mZig

This paper presents mZig, a novel physical layer design that enables a receiver to simultaneously decode multiple packets from different transmitters in ZigBee. As a low-power and low-cost wireless protocol, the promising ZigBee has been widely used in sensor networks, cyber-physical systems, and smart buildings. Since ZigBee based networks usually adopt tree or cluster topology, the convergecast scenarios are common in which multiple transmitters need to send packets to one receiver. For example, in a smart home, all appliances report data to one control plane via ZigBee. However, concurrent transmissions in convergecast lead to the severe collision problem. The conventional ZigBee avoids collisions using backoff time, which introduces additional time overhead. Advanced methods resolve collisions instead of avoidance, in which the state-of-the-art ZigZag resolves one m-packet collision requiring m retransmissions. We propose mZig to resolve one m-packet collision by this collision itself, so the theoretical throughput is improved m-fold. Leveraging the unique features in ZigBee's physical layer including its chip rate, half-sine pulse shaping and O-QPSK modulation, mZig subtly decomposes multiple packets from one collision in baseband signal processing. The practical factors of noise, multipath, and frequency offset are taken into account in mZig design. We implement mZig on USRPs and establish a seven-node testbed. Experiment results demonstrate that mZig can receive up to four concurrent packets in our testbed. The throughput of mZig is 4.5x of the conventional ZigBee and 3.2x of ZigZag in the convergecast with four or more transmitters.