Pulse or digital communications – Synchronizers – Frequency or phase control using synchronizing signal
Reexamination Certificate
2000-10-26
2003-05-06
Phu, Phuong (Department: 2631)
Pulse or digital communications
Synchronizers
Frequency or phase control using synchronizing signal
C370S514000, C370S509000
Reexamination Certificate
active
06560303
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for synchronizing a data sequence, and more specifically, to a synchronization method where frame synchronization occurs prior to carrier and clock recovery and in accordance with a unique word, and does not require differentially decoded or differentially encoded data.
2. Background of the Prior Art
Prior art frame synchronization has developed over the past several decades. J. Massey, “Optimum frame synchronization,”
IEEE Trans. Commun.,
Vol. COM-20, pp. 115-119, April 1972, the contents of which is incorporated herein by reference, derived the optimum maximum likelihood (ML) rules to locate a periodically inserted frame-synchronization pattern in random data over an additive white Gaussian noise (AWGN) channel with binary phase shift keying (BPSK) signaling. Massey observed that the ML rule performs at least 3 dB better than the prior art correlation rule.
FIG. 1
illustrates a prior art data sequence for a satellite communications system. An overhead portion of the data sequence includes a preamble
1
a
that has a length of N symbols, followed by a unique word
1
b
that has a length of L symbols, and precedes the traffic data field portion
5
of the data sequence. Based on the initial system timing accuracy, prescribed margins n
1
and n
2
are allocated to assure the occurrence of frame. As a result, if the length of the preamble
1
a
is N, only N−n
1
−n
2
symbols of the preamble are actually used for carrier and clock recovery.
FIG. 2
illustrates the prior art data sequence synchronization method. In a first step S
1
, an input signal is transmitted to a receiver that receives the alternating preamble S
3
, and carrier and clock recovery is performed S
3
a
using the prior art alternating preamble. Carrier phase and clock timing are not provided in the incoming data sequence. The unique word is then received S
5
and applied to a frame synchronization process S
5
a
, and marks the time of occurrence of traffic data. The data sequence transmission is then completed S
7
.
H. Meyr, “Optimum Frame Synchronization For Asynchronous Packet Transmission,” Proc. IEEE ICC, Geneva, Switzerland, pp. 826-830, 1993; and R. Balasubramanian, “A Maximum Likelihood Based Unique Word Detector And Performance Evaluation For Next Generation Linkway,” COMSAT Technical Report, 1999, the contents of which are incorporated herein by reference, disclose that prior art frame synchronization methods require the carrier and clock synchronization to be implemented before the frame synchronization. Further, S. I. Sayegh, “Algorithm For Burst Mode Acquisition Using An Alternating Binary Phase Shift Keying Preamble,” COMSAT Invention Disclosure Data, Apr. 8, 1997, and H. Meyr, M. Moeneclaey and S. A. Fechtel, “Digital Communication Receivers: Synchronization, Channel Estimation And Signal Processing,” John Wiley & Sons, Inc., New York, 1998, the contents of which are incorporated herein by reference, disclose the prior art alternating preamble.
The above-discussed prior art synchronization methods have various disadvantages and problems. For example, not all of the symbols in the alternating preamble can be used for carrier and clock recovery, because the frame starting point has not yet been determined. Further, a two-state phase ambiguity exists in the acquired carrier reference, because the polarity of first symbol used for carrier and clock recovery is not known and could have a value of either 1 or 0 with equal probability. This ambiguity is resolved by the unique word detection that follows the alternating preamble. Also, the design configuration of the prior art overhead portion of the data sequence is inefficient at low data rate and short packet transmissions, because the prior art method uses different portions of the prior art overhead to perform different functions (i.e., one for carrier and clock recovery and the other for frame synchronization) that cannot be performed with only the unique word. These prior art methods are inoperative without two separate and different data synchronization signals.
As illustrated in
FIG. 3
, Schmidt et al. (U.S. Pat. No. 6,002,729, hereafter “Schmidt”), the contents of which is incorporated herein by reference, discloses a prior art synchronization method that performs frame synchronization S
13
before frequency and phase synchronization S
15
(i.e., clock and carrier recovery). A data sequence is differentially encoded in a transmitter and sent to the receiver S
8
. The data sequence is then differentially decoded at the receiver S
9
. Frame detection S
13
is achieved by maximizing a correlation between the differentially decoded data sequence and the complex-conjugated, differentially decoded synchronization sequence S
11
, which is known at the receiver. The prior art method disclosed in Schmidt cannot be performed without differentially encoded data at the transmitter and differentially decoded data at the receiver.
Moreover, Schmidt also has various problems and disadvantages. For example, Schmidt differentially encodes the data at transmitter and differentially decodes the data at the receiver for the purpose of synchronization, which causes various problems and creates limitations and disadvantages. For example, the receiver must perform additional processing to differentially encode and differentially decode the data. The scheme of Schmidt has an increased complexity due to the additional processing requirements. Thus, the performance of Schmidt is negatively impacted by the data processing requirements. Further, Schmidt does not disclose being able to use an alternating preamble for carrier and clock recovery, as a single synchronization word is used in Schmidt.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an efficient, robust and implementable synchronization method to overcome the various problems and disadvantages of the prior art.
A further object of the present invention is to provide a synchronization method where the data transmission is preceded by a unique word that can be used to perform frame and clock synchronization as well as carrier phase offset and frequency estimation, and does not require an alternating preamble prior to a unique word, thus providing a more compact overhead.
Another object of the present invention is to provide a synchronization method that allows frame detection and clock synchronization to be conducted prior to carrier phase offset and frequency estimation, and wherein a differentially decoded and differentially encoded data sequence is not required, to provide a more efficient transmission.
Yet another object of the present invention is to provide a synchronization method that allows a unique word to perform frame synchronization, followed by a preamble for carrier and clock recovery for use with high data rate transfer applications.
The present invention includes an algorithm to achieve both the frame synchronization and the carrier and clock recovery by using a unique word for any M-ary phase shift keying (MPSK) modulated signals. More specifically, a method of transmitting a data sequence using the maximum likelihood theory is provided, comprising (a) performing a frame synchronization by processing a sampled unique word in said data sequence without differentially encoding and differentially decoding said data sequence, (b) performing a carrier and clock recovery after performing said frame synchronization, and transmitting a portion of said data sequence in accordance with said performing steps (a) and (b).
Additionally, another method of synchronizing a data sequence received in a receiver is provided, comprising processing a sampled unique word that corresponds to a portion of a data sequence to perform a clock synchronization and a frame detection to estimate a starting point of said frame by determining a maximum correlation value from all possible positions of said unique word on said frame, and performing said carrier and clock recovery process that com
Chakravarthi Prakash
Fan Yigang
Comsat Corporation
Phu Phuong
Sughrue & Mion, PLLC
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