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Advanced quantisation schemes for physical layer security in vehicle communication

Mirko Bottarelli

Published Sep 17, 2026• 0 citations
Publisher

Abstract

Security is a mandatory requirement in wireless communications since their broadcasting nature make them vulnerable to passive eavesdropping, as well as more advanced active threats, such as jamming or man-in-the-middle attacks. Physical layer security protocols have recently been deployed, derived from the characteristics of the medium for randomness extraction and further key generation. This has a two-fold advantage, as it allows for low computational complexity and high energy efficiency while potentially achieving (unconditionally) secure communications. The core part of physical layer security is channel quantisation, where estimates are harvested and converted into candidate keys. However, existing quantisation approaches do not thoroughly consider the trade-off among relevant metrics, such as the bit generation rate, the bit mismatch rate and key entropy. Furthermore, they fail to consider and exploit the peculiar propagation characteristics of vehicular environments, mainly due to their high mobility. Consequently, the current research aims to design a novel quantisation protocol to strike an optimal balance among the aforementioned metrics, using vehicle-to-vehicle communication as the primary use case. The first step consisted of a deep analysis of existing quantisation schemes through a comprehensive critical narrative of both existing and future developments in the field. This ushered in the novel algorithm’s design with holistically optimised metrics. Quantisation thresholds are analytically derived from the probability density function, stemming from channel modelling, which was meticulously tailored for the chosen propagation environment. Experiments showed that the extracted streams have a superior entropy compared to the state-of-the-art quantisation approach. In another contribution, the algorithm was extended to cope with a continuously changing channel reciprocity, a common phenomenon in vehicle communications. A new feedback block was designed to adapt the thresholds to the varying channel noise, attempting to optimise a newly defined metric. Since the latter includes both quantisation and information reconciliation, experiments confirmed that it is able to reach higher performances than fixed approaches, intrinsically exploiting the capability of existing and future error-correcting techniques. Finally, the algorithm was applied to multi-carrier wireless communication, as depicted by the current standards. In doing so, this study considers the estimation of time-varying coherence regions arising from the non-WSSUS behaviour of vehicle communications for the first time in the literature. Such a novelty was used to adapt the probing rate, avoiding sampling the channel for useless correlated estimates and improving the scheme’s efficiency and statistical robustness.

