CN101888253B - Deviation correcting method and system of communication channel baseband frequency - Google Patents
Deviation correcting method and system of communication channel baseband frequency Download PDFInfo
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Abstract
The invention provides a deviation correcting method and system of communication channel baseband frequency. The method of the invention comprises the following steps: detecting the deviation of the central point of input signal spectrum to zero-frequency, outputting an estimated completion indication signal, outputting a frequency deviation, which is equal to zero, before the completion of detection, outputting a constant frequency deviation after the completion of detection; adding a fixed shifting frequency and the frequency deviation to obtain a target frequency, shifting the frequency spectrum of input digital signals to the target frequency to obtain a quasi-baseband signal; controlling the low-pass filter factor to switch between large bandwidth coefficient and small bandwidth coefficient according to the estimated completion indication signal, performing low-pass filter filtering treatment to the quasi-baseband signal to obtain a low-pass signal; and performing small-frequencydeviation correcting treatment to the low-pass signal to obtain a baseband signal. By using the method and system of the invention to perform large frequency deviation correcting and small frequency deviation correcting to the quasi-baseband signal, the center frequency of data is finally shifted to zero-frequency and the method and system has the advantages of convenient and fast operation and high detection accuracy.
Description
Technical field
The present invention relates to the simultaneous techniques of communication channel frequencies, especially relate to a kind of method and system of base band frequency bias correcting, make signal become baseband signal accurately by the correction to frequency shift (FS).
Background technology
In communication system, correcting frequency deviation is a requisite link.Digital receiver needs that after receiving high-frequency signal high-frequency signal is moved base band and carries out subsequent treatment again, if transmitting terminal is consistent with the frequency of receiving terminal, and the sample frequency ideal of receiving terminal, the high-frequency signal that receives like this can be moved zero-frequency accurately.But owing to exist the frequency of transmitting terminal or receiving terminal nonstandard, situations such as drift take place in sample frequency, high-frequency data is difficult to move accurately zero-frequency, thereby can make the synchro system degradation cause the reduction of whole communication system performance like this, so need a correcting frequency deviation device in each receiving system.
Correcting frequency deviation has a lot of methods, and methods such as polarity decision, phase-detection, direct judgement are arranged.But these methods have certain restriction, and namely they can not correct bigger frequency departure.In order to correct big frequency departure, can use the method for frequency sweep that big frequency deviation is detected and corrects.Yet the method for frequency sweep needs consumed time longer, and wanting to cover more big frequency deviation region just needs more long sweep time.
Therefore, how finding a kind of more convenient method fast to go to detect the big frequency deviation that receives signal and have the higher detection precision, is one of technical barrier of current urgent need solution.
Summary of the invention
The invention provides a kind of base band frequency bias correcting method and system, this method is carried out big correcting frequency deviation and little correcting frequency deviation by aiming at base band data, and the centre frequency of data is moved zero-frequency the most at last, has convenient quick, advantage that accuracy of detection is high.
The present invention has adopted following technical scheme to realize: a kind of bias correcting method of communication channel baseband frequency, and it comprises step:
Detect the skew of the relative zero-frequency of central point of input signal spectrum, output estimates to finish index signal, and is output as 0 frequency deviation before detection is finished, and detects the frequency deviation of exporting steady state value after finishing;
Fixed shifting frequency and frequency deviation addition are obtained target frequency, the frequency spectrum shift of supplied with digital signal is obtained accurate baseband signal to this target frequency;
According to estimating that finishing index signal control low-pass filtering coefficient switches selection between big bandwidth factor and little bandwidth factor, aim at baseband signal and carry out low-pass filtering treatment, obtain low-pass signal;
Low-pass signal is carried out the small frequency bias correction handle, obtain baseband signal.
Wherein, the step of described frequency deviation detection comprises:
Low-pass signal after the low-pass filtering is carried out in time uniformly the extraction of different multiples according to different character rates;
Be one group with every N the data that extract and carry out discrete Fourier transform, and ask absolute value and addition to obtain N frequency spectrum accumulated value these data, N point carried out The disposal of gentle filter, the frequency spectrum behind the smothing filtering is being carried out frequency deviation detect, detecting the skew of the relative central point of frequency spectrum.
Wherein, detect from left to right earlier, detect first point that surpasses frequency spectrum maximum 1/4; Detections of then turning left from the right side detects first point above frequency spectrum maximum 1/4; Asking the central point of two points, deducting N/2 again and obtain spectrum offset.
