A Secure Spatial Multiplexing Transmission Scheme in MIMO Amplify-and-Forward Wiretap Relaying Systems Using Deliberate Precoder Randomization

Nowadays, there is a growing awareness of the requirement of low probability of intercept (LPI) in modern battlefield. In this study, a novel LPI optimization algorithm is presented for target tracking in radar network architectures. We first provide radar network signal model. Then, an effective LPI optimization algorithm is proposed, where Schleher intercept factor is utilized as an optimization metric for LPI performance. Our purpose is to minimize the intercept factor for a predetermined minimum mean square error (MMSE) threshold of the estimate of target response matrix. The resulting nonlinear and constrained problem is solved through the genetic algorithm with nonlinear constraint (GANC). Simulation results demonstrate the effectiveness of our proposed algorithm.

This paper investigates the problem of optimal power allocation for radar network to improve the system low probability of intercept (LPI) performance. We first introduce radar network signal model, and derive Schleher intercept factor for radar network. Then, with the defined target threatening level, two optimal power allocation algorithms are proposed. The first one is to optimize transmitting power allocation with a predefined mutual information (MI) threshold, and the second one is to find the optimal power allocation for a given minimum mean-square error (MMSE) threshold. There is a one-to-one correspondence between target threatening level and MI/MMSE threshold, for we need different quantity of target information and estimation accuracy in the condition of different target threatening level. Additionally, the nonlinear programming based genetic algorithm (NPGA) is employed to solve the nonlinear and constrained optimization problems. Simulations demonstrate that these two algorithms lead to the same solution under the same system constraints and can improve LPI performance for radar network effectively.

This letter proposes a robust joint linear precoding scheme based on the minimum mean squared error (MMSE) criterion for amplify-and-forward (AF) multiple-input multiple-output (MIMO) relay broadcast systems with limited feedback, where only the quantized channel direction information (CDI) of the forward channel is available for the base station (BS) and the relay station (RS). The proposed scheme employs an iterative algorithm which alternately optimizes the BS and RS precoders to jointly minimize the expected MSE conditioned on the quantized CDI.

This paper investigates the problem of joint design of relay precoders and receivers based on the minimum mean-square-error (MMSE) criterion in multi-hop bi-directional multiple-input multiple-output (MIMO) amplify-and-forward (AF) relay systems. In contrast to the conventional research on two-way relay systems, we combine a multi-hop relay system with a bi-directional relay system which further enhances the system performance. By decoupling the original highly non-convex problem into three sub-problems, we propose an iterative algorithm to alternatively optimize relay precoders and receivers. Numerical simulation results show that the proposed iterative algorithm outperforms other algorithms in terms of bit-error-rate (BER).

Two different two-way amplify-and-forward (AF) wireless relay systems with transmit power constraints at the relay(s) are studied: one- relay system with multiple N antennas between two sources, and multiple N-relay system with one antenna per relay between two sources. The iterative and explicit relay amplifying matrices (or vectors) with perfect cancelation of self- interference under channel uncertainty are determined based on the minimum mean square error (MMSE) criterion. Both iterative and explicit relay amplifying matrices are numerically investigated through the simulation. Simulation results show that the gain of diversity order with channel uncertainty can occur as N increases, while the loss of diversity order can occur as channel estimation error powers increase.

As Electronic Warfare (EW) relevant technologies develops, more complex and solid defense algorithm is required to keep the military secrets and message in safe. In the wireless communication system, Low Probability of Interception (LPI) communication algorithms, based on Code Division Multiple Access (CDMA), had been introduced to keep the message safe. Frequency Hopping CDMA (FH-CDMA) and Direct Sequence CDMA (DS-CDMA) are the representative methods of the LPI communication algorithm. FH-CDMA is widely used; however, the system has an expensive hardware configuration requirement due to the implementation complexity. In addition, several studies presented improved LPI communication algorithms with a physical-layer level processing; however, they have limitations of unable to well-hide the original signal. To address these problems, we propose an algorithm, called noise communication, which generates Gaussian distributed random numbers that added to In-phase and Quadrature (I/Q) input data independently to hide the original signal. Through Matlab simulation, we compare the conventional DS-CDMA and the proposed algorithm applied DS-CDMA and evaluate the effect of the algorithm with different configurations of PN sequence length and the effective level of the algorithm. Through the evaluations, we observe the algorithm can provide Signal-to-Noise Ratio (SNR) gain than the conventional DS-CDMA and SNR gain has an inverse relationship with system stability. Moreover, we exploit a Software Defined Radios (SDR) hardware to implement the algorithm for the demonstration of a noise-shaped Radio Frequency (RF) signal.

