Software Defined Radio - how does it work? Part 6
Hi, Habr.
In the previous section, we looked at the possibility of transmitting simple signals using GNU Radio. Now we will go further and see how to convey something more complicated. Let's start with amateur radio signals WSPR, and then create a working software QAM-modem.
')
And as in the previous case, we will do this without writing a single line of code, the program will also be cross-platform, and will be able to work both under OSX / Linux and under Windows. I will also show how to debug a modem using GNU Radio, without having any hardware at all.
Continued under the cut.
For those who have not used GNU Radio, it is recommended to read parts 4 and 5 , which described the principles of working with the program.
Let's start with the simpler, with WSPR - this type of communication was specifically made for tests of the propagation of weak signals, i.e. what we need is the power level of LimeSDR devices no more than 100mW. The WSPR signal is transmitted at a very low speed (2 minutes per message of about 30 bytes) in a very narrow band, which allows you to receive it even below the noise level. We transmit such a signal using GNU Radio.
To start the signal must be recorded. To do this, take the WSJT program, set all the necessary parameters (power, call, location, etc.). Specify in the settings as the output audio device Virtual Audio Cable, and record the signal in WAV. Pauses around the edges should be cut in any editor (for example, Cool Edit), as a result we should have a file with a duration of about 2 minutes.
Now create a graph in the GNU Radio Companion.
This method does not claim to maximum efficiency, but it is quite simple and understandable. The WSPR signal is initially located at a frequency of 1500 Hz, the recorded wav file has a sampling frequency of 22050s / s. First we resample the signal 57/5 times to bring the sampling rate to the required 250.000s / s. Then we shift the frequency up by 10kHz (the useful signal will be at a frequency of 11.5KHz), we translate the signal into the complex view required by the receiver, and cut out the excess filter, leaving the frequencies 11-12kHz.
WSPR signals are time-bound and transmitted every even minute (0:00, 0:02, etc.). I started the transfer to GNU Radio manually, “by eye” defining the interval by the hour, those who wish can add a script in Python to automatically start the transfer.
We are waiting for the right time, turn on the transmitter, receiver, and check the result.
If you wish, you can also add automatic generation of a WSPR file based on the input data (callsign, location, transmit power) to the program; examples of WSPR generation for Python can be taken on github.
It is interesting to note that at 432 MHz the frequency drift is already quite noticeable, although the signal is still decoded. But at 1.3GHz frequency, the drift becomes so large that WSPR is no longer accepted - an external reference oscillator with a more stable signal is needed for SDR (or at least software frequency correction during transmission, although this is less convenient).
If the SDR allows you to transmit at low frequencies, then you can try transmission in the HF band. Thus, with HackRF, it was possible to transmit a signal to 1000 km at 14 MHz from a room antenna, which can be considered a good result. Although high frequencies (433 MHz and 1.3 GHz) are perhaps even more interesting for experiments, the signals are transmitted only in direct line of sight, so for such experiments a second participant is needed on the receiving side. The second plus of such tests is that only lazy did not transmit to HF in wspr, but high frequencies are much less mastered. So, with LimeSDR or USRP boards it is possible to conduct quite interesting experiments with reception and transmission at ultrahigh frequencies.
Let's go further. WSPR is a fairly simple format, let's do something more interesting - a full-fledged (well, almost) modem. With quadrature-amplitude modulation, both the amplitude and phase of the signal simultaneously change, which allows data to be transmitted at a higher rate (but the occupied bandwidth is also larger).
Consider the first part - the transmitter.
As you can see, we read data from the data.txt file, then with a sampling frequency of 25KHz we send data to a packet encoder, which converts the stream into a 4-bit code. This stream enters the quadrature modulator, then the sampling frequency is increased to a transmitter frequency of 250 kHz, and the signal is shifted upwards by 80 kHz (many receivers have a peak at zero frequency, and this will interfere). The Constellation Rect component sets the modulation parameters - the number of characters and the phase and amplitude shift.
The first part is ready. Launch the "transfer" and see our signal.
We can test our transmitter, even without any hardware - for this there is a special Channel Model unit - a model of a communication channel. There you can set the noise, frequency shift, etc.
This is what our signal looks like before and after transmission. By the way, as you can see all the "harmonics" of the transmitter below 120 dB went far below the noise level.
Now welcome. Actually the same thing, only in reverse order.
Separately, you can stay on the last block of UDP Sink. It is not clear why, but in GNU Radio there is no component for viewing text data. Therefore, we simply send data via UDP to any local port (I chose 999).
