In this paper, a practical wireless transmission scheme is proposed to transmit confidential messages to the desired user securely and precisely by the joint use of multiple techniques, including artificial noise (AN) projection, phase alignment/beamforming, and random subcarrier selection (RSCS) based on orthogonal frequency division multiplexing (OFDM), and directional modulation (DM), namely RSCS-OFDM-DM. This RSCS-OFDM-DM scheme provides an extremely low-complexity structure for the desired receiver and makes the secure and precise wireless transmission realizable in practice. For illegal eavesdroppers, the receive power of confidential messages is so weak that their receivers cannot intercept these confidential messages successfully once it is corrupted by AN. In such a scheme, the design of phase alignment/beamforming vector and AN projection matrix depends intimately on the desired direction angle and distance. It is particularly noted that the use of RSCS leads to a significant outcome that the receive power of confidential messages mainly concentrates on the small neighboring region around the desired receiver and only small fraction of its power leaks out to the remaining large broad regions. This concept is called secure precise transmission. The probability density function of real-time receive signal-to-interference-and-noise ratio (SINR) is derived. Also, the average SINR and its tight upper bound are attained. The approximate closed-form expression for average secrecy rate is derived by analyzing the first-null positions of the SINR and clarifying the wiretap region. Simulation and analysis show that the proposed scheme actually can achieve a secure and precise wireless transmission of confidential messages in line-of-propagation channel, and the derived theoretical formula of average secrecy rate is verified to coincide with the exact results well for medium and large scale transmit antenna array or in the low and medium SNR regions.

/pdf/secure-and-precise-wireless-transmission-for-random-4kv0n1ak7s.pdf

Secure and Precise Wireless Transmission for Random-Subcarrier-Selection-Based Directional Modulation Transmit Antenna Array

Wireless communication provides a wide coverage at the cost of exposing information to unintended users. As an information-theoretic paradigm, secrecy rate derives bounds for secure transmission when the channel to the eavesdropper is known. However, such bounds are shown to be restrictive in practice and may require exploitation of specialized coding schemes. In this paper, we employ the concept of directional modulation and follow a signal processing approach to enhance the security of multiuser multi-input multioutput (MIMO) communication systems when a multiantenna eavesdropper is present. Security enhancement is accomplished by increasing the symbol error rate at the eavesdropper. Unlike the information-theoretic secrecy rate paradigm, we assume that the legitimate transmitter is not aware of its channel to the eavesdropper, which is a more realistic assumption. We examine the applicability of MIMO receiving algorithms at the eavesdropper. Using the channel knowledge and the intended symbols for the users, we design security enhancing symbol-level precoders for different transmitter and eavesdropper antenna configurations. We transform each design problem to a linearly constrained quadratic program and propose two solutions, namely the iterative algorithm and one based on nonnegative least squares, at each scenario for a computationally efficient modulation. Simulation results verify the analysis and show that the designed precoders outperform the benchmark scheme in terms of both power efficiency and security enhancement.

/pdf/directional-modulation-via-symbol-level-precoding-a-way-to-5baaist6tq.pdf

Directional Modulation Via Symbol-Level Precoding: A Way to Enhance Security

Precoding has been conventionally considered as an effective means of mitigating or exploiting the interference in the multiantenna downlink channel, where multiple users are simultaneously served with independent information over the same channel resources. The early works in this area were focused on transmitting an individual information stream to each user by constructing weighted linear combinations of symbol blocks (codewords). However, more recent works have moved beyond this traditional view by: 1) transmitting distinct data streams to groups of users and 2) applying precoding on a symbol-per-symbol basis. In this context, the current survey presents a unified view and classification of precoding techniques with respect to two main axes: 1) the switching rate of the precoding weights, leading to the classes of block-level and symbol-level precoding and 2) the number of users that each stream is addressed to, hence unicast, multicast, and broadcast precoding. Furthermore, the classified techniques are compared through representative numerical results to demonstrate their relative performance and uncover fundamental insights. Finally, a list of open theoretical problems and practical challenges are presented to inspire further research in this area 1 . 1 The concepts of precoding and beamforming are used interchangeably throughout this paper.

