A frequency domain (FD) time-reversal (TR) precoder is proposed to perform physical layer security (PLS) in single-input single-output (SISO) system using orthogonal frequencydivision multiplexing (OFDM). To maximize the secrecy of the communication, the design of an artificial noise (AN) signal well-suited to the proposed FD TR-based OFDM SISO system is derived. This new scheme guarantees the secrecy of a communication toward a legitimate user when the channel state information (CSI) of a potential eavesdropper is not known. In particular, we derive an AN signal that does not corrupt the data transmission to the legitimate receiver but degrades the decoding performance of the eavesdropper. A closed-form approximation of the AN energy to inject is defined in order to maximize the secrecy rate (SR) of the communication. Simulation results are presented to demonstrate the security performance of the proposed secure FD TR SISO OFDM system.

Spatial Data Focusing (SDF) is introduced as a novel technique that allows wireless broadcasting of information towards specific spatial locations only. It is shown that this approach allows one to target geographic areas more accurately than traditional power focusing methods, using limited equipment at the transmitter. This paper describes the SDF system model for linear arrays, based on simple modulation techniques and transmitter architectures, both in pure line-of-sight and multipath environments. In particular, the robustness of the scheme is proven for over-the-ground propagation environments. Theoretic results are illustrated by simulations, confirming the increased spatial selectivity of SDF and showing the influence of various design parameters of the scheme on the resulting beam.

An alternative scenario is introduced in order to overcome antenna array's beamwidth limitations due to finite aperture size. The proposed approach aims to focus the transmitted data rather than the transmitted power. This scheme enables wireless broadcast of information to specific spatial locations, using fewer antenna elements compared to classical beamforming techniques. It is shown indeed with numerical examples that focusing the data is spatially more selective than focusing the power. In particular, we show that a linear 2-antenna array using spatial data focusing can exhibit similar beamwidths to a linear 7-antenna array using power focusing.

A frequency domain time-reversal (TR) precoder is proposed to perform physical layer security in single-input single-output (SISO) systems using orthogonal frequency-division multiplexing (OFDM) and artificial noise (AN) injection. This scheme guarantees the secrecy of a communication towards a legitimate user, Bob, by exploiting the frequency diversity selective behaviour in multipath channels. The transmitter, Alice, has imperfect channel state information (CSI) of the legitimate link thanks to the channel reciprocity in time division duplex systems and does not know the instantaneous CSI of a potential eavesdropper, Eve. Three optimal decoding structures at Eve are considered in a block fading environment depending on the handshake procedure between Alice and Bob. Closed-form approximations of the signal-to-noise ratio required at Bob and the maximal CSI error that can be made at Alice, in order to guarantee a communication ergodic secrecy rate (ESR), are derived. Furthermore, the optimal amount of AN energy to inject, considering imperfect CSI, is also given as a closed-form expression. A trade-off on the choice of the spreading factor of the TR precoder is established between maximizing the ESR and decreasing the −achievable secrecy rate. Finally, thanks to these results, Alice can be a priori aware of the ESR over which she can establish a secure communication.INDEX TERMS Artificial noise, block-fading, eavesdropper, ergodic secrecy rate, physical layer security, −achievable secrecy rate , SISO-OFDM, time division duplex , time-reversal.

Time reversal-based indoor positioning system (TRIPS) is a promising technology for the centimeteraccuracy indoor positioning, since it exploits the rich multipath propagation in indoor environments as a specific signature for each location. In TRIPS, a database is first constructed via channel probing. Well-calibrated devices are usually assumed in this process, i.e., no hardware impairments. However, a low cost terminal to be located, whose typical impairment is the I/Q imbalance (IQI) at the front-end transmitter, can significantly influence the TRIPS performance. More specifically, IQI creates an interference image of the signal that reduces the metric value used in TRIPS and hence decreases the localization accuracy. In this paper, we analytically investigate the impact of the IQI on the metric of TRIPS. A closed-form approximation of the localization metric inherent to the IQI is derived. In order to improve the TRIPS performance, an effective IQI mitigation method is proposed. Numerical simulations are carried out to validate the derived analytical expression under the IQI impact and the proposed compensation method. Index terms-Indoor positioning system, time reversal, I/Q imbalance, OFDM.