Distribution of digital terrestrial television (DTT) and mobile digital television (MDTV) frequently utilizes single frequency networks (SFN). In an SFN, several transmitters simultaneously send the same signal over the same frequency channel.
The transmitters in an SFN must be synchronized to send their signals at the same time to avoid interference at the receiving antennas. This is commonly achieved by installing global positioning system (GPS) receivers at all transmitter sites. GPS receivers, however, may be easily intentionally or unintentionally jammed, or fail for other reasons such as equipment failure, and represent an additional cost in the network in terms of equipment and supervision. Further, the military control of the GPS may be an issue.
Also known are techniques for time synchronization of network nodes without utilization of GPS. For instance, the network time protocol (NTP) may be used to synchronize the clocks of network nodes to a master node or a reference clock using time stamps. However, the accuracy of NTP, at least in non-dedicated networks, is far too limited for the purpose of time synchronization in digital television (DTV) distribution networks.
WO 2008/103170 A discloses a method of a network client for extracting a reference frequency carried in the physical layer of a network signal originating from a server, and for using it to stabilize an oscillator of a clock of the client. The method also includes determining a clock correction value based on a server time stamp and a client time stamp.
U.S. Pat. No. 7,535,931 B discloses a two-way time transfer protocol for estimating a time error between the clocks of two network nodes.