Pulse or digital communications – Synchronizers – Frequency or phase control using synchronizing signal
Reexamination Certificate
2000-01-26
2003-09-09
Chin, Stephen (Department: 2634)
Pulse or digital communications
Synchronizers
Frequency or phase control using synchronizing signal
C375S366000, C370S350000, C370S510000, C370S512000
Reexamination Certificate
active
06618458
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and apparatus for signal receiving synchronization and more particularly to the method and apparatus capable of providing the signal receiving synchronization by estimating timing and frequencies for synchronization with high accuracy even when communication lines are in a bad state.
2. Description of the Related Art
In a portable radio line of a mobile radio communication system and in a satellite radio line using a TDMA (Time Division Multiple Access) system, in order for a signal receiver to successfully receive sending information transmitted from a signal sender i.e., to establish good synchronization between them, it is necessary to match signal receiving timing and frequency of a signal receiving apparatus to those of signals to be received.
Therefore, conventionally, in order for the signal receiving apparatus to provide synchronization for receiving a signal fed from the sender, a multiplexed signal for synchronization contained in a control signal sent through a radio control channel is sent from the signal sender to the signal receiver. The multiplexed signal containing frames, each being composed of two or more slots (also referred to as “bursts”), contains a CCS (Common Channel Signaling) signal containing a synchronization word composed of several tens of symbols inserted in one slot and a frequency estimating signal inserted in the other slot (i.e., in any of slots other than the slot into which the CCS signal is inserted). Data for controlling is inserted into a part having no synchronization word in the CCS signal.
A peak value is obtained by taking a moving average of levels of signals received through the control channel in each of the frames of the control signal, and a time when a largest peak value occurs is defined to be the time when a rough timing estimation is achieved, i.e., to be the time when a receiving signal (frequency estimating signal) is detected.
Though the frequency estimating signal is obtained by detection of the receiving signal described above, it also means that the frequency estimating signal having the frequency of the control signal affected by the state of radio is propagated. The detected frequency estimating signal is converted into spectrum data by using FFT (Fast Fourier Transform) and an estimated frequency error (i.e., a difference between the frequency assigned to the control channel by the signal receiver and the frequency of the receiving signal) is calculated and, by using the estimated frequency error, the frequency of the synchronization word in the CCS signal contained in a predetermined slot in the frame is corrected. The correlation between the received signal with its corrected frequency and a synchronization word (i.e., pre-specified bit series) is examined over a predetermined range of the signal and whether the correlated value exceeds a predetermined threshold is observed in order to check the appropriateness of the rough estimating of timing. If the correlated value exceeds the threshold, it is defined that the synchronization is established, i.e., the time when a peak correlation value occurs is defined to be the time when the timing is provided between the receiver and the sender. This timing is used for demodulation by the receiver.
The conventional method in which signals received in only one frame are used for establishing synchronization can achieve a desired purpose if radio propagation is in a comparatively good state. However, if the state of radio propagation becomes worse than expected in the conventional method, larger errors occur in the detection of received signals, estimating of the signal frequency and the correlation, thus resulting in a decrease in the accuracy of the signal synchronization process. Accordingly, it is impossible for the conventional method to satisfactorily achieve the synchronization purpose as desired.
A synchronization method for received signals that may solve such a shortcoming as described above is disclosed in European Patent Application (EP 0 809 377 A2). By referring to
FIG. 12
, the synchronization method disclosed in the above European Patent Application is described below.
In this method, a multiplexed signal for synchronization is sent through a radio control channel from a signal transmitter to a signal receiver. The multiplexed signal having frame structure, each being composed of two or more slots (also referred to as “bursts”), contains a CCS (Common Channel Signaling) signal containing a synchronization word composed of several tens of symbols inserted in one predetermined slot and a frequency estimating signal in the other predetermined slot (i.e., in any of slots other than the slot into which the CCS signal is inserted).
A peak value is obtained by taking a moving average of levels of signals received through the control channel in each of frames of the control signal, and the time when a largest peak value occurs is defined to be the time when a rough timing estimation is achieved, i.e., to be the time when a receiving signal (i.e., frequency estimating signal) is detected. By using the roughly estimated timing, the receiving signal (i.e., frequency estimating signal) is detected. This rough estimating of the timing is performed in the signal receiving synchronization method disclosed in the European Patent Application as follows.
The measurement of peak values of signal levels obtained by taking a moving average of levels of signals received is performed on signals received in two frames (at Step SD
1
in
FIG. 12
) and two or more peak values corresponding to each other are selected and, if a time interval between these peak values is appropriate, it is presumed that the timing for synchronization is provided by using the roughly estimated timing. With the use of this roughly estimated timing, in the same manner as in the conventional method, the receiving signal is detected (i.e. “Detected or not”, at Step SD
2
).
When it is impossible to perform the rough estimating of the timing for synchronization and also impossible to detect the receiving signal (in the case of “Not Detected” at Step SD
2
in FIG.
12
), in the same manner as in the conventional method, the same processing as above is carried out on the next frequency assigned to the control channel (i.e. “To next CCS frequency”, at Step SD
3
).
The frequency is estimated by the FFT-conversion of the frequency estimating signal obtained based on roughly estimated timing and by detecting the receiving signal (in the case of “Detected” at Step SD
2
). The estimating of this frequency is made in the method disclosed in the European Patent Application as follows. That is, the above estimating of the frequency is performed on signals received in two frames of the control signal in accordance with a carrier to noise ratio (C/N ratio). If the estimating of the frequency is made twice, resulting values are averaged to finally obtain the estimated frequency (i.e. “Estimating of frequency error”, at Step SD
4
).
The frequency estimating error at the time of receiving signals is estimated and the frequency of the synchronization word in the CCS signal contained in the next slot is corrected by using the estimated above frequency error. Then, the correlation between the synchronization word with its frequency corrected and the synchronization word (synchronization word known to the signal receiver) set to the control channel from the signal receiver side is examined over a predetermined range of the signal (i.e. “Judgement of correlation between signals”, at Step SD
5
).
By checking whether the peak correlation value exceeds a predetermined threshold value, a fine estimating is made instead of the rough estimating of the timing. If the peak correlation value described above exceeds the threshold value (in the case of “Large correlation value” at Step SD
6
), it is presumed that the synchronization between the receiver and the sender has been established, i.e., the time when the peak correlation value is obtained
Chin Stephen
Dickstein Shapiro Morin & Oshinsky LLP.
Ha Dac V.
NEC Corporation
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