JP2005204079A - Receiving method and apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To receive excellently a spread spectrum signal like a signal for GPS. <P>SOLUTION: In receiving the spread spectrum signal whose tip rate is known or in detecting the spread spectrum signal of a spreading code of a predetermined tip rate, a sampling frequency is adjusted on the basis of the tip rate of the spreading code to sample the sampling frequency almost integer times as the tip rate of the spreading signal, and correlation detection is performed from the sampled signal, so that an error between the integer times of the tip rate of the spread spectrum signal and the sampling frequency disappears and, with respect to the tip rate of the spread spectrum signal, the spread spectrum signal which is received normally with a fixed ratio sampling frequency can be sampled. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a receiving method and a receiving apparatus for receiving a spread spectrum signal, and more particularly, to a receiving method and a receiving apparatus for receiving a signal from a satellite in a so-called GNSS (Global Navigation Satellites System) and calculating its own position and the like. It relates to a suitable thing.

  In recent years, GNSS systems that measure the position of a moving object on the ground using an artificial satellite are becoming widespread. As this GNSS system, for example, there is a global positioning system (hereinafter referred to as GPS). In this GPS system, a GPS receiver that receives signals from GPS satellites receives signals from at least four GPS satellites, calculates the position of the GPS receiver based on the received signals, and It is a basic function to notify to.

  In other words, the GPS receiver demodulates the signal from each GPS satellite to obtain the orbit information of each GPS satellite, and based on the orbit and time information of each GPS satellite and the delay time of the received signal, the GPS receiver Is derived by simultaneous equations. In the GPS system, at least four GPS satellites for obtaining received signals are required because there is an error between the internal time due to the clock provided in the GPS receiver and the time due to the atomic clock provided in the GPS satellite. This is because the pseudo distances from at least four GPS satellites are necessary to calculate the four unknown parameters of the three-dimensional position and the accurate time from which the influence of the above is removed.

  In a GPS system, when a consumer GPS receiver is used, a spread spectrum signal radio wave called a C / A (Clear and Acquisition) code is received from a GPS satellite and a positioning calculation is performed.

  The transmission signal called the L1 band and C / A code has a transmission signal speed, that is, a chip rate of 1.023 MHz, and a pseudo-random noise (PN) having a code length of 1023 such as a so-called Gold code. ) Binary phase shift keying (hereinafter referred to as BPSK modulation method) with respect to a carrier having a frequency of 1575.42 MHz (hereinafter referred to as carrier) using a signal obtained by directly spreading 50 bps data with a sequence spreading code. It is a signal that has been modulated based on. In this case, since the code length is 1023, the C / A code has a spreading code of 1023 chips as one cycle, that is, one cycle = 1 millisecond (msec) as shown in the first stage in FIG. Will be repeated.

The spreading code of this C / A code is different for each GPS satellite, but which GPS satellite uses which spreading code is registered in advance in the GPS receiver.
Further, the GPS receiver can grasp which GPS satellite can receive a signal at that point and at that point by a navigation message described later. Therefore, for example, in the case of three-dimensional positioning, a GPS receiver receives radio waves from at least four or more GPS satellites that can be acquired at that point and at that time, performs spectrum despreading, and performs positioning calculation Thus, the position of itself is calculated.

  One bit of the signal data from the GPS satellite is transmitted for 20 cycles of the spread code, that is, in units of 20 milliseconds, as shown in the second stage in the figure. That is, the data transmission rate is 50 bps as described above. Further, 1023 chips for one cycle of the spread code are inverted when the bit is “1” and when it is “0”.

  Further, the signal from the GPS satellite forms one word with 30 bits, that is, 600 milliseconds, as shown in the third row in FIG. Furthermore, the signal from the GPS satellite forms one subframe in 10 words, that is, 6 seconds, as shown in the fourth row in FIG. In the signal from the GPS satellite, as shown in the fifth row in the figure, a preamble that always has a prescribed bit pattern is added to the first word of one subframe even when data is updated. Inserted, and data is transmitted following this preamble.

  Furthermore, signals from GPS satellites form 5 subframes, ie, 1 frame in 30 seconds. And in the signal from a GPS satellite, the navigation message mentioned above is transmitted in the data unit of this 1 frame.

  The first three subframes of the data of one frame are information specific to GPS satellites called ephemeris information. The ephemeris information includes a parameter for obtaining the orbit of the GPS satellite and a transmission time of the signal from the GPS satellite.

  All GPS satellites use the common time information by providing an atomic clock, and the transmission time of the signal from the GPS satellite included in the ephemeris information is in units of one second of the atomic clock. Further, the spread codes of GPS satellites are generated in synchronization with the atomic clock.

