JP4375032B2 - QAM transmission system and QAM receiver - Google Patents

Description

  The present invention relates to a QAM (Quadrature Amplitude Modulation) transmission system and a QAM receiver, and in particular, a QAM transmission system that switches between a current QAM transmitter and a spare QAM transmitter, and a transmission sent from the QAM transmission system. The present invention relates to a QAM receiving apparatus that receives a signal.

  In a QAM transmission system that switches between the active system and the standby system in hot standby, in order to reduce errors on the receiving side that occur when switching the transmission system, the data and clock phase of the active and standby modulation waves are reduced. The instantaneous interruption time of the modulated wave is shortened by switching at a high speed with the changeover switch. Further, the QAM receiver side measures such as correcting an error that has occurred by an error correction circuit.

  However, since it is extremely difficult to match the phase difference between the active system and the standby system waves between the active system and the standby system, the carrier phase suddenly changes during switching. In QAM, particularly multi-level QAM, when the phase of the regenerative carrier is largely shifted due to a sudden change in the carrier phase, it takes time to correct the phase error because the detection gain of the carrier phase error is low. For this reason, various contrivances have been made to shorten the synchronization recovery on the QAM receiver side as much as possible.

  For example, in Patent Document 1, a C / N (signal-to-noise ratio) value of a line is obtained, and when it is determined that the frequency difference between a received carrier wave and a regenerated carrier wave is close based on the C / N value, a carrier sweep circuit is instantaneously obtained. And a QAM demodulation system is disclosed in which the reproduction carrier is synchronized with the received carrier.

  Further, in Patent Document 2, in order to synchronize the frequency of the reception carrier wave and the regenerated carrier wave of the quadrature amplitude modulation signal having a large multi-value number, the reception symbol point having the largest amplitude value or the reception having the second largest amplitude value is disclosed. A technique for establishing frequency synchronization in a short time by rotating the orthogonal coordinate axis so that the received symbol point is moved on a straight line connecting the origin O and the ideal symbol point having the maximum symbol point when the symbol point is received. It is disclosed.

Japanese Patent Laid-Open No. 2002-118614 (FIG. 1) JP-A-9-238173 (FIGS. 1 and 53)

  However, the conventional known technique can only shorten the synchronization recovery on the receiving device side as much as possible. That is, when a transmission signal sent from a QAM transmission system that switches between the active system and the standby system is received, the receiver apparatus operates so as to attempt synchronization recovery from the signal after switching. Therefore, it takes a long time to correct the phase error, and there is a possibility that the error time increases and the frame synchronization is lost.

  An object of the present invention is to provide a QAM transmitting system and a QAM receiving apparatus in which a QAM receiving apparatus that receives a transmission signal transmitted from a QAM transmission system that switches between an active system and a standby system does not lose frame synchronization at the time of switching. It is in.

In order to achieve the above object, according to a first aspect, a QAM communication system according to the present invention adds an error correction code to transmission data and performs error correction at a specific position in a frame constituting the error correction code. Two QAM transmission apparatuses are provided that insert a signal of a specific symbol of QAM into transmission data to which a code is added and transmit a signal with the specific symbol inserted. Further, the transmission signals from the two QAM transmission apparatuses are switched by the switching signal. Furthermore, a QAM receiving apparatus that performs an operation of obtaining a phase difference between two adjacent specific symbols included in a signal received from the QAM transmitting apparatus and correcting the phase difference is provided.

In addition, according to the second aspect, the QAM communication system according to the present invention is a signal transmitted from the active QAM transmitter, the spare QAM transmitter, the active QAM transmitter, and the spare QAM transmitter. And a QAM receiver . The QAM transmission apparatus supports an error correction encoder that adds an error correction code to transmission data, and a QAM specific symbol in transmission data to which the error correction code is added at a specific position in a frame constituting the error correction code. A specific symbol signal inserter that inserts a signal to be transmitted, a modulator that modulates the signal in which the specific symbol is inserted by QAM, and a transmitter that transmits the modulated signal. Further, the QAM receiving apparatus calculates the phase difference between two adjacent specific symbols included in the signal received from the QAM transmitting apparatus, and performs an operation of correcting the phase difference.

