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IEEE Transactions on Instrumentation and Measurement

SDR-Based Bidirectional Precise Wireless Channel Sounding for Physical Layer Key Generation

Ji Hong Gu, Liang Yin, Xin Zhang, Hongwen Yang

Published Jan 1, 2026• 0 citations
Publisher

Abstract

Wireless channel state information has been widely utilized in physical layer key generation (PLKG) as shared random sources. The temporal randomness and reciprocity of the channel parameters are utilized, posing a requirement for high-precision time-division duplexing (TDD) channel measurement. However, existing real channel measurement-based PLKG studies suffer from significant channel estimation errors, which degrade the measured channel reciprocity and increase the key disagreement rate. Furthermore, PLKG in static environments has aroused extensive attention. In static environments, the channel temporal variation is very weak, and is generally considered insufficient for reliable key generation. To address these challenges, in this paper, a high-precision bidirectional wireless channel fluctuation measuring platform was designed and implemented based on generic software-defined radio (SDR) architectures. The hardware impairments of multiple SDR platforms with different architectures were systematically analyzed. With such hardware impairments, traditional channel estimation methods, such as least squares (LS) and minimum mean square error (MMSE), were found suboptimal for measurement of the received signal strength indication (RSSI), and a tailored TDD RSSI measurement algorithm was proposed for generic SDR devices. With the proposed algorithm, TDD measurement with 200∼500μs period was achieved with root mean square (RMS) RSSI measurement error of less than 0:002dB on all tested devices, significantly outperforming the state-of-the-art result in the same field. The wireless channel fluctuations in indoor office static environments were measured using the proposed system, and results have shown that RSSI fluctuations smaller than 0:5dB can be detected with high reciprocity between opposite directions, which provides fine-grained RSSI data for PLKG. Through PLKG experiment based on the measurement data, a high KGR of 80bit/s can be achieved with an acceptable KDR of 0:05∼0:2, highlighting the critical role of precise and fine-grained RSSI measurements in enabling efficient PLKG.

