Pulse or digital communications – Synchronizers – Synchronizing the sampling time of digital data
Reexamination Certificate
1998-12-11
2001-11-27
Pham, Chi (Department: 2631)
Pulse or digital communications
Synchronizers
Synchronizing the sampling time of digital data
Reexamination Certificate
active
06324234
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention relates generally to data communications and more specifically to a method and system for synclshonization and clock recovery.
BACKGROUND OF THE INVENTION
In certain data communications systems, a frame synchronization pattern is periodically inserted into the data stream by the transmitter. A conventional clock and frame recovery scheme at the receiver uses samples of an M-ary phase-shift-keying (MPSK) signal at the output of a Nyquist filter to reconstruct the data clock. The search mechanism used often requires a serial search of the received signal. Upon recovery of the data clock, the received bit or symbol streams are demodulated and the resulting bits or symbols are forwarded to a frame recovery mechanism that searches for a unique word (UW) of length L symbols which follows the synchronization pattern in the preamble of the MPSK signal. The length L of the UW should be long enough so that as compared to the frame length, the probability of the UW appearing in the data stream is minimized.
This scheme requires a reasonably accurate clock timing recovery before frame synchronization (or any form of communication) can take place. [See, for example, T. T. Ha,
Digital Satellite Communications,
2
nd
edition, McGraw-Hill, New York, 1990, pp. 230-241]. Any false or missed detection of the UW or of the synchronization pattern results in a loss of the data frame that follows the preamble. This in turn may result in large delays in, for example, a Time Division Multiple Access (TDMA) system operating in a less than ideal environment.
Moreover, the above scheme requires a synchronization pattern followed by a fairly long UW sequence to provide for synchronization at the receiver. An additional deficiency of the UW search method used in practice is the sub-optimum nature of the search mechanism. From a detection theory standpoint, the conventional method of recovering the UW of an MPSK signal, which uses a threshold test at the output of the sequence correlator, is a sub-optimum solution to the problem of codeword recovery in the face of additive Gaussian noise. One reason for this simplification is the prohibitively large computational complexity of an optimum codeword search.
SUMMARY OF THE INVENTION
The present invention provides for joint estimation of data clock and frame timing in, for example, an M-ary phase-shift-keying (MPSK) signaling scheme with raised-cosine (RC) baseband pulse shaping. Asynchronous samples of the MPSK signal are obtained at a sampling rate greater than the Nyquist rate. A Fast Fourier Transform (FFT) processor processes the acquired samples, and the outcome passed on to an absolute-value device followed by a peak detector. The present invention then operates to extract clock and frame information from the MPSK signal using a dotting pattern. A generalized case of the dotting pattern used has the form &phgr;, &phgr;+&pgr;, &phgr;, &phgr;+&pgr;, . . . where &phgr; is selected from the MPSK signal constellation.
In addition to clock recovery, the present invention provides for frame synchronization using the dotting sequence described hereinabove. The present invention thus eliminates the need for a long unique work (UW) frame synchronization pattern often included in the preamble of, for example, a time-division multiple-access (TDMA) signal to establish frame synchronization.
These and other features of the invention that will be apparent to those skilled in the art from the following detailed description of the invention, taken together with the accompanying drawings.
REFERENCES:
patent: 5995569 (1999-11-01), Shimoi et al.
“A Method for Coarse Frequency Acquisition for Nyquist Filtered MPSK”, Ahmed, et. al.,IEEE Transactions on Vehiculat Technology, vol. 45, No. 4, 11/96, pp. 720-731.
Brady III W. James
Moore J. Dennis
Nguyen Dung X.
Pham Chi
Telecky , Jr. Frederick J.
LandOfFree
Synchronization and clock recovery does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Synchronization and clock recovery, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Synchronization and clock recovery will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2604715