  In the present invention, preferably, a signal of a specific symbol may be inserted at a specific position at least after the switching time.

  In the present invention, preferably, a signal of a specific symbol may be inserted at a specific position before and after the switching point.

  Furthermore, in the present invention, it is preferable that the signal transmitted from the QAM transmission apparatus may be switched so that the switching point comes after a predetermined time has elapsed after the signal of the specific symbol is inserted.

  In the present invention, preferably, the number of insertions and insertion time of a signal of a specific symbol in one frame may be within a range that can be corrected when an error correction code is decoded.

  Further, in the present invention, it is preferable that the specific symbol may be a symbol corresponding to one of four signal points generating the maximum amplitude of QAM.

  In addition, according to the third aspect, the QAM receiver according to the present invention obtains a demodulated baseband signal by orthogonally demodulating the received signal with the local oscillation output signal, and obtains a normal signal on the phase plane of the demodulated baseband signal. The present invention is applied to a QAM receiving apparatus that controls a local oscillation output signal based on a phase error from a point and determines QAM data from a demodulated baseband signal and outputs demodulated data. It is always detected that a signal of a specific symbol is inserted into a QAM signal point at a specific position in a frame of an error correction code detected from demodulated data, and the detected signal point of the specific symbol is detected from a corresponding normal signal point. If shifted, the phase error is corrected so as to correct the shift.

  Furthermore, according to the fourth aspect, the QAM receiver according to the present invention obtains a demodulated baseband signal by orthogonally demodulating the received signal with the local oscillation output signal, and obtains a normal signal on the phase plane of the demodulated baseband signal. The present invention is applied to a QAM receiving apparatus that controls a local oscillation output signal based on a phase error from a point and determines QAM data from a demodulated baseband signal and outputs demodulated data. The QAM receiver includes an error correction circuit that detects a frame of an error correction code from demodulated data, corrects an error in the demodulated data, and outputs the corrected data. Also, a specific symbol for detecting a phase difference between the phase of the signal of the specific symbol of QAM detected at a specific position in the frame of the error correction code and the phase corresponding to the signal of the specific symbol that has been received and fixed A signal detector is provided. Further, a phase shift circuit that calculates a shift amount corresponding to the phase difference detected by the specific symbol signal detector is provided, and the phase error is corrected based on the shift amount.

  In addition, according to the fifth aspect, the QAM receiver according to the present invention performs a complex operation of a received signal separated into two orthogonal components and two local oscillation output signals having a phase difference of 90 °. And a complex multiplier for outputting a demodulated baseband signal composed of two orthogonal signals. In addition, a phase detector that detects a phase error from a normal signal point in a demodulated baseband signal and outputs a carrier phase error signal, a loop filter that extracts and outputs a low-frequency component of the carrier phase error signal, and a loop filter A numerical control transmitter that outputs a phase signal whose rotation speed varies depending on the magnitude of the output signal. Furthermore, a data converter that generates two local oscillation output signals according to the angle of the phase signal is provided. A data determination circuit that determines QAM data from the demodulated baseband signal and outputs the demodulated data; an error correction circuit that detects a frame of an error correction code from the demodulated data and corrects and outputs an error in the demodulated data; Is provided. Furthermore, a specific symbol for detecting a phase difference between a phase of a signal of a specific symbol of QAM detected at a specific position in a frame of an error correction code and a phase corresponding to a signal of a specific symbol that has been received and fixed A signal detector is provided. In addition, a phase shift circuit that calculates a shift amount corresponding to the phase difference detected by the specific symbol signal detector is provided. Further, an adder for adding the shift amount and the angle of the phase signal output from the numerically controlled oscillator and inputting the addition result to the data converter is provided.

