A low bit rate signal demodulation and bit error rate testing device and testing method thereof

Abstract

The present invention provides a low bit rate signal demodulation and bit error rate test device, including a low bit rate modulation signal bit error generation unit, a low bit rate modulation signal signal-to-noise ratio calibration unit, a signal demodulation unit and a bit error rate test unit. The test method thereof includes: S1, bit error generation; S2, signal-to-noise ratio calibration; S3, signal demodulation and bit error rate test: S4, bit error test result verification. The hardware equipment acquisition + software radio demodulation scheme of the present invention has the characteristics of simple structure, good synchronization performance, strong scalability, etc. It is not only easy to implement but also has low hardware cost. The polarity switching method and the translation adjustment method can quickly solve the "inverted π" phenomenon and phase synchronization problem in the demodulation process, and the calculation amount is low. The Butterworth secondary filtering method greatly improves the signal-to-noise ratio of the demodulated signal, improves the overall decoding performance of the system, and reduces the system bit error rate. The method can also be extended to the test and verification of the bit error rate of other digital modulation signals.

Description

Technical Field

[0001] The invention relates to the technical field of national defense military radio electronics, and in particular to a low bit rate signal demodulation and bit error rate testing device and a testing method thereof. Background Art

[0002] Deep space exploration is of great significance in science, technology, economy and other aspects, and has become a hot topic of current research. Deep space communication is an important guarantee for achieving deep space exploration. Compared with traditional wireless communication, it will face problems such as long transmission distance, limited detector energy and variable movement. In particular, deep space communication needs to solve the communication problem of extremely low signal-to-noise ratio (SNR), that is, how to effectively and reliably realize demodulation under extremely low SNR, and quickly rebuild the deep space communication link after transmission interruption to achieve high-quality signal transmission, so as to ensure that the instructions reaching the spacecraft are accurate. The bit error rate (BER) of the communication system is an indicator to measure the accuracy of data transmission within a specified time, and is the main basis for judging the reliability of a digital system. Different modulation and demodulation systems may be interfered by noise during transmission in an additive Gaussian noise channel, resulting in bit errors at the receiving end. Traditional bit error meters can only perform bit error rate tests on baseband digital sequences, and do not have the demodulation function of digital modulation signals of deep space exploration communication systems. By testing the bit error rate of deep space exploration communication systems, an effective demodulation and bit error rate testing method for low bit rate signals is studied to improve its anti-noise performance, which can provide a reference for the improvement and design of new communication systems. Summary of the invention

[0003] The object of the present invention is to provide a low bit rate signal demodulation and bit error rate testing device and a testing method thereof, so as to solve the problems existing in the prior art.

[0004] The technical solution of the present invention is: to provide a low bit rate signal demodulation and bit error rate test device, including a low bit rate modulation signal bit error generation unit, a low bit rate modulation signal signal-to-noise ratio calibration unit, a signal demodulation unit and a bit error rate test unit;

[0005] The low bit rate modulation signal error generating unit includes a first arbitrary wave generator, a second arbitrary wave generator and a combiner; the low bit rate modulation signal noise ratio calibration unit includes a voltmeter; the signal demodulation unit includes an oscilloscope and a computer; the bit error rate testing unit is placed in the computer;

[0006] The first arbitrary waveform generator generates a low bit rate baseband signal, which is input into the external modulation port of the second arbitrary waveform generator, and the modulation signal form, signal frequency, and signal amplitude are set, and the second arbitrary waveform generator channel 1 outputs a modulation signal waveform; the second arbitrary waveform generator channel 2 is set to output Gaussian white noise, and the signals output by the second arbitrary waveform generator channels 1 and 2 are output as a modulation signal with Gaussian white noise after passing through a combiner; the low bit rate modulation signal-to-noise ratio calibration unit calibrates the modulation signal with Gaussian white noise output by the combiner;

[0007] The modulated signal with Gaussian white noise output by the combiner is input to the oscilloscope channel 1, and the oscilloscope performs high-speed sampling, and the sampling result is input to the computer, and the bit error rate test unit performs demodulation to obtain a baseband signal, and performs digital-to-analog conversion on the demodulated baseband signal and the reference baseband signal and compares them bit by bit to obtain a bit error rate test result.

[0008] Furthermore, the first arbitrary wave generator generates a low code rate baseband signal, and sets the PN code type, code element width, code element period, and baseband signal amplitude parameters.