Wherein, determined to carry out the multiple of data pick-up by the character rate of low-pass signal.
Wherein, the step of described data pick-up comprises:
The multiple that specified data extracts, and an initial value is set is 0 counter;
Character rate by low-pass signal adds 1 counting by counter when data are adopted in one of every input, carry out data pick-up when counter reaches extracting multiple one time, and the sampled data output with input obtains an effective extracted data, the unison counter zero clearing.
Wherein, before the frequency deviation detection is finished, estimate to finish the index signal output low level, control selects big bandwidth factor aligning baseband signal to carry out low-pass filtering treatment; After the frequency deviation detection is finished, estimate to finish index signal output high level, control selects little bandwidth factor aligning baseband signal to carry out low-pass filtering treatment.
In addition, the present invention discloses a kind of bias correcting system of communication channel baseband frequency, and it comprises:
The frequency deviation detection module, for detection of the skew of the relative zero-frequency of central point of input signal spectrum, output estimates to finish index signal, and is output as 0 frequency deviation before detection is finished, and detects the frequency deviation of exporting steady state value after finishing;
Digital Down Converter Module is used for fixed shifting frequency and frequency deviation addition are obtained target frequency, and the frequency spectrum shift of supplied with digital signal is obtained accurate baseband signal to this target frequency;
Low-pass filtering bandwidth control module is used for according to estimating that finishing the switching between big bandwidth factor and little bandwidth factor of index signal control low-pass filtering coefficient selects;
Low-pass filtering module is used for aiming at baseband signal and carries out low-pass filtering treatment, obtains low-pass signal;
Little correcting frequency deviation module is used for that low-pass signal is carried out the small frequency bias correction and handles, and obtains baseband signal.
Wherein, the frequency deviation detection module comprises:
The data adaptive abstraction module is used for the low-pass signal after the low-pass filtering is carried out the extraction of different multiples in time uniformly according to different character rates;
Big frequency deviation estimating modules, be used for being one group with every N the data that extract and carry out discrete Fourier transform, and ask absolute value and addition to obtain N frequency spectrum accumulated value these data, N point carried out The disposal of gentle filter, frequency spectrum behind the smothing filtering is carried out frequency deviation detect, detect the skew of the relative central point of frequency spectrum.
Wherein, described data adaptive abstraction module comprises:
Initial value is 0 counter, is used for when data are adopted in one of every input, adding 1 counting by counter by the character rate of low-pass signal, and in the extracted data of one of input, unison counter zero clearing;
Data extraction module is used for carrying out one time data pick-up when counter reaches extracting multiple, the sampled data output with input obtains an effective extracted data.
Compared with prior art, the present invention has following beneficial effect:
Correcting frequency deviation method of the present invention has been owing to adopted the method that data are carried out bandwidth varying low-pass filtering and adaptive decimation and after big frequency deviation estimation is finished data in time corrected, and can make the scope of correcting frequency deviation of system big, accuracy is high and simple to operate.
Description of drawings
Fig. 1 is system configuration schematic diagram of the present invention;
Fig. 2 is the structural representation of Digital Down Convert construction module;
Fig. 3 A, Fig. 3 B and Fig. 3 C are respectively the spectrum diagram of three embodiment of low-pass filtering;
Fig. 4 is the structural representation of low-pass filtering module;
Fig. 5 is the spectrogram of the invention process instance data after through 2 times of extractions;
Fig. 6 is the spectrogram of the invention process instance data after through 3 times of extractions;
Fig. 7 extracts schematic flow sheet for data adaptive of the present invention;
Fig. 8 is data pick-up construction module schematic diagram of the present invention;
Fig. 9 estimates the construction module schematic diagram for the big frequency deviation of the present invention;
Figure 10 is the schematic diagram of The disposal of gentle filter of the present invention.
Embodiment
Be that example illustrates operation principle of the present invention with the CATV transmission system.
Fig. 1 is the structural representation of CATV transmission system, and it comprises Digital Down Converter Module 101, low-pass filtering module 102, data adaptive abstraction module 103, big frequency deviation estimating modules 104, low-pass filtering bandwidth control module 105 and little correcting frequency deviation module 106.
As shown in Figure 2, Digital Down Converter Module 101 comprises: be used for fixed shifting frequency and the adder of estimating that big frequency deviation is carried out add operation; And the frequency spectrum shift of digital medium-frequency signal that is used for entering Digital Down Converter Module 101 is to the frequency spectrum shift module of accurate baseband signal, and its frequency of moving is fixed shifting frequency and estimate big frequency deviation sum.In addition, estimating that the value of big frequency deviation was 0 before frequency deviation is estimated not finish, is a constant value after estimation is finished.