In the present invention, the specific position in the QAM receiving apparatus is preferably a specific position in the QAM communication system.

In the present invention, the specific symbol in the QAM receiving apparatus is preferably a specific symbol in the QAM communication system.

In the present invention, QAM receiver device is preferably configured to receive a transmission signal transmitted from the QAM transmission system.

  According to the QAM transmission system and the QAM receiving apparatus according to the present invention, in the QAM receiving apparatus, the phase of the QAM signal point after switching the QAM transmitting apparatus is grasped at a specific position and the phase of the received signal is corrected. It is possible to prevent out of sync.

  Next, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram of a QAM transmission system according to an embodiment of the present invention. In FIG. 1, the QAM transmission system uses a switching signal 23 to transmit transmission waves output from a working QAM transmitting apparatus 10a, a spare QAM transmitting apparatus 10b, a working QAM transmitting apparatus 10a, and a spare QAM transmitting apparatus 10b. And a switching device 15 for switching.

  The spare QAM transmitter 10b is switched when a reason, for example, a failure occurs in the active QAM transmitter 10a. The active QAM transmitter 10a and the spare QAM transmitter 10b are: It is the same configuration. Therefore, only the QAM transmitter 10a will be described.

  The QAM transmission apparatus 10a includes an error correction encoder 11, a specific symbol signal inserter 12, a modulator 13, and a transmitter 14. A frame signal 21 and a data signal 22 are input to the error correction encoder 11, and the frame signal 21 and an error correction code synchronized with the frame signal 21 are inserted into the input data signal 22 to insert a specific symbol signal. Is output to the device 12.

  The specific symbol signal inserter 12 receives the data signal 22, the frame signal 21, and the switching signal 23 into which the error correction code is inserted. The specific symbol signal inserter 12 inserts a QAM specific symbol signal into the transmission data input to the specific symbol signal inserter 12 at a specific position in the frame signal 21 constituting the error correction code. Note that this specific position is a predetermined position in the frame signal 21 and is provided at least one place in one frame. In addition, the number of insertions or insertion time of a signal of a specific symbol in one frame is determined to be within a range that can be corrected when an error correction code is decoded.

  It should be noted that the insertion of a specific symbol signal may always be performed regardless of switching between the active / standby QAM transmitters. In other words, the occurrence of an error due to the insertion of a signal of a specific symbol is within the range that can be corrected when decoding, and thus can always be corrected by the QAM receiver. Further, a signal of a specific symbol may be inserted before and after switching. However, in order to quickly recover the synchronization in the QAM receiver, it is necessary that a signal of a specific symbol is inserted at least after switching.

  In general, the generation time of the switching signal 23 can be changed to some extent. In this case, it is desirable to input the switching signal 23 after a predetermined time has elapsed after inserting the signal of the specific symbol. That is, by allowing a predetermined time to elapse, it is possible to ensure the phase detection of the signal of the inserted specific symbol in the QAM receiver.

  The modulator 13 modulates the signal output from the specific symbol signal inserter 12 by QAM.

  The transmitter 14 outputs the modulation signal output from the modulator 13 to the switch 15 as a transmission wave signal.

  The switch 15 switches the transmission wave signal output from the active QAM transmitter 10a and the spare QAM transmitter 10b by the switch signal 23 and outputs it from the output terminal 24.

  Next, a QAM receiving apparatus that receives a QAM signal will be described. The QAM receiver according to the embodiment of the present invention obtains a demodulated baseband signal by orthogonally demodulating a received signal with a local oscillation output signal, and generates a phase error from a normal signal point on the phase plane of the demodulated baseband signal. In addition, the present invention is applied to a QAM receiving apparatus that controls the local oscillation output signal and determines QAM data from the demodulated baseband signal and outputs demodulated data.

