An EM Body Model for Device-Free Localization with Multiple Antenna Receivers: A First Study

TL;DR

This work addresses device-free localization by leveraging an electromagnetic (EM) body model extended to a multi-antenna receiver. It generalizes a prior body model to a Uniform Linear Array of size $2M+1$, deriving per-antenna excess attenuation $A_T^{(m)}=|E_R^{(m)}/E^{(m)}|^2$ and presenting the diffraction-based expressions that connect target position to the measured perturbations, with the $M=0$ case reducing to the single-antenna scenario. The authors integrate this model with array processing by formulating the received signal $y(t,\mathcal{S})=\mathbf{w}^H\mathbf{r}(t,\mathcal{S})$, and derive how beamforming interacts with the near-field EM perturbations to produce a target-induced attenuation metric $A_T$ that depends on the steering vector $\mathbf{a}$ and the array response. They analyze the array factor $F_a$ and show how the main-lobe width scales with array size and element spacing, accompanied by preliminary simulations at $f_c=2.4868$ GHz that demonstrate DoA-based separation of targets inside the first Fresnel ellipsoid and the potential gains of multi-antenna processing for WLAN-based DFL. Overall, the paper provides a foundation for deploying multi-antenna DFL systems and motivates future beamforming and array-processing algorithms in ISAC-enabled networks.

Abstract

Device-Free Localization (DFL) employs passive radio techniques capable to detect and locate people without imposing them to wear any electronic device. By exploiting the Integrated Sensing and Communication paradigm, DFL networks employ Radio Frequency (RF) nodes to measure the excess attenuation introduced by the subjects (i.e., human bodies) moving inside the monitored area, and to estimate their positions and movements. Physical, statistical, and ElectroMagnetic (EM) models have been proposed in the literature to estimate the body positions according to the RF signals collected by the nodes. These body models usually employ a single-antenna processing for localization purposes. However, the availability of low-cost multi-antenna devices such as those used for WLAN (Wireless Local Area Network) applications and the timely development of array-based body models, allow us to employ array-based processing techniques in DFL networks. By exploiting a suitable array-capable EM body model, this paper proposes an array-based framework to improve people sensing and localization. In particular, some simulations are proposed and discussed to compare the model results in both single- and multi-antenna scenarios. The proposed framework paves the way for a wider use of multi-antenna devices (e.g., those employed in current IEEE 802.11ac/ax/be and forthcoming IEEE 802.11be networks) and novel beamforming algorithms for DFL scenarios.

Figures (7)

• Figure 1: 3-D deployment of the radio link using an ULA-like array of $2M+1$ antennas. The segment where the array is deployed is placed at distance $d=d_{0}$ from the TX and it is orthogonal to the LoS path connecting the $TX$ node with the $RX_{0}$ one.
• Figure 2: 2-D layout of the radio link of Fig. \ref{['fig:array_layout']}. The point $O_{m}^{'}$ is the projection of the barycenter $P$ of the target $S$ over the m-th LoS path having length $d_{m}$.
• Figure 3: Array factor for an ULA composed by $9$ antennas ($M=4$) uniformly spaced from $0.1\,\lambda$ up to $0.5\,\lambda$.
• Figure 4: Link layout used for the simulations: the antenna array is composed by 5 antennas ($M=2$) while the target $S$, with size $a_z=0.9$ m and $a_y=0.275$ m, is placed in different positions along the line having distance $x=1$ m from the TX.
• Figure 5: From top to bottom: a) target placement used for this test; b) excess attenuation expressed in terms of the DoA $\gamma$ of the array when the target $S$ is placed along the segment having distance $x=1$ m from the TX. Three different positions of the target $S$ are simulated at $(x,y)$: $(1.0,-0.25)$, $(1.0,0.0)$, and $(1.0,0.25)$; c) excess attenuation as observed on each single antenna without any array processing and for corresponding positions of the targets as above.