JP2012094992A - Base station - Google Patents

Abstract

PROBLEM TO BE SOLVED: To correct a deviation of timing while keeping synchronization with a communication terminal when a reference signal is restored.SOLUTION: A base station comprises: a timing generation unit 114 that generates a timing signal about which a clock signal of an oscillator 113 is synchronized with a reference signal obtained from GPS satellites, and that lets a timing signal run freely when hold-over occurs, and that generates a timing teacher signal about which a clock signal of the oscillator 113 is synchronized with a restored reference signal when the reference signal is restored; a synchronization unit 121 that outputs a transmission signal based on a timing signal; a GI addition unit 124 that adds a guard interval (GI) to a transmission signal; and a timing control unit 115 that adjusts the cycle of a timing signal and the length of a GI such that a deviation between the timing signal and a timing teacher signal will become smaller when a reference signal is restored.

Description

  The present invention relates to a base station that transmits a transmission signal based on a timing signal synchronized with a reference signal acquired from a radio wave of a GPS satellite.

  Conventionally, a method using a GPS receiver that receives a signal from a GPS (Global Positioning System) satellite is known in order to synchronize the frame timing between base stations. The GPS receiver extracts a 1 pps (Pulse Per Second) reference signal synchronized with UTC (Coordinated Universal Time) from the GPS signal received from the GPS satellite, and the base station performs data synchronization with this reference signal. Send. However, the base station may not be able to accurately receive the radio wave from the GPS satellite due to an interference wave or the like, or the radio wave may not be transmitted from the GPS satellite, and the reference signal may not be captured. This state in which the reference signal cannot be captured is called holdover.

  Even when such a holdover occurs, a high-precision oscillator is used together so that a signal synchronized with the reference signal can be output, and the clock signal output from the oscillator is divided to have the same frequency and phase as the reference signal. There is known a clock synchronization method for outputting a signal synchronized with the received signal (see, for example, Patent Document 1).

JP 2001-339373 A

  However, the conventional base station can maintain the frequency accuracy by causing the internal oscillator to self-run when a holdover occurs, but the frame timing caused by letting the oscillator self-run for a long time can be maintained. The shift could not be corrected automatically. If this deviation is within an allowable range, the base station can continue to operate, but if the reference signal recovers from holdover during this time, it is desirable to return to the original frame timing.

  FIG. 5 is a flowchart showing the operation of the conventional base station when the reference signal is restored. The conventional base station determines whether or not the reference signal has recovered from the holdover (step S201). If the reference signal recovers from the holdover, is there a difference between the current frame timing and the original frame timing? It is determined whether or not (step S202). If there is a frame timing shift, the base station performs a transition procedure to the non-operational state (step S203), changes to the non-operational state (step S204), and then corrects the frame timing (step S205). ), The operation state is changed again (step S206). Although this restoration method can return to the original frame timing at once, there is a problem that communication between the base station and the communication terminal is once interrupted, so that the communication terminal needs to re-synchronize the frame.

  In order to solve the above problem, an object of the present invention is to provide a base station capable of correcting a frame timing shift while maintaining synchronization with a communication terminal when a reference signal is restored.

  In order to solve the above problems, a base station according to the present invention is a base station that transmits a transmission signal based on a timing signal synchronized with a reference signal acquired from a radio wave of a GPS satellite, and is output from an oscillator. A timing signal is generated by synchronizing a clock signal with a reference signal acquired from a radio wave of a GPS satellite, and when a holdover occurs, the timing signal is caused to self-run without being synchronized with the reference signal. When the reference signal recovers from holdover, a timing generator that generates a timing teaching signal synchronized with the reference signal recovered from the clock signal output from the oscillator, and outputs a transmission signal based on the timing signal And a guard for adding a guard interval to the transmission signal. When the interval adding unit and the reference signal recover from holdover, the guard interval adding unit detects the amount of deviation of the self-running timing signal from the timing teacher signal and reduces the amount of deviation. And a timing control unit that adjusts the length of the guard interval to be performed.

  Further, in the base station according to the present invention, the timing control unit may perform timing based on the synchronization unit such that the larger the deviation amount, the longer the time until the deviation amount becomes less than one sample length. The period of the signal and the length of the guard interval added by the guard interval adding unit are adjusted.

