Enhancement of RF tag backscatter efficiency with low-power reflection amplifiers

Increasing backscatter tag communication ranges is crucial for the development of low-power long-range wireless sensor networks. A major limitation for increasing the signal-to-noise ratio (SNR) for RF identification tags lies in the fact that tag antennas are terminated with passive loads for modulation, which yields reflection-coefficient values less than unity. Recent work in the field has exploited reflection amplifiers that achieve reflection-coefficient values larger than unity to increase the communication range. However, most of these systems rely on increasing the reflection coefficient at one modulation state only, which is suboptimal. In this paper, an analysis is given for the optimal way to utilize a reflection amplifier and how this compares to suboptimal practices. To demonstrate the concept, a tag is designed that achieves reflection-coefficient values higher than unity for both states in the 900-930-MHz band. The two values are antipodal, thus maximizing the tag SNR for a given amplifier. The system comprises of an ultra-low-power reflection amplifier with up to 10.2-dB gain and sub-milliwatt power consumption, and a phase-shift modulator that selectively alternates the phase of the backscatter signal between 0 and 180. The reflection amplifier-phase modulator system is experimentally characterized in terms of gain, power consumption, and backscatter efficiency.