CN115190048A - Low-bit-rate signal demodulation and bit error rate testing device and testing method thereof - Google Patents
Low-bit-rate signal demodulation and bit error rate testing device and testing method thereof Download PDFInfo
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Abstract
The invention provides a low-code-rate signal demodulation and bit error rate test device which comprises a low-code-rate modulation signal bit error generation unit, a low-code-rate modulation signal to noise ratio calibration unit, a signal demodulation unit and a bit error rate test unit. The test method comprises the following steps: s1, generating error codes; s2, calibrating a signal-to-noise ratio; s3, signal demodulation and bit error rate test: and S4, verifying error code test results. The scheme of hardware equipment acquisition and software radio demodulation has the characteristics of simple structure, good synchronization performance, strong expandability and the like, and is easy to realize and low in hardware cost. The polar switching method and the translation adjusting method are adopted, so that the problems of 'inverted pi' phenomenon and phase synchronization in the demodulation process can be quickly solved, and the calculated amount is low. The Butterworth secondary filtering method greatly improves the signal-to-noise ratio of the demodulated signal, improves the integral decoding performance of the system, and reduces the error rate of the system.
Description
Technical Field
The invention relates to the technical field of national defense and military radio electronics, in particular to a low-code-rate signal demodulation and bit error rate testing device and a testing method thereof.
Background
Deep space exploration has great significance in the aspects of science and technology, economy and the like, and becomes a hot spot of current research. Deep space communication is an important guarantee for realizing deep space detection, and compared with traditional wireless communication, the deep space communication method has the advantages that the transmission distance is long, the energy of a detector is limited, the movement is variable, and the like. In particular, deep space communication needs to solve the communication problem of an extremely low signal to noise ratio (SNR), that is, how to effectively and reliably implement demodulation and fast reconstruction of a deep space communication link after transmission interruption under the extremely low SNR to achieve high-quality transmission of signals, and the like, so as to ensure that an instruction reaching a spacecraft is accurate. The Bit Error Rate (BER) of a communication system is an index for measuring the data transmission accuracy of data in a specified time, and is a main judgment basis for measuring the reliability of a digital system. Different modulation and demodulation systems are likely to be interfered by noise in the transmission process in an additive Gaussian noise channel, so that an error code phenomenon occurs at a receiving end. The traditional error code meter can only test the bit error rate of a baseband digital sequence and does not have the function of demodulating a digital modulation signal of a deep space exploration communication system. By testing the bit error rate of the deep space exploration communication system, an effective demodulation and bit error rate testing method for low-code-rate signals is researched, the anti-noise performance is improved, and reference can be provided for improvement and design of a novel communication system.
Disclosure of Invention
The invention aims to provide a low-code-rate signal demodulation and bit error rate test device and a test method thereof, which are used for solving the problems in the prior art.
The technical scheme of the invention is as follows: the low-code-rate signal demodulation and bit error rate test device comprises a low-code-rate modulation signal bit error generation unit, a low-code-rate modulation signal to noise ratio calibration unit, a signal demodulation unit and a bit error rate test unit;
the low-code-rate modulation signal error code generating unit comprises a first arbitrary wave generator, a second arbitrary wave generator and a combiner; the signal-to-noise ratio calibration unit of the low-code-rate modulation signal comprises a voltmeter; the signal demodulation unit comprises an oscilloscope and a computer; the error rate testing unit is arranged in the computer;
the first arbitrary wave generator generates a low-bit-rate baseband signal, inputs the low-bit-rate baseband signal into an external modulation port of the second arbitrary wave generator, sets a modulation signal form, a signal frequency and a signal amplitude, and outputs a modulation signal waveform from a channel 1 of the second arbitrary waveform generator; setting the second arbitrary waveform generator channel 2 to output Gaussian white noise, wherein the signals output by the second arbitrary waveform generator channels 1 and 2 pass through a combiner and then output modulation signals with the Gaussian white noise; the low-code-rate modulation signal-to-noise ratio calibration unit calibrates the modulation signal with the Gaussian white noise output by the combiner;
and the modulation signal with white Gaussian noise output by the combiner is input into the oscilloscope channel 1, the oscilloscope performs high-speed sampling, the sampling result is input into the computer, the error rate test unit performs demodulation to obtain a baseband signal, and the demodulated baseband signal and the reference baseband signal are subjected to digital-to-analog conversion and are compared bit by bit to obtain an error rate test result.
Further, the first arbitrary wave generator generates a low-code-rate baseband signal, and sets PN code type, code element width, code element period and baseband signal amplitude parameters.
