Pulse or digital communications – Repeaters – Testing
Patent
1990-10-02
1993-02-23
Chin, Stephen
Pulse or digital communications
Repeaters
Testing
375101, 328155, H03H 730
Patent
active
051896842
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a data modem for a detecting and cancelling line deterioration factor and particularly to a line signal deterioration removing system to be employed for data modem which effectively removes phase jitter factor.
2. Description of the Related Art
Data transmission utilizing a telephone line having a transmission band in the voice frequency range is recently spreading. In such a data transmission system, a so-called modem usually providing a modulator in the sending side and a demodulator in the receiving side is provided and various line deterioration factors generated in the line between the modems are removed by such modems provided in the sending side and receiving side.
Particularly, since the recent modulation system has improved the data transmission rate through introduction of the multiphase modulation system, it is desirable to equalize, with high accuracy, various kinds of signal deteriorations in relation to the phases of the signals to be transmitted. Such signal deteriorations include intersymbol interferences effectuated on the line, frequency offset generated due to a difference of carrier frequencies for modulation and demodulation, and phase jitter generated due to a fluctuation of power source voltage.
When the telephone line in the frequency range of 0.3.about.3.4 kHz is used, it is known that following typical deterioration factors must be considered.
For the equalization of characteristics which are considered as the deterioration factors, signal deterioration has been compensated in the receiving side with the structure shown in FIG. 11.
FIG. 11 is a structural diagram of an algorithm within the modem of receiving side of the prior art. Such structure is executed by programs using digital signal processing LSIs and microprocessors.
In the same figure, reference numeral 10 designates an automatic equalizer or (AEQ) 10 which is for example formed by a transversal filter. To an automatic equalizer 10, the base band signal after the demodulation or the pass band signal before demodulation which is received from the transmission line is input as the coordinate data (vector data) of signal points on the complex plane. The equalized output obtained by removing the intersymbol interference element which is the first deterioration factor 1 from such input signal is thus output.
In the successive stage of equalizer 10, a carrier phase controller 11 comprising a carrier automatic phase control circuit (or CAPC) 12 and a hard (or non-probabilistic) decision circuit 26 is provided. CAPC 12 foresees frequency offset and phase error included in the equalized output of equalizer (or automatic equalizer) 10 with the integral operation of quadratic form.
The output of CAPC 12 is multiplied, in vector form, with the equalized output of equalizer 10 and with the feedback signal to the equalizer 10 at the position where the conjugate vector * is indicated by the symbol at .circle. which denotes a multiplying point. The multiplication (second occurrence only) of the CAPC 12 output to the output of automatic equalizer 10 means stops phase rotation, i.e. demodulation and multiplicaiton, while the multiplication of the CAPC 12 output and the by feedback signal to the automatic equalizer 10 causes addition of phase rotation, i.e. modulation. The principle of removing carrier frequency offset with such structure is based on the principle that the phase of output of the automatic equalizer 10 rotates at a constant angular velocity due to the influence of carrier frequency offset. This rotation can be ceased by multiplying an output of CAPC 12 with the output of the equalizer 10.
As an input to the carrier automatic phase control circuit 12, a hard non-probabilistic decision error is given as a phase difference between the non-probabilistic decision point and receiving point by a non-probabilistic decision circuit 26. CAPC 12 computes frequency offset and phase error element included in the output of equalizer 10 based on such h
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IBM Technical Disclosure Bulletin, vol. 18, No. 8, Jan. 1976, pp. 2548-2552; Desblache & Godard: "Phase Jitter Attenuation using a Kalman Predictor".
Hirao Kyoko
Kaku Takashi
Kishimoto Hiroki
Chin Stephen
Fujitsu Limited
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