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IEEE Transactions on Signal Processing

Analysis and Optimization of Transceiver IQ Imbalances in Artificial Noise Shielded FH Communication

Changqing Song, Hongzhi Zhao, Lizhi Qin, Rong Wen, Shihai Shao

Published Jan 1, 2022• 20 citations
Publisher

Abstract

The artificial noise (AN) shielded frequency-hopping (FH) architecture can effectively resist wiretapping and electromagnetic interference, but its wide hopping bandwidth will raise a non-negligible in-phase and quadrature (IQ) imbalance at transceiver oscillators, yielding significant signal distortions during IQ mixing. Considering this, first, signal distortions raised by transceiver IQ imbalances are modelled in this study, and their characteristics are mathematically investigated as well. Next, the signal to distortion-plus-noise ratio (SDNR) is extracted to measure the quality of the obtained signal after AN cancellation, and the secrecy capacity is subsequently derived to evaluate system secrecy under multiple wiretappers. Then, based on the performance evaluations, the transmitting power ratio of AN to information signals is optimized to counteract the secrecy degradation raised by transceiver IQ imbalances. Simulations show that both the AN cancellation and system secrecy performance will decline sharply as the amplitude or phase imbalance deteriorates. Besides, compared with the case where the power ratio of AN to information signals is 2, the average secrecy enhancements of an equal power scheme, a power scheme ignoring IQ imbalances, and our proposed power scheme approximate 8.4%, 9.66%, and 11.05% for a fixed phase imbalance, and approximate 10.88%, 12.61%, and 14.26% for a fixed amplitude imbalance, respectively.

DOI
10.1109/tsp.2022.3179209

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The artificial noise (AN) shielded frequency-hopping (FH) architecture can effectively resist wiretapping and electromagnetic interference, but its wide hopping bandwidth will raise a non-negligible in-phase and quadrature (IQ) imbalance at transceiver oscillators, yielding significant signal distortions during IQ mixing. Considering this, first, signal distortions raised by transceiver IQ imbalances are modelled in this study, and their characteristics are mathematically investigated as well. Next, the signal to distortion-plus-noise ratio (SDNR) is extracted to measure the quality of the obtained signal after AN cancellation, and the secrecy capacity is subsequently derived to evaluate system secrecy under multiple wiretappers. Then, based on the performance evaluations, the transmitting power ratio of AN to information signals is optimized to counteract the secrecy degradation raised by transceiver IQ imbalances. Simulations show that both the AN cancellation and system secrecy performance will decline sharply as the amplitude or phase imbalance deteriorates. Besides, compared with the case where the power ratio of AN to information signals is 2, the average secrecy enhancements of an equal power scheme, a power scheme ignoring IQ imbalances, and our proposed power scheme approximate 8.4%, 9.66%, and 11.05% for a fixed phase imbalance, and approximate 10.88%, 12.61%, and 14.26% for a fixed amplitude imbalance, respectively.
The artificial noise shielded frequency-hopping (AN-FH) architecture can secure wireless communications against interference and eavesdropping. However, the AN suppression performance is highly sensitive to the radio frequency (RF) impairments, and the transceiver in-phase and quadrature (IQ) imbalances and phase noise in FH systems are more severe for the wide FH bandwidth. In this study, the AN-FH transceivers under RF impairments are mathematically modeled. Subsequently, distortions for transceiver phase noise are represented by the common phase error, distortions for transceiver IQ imbalances are represented by the mirror image component, and closed-form expressions for their respective power are derived and compared with the thermal noise power. Finally, the AN suppression capability is evaluated via the AN suppression ratio (ANSR), defined as the ratio of the AN-plus-noise power before and after suppression. It is found that when only phase noise exists, shortening the channel compensation cycle can enhance ANSR and reduce ANSR degradation; when the distortions’ power is smaller than the thermal noise power, it is also recommended to enhance the transmitting power and the power ratio of AN to information signals. When only IQ imbalances exist, increasing the power ratio of AN to information signals is suggested; when the distortions’ power is smaller than the thermal noise power, enhancing the transmitting power is also suggested.
This paper adopts an artificial noise (AN) shielded frequency hopping (FH) architecture to counteract both wire-tapping and hostile attack, but the inevitable phase noise will degrade both the AN cancellation and secrecy performance. Considering this, first, the AN cancellation performance is analyzed by separating the received AN into the cancellable common phase component (CPC) and non-cancellable inter-carrier interference (ICI). On top of this, the system secrecy is analyzed via secrecy capacity, and the secrecy degradation for phase noise is extracted in terms of 3 dB coherence bandwidth of the adopted oscillators, transmitting power ratio of AN to expected signal (ES), propagation channel quality, and parameter estimation period. Numerical and simulation results show that phase noise will degrade both the AN cancellation and secrecy performance, and this degradation can be diminished by shortening the parameter estimation period, optimizing the power ratio of AN to ES, and improving the propagation channel quality.
Transmitting the information-bearing signal simultaneously with artificial noise (AN) can guarantee secure data exchange on the open wireless channel. One of the key technologies is AN reconstruction and cancellation at the legitimate receiver, but its performance will be deteriorated by the ineluctable clock jitter at the analog-to-digital converter (ADC). In this paper, first, the received signal distorted by clock jitter is mathe-matically modelled. Then, the closed-form expression of signal to interference-plus-noise ratio (SINR) after AN cancellation is deduced, and the secrecy capacity is derived to evaluate the system security. Furthermore, to alleviate the effect of clock jitter, the optimized transmitting power allocation scheme for AN and information signal is provided. Theoretical and simulation results demonstrate that the secrecy performance is limited by the 3 dB coherence bandwidth of the phase noise caused by clock jitter, time synchronization period, and transmitting power allocation factor. With the optimized power scheme, the secrecy capacity can be increased by up to 11 % in comparison to the equal power allocation, and the optimal power allocation factor equals one when there is no clock jitter.
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.
Artificial noise (AN) can degrade eavesdroppers' signal-to-noise ratio (SNR) in anti-terrorism emergency communications to secure tactical data transmissions. However, system secrecy performance depends on licensed receivers' AN cancellation capabilities. Imperfect channel state information (CSI) will induce residual AN at licensed receivers, thereby degrading system secrecy performance. Unlike the previous methods of simply optimizing power allocation at the transmitter, this study proposes a joint transceiver optimization scheme to mitigate the impact of channel estimation error. Firstly, we present an ANaided anti-terrorism emergency communication model and theoretically derive the statistical properties of the channel estimation error and the closed-form expression of the achievable secrecy rate. Then, a joint optimization scheme for power allocation at the transmitter and cancellation factor at the receiver is presented to maximize the achievable secrecy rate. On this basis, the optimal values of power allocation weight and cancellation factor are derived. The simulation and analysis results prove that the proposed optimization scheme can effectively improve the system security performance in the presence of channel estimation error, and its achievable secrecy rate can be improved by about 16% compared with the power allocation alone.