For reception we will write the simple program in Python.
Result: we run the script, we launch GNU Radio, and we see received messages in the console.
As you can see, everything works, and you can have neither a receiver nor antennas :)
For those who want to repeat the experiments, project grc-file under the spoiler. It should work under Linux and under Windows.
A good guide to creating a more sophisticated modem is on the GNU Radio site , but they use a custom block for demodulation, which was only launched under Linux. In the above example, this is no problem.
As you can see, GNU Radio is a rather interesting program for different work with signals, in which you can do many different interesting things. If the audience doesn’t lose interest (there is a feeling that I’m delving into narrow topics that most people are not interested in), you can try to consider the transfer of something more interesting, such as video.
In the previous section, we looked at the possibility of transmitting simple signals using GNU Radio. Now we will go further and see how to convey something more complicated. Let's start with amateur radio signals WSPR, and then create a working software QAM-modem.
')
And as in the previous case, we will do this without writing a single line of code, the program will also be cross-platform, and will be able to work both under OSX / Linux and under Windows. I will also show how to debug a modem using GNU Radio, without having any hardware at all.
Continued under the cut.
For those who have not used GNU Radio, it is recommended to read parts 4 and 5 , which described the principles of working with the program.
Wspr
Let's start with the simpler, with WSPR - this type of communication was specifically made for tests of the propagation of weak signals, i.e. what we need is the power level of LimeSDR devices no more than 100mW. The WSPR signal is transmitted at a very low speed (2 minutes per message of about 30 bytes) in a very narrow band, which allows you to receive it even below the noise level. We transmit such a signal using GNU Radio.
To start the signal must be recorded. To do this, take the WSJT program, set all the necessary parameters (power, call, location, etc.). Specify in the settings as the output audio device Virtual Audio Cable, and record the signal in WAV. Pauses around the edges should be cut in any editor (for example, Cool Edit), as a result we should have a file with a duration of about 2 minutes.
Now create a graph in the GNU Radio Companion.
This method does not claim to maximum efficiency, but it is quite simple and understandable. The WSPR signal is initially located at a frequency of 1500 Hz, the recorded wav file has a sampling frequency of 22050s / s. First we resample the signal 57/5 times to bring the sampling rate to the required 250.000s / s. Then we shift the frequency up by 10kHz (the useful signal will be at a frequency of 11.5KHz), we translate the signal into the complex view required by the receiver, and cut out the excess filter, leaving the frequencies 11-12kHz.
WSPR signals are time-bound and transmitted every even minute (0:00, 0:02, etc.). I started the transfer to GNU Radio manually, “by eye” defining the interval by the hour, those who wish can add a script in Python to automatically start the transfer.
We are waiting for the right time, turn on the transmitter, receiver, and check the result.
If you wish, you can also add automatic generation of a WSPR file based on the input data (callsign, location, transmit power) to the program; examples of WSPR generation for Python can be taken on github.
It is interesting to note that at 432 MHz the frequency drift is already quite noticeable, although the signal is still decoded. But at 1.3GHz frequency, the drift becomes so large that WSPR is no longer accepted - an external reference oscillator with a more stable signal is needed for SDR (or at least software frequency correction during transmission, although this is less convenient).
If the SDR allows you to transmit at low frequencies, then you can try transmission in the HF band. Thus, with HackRF, it was possible to transmit a signal to 1000 km at 14 MHz from a room antenna, which can be considered a good result. Although high frequencies (433 MHz and 1.3 GHz) are perhaps even more interesting for experiments, the signals are transmitted only in direct line of sight, so for such experiments a second participant is needed on the receiving side. The second plus of such tests is that only lazy did not transmit to HF in wspr, but high frequencies are much less mastered. So, with LimeSDR or USRP boards it is possible to conduct quite interesting experiments with reception and transmission at ultrahigh frequencies.
QAM Modem
Let's go further. WSPR is a fairly simple format, let's do something more interesting - a full-fledged (well, almost) modem. With quadrature-amplitude modulation, both the amplitude and phase of the signal simultaneously change, which allows data to be transmitted at a higher rate (but the occupied bandwidth is also larger).
Consider the first part - the transmitter.
As you can see, we read data from the data.txt file, then with a sampling frequency of 25KHz we send data to a packet encoder, which converts the stream into a 4-bit code. This stream enters the quadrature modulator, then the sampling frequency is increased to a transmitter frequency of 250 kHz, and the signal is shifted upwards by 80 kHz (many receivers have a peak at zero frequency, and this will interfere). The Constellation Rect component sets the modulation parameters - the number of characters and the phase and amplitude shift.
