Shichuan Ma

Distributed MIMO technologies in cooperative wireless networks

Multiple-input multiple-output techniques enhance wireless communications performance by taking advantage of spatial diversity. However, most traditional MIMO systems hardly achieve large spatial diversity because of limited terminal size. In this article, we present recent advances of the distributed MIMO technologies in cooperative wireless networks. We also compare and discuss various relay protocols and cooperative strategies. Our simulation results indicate that distributed MIMO systems can provide larger spatial diversity, and the data rate in cooperative networks can be significantly increased.

Impact of channel estimation errors on effectiveness of Eigenvector-based jamming for physical layer security in wireless networks

In this paper, we present our study of an Eigenvector-based artificial noise-based jamming technique developed to provide increased wireless physical layer security in transmit-receive diversity systems and analyze the impact of channel estimation errors on system performance. Our simulation results showed that with knowledge of perfect channel state information, the technique provided secrecy capacity of approximately 7 bits/s/Hz for a normalized transmit power of 25 dB for a variety of transmit, receive, and eavesdropper configurations. We also describe a novel method to simulate generalized channel state information estimation errors. While other publications neglect the impact of these estimation errors, our simulations show that the secrecy capacity decreased rapidly as the channel estimation errors increased. For instance, at 25% error the secrecy capacity of the jamming technique was only slightly better than the non-jamming case. Our paper also outlines upcoming research efforts to further explore error sensitivity and channel state temporal stability through experimentation.

An approach to secure wireless communications using randomized eigenvector-based jamming signals

In this paper, we propose a novel approach for providing secure wireless communications in transmit-receive diversity systems. In this approach, we precode the information-bearing signal with a uniquely generated randomized eigenvector-based jamming signal to impair the eavesdroppers received signal, while the main channel, the link between the transmitter and the desired receiver, remains unaffected. Unlike existing methods, our approach can be applied to any antenna array configuration, even when the receiver and eavesdropper utilize more antennas than the transmitter, with no benefits to the eavesdroppers information detection capability from employing more antenna elements. Moreover, our proposed approach can provide more degrees of freedom for the jamming signal, significantly increasing the provided level of security. Additionally, our scheme does not assume any knowledge about the eavesdropper or even the number of collaborating eavesdroppers. Our simulation results show a secrecy capacity increase of about 7 bits/s/Hz for a 4 x 4 antenna configuration under typical transmit power constraints, which results in significant improvement in security performance and enables physically secure wireless communications.

A new approach to null space-based noise signal generation for secure wireless communications in transmit-receive diversity systems

An effective method was recently proposed to increase secrecy capacity in MIMO wireless communication systems. By adding artificially generated noise signal into the information-bearing signal, this method can degrade the eavesdroppers receiving signal, but does not affect the desired receiver. This method, however, can only be used in systems where the transmit antenna has more elements than the receive antenna. In this paper, we present a novel noise signal generation approach to enable secure wireless communications in a transmit-receive diversity system. In this approach, we generate our jamming noise signal based on the null space of an equivalent channel of the system. The presented wireless security mechanism can be applied to systems with arbitrary antenna configurations, unlike existing methods that only work for specific array configurations. In addition, the eavesdropper cannot increase its information detection capability by employing more antenna elements. With this scheme, we also enable more degrees of freedom for selecting parameters for jamming noise signal generation, making the decoding at the eavesdropper even harder due to the increased search space. We performed Monte Carlo simulations to evaluate the proposed method in an urban macro cellular environment using the 3rd Generation Partnership Project (3GPP) spatial channel model. Our simulation results show that the proposed approach can significantly increase secrecy capacity, which is independent of the eavesdroppers position and the element number of its antenna.

MIMO self-encoded spread spectrum with iterative detection over Rayleigh fading channels

Self-encoded spread spectrum (SESS) is a novel communication technique that derives its spreading code from the randomness of the source stream rather than using conventional pseudorandom noise (PN) code. In this paper, we propose to incorporate SESS in multiple-input multiple-output (MIMO) systems as a means to combat against fading effects in wireless channels. Orthogonal space-time block-coded MIMO technique is employed to achieve spatial diversity, and the inherent temporal diversity in SESS modulation is exploited with iterative detection. Simulation results demonstrate that MIMO-SESS can effectively mitigate the channel fading effect such that the system can achieve a bit error rate of 10-4 with very low signal-to-noise ratio, from 3.3 dB for a 2×2 antenna configuration to just less than 0 dB for a 4×2 configuration under Rayleigh fading. The performance improvement for the 2 × 2 case is as much as 6.7 dB when compared to an MIMO PN-coded spread spectrum system.

