Pulse or digital communications – Synchronizers – Synchronizing the sampling time of digital data
Patent
1996-08-13
1998-03-03
Chin, Stephen
Pulse or digital communications
Synchronizers
Synchronizing the sampling time of digital data
375222, 375344, 375371, H04B 138, H04L 700, H04L 2706, H04L 2540
Patent
active
057243972
DESCRIPTION:
BRIEF SUMMARY
This application claim benefit of international application PCT/FI95/00067, filed Feb. 14, 1995.
BACKGROUND OF THE INVENTION
The present invention relates to a method for synchronizing a receiver with the frequency and phase of a received signal, this method comprising the steps of taking a sample of the received signal at predetermined intervals, at least two samples being taken during the duration of one received symbol; selecting a decision sample from the samples taken; selecting a first reference sample from the samples preceding the decision sample; and selecting a second reference sample from the samples subsequent to the decision sample.
The invention particularly relates to receivers operating in connection with a baseband PAM (Pulse Amplitude Modulation) line code, and it is particularly advantageous in connection with synchronizing a four-wire baseband modem in a multipoint network.
The invention relates to establishing and maintaining a connection between a transmitter and a receiver, for instance between two baseband modems, it being thus necessary to adjust the sampling frequency of the receiver in accordance with the symbols transmitted by the transmitter so that the receiver will be able to make correct decisions on the symbols transmitted by the transmitter. In the initial situation, the transmitter transmits symbols at its own frequency, and the receiver takes samples of the signal applied to it at its own sampling frequency, which is with a certain accuracy (e.g. 200 ppm) the same as the multiple of the symbol frequency used by the transmitter. The clock of the transmitter may vary (jitter) within certain limits in comparison with the basic clock of the receiver. The purpose of the synchronization of the receiver is to make the symbol clock of the receiver follow as accurately as possible the symbol clock of the transmitter. This is carried out by extending or reducing the clock cycles of the symbol clock of the receiver. The phase of the symbol clock of the receiver is at its optimum in relation to the symbol clock of the transmitter when the receiver makes decisions regarding the transmitted symbols on the basis of that point of the pulse response which is the global maximum of the absolute value of the pulse response.
A decision-using method is previously known for synchronizing the sampling frequency of a receiver with the frequency of a received signal, in which method the receiver takes samples of the signal applied to it at a symbol frequency. In this method, the ratio of the first maximum of the absolute value of the pulse response to the value preceding it by the duration of one symbol is used as synchronization information. In practice, this is obtained, for instance, as a ratio of the received symbol to the error of the previous symbol (an error refers in this context to the difference between the output of the equalizer and the decision made by the detector).
In the above-mentioned known method, the symbol clock of the receiver is adjusted in such a manner that the ratio remains as a predetermined constant on the average. If the calculated value deviates from the reference value in the same direction sufficiently often, the sampling frequency used by the receiver is adjusted for instance by decelerating or accelerating the clock of the receiver.
This previously known synchronization method works well when the phase of the symbol clock is correct at the outset, or nearly correct. If the phase deviates greatly from the target in the initial situation, for instance at the beginning of a situation where a new connection is established, it may occur at the synchronization stage that all symbols cannot be received without errors, even if the equalizers are adjusted in accordance with the new channel and the given sampling moment.
SUMMARY OF THE INVENTION
The object of the present invention is to solve the above-mentioned problem and provide a method by means of which the receiver can seize the symbol sync of the transmitter and move the phase of the receiver towards the optim
REFERENCES:
patent: 4344176 (1982-08-01), Qureshi
patent: 4805191 (1989-02-01), Burch et al.
patent: 5317600 (1994-05-01), Kansakoski
Chin Stephen
Deppe Betsy L.
Nokia Telecommunications Oy
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