[0009] Another technical solution of the present invention provides a test method using the above-mentioned low bit rate signal demodulation and bit error rate test device, comprising the following steps:

[0010] S1. Error generation:

[0011] The first arbitrary waveform generator in the low bit rate modulation signal error generating unit generates a low bit 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 are output as a modulation signal with Gaussian white noise after passing through a combiner;

[0012] S2. Signal-to-noise ratio calibration:

[0013] Respectively measuring the ratio of useful modulation signal power to noise power in the modulation signal with Gaussian white noise output by the combiner;

[0014] S3, signal demodulation and bit error rate test:

[0015] S3-1, using software radio technology to implement the square loop method to extract the carrier to complete the carrier synchronization of the BPSK demodulator;

[0016] S3-2, obtain the carrier, complete the coherent demodulation through the multiplier, and then obtain the baseband signal through low-pass filtering;

[0017] S3-3, performing secondary filtering using a Butterworth filter, wherein the cutoff frequency of the Butterworth filter is set to the baseband signal frequency;

[0018] S3-4, solve the phase 'inverted π' problem in the carrier recovery process by using the polarity switching method;

[0019] S3-5, performing symbol shifting on the demodulated baseband signal by a time domain shift adjustment method until the symbol is aligned with the symbol of the original baseband signal, thereby removing the phase difference between the demodulated baseband symbol signal and the original baseband symbol signal of the reference channel;

[0020] S3-6, after the signal is aligned, a bit-by-bit comparison is performed to extract a bit error rate parameter;

[0021] S4. Verify the error test results.

[0022] Furthermore, in step S2, the ratio of the useful modulation signal power to the noise power in the modulation signal with Gaussian white noise output by the combiner is measured:

[0023] First, turn off the noise signal and use a voltmeter to measure the useful signal output power Psignal (dBm). Then remove the modulation signal and use a voltmeter to measure the noise output power Pnoise (dBm). The signal-to-noise ratio S / N measurement value is obtained:

[0024] S / N=Psignal(dBm)-Pnoise(dBm).

[0025] Furthermore, 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 its amplitude-frequency characteristic response expression is:

[0026] |H(ω)| 2 =1 / [1+(ω / ω c ) 2n ]

[0027] Where n=1,2,3…is the order of the filter, ω c is the cut-off frequency of the filter.

[0028] Furthermore, in 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 signal phase remains unchanged; if the demodulated signal is opposite to the reference signal, the polarity switch is turned on and the phase must be flipped 180° after multiplication.

[0029] Furthermore, in step S3-6, the bit error rate is calibrated by a comparison method; the number of bit errors of the signal is calculated using an XOR method, and the demodulated baseband signal codewords are XORed with the original baseband signal codewords. When the codewords are consistent, the XOR result is 0, and when the codewords are inconsistent, the XOR result is 1. Therefore, all XOR results are accumulated to obtain the number of bit errors, and the bit error rate is obtained by dividing the number of bit errors by the number of codewords of the signal.

[0030] Furthermore, in step S3-6, a cyclic accumulation method is adopted to accumulate the baseband signal code elements obtained by multiple data acquisitions of the oscilloscope and calculate the number of bit errors, thereby obtaining the bit error rate.

[0031] Furthermore, in step S4, a coherent demodulation method is used, and the BPSK / QPSK bit error rate performance is theoretically p e As shown below:

[0032]

[0033] In the formula, E b / N0 is the normalized signal-to-noise ratio, and erf is the error function as shown below:

[0034]

[0035] In the formula, z is

[0036] The low bit rate signal demodulation and bit error rate testing device and the testing method thereof provided by the present invention have the following beneficial effects:

[0037] The hardware equipment acquisition + software radio demodulation scheme of the present invention has the characteristics of simple structure, good synchronization performance, strong scalability, etc. It is not only easy to implement but also has low hardware cost. The polarity switching method and the translation adjustment method can quickly solve the "inverted π" phenomenon and phase synchronization problem in the demodulation process, and the calculation amount is low. The Butterworth secondary filtering method greatly improves the signal-to-noise ratio of the demodulated signal, improves the overall decoding performance of the system, and reduces the system bit error rate. This method can also be extended to the test and verification of the bit error rate of other digital modulation signals. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] The invention will be further described below in conjunction with the accompanying drawings:

[0039] Figure 1 It is a schematic diagram of a low bit rate modulation signal error generating unit provided by the present invention;

[0040] Figure 2 It is a working schematic diagram of the low bit rate modulation signal noise ratio calibration unit provided by the present invention;

[0041] Figure 3It is a working schematic diagram of the signal demodulation unit and the bit error rate test unit provided by the present invention;

[0042] Figure 4 It is a schematic diagram of the test flow of the signal demodulation unit and the bit error rate test unit provided by the present invention;

[0043] Figure 5 Schematic diagram of the hardware composition of the low bit rate signal demodulation and bit error rate test device in an embodiment of the present invention;

[0044] Figure 6 A schematic diagram of baseband signal generation in an embodiment of the present invention;

[0045] Figure 7 A schematic diagram of generating a BPSK modulation signal in an embodiment of the present invention;