  In the QAM receiver as described above, it is always detected that a signal of a specific symbol is inserted into a QAM signal point at a specific position in a frame of an error correction code detected from demodulated data. If the signal point deviates from the corresponding normal signal point, the phase error is corrected so as to correct the deviation.

  Next, the QAM receiver will be described in more detail. FIG. 2 is a block diagram of the QAM receiving apparatus according to the embodiment of the present invention. In FIG. 2, the QAM receiver includes multipliers 31 and 32, local oscillator 33, phase shifter 34, analog / digital converters 35 and 36, low-pass filters (digital LPF) 37 and 38, complex multiplier 39, and clock recovery. A circuit 40, a data determination circuit 41, an error correction circuit 42, a phase detector 43, a loop filter 44, a numerical control transmitter 45, a specific symbol signal detector 46, a phase shift circuit 47, an adder 48, and a data converter 49 are provided. .

  A signal transmitted from the QAM transmission system is received and input from the signal input terminal 30 as a multilevel QAM modulated wave. At this time, for example, if the received signal is an RF signal, the signal is converted into an IF frequency and input. A signal 130 which is a multi-level QAM modulated wave is supplied to multipliers 31 and 32. The signal 130 input to the multiplier 31 is multiplied in the multiplier 31 by the carrier signal 133 input from the local oscillator 33 that oscillates at a fixed frequency. The signal 130 input to the multiplier 32 is multiplied by the multiplier 32 with the carrier signal 134 output from the local oscillator 33 and given a phase difference of 90 ° by the phase shifter 34.

  Further, the output signals 131 and 132 of the multipliers 31 and 32 are converted into digital signals 135 and 136 by analog / digital converters 35 and 36, respectively, and then spectrally shaped by digital low-pass filters 37 and 38, respectively. , 137 and 138 are input to the complex multiplier 39. The complex multiplier 39 performs complex multiplication of the signals 137 and 138 input from the low-pass filters 37 and 38 and the local oscillation output signals 149 and 152 from the data converter 49, and outputs demodulated baseband signals 139 and 151. To do. In the complex multiplication, for example, if the values of the signals 139, 151, 137, 138, 149, 152 are expressed as X, Y, U, V, sin θ, cos θ, X = U cos θ + V sin θ and Y = −U sin θ + V cos θ are calculated.

  Demodulated baseband signals 139 and 151 are distributed into four. One is supplied to the clock recovery circuit 40 to extract the symbol timing component in the signal and feed back to the input of the analog / digital converters 35 and 36 as the converted clock signal 140 of the analog / digital converters 35 and 36.

  The demodulated baseband signals 139 and 151 are input to the data determination circuit 41. The data determination circuit 41 determines and demodulates QAM data from the demodulated baseband signals 139 and 151, and outputs the demodulated data 141 to the error correction circuit 42. The demodulated data 141 input to the error correction circuit 42 is output from the output terminal 50 as output data 150 after error correction.

  Further, the demodulated baseband signals 139 and 151 are input to the phase detector 43. The phase detector 43 detects a phase error from the normal signal point and outputs a carrier phase error signal 143. The carrier phase error signal 143 is input as a signal 144 to the numerical control transmitter 45 via the loop filter 44. The numerical control transmitter 45 outputs a rotating phase signal 145 whose rotation speed varies depending on the magnitude of the input signal 144. That is, it operates like a voltage controlled oscillator (VCO) in an analog circuit. The output signal 145 of the numerical control transmitter 45 is added to the output signal 147 of the phase shift circuit 47 in the adder 48 and then input to the data converter 49 as the signal 148. The data converter 49 outputs the values of sin θ and cos θ as the local oscillation output signals 149 and 152 to the complex multiplier 39 with the input signal 148 as the angle θ.