The first part is ready. Launch the "transfer" and see our signal.
We can test our transmitter, even without any hardware - for this there is a special Channel Model unit - a model of a communication channel. There you can set the noise, frequency shift, etc.
This is what our signal looks like before and after transmission. By the way, as you can see all the "harmonics" of the transmitter below 120 dB went far below the noise level.
Now welcome. Actually the same thing, only in reverse order.
Separately, you can stay on the last block of UDP Sink. It is not clear why, but in GNU Radio there is no component for viewing text data. Therefore, we simply send data via UDP to any local port (I chose 999).
For reception we will write the simple program in Python.
import socket UDP_IP = "127.0.0.1" UDP_PORT = 999 sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM) # UDP sock.bind((UDP_IP, UDP_PORT)) sock.settimeout(1.0) while True: try: data, addr = sock.recvfrom(64) # buffer size is 64 bytes print("Msg:", data) except socket.timeout: pass Result: we run the script, we launch GNU Radio, and we see received messages in the console.
As you can see, everything works, and you can have neither a receiver nor antennas :)
For those who want to repeat the experiments, project grc-file under the spoiler. It should work under Linux and under Windows.
qam_test.grc
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<key>minoutbuf</key> <value>0</value> </param> <param> <key>num_inputs</key> <value>2</value> </param> <param> <key>vlen</key> <value>1</value> </param> </block> <block> <key>blocks_throttle</key> <param> <key>alias</key> <value></value> </param> <param> <key>comment</key> <value></value> </param> <param> <key>affinity</key> <value></value> </param> <param> <key>_enabled</key> <value>1</value> </param> <param> <key>_coordinate</key> <value>(96, 196)</value> </param> <param> <key>_rotation</key> <value>0</value> </param> <param> <key>id</key> <value>blocks_throttle_1</value> </param> <param> <key>ignoretag</key> <value>True</value> </param> <param> <key>maxoutbuf</key> <value>0</value> </param> <param> <key>minoutbuf</key> <value>0</value> </param> <param> <key>samples_per_second</key> <value>25000</value> </param> <param> <key>type</key> <value>byte</value> </param> <param> <key>vlen</key> <value>1</value> </param> </block> <block> <key>blocks_udp_sink</key> <param> <key>alias</key> <value></value> </param> <param> <key>comment</key> <value></value> </param> <param> <key>affinity</key> <value></value> </param> <param> <key>ipaddr</key> <value>127.0.0.1</value> </param> <param> <key>port</key> <value>999</value> </param> <param> <key>_enabled</key> <value>1</value> </param> <param> <key>_coordinate</key> <value>(680, 660)</value> </param> <param> <key>_rotation</key> <value>0</value> </param> <param> <key>id</key> <value>blocks_udp_sink_0</value> </param> <param> <key>type</key> <value>byte</value> </param> <param> <key>psize</key> <value>64</value> </param> <param> <key>eof</key> <value>True</value> </param> <param> <key>vlen</key> <value>1</value> </param> </block> <block> <key>channels_channel_model</key> <param> <key>alias</key> <value></value> </param> <param> <key>block_tags</key> <value>False</value> </param> <param> <key>comment</key> <value></value> </param> <param> <key>affinity</key> <value></value> </param> <param> <key>_enabled</key> <value>1</value> </param> <param> <key>epsilon</key> <value>1.0</value> </param> <param> <key>freq_offset</key> <value>0.0</value> </param> <param> <key>_coordinate</key> <value>(504, 284)</value> </param> <param> <key>_rotation</key> <value>180</value> </param> <param> <key>id</key> <value>channels_channel_model_0</value> </param> <param> <key>maxoutbuf</key> <value>0</value> </param> <param> <key>minoutbuf</key> <value>0</value> </param> <param> <key>noise_voltage</key> <value>0.1</value> </param> <param> <key>seed</key> <value>0</value> </param> <param> <key>taps</key> <value>1.0 + 1.0j</value> </param> </block> <block> <key>digital_qam_demod</key> <param> <key>alias</key> <value></value> </param> <param> <key>comment</key> <value></value> </param> <param> <key>affinity</key> <value></value> </param> <param> <key>differential</key> <value>True</value> </param> <param> <key>_enabled</key> <value>1</value> </param> <param> <key>excess_bw</key> <value>0.35</value> </param> <param> <key>freq_bw</key> <value>6.28/100.0</value> </param> <param> <key>_coordinate</key> <value>(672, 