Cluster Transmission Time Synchronization for Cooperative Transmission Using Software-Defined Radio

Abstract-We consider the time synchronization of concurrent transmissions from a cluster of cooperating software-defined radios (SDRs). In this type of cooperative transmission (CT), each cooperating node derives its timing autonomously from a signal received from a source or another cluster. Timing errors, caused by SDR processing delays, cause excessively high delay spreads of the signal received at the destination node. To reduce these timing errors, we propose that relays use embedded time stamps to hold the packet for a fixed period before firing. The method has been implemented in an indoor non-line-of-sight channel, for a twohop network (source-cluster-destination), using 64 kbps BFSK, non-coherent demodulation, and up to four diversity channels based on orthogonal carriers. The bit error rate (BER) and the measured cumulative distribution function (CDF) for the rms transmit time spreads (RTTSs) are presented, as functions of transmit power and number of cooperators. Mean RTTSs on the order of 50 ns are achieved. Also presented are the results of a “ping pong” experiment, in which a packet is transmitted back and forth between two clusters. CDFs of the RTTS for the first 10 hops are given, showing stable statistics after the second hop.