In this paper, we design a zero-forcing (ZF) Tomlinson-Harashima precoding (THP) and a minimum mean square error (MMSE) THP for multiuser MIMO relay system with direct link. Based on these precoding structures, we propose the optimal combining weights of the ZF THP and the MMSE THP in half duplex relay system, and the sub-optimal weights of MMSE THP to reduce feedback channel state information (CSI). Through the Monte Carlo simulation, we verify that the optimal weights combining method outperforms the constant weights combining method. In addition, we confirm that the performance of the sub-optimal weights combining method is close to the optimal method, while reducing the feedback CSI simultaneously.

The space-time coding can provide a diversity gain for the multiple-antenna system while spatial multiplexing provides higher data rates. In this paper we are combining space-time block code (STBC) with spatial multiplexing technique over space to take advantages of both schemes. The transmit antennas are divided into groups, with one of the groups transmitting STBC code given by Alamouti and other transmits spatial multiplexing data. At the receiver, a minimum mean square error (MMSE) detector with interference cancelation (IC) is integrated with the space-time decoder to reconstruct the desired signal effectively. MMSE decoder is used for compared to only MMSE method. STBC coded symbols (estimation of interfering terms and then space-time decoder is applied. 4×1 system gives slightly higher diversity gain with rate=3 as s1s2) are giving 8 dB gain as compared to purely orthogonal code (2×1 MMSE-SIC). Rate of 3×1 system is less but it is performing better in terms of reliability as compared to 4×1 system.

Multimedia broadcast over a single frequency network (MBSFN) has attracted a lot of interest recently. This paper is on Golden coded spatial multiplexing MBSFN transmission using a soft-output sphere decoder (SOSD) and the impact of such a transceiver on the block error rate (BLER) system performance. In total, we consider three transceiver types and evaluate their performance in cellular orthogonal frequency division multiplexing (OFDM) based MBSFN scenarios with urban characteristic taking into account single-cell broadcast and multi-cell broadcast with and without guard interval (GI) excess. In addition to the Golden coded system, we consider horizontal BLAST (H-BLAST) with decoded-feedback minimum mean-squared error (MMSE) successive interference cancelation (SIC) detector as a low complexity reference system, and a pure spatial multiplexing system with SOSD. The Golden coded transmission shows an increased overall robustness in an MBSFN network in comparison to the H-BLAST MMSE SIC reference system and in comparison to spatial multiplexing with SOSD, particularly in scenarios with high spectral efficiencies, i.e., high rates of the channel code. We demonstrate that this also applies for the critical case of an exceeded GI.

In traditional multiple-input-multiple-output (MIMO) channel and MIMO one-way relay system, nonlinear precoding design has shown significant performance gain over linear design. In this paper, we aim to study nonlinear precoding design for MIMO amplify-and-forward (AF) two-way relay systems, where nonlinear minimal mean square error (MMSE) decision feedback equalizers (DFEs) are used in two destinations, and linear transmit precoding is applied at the source and relay nodes. We first investigate nonlinear precoding design, where the precoding is only conducted at two sources for a fixed-relay precoder. After some transformations, we prove that this design problem is convex, and an efficient algorithm is provided to find the optimal solution. Then, we consider the nonlinear joint precoding design to further incorporate relay precoding. Due to the nonconvexity of this problem, we first propose an iterative algorithm (Algorithm I) to approach the optimal solution. It is proven that Algorithm I is convergent and can converge to a stationary point of the joint design problem. Moreover, we present a simplified iterative algorithm (Algorithm II) for joint precoding design to reduce the design complexity. It is found that Algorithm II almost achieves the same performance as Algorithm I in most cases. Our simulation results show that the proposed nonlinear joint precoding design significantly outperforms the linear joint precoding design. It is also shown that the choice between the proposed nonlinear source precoding design and the linear relay precoding design is dependent on specific conditions.

