Synchronization for multi-hop distributed MIMO-OFDM

Multi-hop or cascaded distributed multi-input and multi-output (DMIMO) occurs in a series of clusters of singleantenna radios, when the clusters relay to each other, using concurrent transmission one cluster to the next, within a cooperative route. We address the case of no external synchronization, where one cluster derives its transmit synchronization from a packet received from the previous cluster. For DMIMO system based on orthogonal frequency division multiplexing (OFDM) transmission, frequency offsets between cooperating transmitters or cluster members induce inter-carrier-interference (ICI) in the receivers of the next cluster. Also, the timing offsets between transmitters create additional delay spread potentially necessitating a longer guard interval. Rather than estimating each of the multiple offsets, each receiver estimates just one offset, which implicitly is a channel gain-weighted average, known from our previous work to yield statistically stable synchronization from hop to hop. To effectively suppress the timing and frequency offsets between distributed transmitters, we designed a novel preamble which facilitates this one-step estimation without explicit knowledge of channel gains. The performance improvement of the estimation algorithm is evaluated through simulation as well as experiment on GNU Radio and USRPs.