Holography of Wi-fi Radiation

Wireless data transmission systems such as wi-fi or Bluetooth emit coherent light—electromagnetic waves with a precisely known amplitude and phase. Propagating in space, this radiation forms a hologram—a two-dimensional wave front encoding a three-dimensional view of all objects traversed by the light beam. Here we demonstrate a scheme to record this hologram in a phase-coherent fashion across a meter-sized imaging region. We recover three-dimensional views of objects and emitters by feeding the resulting data into digital reconstruction algorithms. Employing a digital implementation of dark-field propagation to suppress multipath reflection, we significantly enhance the quality of the resulting images. We numerically simulate the hologram of a 10-m-sized building, finding that both localization of emitters and 3D tomography of absorptive objects could be feasible by this technique.

Figure 1

Experimental setup. Stray radiation from a commercial router is employed to image meter-sized objects (gray cross) by digital holography. We record a hologram of wi-fi radiation by a synthetic aperture approach. A wi-fi antenna (scanning antenna) is moved across a 3 by 2 meter plane, pointwise registering wi-fi signals. The signal phase is recovered by a homodyne scheme, normalizing the signals to the signal of a stationary antenna (reference antenna).

Figure 2

Homodyne scheme for phase recovery. (a) The time-domain signals of the scanning and stationary antenna are Fourier transformed and (b) normalized to each other to obtain amplitude attenuation $I$ and the phase delay $\mathrm{\Phi }$ imparted by the propagation for every frequency $f$ within the wi-fi bandwidth. (c),(d) Holograms are obtained by doing this for every pixel. (e) Multipath reflections in the building lead to a strong modulation with $\mathrm{VSWR}=9.7$ and correlated artifacts in the phase. $f_c\approx 2.4\mathrm{GHz}$ denotes the carrier frequency. (a)–(d) have been recorded using a 5 GHz wi-fi emitter, and (e) using a 2.4 GHz emitter.

Figure 3

Recovery of 3D holographic views by numerical backpropagation of the hologram. (a) A commercial router is used to illuminate a cross-shaped phantom object. (b) Backpropagation into the emitter plane reliably reveals the router as a single bright spot. (c) Multipath reflections create a speckle pattern that (d) can be canceled by incoherently averaging over holograms obtained at different transmission frequencies.

Figure 4

Reconstruction of objects. (a) Sketch of the dark-field algorithm to enhance shadow contrast: A direct backpropagation into the object plane (b) is subtracted from the forward scattered radiation at this point, obtained by masking the emitter in its plane and forward propagating the result. (c) The resulting dark-field image; the contrast of objects is visibly enhanced. (d) Objects appear blurred in defocused planes, underlining the three-dimensional information contained in the hologram. (e) Simulation model of a full building on the left and reconstructed planes using our algorithm on the right. The black ring in the emitter plane shows the FWHM in intensity and the white dot the true emitter location.