The combination of space-time block codes (STBC) and spatial multiplexing (SM) schemes has the advantages of diversity gain and high data rate. For such STBC/SM hybrid systems, we propose a new detector based on the block structure of STBC and the sphere decoder (SD). By employing the SD, optimal detection is performed with computational complexity much less than the brute-force maximum likelihood (ML) search. Simulation results show that the new decoder significantly outperforms the previously developed zero-forcing (ZF) group/STBC detector, the minimum mean square error (MMSE) group/STBC detector, and the MMSE and ML detectors for equivalent SM systems.

Recent research reveals that information security and information-hiding capabilities can be enhanced by proper exploitation of space-time techniques. Meanwhile, intrinsic properties of ultra wideband (UWB) signals make it an outstanding candidate for secure applications. In this paper, we propose a unitary space-time coding scheme for impulse radio UWB systems. Its transmission secrecy, including low probability of intercept (LPI), low probability of detection (LPD) and anti-jamming performance, is analyzed. Theoretical and simulation results show its superiority in wireless secure communications over other concurrent schemes.

We consider Multiple-input multiple-output (MIMO) wiretap channel with one transmitter, one receiver, one cooperative jammer, and one eavesdropper, where each node equips with multiple antennas. Eigenbeam-space division multiplexing (E-SDM) and cooperative jamming are known as the techniques improving secrecy rate. E-SDM enables the transmitter and the legitimate receiver to communicate with each other in parallel using channel state information (CSI) known to them to maximize the legitimate receiver's mutual information. On the other hand, cooperative jamming is to jam only the eavesdropper to degrade eavesdropper's mutual information. However, co-channel interference and residual interference from the cooperative jammer are caused by channel estimation error, which degrades legitimate receiver's mutual information. How these interference influence on the secrecy rate has not been clarified. In this paper, we derive lower and upper bounds on secrecy rate using legitimate receiver's mutual information in the presence of channel estimation error and eavesdropper's mutual information who uses minimum mean square error (MMSE) receiver. We analyze the effect of co-channel interference and residual interference from the cooperative jammer on the secrecy rate.

Multiple transmit (Tx) and multiple receive (Rx) antenna systems, referred to as multiple input multiple output (MIMO) systems, have been proposed to achieve higher data rates in wireless communication systems. In this paper, we investigate joint optimization of transmitter and receiver for MIMO system when the channel information is available at both transmitter and receiver. We discuss the problem of designing Tx and Rx filter based on minimum bit error probability criterion. We first derive the optimum Tx and Rx filters for the special case of two input data symbols. Based on this optimum filter, we propose a Tx and Rx filter optimization for the general case of input data symbols. The performance of the proposed Tx and Rx filters is compared with that of the conventional minimum mean-squared error (MMSE) filter. Performance analysis shows that the proposed filter provides significant improvement over the MMSE filter in bit error probability.

In this paper, we investigate the optimization problem of joint source and relay beamforming matrices for a two-way amplify-and-forward (AF) multi-input multi-output (MIMO) relay system. The system, consisting of two source nodes and two relay nodes, is considered, and the linear minimum mean-square-error (MMSE) is employed at both receivers. We assume individual relay power constraints and study an important design problem, a so-called determinant maximization (DM) problem. Since this DM problem is nonconvex, we consider an efficient iterative algorithm by using an MSE balancing result to obtain at least a locally optimal solution. The proposed algorithm is developed based on QL, QR, and Choleskey decompositions which differ in complexity and performance. Analytical and simulation results show that the proposed algorithm can significantly reduce computational complexity compared with their existing two-way relay systems and have equivalent bit-error-rate (BER) performance to the singular value decomposition (SVD) based on a regular block diagonal (RBD) scheme.