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International Symposium on Networks, Computers and Communications (ISNCC)

Analysis of IQ Imbalance Effects on Physical Layer Secure Key Generation in mmWave Systems

Navaneetha Channiganathota Manjappa, Nebojša Maletić, Lara Wimmer, Eckhard Grass

Published Oct 22, 2024• 1 citation
Publisher

Abstract

The performance of practical millimetre wave (mmWave) communication systems is affected by different hardware impairments. This can significantly impact the channel reciprocity-based physical layer security (PLS) methods. This paper studies the impact of in-and quadrature-phase (IQ) imbalance on the physical layer secure key generation algorithm performance. Numerical analysis is performed in MATLAB by modelling and simulating the IQ imbalance, which is then applied to the 60 GHz channel used to generate secret keys. The study's outcome shows that IQ imbalance increases bit mismatches up to an average of 5% for an amplitude imbalance of 1.4 dB and a phase imbalance of 7°. However, additional iterations of parity bit sequence exchanges can recover these in the reconciliation phase. As a result, the information is slightly more exposed to an eavesdropper.

DOI
10.1109/isncc62547.2024.10759029

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The performance of practical millimetre wave (mmWave) communication systems is affected by different hardware impairments. This can significantly impact the channel reciprocity-based physical layer security (PLS) methods. This paper studies the impact of in-and quadrature-phase (IQ) imbalance on the physical layer secure key generation algorithm performance. Numerical analysis is performed in MATLAB by modelling and simulating the IQ imbalance, which is then applied to the 60 GHz channel used to generate secret keys. The study's outcome shows that IQ imbalance increases bit mismatches up to an average of 5% for an amplitude imbalance of 1.4 dB and a phase imbalance of 7°. However, additional iterations of parity bit sequence exchanges can recover these in the reconciliation phase. As a result, the information is slightly more exposed to an eavesdropper.
Physical layer security is one of the trending research areas in tackling data security issues of wireless networks. The main focus of this work is to explore practically the possibilities and performance of secret key generation solutions exploiting wireless channel randomness. This paper describes the implementation of a real-time secure key generation algorithm in a 60 GHz millimetre wave (mmWave) communication system. The performance of the algorithm is evaluated systematically for different parameter combinations to find the optimal parameter values for the system in an indoor static office environment.
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.
Physical layer security (PLS) can be used to pro-vide keyless and innately secure communications by exploiting the propagation characteristics of the wireless channel at the physical layer (PHY) layer to co-generate cipher keys for symmetric encryption. While existing methods use channel state information as input for secret key generation, we consider a novel approach that derives features from frequency-time domain representation of in-phase and quadrature (IQ) samples to derive initial keys with high agreement rate. A recurrent neural network (RNN) with a quadruplet loss function is trained based on spectrograms collected from bi-directional time-division duplex channel probing between the two parties and on spectrograms collected from a passive eavesdropper. Information reconciliation is achieved using Reed Solomon codes. At the last step of the protocol, a 512-bit cryptographic key is generated by feeding the reconciled bit sequence to the Secure Hash Algorithm 3. Our experiments with software-defined radios in both indoor and outdoor environments evaluate the proposed key generation system under different channel propagation environments and hardware inaccuracies. We demonstrate that the proposed RNN-assisted channel feature extractor trained on indoor channel probe data operates successfully in two outdoor setups and produces up to 70 amplified keys out of 100 probes exchanged between two trusted parties i.e., Alice and Bob, while the passive eavesdropper fails to directly predict any of the generated keys.
The wireless channel is inherently an open medium which exposes wireless communication users to the risk of interception. The vulnerabilities of the wireless channel have led to the search for innovative and efficient security solutions at different levels of the network protocol. In particular, the physical layer level allows for implementing some security functions, such as key generation and exchange, by exploiting the intrinsic randomness of the wireless communication channel. In this paper, we analyze the effect of wireless channel and of antenna characteristics on physical layer security (PLS) key generation. The aim is to put in evidence the relation between physical quantities, such as radio channel characteristics or antenna directivity, with higher level secrecy performance indicators, such as the key mismatch between the different communication nodes and the key randomness. For this purpose, we developed a comprehensive numerical model, which implements all the different steps, from signal generation and wireless propagation modeling to key generation through spectral quantization. The reported results show that the multipath richness of the radio channel, as in outdoor urban scenarios, improves the PLS secrecy performances with respect to simpler propagation scenarios. Moreover, we show that the use of more directive antennas in the legitimate communication link improves secrecy. An opposite effect is achieved when the directivity of the eavesdropper antenna is increased, thus improving its ability to perform passive attacks.
Radio frequency front ends constitute a fundamental part of both conventional and emerging wireless systems. However, in spite of their importance, they are often assumed ideal, although they are practically subject to certain detrimental impairments, such as amplifier nonlinearities, phase noise, and in-phase and quadrature (I/Q) imbalance (IQI). This letter is devoted to the quantification and evaluation of the effects of IQI on the secrecy capacity of a wiretap channel. Novel closed-form expressions are derived for the secrecy outage probability for the case of ideal transmitter and I/Q imbalanced legitimate user's and eavesdropper's receivers. Numerical results and simulations illustrate the detrimental effects of IQI on the physical layer security performance, and reveal that IQI should be seriously taken into consideration in the design of secure wireless systems.
Secret key generation based on wireless channel reciprocity is a promising area of research towards ensuring secure wireless communication. In this paper, we propose a method of generating secret keys using channel diversity obtained via millimeter wave (mmWave) beamforming at 60 GHz. The pro-posed method is simulated using the QuaDRiGa channel model considering an industrial nonline of sight (NLOS) environment. We verify the method by conducting experiments using a 60 GHz software-defined-radio (SDR) communication system with a linear phased-array-antenna of 16 dual patch elements. Further, we present a composite key generation method in which both the real and imaginary parts of the channel state information (CSI) are utilised, and several sub-keys obtained from individual beam directions are appended. As a result, we can generate long composite keys with high entropy and secrecy. The keys randomness are assessed employing the National Institute of Standards and Technology (NIST) statistical test suite, showing the generated keys are highly random.