1. Field of the Invention
The present invention relates to the filed of the digital signal processing, and more specifically, to the field of phase equivocation in burst modems.
2. Discussion of the Prior Art
When the communication exists for a long period of time, it is possible to use a conventional phase-lock loop (PLL) in a receiver to recover the carrier from a continuously transmitted signal. The PLL that is specifically designed to recover the carrier is termed as a phase carrier loop.
However, in a time-division multiple access (TDMA) communication system, the available spectrum is used by letting each user to have access to the whole band for a short time (traffic burst), during which time the user transmits data as fast as the user can. The user's frequency allocation is shared with the other users who have time slots allocated at other times.
Thus, in a TDMA-type burst communication systems, the signals exist for a short period of time. Consequently, there is little time in which the phase carrier loop can slew to the proper phase in order to recover the carrier from the burst signal.
This situation is exacerbated for a TDMA-type burst commination system if the phase of the incoming symbol is very close to the border between the plant points. If this is the case, the slicer in the phase carrier loop can not determine to which quadrant the incoming symbol point having such phase should belong because such symbol can be equally located in either of two quadrants. Such symbol point is further referred to as a symbol point having a phase equivocation, and the phase carrier loop can not lock fast enough on an incoming signal having a phase equivocation characteristic.
What is needed is to design a carrier phase loop system that is capable of resolving the phase equivocation of the incoming symbols, thus placing any incoming symbol point to a certain quadrant for further processing by the loop, thus increasing the speed of locking of the carrier phase loop on the carrier.