Design and analysis of low baudrate modem using microcontrollers

Special technique is proposed for the design of soft modems that would be appropriate to transfer text messages at up to 300 bauds. Modulation scheme used is audio frequency shift keying, which is suitable for transport of text messages through the voice channel over HF and VHF radios. The soft modem is primarily designed to be implemented using low cost microcontrollers, and without the need of analog circuits such as active filters or a phase-locked loops. In addition to the physical layer, the proposed modem performs data link layer tasks such as framing and error detection and automatic request. Though synchronous transmission could be applied, asynchronous serial communication protocol is used to minimize design complexity, and be in essence to asynchronous FSK demodulation. The FSK modulation technique used is continuous phase FSK to eliminate phase jitters problem. Minimum shift keying can be applied, but with limitation for flexibility in changing baudrate and carriers frequencies. Proposed FSK demodulation is not a synchronous nor a PLL or a filter-based detector. Instead, a sort of frequency counter is proposed in this paper. Matlab simulations are made to validate the algorithm and observe features and limitations, such as the bit error rate and maximum bitrate. Later, the algorithm is examined by the aid of a code running on a virtual microcontroller, before being running on a real one.