A microwave spatial modulator to improve in-home WiFi

[N. Kaina et al., Sci. Rep. (2014)]

Wavefront shaping is not limited to optical waves. Similar techniques can be used for any kind of wave for which one can control dynamically the phase over a large number of independent elements. In [N. Kaina et al., Sci. Rep. (2014)], the authors demonstrate the use of their Spatial Microwave Modulator (SMM) to control the propagation of radiofrequency waves inside a room to improve the WiFi signal at any chosen position. The system is passive as there is no energy transfer from the modulator to the WiFi signal, it only controls the local phase of the waves reflected off the modulator. The device is thin and has the typical size of a small poster, it can be conveniently placed on the wall of a typical room without any loss of space.

The authors built what is the closest equivalent of a spatial light modulator for microwaves; an array of 102 independent electromagnetic reflectors at the working frequency of 2.47 GHz. The phase of each element can be controlled electronically. It is 1.5 mm thick and the total surface is ~0.4 m², so it can be hidden behind (or be!) a decorative element. They decided to use it as a binary phase modulator, i.e. they can switch the phase of the wave on each element from 0 to \(\pi\). To do so, each element of this so-called "metasurface" is a resonant cell. The phase of the reflection can be changed by switching the resonant frequency above or below 2.47 GHz (well, it is slightly more complicated than that but it is the main idea).

Similar to previous optical experiments, the phase modulator can be used to focus the energy at a given target in a complex medium. Instead of a purely random scattering medium, they chose a more realistic environment for microwaves; a complex medium with both reverberations and scattering, i.e. a typical office room. Reverberations are due to the walls and scattering comes from the presence of various objects in the room (chair, desk, ...). A schematic representation of the room used is shown in Figure 1.

 Figure 1. Schematic view of the setup and its surrounding environment. Image from [N. Kaina et al., Sci. Rep. (2014)].

To improve the signal at one position, a receptor is placed there and all the elements of the SMM are initially set to the 0 phase state. Iteratively, each element is switched to \(\pi\), the state is kept if it improves the signal at the target position or switched back to 0 otherwise. In a nutshell, the contributions coming from all the elements are put in phase to create a constructive interference at the target position.

Figure 2. (a) Intensity enhancement over the iteration number. In the inset is shown the image of the optimized state of the modulator, there is a \(\pi\) phase shift between black and white elements. (b) Intensity spectrum for the initial mask (black line) and the optimized one (red line). Initial (c) and final (d) intensity map in the room. The target is placed at the \((x=0,y=0)\) position. Image from [N. Kaina et al., Sci. Rep. (2014)].

A typical result is shown in Figure 2. showing an enhancement of 8.5 dB (which corresponds to the average enhancement measured over 30 realizations). One notices that the focal spot in Figure 2.d. seems isotropic, whereas the modulators are not located all around the room. This shows that the focusing involves reverberating waves that arrive on the target from all angles. This important point is common in optical experiments in complex media, it means that the quality of the focal spot does not depend on the size of the modulator, as it is the case for a lens focusing light in free space. The enhancement is proportional to the number of independent elements on the SMM and the focal spot size is always of the order of \(\lambda/2\). 

The authors also show that the same procedure can be used to cancel the WiFi signal at a given position by creating destructive interference.

In addition to its obvious applications for improving indoor wireless signals, such a device can be a very useful tool for the fundamental study of wave propagation in reverberating or through multiple scattering media.