Perfectly Synchronized Streaming From Multiple Digitally Modulated Backscatter Sensor Tags

This paper proposes multiple subcarrier multiple access (MSMA), a multiple access method for perfectly synchronized concurrent data collection from a group of passive backscatter wireless sensor tags, and its experimental evaluation. By achieving less than 128 μs synchronization error, backscatter communications can be applied to the structural health monitoring of artifacts, such as civil structures and machinery. In MSMA, each sensor tag uses a dedicated subcarrier frequency to produce a modulated backscatter. The sensor data is superposed onto the subcarrier with either an analog or digital modulation. The inevitable harmonics among the subcarriers, stemming from the backscatter principle, can be rejected by numerically calculating the harmonic replicas and subtracting them from the observed signal using a software defined receiver. A frame-based signal processing in the receiver results in no relative synchronization error among subcarriers, even after the interference rejection. Because the interference rejection can be performed before the demodulation and decoding, the concurrency can be secured irrespective of the choice of modulation method. We developed a prototype of MSMA using LabVIEW communications software defined radio environment, and prototype sensor tags using discrete electrical parts. The performance and limitations of MSMA using digitally modulated subcarriers are evaluated both in wired and wireless environments with up to four backscatter sensors. It is confirmed that the interference rejection can achieve about 7 dB processing gain. An experiment to measure the vibration of a bridge under weekday traffic with four sensors revealed the results obtained by MSMA and wired sensor agree well.