The effect of low pass filter 102 is to filter high-frequency noise, and designed low pass filter has two cover low-pass filtering coefficients, the big bandwidth factor of a cover, the little bandwidth factor of a cover.Fig. 3 A-Fig. 3 C has illustrated the difference of big bandwidth filtering and little bandwidth filtering.The big bandwidth filtering of 301 expressions, 302 expression signal spectrums, 303 expression band noises, the little bandwidth filtering of 304 expressions.It is in order to estimate big frequency shift (FS), shown in Fig. 3 A and Fig. 3 B that big bandwidth factor is set.Bandwidth such as signal is 7Mhz, big bandwidth can be set to 10Mhz, even the spectrum offset of signal is like this ± 1Mhz also within the coverage of big bandwidth filter, estimate big skew between input signal and the zero-frequency so big frequency deviation estimating modules 104 just can be correct.In order to guarantee that frequency deviation estimates to finish the performance of back system operation, after frequency deviation is estimated to finish, the low-pass filtering coefficient is switched to little bandwidth factor, influence valid data with regard to not having more noise like this, shown in Fig. 3 C.
The structure of described low-pass filtering module 102 as shown in Figure 4.The shift register 401 of the accurate baseband signal input low-pass filtering module 102 after the down-conversion, each timeticks, shift register 401 is to data of right translation.Data selector of 402 expressions, by estimating to finish index signal control: when estimating that finishing index signal is 0, big bandwidth factor is selected in expression; When estimating that finishing index signal is 1, little bandwidth factor is selected in statement.Processings of multiplying each other in multiplier 403 of the coefficient of the data in the shift register 401 and data selector 402 outputs operated the data that obtain after the low-pass filtering thereby resulting multiplied result is summed up by adder 408.
Data adaptive abstraction module 103 is that the different character rate of the data based data after the low-pass filtering is carried out the extraction of different multiples, so-called different multiples extracts, be because the speed is-greater-than symbol speed of data after over-sampling, can suitably extract sampled data, the data sampling speed after the extraction is slightly larger than character rate.Such as 2 times of extractions, be exactly that per two points of sampled data are extracted data as valid data, the extraction of data is uniform in time.
Therefore, data adaptive abstraction module 103 is in order to improve the precision that big frequency deviation is estimated.The character rate of data may be different in different systems, in order to improve big accuracy of frequency offset estimation to greatest extent, can carry out different multiples to the data of distinct symbols speed and extract.Certainly, the data rate after the extraction still is greater than character rate.So just can guarantee that namely can not produce the data aliasing also can satisfy required precision.For example, the sampling rate of data is 30.4Mhz, and character rate is 6.9Mhz, if data are carried out 2 times to be extracted then the sum frequency of N point DFT representative is 15.2Mhz, frequency resolution (frequency of point-to-point transmission representative) is (30.4/2/N) Mhz, as Fig. 5 (abscissa is the time, and ordinate is frequency); Extract then the sum frequency of N point DFT representative is 10.133Mhz if data are carried out 3 times, frequency resolution (frequency of point-to-point transmission representative) is (30.4/3/N) Mhz, as Fig. 6.Frequency resolution is more high, and it is more accurate that frequency deviation is estimated.The precision that data are carried out 3 times of extractions extracts high than 2 times like this.So, can carry out the extraction of different multiples for the data of distinct symbols speed, the mode of extraction is evenly to extract in time.
The execution flow process that described data adaptive extracts as shown in Figure 7.Sampled data 702 digit rates are 30.4Mhz, the character rate of data at 1Mhz between the 10Mhz.Extracting multiple by the data symbol rate determination data of importing.When 7＜character rate＜10, system carries out 3 times of extractions to sampled data; When 5.5＜character rate＜7, system carries out 4 times of extractions to sampled data; When 4.5＜character rate＜5.5, system carries out 5 times of extractions to sampled data; When 3＜character rate＜4.5, system carries out 6 times of extractions to sampled data; When 1＜character rate＜3, system carries out 8 times of extractions to sampled data.Data after obtaining at last extracting.
Specifically, the extraction process of data as shown in Figure 8.After extracting multiple was determined, counter 801 adds 1 since 0 to be counted, and when counter reaches extracting multiple, just carries out data pick-up one time, is about to the sampled data output of input, obtains an effective extracted data, the unison counter zero clearing.