[0046] Figure 8 A schematic diagram of a baseband sequence obtained by sampling an oscilloscope in an embodiment of the present invention;

[0047] Fig. 9 A schematic diagram of a computer reading an original baseband signal in an embodiment of the present invention;

[0048] Fig.10 A schematic diagram of a BPSK signal modulated by a PN code acquired by a computer in an embodiment of the present invention;

[0049] Fig.11 is the spectrum of the BPSK signal modulated by the PN code in the embodiment of the present invention;

[0050] Fig.12 Schematic diagram of a coherent demodulation signal in an embodiment of the present invention (without noise);

[0051] Fig.13 Schematic diagram of waveform after low-pass filtering in an embodiment of the present invention (noise-free);

[0052] Fig.14 Schematic diagram of waveform after filtering and noise reduction by the Butterworth filter in an embodiment of the present invention (without noise);

[0053] Fig.15 Schematic diagram of a demodulated baseband signal in an embodiment of the present invention (excluding noise);

[0054] Fig.16a Schematic diagram of waveform before translation adjustment in an embodiment of the present invention (without noise);

[0055] Fig.16b Schematic diagram of a waveform after translation adjustment in an embodiment of the present invention (without noise);

[0056] Fig.17Schematic diagram of a BPSK modulation waveform containing noise in an embodiment of the present invention;

[0057] Fig.18 is a schematic diagram of a coherent demodulation signal containing noise in an embodiment of the present invention;

[0058] Fig.19 Schematic diagram of a waveform after the first low-pass filtering containing noise in an embodiment of the present invention;

[0059] Fig. 20 This is a schematic diagram of a waveform after secondary filtering and noise reduction of a Butterworth filter in an embodiment of the present invention;

[0060] Fig.21a It is a schematic diagram of a waveform without using a polarity switch after shaping in an embodiment of the present invention;

[0061] Figure 21b Schematic diagram of the original PN code waveform in an embodiment of the present invention;

[0062] Fig.21c A schematic diagram of a waveform demodulated after using a polarity switch in an embodiment of the present invention;

[0063] Fig.22a is a schematic diagram of a demodulated waveform without time domain shift adjustment in an embodiment of the present invention;

[0064] Figure 22b This is a schematic diagram of the bit error rate calculation result without translation adjustment in an embodiment of the present invention, and the bit error rate adjustment parameter is 0;

[0065] Fig.23 This is a schematic diagram after translation adjustment in an embodiment of the present invention, with a bit error rate adjustment parameter of -50;

[0066] Fig.24 is a schematic diagram of a demodulated baseband waveform after translation adjustment in an embodiment of the present invention;

[0067] Fig.25 Schematic diagram of the bit error rate measurement result (9.6dB) of an embodiment of the present invention;

[0068] Fig.26 Schematic diagram of the bit error rate measurement result (8.35dB) of an embodiment of the present invention;

[0069] Fig. 27 Schematic diagram of the bit error rate measurement result (6.75dB) of an embodiment of the present invention;

[0070] Fig.28 FIG. 4 is a schematic diagram of a bit error rate measurement result (3dB) according to an embodiment of the present invention. DETAILED DESCRIPTION

[0071] 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 below in conjunction with the accompanying drawings and specific embodiments. The advantages and features of the present invention will become clearer according to the following description and claims. It should be noted that the accompanying drawings are all in a very simplified form and use non-precise ratios, which are only used to conveniently and clearly assist in explaining the purpose of the embodiments of the present invention.

[0072] The core idea of ​​the present invention is that the present invention proposes a low-rate signal demodulation and bit error rate test device and method, which can realize the demodulation and decoding of low-rate modulation signals in a Gaussian white noise environment based on the principle of software radio and combined with hardware. The low-rate signal demodulation and bit error rate test device includes a low-rate modulation signal error generation unit, a low-rate modulation signal signal-to-noise ratio calibration unit, a signal demodulation unit and a bit error rate test unit. The present invention collects the modulated signal data containing noise by an oscilloscope, first extracts the carrier by the square loop method on the computer to perform carrier recovery, and then obtains the baseband signal by coherent demodulation and low-pass filtering. The noise in the baseband signal is further reduced by the Butterworth filter secondary filtering method, and the information of the PN code element is obtained by sampling judgment and integer. The "inverted π" phenomenon in the signal carrier recovery process is solved by the polarity switching method, and the "alignment" problem of the PN code is solved by the time domain translation adjustment method, thereby realizing the accurate restoration of the PN code. The experimental results show that the system demodulation performance is improved and the bit error is reduced through the Butterworth filter secondary filtering method, polarity switching method and time domain shift adjustment method. This method has obvious improvement in the bit error rate test results in a low signal-to-noise ratio communication environment.