  Further, the demodulated baseband signals 139 and 151 are input to the specific symbol signal detector 46. The specific symbol signal detector 46 obtains a specific position from the frame period signal 142 of the error correction code output from the error correction circuit 42. If a signal of a specific symbol is detected at a specific position, the specific symbol signal is extracted, carrier phase detection is performed, and a control signal 146 for correcting the carrier phase error is output to the phase shift circuit 47. The phase shift circuit 47 outputs a carrier phase error signal 147 to the adder 48 based on the control signal 146. In the adder 48, the carrier phase error is corrected by adding the carrier phase error signal 147 to the phase signal 145 of the reproduced carrier.

  Here, the specific position and the specific symbol are determined in advance and coincide with the specific position and the specific symbol in the transmission signal transmitted from the QAM transmission system.

  The QAM transmission system and the QAM receiver according to the embodiment of the present invention are configured as described above. In the QAM transmission system, when the active QAM transmission apparatus 10a and the spare QAM transmission apparatus 10b are switched by the switching signal 23, QAM signal points are transmitted as specific symbol signals at least at a specific position after switching. On the other hand, the QAM receiver side always recognizes that a specific symbol signal is inserted into a QAM signal point at a specific position during frame synchronization, and if the phase of the signal point of the specific symbol is shifted, Operates to correct the phase. Therefore, synchronization can be recovered in a short time from the loss of synchronization that occurs after switching, synchronization in units of frames can be maintained, and loss of frame synchronization can be prevented.

  Next, embodiments of the present invention will be described with reference to the drawings. FIG. 3 is a block diagram showing the configuration of the specific symbol signal inserter according to the embodiment of the present invention. The specific symbol signal inserter 12 includes a specific position detection circuit 51 and an outermost angle signal insertion circuit 52. The frame signal 21 and the switching signal 23 are input to the specific position detection circuit 51. The specific position detection circuit 51 detects a specific position in the frame signal 21. In addition, the specific position in the frame signal 21 after the switching signal 23 is input is detected. Then, the outermost angle signal insertion circuit 52 inserts a signal of a specific symbol into the signal output from the error correction encoder 11 at the specific position output from the specific position detection circuit 51 and outputs the signal to the modulator 13. At this time, at least a specific symbol signal is inserted at a specific position after the switching signal 23 is input.

  Next, the specific symbol will be described. FIG. 4 is a diagram showing symbol points in the case of 64QAM. When the QAM is 64QAM, 64 symbols are arranged in a lattice pattern on the phase plane of the IQ axis as shown in FIG. The outermost angle signal insertion circuit 52 inserts a signal of a specific symbol, and this specific symbol is one of the four points A, B, C, and D (outermost angle signal) shown in FIG. 4 having the maximum amplitude on the phase plane. It is desirable. This is because when the received signal has a phase shift, that is, when the four points A, B, C, and D are rotated on the phase plane, it is most easily discriminated when identifying the original symbol before the rotation.

  Next, the specific position will be described. FIG. 5 is a diagram illustrating an example of the insertion position of the specific symbol according to the embodiment of the present invention. FIG. 5 shows an example where there are three specific positions (R1, R2, R3) in the frame period. An error occurs in the QAM receiving apparatus due to the insertion of a specific symbol. In this case, the number of insertion points in one frame and the insertion time of a signal of a specific symbol are within a range that can be corrected by the error correction code.

  In FIG. 5, when transmitting transmission data P, a signal of a specific symbol (outermost angle signal) is inserted at the position of R1. The phase of the signal of the transmission data P and the phase of the signal of the specific symbol inserted at the position R1 are the same phase, and the QAM receiving apparatus continues the synchronization state.

  After that, it is assumed that the switch 15 switches the transmission signal from the current QAM transmission apparatus 10a and the transmission signal from the spare QAM transmission apparatus 10b. Transmission data Q having a carrier phase different from that of transmission data P is transmitted by transmission switching. At that time, in the QAM receiving apparatus, due to the sudden change of the phase, in most cases, the synchronization is lost and the asynchronous state is set.

