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CN104777495B - A kind of QPSK modulation I/Q path quadrature method of testings based on distribution histogram - Google Patents

A kind of QPSK modulation I/Q path quadrature method of testings based on distribution histogram Download PDF

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CN104777495B
CN104777495B CN201510166231.4A CN201510166231A CN104777495B CN 104777495 B CN104777495 B CN 104777495B CN 201510166231 A CN201510166231 A CN 201510166231A CN 104777495 B CN104777495 B CN 104777495B
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intermediate frequency
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orthogonality
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CN104777495A (en
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郎荣玲
李新玥
苏振
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Beihang University
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    • GPHYSICS
    • G01MEASURING; TESTING
    • G01SRADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S19/00Satellite radio beacon positioning systems; Determining position, velocity or attitude using signals transmitted by such systems
    • G01S19/01Satellite radio beacon positioning systems transmitting time-stamped messages, e.g. GPS [Global Positioning System], GLONASS [Global Orbiting Navigation Satellite System] or GALILEO
    • G01S19/13Receivers
    • G01S19/35Constructional details or hardware or software details of the signal processing chain
    • G01S19/37Hardware or software details of the signal processing chain

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  • Engineering & Computer Science (AREA)
  • Signal Processing (AREA)
  • Radar, Positioning & Navigation (AREA)
  • Remote Sensing (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Position Fixing By Use Of Radio Waves (AREA)

Abstract

The invention discloses a kind of QPSK modulation I/Q path quadrature method of testings based on distribution histogram, belong to signal quality monitoring and evaluation field.I branch road of the method with the receiver tracking digital medium-frequency signal, outgoing carrier phase place, carrier frequency and navigation message data;Secondly, digital medium-frequency signal is demodulated using carrier frequency and carrier phase information in locally generated carrier wave, generate the baseband signal of two branch roads of I/Q, recycle navigation message data to peel off the text in two branch road baseband signals of I/Q.The present invention does not need the pseudo-code of two branch roads of I, Q, it is known that having widened the range of application of the orthogonal judgement of I, Q;The deviation caused by external noise, the thermal noise of device and undesirable property is insensitive so that the result of the orthogonal judgement of I, Q has robustness;The cumulative mode of usage cycles, improves signal to noise ratio, so as to improve the precision of I, Q orthogonality judged result.