  Further, in the base station according to the present invention, the timing control unit determines the number of symbol signals whose length of the guard interval is adjusted per period of the timing signal and the amount of adjustment of the guard interval length. Features.

  In the base station according to the present invention, the timing control unit determines a symbol period of a symbol signal in which a length of the guard interval is adjusted and an adjustment amount of the guard interval length.

  According to the present invention, when the reference signal recovers from the holdover, the frame timing shift due to the holdover can be corrected while maintaining the synchronization with the communication terminal by adjusting the length of the guard interval. It becomes like this.

It is a block diagram which shows the structure of the base station of one Embodiment by this invention. It is a figure which shows the OFDM symbol signal by an OFDM modulation system. It is a flowchart which shows the operation | movement at the time of the reference signal recovery | restoration of the base station of one Embodiment by this invention. It is a timing chart at the time of reference signal restoration of a base station of one embodiment by the present invention. It is a flowchart which shows the operation | movement at the time of the reference signal recovery | restoration of the conventional base station.

  Hereinafter, an embodiment of a base station according to the present invention will be described in detail with reference to the drawings.

  FIG. 1 is a block diagram illustrating a configuration of a base station according to an embodiment of the present invention. In this embodiment, an OFDM base station will be described as an example. The OFDM base station 1 includes a frame synchronization unit 11 and an OFDM signal generation unit 12. The frame synchronization unit 11 includes a GPS reception unit 111, an oscillator control unit 112, an oscillator 113, a timing generation unit 114, and a timing control unit 115. The OFDM signal generation unit 12 includes a synchronization unit 121, a mapping unit 122, an IFFT (Inverse Fast Fourier Transform) unit 123, a GI addition unit 124, and a D / A (Digital / Analog). ) A conversion unit 125, a transmission unit 126, and an antenna 127 are provided.

  The GPS receiver 111 receives a GPS satellite radio wave, outputs a reference signal (1 pps signal) acquired from the GPS satellite radio wave to the oscillator control unit 112 and the timing generation unit 114, and captures the reference signal. A lock signal indicating this is output to the timing control unit 115.

  The oscillator control unit 112 outputs an oscillator control signal for controlling the oscillator 113 to the oscillator 113.

  The oscillator 113 outputs an internal clock signal to the timing generation unit 114 based on the oscillator control signal input from the oscillator control unit 112. The oscillator 113 is configured by, for example, OCXO (Oven Controlled Crystal Oscillator).

  The timing generation unit 114 divides the internal clock signal input from the oscillator 113 and outputs a timing signal synchronized with the reference signal input from the GPS reception unit 111 to the timing control unit 115 and the synchronization unit 121.

  When a holdover occurs, the timing generator 114 outputs a self-running timing signal that is not synchronized with the reference signal. Thereafter, when the reference signal recovers from the holdover, the timing generation unit 114 outputs a timing teaching signal synchronized with the reference signal (that is, the original frame timing) to the timing control unit 115.

  The timing control unit 115 determines whether or not a holdover has occurred based on a lock signal input from the GPS reception unit 111. When the timing control unit 115 detects that the holdover has occurred, the timing control unit 115 waits for the reference signal to recover from the holdover, and at the time of recovery of the reference signal, the timing signal in the free-running state input from the timing generation unit 114 A deviation amount with respect to the timing teacher signal input from the timing generation unit 114 is detected. Then, the timing control unit 115 generates a GI sample number control signal in order to control a synchronization unit 121 and a GI addition unit 124, which will be described later, so that the shift amount becomes small.

  The synchronization unit 121 outputs a transmission signal to the mapping unit 122 in synchronization with the timing signal input from the timing generation unit 114. The synchronization unit 121 adjusts the period of the timing signal according to the GI sample number control signal input from the timing control unit 115 when the reference signal is recovered from the holdover.

  Mapping section 122 maps the transmission signal input from synchronization section 121 to the IQ plane according to a predetermined modulation scheme for each subcarrier, and outputs the mapping signal to IFFT section 123.

  IFFT section 123 outputs an effective symbol signal generated by performing inverse fast Fourier transform processing on the mapping signal input from mapping section 122 to GI adding section 124.

