Digital Holographic Imaging With Wi-Fi

Holography is a three-dimensional imaging method that can achieve high spatial resolution based on two-dimensional complex amplitude measurements.Microwave holography is the application of holography in the microwave band with good penetrating performance.It has been widely used in non-destructive testing,security inspection,medical imaging,radar imaging and through-the-wall imaging.However,most existing microwave holographic imaging methods are active,unable to meet the hidden needs of some application scenarios,such as anti-terrorism,surveillance and military detection.This thesis focuses on quasi-passive microwave holographic imaging using the Wi-Fi signals in the indoor environment as illuminating waves.Firstly,the Wi-Fi digital holographic imaging algorithms based on scalar wave approximation are studied.From the angle of antenna radiation,the existing Wi-Fi digital holographic imaging algorithm based on measuring the wavefront complex amplitude by dual-antenna synchronous sampling is re-derived and optimized,and it is proved that the algorithm is only applicable to the case without the co-frequency interference waves.When there are the co-frequency interference waves,the interference waves and the illuminating waves interfere with each other,so that the recorded phase is disorderly distributed and cannot be used for holographic reconstruction.In order to solve this problem,we proposed a Wi-Fi digital holographic imaging algorithm based on phase calculation.Then,based on the above two algorithms,the numerical simulation of single-frequency Wi-Fi digital holographic imaging was performed.The simulation results show that when the illuminating source is a emitter with multiple radiation antennas distributed at different locations from and there are no co-frequency interference waves,multiple reconstructed images of same target with a certain position difference will be obtained.These images are overlapped to form the final reconstructed image;when the illuminating source is two emitters with multiple radiation antennas distributed at different locations,the phase distribution within the recording aperture is significantly different from that of a single emitter,and the holographic reconstruction fails;one of the two emitters is selected as the illuminating source,the theoretical phase distribution within the recording aperture is calculated based on the phase propagation factor e^ikR in the formula of the antenna radiation field in free space,and is used to replace the true phase distribution to reconstruct the complex hologram.Based on that,the images of the illumination source and the target object can be reconstructed successfully,and the qualities of the reconstructed images are related to the number of antennas selected as the illumination source,and those are best in the case of two antennas;the reconstructed images of two different angles of view can be obtained by selecting different emitters as the irradiation source.Then the Wi-Fi digital holographic imaging experiment system was designed according to the two Wi-Fi digital holographic imaging algorithms and the characteristics of Wi-Fi signals.In order to reduce the imaging time,two automatic two-dimensional scanning sampling schemes are proposed in this thesis.Since the wave packets of Wi-Fi signals are discretely distributed in the time domain,it is necessary to extract the wave packet information in order to obtain stable illumination waves.This thesis studies two methods for extracting Wi-Fi wave packets.Finally,the indoor Wi-Fi digital holographic imaging experiments are performed in the environment with and without the co-frequency interference.In the absence of co-frequency interference,the speckle-like noise is apparent in the single frequency reconstructed images of the illumination source and the target object obtained based on the angular spectrum diffraction reconstruction method.Based on the incoherent white-light holography method,holographic reconstructions are performed based on the holograms matched the selected 5,10,and 20 equally spaced frequency points,and the reconstructed images corresponding to all the frequency points are incoherently superposed to obtain a final multifrequencies reconstructed image.With the increase of the number of selected frequency points,the speckle noise in the reconstructed image gradually weakens and the outline of the target object becomes clearer.This shows that the incoherent white-light holography method can suppress the effect of multipath scatterings on indoor holographic imaging,and obtain more detailed target object information at the same time.In the presence of co-frequency interference,the phase of the complex hologram obtained by the same method becomes a disorderly distribution,and the holographic reconstruction fails.By selecting any emitter as the illumination source,the complex hologram is reconstructed by the same way as that in the simulation part to obtain a multi-frequency reconstructed image.Compared with the method measuring the wavefront complex amplitude,the speckle noise and interference fringe in the reconstructed image obtained by the method calculating the wavefront phase are significantly more weakened,and it can be inferred that the multipath scatterings affect the holographic imaging mainly by distorting the phase of holograms.However,the contrast of the target and the background in the reconstructed image obtained by the method calculating the wavefront phase becomes lower,because the influence of the target scattering is not considered when calculating the phase of holograms.By selecting different emitters as the radiation source for phase calculation and holographic reconstruction,the reconstructed images of multiple angles of view can be obtained.It indicates that the Wi-Fi digital holographic imaging algorithm based on wavefront phase calculation has the potential of multi-view imaging.