Big frequency deviation estimating modules 104 is operated in before frequency deviation do not finish, in case frequency deviation is estimated to finish then quit work.Be input to big frequency deviation estimating modules 104 through the data behind the adaptive decimation, N data of each input, be one group, this N data are carried out discrete Fourier transform (DFT, Discrete Fourier Transform), altogether 16 groups of data are DFT, again these data are asked absolute value and addition to obtain N frequency spectrum accumulated value.In order to make frequency spectrum more regular, this N point carried out The disposal of gentle filter.Smothing filtering can be that contiguous several values is averaging also can be low pass filter with a low order, and its structure as shown in figure 10.Frequency spectrum behind the smothing filtering is carried out frequency deviation detect, detect the skew of the relative central point of frequency spectrum, what it was concrete is operating as: detect from left to right earlier, detect first point that surpasses frequency spectrum maximum 1/4; Detections of then turning left from the right side detects first point above frequency spectrum maximum 1/4; Asking the central point of two points, deducting N/2 again and obtain spectrum offset, the spectrum offset that obtains can also can be negative for positive number, and the frequency deviation data that frequency spectrum detection obtains are exported to Digital Down Converter Module 101.
Described low-pass filtering bandwidth control module 105 is used for controlling low pass filter 102 and carries out can switching different filter factors in different operating states: it is low that the initial condition Time Bandwidth is selected signal, and expression control low pass filter 102 is selected big bandwidth factor; After big frequency deviation was estimated to finish, bandwidth selected signal to become height, and expression control low pass filter 102 is selected little bandwidth factor.Because Digital Down Converter Module 101 just can be immediately to correcting frequency deviation after receiving frequency shift (FS), the skew of the data after the down-conversion and zero-frequency is just smaller like this, the little bandwidth factor of low pass filter 102 uses this moment can filter more noise, improves the performance of system.
Little correcting frequency deviation module 106 is in order to correct remaining small frequency deviation, and it can have diverse ways in different systems, such as polarity decision, direct method such as judgement.The scope of polarity decision method correcting frequency offset is little, for its frequency deviation region of entangling of signal of 7Mhz character rate is approximately ± 250Khz, along with the correcting frequency deviation scope that reduces it of character rate is reducing thereupon.Make inherent spurious frequency deviation less so before polarity decision, add a big correcting frequency deviation.Inherent spurious frequency deviation after excessive correcting frequency deviation has just dropped within the scope that polarity decision can correct like this, thereby reaches the purpose of correcting frequency deviation.
Can see from above-mentioned example, for the accurate baseband signal that has frequency departure, by the associated working of big correcting frequency deviation and little correcting frequency deviation, can make intermediate-freuqncy signal move the base band of standard fast and very high accuracy is arranged.
To sum up, correcting frequency deviation method of the present invention has been owing to adopted the method that data are carried out bandwidth varying low-pass filtering and adaptive decimation and after big frequency deviation estimation is finished data in time corrected, and can make the scope of correcting frequency deviation of system big, accuracy is high and simple to operate.
Claims (2)
1. the bias correcting method of a communication channel baseband frequency is characterized in that, comprises step:
Detect the skew of the relative zero-frequency of central point of input signal spectrum, output estimates to finish index signal, and is output as 0 frequency deviation before detection is finished, and detects the frequency deviation of exporting steady state value after finishing;
Fixed shifting frequency and frequency deviation addition are obtained target frequency, the frequency spectrum shift of supplied with digital signal is obtained accurate baseband signal to this target frequency;
According to estimating that finishing index signal control low-pass filtering coefficient switches selection between big bandwidth factor and little bandwidth factor, namely in order to guarantee that frequency deviation estimates to finish the performance that the back system moves, after frequency deviation is estimated to finish, the low-pass filtering coefficient is switched to little bandwidth factor, when estimating that finishing index signal is 0, big bandwidth factor is selected in expression; When estimating that finishing index signal is 1, little bandwidth factor is selected in statement, aims at baseband signal and carries out low-pass filtering treatment, obtains low-pass signal;
Low-pass signal is carried out the small frequency bias correction handle, obtain baseband signal;
Wherein, before the frequency deviation detection is finished, estimate to finish the index signal output low level, control selects big bandwidth factor aligning baseband signal to carry out low-pass filtering treatment; After the frequency deviation detection is finished, estimate to finish index signal output high level, control selects little bandwidth factor aligning baseband signal to carry out low-pass filtering treatment;
The step that described frequency deviation detects comprises:
Low-pass signal after the low-pass filtering is carried out in time uniformly the extraction of different multiples according to different character rates;
Be one group with every N the data that extract and carry out discrete Fourier transform, and ask absolute value and addition to obtain N frequency spectrum accumulated value these data, N point carried out The disposal of gentle filter, the frequency spectrum behind the smothing filtering is being carried out frequency deviation detect, detecting the skew of the relative central point of frequency spectrum;
Detect from left to right earlier, detect first point that surpasses frequency spectrum maximum 1/4; Detections of then turning left from the right side detects first point above frequency spectrum maximum 1/4; Asking the central point of two points, deducting N/2 again and obtain spectrum offset;
Determined to carry out the multiple of data pick-up by the character rate of low-pass signal;
The step of described data pick-up comprises:
The multiple that specified data extracts, and an initial value is set is 0 counter;
Character rate by low-pass signal adds 1 counting by counter when data are adopted in one of every input, carry out data pick-up when counter reaches extracting multiple one time, and the sampled data output with input obtains an effective extracted data, the unison counter zero clearing.