  The GI addition unit 124 outputs an OFDM symbol signal to which the guard interval obtained by copying the latter half of the effective symbol signal is added to the head of the effective symbol signal input from the IFFT unit 123 to the D / A conversion unit 125. The guard interval is inserted in order to reduce interference between symbols when receiving an OFDM signal.

  FIG. 2 is a diagram illustrating an OFDM symbol signal according to the OFDM modulation scheme. The OFDM signal is transmitted in symbol units called OFDM symbol signals. The OFDM symbol signal includes an effective symbol signal output from the IFFT unit 123 and a guard interval obtained by copying the waveform of the latter half of the effective symbol as it is. The guard interval length is set to a time range in which a multipath delayed wave exists, and is the length of the number of samples predetermined by the system. In the following description, it is assumed that a predetermined guard interval length is N sample lengths.

  An OFDM signal receiver calculates an autocorrelation value between an OFDM symbol signal and a signal obtained by delaying the OFDM symbol signal by an effective symbol length. Since the guard interval has the same waveform and the autocorrelation value is maximized in the guard interval part, the OFDM signal receiver detects the symbol timing (FFT window) by detecting the maximum value of the autocorrelation value.

  The GI adding unit 124 adjusts the guard interval length according to the GI sample number control signal input from the timing control unit 115 when the reference signal is recovered from the holdover.

  The D / A conversion unit 125 converts the digital signal input from the GI addition unit 124 into an analog signal, and outputs the analog signal to the transmission unit 126.

  The transmission unit 126 converts the analog signal input from the D / A conversion unit 125 into a transmission frequency and outputs a signal obtained by power amplification to the antenna 127.

  The antenna 127 radiates the signal input from the transmission unit 126 into the air.

  Next, an operation when the reference signal is restored will be described. FIG. 3 is a flowchart showing the operation of the OFDM base station 1 according to this embodiment when the reference signal is restored.

  In the OFDM base station 1, the timing control unit 115 determines whether or not the reference signal has recovered from holdover (step S101). When the reference signal recovers from holdover, the timing signal in the free-running state is timed. It is determined whether or not there is a deviation from the teacher signal (step S102).

  FIG. 4 is a timing chart when the reference signal is restored in the base station of the OFDM system 1 according to the present embodiment. The timing signal output from the timing signal generation unit 114 is in a free-running state because it cannot be synchronized with the reference signal due to the occurrence of holdover. The timing teacher signal output from the timing signal generation unit 114 is a signal indicating the original frame timing synchronized with the reference signal.

  If the timing control unit 115 determines that the timing signal is deviated from the timing teacher signal, the OFDM base station 1 detects the frame timing deviation a shown in FIG. The GI sample number control signal is generated in order to make the timing signal close to the timing teaching signal synchronized with the reference signal (step S103). FIG. 4 shows a state in which a shift amount a occurs between the timing signal and the timing teacher signal when the reference signal is restored, and the shift amount gradually decreases.

  In the OFDM base station 1, the GI adding unit 124 adjusts the guard interval length based on the GI sample number control signal (that is, adjusts the OFDM symbol length), and the synchronization unit 121 adjusts the timing based on the GI sample number control signal. The period of the signal is adjusted (step S104).

  When the frame timing deviation amount a is M sample length, the guard interval length is changed from N sample length to (N ± M) sample length, and the timing signal period is changed by ± M sample length, The timing signal can be matched with the timing teacher signal. However, if the guard interval length is changed at a stretch, the autocorrelation value calculated when the symbol timing is detected on the receiving side of the OFDM signal will be affected. Further, if the guard interval length is shortened at a stretch, there is a possibility that a failure due to a delayed wave (multipath wave) occurs on the receiving side of the OFDM signal.

  Therefore, in order to return the timing signal to the original frame timing slowly, the timing control unit 115 increases the time until the shift amount “a” becomes less than one sample length as the shift amount “a” increases. 121 adjusts the period of the timing signal based on 121 and the length of the guard interval added by the GI adding unit 124.