  The trajectory information included in the ephemeris information is updated every few hours, but is the same information until the update is performed. Therefore, the GPS receiver can use the same orbit information accurately for several hours by holding the orbit information included in the ephemeris information in the memory. Note that the signal transmission time from the GPS satellite is updated every 6 seconds as TOW (Time Of Week) information.

  On the other hand, the navigation messages of the remaining two subframes in one frame of data are information transmitted in common from all GPS satellites called almanac information. This almanac information is required for 25 frames in order to acquire all information, and is composed of approximate position information of each GPS satellite, information indicating which GPS satellite can be used, and the like. This almanac information is updated every few days, but is the same information until the update is performed. Therefore, the GPS receiver can use the same information accurately for several days by holding the almanac information in the memory. However, the GPS receiver can use the same almanac information for several months, although the accuracy is somewhat reduced.

  In order to receive the signal from the GPS satellite and obtain the above-mentioned data, the GPS receiver first removes the carrier and then uses the same spreading code as the C / A code used by the GPS satellite to be received. Using the signal from the GPS satellite, the signal from the GPS satellite is captured by phase-synchronizing the C / A code and the spectrum is despread. When the GPS receiver performs spectrum despreading in phase synchronization with the C / A code, bits are detected, and a navigation message including time information and the like can be acquired based on a signal from a GPS satellite. Become.

  The GPS receiver captures a signal from a GPS satellite by a phase-locked search of a C / A code. As this phase-locked search, a correlation between a spread code generated by itself and a spread code of a received signal from a GPS satellite For example, when the correlation value of the correlation detection result is larger than a predetermined value, it is determined that the two are synchronized. If the GPS receiver determines that synchronization is not achieved, it uses some synchronization technique to control the phase of the spreading code generated by itself and synchronize it with the spreading code of the received signal. Yes.

  By the way, as described above, the signal from the GPS satellite is a signal obtained by modulating the carrier based on the BPSK modulation method with the signal obtained by spreading the data with the spreading code. Therefore, in order to receive a signal from a GPS satellite, the GPS receiver needs to synchronize not only the spreading code but also the carrier and data, but cannot synchronize the spreading code and the carrier independently.

  Further, the GPS receiver usually converts the received signal into an IF signal by converting the carrier frequency of the received signal into an intermediate frequency (hereinafter referred to as IF) within several MHz, and the IF signal described above. Perform synchronization detection processing. The carrier in the IF signal (hereinafter referred to as IF carrier) mainly includes a frequency error due to Doppler shift according to the moving speed of the GPS satellite, and an internal part of the GPS receiver when the received signal is converted into an IF signal. The frequency error of the local oscillator to be generated is included.

  Therefore, in the GPS receiver, since the IF carrier frequency is unknown due to these frequency error factors, it is necessary to search for the frequency. In addition, since the synchronization point (synchronization phase) within one cycle of the spread code is unknown due to dependence on the positional relationship between the GPS receiver and the GPS satellite, the GPS receiver described above Thus, some kind of synchronization method is required.

  A conventional GPS receiver uses a synchronization method that combines a frequency search for a carrier, synchronization acquisition by a sliding correlator, and synchronization maintenance by a DLL (Delay Locked Loop) and a Costas loop. Hereinafter, this synchronization method will be described.

  As a clock for driving a spread code generator of a GPS receiver, a clock obtained by dividing a reference frequency oscillator prepared in the GPS receiver is generally used. A high-precision crystal oscillator is used as the reference frequency oscillator, and the GPS receiver uses a local oscillation signal used to convert a received signal from a GPS satellite into an IF signal based on the output of the reference frequency oscillator. Generate.

  Here, FIG. 5 shows the processing contents for the frequency search. The GPS receiver performs a phase synchronization search for a spread code when the frequency of the clock signal that drives the spread code generator is a certain frequency f1. That is, the GPS receiver detects the correlation between the received signal from the GPS satellite and the spreading code at each chip phase by sequentially shifting the phase of the spreading code by one chip, and detects the correlation peak. Thus, a phase that can be synchronized is detected. In addition, when the frequency of the clock signal is f1, the GPS receiver changes the frequency division ratio with respect to the reference frequency oscillator, for example, if there is no phase that is synchronized in all the phase searches for 1023 chips. The frequency of the signal is changed to another frequency f2, and the phase search for 1023 chips is similarly performed. The GPS receiver implements a frequency search by repeating such an operation by changing the frequency of the clock signal stepwise.

  When the GPS receiver detects the frequency of the clock signal that can be synchronized by performing such a frequency search, the GPS receiver performs phase synchronization of the final spread code at the frequency of the clock signal. As a result, the GPS receiver can capture a signal from a GPS satellite even when there is a deviation in the oscillation frequency of the crystal oscillator.

