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Procedia Computer Science

Investigation of Secret Key Capacity in MIMO-OFDM Wireless Systems

Chakka Sai Pradeep, R. Ramanathan

Published Jan 1, 2018• 5 citations
Publisher

Abstract

MIMO and MIMO-OFDM systems are the enabling technologies for the next generation wireless systems. However, the radio signals transmitted are available for reception by a illegitimate user leading to eavesdropping. This raises severe concerns on the security of the transmission. Physical Layer security aims to exploit the wireless channel characteristics and in turn provide secure transmission robust against eavesdropping. In this paper, we investigate the Secret Key Capacity (SKC) in MIMO-OFDM systems. We derive expressions for SKC in MIMO systems and extend further to OFDM waveforms. Numerical simulations are performed for various transmit and receive antenna configurations without any estimation errors.

DOI
10.1016/j.procs.2018.10.389

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MIMO and MIMO-OFDM systems are the enabling technologies for the next generation wireless systems. However, the radio signals transmitted are available for reception by a illegitimate user leading to eavesdropping. This raises severe concerns on the security of the transmission. Physical Layer security aims to exploit the wireless channel characteristics and in turn provide secure transmission robust against eavesdropping. In this paper, we investigate the Secret Key Capacity (SKC) in MIMO-OFDM systems. We derive expressions for SKC in MIMO systems and extend further to OFDM waveforms. Numerical simulations are performed for various transmit and receive antenna configurations without any estimation errors.
MIMO-OFDM as well as MIMO systems are two steering technologies for 5G and beyond communication systems. The key problem with conventional method is that the security protocols used does not take any advantage of the wireless channel features, making eavesdropping a serious issue. This poses security issues at the physical layer. In this paper, we use SecretKey Capacity (SKC) on MIMO and MIMO-OFDM systems with more than one receiver antennas for the eavesdropper. We investigate the impact of channel estimation errors on the SKC performance. We derive the closed form expressions for SKC for both the MIMO and MIMO-OFDM channel The effect of channel estimation errors at the legitimate receiver end is also investigated for different MIMO-OFDM configurations. MIMO-OFDM channels can achieve positive SKC even when eavesdropper has higher number of receiver antenna than legitimate receiver, which is not possible in the case of MIMO channels. Increase in the channel estimation errors can increase SKC of the system. SKC results are discussed here with different number of transmit and receive antenna for eavesdropper and legitimate receiver.
In physical-layer security, secret bits are extracted from wireless channels. With the assumption of channel reciprocity, the legitimate users share the same channel which is independent of the channels between the legitimate users and the eavesdropper, leading to secure transmissions. However, practical implementation of the physical layer security faces many challenges. First, for the correlated channel such as the multiple-input and multiple-output (MIMO) channel, the security is decreased due to the correlation between the generated secret bits. Second, the nearby eavesdropper posts a security threat due to observing the same channel as the legitimate user's. Third, the eavesdroppers might try to reconstruct the wireless environments. In this paper, we propose two practical physical layer security schemes for the MIMO orthogonal frequency-division multiplexing (MIMO-OFDM) systems: the precoding matrix index (PMI)-based secret key generation with rotation matrix (MOPRO) and the channel quantization-based (MOCHA) scheme. The former utilizes PMI and rotated reference signals to prevent the eavesdroppers from learning the secret key information and the latter applies channel quantization in order to extract more secret key bits. It is shown that not only the secure communication but also the MIMO gain can be guaranteed by using the proposed schemes.
Dynamic secret key generation from wireless channel characteristics is a promising technique for physical layer security. One of the important issues in this field is extending the secret key’s length while preserving its uniformity and randomness. This paper proposes a key generation method based on time-varying and the reciprocity of wireless channels for Multiple-Input Multiple-Output Orthogonal Frequency Division Multiplexing (MIMO-OFDM) wireless communication systems. In the proposed technique, the complex impulse response (CIR) of the estimated channel state information (CSI) is extracted, and a quantization algorithm is designed to convert the maximum peaks of the CIR into secret key bits. The effectiveness of the proposed key generation technique is assessed in terms of the randomness of the produced key bits with different key lengths by using a statistical test suite of the National Institute of Standards and Technology (NIST). The proposed technique is employed in the MIMO-OFDM systems with different modulation schemes through Additive White Gaussian Noise (AWGN) and Rayleigh channels. The simulation results show that the secret keys with various key lengths generated from the proposed technique for the MIMO-OFDM systems guarantee randomness. Moreover, the proposed CSI-based key generation technique provides better effectiveness in terms of security when compared to some existing techniques.
Latest developments in wireless communication networks push the limits of conventional security methods. Security can be improved with a secret key that is dynamically generated based on the surrounding physical environment. With this objective, physical layer security approaches can be implemented by means of simple signal processing methods. In this paper, we propose novel key extraction approaches based on spatial and frequency characteristics of the wireless environment. In a 2 × 2 MIMO-OFDM system, low key disagreement rates and key error rates are observed, proving the potential of physical layer key extraction techniques.
Recent research on the physical layer security of wireless systems focuses on artificial noise (AN) aided security. The main metric for analysis of such systems is the secrecy capacity of the system. Most of the AN schemes proposed in recent research are based on a hypothesis that the number of transmit antennas is larger than that of the receive antennas. Under this assumption, the system can utilize all eigensubchannels, equal to the number of the receive antennas, to send secret messages. The remaining null spaces are used for transmitting AN signals. These AN signals null out at the legitimate receivers and degrade illegitimate receiver's channels. However, this strategy can significantly impair the secrecy capacity of the system if the number of transmit antennas is constrained or even smaller than the number of receive antennas. Recently, a new strategy has been proposed, where messages are encoded in s (which is a variable) strongest eigen-subchannels based on ordered eigenvalues of Wishart matrices, while AN signals are generated in remaining spaces. This paper extends this strategy to Rician channels using the complex non-central Wishart distribution. A closed form expression for secrecy capacity of such system is computed using majorization theory. Performance of a MIMO communication system in Rician fading environment is simulated and effect of the Rician factor on secrecy capacity is studied. The same approach is then extended to decode and forward relay network. Secrecy capacity of the said network is computed, and the effect of AN is studied using the same methodology.
Physical layer-based secret key generation (PHY-SKG) schemes have attracted significant attention in recent years due to their lightweight implementation and ability to achieve information-theoretical security. In this paper, we study a channel frequency response (CFR)-based SKG scheme for in-band full-duplex (IBFD)-multi-input and multi-output (MIMO) systems. We formulate the intrinsic practical imperfections and derive their effects on the probing errors. Then we derive closed-form expressions for the secret key capacity (SKC) in the presence of a passive eavesdropper accordingly. We analyze the asymptotic behavior of the SKC in the high-SNR regime and reveal the fundamental limits for IBFD and HD probing. Based on the asymptotic SKC, we investigate the conditions under which IBFD can outperform HD. Numerical results illustrate that effective analog self-interference cancellation (ASIC) depth is the basis for IBFD probing to gain benefits over HD. Finally, we analyze the properties of the collected samples of the CFR-based SKG scheme and propose an averaging pre-processing and a segmental quantization, which reduce the key disagreement rate and remove the effects of large-scale fading to guarantee randomness. 3GPP specification-based simulations and the National Institute of Standards and Technology (NIST) test suite verify the theoretical analysis and the effectiveness of the proposed SKG scheme.