Wireless Time and Frequency Clock Synchronization Technology for Software Defined Radios
Daniel T. Goff, W. Todd Faulkner, ENSCO, Inc; and Olukayode K. Okusaga, JHU APL
Date/Time: Wednesday, Sep. 9, 11:10 a.m.
Wireless Stratum-1 time and frequency synchronization of distributed systems in real-time creates new possibilities for coherent operation of warfighting systems that leverage software defined radios (SDRs). ENSCO has developed the Coherent Link Technology (CLT) application for SDRs that enables rapid synchronization of distributed systems using an existing wireless communication and ranging link. CLT is a precise time and frequency synchronization software application that uses round-trip RF measurements of relative time, frequency, distance, and distance-rate to actively estimate and synchronize clock frequency and phase across a network of communicating radio platforms.
The CLT application consumes real-time RF measurements from a timing, communications, and ranging (TCR) IP core on each communicating SDR. ENSCO integrated the CLT application onto an SDR platform utilizing commercial off the shelf (COTS) temperature compensated crystal oscillators (TCXO) and featuring digital synthesis capability for the RF local oscillator and baseband processor – A common architecture for COTS, handheld SDRs. In this configuration, the TCR element provides RF measurements at up to 400 Hz, enabling rapid synchronization and support for mobile platform synchronization and dynamic operations. With active clock synchronization enabled through the CLT application, wireless synchronization performance of 10 MHz reference signals on two SDRs was measured to Stratum-1 (10^-11 fractional frequency) synchronization in less than 1-second and time synchronization maintained to 12 picoseconds 1-sigma over a three hour measurement duration.
The CLT application can also enable the dynamic creation of sparse arrays with arbitrary placement of array elements when additional sensor data from an inertial measurement unit (IMU) along with the RF measurements provided by the TCR element are combined. Real-time relative position estimation of the communicating radios supports the formation and real-time control of a wirelessly coherent sparse array by enabling the determination of an array manifold vector for the array. Simulation results will be presented that show theoretical performance of a synchronized system with the CLT application also estimating relative position based on the fusion of RF measurements and a cellphone-grade IMU. Simulation results show millimeter-level array manifold accuracy is possible with mobile SDR platforms.