Quadrature amplitude modulation type digital radio communication device and method for preventing abnormal synchronization in demodulation system

• US 5,438,591 A
• Filed: 07/30/1992
• Issued: 08/01/1995
• Est. Priority Date: 07/31/1991
• Status: Expired due to Fees

• Current Assignee: Kabushiki Kaisha Toshiba (Toshiba Corporation)
• Original Assignee: Kabushiki Kaisha Toshiba (Toshiba Corporation)
• Inventors: Hirama, Mitsuru, Oie, Yasunori, Fujii, Yasushi, Tanaka, Hidekazu
• Primary Examiner(s): Chin, Stephen
• Assistant Examiner(s): Le, Amanda T.
• Application Number:
  US07/921,687
• Time in Patent Office: 1,097 Days
• Field of Search: 375/10, 375/119, 375/20, 375/80, 375/81, 375/39, 375/97, 375/93, 375/120, 375/75, 329/306, 329/308, 329/319, 329/320, 329/349, 455/208, 455/214, 455/259, 455/263, 455/265
• US Class Current: 375/261
• CPC Class Codes: H04L 27/3827 in which the carrier is recovered using only the demodulated baseband signals

Abstract

This invention relates to an abnormal synchronization preventing device used in a multiple value quadrature modulation type radio device and the abnormal synchronization preventing device includes a monitoring circuit for monitoring the carrier pull-out based on demodulated base band signals to output a carrier pull-out alarm signal, an abnormal synchronization detection circuit for sampling the reception signal level at a speed twice the transmission speed of reception data according to identification data and detecting abnormal synchronization according to whether or not the detection area of sampled data lies in a specified area, an abnormal synchronization preventing circuit for generating a signal for a preset period of time when abnormal synchronization is detected in a case where the carrier pull-out alarm signal is not output, and a selection circuit for receiving a reference signal used for generating a reference carrier frequency signal having a frequency close to the frequency of the carrier wave and a phase difference signal, selecting the phase difference signal as the control signal in the normal state, and selects the reference signal as the control signal while receiving an output signal of the abnormal synchronization preventing circuit and supplying the selected signal to an oscillator.

Claims

1. A demodulation device of a quadrature amplitude modulation type digital radio device having an oscillator means with an oscillation frequency controlled by a control signal, wherein identifying means for identifying levels of demodulated base band signals to derive a value corresponding to the levels, phase difference detection means for deriving a phase difference signal of a reproduced carrier wave based on the value, the phase difference signal is used as the control signal to control the oscillation frequency so as to derive an oscillation frequency corresponding to the phase difference signal, an output signal having the thus derived frequency is used as the reproduced carrier wave, the reproduced carrier wave is subjected to a 90°

phase-shifting process to create two reproduced carrier waves with 90°

phase difference which are separately multiplied with a reception signal to create demodulated base band signals, clock reproducing means for reproducing a transmission clock signal from the demodulated base band signals to define timings for identification of the levels of the demodulated base band signals, and the reception signal is restored based on the value derived by the identification, comprising;

monitoring means for monitoring the carrier pull-out based on the demodulated base band signals and outputting a carrier pull-out alarm signal when the carrier pull-out occurs;

abnormal synchronization detecting means for sampling the reception signal level at a speed twice the transmission speed of the reception data based on the identification value derived by the identification and for detecting abnormal synchronization when a detection area of the sampled data derived by abnormal synchronization detecting means corresponds to a predetermined area of a plurality of areas;

abnormal synchronization signal generating means for generating a signal for a preset period of time, when the abnormal synchronization is detected and the carrier pull-out alarm signal is not generated; and

selection means for receiving a reference signal used for generating a signal having a reference carrier frequency nearly equal to the frequency of the carrier and the phase difference signal, selecting the phase difference signal as the control signal in the normal operation state, selecting the reference signal as the control signal and supplying the same to the oscillator means while receiving an output signal of said abnormal synchronization signal generating means;

wherein said identifying means is operated at a speed twice the transmission clock to effect the identification.

2. A demodulation device of a quadrature amplitude modulation type digital radio device according to claim 1, wherein said oscillator means is a voltage-controlled oscillator.

3. A demodulation device of a quadrature amplitude modulation type digital radio device according to claim 1, wherein said identifying means includes a first A/D converting means for subjecting a component of one of two quadrature axes for quadrature modulation to the analog-digital conversion and a second A/D converting means for subjecting a component of the other axis to the analog-digital conversion and said first A/D converting means and second A/D converting means effect the sampling operation by use of a clock twice the reproducing transmission clock.