Another technical solution of the present invention provides a testing method using the low-bit-rate signal demodulation and bit error rate testing apparatus, including the following steps:
s1, error code generation:
a first arbitrary waveform generator in the low-code-rate modulation signal error code generating unit generates a low-code-rate baseband signal; the second arbitrary waveform generator channel 2 outputs Gaussian white noise, and the signals output by the second arbitrary waveform generator channels 1 and 2 output modulation signals with the Gaussian white noise after passing through the combiner;
s2, signal-to-noise ratio calibration:
respectively measuring the ratio of useful modulation signal power to noise power in the modulation signals with the Gaussian white noise output by the combiner;
s3, signal demodulation and bit error rate test:
s3-1, extracting a carrier wave by using a square loop method by using a software radio technology to complete carrier synchronization of the BPSK demodulator;
s3-2, acquiring a carrier, completing coherent demodulation through a multiplier, and acquiring a baseband signal through low-pass filtering;
s3-3, performing secondary filtering by using a Butterworth filter, wherein the cut-off frequency of the Butterworth filter is set to be the frequency of the baseband signal;
s3-4, solving the problem of 'inverted pi' of the phase in the carrier recovery process by a polarity switching method;
s3-5, performing code element translation on the baseband signal obtained by demodulation through a time domain translation adjustment method until the baseband signal is aligned with a code element of the original baseband signal, so as to remove the phase difference between the baseband code element signal obtained by demodulation and the original baseband code element signal of the reference channel;
s3-6, carrying out bit-by-bit comparison after the signals are aligned so as to extract error rate parameters;
and S4, verifying error code test results.
Further, in step S2, a ratio of useful modulation signal power to noise power in the modulation signal with white gaussian noise output by the combiner is measured:
firstly, the noise signal is closed, the useful signal output power is measured to be Psignal (dBm) by a voltmeter, then the modulation signal is removed, the noise output power is measured to be Pnoise (dBm) by the voltmeter, and then the measured value of the signal-to-noise ratio S/N is obtained:
S/N＝Psignal(dBm)-Pnoise(dBm)。
further, in step S3-3, the butterworth filter approximates the ideal rectangular characteristic of the filter in the form of the highest order taylor series, and the amplitude-frequency characteristic response expression thereof is:
|H(ω)| 2 ＝1/[1+(ω/ω c ) 2n ]
wherein n =1,2,3 \ 8230is the order of the filter, ω c Is the cut-off angular frequency of the filter.
Further, in the step S3-4, if the demodulated signal is in phase with the carrier signal, the polarity switch does not need to be turned on, and the phase of the signal is unchanged; if the demodulated signal is opposite to the reference signal, the polarity switch is turned on and the phase must be reversed by 180 ° after multiplication.
Further, in the step S3-6, a comparison method is adopted to calibrate the error rate; and calculating the error code number of the signal by using an exclusive OR mode, carrying out exclusive OR operation on the demodulated baseband signal code element and the original baseband signal code element, wherein the exclusive OR result is 0 when the code elements are consistent, and the exclusive OR result is 1 when the code elements are not consistent, so that the error code number can be obtained by accumulating all the exclusive OR results, and the error code number is divided by the code element number of the signal, thereby obtaining the error code rate.
Further, in step S3-6, a cyclic accumulation mode is adopted to accumulate the baseband signal code elements obtained by multiple data acquisition of the oscilloscope and calculate the error code number thereof, thereby obtaining the error rate.
Further, in step S4, a coherent demodulation method is used, and theoretically BPSK/QPSK bit error rate performance p is used e As shown in the following formula:
in the formula, E b /N 0 For normalized signal-to-noise ratio, erf is the error function as shown below:
in the formula, z is
The low-bit-rate signal demodulation and bit error rate testing device and the testing method thereof provided by the invention have the beneficial effects that:
the scheme of hardware equipment acquisition and software radio demodulation has the characteristics of simple structure, good synchronization performance, strong expandability and the like, and is easy to realize and low in hardware cost. The polar switching method and the translation adjusting method are adopted, so that the problems of 'inverted pi' phenomenon and phase synchronization in the demodulation process can be quickly solved, and the calculated amount is low. The Butterworth secondary filtering method greatly improves the signal-to-noise ratio of the demodulated signal, improves the integral decoding performance of the system, and reduces the error rate of the system.