  After that, when transmitting transmission data Q, a signal of a specific symbol (outermost angle signal) is inserted at the position of R2. The QAM receiver knows in advance which specific symbol is inserted at which position in the frame period. Therefore, the signal of the specific symbol can be received at the position of R2, and the difference between the phase of the signal of the specific symbol at the position of the transmission data P or R1 and the phase of the signal of the specific symbol inserted at the position of R2 Can be requested. By receiving the transmission data Q by correcting the phase of the signal of the transmission data Q using the obtained phase difference, the QAM receiving apparatus recovers the synchronization after the position of R2 and enters a synchronization state.

  Thereafter, when transmitting transmission data Q, a signal of a specific symbol is inserted at the position of R3. The phase of the signal of the transmission data Q and the phase of the signal of the specific symbol inserted at the position R3 are the same phase, and the QAM receiving apparatus continues the synchronization state.

  Next, the specific symbol signal detector 46 of the QAM receiver will be described. FIG. 6 is a block diagram illustrating a specific symbol signal detector according to an embodiment of the present invention. In FIG. 6, the specific symbol signal detector 46 includes a phase detection circuit 61, a specific position detection circuit 62, and a phase difference detection circuit 63. The phase detection circuit 61 receives the output signals 139 and 151 from the complex multiplier 39 and outputs the phase. The frame signal 142 output from the error correction circuit 42 is input to the specific position detection circuit 62, and specific positions (R1, R2, R3 in the example of FIG. 5) within the frame period are detected based on the frame signal 142. Is done.

  The phase difference detection circuit 63 receives the phase output from the phase detection circuit 61 at the specific position detected by the specific position detection circuit 62. Furthermore, the difference between the phase that has been normally received as a synchronized state and the detected phase is detected. The detected phase difference signal 146 is output to the phase shift circuit 47.

  Referring to the example of FIG. 5 above, when receiving a transmission data P, if a signal of a specific symbol (outermost angle signal) is received at the position of R1, the phase of the signal of transmission data P and the position of R1 Therefore, the phase difference detection circuit 63 outputs that the phase difference is almost zero.

  Then, it is assumed that switching is performed by transmission switching. As a result of the switching, transmission data Q having a carrier phase different from that of transmission data P is received. At that time, in the QAM receiver described above, in FIG. 2, the complex multiplier 39, the clock recovery circuit 40, the phase detector 43, the loop filter 44, the numerical control transmitter 45, the adder 48, and the data converter 49 transmit. The operation is locked to the phase of the data P. Therefore, when the QAM receiving apparatus receives the transmission data Q, in most cases, the synchronization is lost due to a sudden change in phase, and the QAM receiving apparatus is in an asynchronous state.

  After that, when the transmission data Q is being received, the signal of the specific symbol (outermost angle signal) is received at the position of R2. QAM receiver, since any particular symbol know whether it is inserted in advance which position, by receiving the signal of a particular symbol at the position of R2, the signal of a particular symbol at the position of the transmission data P or R1 The difference between the phase and the phase of the signal of the specific symbol inserted at the position of R2 can be obtained. The phase difference detection circuit 63 outputs the obtained phase difference to the phase shift circuit 47 so that the transmission data Q can be received by correcting the phase of the transmission data Q signal. Therefore, the QAM receiving apparatus recovers synchronization and enters a synchronized state.

  This is shown in FIG. FIG. 7 is a diagram illustrating how the outermost angle signal of QAM rotates. In FIG. 7, it is assumed that the outermost angle signal in the first quadrant corresponding to the phase of the signal of the specific symbol at the position of the transmission data P or R1 is A0. When the signal of the specific symbol is detected as A1 at the position R2, the phase difference φ is obtained by the phase difference detection circuit 63. By correcting the phase difference φ with respect to the reception signal, the transmission data Q can be received correctly.