[0073] The Butterworth filter secondary filtering method is used for noise reduction under low signal-to-noise ratio conditions. The signal-to-noise ratio of the signal in a strong background noise environment is relatively low. After the low-pass filtering removes the carrier signal, the signal will still be submerged in the noise. The single filtering method is difficult to achieve a noise reduction effect under low signal-to-noise ratio conditions. The secondary filtering can reduce the impact of noise, obtain a smoother baseband signal curve, realize the bit error rate test under low signal-to-noise ratio conditions, greatly improve the signal-to-noise ratio, and obtain better bit error rate test results under the same signal-to-noise ratio conditions, thereby improving the decoding performance of the system.

[0074] The polarity switch method is mainly used to solve the "inverted π" phenomenon in the signal carrier recovery process. The low-code rate baseband signal of deep space communication is usually a BPSK signal, and the demodulation method for this signal is a coherent demodulation method. Since the BPSK signal itself uses phase to transmit information, the phase information of the signal must be used to demodulate the signal at the receiving end. In the actual communication system, the coherent carrier needs to be recovered from the received modulated signal using a phase-locked loop. This process increases the complexity of the system. At the same time, the recovered carrier may have a 180-degree phase deviation from the carrier during modulation. The recovered local carrier may be in phase or in reverse phase with the required coherent carrier. The uncertainty of this phase relationship will cause the demodulated digital baseband signal to be exactly opposite to the transmitted digital baseband signal, that is, '1' becomes '0', '0' becomes '1', and the digital signal output by the decision device is all wrong. This phenomenon is called the "inverted π" phenomenon of the BPSK method. In order to solve the "inverted π" phenomenon, this method solves this problem by designing a polarity switch adjustment during the demodulation process. The polarity switch changes the direction of the digital baseband signal, that is, when the phase shows the "inverted π" phenomenon, it changes "1" to "0" and "0" to "1", thus adjusting the phase of the erroneous code element.

[0075] The time domain shift adjustment method is used to accurately align the PN code and achieve bit synchronization in the case of unknown prior information. In modulation and demodulation technology, due to different system transmission paths, there is a certain delay in the received signal, and there is a phase difference between the demodulated baseband signal and the baseband signal of the reference channel. This phase difference will affect the judgment of the bit error rate. In order to solve the problem of phase synchronization and obtain an accurate bit error rate, this phase difference should be eliminated first. The traditional sliding correlation method requires a large number of multiplication and accumulation operations, the search speed is slow, and the average PN code capture time is long. The existence of the phase difference causes the two code sequences to appear to have relative sliding in terms of timing. Therefore, this method uses the time domain shift adjustment method to remove this phase difference, that is, the demodulated baseband signal is shifted until the code element is aligned with the code element of the original baseband signal, and then a bit-by-bit comparison is performed after the signal is aligned to extract the bit error rate parameter.

[0076] Example 1

[0077] The present embodiment provides a low bit rate signal demodulation and bit error rate testing device, comprising a low bit rate modulation signal bit error generating unit, a low bit rate modulation signal signal-to-noise ratio calibration unit, a signal demodulation unit and a bit error rate testing unit;

[0078] The low bit rate modulation signal error generating unit includes a first arbitrary wave generator, a second arbitrary wave generator and a combiner; the low bit rate modulation signal noise ratio calibration unit includes a voltmeter; the signal demodulation unit includes an oscilloscope and a computer; the bit error rate testing unit is placed in the computer;

[0079] The first arbitrary waveform generator generates a low bit rate baseband signal, which is input into the external modulation port of the second arbitrary waveform generator, and the modulation signal form, signal frequency, and signal amplitude are set, and the second arbitrary waveform generator channel 1 outputs a modulation signal waveform; the second arbitrary waveform generator channel 2 is set to output Gaussian white noise, and the signals output by the second arbitrary waveform generator channels 1 and 2 are output as a modulation signal with Gaussian white noise after passing through a combiner; the low bit rate modulation signal-to-noise ratio calibration unit calibrates the modulation signal with Gaussian white noise output by the combiner;

[0080] The modulated signal with Gaussian white noise output by the combiner is input to the oscilloscope channel 1, and the oscilloscope performs high-speed sampling, and the sampling result is input to the computer, and the bit error rate test unit performs demodulation to obtain a baseband signal, and performs digital-to-analog conversion on the demodulated baseband signal and the reference baseband signal and compares them bit by bit to obtain a bit error rate test result.

[0081] The testing method using the low bit rate signal demodulation and bit error rate testing device comprises the following steps:

[0082] Step 1: Bit error occurs.