DOI
10.1109/tim.2026.3670582

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Wireless channel state information has been widely utilized in physical layer key generation (PLKG) as shared random sources. The temporal randomness and reciprocity of the channel parameters are utilized, posing a requirement for high-precision time-division duplexing (TDD) channel measurement. However, existing real channel measurement-based PLKG studies suffer from significant channel estimation errors, which degrade the measured channel reciprocity and increase the key disagreement rate. Furthermore, PLKG in static environments has aroused extensive attention. In static environments, the channel temporal variation is very weak, and is generally considered insufficient for reliable key generation. To address these challenges, in this paper, a high-precision bidirectional wireless channel fluctuation measuring platform was designed and implemented based on generic software-defined radio (SDR) architectures. The hardware impairments of multiple SDR platforms with different architectures were systematically analyzed. With such hardware impairments, traditional channel estimation methods, such as least squares (LS) and minimum mean square error (MMSE), were found suboptimal for measurement of the received signal strength indication (RSSI), and a tailored TDD RSSI measurement algorithm was proposed for generic SDR devices. With the proposed algorithm, TDD measurement with 200∼500μs period was achieved with root mean square (RMS) RSSI measurement error of less than 0:002dB on all tested devices, significantly outperforming the state-of-the-art result in the same field. The wireless channel fluctuations in indoor office static environments were measured using the proposed system, and results have shown that RSSI fluctuations smaller than 0:5dB can be detected with high reciprocity between opposite directions, which provides fine-grained RSSI data for PLKG. Through PLKG experiment based on the measurement data, a high KGR of 80bit/s can be achieved with an acceptable KDR of 0:05∼0:2, highlighting the critical role of precise and fine-grained RSSI measurements in enabling efficient PLKG.
Physical layer key generation (PLKG) is a promising technology to obtain symmetric keys between a pair of wireless communication users in a plugand- play manner. The shared entropy source almost entirely comes from the intrinsic randomness of the radio channel, which is highly dependent on the wireless environments. However, in some static/ block fading wireless environments, it is hard to guarantee the intrinsic randomness of the wireless channel. Thanks to reconfigurable intelligent surfaces (RISs) with their excellent ability on electromagnetic wave control, the wireless channel environment can now be customized. In this article, we overview the RIS-aided PLKG in static indoor environments, including its channel model and hardware architectures. Then we propose potential application scenarios and analyze the design challenges of RIS-aided PLKG, including channel reciprocity, RIS reconfiguration speed, and RIS deployment via proof-of-concept experiments on a RIS-aided PLKG prototype system. In particular, our experimental results show that the key generation rate is 15-fold higher than that without RIS in a static indoor environment. Next, we design a RIS jamming attack via a prototype experiment and discuss its possible attack-defense countermeasures. Finally, several conclusions and future directions are identified.
Physical Layer Secret Key Generation (PLSKG), which utilizes wireless channel reciprocity, is increasingly adopted to secure wireless communications. In Time Division Duplexing (TDD) systems, the inherent channel reciprocity facilitates the generation of cryptographic keys without requiring a key exchange process. Conversely, Frequency Division Duplexing (FDD) systems present greater challenges for key generation due to the use of distinct frequency bands for uplink and downlink, resulting in differing frequency responses. In this paper, we address this challenge by modeling the channel along with connecting cables as a two-port network and utilizing the reciprocity principles applicable to such networks within the same frequency range. For key generation, we construct the channel profile for each FDD band by utilizing the amplitude of the bidirectional scattering parameters S12and S21in two closely spaced FDD bands, thereby ensuring the required reciprocity in FDD systems. To assess the practicality of the proposed scheme, we analyze its performance across various indoor scenarios and compare it with other key generation schemes using multiple performance indicators. The comparative results confirm the feasibility and effectiveness of our scheme in terms of randomness and key disagreement ratio.
Physical layer encryption is a promising solution to provide transmission security for Internet-of-Things (IoT). To realize physical layer encryption, channel state information (CSI) as a random source to generate secret keys should be exploited. The reciprocity of wireless channel between legitimate communication nodes which is typically assumed for time division duplex (TDD) systems, is of critical importance to ensure the consistency in generated keys at two nodes. However, in practical TDD systems, due to the asynchronous measurement of CSI, the receiver noise and the the difference in hardware, etc., the reciprocity of channel measurements between legitimate parties is hard to be guaranteed, which makes physical layer key generation approach infeasible. To address this issue, we propose a wireless key generation scheme based on deep learning techniques. Firstly, a feature extraction network, which consists of two auto-encoders trained jointly, is developed to extract the common feature from the different yet correlated channel observations at two legitimate nodes, which will be used as the source to generate keys. Then a quantizer with an adaptive guard band width is devised to output a binary secret-key sequence from the feature vector. Numerical results show that the proposed scheme achieves remarkable improvement in terms of key disagreement rate (KDR) and key generation rate (KGR) compared existing counterparts in the literature.
Physical layer key generation (PLKG) has emerged as a promising strategy for securing wireless communication using shared channel knowledge between two legitimate users. Traditional PLKG systems rely on time-frequency domain channels using metrics such as Received Signal Strength Indicator (RSSI) or Channel State Information (CSI) to generate secure keys. As a result of the rapid variation in the time-frequency domain channels, these systems are prone to error and suffer from a high reconciliation overhead requirement. Conversely, the same channels have a sparse and static representation in the delay-Doppler domain, which is utilized by the recently introduced Orthogonal Time Frequency Space (OTFS) modulation scheme. Furthermore, the channel representation in the delay-Doppler domain is uniquely determined by the scattering environment between the transmitter-receiver pair. In this paper, we leverage the sparsity of the delay-Doppler domain channel as well as the uniqueness of the channel representation to introduce two OTFS based PLKG schemes. We show through extensive simulations that the introduced OTFS schemes can get < 10% bit disagreement ratio (BDR) in scenarios where time-frequency domain based RSSI methods break down.
Physical layer key generation technology which leverages channel randomness to generate secret keys has attracted extensive attentions in long range (LoRa)-based networks recently. We in this paper develop a software-defined radio (SDR) based LoRa communications platform using GNU Radio on universal software radio peripheral (USRP) to implement and evaluate typical physical layer key generation schemes. Thanks to the flexibility and configurability of GNU Radio to extract LoRa packets, we are able to obtain the fine-grained channel frequency response (CFR) through LoRa preamble based channel estimation for key generation. Besides, we propose a low-complexity preprocessing method to enhance the randomness of quantization while reducing the secret key disagreement ratio. The results indicate that we can achieve 367 key bits with a high level of randomness through just a single effective channel probing in an indoor environment at a distance of 2 meters under the circumstance of a spreading factor (SF) of 7, a preamble length of 8, a signal bandwidth of 250 kHz, and a sampling rate of 1 MHz.
Wireless information security generates shared secret keys from reciprocal channel dynamics. Current solutions are mostly based on temporal per-frame channel measurements of signal strength and suffer from low key generate rate (KGR), large budget in channel probing, and poor secrecy if a channel does not temporally vary significantly. This paper designs a cross-layer solution that measures noise-free per-symbol channel dynamics across both time and frequency domain and derives keys from the highly fine-grained per-symbol reciprocal channel measurements. This solution consists of merits that: (1) the persymbol granularity improves the volume of available uncorrelated channel measurements by orders of magnitude over per-frame granularity in conventional solutions and so does KGR; 2) the solution exploits subtle channel fluctuations in frequency domain that does not force users to move to incur enough temporal variations as conventional solutions require; and (3) it measures noise-free channel response that suppresses key bit disagreement between trusted users. As a result, in every aspect, the proposed solution improves the security performance by orders of magnitude over conventional solutions. The performance has been evaluated on both a GNU SDR testbed in practice and a local GNU Radio simulator. The cross-layer solution can generate a KGR of 24.07 bits per probing frame on testbed or 19 bits in simulation, although conventional optimal solutions only has a KGR of at most one or two bit per probing frame. It also has a low key bit disagreement ratio while maintaining a high entropy rate. The derived keys show strong independence with correlation coefficients mostly less than 0.05. Furthermore, it is empirically shown that any slight physical change, e.g. a small rotation of antenna, results in fundamentally different cross-layer frequency measurements, which implies the strong secrecy and high efficiency of the proposed solution.