  However, such a conventional synchronization method is not suitable for high-speed synchronization in principle. In the GPS receiver, if it takes time to synchronize the spread code and the IF carrier, the reaction becomes slow and inconvenience occurs in use. Therefore, in an actual GPS receiver, in order to compensate for this disadvantage, the time until synchronization acquisition is shortened by parallel processing by increasing the number of channels.

  On the other hand, as a technique for performing high-speed acquisition of a spread spectrum signal instead of the above-described technique using sliding correlation, a matched filter is used. The matched filter can be digitally realized by a so-called transversal filter. In addition, as a matched filter, a spread code can be generated by a digital matched filter using a fast Fourier transform (hereinafter referred to as “FFT”) due to an improvement in hardware capability represented by a recent DSP (Digital Signal Processor). A technique for performing synchronization at high speed has been realized. The latter is based on a correlation calculation speed-up method that has been known for a long time.

  By using these matched filters, the GPS receiver detects a correlation peak, for example, as shown in FIG. 6 for one period of the output waveform when there is a correlation. The position of this peak indicates the beginning of the spread code. By detecting this peak, the GPS receiver can acquire synchronization, that is, detect the phase of the spread code in the received signal. Further, the GPS receiver can detect the IF carrier frequency together with the phase of the spread code by using, for example, the above-described digital matched filter using FFT. Then, the GPS receiver converts the phase of the spread code into a pseudorange, and can calculate the position of the GPS receiver when at least four GPS satellites are detected. Based on this, the speed of the GPS receiver can be calculated.

Here, a configuration example of a GPS receiver using the matched filter is shown in FIG. This GPS receiver, as means for generating a clock, as shown in the figure, a crystal oscillator (X'tal Oscillator; hereinafter referred to as XO) 42 for generating an oscillation signal D1 having a predetermined oscillation frequency, and this A temperature compensated crystal oscillator (hereinafter referred to as TCXO) 46 that generates an oscillation signal having a predetermined oscillation frequency F _OSC different from that of the XO 42, and an oscillation signal D2 supplied from the TCXO 46 is multiplied. And a multiplier / divider 47 for multiplying and / or dividing.

  The XO 42 generates an oscillation signal D1 having a predetermined oscillation frequency of about 32.768 kHz, for example. The XO 42 supplies the generated oscillation signal D1 to an RTC (Real Time Clock) 43 described later.

The TCXO 46 generates an oscillation signal D2 having a predetermined oscillation frequency F _OSC that is different from the XO 42, for example, about 16.384 MHz. The multiplier / divider 47 multiplies the oscillation signal D2 supplied from the TCXO 46 at a predetermined multiplication rate based on a control signal supplied from a CPU (Central Processing Unit) 27 described later, and / or Divide by the division ratio. The multiplier / frequency divider 47 supplies the multiplied and / or frequency-divided oscillation signal D4 to a CPU 41, timer 44, memory 45, synchronization acquisition unit 20, synchronization holding unit 30, frequency synthesizer 16, and the like which will be described later.

Next, the configuration for receiving a signal will be described. This receiver, as the frequency converter 10, receives an antenna 11 that receives an RF (Radio Frequency) signal transmitted from a GPS satellite, and the antenna 11 receives the signal. A low noise amplifier (LNA) 12 that amplifies the received RF signal and a band pass filter (Band Pass Filter) that passes a predetermined frequency band component of the amplified RF signal amplified by the LNA 12. (Hereinafter referred to as BPF) 13, an amplifier 14 for further amplifying the amplified RF signal passed by the BPF 13, and a local oscillation signal having a predetermined frequency F _LO based on the oscillation signal D 2 supplied from the TCXO 46. The frequency synthesizer 16 and the predetermined frequency F _RF amplified by the amplifier 14 are A mixer 15 that multiplies the amplified RF signal having a local oscillation signal supplied from the frequency synthesizer 16, and an intermediate frequency (Intermediate Frequency) having a predetermined frequency F _IF down-converted by being multiplied by the mixer 15. Hereinafter, the amplifier 17 that amplifies the signal, the BPF 18 that passes a predetermined frequency band component of the amplified IF signal amplified by the amplifier 17, and the analog amplified IF signal that is passed by the BPF 18 And an analog / digital converter (hereinafter referred to as A / D) 19 for converting the digital IF into an amplified IF signal.

  The antenna 15 receives an RF signal which is a so-called spread spectrum signal in which a carrier having a frequency of 1575.42 MHz transmitted from a GPS satellite is spread with a C / A code. The BPF 13 includes a so-called SAW (Surface Acoustic Wave) filter.

The frequency synthesizer 16 generates a local oscillation signal having a predetermined frequency F _LO based on the oscillation signal D2 supplied from the TCXO 46 under the control of the control signal supplied from the CPU 41. At this time, the frequency synthesizer 16 makes the setting variable according to the oscillation frequency F _OSC of the oscillation signal D2 generated by the TCXO 46.

