| Battery-free backscatter systems can harvest energy from the environment to support information transfer.This significantly reduces the deployment and maintenance costs of devices caused by batteries,making it one of the most popular low-power solutions in the Internet of Things（Io T）.Simultaneous transmission of multiple tags is the key to increase the number of accessed tags and improve network throughput.However,existing solutions face the issues in the multi-tag scenario:low efficiency in energy transmission and vulnerability to signal interference in information transmission.Therefore,this thesis proposes three novel schemes for for battery-free tags:a distributed energy beamforming scheme,a frequency-shift multi-tag transmission scheme,and an in-band multi-tag transmission scheme.The main contributions are summarized as follows:1.To address the challenge of low efficiency in energy transmission,this thesis pro-poses a distributed energy beamforming scheme that leverages the big gap between energy and communication sensitivity,to help the channel estimation.Based on this,the thesis presents a cold start method based on distributed orthogonal frequency signal to initialize the system.Additionally,it designs a channel classifier based on spatial diversity to correlate the tag and the estimated channel.Moreover,the relative power is proposed as the indicator for the phase alignment algorithm in the beamforming process to eliminate the effect of un-known feedback channels.This thesis implement a prototype to evaluate the performance.The results shows that the proposed scheme improves the power gain by 5.72 d B.2.To overcome the interference from the narrowband signal,this thesis explores the feasibility of information transmission between heterogeneous devices and propose a frequency-shift multi-tag transmission scheme.The scheme transfer the interference to the carrier of the backscatter by reflecting the narrowband signal to a wideband receiver.To achieve this,it in-vestigates the heterogeneous physical layer between the narrowband and wideband protocol and present a cross-technology communication model.Additionally,it designs a frequency-shift modulation that reflects the narrowband signal to support multi-tag transmission.It further analyze the interference of the heterogeneous signal on the channel state information at the receiver and design a parallel decoding method.The prototype can support up to 6tags with BER of 10-4.3.To eliminate the interference from the wideband signal,this thesis presents an in-band multi-tag transmission scheme that leverages the multi-antenna feature in the ubiqui-tous access points in the environment.It builds a hybrid-user multiple-input multiple-output model to simultaneously transfer signals from multiple ambient devices and tags.Based on this model,it modulates the in-band backscatter signal under the restriction of imperfect hardware,and further design a low-power preamble structure to support channel estimation process.Additionally,it analyzes the coupling effect of the channel between the direct link and the backscatter links and the signal between the battery-free tags and ambient devices.It respectively decomposes the coupled channel and decode the coupled signal.It implements a prototype that shows the BER achieves 10-3at the multi-tag scenario.This thesis systematically investigates the key technologies of energy transfer and infor-mation delivery for multiple tags,providing theoretical and technical support for battery-free backscatter system,which is expected to promote the further development of backscatter in low-power Internet of Things. |
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