A USB signal demodulation method based on software radio

Abstract

The present invention relates to the field of communication software radio, and in particular to a USB signal demodulation method based on software radio. The method uses digital down-conversion to remove the carrier and reduce the sampling rate, and then directly uses a digital frequency synthesizer to generate a digital mixing local oscillator signal. The mixer calculates the frequency control word by the software end and sends it to the mixer. It can obtain higher mixing accuracy and a larger mixing range to determine the carrier frequency with fewer resources. At the same time, through the soft and hard collaborative processing, the hardware of the mixer can be reused and adjusted to the position of BW / 4Hz, which reduces the processing pressure of the subsequent mixer; the mixed IQ signal is low-pass filtered to filter out the high-frequency component of the signal, and then the imaginary part of the signal is removed, which can not only recover the frequency domain symmetric signal, but also save hardware resources for the subsequent FFT conversion, reduce spectrum waste, and improve processing efficiency.

Description

Technical Field

[0001] The invention relates to the field of communication software radio, and in particular to a USB signal demodulation method based on software radio. Background Art

[0002] like Figure 1 As shown, in the digital intermediate frequency receiver, after the antenna receives the air signal, the signal is converted to an intermediate frequency through the RF module, and the analog intermediate frequency signal is output as a high-speed digital intermediate frequency signal through ADC acquisition, and then the high-speed signal is converted into a low-speed baseband signal through a digital down-converter, and finally the baseband signal is sent to the demodulation module for demodulation, so as to obtain the required message signal.

[0003] The core content of the communication system is modulation and demodulation. Using modulation technology, the transmitter converts the baseband message signal into the required frequency band signal before it can be transmitted in a specific frequency band channel. Demodulation is the reverse process of modulation, and its purpose is to restore the message signal sent by the transmitter. Although various new radio communication systems are constantly emerging, shortwave communication, a traditional communication method, is still widely valued by various countries, especially in military communication, emergency communication, and sudden disaster relief. Shortwave communication plays an irreplaceable role.

[0004] The USB signal is a type of single sideband modulation (SSB) signal, generally the upper sideband of the SSB signal. The demodulation method currently used for this signal is coherent demodulation, that is, by performing FFT transformation on the received modulated signal. Since the pilot component is added, the peak value in the spectrum is the corresponding pilot frequency. Thus, the coherent demodulation process is performed after the carrier frequency is determined. Since the USB signal is symmetrical in the spectrum, performing FFT transformation first will undoubtedly cause a waste of spectrum resources and hardware resources. And as the signal sampling rate increases, it is undoubtedly more difficult to calculate the carrier frequency through the spectrum peak. Summary of the invention

[0005] The purpose of the present invention is to provide a USB signal demodulation method based on software radio to solve the problems of waste of spectrum resources and hardware resources caused by the symmetry of the spectrum in the existing USB signal demodulation process.

[0006] A USB signal demodulation method based on software radio comprises the following steps:

[0007] Step 1: Use ADC to collect the intermediate frequency signal output by the RF front end and convert the analog signal into a digital signal; the digital signal is moved to the vicinity of zero frequency after orthogonal mixing to obtain the in-phase signal Mix_I and the orthogonal signal Mix_Q;

[0008] Step 2: Digitally filter the in-phase signal Mix_I and the quadrature signal Mix_Q obtained in step 1 and reduce the sampling rate to obtain quadrature baseband signals FIR_I and FIR_Q;

[0009] Step 3: Extract the sampling rate and bandwidth from the orthogonal baseband signals FIR_I and FIR_Q; upload the sampling rate and bandwidth to the software end, calculate the frequency control word using formula (1) on the software end and send it to the digital mixer DDS; the digital mixer DDS generates a local oscillator signal according to the frequency control word, and uses the local oscillator signal to adjust the baseband signals FIR_I and FIR_Q downward to the position of BW / 4Hz, thereby obtaining digital mixing local oscillator signals g1(t) and g2(t), where BW refers to the bandwidth of the input signal;

[0010] f out =(frequency control word*f s ) / 2 width Hz (1)

[0011] In formula (1), f out is the local oscillator signal generated by the digital mixer DDS, f s is the input signal sampling rate;

[0012] Step 4: Use a FIR low-pass filter to filter the digital mixing local oscillator signals g1(t) and g2(t) obtained in step 3 to solve the aliasing problem caused by the reduction of the sampling rate, meet the filtering requirements, and obtain two signals I1(t) and I2(t);

[0013] Step 5: Send the two signals I1(t) and I2(t) obtained in step 4 to the digital mixer, and use the digital mixer to adjust them upward to the position of BW / 4Hz to obtain two signals P1(t) and P2(t);

[0014] Step 6: Remove the imaginary part of the signal obtained in step 5, that is, remove P2(t) to obtain the output signal P1(t), and the signal P1(t) is the original USB signal.