The mixer 15 down-converts the amplified RF signal by multiplying the amplified RF signal having the predetermined frequency F _RF amplified by the amplifier 14 by the local oscillation signal supplied from the frequency synthesizer 16, for example, 1.023 MHz. An IF signal having a predetermined frequency F _IF is generated, and the generated IF signal is supplied to the amplifier 21.

  The amplified IF signal converted by the A / D 19 is supplied to the synchronization acquisition unit 20 and the synchronization holding unit 30.

  The synchronization acquisition unit 20 includes a digital matched filter using FFT, and captures synchronization between the spreading code generated by itself and the spreading code in the amplified IF signal supplied from the A / D 19 and detects the carrier frequency in the amplified IF signal. I do. The synchronization holding unit 30 holds the synchronization between the spread code and the carrier in the amplified IF signal supplied from the A / D 19 and demodulates the message. The CPU 41 performs various arithmetic processes by comprehensively controlling each unit in the receiver. Further, an RTC 43 that measures time based on the oscillation signal D1 supplied from the XO 42, a timer 44 as an internal clock of the CPU 41, and a memory 45 including a RAM (Random Access Memory), a ROM (Read Only Memory), and the like. Prepare.

  The satellite number for identifying the GPS satellite detected by the synchronization acquisition unit 20, the phase of the spread code, and the carrier frequency are supplied to the synchronization holding unit 30 and the CPU 41.

For example, a transversal filter is used as a filter that the synchronization holding unit 30 includes for synchronization holding processing. FIG. 8 is a diagram illustrating a configuration example of a transversal filter. This transversal filter supplies inputs to delay circuits 31 ₁ , 31 ₂ ,... 31 n connected in a plurality of stages, and the signals before being delayed and the respective delayed signals are coefficient multipliers 32 ₁ , 32 ₂ ,. ... Are supplied to the adder 33 via 32m and added. Then, the output of the adder 33 is output via the 1 / M circuit 34. In this case, the coefficient set in each coefficient multiplier 32 ₁ , 32 ₂ ,... 32 m is set to +1 or −1, and is set in the opposite direction to the received PN code.

  Returning to the explanation of FIG. 7, the CPU 41 acquires the phase of the spread code, the carrier frequency, and the navigation message supplied from the synchronization holding unit 20, and determines the position and speed of the receiver based on these various information. In addition to the calculation, various arithmetic processes related to GPS such as correcting the time information of the mobile phone 10 based on the accurate time information of the GPS satellite obtained from the navigation message are performed. In addition, the CPU 41 performs overall control related to input / output (Input / Output) between each unit of the receiver and the outside.

  The RTC 43 measures time based on the oscillation signal D1 supplied from the XO42. The time information measured by the RTC 43 is substituted until the accurate time information of the GPS satellite is obtained, and the CPU 41 that has obtained the accurate time information of the GPS satellite controls the XO 42. Corrected as appropriate.

  The timer 44 functions as an internal clock of the CPU 41, and is used for generation of various timing signals necessary for the operation of each unit and time reference.

  The memory 45 includes a RAM, a ROM, and the like. In the memory 45, a RAM is used as a work area when performing various processes by the CPU 41 and the like, and when buffering various input data and holding intermediate data and calculation result data generated in the calculation process. Also, a RAM is used. In the memory 45, a ROM is used as means for storing various programs, fixed data, and the like.

Patent Literature 1 describes a receiver that performs synchronization acquisition using such a digital matched filter.
JP 2002-53898 A

By the way, in the GPS receiver using the C / A code described above, the C / A code has a chip rate of 1.023 MHz, a code length of 1,023, a period of 1 ms (1 kHz), and a signal multiplied by the navigation message in the satellite. Is modulated using a carrier wave of f _RF = 1575.42 MHz. A signal emitted from a GPS satellite has a Doppler effect due to the movement of the satellite itself and the GPS receiver. At this time, if the Doppler shift amount with respect to the carrier wave is Δf _D , the period of the C / A code is shifted to (1−Δf _D / f _RF ) ms. If one chip length deviates by about 1 μs within the analysis unit, the correlation shown in FIG. 6 cannot be obtained. Even if the deviation falls within one chip length within the analysis unit, the results of multiple analysis units are added. The width of the correlation peak widens, the peak value decreases, and the S / N deteriorates.

As already described, in the GPS receiver, the received signal is converted into an intermediate frequency signal (IF signal), and sampling and correlation calculation are performed. Normally, the clock is an oscillation signal supplied from such TCXO46 7 includes an error Delta] f _OSC for a nominal frequency f _OSC, the frequency f _IF of the carrier in the IF during IF signal conversion, IF is the time of sampling The frequency of the carrier wave and the period of the C / A code are shifted by this error.

