| Battery-free backscatter communications has been considered a cutting-edge technology for the Internet of Things(Io T)due to the advantages of ultra-low power consumption and ultra-low hardware cost.The design philosophy of this novel communication paradigm is that the backscatter tag transmits information by reflecting and re-modulating the ambient wireless signal instead of generating the signal itself.However,due to its minimalist design and the cascaded fading property of its channel,backscatter communications faces application challenges of low transmission rate and poor transmission reliability,which hinder its wide application and further development.Multiple-antenna technology is an effective way to improve the transmission rate and transmission reliability.However,due to the characteristics of impedance modulation and signal coupling properties of the backscatter,multipleantenna technology cannot be directly applied to backscatter communication systems.To solve this problem,this paper tries to extend the mapping dimensions of backscatter information bits and design multiple-antenna backscatter transmission schemes.The advantages and feasibility of these proposed schemes are verified through theoretical analysis,simulation,and experimental results.The main contributions are summarized as follows:To enhance the transmission rate of backscatter tags,a spatial modulation(SM)-based backscatter transmission scheme,namely SM-Backscatter,is proposed.In SM-Backscatter,the index of the activated antenna is mapped as a spatial constellation point to transmit additional information bits.The simulation results show that the throughput of the proposed scheme is three times that of the traditional single-antenna case when only a single antenna is activated in each time slot.Moreover,a unified backscatter hardware prototype and experimental platform are designed,and the application feasibility of the proposed scheme is preliminarily verified through experimental results.To enhance the transmission reliability of backscatter tags while maintaining a high transmission rate,a unified generalized space-time shift keying(GSTSK)-based backscatter transmission scheme is proposed.Benefiting from the space-time modulation principle,GSTSK-Backscatter can strike a flexible tradeoff between the attainable diversity and multiplexing gains by activating different dispersion matrices in an efficient but low-complexity manner.The simulation results show that the proposed scheme can bring a BER gain of about4 d B compared with the existing schemes with the same transmission rate,which verifies the advantages of the proposed scheme in both gains.Moreover,with the help of the designed backscatter hardware prototype and experimental platform,the application feasibility of the proposed scheme is preliminarily verified through experimental results.To minimize the pilot overhead required by the multiple-antenna backscatter tag,differential coding-based transmission schemes are proposed for SM-Backscatter and GSTSKBackscatter,referred to as DSM-Backscatter and DGSTSK-Backscatter.DSM-Backscatter employs the activated square antenna matrixes’ index to improve the tag’s transmission rate.DGSTSK-Backscatter applies Cayley transform to transform the dispersion matrix into a unitary matrix form.Next,the backscatter signal is mapped to the index and space-time dimensions using the activated unitary dispersion matrixes.As a result,DGSTSK-Backscatter can maintain its high transmission rate and reliability while implementing differential coding.Finally,the application feasibility of the proposed differential multiple-antenna backscatter schemes without cascaded backscatter channel estimation is verified by using the designed backscatter hardware prototype and experimental platform.In conclusion,the proposed schemes effectively enhance the transmission rate and reliability of backscatter communications by expanding the mapping dimension of the tag information bits.Furthermore,these schemes have been extended to scenarios where cascaded backscatter channel estimation is not required,resulting in reduced pilot overhead.Overall,these schemes provide a solid theoretical basis and technical support for the development and application of future backscatter communications. |
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