# Time Reversal in Ray Tracing Indoor Radio Propagation Channel for Physical Layer Security

Lap Phuong Uyen Nguyen, Philippe Rostaing, Roland Gautier

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Lap Phuong Uyen Nguyen, Philippe Rostaing, Roland Gautier. Time Reversal in Ray Tracing Indoor Radio Propagation Channel for Physical Layer Security. 2025 International Conference on Emerging Technologies and Computing (IC_ETC), Jun 2025, Brest, France. pp.1-2, 10.1109/IC_ETC65981.2025.11141243 . hal-05495219

HAL Id: hal-05495219

https://hal.science/hal-05495219v1

Submitted on 5 Feb 2026

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# Time Reversal in Ray Tracing Indoor Radio Propagation Channel for Physical Layer Security

Lap Phuong Uyen NGUYEN¹,²,³, Philippe ROSTAING¹, Roland GAUTIER¹

¹Univ Brest, CNRS, Lab-STICC, CS 93837, 6 avenue Le Gorgeu, 29238 Brest Cedex 3, France

²School of Electrical Engineering, International University, Ho Chi Minh City 700000, Vietnam

³Vietnam National University, Ho Chi Minh City 700000, Vietnam

Email: lap-phuong-uyen.nguyen@univ-brest.fr, philippe.rostaing@univ-brest.fr, roland.gautier@univ-brest.fr

**Abstract**—This study investigates Time Reversal in Ray tracing indoor Radio propagation channel which is applied to SISO/OFDM system. In addition, the ray tracing technique is analyzed by simulating an indoor environment with an adjustable distance between Bob (legitimate receiver) and Eve (eavesdropper). Finally, the two metrics i) received power and ii) Bit-Error-Rate for Bob versus Eve are evaluated to verify the advantage of Time Reversal for physical layer secure transmission.

**Index Terms**—Time Reversal, ray tracing, SISO/OFDM.

## I. INTRODUCTION

Time Reversal (TR) technique can work effectively in a cluttered environment with multiple reflections. Particularly, the system with TR can increase the spatial focusing by utilizing multipath propagation. It aims to minimize the interference and prevent eavesdropping. A ray tracing approach is used to simulate paths through the environment to obtain the Channel Impulse Response (CIR). The ray paths are computed via passing through a virtual environment, reflecting off surfaces, and interacting with materials. Due to the advantages of TR for multipath channels, especially ray tracing technique, the proposed method is explored in the UBO room's indoor environment to evaluate the performance in Bit-Error-Rate (BER) and the power density at Bob's location.

## II. TIME REVERSAL IN RAY TRACING

Alice at the transmitter tries to send the message X to Bob at the expected receiver, while Eve eavesdrops on this message in their communication. Bob is assumed to have full knowledge of channel, so he can use it to decipher the encoded data. On the other hand, due to a lack of information about channel, Eve cannot decode the data. For the SISO/OFDM system [1], the input data is separated into many data blocks X depicted in Fig. 1. Each data block contains a N QAM symbols, which is transmitted on Q subcarriers. Each data symbol $X_n$ is carried on by ($U = Q/N$) different subcarriers. Bob's channel, denoted as $H_B$, and Eve's channel, denoted as $H_E$, are represented by diagonal matrices within the SISO/OFDM model. In this scheme, the TR is defined as $H_B^*$. The matrix $S(Q \times N)$ consists of the vertical concatenation of identity matrices $I_n(N \times N)$. The diagonal matrices $I_n$ are generated by randomly selecting two values $\{-1, 1\}$ with i.i.d. The diversity gains at Bob and Eve are determined as $G_{n,B} = \sum_{i=0}^{U-1} |H_{n+iN,B}|^2$ and $G_{n,E} = \sum_{i=0}^{U-1} H_{n+iN,E} H_{n+iN,B}^*$, respectively and $n = 0, \dots, N-1$. It demonstrates that as

<figure><img src="image_2.png" /><figcaption>Fig. 1. SISO/OFDM scheme with Time Reversal.</figcaption></figure>

$U$ increases, the diversity gain at Bob grows due to the spreading matrix $S$ with TR, while increasing $U$ has no effect on the diversity gain at Eve. The channel frequency gain for SISO/OFDM with Q-subcarriers is obtained as follows:

$$H_k \triangleq H(\frac{k}{Q}) = \sum_{i=0}^{L-1} \alpha_i e^{-j2\pi \frac{k}{Q} \tau_i f_s}, \quad (1)$$

where $L$ is the number of valid rays, $f_s$ is the sampling frequency. In Fig. 2, a ray tracing analysis between the

<figure><img src="image_3.png" /><figcaption>Fig. 2. Ray tracing analysis with a) UBO room's indoor environment simulation and b) CIR.</figcaption></figure>

transmitter and receiver is performed in a UBO room's indoor environment. The surface material is specified as wood with up to five reflection bounces. The CIR is illustrated in b) where each path corresponds to a time delay representing the difference between its arrival time and that of the first arriving path (the LOS path).
III. CONCLUSION

This study will examine the BER performance when chang-
ing the position between the legitimate receiver (Bob) and
the eavesdropper (Eve), especially Bob benefits from a better
signal quality compared to Eve when increasing distance.
Also, thanks to TR, the power density around Bob will be

enhanced, even with Alice nearby.

REFERENCES

[1] S. Golstein, T.-H. Nguyen, F. Horlin, P. De Doncker, and J. Sarrazin, "Physical layer security in frequency-domain time-reversal siso ofdm communication," in *2020 International Conference on Computing, Networking and Communications (ICNC)*. IEEE, 2020, pp. 222-227.