FH-Code Phase Synchronization in a Wireless Multi-Hop FH/DSSS Adhoc Network
Teemu Vanninen, Harri Saarnisaari, Matti Raustia, Timo Koskela · 2006
A military ad hoc network has to have a good LPI (low probability of intercept) and LPJ (low probability of jamming) capabilities. These requirements can be met by using a combined frequency hopping/direct sequence (FH/DS) spread spectrum (SS) system. Due to characteristics of an ad hoc network, the network wide synchronization has to be achieved distributely, which makes especially FH-code phase synchronization a challenging task. Here, we present an FH-code phase synchronization method suitable for very long FH-codes and unknown code-patterns. The synchronization data is spread using a specific synchronization DS-code (DS(s)-code), i.e., the control channel is in the code-space. Synchronization messages are broadcasted periodically to allow single nodes or subnetworks with different FH-code phases to synchronize as a one network. Divergent synchronization hops are not needed, thus providing excellent LPI & LPJ properties. In order to achieve and maintain a common FH-code phase a network time synchronization algorithm has to be used. Simulations of an ad hoc network with a simple network time synchronization algorithm has been made and the results are presented in this paper. The simulations prove that the chosen algorithm can maintain a common timing at a satisfactory level as long as the connectiveness in a network is tolerable. The effects of clock drift and the synchronization message size have also been studied