Assuming that the difference between the Doppler shift amount Δf _D and the IF carrier frequency due to clock error is ΔF _IF , the ratio of one period length of the C / A code in the IF received signal to 1 ms counted at the nominal oscillation frequency of the clock is 1− In ΔF _IF / F _RF , a deviation occurs between the received signal shown in FIG. 9A and the sampling period shown in FIG. 9B, and −T × ΔF _IF / F during T seconds. _RF seconds will shift. For example, assuming that the carrier shift due to the Doppler effect is 4 [kHz] and the clock error is 3 [ppm], the phase of the C / A code is shifted by 5.7 [chip] in 1 second in the sampling operation.

  In Patent Document 1 described above, this shift is corrected by resetting the clock timing for each cycle length of the C / A code. However, if the clock timing is reset and corrected for each cycle length of the C / A code in this way, the timing of sampling the received signal becomes discontinuous at the time of resetting, and the sampling timing of the received signal is not always appropriate. There was a possibility that the reception characteristics deteriorated.

  The present invention has been made in view of such circumstances, and an object of the present invention is to make it possible to satisfactorily receive a spread spectrum signal such as a GPS signal.

  The present invention, when receiving a spread spectrum signal with a known spread code chip rate, adjusts the sampling frequency based on the known spread code chip rate so that the sampling frequency is substantially an integer of the spread code chip rate. Sampling is performed as a double, and correlation detection is performed from the sampled signal.

  According to this invention, there is no error between the spread code cycle of the spread spectrum signal and an integral multiple of the sampling cycle, and the spread spectrum signal received at a constant sampling frequency can always be sampled with respect to the chip rate of the spread spectrum signal. become.

According to the present invention, an error between the spread spectrum signal period and an integral multiple of the sampling period is eliminated, and a spread spectrum signal received at a constant sampling frequency can be sampled at a constant ratio with respect to the chip rate of the spread spectrum signal. Therefore, for example, the phase at which the correlation peak is detected by the digital matched filter is always the same, and correction of the C / A code shift is unnecessary. In addition, when the correlation calculation results are added a plurality of times, for example, if signal acquisition is performed at a timing synchronized with 1,023 [kHz] × (1 + ΔF _IF / ΔF _RF ), the signal acquisition is continuous in time / The effect is the same regardless of whether it was done in a discontinuous manner.

  The adjustment of the sampling frequency is particularly effective when there is error information of the satellite Doppler frequency and the clock, and the correlation calculation can be easily performed over a long time, so that it is easy to increase the sensitivity. Even if the accuracy of the error information of the satellite Doppler frequency and the clock is not high in order to determine the sampling frequency, the deviation is considerably smaller than the fixed case. The Doppler frequency of the satellite can be calculated from the approximate own position and the ephemeris information or almanac information. As clock error information, for example, information calculated at the time of previous positioning or information at the time of shipping adjustment can be used.

  In addition, there is no error information of the satellite Doppler frequency and clock to determine the sampling frequency, and even when searching the satellite while changing the carrier frequency or the like, it is easy to increase the sensitivity by changing the sampling frequency according to the frequency to be searched.

  Hereinafter, an embodiment to which the present invention is applied will be described in detail with reference to FIGS.

  In this embodiment, a global positioning system (GPS), which is a kind of so-called GNSS system that measures the position of a moving object on the ground using an artificial satellite, is applied. From at least four or more GPS satellites It is a GPS receiver that receives the above signal and calculates its position based on the received signal. This GPS receiver receives a spread spectrum signal radio wave called a C1 / A (Clear and Acquisition) code as a received signal.

  The basic configuration of the GPS receiver of this example is the same as the receiver shown in FIG. 7 described as the conventional example, and the received signal is sampled by the analog / digital converter (A / D) 19 of FIG. It is characterized by the setting of the clock frequency and its peripheral processing and configuration.

  FIG. 1 is a diagram showing an overview of the configuration up to this A / D 19 and a configuration example of a synchronization capturing unit 20 that performs synchronization capturing from the IF signal obtained by the A / D 19. The IF signal supplied from the A / D 19 to the synchronization acquisition unit 20 performs high-speed acquisition of the spread code and detection of the carrier frequency in the IF signal in the synchronization acquisition unit 20. The synchronization acquisition unit 20 uses a matched filter to perform synchronization acquisition of the spread code at high speed. Specifically, the synchronization acquisition unit 20 configures the synchronization acquisition unit 20 as a matched filter, for example, as shown in FIG. 1, as a digital matched filter using a fast Fourier transform (hereinafter referred to as FFT). .

  Specifically, as shown in the figure, the synchronization acquisition unit 20 that is a digital matched filter converts the amplified IF signal obtained by the processing from the antenna 11 to the filter 18 to the oscillation signal D2 generated by the TCXO 46. In order to sample the input signal at a clock frequency (sampling frequency) based on the input, the input signal is sampled by the A / D 19 and input.

