Rapid frequency-hopping time synchronization

Abstract

A method is disclosed for synchronizing the operation of frequency hopping, spread spectrum communication devices in a network. One device (the 'NTU') is deemed to be keeping the network time, to which the other devices must synchronize in order to communicate over the network. In a broadcast mode of the method an unsynchronized unit (a 'RTU') operates in an idle state to receive on a discrete set of receive frequencies. The total communication time period is divided into intervals, and a unique synchronization code is generated for each time interval which defines the idle state frequencies and time information transmit frequencies to be employed in accordance with the method. The time kept by the RTU is assumed to within a predetermined number of time interval of the present network time interval. In a broadcast mode the NTU transmits sequences of synchronization messages, each sequence including a message transmitted at a receive frequency for each of the past, present and future time intervals necessary to cover the time uncertainty time. This sequence is transmitted at a transmit hop rate substantially higher than the RTU receive hop rate, so that all possible receive frequencies and synchronization codes are bracketed by the transmission sequence. Each message include information enabling the RTU to predict the time and frequency at which network time information will be transmitted byt the NTU. The RTU receives one of the messages and tunes to the appropriate receive frequency to receive information enabling it to synchronize to the network time.