2. the bias correcting system of a communication channel baseband frequency is characterized in that, comprising:
The frequency deviation detection module, this module comprises the data adaptive abstraction module again, is used for the low-pass signal after the low-pass filtering is carried out the extraction of different multiples in time uniformly according to different character rates; With big frequency deviation estimating modules, be used for being one group with every N the data that extract and carry out discrete Fourier transform, and ask absolute value and addition to obtain N frequency spectrum accumulated value these data, N point carried out The disposal of gentle filter, frequency spectrum behind the smothing filtering is carried out frequency deviation detect, detect the skew of the relative central point of frequency spectrum; The multiple of data pick-up is determined by the character rate of low-pass signal;
Described data adaptive abstraction module comprises:
Initial value is 0 counter, is used for when data are adopted in one of every input, adding 1 counting by counter by the character rate of low-pass signal, and in the extracted data of one of input, unison counter zero clearing;
Data extraction module is used for carrying out one time data pick-up when counter reaches extracting multiple, the sampled data output with input obtains an effective extracted data;
The frequency deviation detection module detects earlier from left to right for detection of the skew of the relative zero-frequency of central point of input signal spectrum, detects first point that surpasses frequency spectrum maximum 1/4; Detections of then turning left from the right side detects first point above frequency spectrum maximum 1/4; Asking the central point of two points, deducting N/2 again and obtain spectrum offset;
Digital Down Converter Module is used for fixed shifting frequency and frequency deviation addition are obtained target frequency, and the frequency spectrum shift of supplied with digital signal is obtained accurate baseband signal to this target frequency;
Low-pass filtering bandwidth control module, be used for according to estimating that finishing the switching between big bandwidth factor and little bandwidth factor of index signal control low-pass filtering coefficient selects, namely in order to guarantee that frequency deviation estimates to finish the performance that the back system moves, after frequency deviation is estimated to finish, the low-pass filtering coefficient is switched to little bandwidth factor, when estimating that finishing index signal is 0, big bandwidth factor is selected in expression; When estimating that finishing index signal is 1, little bandwidth factor is selected in statement;
Low-pass filtering module is used for aiming at baseband signal and carries out low-pass filtering treatment, obtains low-pass signal;
Little correcting frequency deviation module is used for that low-pass signal is carried out the small frequency bias correction and handles, and obtains baseband signal;
Wherein, before the frequency deviation detection is finished, estimate to finish the index signal output low level, control selects big bandwidth factor aligning baseband signal to carry out low-pass filtering treatment; After the frequency deviation detection is finished, estimate to finish index signal output high level, control selects little bandwidth factor aligning baseband signal to carry out low-pass filtering treatment.
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| CN1719818A (en) * | 2005-07-08 | 2006-01-11 | 广州海格通信有限公司 | A sampling frequency deviation tracking signal processing device and method for an orthogonal frequency division multiplexing system |
| CN101414988A (en) * | 2008-11-19 | 2009-04-22 | 西安电子科技大学 | Low-complex-degree timing recovery method for TDS-OFDM system |
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| CN101414988A (en) * | 2008-11-19 | 2009-04-22 | 西安电子科技大学 | Low-complex-degree timing recovery method for TDS-OFDM system |
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