  Thereafter, when transmitting transmission data Q, a signal of a specific symbol is inserted at the position of R3. The phase of the signal of the transmission data Q and the phase of the signal of the specific symbol inserted at the position R3 are in phase, and the QAM receiver continues to be in a synchronized state.

  As described above, the phase difference detection circuit 63 of the QAM receiver always grasps the QAM signal point at a specific position, and if the phase of the signal point is shifted, the phase difference detection circuit 63 corrects the phase of the received signal. It operates to output to the phase shift circuit 47. The phase shift circuit 47 provides a phase shift in the opposite direction to the shift so that the phase shift is canceled. As a result, the phase of the reproduced carrier matches within the time of one frame with respect to the carrier phase suddenly changed by switching. Therefore, the error occurrence time can be suppressed within one frame, and no loss of frame synchronization occurs.

  There is provided a QAM receiver that does not cause a reception failure due to switching between an active system and a standby system in a QAM transmission system.

1 is a block diagram of a QAM transmission system according to an embodiment of the present invention. It is a block diagram of the QAM receiver which concerns on embodiment of this invention. It is a block diagram of a specific symbol signal inserter according to an embodiment of the present invention. It is a figure showing the symbol point in the case of 64QAM. It is a figure which shows the example of the insertion position of the specific symbol which concerns on the Example of this invention. It is a block diagram of a specific symbol signal detector according to an embodiment of the present invention. It is a figure showing a mode that the outermost angle signal of QAM rotates.

Explanation of symbols

10a, 10b QAM transmitter 11 Error correction encoder 12 Specific symbol signal inserter 13 Modulator 14 Transmitter 15 Switcher 21 Frame signal 22 Data signal 23 Switch signal 24 Output terminal 30 Input terminal 31, 32 Multiplier 33 Local oscillator 34 Phase shifter 35, 36 Analog / digital converter 37, 38 Low pass filter 39 Complex multiplier 40 Clock recovery circuit 41 Data decision circuit 42 Error correction circuit 43 Phase detector 44 Loop filter 45 Numerical control transmitter 46 Specific symbol signal detection Unit 47 phase shift circuit 48 adder 49 data converter 50 output terminal 51, 62 specific position detection circuit 52 outermost angle signal insertion circuit 61 phase detection circuit 63 phase difference detection circuit 130, 137, 138, 144 signal 131, 132, 147 Output signal 133, 34 Carrier signal 135, 136 Digital signal 139, 151 Demodulated baseband signal 140 Conversion clock signal 141 Demodulated data 142 Frame period signal 143, 147 Carrier phase error signal 149, 152 Local oscillation output signal 145 Phase signal 146 Control signal 148 Input signal 150 output data

Claims (13)

