[Paper Review] Secrecy Performance Analysis of Location-Based Beamforming in Rician Wiretap Channels

Shihao Yan, Robert Malaney|arXiv (Cornell University)|Dec 22, 2014

Wireless Communication Security Techniques41 references18 citations

TL;DR

This paper proposes a novel location-based beamforming (LBB) scheme for physical layer security in Rician wiretap channels, where beamforming weights are optimized using only the known locations of the legitimate receiver and eavesdropper, without requiring instantaneous channel state information (CSI). The key contribution is a closed-form expression for secrecy outage probability and an optimal beamformer that minimizes this outage, achieving strong security performance even in practical scenarios with limited CSI.

ABSTRACT

We propose a new optimal Location-Based Beamforming (LBB) scheme for the wiretap channel, where both the main channel and the eavesdropper's channel are subject to Rician fading. In our LBB scheme the two key inputs are the location of the legitimate receiver and the location of the potential eavesdropper. Notably, our scheme does not require as direct inputs any channel state information of the main channel or the eavesdropper's channel, making it easy to deploy in a host of application settings in which the location inputs are known. Our beamforming solution assumes a multiple-antenna transmitter, a multiple-antenna eavesdropper, and a single-antenna receiver, and its aim is to maximize the physical layer security of the channel. To obtain our solution we first derive the secrecy outage probability of the LBB scheme in a closed-form expression that is valid for arbitrary values of the Rician K-factors of the main channel and the eavesdropper's channel. Using this expression we then determine the location-based beamformer solution that minimizes the secrecy outage probability. To assess the usefulness of our new scheme, and to quantify the value of the location information to the beamformer, we compare our scheme to other schemes, some of which do not utilize any location information. Our new beamformer solution provides optimal physical layer security for a wide range of location-based applications.

Motivation & Objective

• To address the practical challenge of physical layer security in MIMO wiretap channels where perfect CSI of the main or eavesdropper channel is often unavailable.
• To explore whether location information of the legitimate user and eavesdropper can be leveraged to enhance secrecy performance without relying on CSI.
• To design and analyze a new beamforming scheme—Location-Based Beamforming (LBB)—that uses only location inputs to maximize secrecy outage performance.
• To compare the LBB scheme against non-beamforming and full-CSI schemes to quantify the value of location information in secrecy performance.
• To derive a closed-form expression for secrecy outage probability under arbitrary Rician K-factors for both main and eavesdropper channels.

Proposed method

• Derives a closed-form expression for the secrecy outage probability of the LBB scheme under Rician fading, valid for any Rician K-factors of the main and eavesdropper channels.
• Models the system with a multiple-antenna transmitter, a single-antenna legitimate receiver, and a multiple-antenna eavesdropper, assuming line-of-sight (LOS) components in both channels.
• Uses location information of Bob (receiver) and Eve (eavesdropper) as inputs to compute the optimal beamformer that minimizes secrecy outage probability.
• Applies Rician fading modeling to the effective channel gains, incorporating both LOS and scattered components via Rician K-factors.
• Approximates the distribution of the eavesdropper’s channel gain using a non-central chi-squared distribution and derives the secrecy outage probability via integration over the eavesdropper’s SNR.
• Compares the LBB scheme against a non-beamforming (NB) scheme and a full-CSI beamforming scheme to evaluate performance gains from location-based design.

Experimental results

Research questions

• RQ1 Can location information alone be used to design an optimal beamformer in a Rician wiretap channel without requiring CSI of the main or eavesdropper channel?
• RQ2 What is the secrecy outage probability of the proposed LBB scheme under arbitrary Rician K-factors for both the main and eavesdropper channels?
• RQ3 How does the secrecy performance of the LBB scheme compare to non-beamforming and full-CSI beamforming schemes in terms of outage probability and diversity gain?
• RQ4 What is the impact on secrecy performance when the eavesdropper’s location is inaccurate or untrustworthy?
• RQ5 What is the minimum achievable secrecy outage probability under the LBB framework, and how does it scale with system parameters like array size and Rician K-factors?

Key findings

• The secrecy outage probability of the LBB scheme is derived in closed-form for arbitrary Rician K-factors of both the main and eavesdropper channels, enabling precise performance evaluation.
• The proposed LBB beamformer achieves optimal secrecy performance by minimizing the secrecy outage probability using only location inputs, without requiring CSI of either channel.
• Numerical results show that the LBB scheme significantly outperforms the non-beamforming (NB) scheme, especially in high-SNR regimes and when the eavesdropper is close to the transmitter.
• The secrecy diversity order of the LBB scheme is shown to be $ N_A m_B $, matching that of the full-CSI scheme, indicating full diversity gain despite the absence of CSI.
• The performance gain of LBB over NB is quantitatively significant: for example, at a secrecy outage probability of $ 10^{-3} $, the LBB scheme achieves a 6–8 dB SNR gain over NB under moderate Rician fading.
• When the eavesdropper’s location is untrustworthy, the secrecy outage probability increases, but the LBB scheme remains robust and outperforms NB, demonstrating the value of accurate location information.