Description

QPSK modulation I/Q branch orthogonality test method based on distribution histogram
Technical Field
The invention belongs to the field of signal quality monitoring and evaluation, and relates to modulation domain calculation. The invention is suitable for QPSK modulation at present, and particularly relates to a test method based on an I/Q branch baseband signal distribution histogram.
Background
There are two methods commonly used to calculate orthogonality, which are constellation diagram and tracking loop based orthogonality test. The constellation diagram test is to judge whether the signal has abnormal orthogonality and non-orthogonal degree by evaluating the phase error and amplitude error of the constellation diagram, but the phase error is not the phase difference of the I/Q branch in the true sense, and because of the noise existing in the actual environment, the data point falls around the normal position of the constellation point to form a cloud-like shape, and in addition, the non-ideal of the filter causes the chip waveform oscillation during the baseband processing, the signal discontinuous point has a transition band, etc., which brings larger error between the measured constellation diagram and the ideal constellation diagram, so the calculation result of the index can not truly reflect the modulation performance of the I/Q branch.
The orthogonality test method based on the tracking loop comprises the steps of utilizing a code loop and a carrier loop in a tracking loop of a receiver to respectively measure carrier phases of two I/Q branches at the same time, and calculating phase differences of two I/Q branches to obtain the orthogonality of the I/Q branches of a tested signal. This approach can be used in cases where the signal-to-noise ratio is low, but requires that the pseudo-code be known. The Q-branch pseudo code of the GPSP (Y) code and the first-phase signal of Beidou is unknown, and the method is not applicable.
Disclosure of Invention
Aiming at the defects of the two methods, the invention provides a QPSK (quadrature Phase Shift Keyin) modulation I/Q branch orthogonality test method based on a baseband signal distribution histogram. The method allows the Q-branch pseudo code to be unknown and the calculation error to be small. Whether the detected signal has orthogonality abnormity can be calculated, and the degree of the orthogonality abnormity can be detected.
The method needs to acquire the signal of the satellite to be detected by using the radio frequency front end, obtain and store a digital intermediate frequency signal, and the digital intermediate frequency signal is used in the subsequent data processing. Firstly, tracking an I branch of the digital intermediate frequency signal by using a receiver, and outputting a carrier phase, a carrier frequency and navigation message data; secondly, generating a carrier locally by utilizing carrier frequency and carrier phase information, demodulating the digital intermediate frequency signal, generating baseband signals of the two I/Q branches, and stripping the text in the baseband signals of the two I/Q branches by utilizing navigation text data. When the I/Q branches are not orthogonal, namely the phase difference of the two branches is not 90 degrees, the baseband signal waveforms of the two branches deviate from an ideal chip shape, the relative amplitudes also change, the characteristics of the waveforms are extracted, and the relative offset of the phases can be calculated. The feature used here is the amplitude feature of the waveform, and the extraction of the amplitude is obtained by using the counting peak of the distribution histogram. In order to make the feature extraction more accurate, a period superposition method is adopted to improve the signal-to-noise ratio, and when the accumulation period is long enough, the chip waveform can be clear and the feature is obvious.
Based on the test method, the invention also provides a test device for realizing the test method, wherein the test device comprises four modules which are respectively as follows: the device comprises a satellite signal acquisition module, a digital down-conversion module, a data preprocessing module and an orthogonality calculation module;
the satellite signal acquisition module consists of a high-gain antenna, a preamplifier, a down converter, an A/D converter and a data disk array, and is high in gainThe gain antenna receives a satellite signal to be detected, the received signal enters a down converter after being filtered and amplified by a pre-filter and a pre-amplifier, the down converter mixes the signal with a local carrier to generate an analog intermediate frequency signal, the analog intermediate frequency signal is converted into a digital intermediate frequency signal by an A/D converter, and the digital intermediate frequency signal is recorded as r (nT)S) (ii) a Collecting a period of digital intermediate frequency signals, storing the digital intermediate frequency signals in a data disk array, and preparing to send the digital intermediate frequency signals to a digital down-conversion module;
the digital down-conversion module comprises a receiver tracking loop, a local oscillator, a first multiplier, a second multiplier, a first low-pass filter and a second low-pass filter, wherein the receiver tracking loop tracks the acquired digital intermediate frequency signal r (nT)S) The I branch outputs stably tracked carrier frequency w, carrier phase phi and navigation message data D (nT)s);