  The synchronization acquisition unit 20 includes a memory 21 that buffers the IF signal sampled by the A / D 19, an FFT processing unit 22 that reads the IF signal buffered by the memory 21 and performs an FFT process, and the FFT processing unit 22, a memory 23 for buffering a frequency domain signal obtained by performing FFT processing, a spreading code generator 25 for generating the same spreading code as a spreading code in an RF signal from a GPS satellite, and the spreading code generator FFT processing unit 26 that performs FFT processing on the spread code generated by 25, memory 27 that buffers the frequency domain signal obtained by performing FFT processing by this FFT processing unit 26, and buffered in memory 23 The frequency domain signal being ringed and buffered in the memory 27 A multiplier 24 that multiplies one of the complex conjugates of the frequency domain signal and the other, and an inverse fast Fourier transform (hereinafter referred to as IFFT) for the frequency domain signal multiplied by the multiplier 24. An IFFT processing unit 28 that performs processing, a spreading code in an RF signal from a GPS satellite based on a correlation function obtained by performing IFFT processing by the IFFT processing unit 28, and a spreading code generated by the spreading code generator 25 And a peak detector 29 for detecting a correlation peak.

  Note that the synchronization acquisition unit 20 having such a digital matched filter configuration includes each of the FFT processing units 22 and 26, the multiplier 24, the spread code generator 25, the IFFT processing unit 28, and the peak detector 29 as a DSP (Digital Signal). It can also be implemented as software executed by a processor.

  Based on the phase and carrier frequency of the spread code for at least four or more GPS satellites detected by the synchronization acquisition unit 20, the position and velocity of the receiver can be calculated by calculation in the CPU 41 or the like.

  Here, in this example, when the oscillation signal supplied from the TCXO 46 as the source oscillator shown in FIG. 7 is set to a predetermined frequency by the multiplier / divider 47, the multiplier / divider 47 is shown in FIG. The NCO (Numeric Controlled Oscillator) shown in FIG. 2 is configured so that the clock frequency that determines the sampling frequency supplied to the A / D 19 can be adjusted.

Next, the configuration of this NCO will be described with reference to FIG. Here, since the received RF signal is a spread spectrum signal, the NCO is divided according to the oscillating frequency F _{OSC of the} TCXO 46 so as to cover the vicinity of 1.023 MHz which is the chip rate of the C / A code which is a spread code. Set the ratio. That is, as shown in the figure, the NCO has a resolution of values held in frequency dividers 51 and 52 that divide the oscillation signal D2 supplied from the TCXO 46 by a predetermined frequency division ratio and a register 55 described later. A register value setting unit 53 in which a K-bit register value to be expressed is set, an adder 54 for accumulating the register value set in the register value setting unit 53 and the value read from the register 55, and the addition A finite-length register 55 that holds the cumulative addition value supplied from the frequency divider 54, a frequency divider 56 that divides the frequency-divided oscillation signal supplied from the frequency divider 52 at a predetermined frequency division ratio, and the CPU 41. And a storage element 57 such as a register or a memory for holding the set values N, M ₁ , M ₂ , and M ₃ set by.

The frequency divider 51 divides the oscillation signal D2 supplied from the TCXO 46 by a frequency division ratio 1 / M ₁ expressed using the set value M ₁ set in the storage element 57 by the CPU 41. The frequency divider 51 supplies the frequency-divided signal to the register 55.

Divider 52, depending on the value read from the register 55, the frequency division ratio of the oscillation signal D2 supplied from TCXO46, represented using the set value _{M 2} which is set in the storage device 57 by the CPU41 The frequency is divided by 1 / (M ₂ −1) or a division ratio 1 / M ₂ or a division ratio 1 / (M ₂ +1). The frequency divider 52 supplies the frequency-divided signal to the frequency divider 56.

  In the register value setting unit 53, the set value N set in the storage element 57 by the CPU 41 is set as a K-bit register value indicating the resolution of the value held in the register 55. This register value N is represented by 2's complement and takes a positive or negative value. The register value N set in the register value setting unit 53 is supplied to the adder 54.

  The adder 54 adds the register value N set in the register value setting unit 53 and the value read from the register 55. The adder 54 supplies the accumulated addition value obtained by the addition to the register 55. As a result, the value held in the register 55 is a cumulative addition value of the register value N.

  The register 55 has a finite length of K bits, and holds the cumulative added value supplied from the adder 54 based on the timing when the divided oscillation signal supplied from the frequency divider 51 is used as a gate signal. The accumulated addition value held in the register 55 is used to determine the frequency division ratio in the frequency divider 52.

The frequency divider 56 divides the oscillation signal supplied from the frequency divider 52 by a frequency division ratio 1 / M ₃ expressed using the set value M ₃ set in the storage element 57 by the CPU 41. The frequency divider 56 outputs the frequency-divided signal as a sampling clock.