DOI
10.82444/warw.33910417

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Security is a mandatory requirement in wireless communications since their broadcasting nature make them vulnerable to passive eavesdropping, as well as more advanced active threats, such as jamming or man-in-the-middle attacks. Physical layer security protocols have recently been deployed, derived from the characteristics of the medium for randomness extraction and further key generation. This has a two-fold advantage, as it allows for low computational complexity and high energy efficiency while potentially achieving (unconditionally) secure communications. The core part of physical layer security is channel quantisation, where estimates are harvested and converted into candidate keys. However, existing quantisation approaches do not thoroughly consider the trade-off among relevant metrics, such as the bit generation rate, the bit mismatch rate and key entropy. Furthermore, they fail to consider and exploit the peculiar propagation characteristics of vehicular environments, mainly due to their high mobility. Consequently, the current research aims to design a novel quantisation protocol to strike an optimal balance among the aforementioned metrics, using vehicle-to-vehicle communication as the primary use case. The first step consisted of a deep analysis of existing quantisation schemes through a comprehensive critical narrative of both existing and future developments in the field. This ushered in the novel algorithm’s design with holistically optimised metrics. Quantisation thresholds are analytically derived from the probability density function, stemming from channel modelling, which was meticulously tailored for the chosen propagation environment. Experiments showed that the extracted streams have a superior entropy compared to the state-of-the-art quantisation approach. In another contribution, the algorithm was extended to cope with a continuously changing channel reciprocity, a common phenomenon in vehicle communications. A new feedback block was designed to adapt the thresholds to the varying channel noise, attempting to optimise a newly defined metric. Since the latter includes both quantisation and information reconciliation, experiments confirmed that it is able to reach higher performances than fixed approaches, intrinsically exploiting the capability of existing and future error-correcting techniques. Finally, the algorithm was applied to multi-carrier wireless communication, as depicted by the current standards. In doing so, this study considers the estimation of time-varying coherence regions arising from the non-WSSUS behaviour of vehicle communications for the first time in the literature. Such a novelty was used to adapt the probing rate, avoiding sampling the channel for useless correlated estimates and improving the scheme’s efficiency and statistical robustness.
In intelligent transportation systems (ITS), communications between vehicles, i.e. vehicle-to-vehicle (V2V) communications are of greatest importance to facilitate autonomous driving. The current state-of-the-art for secure data exchange in V2V communications relies on public-key cryptography (PKC) consuming significant computational and energy resources for the encryption/decryption process and large bandwidth for the key distribution. To overcome these limitations, physical-layer security (PLS) has emerged as a lightweight solution by exploiting the physical characteristics of the V2V communication channel to generate symmetric cryptographic keys. Currently, key-generation algorithms are designed via empirical parameter settings, without resulting in optimum key-generation performance. In this paper, we devise a key-generation algorithm for PLS in V2V communications by introducing a novel channel response quantisation method that results in optimum performance via analytical parameter settings. Contrary to the current state-of-the-art, the channel responses incorporate all V2V channel attributes that contribute to temporal variability, such as three dimensional (3D) scattering and scatterers' mobility. An extra functionality, namely, Perturbe-Observe (PO), is further incorporated that enables the algorithm to adapt to the inherent non-reciprocity of the V2V channel responses at the legitimate entities. Optimum performance is evidenced via maximisation of the key bit generation rate (BGR) and key entropy (H) and minimisation of the key bit mismatch rate (BMR). A new metric is further introduced, the so-called secret-bit generation rate (SBGR), as the ratio of the number of bits which are successfully used to compose keys to the total amount of channel samples. SBGR unifies BGR and BMR and is thus maximised by the proposed algorithmic process.
Securing data exchange between intercommunicating terminals, e.g., vehicle-to-everything, constitutes a technological challenge that needs to be addressed. Security solutions must be computationally efficient and flexible enough to be implemented in any wireless propagation environment. Recently, physical layer security has gained popularity, which exploits the randomness of wireless channel responses for extracting high entropy secret cryptographic keys. The current state-of-the-art relies on the independently varying channel sources of randomness, e.g., received signal strength (RSS) and phase. However, the limited capability of RSS-based extraction techniques has motivated researchers to investigate alternative approaches. Although phase-based approaches have emerged in many studies, optimising the extraction performance by adapting the algorithm to the non-reciprocal components of static and dynamic channels remains a challenge. In this paper, we propose an adaptive multilevel quantisation approach that adjusts the size of the quantisation region to the channel responses’ non-reciprocity parameters, thus optimising the trade-off between the bit generation rate (BGR) and the bit mismatch rate (BMR). The probability of error has been theoretically formulated. Accordingly, the order of the quantisation process is adapted for acceptable mismatching probability. Moreover, simulation analysis is conducted to prove the ability of the proposed approach to provide flexible adaptation of the quantisation order at different signal-to-noise ratios (SNRs), achieving fast secret bit generation rates 1. 1$\sim2.85bits/packetatSNRsof102.85bits/packet at SNRs of 102.85bits/packetatSNRsof10\sim$25 dB for acceptable BMR ≤0.1\leq 0.1≤0.1.
