Chained, Repeated IQ-Based Data Separation for Streaming from Multiple Tags

This paper describes a simple demodulation technique based on chained, repeating functions for separating and demodulating multiple tags that are simultaneously sending data. When multiple tags transmit simultaneously, the data streams collide often causing data loss. Most protocols detect such collisions and request that devices re-transmit data. However, it is possible to separate the individual tag data streams from the received samples. In a backscatter communication system, all nodes share a common, phase-coherent carrier signal. This work uses this unique property of the backscatter channel to receive data from simultaneously-transmitting tags by demodulating received samples in the complex baseband, removing estimates of the received bits from the incoming data stream and continuing further demodulation through repeated application of the demod-ulation function until all tag transmissions are removed. While similar to recursion, this is a chained, repeating demodulation process and can demodulate several tags that are simultaneously communicating. This paper presents an overview of the signals in a backscatter communication system and measured results from a bench-top over-the-air test using two tags placed 30 cm apart from each other and 1.67 m away from a +27 dBm, 915 MHz, reader. The data streams from the two tags fully overlap, but are correctly recovered through the chained, repeated demodulation process.