The storage element 57 holds at least four set values N, M ₁ , M ₂ , and M ₃ set by the CPU 41. Set value N held in the storage element 57, as described above, is used as a register value set in the register value setting unit 53, the set value M ₁ determines the division ratio of the divider 51 The set value M ₂ is used to determine the frequency division ratio in the frequency divider 52, and the set value M ₃ is used to determine the frequency division ratio in the frequency divider 53.

It is assumed that the NCO configured as described above is variably set by the CPU 41, instead of setting the four set values N, M ₁ , M ₂ , and M ₃ as fixed values. Thereby, in NCO, a frequency and a variable width can be set. Here, in the NCO, since the register 55 has a finite length, overflow may occur. Therefore, in the NCO of this example, when the register value N set in the register value setting unit 53 is cumulatively added by the adder 54, if there is no overflow of the value held in the register 55, The frequency divider 52 divides the frequency by a division ratio of 1 / M ₂ , but when a negative digit overflow occurs, the frequency division ratio is increased by incrementing a counter (not shown) of the frequency divider 52 by “1”. When the frequency is divided by 1 / (M ₂ +1) and a positive overflow occurs, the frequency division ratio 52 is decreased by “1” to reduce the frequency division ratio 1 / (M ₂ −1). Divide by. Therefore, when the register value set in the register value setting unit 53 is set to the N = 0, the oscillation frequency _{F OSC} a _{1 / (M 2 × M 3} ) of the oscillation signal D2 supplied from TCXO46 multiplied by the frequency When the register value is N> 0, a reproduction carrier having a higher frequency than that when the register value is N = 0 is output, and the register value is N <0. In this case, a clock having a lower frequency than that when the register value is N = 0 is output.

The frequency range of the NCO covers the range obtained by adding the error of the oscillation frequency F _OSC of the TCXO 46 to the spread code chip rate and the Doppler shift amount generated by the relative speed between the GPS satellite on the transmission side and the reception side. Set as follows. For example, assuming that the TCXO 46 is used, if the oscillation frequency error is within about ± 3 ppm, the Doppler shift amount is within about ± 3 ppm, so that the variable range of the NCO as a whole is at least about 1.023 MHz ± 6 ppm. It only has to be included.

Using the NCO configured as described above, the frequency of the clock for setting the sampling frequency for sampling the IF signal by the A / D 19 is adjusted. A specific adjustment state will be described. The clock in the receiver supplied by the TCXO 46 includes an error Δf _OSC with respect to the nominal frequency f _OSC , and the frequency f _IF of the carrier wave in the _IF is sampled at the time of IF signal conversion. Sometimes the sampling frequency shifts due to this error.

Here, if the total of the IF carrier frequency shift due to the Doppler shift amount Δf _D and the error of the clock is ΔF _IF , one period length of the C / A code in the IF reception signal and 1 ms counted by the nominal oscillation frequency of the clock. The ratio is 1−ΔF _IF / F _RF , and a shift occurs as shown in FIG. 9 described above, and −T × ΔF _IF / F _RF seconds are shifted during T seconds.

In this example, the sampling frequency is set to a value that takes into account the Doppler shift amount and the clock error, thereby correcting the deviation of the chip rate of the C / A code. That is, the conventional sampling frequency is fixed to an integral multiple of 1023 [kHz] according to the chip rate of the C / A code, but in this example, 1023 [kHz] / (1−ΔF _IF / F _RF ) ≈1023 [kHz] ] × (1 + ΔF _IF / F _RF ) is adjusted to an integral multiple to cancel the C / A code chip rate deviation. F _RF is 1575.42 MHz and ΔF _IF is obtained by ΔF _IF = ΔF _D −N × ΔF _OSC . N is a constant (N >> 1), and there is a relationship of local oscillation frequency F _LO and F _LO = N × F _OSC .

  FIG. 3 shows an example in which the sampling frequency is corrected and the received signal is sampled as described above. When an integer multiple of the chip rate of the spread code and the sampling frequency include an error, a constant sampling clock is used. For example, as shown in FIG. 3B, depending on the sampling time length, a fraction may be generated in the number of samples per one period of the spread code, and the timing may shift as time passes. This phenomenon is undesirable when signal processing is performed over multiple cycles of spreading codes.

  Here, in the case of this example, by adjusting so that an integral multiple of the basic period of the spreading code becomes the sampling frequency, as shown in FIG. Thus, sampling can be performed continuously at a fixed timing.