Drawings
The invention is further described with reference to the accompanying drawings:
FIG. 1 is a schematic diagram of an error code generating unit of a low bit rate modulation signal according to the present invention;
FIG. 2 is a schematic diagram of the signal-to-noise ratio calibration unit of the low-bit-rate modulation signal provided by the present invention;
FIG. 3 is a schematic diagram of the signal demodulation unit and the BER test unit provided in the present invention;
FIG. 4 is a schematic diagram illustrating a testing process of the signal demodulation unit and the BER testing unit according to the present invention;
FIG. 5 is a hardware configuration diagram of a low code rate signal demodulation and bit error rate testing apparatus according to an embodiment of the present invention;
FIG. 6 is a diagram illustrating baseband signal generation according to an embodiment of the present invention;
fig. 7 is a schematic diagram of BPSK modulated signal generation in an embodiment of the invention;
FIG. 8 is a schematic diagram of a baseband sequence sampled by an oscilloscope in an embodiment of the present invention;
FIG. 9 is a diagram illustrating a computer reading a raw baseband signal according to an embodiment of the present invention;
fig. 10 is a schematic diagram of a PN code modulated BPSK signal obtained by a computer according to an embodiment of the present invention;
fig. 11 shows the spectrum of a PN code modulated BPSK signal in an embodiment of the present invention;
FIG. 12 is a diagram of a coherent demodulated signal (without noise) in an embodiment of the present invention;
FIG. 13 is a schematic diagram of a low-pass filtered waveform (without noise) in an embodiment of the present invention;
FIG. 14 is a schematic diagram of a waveform filtered and de-noised by a Butterworth filter according to an embodiment of the present invention (without noise);
FIG. 15 is a diagram of a demodulated baseband signal (without noise) according to an embodiment of the present invention;
FIG. 16a is a schematic diagram of waveforms before panning (without noise) in an embodiment of the present invention;
FIG. 16b is a schematic diagram of the waveform after the translational adjustment (without noise) in the embodiment of the present invention;
fig. 17 is a schematic diagram of a BPSK modulation waveform with noise according to an embodiment of the present invention;
FIG. 18 is a diagram illustrating a coherent demodulated signal with noise according to an embodiment of the present invention;
FIG. 19 is a waveform illustrating a first low pass filtered waveform of noise in an embodiment of the present invention;
FIG. 20 is a schematic diagram of a waveform after two-pass filtering and noise reduction by a Butterworth filter according to an embodiment of the present invention;
FIG. 21a is a waveform illustrating an exemplary waveform without using polarity switches after shaping;
FIG. 21b is a diagram illustrating an original PN code waveform in an embodiment of the present invention;
FIG. 21c is a schematic diagram of a waveform demodulated using a polarity switch according to an embodiment of the present invention;
FIG. 22a is a diagram illustrating a demodulation waveform without time-domain shift adjustment according to an embodiment of the present invention;
fig. 22b is a schematic diagram of a bit error rate calculation result without performing translation adjustment according to the embodiment of the present invention, where the bit error rate adjustment parameter: 0;
fig. 23 is a diagram of the post-translational adjustment in the embodiment of the present invention, where the error rate adjustment parameter: -50;
FIG. 24 is a diagram illustrating baseband waveforms demodulated after panning in an embodiment of the invention;
FIG. 25 is a graph illustrating bit error rate measurements (9.6 dB) according to an embodiment of the present invention;
FIG. 26 is a graph showing the measurement results of the bit error rate ((8.35 dB);
FIG. 27 is a graph illustrating bit error rate measurements (6.75 dB) in accordance with an embodiment of the present invention;
fig. 28 is a schematic diagram of the measurement result (3 dB) of the ber according to the embodiment of the present invention.
Detailed Description
The low-bit-rate signal demodulation and bit error rate test device and the test method thereof proposed by the present invention are further described in detail with reference to the accompanying drawings and the specific embodiments. Advantages and features of the present invention will become apparent from the following description and from the claims. It is to be noted that the drawings are in a very simplified form and are all used in a non-precise ratio for the purpose of facilitating and distinctly aiding in the description of the embodiments of the invention.
The core idea of the invention is that the invention provides a low-code-rate signal demodulation and bit error rate test device and method, which can realize demodulation and decoding of a low-code-rate modulation signal under the white Gaussian noise environment by combining hardware based on the software radio principle. The low-code-rate signal demodulation and bit error rate test device comprises a low-code-rate modulation signal bit error generation unit, a low-code-rate modulation signal to noise ratio calibration unit, a signal demodulation unit and a bit error rate test unit. The invention collects modulation signal data containing noise from an oscilloscope, firstly adopts a square loop method to extract carrier waves on a computer for carrier recovery, and then obtains baseband signals through coherent demodulation and low-pass filtering. Noise in the baseband signal is further reduced by a Butterworth filter secondary filtering method, and information of the PN code element is obtained through sampling judgment and shaping. The 'inverted pi' phenomenon in the signal carrier recovery process is solved through a polarity switching method, and the 'alignment' problem of the PN code is solved through a time domain translation adjustment method, so that the accurate reduction of the PN code is realized. The test result proves that the system demodulation performance is improved and the error code is reduced by the aid of a Butterworth filter secondary filtering method, a polarity switching method and a time domain translation adjusting method, and the error code rate test result is obviously improved in a low signal-to-noise ratio communication environment.
The Butterworth filter secondary filtering method is used for noise reduction under the condition of low signal-to-noise ratio. The signal to noise ratio of the signal in the strong background noise environment is low, the signal can still be submerged in the noise after the carrier signal is removed by low-pass filtering, and the noise reduction effect is difficult to achieve by the single filtering method under the condition of low signal to noise ratio. The influence caused by noise can be reduced through secondary filtering, a smoother baseband signal curve is obtained, the error rate test under the condition of low signal-to-noise ratio is realized, the signal-to-noise ratio is greatly improved, a better error rate test result can be obtained under the condition of the same signal-to-noise ratio, and the decoding performance of the system is improved.