[0083] like Figure 1 As shown, the arbitrary waveform generator 1 in the error generating unit generates a low code rate baseband signal, and sets parameters such as the PN code type, code element width, code element period, and baseband signal amplitude. The baseband signal output by the arbitrary waveform generator 1 is input to the external modulation port of the arbitrary waveform generator 2, and the modulation signal form, signal frequency, and signal amplitude are set. The arbitrary waveform generator 2 channel 1 outputs the modulation signal waveform. The arbitrary waveform generator 2 channel 2 is set to output Gaussian white noise, and the two channel signals are output as a modulated signal with Gaussian white noise after passing through the combiner.

[0084] The present invention uses an arbitrary waveform generator to generate a PN code baseband signal, a Gaussian white noise signal, and complete the modulation function of the carrier signal. In a digital communication system, E s / N0 (or its normalized form (E b / N0) is used as the performance indicator of signal-to-noise ratio. s is the energy per symbol, which is equal to the product of the signal power S and the duration per symbol Ts; N0 is the noise power spectral density, which is equal to the ratio of the noise power N to the bandwidth B; and because the duration per symbol T b It is the reciprocal of the symbol rate Rs and can be expressed as 1 / T b Substituting for Rs, the following expression holds:

[0085]

[0086] For an ideal channel with only additive white Gaussian noise, when E b After / N0 is determined, the signal-to-noise ratio S / N required to achieve a certain bit error rate value is:

[0087] S / N=(E s / N0)×(R / B) (2)

[0088] Where: E b is the energy per bit of the signal, E b =E s / f s , f s is the symbol rate. N0 is the noise power spectrum density of the transmission channel; B is the equivalent noise bandwidth of the detection filter; R is the bit rate, which represents the spectrum efficiency of the transmission signal.

[0089] E s / N0=E b / N0×(R / R s )=E b / N0×log2(M) (3)

[0090] Where M is the number of constellation points of the modulated signal. The constellation point number of BPSK modulation is 2, and its bit rate is the same as the symbol rate. The constellation point number of QPSK modulation is 4, and its bit rate is twice the symbol rate. The constellation point number of 8PSK modulation is 8, and its bit rate is three times the symbol rate.

[0091] Step 2: Signal-to-noise ratio calibration.

[0092] like Figure 2 As shown, measure the ratio of useful modulated signal power to noise power in the combiner output signal. First, turn off the noise signal and use a voltmeter to measure the useful signal output power as Psignal (dBm). Then remove the modulated signal and use a voltmeter to measure the noise output power as Pnoise (dBm). The signal-to-noise ratio S / N measurement value is obtained:

[0093] S / N Psignal(dBm)-Pnoise(dBm) (4)

[0094] The present invention adopts the method of measuring the modulated signal power and the noise power separately to realize the calibration of the signal-to-noise ratio. The spectrum of the BPSK signal has the following characteristics: (1) The spectrum shape of the BPSK signal and the baseband modulated signal is exactly the same, except that the frequency axis is shifted right by a carrier frequency; (2) When the probability of transmitting code elements 1 and 0 is equal, there is no discrete spectrum of the carrier component in the BPSK signal power spectrum. At this time, the BPSK signal is equivalent to a double-sideband signal with suppressed carrier. This modulation completely converts the carrier energy into signal energy, and the carrier energy C is equal to the signal energy S. Therefore, the calibration of the signal-to-noise ratio can be realized by measuring the modulated signal power and the noise power separately.

[0095] Step 3: Signal demodulation and bit error rate test.

[0096] like Figure 3 As shown, an oscilloscope and a computer are used to form a signal demodulation and bit error rate test device. One path of the PN code baseband signal generated by arbitrary waveform generator 1 is input to the external modulation port of arbitrary waveform generator 2 for modulation as the signal to be tested, and the other path is input to oscilloscope channel 2 as the reference baseband signal. The two signals are set to be output synchronously. The modulated signal containing noise output by the combiner is input to oscilloscope channel 1, and then the oscilloscope performs high-speed sampling on the two signals, and the sampling results are input to the computer. The modulated signal is demodulated by software to obtain a baseband signal, and the demodulated baseband signal and the reference baseband signal are converted into digital-to-analog and compared bit by bit to obtain the bit error rate test result. The flow chart of the signal demodulation unit and the bit error rate test unit is shown in the figure. Figure 4 shown.

[0097] First, the present invention uses software radio technology to implement the square loop method to extract the carrier to complete the carrier synchronization of the BPSK demodulator. The square loop method is used to implement the carrier synchronization of the BPSK digital receiver, which has the characteristics of simple structure, high efficiency, good synchronization performance, etc. Assume that the BPSK signal form is:

[0098] m(t)=s(t)cos2πf c t (5)

[0099] The probability of s(t) = ±1 is equal. After squaring the above signal, we can get:

[0100]

[0101] From the above, we can see that s 2 (t) is 1, and a narrowband filter can be used to extract 2f from y(t). c Frequency component, using a frequency divider can get the carrier frequency f of the BPSK signal c , this is the working principle of the square ring to extract the carrier.