4. A demodulation device of a quadrature amplitude modulation type digital radio device according to claim 1, wherein said identifying means includes a first A/D converting means for subjecting a component of one of two quadrature axes for quadrature modulation to the analog-digital conversion and a second A/D converting means for subjecting a component of the other axis to analog-digital conversion, said first A/D converting means and said second A/D converting means are constructed to effect the sampling operation by use of a clock twice the reproducing transmission clock and at least said first A/D converting means has an overflow/underflow terminal for generating a signal when the demodulated baseband signal of a level exceeding a predetermined level range is supplied;

said abnormal synchronization generating means includes a first shift register for shifting first-pass output data of said first A/D converting means, a second shift register for shifting an overflow/underflow output of said first A/D converting means, logical processing means for logically processing output data of plural passes of said second A/D converting means to determine whether or not data is detected in a specified area which is previously determined on the phase plane,

a first gate for deriving a logical AND of outputs of said first and second shift registers and an output of said logical processing means, a second gate for deriving a logical AND of an output of said first gate and the pull-out alarm signal, and switching signal generating means for supplying a switching signal to said selection means while receiving an output of said second gate so as to cause said selection means to select the reference signal as the control signal.

5. An abnormal synchronization preventing method used at a time of restoring a reception signal in a demodulation device of a quadrature amplitude modulation type digital radio device in which a phase difference signal is derived based on levels of demodulation base band signals derived by identifying means, an oscillation frequency corresponding to the phase difference signal is determined by using the phase difference signal as a control signal for controlling the oscillation frequency and is used as a reproduced carrier wave, the reproduced carrier wave is subjected to a 90°

phase-shifting process to create two reproduced carrier waves which are separately multiplied with a reception signal to create demodulated base band signals, a transmission clock of the reception signal is reproduced from the demodulated base band signals to define timings for identification of the levels of the demodulated base band signals, and the reception signal is restored based on a value derived by the identification, an abnormal synchronization preventing method used at the time of restoring the reception signal based on the value derived by the identification, comprising the steps of;

monitoring the carrier pull-out based on the demodulated base band signals and outputting a carrier pull-out alarm signal when the carrier pull-out occurs;

causing said identifying means to effect the sampling operation at a speed twice that of the transmission clock and generate the value;

detecting the level of the reception signal based on the value derived by the identification and detecting abnormal synchronization according to whether or not the detected area lies in a predetermined area;

generating an abnormal synchronization detection signal for a preset period of time when the abnormal synchronization is detected in a case where the carrier pull-out alarm signal is not generated; and

selecting the phase difference signal among a reference signal used for generating a signal having a reference carrier frequency nearly equal to the frequency of the carrier and the phase difference signal as the control signal in the normal operation state and using the reference signal as the control signal while the abnormal synchronization detection signal is generated.

6. An abnormal synchronization preventing method used at a time of demodulation in the multiple value quadrature modulation type digital radio communication which uses a Costas type carrier reproducing loop and in which two reproduced carrier waves having a 90°

phase difference from each other are derived based on a reproduced carrier wave, the two reproduced carrier waves are multiplied with a reception signal to demodulate the reception signal and derive demodulated base band signals, a transmission clock is reproduced based on the demodulated base band signals, the demodulated base band signals are sampled in synchronism with the reproduced clock signal, the sampled signals are converted to identification data items corresponding to the levels thereof, and data is restored based on the identification data items, comprising;

a step of multiplying the reproduced clock by 2;

a step of sampling the demodulated base band signals in synchronism with the two-multiplied reproduced clock which is multiplied by 2 in said step of multiplying;

a step of extracting data items at points other than synchronized points of the reproduced clock from data items obtained in said step of sampling;

a carrier pull-out monitoring step for detecting the pull-out of the reproduced carrier wave having the same frequency as a carrier wave of the reception signal;

a step of monitoring abnormal synchronization corresponding to a synchronized state set up by a carrier wave having a frequency different from that of the reproduced wave;

a step of monitoring whether or not the carrier wave produced by oscillation means is out of phase with the carrier wave of the input signal;

a step of detecting a normal synchronization achieved by the carrier wave produced by said oscillation means and having the same frequency of the carrier wave of the input signal, and an abnormal synchronization caused by the carrier wave produced by said oscillation means and having a frequency different from that of the carrier wave of the input signal; and

a step of stopping said carrier reproducing loop for a preset period when an abnormal synchronization is detected, causing said oscillation means to oscillate at a frequency extremely close to that of the carrier wave of the input signal, and driving said carrier reproducing loop, thereby achieving normal synchronization.