  Specifically, the timing control unit 115 determines the number of OFDM symbol signals whose guard interval length per period of the timing signal is adjusted, and the amount of adjustment of the guard interval length. For example, when the shift amount a is 10 samples long and the timing signal is delayed with respect to the timing teaching signal, the timing control unit 115 adds one OFDM symbol signal to the guard interval length per cycle of the timing signal. Is set to (N + 1) sample length, and this is repeated 10 times. By this processing, the phase of the timing signal coincides with the timing teacher signal after 10 cycles (the shift amount becomes less than one sample length). In addition, when the timing control unit 115 controls one OFDM symbol signal so that the guard interval length becomes (N + 2) sample length per cycle of the timing signal, the timing signal phase is set to the timing teacher after 5 cycles. Match the signal.

  Note that, among the plurality of OFDM symbol signals (# 0, # 1,..., #N) included in one period of the timing signal, which OFDM symbol signal has a variable guard interval length is determined in advance. Alternatively, it may be designated by a GI sample number control signal.

  In addition, the timing control unit 115 may determine the symbol period of the OFDM symbol signal for which the guard interval length is controlled and the adjustment amount of the guard interval length. For example, when the timing shift amount a is 10 samples long and the timing signal is delayed with respect to the timing teaching signal, the timing control unit 115 performs 100 symbol periods, that is, one OFDM symbol signal per 100 OFDM symbol signals. Is controlled so that the guard interval length becomes (N + 1) sample length, and this is repeated 10 times, thereby matching the phase of the timing signal with the timing teacher signal.

  Thus, according to the OFDM base station 1 of the present embodiment, the timing controller 115 calculates the amount of timing deviation when the reference signal is restored, and adjusts the guard interval length and the timing signal period, The frame timing shift due to holdover can be corrected while maintaining synchronization with the communication terminal. Such an OFDM base station 1 can be implemented easily and without increasing cost because there are few changes from the conventional OFDM base station.

  Also, according to the OFDM base station 1 of the present embodiment, the timing controller 115 slowly returns the frame timing to the original timing, thereby improving the tolerance to delayed waves on the receiving side of the OFDM signal and autocorrelation. The influence on the value can be minimized.

  Although the above embodiments have been described as representative examples, it will be apparent to those skilled in the art that many changes and substitutions can be made within the spirit and scope of the invention. Therefore, the present invention should not be construed as being limited by the above-described embodiments, and various modifications and changes can be made without departing from the scope of the claims. In the above-described embodiment, the OFDM scheme has been described as an example. However, the present invention is not limited to the OFDM scheme, and may be anything as long as it uses a radio signal including a guard interval.

1 OFDM base station 11 Frame synchronization unit 12 OFDM signal generation unit 111 GPS reception unit 112 Oscillator control unit 113 Oscillator 114 Timing generation unit 115 Timing control unit 121 Synchronization unit 122 Mapping unit 123 IFFT unit 124 GI addition unit 125 D / A Conversion unit 126 Transmission unit

Claims (4)

A base station that transmits a transmission signal based on a timing signal synchronized with a reference signal acquired from a radio wave of a GPS satellite,
A timing signal is generated by synchronizing a clock signal output from an oscillator with a reference signal acquired from a radio wave of a GPS satellite, and when a holdover occurs, the timing signal is not synchronized with the reference signal. A timing generation unit that generates a timing teaching signal that is synchronized with a reference signal that is recovered from a clock signal that is output from the oscillator when the reference signal is recovered from holdover, and is free-running in a state;
A synchronization unit that outputs a transmission signal based on the timing signal;
A guard interval adding unit for adding a guard interval to the transmission signal;
When the reference signal recovers from holdover, the amount of deviation of the self-running timing signal from the timing teacher signal is detected, and the guard interval addition unit adds a guard interval length so that the amount of deviation is small. A timing control unit for adjusting the thickness,
A base station comprising:   The timing control unit includes a period of the timing signal used as a reference by the synchronization unit and the guard interval adding unit so that the larger the shift amount, the longer the time until the shift amount becomes less than one sample length. The base station according to claim 1, wherein the length of the guard interval added by is adjusted.   The timing control unit according to claim 2, wherein the timing control unit determines the number of symbol signals whose length of the guard interval is adjusted per period of the timing signal and an adjustment amount of the guard interval length. base station. The base station according to claim 2, wherein the timing controller determines a symbol period of a symbol signal in which a length of the guard interval is adjusted and an adjustment amount of the guard interval length.

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