456)</value> </param> <param> <key>_rotation</key> <value>0</value> </param> <param> <key>mod_code</key> <value>"gray"</value> </param> <param> <key>id</key> <value>digital_qam_demod_0</value> </param> <param> <key>log</key> <value>False</value> </param> <param> <key>maxoutbuf</key> <value>0</value> </param> <param> <key>minoutbuf</key> <value>0</value> </param> <param> <key>constellation_points</key> <value>4</value> </param> <param> <key>phase_bw</key> <value>6.28/100.0</value> </param> <param> <key>samples_per_symbol</key> <value>4</value> </param> <param> <key>timing_bw</key> <value>6.28/100.0</value> </param> <param> <key>verbose</key> <value>False</value> </param> </block> <block> <key>digital_qam_mod</key> <param> <key>alias</key> <value></value> </param> <param> <key>comment</key> <value></value> </param> <param> <key>affinity</key> <value></value> </param> <param> <key>differential</key> <value>True</value> </param> <param> <key>_enabled</key> <value>True</value> </param> <param> <key>excess_bw</key> <value>0.35</value> </param> <param> <key>_coordinate</key> <value>(384, 116)</value> </param> <param> <key>_rotation</key> <value>0</value> </param> <param> <key>mod_code</key> <value>"gray"</value> </param> <param> <key>id</key> <value>digital_qam_mod_0</value> </param> <param> <key>log</key> <value>False</value> </param> <param> <key>maxoutbuf</key> <value>0</value> </param> <param> <key>minoutbuf</key> <value>0</value> </param> <param> <key>constellation_points</key> <value>4</value> </param> <param> <key>samples_per_symbol</key> <value>4</value> </param> <param> <key>verbose</key> <value>False</value> </param> </block> <block> <key>low_pass_filter</key> <param> <key>beta</key> <value>6.76</value> </param> <param> <key>alias</key> <value></value> </param> <param> <key>comment</key> <value></value> </param> <param> <key>affinity</key> <value></value> </param> <param> <key>cutoff_freq</key> <value>12000</value> </param> <param> <key>decim</key> <value>1</value> </param> <param> <key>_enabled</key> <value>True</value> </param> <param> <key>type</key> <value>fir_filter_ccf</value> </param> <param> <key>_coordinate</key> <value>(320, 460)</value> </param> <param> <key>_rotation</key> <value>0</value> </param> <param> <key>gain</key> <value>1</value> </param> <param> <key>id</key> <value>low_pass_filter_0</value> </param> <param> <key>interp</key> <value>1</value> </param> <param> <key>maxoutbuf</key> <value>0</value> </param> <param> <key>minoutbuf</key> <value>0</value> </param> <param> <key>samp_rate</key> <value>samp_rate</value> </param> <param> <key>width</key> <value>1000</value> </param> <param> <key>win</key> <value>firdes.WIN_HAMMING</value> </param> </block> <block> <key>qtgui_const_sink_x</key> <param> <key>autoscale</key> <value>False</value> </param> <param> <key>axislabels</key> <value>True</value> </param> <param> <key>alias</key> <value></value> </param> <param> <key>comment</key> <value></value> </param> <param> <key>affinity</key> <value></value> </param> <param> <key>_enabled</key> <value>1</value> </param> <param> <key>_coordinate</key> <value>(456, 20)</value> </param> <param> <key>gui_hint</key> <value></value> </param> <param> <key>_rotation</key> <value>0</value> </param> <param> <key>grid</key> <value>False</value> </param> <param> <key>id</key> <value>qtgui_const_sink_x_0</value> </param> <param> <key>legend</key> <value>True</value> </param> <param> <key>alpha1</key> <value>1.0</value> </param> <param> <key>color1</key> <value>"blue"</value> </param> <param> <key>label1</key> <value></value> </param> <param> <key>marker1</key> <value>0</value> </param> <param> <key>style1</key> <value>0</value> </param> <param> <key>width1</key> <value>1</value> </param> <param> <key>alpha10</key> <value>1.0</value> </param> <param> <key>color10</key> <value>"red"</value> </param> <param> <key>label10</key> <value></value> </param> <param> <key>marker10</key> <value>0</value> </param> <param> <key>style10</key> <value>0</value> </param> <param> <key>width10</key> <value>1</value> </param> <param> <key>alpha2</key> <value>1.0</value> </param> <param> <key>color2</key> <value>"red"</value> </param> <param> <key>label2</key> <value></value> </param> <param> <key>marker2</key> <value>0</value> </param> <param> <key>style2</key> <value>0</value> </param> <param> <key>width2</key> <value>1</value> </param> <param> <key>alpha3</key> <value>1.0</value> </param> <param> <key>color3</key> <value>"red"</value> </param> <param> <key>label3</key> <value></value> </param> <param> <key>marker3</key> <value>0</value> </param> <param> <key>style3</key> <value>0</value> </param> <param> <key>width3</key> <value>1</value> </param> <param> <key>alpha4</key> <value>1.0</value> </param> <param> <key>color4</key> <value>"red"</value> </param> <param> <key>label4</key> <value></value> </param> <param> <key>marker4</key> <value>0</value> </param> <param> <key>style4</key> <value>0</value> </param> <param> <key>width4</key> <value>1</value> </param> <param> <key>alpha5</key> <value>1.0</value> </param> <param> <key>color5</key> <value>"red"</value> </param> <param> <key>label5</key> <value></value> </param> <param> <key>marker5</key> <value>0</value> </param> <param> <key>style5</key> <value>0</value> </param> <param> <key>width5</key> <value>1</value> </param> <param> <key>alpha6</key> <value>1.0</value> </param> <param> <key>color6</key> <value>"red"</value> </param> <param> <key>label6</key> <value></value> </param> <param> <key>marker6</key> <value>0</value> </param> <param> <key>style6</key> <value>0</value> </param> <param> <key>width6</key> <value>1</value> </param> <param> <key>alpha7</key> <value>1.0</value> </param> <param> <key>color7</key> <value>"red"</value> </param> <param> <key>label7</key> <value></value> </param> <param> <key>marker7</key> <value>0</value> </param> <param> <key>style7</key> <value>0</value> </param> <param> <key>width7</key> <value>1</value> </param> <param> <key>alpha8</key> <value>1.0</value> </param> <param> <key>color8</key> <value>"red"</value> </param> <param> <key>label8</key> <value></value> </param> <param> <key>marker8</key> <value>0</value> </param> <param> <key>style8</key> <value>0</value> </param> <param> <key>width8</key> <value>1</value> </param> <param> <key>alpha9</key> <value>1.0</value> </param> <param> <key>color9</key> <value>"red"</value> </param> <param> <key>label9</key> <value></value> </param> <param> <key>marker9</key> <value>0</value> </param> <param> <key>style9</key> <value>0</value> </param> <param> <key>width9</key> <value>1</value> </param> <param> <key>name</key> <value>""</value> </param> <param> <key>nconnections</key> <value>1</value> </param> <param> <key>size</key> <value>1024</value> </param> <param> <key>tr_chan</key> <value>0</value> </param> <param> <key>tr_level</key> <value>0.0</value> </param> <param> <key>tr_mode</key> <value>qtgui.TRIG_MODE_FREE</value> </param> <param> <key>tr_slope</key> <value>qtgui.TRIG_SLOPE_POS</value> </param> <param> <key>tr_tag</key> <value>""</value> </param> <param> <key>type</key> <value>complex</value> </param> <param> <key>update_time</key> <value>0.10</value> </param> <param> <key>xmax</key> <value>2</value> </param> <param> <key>xmin</key> <value>-2</value> </param> <param> <key>ymax</key> <value>2</value> </param> <param> <key>ymin</key> <value>-2</value> </param> </block> <block> <key>rational_resampler_xxx</key> <param> <key>alias</key> <value></value> </param> <param> <key>comment</key> <value></value> </param> <param> <key>affinity</key> <value></value> </param> <param> <key>decim</key> <value>1</value> </param> <param> <key>_enabled</key> <value>True</value> </param> <param> <key>fbw</key> <value>0</value> </param> <param> <key>_coordinate</key> <value>(648, 148)</value> </param> <param> <key>_rotation</key> <value>0</value> </param> <param> <key>id</key> <value>rational_resampler_xxx_0</value> </param> <param> <key>interp</key> <value>10</value> </param> <param> <key>maxoutbuf</key> <value>0</value> </param> <param> <key>minoutbuf</key> <value>0</value> </param> <param> <key>taps</key> <value></value> </param> <param> <key>type</key> <value>ccc</value> </param> </block> <block> <key>rational_resampler_xxx</key> <param> <key>alias</key> <value></value> </param> <param> <key>comment</key> <value></value> </param> <param> <key>affinity</key> <value></value> </param> <param> <key>decim</key> <value>10</value> </param> <param> <key>_enabled</key> <value>True</value> </param> <param> <key>fbw</key> <value>0</value> </param> <param> <key>_coordinate</key> <value>(480, 