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Summary

AudioModem is a narrowband digital transmitter and receiver implemented using MATLAB®. It is intended to be a demonstration of a practical system and not a simulation. Using single carrier QAM digital modulation, an image can be transmitted from one computer to another with a typical audio card, speaker and microphone. The performance will vary depending on your hardware. Typically a bitrate of 8 kbit/s can be achieved.

Features

This transceiver demonstrates the following digital communication concepts:

  • Digital modulation
    • BPSK (preamble only)
    • QPSK
    • 16-QAM (not reliable)
    • 64-QAM (not reliable)
  • Convolutional coding
    • 1/2 rate without puncturing
    • 2/3 rate with puncturing
    • 3/4 rate with puncturing
  • Interleaving
  • Scrambling
  • Timing synchronization
  • Phase synchronization
  • Frame synchronization

Dependencies

A recent version of MATLAB is required and only version 2014a and 2014b have been tested. In addition, this project uses System objects™ extensively. Therefore, the Communications Systems Toolbox and DSP System Toolbox are also required.

Usage

Quick Start

When running any of the provided examples an open file dialog will appear on the transmitter for you to select an image for transfer. On the receiver a constellation and spectrum plot will be shown. After the file transfer is complete the received image will be displayed. Some status messages will be printed to the console. A warning will be printed if the CRC checksum for the image is not correct.

To run a simple loop back simulation execute runSimulateImage.m from a MATLAB command prompt:

>> runSimulateImage

To transfer an image execute runReceiveImage.m:

>> runReceiveImage

Then from another computer execute runTransmitImage.m:

>> runTransmitImage

It is suggested you run the transmitter and receiver applications on separate computers. It is certainly possible to run two separate MATLAB instances on the same computer, but most audio cards have significant internal leakage between the input and output paths causing the SNR to be much higher then in a realistic scenario.

Configuration

All of the examples provided use the SystemConfiguration function to return a structure of configuration parameters. Have a look at this file and edit only the basic configuration parameters unless you know what you are doing. Note that almost all of the parameters correspond to options of the MATLAB provided System objects used in the system.

Details

The system is implemented using custom and MATLAB provided System objects. Below are some short descriptions of each and some delightful flow graphs.

System

Transmitter

The transmitter class implements transmit physical layer (PHY). The input is the payload to be transmitted as a column vector of bytes and the output is the complex baseband samples that are to be transmitted. The transmitter consists of a frame generator, root-raised cosine (RRC) pulse shaping filter and a digital upconverter (DUC).

Transmitter

Receiver

The receiver class implements the receive physical layer. The data flow is essentially the reverse of the transmitter, but with the added complexity of synchronization. The receiver consists of a digital downconverter (DDC), an RRC matched filter, a symbol timing synchronizer and a frame synchronizer.

Receiver