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IEEE Journal on Selected Areas in Communications

Practical Physical Layer Security Schemes for MIMO-OFDM Systems Using Precoding Matrix Indices

Chih-Yao Wu, Pang-Chang Lan, Ping‐Cheng Yeh, Chia‐Han Lee, Chen-Mou Cheng

Published Aug 23, 2013• 86 citations
Publisher

Abstract

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.

DOI
10.1109/jsac.2013.130904

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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.
In the literature, J.-P. Cheng et al. have proposed the MIMO-OFDM PHY integrated (MOPI) scheme for achieving physical-layer security in practice without using any cryptographic ciphers. The MOPI scheme uses channel sounding and physical-layer network coding (PNC) to prevent eavesdroppers from learning the channel state information (CSI). Nevertheless, due to the use of multiple antennas for PNC at transmitter and beamforming at receiver, it is not possible to have spatial multiplexing nor use space-time codes in our previous MOPI scheme. In this paper, we propose a variant of the MOPI scheme, called P-MOPI, that works with a cryptographic cipher and utilizes precoding matrix index (PMI) as an efficient key-exchange mechanism. With channel sounding, the PMI is only known between the transmitter and the legal receiver. The shared key can then be used, e.g., as the seed to generate pseudo random bit sequences for securing subsequent transmissions using a stream cipher. By applying the same techniques at independent subcarriers of the OFDM system, the P-MOPI scheme easily allows two communicating parties to exchange over 100 secret bits. As a result, not only secure communication but also the MIMO gain can be guaranteed by using the P-MOPI scheme.
Secret key establishment is considered to be one of the main challenging issues in cryptography. Many security algorithms are implemented in practice using complicated mathematical methods to exchange secret keys, but those methods are not desirable in power limited terminals such as cellular and sensor networks. In this paper, we propose a physical layer method for exchanging secret key bits in precoding based multi-input multi-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems. The proposed method uniquely relates the key bits to the indices of the precoding matrix used for MIMO channel precoding. The basic idea of the technique is to utilize a MIMO-OFDM precoding codebook. Comparative analysis with respect to the average number of mismatch bits, named key error rate (KER), shows an interesting lead for the new method relative to existing work. In addition, it will be shown that the proposed technique requires lower computation per byte per secret key.
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.
Physical layer security has been considered as an important security approach in wireless communications to protect legitimate transmission from passive eavesdroppers. This paper investigates the physical layer security of a wireless multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) communication system in the presence of a multiple-antenna eavesdropper. We first propose a transmit-filter-assisted secure MIMO-OFDM system which can destroy the orthogonality of eavesdropper's signals. Our proposed transmit filter can disturb the reception of eavesdropper while maintaining the quality of legitimate transmission. Then, we propose another artificial noise (AN)-assisted secure MIMO-OFDM system to further improve the security of the legitimate transmission. The time-domain AN signal is designed to disturb the reception of eavesdropper while the legitimate transmission will not be affected. Simulation results are presented to demonstrate the security performance of the proposed transmit filter design and AN-assisted scheme in the MIMO-OFDM system.
Recently, MIMO (multiple-input-multiple-output) physical layer secure transmission has attracted great attentions. However, current schemes cannot defend against the passive eavesdroppers with arbitrary number of antennas. To address this problem, in this work, we propose a practical physical layer MIMO secure communication scheme (PLSC) to defend against such an eavesdropper with arbitrary number of antennas. In the proposed scheme, the transmitter first independently generates a random binary sequence as the “key bits (KB)” to “encrypt” (XOR) the confidential information. After that, the transmitter sends the “encrypted information” over the wireless channel, along with mapping key bits to the legitimate receiver simultaneously. The key principle lies in that the KB information is coded in the indexes of the activated/non-activated antennas combination of the legitimate user. Then, the legitimate receiver first observes his/her activated antenna indexes to obtain the corresponding key bits. After that, he/she demodulates the “encrypted information” at the activated antennas, and finally “decrypts” (XOR) the confidential information by using the observed key bits. However, due to the uniqueness and independence of MIMO wireless channel, for any other eavesdroppers who suffer an independent channel from legitimate users, we prove that it cannot observe any information about KB from the received signals, regardless of how many antennas it has used. Consequently, without knowledge of KB, it cannot decrypt any information about the confidential information, too. The reliability and security of PLSC are theoretically demonstrated. The simulation and numerical results fully verified the validity and effectiveness of the proposed scheme.
The demands for higher data rates and network decentralized nodes in wireless communication systems urge the need for efficient methods to preserve the security levels. Many of the existing methods to share a private secret key between the connected nodes, tend to have high computational complexity. Furthermore, most physical layer approaches assume TDD systems to use the intrinsic feature of the reciprocal channels. In this paper, we propose a new method for exchanging secret key bits using a private random precoding based in MIMO FDD systems. The principle of this method is to exploit the PMI in a manner that produces low correlation at the intruder. By uniquely relating the secret key bits to the channel precoding matrix in a private version of the universal codebook, a robust key exchange between the transmitter and the receiver is then performed.