  In addition, in order to increase the detection sensitivity of the correlation peak, the correlation calculation results obtained by the signals captured multiple times at different timings may be added. In such a case, conventionally, it has been necessary to correct the Doppler effect, clock error, and shift of the correlation peak position according to the elapsed time during the addition. By eliminating an error in the integral multiple of the chip rate and the sampling frequency, the processing for correcting and adding the shift of the correlation peak position becomes unnecessary. That is, in this example, the phase at which the correlation peak is detected is always the same, and correction of the phase shift of the correlation peak at the time of addition becomes unnecessary.

In addition, when signal capture is performed at a timing synchronized with 1023 [kHz] × (1 + ΔF _IF / ΔF _RF ), the processing of this example is performed regardless of whether the signal capture is performed continuously or discontinuously. The same effect is exhibited.

The adjustment of the sampling frequency as in this example is particularly effective when there is error information of the satellite Doppler frequency and the clock, and the correlation calculation can be easily performed over a long time, so that it is easy to increase the sensitivity. Even if the accuracy of the error information of the satellite Doppler frequency and the clock is not high in order to determine the sampling frequency, the deviation is considerably smaller than the fixed case. Also, when there is no such information, even when searching for a satellite while changing ΔF _IF , which is the difference between the Doppler shift amount Δf _D and the IF carrier frequency due to a clock error, the sampling frequency is set in accordance with the ΔF _IF to be searched. High sensitivity can be achieved by changing.

In the above-described embodiment, the matched filter for correlation detection is a digital matched filter using fast Fourier transform (FFT). However, for example, as a digital matched filter using a transversal filter as shown in FIG. Also good.
Further, as described above, when the clock timing is reset and corrected for each cycle length, the cycle of the reset timing can be adjusted based on the known spread code chip rate.

  Further, the present invention is not limited to the above-described embodiment. For example, although the above-described embodiment has been described as a single GPS receiver, it may be a GPS receiver built in other various electronic devices. In the above-described embodiment, the GPS spread spectrum signal is described as being demodulated. However, the present invention can also be applied to a process of receiving a spread spectrum signal of another system.

  Further, when applied to a positioning system, any electronic device incorporating a function of a positioning system other than a GPS receiver, that is, a receiver to which a GNSS system is applied may be applied. it can. Examples of the GNSS system include the above-described GPS system in the United States, GLONASS (Global Navigation Satellites System) in the former Soviet Union, and GALILEO being developed mainly in Europe. The system can be applied.

It is a block diagram which shows the structural example of the synchronous acquisition part by one embodiment of this invention. It is a block diagram which shows the structural example of NCO by one embodiment of this invention. It is a timing diagram which shows the example of a correction of the sampling timing by one embodiment of this invention. It is explanatory drawing which shows the signal from a GPS satellite. It is explanatory drawing which shows the frequency search example of the signal from a GPS satellite. It is explanatory drawing which shows the example of the output waveform of a matched filter. It is a block diagram which shows the structural example of a GPS receiver. It is a block diagram which shows the structural example of the matched filter by a transversal filter. It is explanatory drawing which shows the shift | offset | difference of sampling frequency.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 ... Frequency conversion part, 11 ... Antenna, 12 ... LNA, 13, 18 ... BPF, 14, 17 ... Amplifier, 15 ... Mixer, 16 ... Frequency synthesizer, 19 ... A / D, 20 ... Synchronization acquisition part, 21, 23 , 27 ... Memory, 22, 26 ... FFT circuit, 24 ... Mixer, 25 ... Spread code generator, 28 ... IFFT circuit, 29 ... Peak detection circuit, 30 ... Synchronization holding unit, 41 ... CPU, 42 ... XO, 43 ... RTC 44 ... Timer 45 ... Memory 46 ... TCXO 47 ... Multiplier / Divider

Claims (4)

In a receiving method for receiving a spread spectrum signal,
When receiving a spread spectrum signal with a known chip rate of the spreading code, or when detecting a spread spectrum signal of a spreading code with a specific chip rate, the sampling frequency is approximately an integer multiple of the chip rate of the spreading code. Adjust and sample
A reception method for performing correlation detection from the sampled signal. The receiving method according to claim 1,
The spread spectrum signal is a positioning signal transmitted from a satellite,
A receiving method for capturing the correlation-detected spreading code and performing positioning calculation based on the captured signal. In a receiving device that receives a spread spectrum signal,
A sampling frequency adjusting means that adjusts a sampling frequency based on a chip rate of a spreading code whose chip rate is known, and sets the sampling frequency to an integer multiple of the chip rate of the spreading code;
Sampling means for performing sampling at a frequency adjusted by the sampling frequency adjusting means;
A receiving apparatus comprising: correlation detecting means for detecting correlation from the signal sampled by the sampling means. The receiving device according to claim 3,
The spread spectrum signal sampled by the sampling means is a positioning signal transmitted from a satellite,
A receiving apparatus that captures a spread code detected by the correlation detection means and performs a positioning calculation based on the captured signal.

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