[0102] After acquiring the carrier, coherent demodulation is completed through a multiplier, and then the baseband signal is obtained through low-pass filtering. Due to reasons such as channel noise, the amplitude of the rectangular pulse signal still fluctuates. In the present invention, in order to make the signal obtained at the receiving end have a maximized signal-to-noise ratio, a FIR filter is first used in the digital receiving system to filter out certain noise components, and then a Butterworth filter is used for secondary filtering. This method is beneficial to improving the demodulation signal-to-noise ratio and system error performance. The Butterworth low-pass filter uses the Butterworth function as the transfer function of the filter, and has a maximum flat amplitude characteristic. The filter approximates the ideal rectangular characteristic of the filter in the form of the highest-order Taylor series. Its amplitude-frequency characteristic response expression is:

[0103] |H(ω)| 2 =1 / [1+(ω / ω c ) 2n ] (7)

[0104] Where n=1,2,3…is the order of the filter, ω c is the cut-off frequency of the filter.

[0105] The Butterworth filter needs to be properly adjusted to obtain a standard waveform. The filter order controls whether its output graph has a delay. During the debugging process, the cutoff frequency of the filter is set to the baseband signal frequency.

[0106] After filtering, the present invention solves the phase 'inversion π' problem 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 turned on, and the signal phase remains unchanged; if the demodulated signal is opposite to the reference signal, the polarity switch is turned on, and the phase must be flipped 180° after multiplication, which is the same as the reference signal phase.

[0107] After the phase is consistent, the present invention adopts a translation adjustment method to solve the problem of a certain delay in the received signal caused by different system transmission paths. By setting the position of the translation adjustment, the demodulated baseband signal is translated until it is aligned with the code element of the original baseband signal, thereby removing the phase difference between the demodulated baseband code element signal and the original baseband code element signal of the reference channel.

[0108] Finally, after the signal is aligned, a bit-by-bit comparison is performed to extract the bit error rate parameter. The bit error rate is defined as the ratio of the erroneous bit to the total number of bits sent, and the bit error rate is related to the signal-to-noise ratio at the input. The present invention uses a comparison method to calibrate the bit error rate. The number of bit errors of the signal is calculated using an XOR method, and the baseband signal code element obtained by demodulation is XORed with the original baseband signal code element. When the code element is consistent, the XOR result is 0, and when the code element is inconsistent, the XOR result is 1. Therefore, all XOR results are accumulated to obtain the number of bit errors, and the number of bit errors divided by the number of code elements of the signal is the bit error rate. Since the bit error test is performed under extremely low bit rate conditions, the number of code elements collected by the oscilloscope at one time is small, so a cyclic accumulation method is adopted to accumulate the baseband signal code elements obtained by multiple data collections of the oscilloscope and calculate their number of bit errors, thereby obtaining the bit error rate.

[0109] Step 4: Verify the error test results.

[0110] like Figure 5 As shown, the performance of the bit error rate test device of the present invention is verified by comparing with the theoretical value.

[0111] After adding Gaussian white noise, different PSK modulation and demodulation methods have different bit error rate performance. Taking BPSK / QPSK as an example, using the coherent demodulation method, the theoretical BPSK / QPSK bit error rate performance is p e As shown below:

[0112]

[0113] In the formula, E b / N0 is the normalized signal-to-noise ratio, and erf is the error function as shown below:

[0114]

[0115] 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. Due to channel noise and other reasons, the amplitude of the rectangular pulse signal after correlation demodulation and low-pass filtering in the traditional method fluctuates. In the present invention, in order to reduce the bit error rate, the signal obtained at the receiving end has a maximized signal-to-noise ratio. The invention uses a Butterworth filter in the receiving system and filters again. After passing through the Butterworth filter, the noise reduction effect is obvious, thereby reducing the impact of the bit error rate of the system due to the poor signal-to-noise ratio. It has been verified by experiments that through the method of the present invention, under the condition of inputting the same signal-to-noise ratio, by improving the signal-to-noise ratio of the demodulated signal, the system bit error rate can be greatly reduced, and the decoding performance of the system can be improved.

[0116] Technical effect verification:

[0117] Figure 5This is a hardware composition diagram of the low bit rate signal demodulation and bit error rate testing device described in the present invention.

[0118] The specific system hardware components are shown in Table 1.