Combining the output carrier frequency and carrier phase to generate a carrier signal cos (w.nT) in phase with the I-branch carrier in a local oscillators+ phi) and a carrier signal sin (w.nt) orthogonal to the I-branch carriers+ phi); carrier signal cos (w.nT)s+ phi) is summed with the digital intermediate frequency signal r (nT) in a first multipliers) After multiplication, an I branch baseband signal is obtained through a first low-pass filter and is marked as Ir(nTs) Analogously, the carrier signal sin (w.nT)s+ phi) is summed with the digital intermediate frequency signal r (nT) in the second multipliers) After multiplication, a Q branch baseband signal is obtained through a second low-pass filter and is marked as Qr(nTs),Ir(nTs)、Qr(nTs) A baseband signal for local restoration;
the data preprocessing module comprises a text stripping module, a period superposition module and a normalization processing module, wherein the text stripping module is used for stripping navigation text data of two paths, then performing data segment superposition in the period superposition module, and finally performing normalization calculation in the normalization processing module;
and the orthogonality calculation module is used for extracting level values of the distribution histogram and calculating orthogonality.
The invention has the advantages and positive effects that:
(1) pseudo codes of two branches of I, Q are not required to be known, so that the application range of I, Q orthogonal judgment is widened;
(2) is insensitive to external noise, thermal noise of the device and deviation caused by non-ideality, so that I, Q orthogonal judgment is realized
The results of (a) are robust;
(3) by using the periodic accumulation mode, the signal-to-noise ratio is improved, and therefore the accuracy of the I, Q orthogonality judgment result is improved.
Drawings
Fig. 1 is a flowchart of a quadrature test method for QPSK modulation I/Q branches based on a distribution histogram provided in the present invention.
Detailed Description
The present invention will be described in further detail below with reference to the accompanying drawings.
The invention provides a QPSK modulation I/Q branch orthogonality test method based on a distribution histogram and also provides a test device for realizing the test method, wherein the test device comprises four modules which are respectively as follows: the system comprises a satellite signal acquisition module, a digital down-conversion module, a data preprocessing module and an orthogonality calculation module, and each module and a test method thereof are described in detail below.
1. The satellite signal acquisition module acquires signals;
as shown in figure 1, the satellite signal acquisition module is composed of a high-gain antenna, a preamplifier, a down converter, an A/D (analog/digital) converter and a data disk array, and mainly completes off-line data acquisition of satellite navigation signalsAnd (4) collecting for later module processing and analysis. The high-gain antenna receives a satellite signal to be detected, the received signal enters a down converter after being filtered and amplified by a pre-filter and a pre-amplifier, the down converter mixes the signal with a local carrier to generate an analog intermediate frequency signal, the analog intermediate frequency signal is converted into a digital intermediate frequency signal by an A/D converter, and the digital intermediate frequency signal is recorded as r (nT)S). And collecting a period of digital intermediate frequency signals, storing the digital intermediate frequency signals in a data disk array, and preparing to send the digital intermediate frequency signals to a digital down-conversion module.
A QPSK-modulated digital intermediate frequency signal in an ideal case without considering noise or the like, the expression is as follows:
where n is the number of the sample sequence, TsFor the sampling period, w is the intermediate frequency, phi is the initial phase, PIIs the power of the I branch signal, PQIs the power of the Q branch signal, c (nT)S) For the sampling value of the pseudo code of branch I, P (nT)S) For sampling Q-branch pseudo-code, D (nT)s) Is the sampling value of the navigation message. Under normal conditions, the phase difference of the carrier waves of the I/Q branch is 90 degrees, and when the modulated digital intermediate frequency signal generates non-orthogonality abnormity, the non-orthogonality existsThe signal may be expressed as:
the above formula (2) can be arranged as follows:
2. and the digital down-conversion module completes signal demodulation to obtain two paths of I/Q baseband signals.
(1) Firstly, a tracking loop of a receiver is utilized to track a collected digital intermediate frequency signal r (nT)S) Outputs a stably tracked carrier frequency w, a carrier phase phi and navigation text data D (nT)s)。
(2) Combining the output carrier frequency and carrier phase to generate a carrier signal cos (w.nT) in phase with the I-branch carrier in a local oscillators+ phi) and a carrier signal sin (w.nt) orthogonal to the I-branch carriers+ phi). Carrier signal cos (w.nT)s+ phi) is summed with the digital intermediate frequency signal r (nT) in a first multipliers) After multiplication, an I branch baseband signal is obtained through a first low-pass filter and is marked as Ir(nTs) The cut-off frequency of the first low-pass filter is larger than c (nT)s) And P (nT)s) The bandwidth of the two pseudo codes ensures that the two pseudo codes are contained in the time domain. Analogously, the carrier signal sin (w.nT)s+ phi) is summed with the digital intermediate frequency signal r (nT) in the second multipliers) After multiplication, a Q branch baseband signal is obtained through a second low-pass filter and is marked as Qr(nTs) The cut-off frequency of the second low-pass filter is greater than P (nT)s) The bandwidth of the pseudo code is only needed. I isr(nTs)、Qr(nTs) For locally recovered baseband signals, a distinction is made from satellite baseband signals.