The polar switching method is mainly used for solving the 'inverted pi' phenomenon in the signal carrier recovery process. The low-bit-rate baseband signal for deep space communication is usually a BPSK signal, and the demodulation method for the signal is a coherent demodulation method. Since BPSK signals themselves use phase to convey information, the phase information of the signal must be used at the receiving end to demodulate the signal. In an actual communication system, a coherent carrier needs to be recovered from a received modulated signal by using a phase-locked loop, the complexity of the system is increased by the process, meanwhile, a phase deviation of 180 degrees may exist between the recovered carrier and a carrier during modulation, the recovered local carrier and a required coherent carrier may be in phase or in phase opposition, uncertainty of the phase relationship will cause that a demodulated digital baseband signal is exactly opposite to a transmitted digital baseband signal, namely, '1' becomes '0', '0' becomes '1', and all errors of a digital signal output by a decision device are caused, which is called as a 'inverse pi' phenomenon of a BPSK method. In order to solve the 'inverse pi' phenomenon, the method solves the problem by designing a polarity switch for adjustment in the demodulation process. The polarity switch plays a role of changing the direction of the digital baseband signal, namely when the phase has 'inverted pi' phenomenon, the '1' is changed into '0', the '0' is changed into '1', and the phase of the error code element is adjusted.
The time domain translation adjustment method is used for accurately aligning PN codes and realizing bit synchronization under the condition of unknown prior information. In the modulation and demodulation technology, due to different system transmission paths, a received signal has a certain delay, a phase difference exists between a baseband signal obtained by demodulation and a baseband signal of a reference channel, and the phase difference can influence the judgment of an error rate. To solve the problem of phase synchronization and obtain an accurate bit error rate, the phase difference should be eliminated first. The traditional sliding correlation method needs a large amount of multiplication and accumulation operations, the searching speed is low, and the average PN code capturing time is long. The existence of the phase difference causes the two code sequences to appear to have relative sliding in time sequence, therefore, the method removes the phase difference by adopting a time domain translation adjustment mode, namely, the demodulated baseband signal is subjected to code element translation until the demodulated baseband signal is aligned with the code element of the original baseband signal, and bit-by-bit comparison is carried out after the signals are aligned to extract the error rate parameter.
Example 1
The low-bit-rate signal demodulation and bit error rate test device provided by the embodiment comprises a low-bit-rate modulation signal bit error generation unit, a low-bit-rate modulation signal to noise ratio calibration unit, a signal demodulation unit and a bit error rate test unit;
the low-code-rate modulation signal error code generating unit comprises a first arbitrary wave generator, a second arbitrary wave generator and a combiner; the signal-to-noise ratio calibration unit of the low-code-rate modulation signal comprises a voltmeter; the signal demodulation unit comprises an oscilloscope and a computer; the error rate testing unit is arranged in the computer;
the first arbitrary wave generator generates a low-bit-rate baseband signal, inputs the low-bit-rate baseband signal into an external modulation port of the second arbitrary wave generator, sets a modulation signal form, a signal frequency and a signal amplitude, and outputs a modulation signal waveform from a channel 1 of the second arbitrary waveform generator; setting the second arbitrary waveform generator channel 2 to output Gaussian white noise, wherein the signals output by the second arbitrary waveform generator channels 1 and 2 pass through a combiner and then output modulation signals with the Gaussian white noise; the low-code-rate modulation signal-to-noise ratio calibration unit calibrates the modulation signal with the Gaussian white noise output by the combiner;
and the modulation signal with white Gaussian noise output by the combiner is input into the oscilloscope channel 1, the oscilloscope performs high-speed sampling, the sampling result is input into the computer, the error rate test unit performs demodulation to obtain a baseband signal, and the demodulated baseband signal and the reference baseband signal are subjected to digital-to-analog conversion and are compared bit by bit to obtain an error rate test result.
The test method using the low-bit-rate signal demodulation and bit error rate test device comprises the following steps:
the first step is as follows: an error is generated.
As shown in fig. 1, an arbitrary waveform generator 1 in the error code generation unit generates a low-bit-rate baseband signal, and sets parameters such as PN code type, symbol width, symbol period, and baseband signal amplitude. The baseband signal output by the arbitrary waveform generator 1 is input to an external modulation port of the arbitrary waveform generator 2, the modulation signal form, the signal frequency and the signal amplitude are set, and the arbitrary waveform generator 2 outputs the modulation signal waveform through a channel 1. An arbitrary waveform generator 2 is arranged, a channel 2 outputs Gaussian white noise, and two channel signals output modulation signals with the Gaussian white noise after passing through a combiner.
