File transfer using Packet and BPSK

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THIS IS A WORK IN PROGRESS and a Proof of Concept.
Please leave comments in the Discussion tab above, or on the GNU Radio General Chat channel. See Chat.

This package consists of a packet transmitter, a channel simulation, and a packet receiver. It has been developed and tested with GNU Radio version 3.10.6.0.

At this point, the basic operation has been proven, but the following items need to be done to provide a more functional package:

  1. Write a program to strip the preamble and filler codes.
  2. Accept the input filename in the command line for 'pkt_xmt.py'.
  3. Add the original filename in the preamble information.
  4. Test with two computers using SDR devices (xmt_rcv_switch).


Prerequisites

It is imperative that all of the prerequisites are studied before starting this one.

Download the gr-control software

1. Open a terminal window.
2. Change to the home directory. 

   cd ~/

3. If you don't have 'git', enter 

   sudo apt install git

4. Clone the repository: 

   git clone https://github.com/duggabe/gr-control.git

Operation


NOTE: The order of starting the tasks is important!

Receiving the File

pkt_rcv.py receives the BPSK signal and presents it to the HDLC_Deframer. If the CRC check is valid, the packet is sent to the Correlate Access Code - Tag Stream. That block detects the Access Code and passes the payload to the File_Sink.

Pkt rcv file fg.png

1. Open a new terminal window.
2. Go to the gr-control/Receivers folder. 

   cd ~/gr-control/Receivers

3. Execute the packet receiver. 

   python3 -u pkt_rcv.py

4. A new window will open (titled "pkt_rcv") showing a QT_GUI_Constellation_Sink display and two QT_GUI_Time_Sinks.

Simulating Channel Impairments

There are two flowgraphs included to allow loopback testing of a transmitter and a receiver without using SDR hardware. Either operates **in place of** the `xmt_rcv_switch` program. The `chan_loopback` is for the digital modes such as BPSK packet. It allows introduction of noise, frequency offset, and timing offset.

Chan loopback fg.png

1. Open another terminal window.
2. Go to the gr-control folder. 

   cd ~/gr-control

3. Execute the loopback program. 

   python3 -u chan_loopback.py

4. A new window will open (titled "chan_loopback") showing a chooser for the Sample rate. For the version 3.9 and 3.10 programs, select 768kHz. There are three sliders to introduce impairments, but for initial testing, leave them all as loaded.

Transmitting the File

Pkt xmt file fg.png

The 'EPB: File Source to Tagged Stream' block is an Embedded Python Block which takes the place of a File_Source block, a Stream_to_Tagged_Stream block, and parts of a Burst_Shaper block. The Python block performs the following functions:

  • Send a preamble to allow the receiver to synchronize.
  • Send the selected file with "packet_len" tags.
  • Send a post-file filler to assure that any buffers have been flushed.

The content of the Embedded Python Block is shown here: pkt_xmt_epy_block_0.py

The preamble is composed of the '%' character, followed by 50 capital 'U's, followed by a ']'. It is repeated 100 times to allow the receiver to synchronize. The post-file filler is sent 40 times.

1. Open another terminal window.
2. Go to the gr-control/Transmitters folder. 

   cd ~/gr-control/Transmitters

3. Start GNU Radio Companion (GRC). 

   gnuradio-companion

4. Open the ~/gr-control/Transmitters/pkt_xmt.grc flowgraph.
5. Right click on the 'EPB: File Source to Tagged Stream' block and then click on "Properties".
6. Enter the desired filename to transmit surrounded by quote characters.
7. Click on "OK".
8. Execute the packet transmitter by clicking the 'Execute the flow graph' icon or pressing F6.
9. A new window will open (titled "pkt_xmt") showing a QT_GUI_Time_Sink.
10. The file transmission will begin. The average throughput is 2048 bytes per second.
11. The Time Sink display in the transmitter will show data bits with a 'packet_len' tag.
12. The Time Sink display in the receiver will show data bits with a 'packet_len' tag.
13. When the file is complete, the GRC terminal window will show "End of file".

Shutdown

1. Close the "pkt_xmt" window by clicking the 'X' in the upper right corner.
2. Close the "pkt_rcv" window by clicking the 'X' in the upper right corner.
3. Close the "chan_loopback" window by clicking the 'X' in the upper right corner.

Sending Non-text Files

Any text, binary, or other non-text files (such as PDF or PNG) can be sent. The file "$HOME/gr-control/gr-logo.png" has been provided for testing. It is the GNU Radio logo.

Stripping the Preamble and Filler

The received file will be stored in "$HOME/gr-control/Receivers/output.tmp". That file will contain up to 100 preamble packets, followed by the transmitted file, followed by filler packets.

A Python program will be written to strip those preamble/filler packets, but a hex file editor (such as ghex) can be used until then.

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