[0015] Furthermore, the step 2 comprises the following steps:

[0016] Step 2.1: Use a cascaded comb CIC filter to extract data from the in-phase signal Mix_I and the orthogonal signal Mix_Q, reduce the sampling rate, and suppress the high-frequency components brought by the previous mixing stage;

[0017] Step 2.2: Use FIR filter to pass the signal obtained in step 2 to solve the aliasing problem caused by the reduction of sampling rate, meet the filtering requirements, and output baseband signals FIR_I and FIR_Q.

[0018] With the development of software radio technology and the widespread application of large-scale integrated circuits, communication systems have gradually begun to adopt pure digital architecture processing. The configurable characteristics of programmable devices and the collaborative processing of software and hardware can meet different communication function requirements. Based on this premise, the present invention provides a USB signal demodulation method based on software radio, which uses IQ signal mixing in the time domain and then performs subsequent coherent demodulation to realize USB demodulation. First, a digital down-conversion module (DDC) is used to remove the carrier and reduce the sampling rate, that is, steps 1 and 2, and then a high-precision digital frequency synthesizer (DDS) is directly used to generate a digital mixing local oscillator signal, that is, step 3. The mixer calculates the frequency control word from the software end and sends it to the mixer. It can obtain higher mixing accuracy and a larger mixing range through fewer resources to determine the carrier frequency. At the same time, the hardware reusability of the mixer can be achieved through the collaborative processing of software and hardware, that is, step 5, which reduces the processing pressure of the subsequent mixer. After the mixed IQ signal is low-pass filtered to remove the high-frequency component of the signal, the imaginary part of the signal is removed, that is, step 6. This can not only recover the frequency domain symmetric signal, but also save hardware resources for the subsequent FFT transformation, reduce spectrum waste, and improve processing efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 This is a general block diagram of a digital intermediate frequency receiver;

[0020] Figure 2 It is a block diagram of the specific process implementation of the present invention;

[0021] Figure 3 It is the digital down conversion (DDC) output;

[0022] Figure 4 It is the DDC output IQ down-conversion output;

[0023] Figure 5 Demodulates and outputs USB signals. DETAILED DESCRIPTION

[0024] The implementation case of the present invention is described in detail below. This implementation case is implemented under the premise of the technical solution of the present invention, and a detailed implementation method and specific process are given, but the implementation scope of the present invention is not limited to the described scenario.

[0025] According to this implementation case, the present invention provides a USB signal demodulation method based on software radio, which mainly includes the following steps:

[0026] Step 1: Sample the signal S after AD sampling m (t) = A*cos(ω ct) can be moved to the vicinity of zero frequency through the orthogonal mixing signal Mix(t)=cos(ωt)-j*sin(ωt). Since the intermediate frequency signal (IF) obtained by the RF signal (RF) after being processed by the RF module is already at a fixed frequency, the orthogonal mixing will also produce a mixing signal equal to the intermediate frequency. m (t) Move to baseband to obtain the in-phase signal Mix_I and the quadrature signal Mix_Q;

[0027] Mix_I=A / 2*(cos(ω+ω c )+cos(ω-ω c ))t;

[0028] Mix_Q=A / 2*(sin(ω+ω c )+sin(ω-ω c ))t;

[0029] Step 2: The mixed output signal is filtered by a cascaded comb filter (CIC) to extract the data by 200 times, reducing the data rate from 102.4MSPS to 512KSPS. At the same time, the low-pass characteristic of CIC will filter the high-frequency component sin(ω+ω c ) and cos(ω+ω c ) produces inhibition;

[0030] Step 3: The downsampled signal passes through the FIR filter to solve the aliasing problem caused by the reduction in sampling rate, meet the filtering requirements, and output the baseband IQ signal, FIR_I = cos(ω-ω c )t,FIR_Q=sin(ω-ω c )t;

[0031] Step 4: Extract the sampling rate and bandwidth from the orthogonal baseband signals FIR_I and FIR_Q; upload the sampling rate and bandwidth to the software end, and use formula (1) on the software end to calculate the frequency control word and send it to the digital mixer DDS. The digital mixer (DDS) generates the local oscillator signal cos(ω) according to the frequency control word. BFO t) and sin(ω BFO t), local oscillator signal cos(ω BFO t) and sin(ω BFO t) Multiply by FIR_I and FIR_Q respectively, and move the signal to the mixing frequency position to output g1(t) and g2(t);

[0032] g1(t)=cos(ω-ω c )t*cos(ω BFO t)+sin(ω-ω c )t*sin(ω BFO t);

[0033] g2(t)=sin(ω-ω c )t*cos(ω BFO t)-cos(ω-ω c )t*sin(ω BFO t);

[0034] DDS is a digital mixer. Its essential principle is the Quist sampling theorem. It generates a sinusoidal signal through a table lookup method. The local oscillator signal it generates is f out =(frequency control word*f s ) / 2 width Hz(f s Refers to the input signal sampling rate). In this design, the input signal sampling rate is between 1024MSPS and 8KSPS, the bit width is 16, and when the frequency control word is 1, the theoretical maximum accuracy can be 0.1227Hz. The frequency control word can be sent from the ARM end to the FPGA register and can be changed through the software end to meet the needs of multi-signal bandwidth and sampling rate, so as to obtain a larger mixing range and higher mixing accuracy;