The invention adopts an arbitrary waveform generator to generate a PN code baseband signal and a Gaussian white noise signal and completes the modulation function of a carrier signal. In digital communication systems, E is commonly used s /N 0 (or using its normalized form (E) b /N 0 ) As a performance indicator of signal-to-noise ratio. E s Is the energy per symbol, equal to the product of the signal power S and the duration per symbol Ts; n is a radical of 0 Is the noise power spectral density, equal to the ratio of the noise power N to the bandwidth B; and because of the duration T per symbol b Reciprocal of the symbol rate Rs, which may be 1/T b Instead of Rs, the following expression holds:
for an ideal channel with only additive white Gaussian noise, when E b /N 0 After determination, a certain error rate value is reachedThe signal-to-noise ratio S/N is calculated as:
S/N＝(E s /N 0 )×(R/B) (2)
in the formula: e b For the energy per bit of the signal, E b ＝E s /f s ，f s Is the symbol rate. N is a radical of hydrogen 0 A noise power spectral density for the transmission channel; b is the equivalent noise bandwidth of the detection filter; r is the bit rate, characterizing the spectral efficiency of the transmitted signal.
E s /N 0 ＝E b /N 0 ×(R/R s )＝E b /N 0 ×log2(M) (3)
Where M is the number of constellation points of the modulated signal. The number of constellation points for BPSK modulation is 2, the bit rate is the same as the symbol rate, the number of constellation points for QPSK modulation is 4, the bit rate is twice the symbol rate, the number of constellation points for 8PSK modulation is 8, and the bit rate is three times the symbol rate.
The second step is that: and calibrating the signal-to-noise ratio.
As shown in fig. 2, the ratio of the useful modulated signal power to the noise power in the combiner output signal is measured. First the noise signal is turned off, the useful signal output power is measured as Psignal (dBm) with a voltmeter, then the modulation signal is removed, the noise output power is measured as Pnoise (dBm) with a voltmeter, and then the signal-to-noise ratio S/N measurement is obtained:
S/N Psignal(dBm)-Pnoise(dBm) (4)
the invention realizes the calibration of the signal-to-noise ratio by respectively measuring the power of the modulation signal and the power of the noise. The BPSK signal spectrum has the following characteristics: (1) The spectrum shapes of the BPSK signal and the baseband modulation signal are completely the same, but the frequency axis is shifted to the right by one carrier frequency; (2) Under the condition of equal probability of transmitting symbols l and 0, a discrete spectrum of a carrier component does not exist in a BPSK signal power spectrum, the BPSK signal is equivalent to a double-sideband signal for restraining a carrier, the carrier energy is completely converted into signal energy through modulation, and the carrier energy C is equal to the signal energy S. Therefore, the signal-to-noise ratio can be calibrated by measuring the power of the modulation signal and the power of the noise respectively.
The third step: signal demodulation and error rate testing.
As shown in fig. 3, an oscilloscope and a computer are used to form a signal demodulation and bit error rate test device. One path of PN code baseband signals generated by the arbitrary waveform generator 1 is input to an external modulation port of the arbitrary waveform generator 2 to be modulated to be used as detected signals, the other path of PN code baseband signals is input to an oscilloscope channel 2 to be used as reference baseband signals, and the two paths of PN code baseband signals are synchronously output. The modulation signal containing noise output by the combiner is input to the oscilloscope channel 1, then the oscilloscope performs high-speed sampling on two paths of signals, the sampling result is input to the computer, the modulation signal is demodulated by software to obtain a baseband signal, the demodulated baseband signal and the reference baseband signal are subjected to digital-to-analog conversion and are compared bit by bit to obtain an error rate test result, and the flow schematic diagrams of the signal demodulation unit and the error rate test unit are shown in fig. 4.
Firstly, the invention uses software radio technology to realize the square loop method to extract the carrier wave to complete the carrier synchronization of the BPSK demodulator, and the square loop method is used to realize the carrier synchronization of the BPSK digital receiver, and has the characteristics of simple structure, higher efficiency, good synchronization performance and the like. Let BPSK signal form be:
m(t)＝s(t)cos2πf c t (5)
wherein the probability that s (t) = ± 1 is equal, the above formula signal is squared to obtain:
as can be seen from the above, s 2 (t) has a value of 1, and a narrow band filter is used to extract 2f from y (t) c Frequency component, carrier frequency f of BPSK signal can be obtained by using frequency divider c This is the working principle of extracting the carrier wave by the square loop.
After the carrier is obtained, coherent demodulation is completed through a multiplier, and then a baseband signal is obtained through low-pass filtering. The amplitude of the rectangular pulse signal still fluctuates due to channel noise and the like. In the invention, in order to ensure that the signal obtained by the receiving end has the maximized signal-to-noise ratio, after a digital receiving system firstly uses an FIR filter to filter certain noise components, a Butterworth filter is used for secondary filtering, and the method is favorable for improving the demodulation signal-to-noise ratio and the system error code performance. A butterworth low pass filter is a filter that has the maximum flat amplitude characteristic with the butterworth function as the transfer function of the filter, which approximates the ideal rectangular characteristic of the filter in the form of the highest order taylor series. The amplitude-frequency characteristic response expression is as follows:
|H(ω)| 2 ＝1/[1+(ω/ω c ) 2n ] (7)
wherein n =1,2,3 \ 8230denotes the order of the filter, ω c Is the cut-off angular frequency of the filter.
