This question appears in Mumbai University > Satellite Communication and Network subject

Marks: 10 M

Year: May 2015

Network Synchronization in a TDMA system

• Network synchronization is required to ensure that all bursts arrive at the satellite in their correct time slots. For this purpose, timing markers are provided by the reference bursts, which are tied to a highly stable clock at the reference station and transmitted through the satellite link to the traffic stations
• At any given traffic station, detection of the unique word (or burst code word) in the reference burst signals the start of receiving frame (SORF), the marker coinciding with the last bit in the unique word
• It would be desirable to have the highly stable clock located aboard the satellite because this would eliminate the variations in propagation delay arising from the uplink for the reference station, but this is not practical because of weight and space limitations. Hence, there is a need to carry out network synchronization
• A major advantage of the TDMA mode of operation is that the burst time plan is essentially under software control so that changes in traffic patterns can be accommodated much more readily than is the case with FDMA, where modifications to hardware are required. Against this, implementation of the synchronization is a complicated process
• Corrections must be included for changes in propagation delay which result from the slowly varying position of the satellite
• In general, the procedure for transmit timing control has two stages. First, there is the need for a station just entering, or re-entering after a long delay, to acquire its correct slot position, this being referred to as burst position acquisition. Once the time slot has been acquired, the traffic station must maintain the correct position, this being known as burst position synchronization
• Open-loop timing control and Loopback timing control are two methods that are employed for transmit timing. In open-loop timing control, a station transmits at a fixed interval following reception of the timing markers, according to the burst time plan, and sufficient guard time is allowed to absorb the variations in propagation delay. Loopback refers to the fact that an earth station receives its own transmission, from which it can determine range.