484)</value> </param> <param> <key>_rotation</key> <value>0</value> </param> <param> <key>id</key> <value>rational_resampler_xxx_0_0</value> </param> <param> <key>interp</key> <value>1</value> </param> <param> <key>maxoutbuf</key> <value>0</value> </param> <param> <key>minoutbuf</key> <value>0</value> </param> <param> <key>taps</key> <value></value> </param> <param> <key>type</key> <value>ccc</value> </param> </block> <connection> <source_block_id>analog_sig_source_x_0</source_block_id> <sink_block_id>blocks_multiply_xx_0</sink_block_id> <source_key>0</source_key> <sink_key>0</sink_key> </connection> <connection> <source_block_id>analog_sig_source_x_1</source_block_id> <sink_block_id>blocks_multiply_xx_1</sink_block_id> <source_key>0</source_key> <sink_key>1</sink_key> </connection> <connection> <source_block_id>blks2_packet_decoder_0</source_block_id> <sink_block_id>blocks_udp_sink_0</sink_block_id> <source_key>0</source_key> <sink_key>0</sink_key> </connection> <connection> <source_block_id>blks2_packet_encoder_0</source_block_id> <sink_block_id>digital_qam_mod_0</sink_block_id> <source_key>0</source_key> <sink_key>0</sink_key> </connection> <connection> <source_block_id>blocks_file_source_0</source_block_id> <sink_block_id>blocks_throttle_1</sink_block_id> <source_key>0</source_key> <sink_key>0</sink_key> </connection> <connection> <source_block_id>blocks_multiply_xx_0</source_block_id> <sink_block_id>channels_channel_model_0</sink_block_id> <source_key>0</source_key> <sink_key>0</sink_key> </connection> <connection> <source_block_id>blocks_multiply_xx_1</source_block_id> <sink_block_id>low_pass_filter_0</sink_block_id> <source_key>0</source_key> <sink_key>0</sink_key> </connection> <connection> <source_block_id>blocks_throttle_1</source_block_id> <sink_block_id>blks2_packet_encoder_0</sink_block_id> <source_key>0</source_key> <sink_key>0</sink_key> </connection> <connection> <source_block_id>channels_channel_model_0</source_block_id> <sink_block_id>blocks_multiply_xx_1</sink_block_id> <source_key>0</source_key> <sink_key>0</sink_key> </connection> <connection> <source_block_id>digital_qam_demod_0</source_block_id> <sink_block_id>blks2_packet_decoder_0</sink_block_id> <source_key>0</source_key> <sink_key>0</sink_key> </connection> <connection> <source_block_id>digital_qam_mod_0</source_block_id> <sink_block_id>qtgui_const_sink_x_0</sink_block_id> <source_key>0</source_key> <sink_key>0</sink_key> </connection> <connection> <source_block_id>digital_qam_mod_0</source_block_id> <sink_block_id>rational_resampler_xxx_0</sink_block_id> <source_key>0</source_key> <sink_key>0</sink_key> </connection> <connection> <source_block_id>low_pass_filter_0</source_block_id> <sink_block_id>rational_resampler_xxx_0_0</sink_block_id> <source_key>0</source_key> <sink_key>0</sink_key> </connection> <connection> <source_block_id>rational_resampler_xxx_0</source_block_id> <sink_block_id>blocks_multiply_xx_0</sink_block_id> <source_key>0</source_key> <sink_key>1</sink_key> </connection> <connection> <source_block_id>rational_resampler_xxx_0_0</source_block_id> <sink_block_id>digital_qam_demod_0</sink_block_id> <source_key>0</source_key> <sink_key>0</sink_key> </connection> </flow_graph> A good guide to creating a more sophisticated modem is on the GNU Radio site , but they use a custom block for demodulation, which was only launched under Linux. In the above example, this is no problem.
Conclusion
As you can see, GNU Radio is a rather interesting program for different work with signals, in which you can do many different interesting things. If the audience doesn’t lose interest (there is a feeling that I’m delving into narrow topics that most people are not interested in), you can try to consider the transfer of something more interesting, such as video.
Source: https://habr.com/ru/post/453766/
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