An Adaptive Approach to Estimation and Compensation of Frequency-Dependent I/Q Imbalances in MIMO-OFDM Systems

In this paper, a novel adaptive approach based on Alamouti scheme is proposed to estimate and compensate frequency-dependent In-phase/Quadrature-phase (I/Q) imbalances in space-time coded MIMO-OFDM communication systems. The I/Q imbalances may be present at both ends of transmitter (TX) and receiver (RX). Although different methods have been proposed to utilize the Alamouti scheme in order to obtain spatial diversity in a MIMO-OFDM system, the inaccurate channel estimation caused by I/Q imbalances degrades the system performance. A new combining scheme is introduced to overcome this significant drawback by using joint estimation and compensation of the multipath fading channel and I/Q imbalances. The simulation results show that the proposed adaptive method can effectively mitigate the effect of the frequency-dependent TX and RX I/Q imbalances in the MIMO-OFDM systems.

Multiple-Input Multiple-Output Self-Encoded Spread Spectrum System with Iterative Detection

Self-encoded spread spectrum (SESS) is a novel modulation technique that acquires its spreading sequence from the randomness of an input data stream rather than through the use of the traditional pseudo-random (PN) code generator. The statistical properties of random codes in SESS offer a number of advantages over the deterministic PN codes. In this paper we propose a multiple-input multiple-output (MIMO) SESS system that employs iterative detection to exploit the inherent time diversity in SESS modulation. Alamouti scheme-based spacetime block coded MIMO technique is used to exploit the spatial diversity. The simulation results demonstrate that the proposed system can completely mitigate the fading effect to achieve about 7.5 dB of gain over a PN-coded spread spectrum MIMO system in Rayleigh fading channels.

Performance enhancement in MIMO self-encoded spread spectrum systems by using multiple codes

Self-encoded spread spectrum (SESS) is a novel modulation technique that acquires its spreading sequence from the randomness of the input data stream. In recent work, we incorporated SESS into MIMO system and showed that the system can achieve diversities in both time and space domains. In this paper, we propose a novel multi-code MIMO-SESS system that not only can achieve spatial and temporal diversities, but also can double the throughput compared with previous scheme. Simulation results demonstrate that the performance of the proposed multi-code MIMO-SESS system achieves about 6 dB of gain at 10−4 BER from the combined time and spatial diversities over an Alamouti scheme-based MIMO system under Rayleigh fading channels.

Performance enhancement in limited feedback precoded spatial multiplexing MIMO-OFDM systems by using multi-block channel prediction

Linear precoding plays an important role in the spatial multiplexing multiple-input multiple-output (MIMO)-orthogonal frequency-division multiplexing (OFDM) systems which are considered one of the primary candidates for the physical-layer techniques in the next-generation wireless communication systems. It requires the channel state information (CSI) at the transmitter to adapt the transmitted signal to the channel conditions. In most communication systems, the CSI is estimated at the receiver and fed back to the transmitter. However, the error performance in a precoded MIMO-OFDM system is significantly degraded due to the long feedback delay that causes the outdated CSI at the transmitter. In this paper, a novel approach using multi-block linear channel prediction is proposed to combat the feedback delay in a limited feedback precoded spatial multiplexing MIMO-OFDM system. The time-varying channel is modeled by autoregressive (AR) process, whose coefficients are obtained by linear minimum mean square error (MMSE) method. To increase prediction range, block-based channel samples are used to establish the AR model, and multiple blocks are employed to iteratively predict the CSI. Simulation results show that the performance degradation caused by large feedback delay can well be mitigated.

Securing wireless communications in transmit-beamforming systems by precoding jamming noise signals

Information transmission in wireless communication systems is traditionally protected by cryptographic techniques at the higher layers. Recently, a method of physical layer security has attracted much attention because it can potentially provide lower probability of interception of the signals at the eavesdroppers and hence augment the link security in addition to the conventional encryptions. In this paper, we propose a novel approach to securing transmit-beamforming systems by using uniquely designed jamming noise signal, which can significantly degrade the signal quality at the eavesdropper but not at the intended receiver. The jamming noise signal is generated from the null space of the channel matrix. An eigenvector-based implementation of this theoretical method is also provided. Unlike previous physical layer security methods, the proposed approach can provide secure communications over systems with arbitrary antenna configurations. Our proposed method offers more degrees of freedom to generate the jamming noise signal, resulting in the eavesdropper being unable to decode the information signals. Moreover, the eavesdropper cannot influence the system secrecy capacity by employing more antennas or by moving close to the transmitter. Simulation results show that the secrecy capacity increases significantly, by about 7 bits/s/Hz for a 4 × 4 antenna configuration, under typical transmit power constraints. Copyright