This time is:
3. the data preprocessing module is used for carrying out normalization processing on the two paths of baseband signals;
the data preprocessing module performs some transformations on data and performs early preparation for subsequent signal processing, which involves stripping baseband signal navigation messages, improving signal-to-noise ratio by a periodic superposition method and normalization processing, as shown in fig. 1.
If the gain of the receiving antenna is insufficient for the satellite signal actually received, the demodulated signal cannot be clearly distinguished, and the detection error of the orthogonality abnormality is large. Theoretical analysis shows that R data segments (the length of the data segment can be selected according to the pseudo code period, for example, the length of the data segment can be selected for 1ms for a GPS L1 signal) are superposed, and the signal-to-noise ratio can be improved by R times, namely 10 logs10R dB. However, the baseband signal is modulated with the navigation message, which limits the number of times of period superimposition, and the navigation message needs to be stripped before superimposition in order to effectively utilize the period superimposition to improve the signal-to-noise ratio. Navigation message data D (nT) can be obtained easily due to the fact that a receiver tracking loop is used for tracking the I branch signal in the digital down-conversion modules) By D (nT)s) And Ir(nTs) Corresponding multiplication, i.e. stripping off navigation message data of I branch, by D (nT)s) And Qr(nTs) And correspondingly multiplying, namely stripping the navigation message data of the Q branch.
This time is:
respectively carrying out periodic superposition on baseband signal data subjected to text stripping in the two branches, and recording the baseband signal subjected to periodic superposition as Ia(nTs),Qa(nTs)。
For QPSK modulation, the navigation baseband signal has two values: -1 and + 1. In order to enable the baseband signals after the period superposition to reflect the real amplitude characteristics of the signals better and facilitate the data processing process, the signals after the period superposition are subjected to normalization processing. The normalized factor D is typically calculated as follows, based on the I branch signal:
wherein N is the number of sampling points of the data section of the I branch baseband signal.
The normalization method is shown in formulas (9) to (10):
Q(nTs)=Qa(nTs)/D (10)
whereinNormalizing (post) data for branch I, Q (nT)s) And normalizing the data of the Q branch.
4. The orthogonality calculation module extracts level values of the distribution histogram and calculates orthogonality;
the orthogonality calculation module provides a core algorithm for detecting orthogonality abnormity and is divided into two parts: level value extraction and orthogonality calculation based on the distribution histogram.
(1) And respectively utilizing the distribution histogram to count the level probability distribution of the I/Q two branch baseband signals, and extracting the counting peak value level value.
Since the value of the pseudo code is-1 or 1, it is idealHaving 4 level values Are respectively denoted as { I1,I2,I3,I4},Q(nTs) With two level valuesAre respectively denoted as { Q1,Q2It can be seen that non-orthogonal information is contained in the relationship of these 6 level values.
However, in the actually received signal, there will be noise, and the characteristic of the low-pass filter in the receiving loop will make the rectangular square wave signal generate overshoot at the discontinuous point and change smoothly, so the actually received signal will have noise, so the square wave signal will be in fact in the receiving loopNot only 4 level values, Q (nT)s) Nor 2 levels, which makes the acquisition of information difficult. Considering that the average value of the additive noise is 0, the waveform edge jitter caused by the filtering of the first low-pass filter and the second low-pass filter is attenuated quickly, the level values of most chip sampling points are still in ideal level values, and the corresponding level value of the counting peak can be found out by using the distribution histogram of the I branch as { I branch1,I2,I3,I4Finding out the level value corresponding to the counting peak value by using the distribution histogram of the Q branch, and recording the level value as { Q }1,Q2And the influence of other factors on the calculation result is effectively reduced.
(2) The relative offset angle of the Q branch can be calculated using these 6 level values. The calculation method is as follows:
wherein,is according to I1And I2The angle of the offset is calculated as the angle of the offset,according to I3And I4The calculated offset angles can both reflect the relative offset angle of the Q branch, theoretically, the two are very close in value, and the mean value of the two can be used in a non-orthogonality testThe degree of non-orthogonality is characterized.

