1. A method for operating a slave transmitter to provide coordinated transmission with a master transmitter, the method comprising: forming a signal for concurrent modulation from the slave transmitter with a transmission of the signal from the master transmitter; maintaining an estimate of a time-varying phase offset between modulation of a transmission from the master and a frequency reference at the slave access point based on transmissions received at the slave transmitter from the master transmitter; modifying the signal prior to modulation according to the estimate of the time-varying phase offset; and modulating the modified signal according to the frequency reference at the slave transmitter.