference CJ at the receiver, and the inter-hop interference (IHI) is introduced to describe the frequency misalignment. In addition, the closed-form expression of the jamming cancellation ratio (JCR) is derived to evaluate the CJ cancellation performance. Theoretical analyses and simulation results show that in the slow FH systems, both the proportions of the ISI and IHI components in the residual CJ are jointly determined by the synchronization error and the hop length. Besides, the JCR increases with the increase of the hop length and the jamming-to-noise ratio (JNR), and decreases with the increase of the synchronization error.