The Butterworth filter can obtain a standard waveform only by proper adjustment, the order of the filter controls whether an output graph of the Butterworth filter has delay, and the cut-off frequency of the filter is set to be the frequency of a baseband signal in the debugging process.
After filtering, the invention solves the problem of phase inversion pi' in the carrier recovery process by designing a polarity switch. If the demodulated signal is in phase with the carrier signal, the polarity switch does not need to be started, and the signal phase is unchanged; if the demodulated signal is opposite to the reference signal, the polarity switch is turned on, and the phase must be reversed by 180 ° after multiplication, which is the same as the phase of the reference signal.
After the phases are consistent, the invention adopts a translation adjustment method to solve the problem that the received signals have certain time delay due to different system transmission paths, and the baseband signals obtained by demodulation are subjected to code element translation until the baseband signals are aligned with the code elements of the original baseband signals by setting the position of translation adjustment, so that the phase difference between the baseband code element signals obtained by demodulation and the original baseband code element signals of the reference channel is removed.
Finally, after the signals are aligned, bit-by-bit comparison is carried out so as to extract the error rate parameter. The bit error rate, which is defined as the ratio of the erroneous bits to the total number of transmitted bits, is related to the signal-to-noise ratio at the input. The invention adopts a comparison method to calibrate the error rate. And calculating the error code number of the signal by using an exclusive-or mode, carrying out exclusive-or operation on the demodulated baseband signal code element and the original baseband signal code element, wherein the exclusive-or result is 0 when the code elements are consistent, and the exclusive-or result is 1 when the code elements are not consistent, so that the error code number can be obtained by accumulating all the exclusive-or results, and the error code number is divided by the code element number of the signal to obtain the error code rate. Because the error code test is carried out under the condition of extremely low code rate, the code element number of data acquisition once carried out by the oscilloscope is less, and a cyclic accumulation mode is adopted to accumulate the baseband signal code elements obtained by the data acquisition of the oscilloscope for many times and calculate the error code number of the baseband signal code elements, thereby obtaining the error code rate.
The fourth step: and verifying the error code test result.
As shown in fig. 5, the performance of the bit error rate testing apparatus of the present invention is verified by comparing with a theoretical value.
After Gaussian white noise is added, different PSK modulation and demodulation modes have different error rate performances. BPSK/QPSK is taken as an example, a coherent demodulation method is used, and the BPSK/QPSK error rate performance p is theoretically achieved e As shown in the following formula:
in the formula, E b /N 0 For normalized signal-to-noise ratio, erf is the error function as shown below:
it can be seen from the above formula that there is a certain relationship between the bit error rate and the signal-to-noise ratio of the BPSK/QPSK modulation system: the lower the signal-to-noise ratio, the greater the bit error rate. In the conventional method, the amplitude of the rectangular pulse signal subjected to correlation demodulation and low-pass filtering fluctuates due to channel noise and the like. In the invention, in order to reduce the error rate, the signal obtained by the receiving end has the maximized signal-to-noise ratio. The invention uses a butterworth filter at the receiving system to re-filter. After passing through the Butterworth filter, the noise reduction effect is obvious, and therefore the influence of the bit error rate of a system caused by poor signal to noise ratio is reduced. Experiments prove that the method can greatly reduce the error rate of the system and improve the decoding performance of the system by improving the signal-to-noise ratio of the demodulation signal under the condition of inputting the same signal-to-noise ratio.
The technical effect is verified:
fig. 5 is a hardware composition diagram of the low code rate signal demodulation and bit error rate testing apparatus according to the present invention.
A list of specific system hardware compositions is shown in table 1.
TABLE 1 hardware composition list of demodulation and bit error rate tester for low code rate signals
(1) BPSK signal demodulation example without adding noise PN code as baseband signal:
the instrument parameters were set as follows:
(a) 33522A Arbitrary wave generator, refer to FIG. 6:
BPSK signal: sine wave frequency 20kHz, amplitude 1Vpp, BPSK phase 180 °
Baseband signals: PN7, the code element width is 1ms, the PN code period is 127ms, and the amplitude is 3.5Vpp;
(b) TDS5032B oscilloscope: sample rate 2.5MS/s, record length 500K, refer to fig. 7:
the baseband signal generated by the arbitrary waveform generator 33522A consists of a pseudo-random code PN7 sequence, with a symbol width of 1ms and a PN code period of 127ms. As shown in fig. 8 and 9, the symbols after 127ms are repeatedly transmitted. PN7 is input as a modulation signal to the BPSK external modulation port of arbitrary waveform generator 33522A, and the resulting BPSK waveform is shown in fig. 10.
