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.
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.
In physical layer security, imperfect time synchronization results in random mismatch between the received cooperative jamming (CJ) and the reconstructed CJ, leading to performance degradation of CJ cancellation at the authorized receiver. In this letter, the CJ cancellation performance with imperfect time synchronization is analyzed and evaluated by jamming cancellation ratio (JCR). Then, by assuming that the timing error is uniformly distributed within the synchronization accuracy, the expectation and cumulative distribution function (CDF) of JCR are provided. Experimental results confirm the theoretical analysis that imperfect time synchronization causes dramatic performance degradation of CJ cancellation, especially for high jamming-to-noise ratio (JNR).
In the presence of some eavesdroppers, cooperative jamming cancellation (CJC) plays a significant role in physical layer security for wireless communications. The assumption that the cooperative jamming is cancelled ideally with perfect frequency and timing synchronization is commonly used in the past literatures. Due to the instability of crystal oscillators and the relative motion between the cooperative node and the receiving node, residual frequency offset always exists between these two nodes, resulting in performance degradation during CJC. In this paper, a scenario in which all nodes equipped with single antenna is considered. To explicitly analyze the effect of frequency offset on CJC, we model frequency offset as an inter-frequency interference (IFI) in the frequency domain, and evaluate the CJC performance by jamming cancellation ratio (JCR), which is derived as the closed-form expression. Theoretical and simulation results show that, for the additive white Gaussian noise (AWGN) channel, when the ideal received jamming-to-noise ratio (JNR) is 60dB, the jamming bandwidth is 10MHz, and the sampling period of analog-digital converter (ADC) is 100ns, the maximum tolerable frequency offsets that incur 0.1dB, 1dB, and 3dB JCR degradation are 0.82Hz, 2.74Hz, and 5.37Hz, respectively.
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.
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.
In physical layer secure communications, time synchronization is crucial to cooperative jamming cancellation. However, perfect time synchronization is unrealistic due to inevitable synchronization error in practical communication systems. Consequently, researchers have done some research on the above problem. Unfortunately, the impact of cooperative jamming wave shaping is neglected. In this paper, taking account of transmission signals with raised cosine-rolloff filter, performance of cooperative jamming cancellation with timing errors is studied. By infinite series expansion, an exact analytical expression of the cooperative-jamming-cancellation ratio (CJCR) is derived, which evaluates the performance of the cooperative jamming cancellation. Furthermore, impact of the normalized time synchronization error and the rolloff factor on CJCR is studied. Simulation results show that the normalized time synchronization error of 0.1 causes a loss of 42dB in CJCR, with the ideal received cooperative-jamming-to-noise ratio of 60dB and the rolloff factor of 0.5. Besides, the communication system with a lower rolloff factor and a higher cooperative jamming power is more sensitive to time synchronization error.
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.