# On the Secrecy Rate of Spatial Modulation Based Indoor Visible Light Communications

## TL;DR

This paper analyzes the physical-layer security of spatial modulation in indoor visible light communication systems, deriving bounds on secrecy rate and proposing schemes to enhance secrecy performance under various optical constraints.

## Contribution

It introduces new bounds on secrecy rate for VLC systems with spatial modulation and proposes adaptive schemes to improve security performance.

## Key findings

- Secrecy rate bounds are tight and accurate at high SNR.
- Proposed greedy scheme outperforms other transmitter selection methods.
- Asymptotic analysis reveals small gaps between bounds at high SNR.

## Abstract

In this paper, we investigate the physical-layer security for a spatial modulation (SM) based indoor visible light communication (VLC) system, which includes multiple transmitters, a legitimate receiver, and a passive eavesdropper (Eve). At the transmitters, the SM scheme is employed, i.e., only one transmitter is active at each time instant. To choose the active transmitter, a uniform selection (US) scheme is utilized. Two scenarios are considered: one is with non-negativity and average optical intensity constraints, the other is with non-negativity, average optical intensity and peak optical intensity constraints. Then, lower and upper bounds on the secrecy rate are derived for these two scenarios. Besides, the asymptotic behaviors for the derived secrecy rate bounds at high signal-to-noise ratio (SNR) are analyzed. To further improve the secrecy performance, a channel adaptive selection (CAS) scheme and a greedy selection (GS) scheme are proposed to select the active transmitter. Numerical results show that the lower and upper bounds of the secrecy rate are tight. At high SNR, small asymptotic performance gaps exist between the derived lower and upper bounds. Moreover, the proposed GS scheme has the best performance, followed by the CAS scheme and the US scheme.