An error correction code is added to the transmission data, a signal of a specific symbol of QAM is inserted into the transmission data to which the error correction code is added at a specific position in a frame constituting the error correction code, and the specific symbol is inserted Two QAM transmitters for transmitting the received signals,
The transmission signals from the two QAM transmission devices are configured to be switched by a switching signal ,
QAM communication system characterized in that it comprises the search of the phase difference of two adjacent specific symbol contained in a signal received from the QAM transmission system, QAM receiving apparatus that performs an operation to correct the phase difference. A working QAM transmitter;
A spare QAM transmitter;
A switch for switching signals transmitted from the active QAM transmitter and the spare QAM transmitter by a switching signal ;
A QAM receiver;
A QAM transmission system comprising:
The QAM transmitter is
An error correction encoder for adding an error correction code to transmission data;
A specific symbol signal inserter for inserting a signal corresponding to a specific symbol of QAM to transmission data to which the error correction code is added at a specific position in a frame constituting the error correction code;
A modulator for modulating the signal in which the specific symbol is inserted with QAM;
A transmitter for transmitting the modulated signal;
Equipped with a,
The QAM communication system characterized in that the QAM receiving apparatus obtains a phase difference between two adjacent specific symbols included in a signal received from the QAM transmitting apparatus and corrects the phase difference . 3. The QAM communication system according to claim 1, wherein a signal of the specific symbol is inserted at the specific position at least after the switching time point. The QAM communication system according to claim 1, wherein a signal of the specific symbol is inserted at the specific position before and after the switching time. 5. The QAM communication system according to claim 4, wherein a signal transmitted from the QAM transmission apparatus is switched so that the switching time point comes after a predetermined time has elapsed after inserting the signal of the specific symbol. The number of insertions and the insertion time of the signal of the specific symbol in one frame are within a range that can be corrected when the error correction code is decoded. The QAM communication system as described. The QAM communication system according to any one of claims 1 to 6, wherein the specific symbol is a symbol corresponding to one of four signal points generating the maximum amplitude of QAM. The received signal is orthogonally demodulated with the local oscillation output signal to obtain a demodulated baseband signal, and the local oscillation output signal is controlled based on the phase error from the normal signal point on the phase plane of the demodulated baseband signal, In a QAM receiver that determines QAM data from the demodulated baseband signal and outputs demodulated data,
It is always detected that a signal of a specific symbol is inserted into a QAM signal point at a specific position in a frame of an error correction code detected from the demodulated data, and the detected normal signal corresponding to the signal point of the specific symbol A QAM receiving apparatus configured to correct the phase error so as to correct a deviation if it deviates from a point. The received signal is orthogonally demodulated with the local oscillation output signal to obtain a demodulated baseband signal, and the local oscillation output signal is controlled based on the phase error from the normal signal point on the phase plane of the demodulated baseband signal, In a QAM receiver that determines QAM data from the demodulated baseband signal and outputs demodulated data,
An error correction circuit for detecting an error correction code frame from the demodulated data and correcting and outputting an error in the demodulated data; and
A specific symbol for detecting a phase difference between a phase of a signal of a specific symbol of QAM detected at a specific position in a frame of the error correction code and a phase corresponding to the signal of the specific symbol that has been received and fixed A signal detector;
A phase shift circuit for calculating a shift amount corresponding to the phase difference detected by the specific symbol signal detector;
With
A QAM receiving apparatus configured to correct the phase error based on the shift amount. A complex multiplier that performs a complex operation on the received signal separated into two orthogonal components and two local oscillation output signals having a phase difference of 90 °, and outputs a demodulated baseband signal composed of the two orthogonal signals; ,
A phase detector that detects a phase error from a normal signal point in the demodulated baseband signal and outputs a carrier phase error signal;
A loop filter that extracts and outputs a low-frequency component of the carrier phase error signal;
A numerically controlled oscillator that outputs a phase signal whose rotational speed changes depending on the magnitude of the signal output from the loop filter;
A data converter for generating the two local oscillation output signals according to the angle of the phase signal;
A data determination circuit for determining QAM data from the demodulated baseband signal and outputting demodulated data;
An error correction circuit for detecting an error correction code frame from the demodulated data and correcting and outputting an error in the demodulated data; and
A specific symbol for detecting a phase difference between a phase of a signal of a specific symbol of QAM detected at a specific position in a frame of the error correction code and a phase corresponding to the signal of the specific symbol that has been received and fixed A signal detector;
A phase shift circuit for calculating a shift amount corresponding to the phase difference detected by the specific symbol signal detector;
An adder that adds the shift amount and the angle of the phase signal output by the numerically controlled oscillator and inputs an addition result to the data converter;
A QAM receiving apparatus comprising: The QAM receiving apparatus according to any one of claims 8 to 10, wherein the specific position is a specific position in the QAM communication system according to any one of claims 1 to 7. The QAM receiver according to any one of claims 8 to 10, wherein the specific symbol is a specific symbol in the QAM communication system according to any one of claims 1 to 7. The QAM receiving apparatus according to any one of claims 8 to 10, which receives a transmission signal transmitted from the QAM transmitting apparatus according to any one of claims 1 to 7.

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