[0119] Table 1 Hardware composition list of low bit rate signal demodulation and bit error rate test device

[0120]

[0121] (1) Example of BPSK signal demodulation without adding noise PN code as baseband signal:

[0122] The instrument parameters are set as follows:

[0123] (a) 33522A arbitrary wave generator, reference Figure 6 :

[0124] BPSK signal: sine wave frequency 20kHz, amplitude 1Vpp, BPSK phase 180°

[0125] Baseband signal: PN7, symbol width 1ms, PN code period 127ms, amplitude 3.5Vpp;

[0126] (b) TDS5032B oscilloscope: sampling rate 2.5MS / s, record length 500K, reference Figure 7 :

[0127] The baseband signal generated by the arbitrary waveform generator 33522A consists of a pseudo-random code PN7 sequence, with a code element width of 1ms and a PN code period of 127ms. Figure 8 and Fig. 9 As shown in the figure, the code element after 127ms is sent repeatedly. PN7 is input as the modulation signal to the BPSK external modulation port of the arbitrary waveform generator 33522A, and the obtained BPSK waveform is as follows Fig.10 shown.

[0128] It can be seen that the phase of the sine wave signal, under the modulation of the baseband signal, produces a 180° phase mutation in each 0, 1 jump process. The spectrum of the BPSK signal is as follows: Fig.11 shown.

[0129] Depend on Fig.11 It can be seen that the BPSK signal modulated by the PN code is also a carrier suppressed signal, and the carrier signal energy at 20kHz is suppressed. After the square ring and two-way frequency division, the carrier information is extracted and coherent demodulation is performed. The signal after the multiplier is as follows Fig.12 shown.

[0130] The signal recovered by coherent demodulation needs to filter out the high-frequency components of the coherent demodulation output to achieve down-conversion, and then perform subsequent response processing on the retained low-frequency signal. According to Nyquist's low-pass sampling theorem, we set the sampling frequency (Fs) of the low-pass filter to 2.5MHz. The demodulated signal obtained after low-pass filtering is as follows: Fig.13 shown.

[0131] Depend on Fig.13 It can be seen that before and after the coherent demodulated signal passes through the low-pass filter, we can clearly see that the high-frequency component of the signal is reduced. Use the Butterworth filter to filter again, and the waveform obtained after filtering is as follows Fig.14 shown.

[0132] It can be seen that after passing through the Butterworth filter, the noise reduction effect is obvious and it becomes a more neat rectangular pulse signal.

[0133] After analog-to-digital conversion, the demodulated waveform is as follows: Fig.15 shown.

[0134] The delay is adjusted by translation adjustment to align the demodulated baseband signal with the code element of the original baseband signal. The bit error rate is calculated, and the comparison waveform between the demodulated signal and the baseband reference signal is as follows: Fig.16a (before translation) and 16b (after translation).

[0135] (2) Bit error rate measurement of BPSK signal with white noise PN code added as baseband signal:

[0136] The instrument parameters are set as follows:

[0137] The signal generator bandwidth is set to 2MHz, and the noise voltage is 1V to 1.5V (peak-to-peak). The signal-to-noise ratio calibrated by the URE3 voltmeter is 6.35dB (the theoretical value of the bit error rate is 1.0×10 -3 ). The waveform of the modulated signal containing noise and the waveform of the demodulated signal are as follows Figure 17-Figure 24 shown.

[0138] Depend on Fig.17 , Fig.18 and Fig.19 It can be seen that after the first filtering, the noise is still large and it is easy to cause bit errors. Fig. 20 It can be seen that after the Butterworth filter, the noise reduction effect is obvious; Fig.21a and 21b It can be seen that the polarity switch waveform is not used to invert the original PN code waveform, so the polarity switch input value is set to -1, and the phase of the demodulated baseband signal is adjusted to be consistent with the original PN code, such as Fig.21c shown by Fig.21cIt can be seen that there may be a delay between the demodulated information and the original information around 0.04s. After zooming in on the details, Figures 22a-22b As shown in the figure, it can be seen that without translation adjustment, the calculated bit error rate is 7.67×10 -5 .

[0139] After the code elements are aligned, the bit error rate is calculated. The waveform after the shift in this period is as follows: Fig.23 After the translation adjustment is performed to align the code elements, the calculated bit error rate is 3.2×10 -5 Compared with the case before the translation adjustment, the bit error rate is reduced. The baseband waveform after demodulation of the present invention is as follows: Fig.24 shown.

[0140] (3) Verification of bit error rate measurement results.

[0141] The comparison results of the bit error rate test device measurement results of this embodiment and the bit error rate theoretical value are shown in Table 2.

[0142] Table 2 Comparison of low bit rate signal demodulation and bit error rate test device measurement results and bit error rate theoretical values

[0143]

[0144] Depend on Figure 25 to Figure 28 It can be seen that under low signal-to-noise ratio conditions, the method of the present invention can effectively improve the signal-to-noise ratio and reduce the bit error rate.

