Extending the precision time protocol to a metropolitan area network : Synchronizing radio base stations

Abstract: When building various types of wide area cellular radio networks there is a need to synchronize all of the base stations within a given system. Today this is typically done by attaching a highly accurate clock to each radio base station. A GPS radio receiver is commonly used as such a clock. This thesis explores the use of the Precision Time Protocol (PTP) to provide synchronization of radio base stations, rather than the current practice of using GPS radio receivers. Advantages of utilizing PTP rather than a GPS radio receiver include the ability to easily locate radio base stations (without the need for connecting the GPS radio receiver to an antenna that has line of sight to a sufficient number of GPS satellites); the system is not vulnerable to interference with or jamming of GPS radio signals; the system is not vulnerable to spoofing of GPS radio signals, and because the new generations of radio base stations are connected to a packet based backhaul link – the system can potentially utilize the existing packet network interface (thus avoiding the need for a serial interface to the GPS receiver and a pulse per second input). At the start of this thesis project it was not known what the limits of PTP are (in terms of utilizing PTP together with radio base stations). Thus it was not clear whether PTP could be extended to much longer distances than it had originally been designed for.<p> This thesis shows that PTP can be used as an accurate timing source to synchronize base stations in networks with up to four switches between the PTP grandmaster and any PTP slave.<p> This project was performed in the Common Transport Feature department at Ericsson.