/pdf/symbol-level-and-multicast-precoding-for-multiuser-19itwisvs4.pdf

Symbol-Level and Multicast Precoding for Multiuser Multiantenna Downlink: A State-of-the-Art, Classification, and Challenges

An orthogonal vector approach is proposed for the synthesis of multi-beam directional modulation (DM) transmitters. These systems have the capability of concurrently projecting independent data streams into different specified spatial directions while simultaneously distorting signal constellations in all other directions. Simulated bit error rate (BER) spatial distributions are presented for various multi-beam system configurations in order to illustrate representative examples of physical layer security performance enhancement that can be achieved.

/pdf/orthogonal-vector-approach-for-synthesis-of-multi-beam-4tjk2fjjuq.pdf

Orthogonal Vector Approach for Synthesis of Multi-Beam Directional Modulation Transmitters

In this paper, metrics for assessing the performance of directional modulation (DM) physical-layer secure wireless systems are discussed. In the paper DM systems are shown to be categorized as static or dynamic. The behavior of each type of system is discussed for QPSK modulation. Besides EVM-like and bit error rate (BER) metrics, secrecy rate as used in information theory community is also derived for the purpose of this QPSK DM system evaluation.

/pdf/establishing-metrics-for-assessing-the-performance-of-40511m1ed8.pdf

Establishing Metrics for Assessing the Performance of Directional Modulation Systems

A new approach to enhance the security of data transmission using direction dependent antenna modulation based on an array with wide element spacing combined with directional array elements is presented. For illustrative purposes, the system investigated, and reported here, consists of an array of only two elements but the approach is general and can be extended to multi-element array systems. It is shown that by increasing the element spacing of an array the directional error rate characteristics of the system, in a given direction, can be significantly improved. Furthermore, the directly modulated system shows superior error rate performance to that of an equivalent, conventionally modulated, system in the grating lobe directions. The error rate characteristics of the proposed system are analysed for two scenarios: firstly when the angular separation between an eavesdropper and the intended recipient is small and secondly for the case when a potential eavesdropper is located much closer to the transmit antenna than the intended recipient.

https://ietresearch.onlinelibrary.wiley.com/doi/epdf/10.1049/iet-map.2012.0644

Enhancing the security of communication via directly modulated antenna arrays

A multidirectional transmission scheme using direct antenna modulation (DAM) based on an array with 2-bit phase control is presented. By switching each element phase properly, the line-of-sight (LOS) communication channels generated by the scheme can be independent from each other at various directions. Compared to previous work, the use of 2-bit phase control offers prospects of lower system cost and simpler phase control. An algorithmic method for calculating the required phase control for simultaneous, independent multiuser data modulation is described, and specific examples are presented.

Simultaneous, Multichannel, Spatially Directive Data Transmission Using Direct Antenna Modulation

An antenna system configured to provide direction dependent modulation is shown to increase the security of data transmission. The antenna is based on an array with wide element spacing combined with directional array elements. The array system investigated here consists of only two elements but the approach is general and can be extended to multi-element array systems. The directional error rate characteristics of the directly modulated array system, in a given direction, can be significantly improved by increasing the element spacing. The error rate performance of the directly modulated array system is compared to that of an equivalent array configured for conventional baseband modulation.

Secure physical-layer communication based on directly modulated antenna arrays

An antenna system configured to provide direction dependent modulation is shown to increase the security of data transmission especially against eavesdropping at directions which are very close to the desired recipient. Such a transmitter system is based on a directly modulated array which can simultaneously send independent signals to multiple spatially separated recipients. In this contribution the performance of an array antenna configured for direction dependent antenna modulation is examined for the case of multi-path transmission. The modulation properties of the directly modulated array system with multipath are compared to those transmitted by an equivalent array with a purely LOS communication link.

Secure communications based on directly modulated antenna arrays combined with multi-path

A direction dependent communication system based on a simple two element array is described. The system is similar to a conventional phased array but the phase shifters have only 2-bit control. The basic concept of directional antenna modulation is presented and illustrative examples are given. Some of the system characteristics of direction dependent modulation are examined and compared to conventional modulation systems. A simple two element array system configured for direction dependent modulation is introduced and numerical examples are given to illustrate how transmission error rates depend on various system parameters.

HongZhe Shi

Papers

Enhancing the security of communication via directly modulated antenna arrays

Simultaneous, Multichannel, Spatially Directive Data Transmission Using Direct Antenna Modulation

Secure physical-layer communication based on directly modulated antenna arrays

Secure communications based on directly modulated antenna arrays combined with multi-path

Characteristics of a two element direction dependent antenna array