[0035] Step 5: Pass the two signals g1(t) and g2(t) in step 4 through the FIR filter to solve the aliasing problem caused by the reduction of the sampling rate, meet the filtering requirements, and output IQ signals I1(t) and I2(t);

[0036] Step 6: The orthogonal IQ signals I1(t) and I2(t) enter the digital mixer, which generates the local oscillator signal cos(ω BFO t) and sin(ω BFO t), multiply them with I1(t) and I2(t) respectively, and move the signals to the mixing frequency position to output P1(t) and P2(t);

[0037] p1(t)=I1(t)*cos(ω BFO t)-I2(t)*sin(ω BFO t);

[0038] p2(t)=I2(t)*cos(ω BFO t)-I1(t)*sin(ω BFO t);

[0039] Step 7: After removing the imaginary part of the orthogonal IQ signal output in step 6, that is, removing p2(t), the output P1(t) is the USB signal result.

[0040] The present invention is not limited to the above-mentioned specific implementation methods. For ordinary technicians in the technical field to which the present application belongs, several simple deductions or substitutions can be made without departing from the inventive concept of the present application.

[0041] Figure 2 The position of the USB demodulator in the digital superheterodyne receiver is shown in Figure 1. Figure 2 As shown in the figure, the signal is moved to the intermediate frequency through the RF module, and the intermediate frequency signal is converted into a digital signal through high-speed ADC sampling, which is convenient for subsequent digital signal processing. The digital down-conversion module DDC performs orthogonal mixing on one real signal and moves the signal to zero intermediate frequency through mixing. The mixed signal becomes two mutually orthogonal IQ channels. The USB demodulator demodulates and outputs the two IQ signals.

[0042] The simulation effect of the present invention is as follows Figure 3 , Figure 4 , Figure 5 As shown, the simulation conditions are: AD sampling rate 102.4MSPS, signal frequency 30MHz, Figure 3 The signal in is the spectrum of the digital down-converted output signal when the sampling rate is reduced to 512KSPS. Figure 4 This is the input and output simulation diagram for down-converting the baseband signal output by the digital down-converter. Figure 5 This is the USB signal demodulation output diagram. Figure 3 , Figure 4 , Figure 5 It can be seen that the USB signal demodulation method based on software radio in this embodiment not only demodulates the correct signal, but also removes the symmetrical repeated signals in the frequency domain, thereby saving hardware resources for the subsequent FFT transformation, reducing spectrum waste, and improving processing efficiency.

Claims

1. A USB signal demodulation method based on software radio, characterized in that: The following steps are involved: Step 1: Use ADC to collect the intermediate frequency signal output by the RF front end and convert the analog signal into a digital signal; the digital signal is moved to the vicinity of zero frequency after orthogonal mixing to obtain the in-phase signal Mix_I and the orthogonal signal Mix_Q; Step 2: digitally filter the in-phase signal Mix_I and the quadrature signal Mix_Q obtained in step 1, and reduce the sampling rate to obtain quadrature baseband signals FIR_I and FIR_Q; Step 3: extract the sampling rate and bandwidth from the orthogonal baseband signals FIR_I and FIR_Q; upload the sampling rate and bandwidth to the software end, and use formula (1) on the software end to calculate the frequency control word and send it to the digital mixer DDS; The digital mixer DDS generates a local oscillator signal according to the frequency control word, and uses the local oscillator signal to adjust the baseband signals FIR_I and FIR_Q downward to the position of BW / 4Hz to obtain digital mixing local oscillator signals g1(t) and g2(t), where BW refers to the bandwidth of the input signal; f out =(frequency control word*f s ) / 2 width Hz (1) In formula (1), f out is the local oscillator signal generated by the digital mixer DDS, f s is the input signal sampling rate; Step 4: Use a FIR low-pass filter to filter the digital mixing local oscillator signals g1(t) and g2(t) obtained in step 3 to obtain two signals I1(t) and I2(t); Step 5: digitally mix the two signals I1(t) and I2(t) obtained in step 4, and adjust them upward to the position of BW / 4 Hz using a digital mixer to obtain two signals P1(t) and P2(t); Step 6: remove the imaginary part of the signal obtained in step 5, that is, remove P2(t) to obtain signal P1(t), and signal P1(t) is the original USB signal.

2. A USB signal demodulation method based on software radio according to claim 1, characterized in that: The step 2 comprises the following steps: Step 2.1, using a cascaded comb CIC filter to extract data from the in-phase signal Mix_I and the orthogonal signal Mix_Q, and reduce the sampling rate, while suppressing the high-frequency components brought by the previous stage of mixing; Step 2.2: Use FIR filter to pass the signal obtained in step 2 to solve the aliasing problem caused by the reduction of sampling rate, meet the filtering requirements, and output baseband signals FIR_I and FIR_Q.

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