It can be seen that the phase of the sine wave signal is modulated by the baseband signal, and the phase changes by 180 ° in each 0,1 jump, and the spectrum of the BPSK signal is shown in fig. 11.
As can be seen from fig. 11, the PN code modulated BPSK signal is also a carrier suppressed signal, and the carrier signal energy at 20kHz is suppressed. After frequency division by a square loop, carrier information is extracted and coherent demodulation is performed, and a signal after multiplication by a multiplier is as shown in fig. 12.
The signals recovered by coherent demodulation need to be filtered to remove high-frequency components output by coherent demodulation, down-conversion is realized, and then subsequent response processing is performed on the retained low-frequency signals. According to the nyquist low-pass sampling theorem, we set the sampling frequency (Fs) of the low-pass filter to 2.5MHz, and the demodulated signal obtained after low-pass filtering is shown in fig. 13.
As can be seen from fig. 13, before and after the coherently demodulated signal passes through the low-pass filter, we can clearly see that the high-frequency component of the signal is reduced. The filtering was repeated using a butterworth filter and the resulting waveform after filtering is shown in fig. 14.
It can be seen that after passing through the Butterworth filter, the noise reduction effect is obvious, and the rectangular pulse signal is relatively regular.
The demodulation waveform obtained after analog-to-digital conversion is shown in fig. 15.
And adjusting the time delay in a translation adjusting mode, and aligning the demodulated baseband signal with the code element of the original baseband signal. And the bit error rate is calculated, and the resulting comparison waveforms of the demodulated signal and the baseband reference signal are shown in fig. 16a (before translation) and 16b (after translation).
(2) Adding a white noise PN code as BPSK signal bit error rate measurement of a baseband signal:
the instrument parameters were set as follows:
the bandwidth of the signal generator is set to be 2MHz, and the noise voltage is 1V-1.5V (peak-to-peak value). The signal-to-noise ratio calibrated by the URE3 voltmeter is 6.35dB (the theoretical value of the error rate is 1.0 multiplied by 10) -3 ). The waveforms of the modulated signal containing noise and the waveform of the demodulated signal are shown in fig. 17 to 24.
As can be seen from fig. 17, 18 and 19, after the first filtering, the noise is still large, and error codes are easily caused; as can be seen from fig. 20, after passing through the butterworth filter, the noise reduction effect is obvious; as can be seen from fig. 21a and 21b, the polarity switch waveform and the original PN code waveform are not used for inversion, so that the polarity switch input value is set to-1, and the phase of the demodulated baseband signal is adjusted to be equal to the original PN code oneThus, as shown in FIG. 21 c; as can be seen from fig. 21c, there may be a delay between the demodulated information and the original information around 0.04s, and details are enlarged as shown in fig. 22a to 22 b. It can be seen that the calculated bit error rate is 7.67 × 10 without performing the translation adjustment -5 。
The bit error rate is calculated after the shift adjustment is performed so that the symbols are aligned, and the waveform after the period shift is as shown in fig. 23. After the translation adjustment is carried out to align the code elements, the calculated error rate is 3.2 multiplied by 10 -5 The error rate is reduced compared with that before the shift adjustment, and the demodulated baseband waveform of the present invention is shown in fig. 24.
(3) And verifying the error rate measurement result.
The comparison result between the measurement result of the bit error rate testing device and the theoretical bit error rate value in this embodiment is shown in table 2.
TABLE 2 comparison of measurement results of low-bit-rate signal demodulation and bit error rate test device and theoretical bit error rate values
As can be seen from fig. 25 to fig. 28, under the condition of low snr, the snr can be effectively improved and the bit error rate can be reduced by using the method of the present invention.
Those not described in detail in this specification are within the skill of the art. It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrative embodiments, and that the present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.
Claims (9)
1. A low-bit-rate signal demodulation and bit error rate test device is characterized by comprising a low-bit-rate modulation signal bit error generation unit, a low-bit-rate modulation signal-to-noise ratio calibration unit, a signal demodulation unit and a bit error rate test unit;
the low-code-rate modulation signal error code generating unit comprises a first arbitrary wave generator, a second arbitrary wave generator and a combiner; the signal-to-noise ratio calibration unit of the low-code-rate modulation signal comprises a voltmeter; the signal demodulation unit comprises an oscilloscope and a computer; the error rate testing unit is arranged in the computer;
the first arbitrary wave generator generates a low-bit-rate baseband signal, inputs the low-bit-rate baseband signal into an external modulation port of the second arbitrary wave generator, sets a modulation signal form, a signal frequency and a signal amplitude, and outputs a modulation signal waveform from a channel 1 of the second arbitrary waveform generator; setting the second arbitrary waveform generator channel 2 to output Gaussian white noise, wherein the signals output by the second arbitrary waveform generator channels 1 and 2 pass through a combiner and then output modulation signals with the Gaussian white noise; the low-code-rate modulation signal-to-noise ratio calibration unit calibrates the modulation signal with the Gaussian white noise output by the combiner;
the modulation signal with white gaussian noise output by the combiner is input to the oscilloscope channel 1, the oscilloscope performs high-speed sampling, the sampling result is input to the computer, the signal demodulation unit performs demodulation to obtain a baseband signal, and the bit error rate test unit performs digital-to-analog conversion and bit-by-bit comparison on the demodulated baseband signal and a reference baseband signal to obtain a bit error rate test result.