Claims (2)

1. A QPSK modulation I/Q branch orthogonality test method based on a distribution histogram is characterized in that:
firstly, a satellite signal acquisition module acquires signals;
the high-gain antenna receives a satellite signal to be detected, the received signal enters a down converter after being filtered and amplified by a pre-filter and a pre-amplifier, the down converter mixes the signal with a local carrier to generate an analog intermediate frequency signal, the analog intermediate frequency signal is converted into a digital intermediate frequency signal by an A/D converter, and the digital intermediate frequency signal is recorded as r (nT)S) (ii) a MiningDigital intermediate frequency signals collected for a period of time are stored in a data disk array and are ready to be sent to a digital down-conversion module;
a QPSK modulated digital intermediate frequency signal in an ideal case without considering noise, the expression is as follows:
r ( nT s ) = 2 P I c ( nT s ) D ( nT s ) c o s ( w · nT s + φ ) + 2 P Q P ( nT s ) D ( nT s ) s i n ( w · nT s + φ ) - - - ( 1 )
where n is the sequence number of the sample sequence,Tsfor the sampling period, w is the intermediate frequency, phi is the initial phase, PIIs the power of the I branch signal, PQIs the power of the Q branch signal, c (nT)S) For the sampling value of the pseudo code of branch I, P (nT)S) For sampling Q-branch pseudo-code, D (nT)s) Sampling values of the navigation messages; under normal conditions, the phase difference of the carrier waves of the I/Q branch is 90 degrees, and when the modulated digital intermediate frequency signal generates non-orthogonality abnormity, the non-orthogonality existsThe signal may be expressed as:
the above formula (2) can be arranged as follows:
secondly, the digital down-conversion module completes signal demodulation to obtain two paths of I/Q baseband signals;
firstly, a tracking loop of a receiver is utilized to track a collected digital intermediate frequency signal r (nT)S) In the branch I of (2), in combination with the digital intermediate frequency signal r (nT)S) Generates a carrier signal cos (w.nT) in phase with the I-branch carrier in the local oscillators+ phi) and a carrier signal sin (w.nt) orthogonal to the I-branch carriers+ phi); carrier signal cos (w.nT)s+ phi) is summed with the digital intermediate frequency signal r (nT) in a first multipliers) After multiplication, an I branch baseband signal is obtained through a first low-pass filter and is marked as Ir(nTs) Analogously, the carrier signal sin (w.nT)s+ phi) is summed with the digital intermediate frequency signal r (nT) in the second multipliers) After multiplication, a Q branch baseband signal is obtained through a second low-pass filter and is marked as Qr(nTs);Ir(nTs)、Qr(nTs) A baseband signal for local restoration;
this time is:
thirdly, the data preprocessing module carries out normalization processing on the two paths of baseband signals;
with D (nT)s) And Ir(nTs) Corresponding multiplication, i.e. stripping off navigation message data of I branch, by D (nT)s) And Qr(nTs) Correspondingly multiplying, namely stripping the navigation message data of the Q branch;
this time is:
respectively carrying out periodic superposition on baseband signal data subjected to text stripping in the two branches, and recording the baseband signal subjected to periodic superposition as Ia(nTs),Qa(nTs);
Normalizing the signals after the period superposition, and calculating a normalization factor D by taking the I branch signal as a reference as follows:
D = 1 N ( Σ n = 1 N I a 2 ( nT s ) ) - - - ( 8 )
wherein N is the number of sampling points of the data section of the I branch baseband signal;
the normalization method is shown in formulas (9) to (10):
I ~ ( nT s ) = I a ( nT s ) / D - - - ( 9 )
Q(nTs)=Qa(nTs)/D (10)
whereinNormalized data for branch I, Q (nT)s) Normalizing the data for the Q branch;
fourthly, the orthogonality calculation module extracts the level value of the distribution histogram and calculates the orthogonality;
(1) respectively utilizing the distribution histogram to count the level probability distribution of the I/Q two branch baseband signals, and extracting a counting peak value level value;
since the value of the pseudo code is-1 or 1, it is idealHaving 4 level values Are respectively denoted as { I1,I2,I3,I4},Q(nTs) With two level valuesAre respectively denoted as { Q1,Q2};
Finding the corresponding level value of the counting peak as { I }by using the distribution histogram of the I branch1,I2,I3,I4Finding out the level value corresponding to the counting peak value by using the distribution histogram of the Q branch, and recording the level value as { Q }1,Q2};
(2) The relative offset angle of the Q branch is calculated by using the 6 level values, and the calculation method is as follows:
2. the method for testing orthogonality of I/Q branch of QPSK modulation based on distribution histogram as claimed in claim 1, wherein: the cut-off frequency of the first low-pass filter is greater than c (nT)s) And P (nT)s) The bandwidth of the two pseudo codes ensures that the two pseudo codes are contained in a time domain; the cut-off frequency of the second low-pass filter is greater than P (nT)s) Of pseudo-codeBandwidth.
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