In intelligent transportation systems (ITS), communications between vehicles, i.e. vehicle-to-vehicle (VV) communications are of greatest importance to facilitate autonomous driving. The current state-of-the-art for secure data exchange in VV communications relies on public-key cryptography (PKC) consuming significant computational and energy resources for the encryption/decryption process and large bandwidth for the key distribution. To overcome these limitations, physical-layer security (PLS) has emerged as a lightweight solution by exploiting the physical characteristics of the VV communication channel to generate symmetric cryptographic keys. Currently, key-generation algorithms are designed via empirical parameter settings, without resulting optimum key-generation performance. In this paper, we devise a key-generation algorithm, namely, Perturbe-Observe (PO), for PLS in VV communications by introducing a novel channel response quantisation method that results optimum performance via analytical parameter settings. Contrary to the current state-of-the-art, the channel responses incorporate all VV channel attributes that contribute to temporal variability, such as three dimensional (3D) scattering and scatterers’ mobility. An extra add-on is further incorporated that enables the algorithm to adapt to the inherent non-reciprocity of the VV channel responses at the legitimate entities. Optimum performance is evidenced via maximisation of the key bit generation rate (BGR) and key entropy (H) and minimisation of the key bit mismatch rate (BMR). A new metric is further introduced, the so-called secret-bit generation rate (SBGR), as the ratio of the number of bits which are successfully used to compose keys to the total amount of channel samples. SBGR unifies BGR and BMR and is thus maximised by the proposed algorithmic process.
The use of wireless networks for communication has grown significantly in recent times, and continues to develop further. The broadcast nature of wireless communications makes them susceptible to a wide variety of security attacks. Unlike traditional solutions, which usually handle security at the application layer, the primary concern of this dissertation is to analyse and develop solutions for secure communication using channel coding techniques at the physical-layer. \nThe topic of physical layer authenticated encryption using high rate key generation through shared randomness is investigated in this work. First, a physical layer secret key generation scheme is discussed exploiting channel reciprocity in wireless systems. In order to address the susceptibility of this family of schemes to active attacks, a novel physical layer authentication encryption protocol is presented along with its extension to multi-node networks in the presence of active adversaries. Unlike previous work in the area of generating secret keys through shared randomness, it is demonstrated that the proposed scheme is semantically secure with respect to chosen plaintext and chosen cipher text attacks. \nSecondly, in order to increase the rate in bits per seconds at which agreed cryptographic keys are been generated, a multi-level quantization algorithm with public feedback is discussed. It is demonstrated that the proposed scheme is superior to direct information distillation approaches and can substantially increase the key generation rates even at low and medium SNRs. Furthermore, the employment of this low-overhead feedback at the information distillation process can largely simplify the information reconciliation process. \nThe proposed secret key generation schemes are tested for randomness such as required for cryptographic keys. The validation test is perfomed with the aid of National Institute of Standards and Technology (NIST) statistical test suite. The P-values obtained in each of the test carried out indicates that the key sequence generated by our algorithm is random.
Due to the sensitivity of the information exchanged in Vehicle to Vehicle (V2V) and Vehicle to Infrastructure (V2I) communication, generating secret keys is critical to secure these communications. As nature is open access, distributed keys are more vulnerable to attacks in the vehicular environment. Physical layer key generation methods using wireless channel characteristics show promise in preventing such attacks, generating keys independently, and removing the need for distribution. In this paper, we present a novel key generation approach in a real vehicular environment based on Channel State Information (CSI), including a new algorithm for key bit extraction. We implemented our algorithm using USRP B210 Software-Defined Radios (SDR) and the industry-standard V2X communication protocol: IEEE 802.11p. The proposed key generation protocol uses the CSI values of each sub-carrier as a source of randomness, from which bits are extracted using a new QAM demodulator quantizer (QAM-Dem-Quan). We compared our technique to state-of-the-art Received Signal Strength (RSS)-based approaches, and show that our method achieves better performance. Moreover, we reached a min-entropy of approximately 70% for the generated keys and a key generation rate of less than150 μs150~\mu \text{s}150 μs/key for key lengths ranging from 16 to 128 bits.
With the rise of connected vehicular networks, Vehicle-to-Vehicle (V2V) communication has become a fundamental enabler for intelligent transportation systems. However, the open nature of wireless channels introduces risks such as eavesdropping and key compromise. To address these issues, we propose a secure V2V communication scheme based on Physical Layer Key Generation (PLKG) using LoRa technology. The proposed system utilizes Received Signal Strength Indicator (RSSI) measurements between two moving vehicles as the entropy source. To improve the channel correlation, a Savitzky-Golay filter is applied as a preprocessing step. Adaptive multibit quantization is then used to convert the RSSI into binary sequences, followed by the Cascade algorithm for error reconciliation. To enhance randomness, privacy amplification is applied using universal hashing and SHA-1. Finally, the generated key is used for AES-128 encryption. The system is implemented on Raspberry Pi 4B using AcSiP EK-S76SXB and RAK2245 LoRa modules. Experimental validation was conducted in three movement scenarios firstly both vehicle move from opposite-direction at10 km/h\mathbf{1 0 ~ k m} / \mathbf{h}10 km/hfor300\mathbf{3 0 0}300meters, secondly both vehicle move at the same-direction at10 km/h\mathbf{1 0 ~ k m} / \mathbf{h}10 km/hfor800\mathbf{8 0 0}800meters, and lastly same as the second scenario but with different speed at20 km/h20 ~\text{km} / \mathrm{h}20 km/h. Results shown that the system achieves a reliable key generation rate (KGR) of up to 176075.73 bit/s while maintaining low computation time, demonstrating its feasibility for secure real-time V2V communication.