2. The apparatus for demodulating low-rate signals and testing error rate according to claim 1, wherein the first arbitrary wave generator generates a low-rate baseband signal, and sets PN code type, symbol width, symbol period, and amplitude parameters of the baseband signal.
3. A test method using the low bit rate signal demodulation and bit error rate test device as claimed in any one of claims 1 or 2, comprising the steps of:
s1, error code generation:
a first arbitrary waveform generator in the low-code-rate modulation signal error code generating unit generates a low-code-rate baseband signal;
s2, signal-to-noise ratio calibration:
measuring the ratio of useful modulation signal power to noise power in the modulation signal with Gaussian white noise output by the combiner;
s3, signal demodulation and bit error rate test:
s3-1, extracting carriers by using a square loop method by using a software radio technology to complete carrier synchronization of the BPSK demodulator;
s3-2, acquiring a carrier, completing coherent demodulation through a multiplier, and acquiring a baseband signal through low-pass filtering;
s3-3, performing secondary filtering by using a Butterworth filter, wherein the cut-off frequency of the Butterworth filter is set to be the frequency of the baseband signal;
s3-4, solving the problem of phase inversion pi' in the carrier recovery process by a polarity switching method;
s3-5, carrying out code element translation on the baseband signal obtained by demodulation through a time domain translation adjustment method until the baseband signal is aligned with the code element of the original baseband signal, and thus removing the phase difference between the baseband code element signal obtained by demodulation and the original baseband code element signal of the reference channel;
s3-6, carrying out bit-by-bit comparison after the signals are aligned so as to extract error rate parameters;
and S4, verifying error code test results.
4. The method for testing the apparatus for demodulating low bit rate signals and testing bit error rate as claimed in claim 3, wherein in step S2, the ratio of useful modulation signal power to noise power in the Gaussian white noise-containing modulation signal outputted from the combiner is measured:
first the noise signal is turned off, the useful signal output power is measured as Psignal (dBm) with a voltmeter, then the modulation signal is removed, the noise output power is measured as Pnoise (dBm) with a voltmeter, and then the signal-to-noise ratio S/N measurement is obtained:
S/N＝Psignal(dBm)-Pnoise(dBm)。
5. the method for testing a low bit rate signal demodulation and bit error rate testing device as claimed in claim 4, wherein in said step S3-3, said Butterworth filter approximates the ideal rectangular characteristic of the filter in the form of the highest order Taylor series, and the amplitude-frequency characteristic response expression is:
|H(ω)| 2 ＝1/[1+(ω/ω c ) 2n ]
wherein n =1,2,3 \ 8230is the order of the filter, ω c Is the cut-off angular frequency of the filter.
6. The method for testing the device for demodulating a low bit rate signal and testing the bit error rate according to claim 4, wherein in the step S3-4, if the demodulated signal is in phase with the carrier signal, the polarity switch does not need to be turned on, and the phase of the signal is not changed; if the demodulated signal is opposite to the reference signal, the polarity switch is turned on and the phase must be reversed by 180 ° after multiplication.
7. The method for testing the apparatus for demodulating low-bit-rate signals and testing bit error rate according to claim 4, wherein in the step S3-6, the bit error rate is calibrated by using a comparison method; and calculating the error code number of the signal by using an exclusive OR mode, carrying out exclusive OR operation on the demodulated baseband signal code element and the original baseband signal code element, wherein the exclusive OR result is 0 when the code elements are consistent, and the exclusive OR result is 1 when the code elements are not consistent, so that the error code number can be obtained by accumulating all the exclusive OR results, and the error code number is divided by the code element number of the signal, thereby obtaining the error code rate.
8. The method as claimed in claim 4, wherein in step S3-6, the error rate is obtained by accumulating the baseband signal code elements obtained by the oscilloscope through multiple data acquisitions in a cyclic accumulation manner and calculating the error code number.
9. Such as rightThe method for testing the apparatus for demodulating low bit rate signal and testing bit error rate of claim 4, wherein in the step S4, a coherent demodulation method is used, and theoretically BPSK/QPSK bit error rate performance p is used e As shown in the following formula:
in the formula, E b /N 0 For normalized signal-to-noise ratio, erf is the error function as shown below:
in the formula, z is
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