[0145] The contents not described in detail in this specification belong to the prior art known to those skilled in the art. It is obvious to those skilled in the art that the present invention is not limited to the details of the above exemplary embodiments, and the present invention can be implemented in other specific forms without departing from the spirit or basic features of the present invention. Therefore, no matter from which point of view, the embodiments should be regarded as exemplary and non-restrictive, and the scope of the present invention is defined by the appended claims rather than the above description, and it is intended that all changes falling within the meaning and scope of the equivalent elements of the claims are included in the present invention.

Claims

1. A low bit rate signal demodulation and bit error rate testing device, characterized in that: It includes a low code rate modulation signal error generating unit, a low code rate modulation signal signal-to-noise ratio calibration unit, a signal demodulation unit and a bit error rate testing unit; The low bit rate modulation signal error generating unit includes a first arbitrary wave generator, a second arbitrary wave generator and a combiner; the low bit rate modulation signal noise ratio calibration unit includes a voltmeter; the signal demodulation unit includes an oscilloscope; the bit error rate testing unit is placed in a computer; The first arbitrary wave generator generates a low bit rate baseband signal, which is input to the external modulation port of the second arbitrary wave generator; the second arbitrary wave generator sets the modulation signal form, signal frequency and signal amplitude; the second arbitrary wave generator channel 1 outputs the modulation signal waveform; the second arbitrary wave generator channel 2 is set to output Gaussian white noise, and the signals output by the second arbitrary wave generator channels 1 and 2 are output as modulation signals with Gaussian white noise after passing through a combiner; the low bit rate modulation signal-to-noise ratio calibration unit calibrates the modulation signal with Gaussian white noise output by the combiner; The modulated signal with Gaussian white noise output by the combiner is input to the oscilloscope channel 1, the oscilloscope performs high-speed sampling, and the sampling result is input to the computer, the bit error rate test unit performs demodulation to obtain a baseband signal, and the bit error rate test unit performs digital-to-analog conversion on the demodulated baseband signal and the reference baseband signal and compares them bit by bit to obtain a bit error rate test result, specifically including: The carrier synchronization of BPSK demodulator is completed by using the square loop method to extract the carrier using software radio technology. Get the carrier, complete coherent demodulation through a multiplier, and then get the baseband signal through low-pass filtering; A Butterworth filter is used for secondary filtering, where the cutoff frequency of the Butterworth filter is set to the baseband signal frequency; Solve the phase 'inversion π' problem in the carrier recovery process through polarity switching method; The demodulated baseband signal is symbol-shifted by a time-domain shift adjustment method until it is aligned with the symbol of the original baseband signal, thereby removing the phase difference between the demodulated baseband symbol signal and the original baseband symbol signal of the reference channel; After the signals are aligned, a bit-by-bit comparison is performed to obtain the bit error rate parameters.

2. A test method using the low bit rate signal demodulation and bit error rate test device as claimed in claim 1, characterized in that: The steps include: S1. Error generation: The first arbitrary waveform generator in the low code rate modulation signal error 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, using software radio technology to implement the square loop method to extract the carrier to complete the carrier synchronization of the BPSK demodulator; S3-2, obtain the carrier, complete the coherent demodulation through the multiplier, and then obtain the baseband signal through low-pass filtering; S3-3, performing secondary filtering using a Butterworth filter, wherein the cutoff frequency of the Butterworth filter is set to the baseband signal frequency; S3-4, solve the phase 'inverted π' problem in the carrier recovery process by using the polarity switching method; S3-5, performing symbol shifting on the demodulated baseband signal by a time domain shift adjustment method until the symbol is aligned with the symbol of the original baseband signal, thereby removing the phase difference between the demodulated baseband symbol signal and the original baseband symbol signal of the reference channel; S3-6, after the signal is aligned, a bit-by-bit comparison is performed to obtain a bit error rate parameter; S4. Verify the error test results.

3. The testing method of the low bit rate signal demodulation and bit error rate testing device according to claim 2, characterized in that: In 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 signal phase remains unchanged; if the demodulated signal is in opposite phase to the reference signal, the polarity switch is turned on and the phase is flipped 180° after multiplication.

4. The testing method of the low bit rate signal demodulation and bit error rate testing device according to claim 2, characterized in that: In step S3-6, a cyclic accumulation method is adopted to accumulate the baseband signal code elements obtained by multiple data acquisitions by the oscilloscope and calculate the number of bit errors, thereby obtaining the bit error rate.

5. The testing method of the low bit rate signal demodulation and bit error rate testing device according to claim 2, characterized in that: In step S3-3, the Butterworth filter approximates the ideal rectangular characteristics of the filter in the form of the highest-order Taylor series, and its amplitude-frequency characteristic response expression is: |H(ω)| 2 =1 / [1+(ω / ω) c ) 2n ] Where n represents the order of the filter, ω c is the cut-off frequency of the filter.

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