Investigation into the use of a four-channel sequency division multiplexing system for the transmission of digital data

Abstract

Data communications in applications such as telecommunications, telemetry and data networks has become an integral part of everyday life and considerable research is conducted into methods of increasing signalling speed and reliability of data communications systems, while reducing costs at the same time. At present many multi-channel systems employ frequency division multiplexing (FDM) and time division multiplexing (TDM) techniques to combine the input signals of several channels onto a common communications link. In this regard the use of sequency division multiplexing (8DM) offers a number of advantages over the more conventional methods of multiplexing. Orthogonal rectangular waveforms are employed as carriers for the modulator and demodulator circuits in a 8DM system. The set of Walsh functions; Wa/(1 ,t) , Wa/(2,t), Wa/(3,t), Wa/(4,t) were used for this purpose. The unit pulse method employing block pulse waveforms was used to generate the set of Walsh functions. Two different techniques of sequency division multiplexing were investigated in this project, viz; multi-level and binary systems of 8DM. During the course of the project an adaptive method of slicing the multi-level modulator output signals was developed to produce binary SDM signals that switch between logic 1 and logic O. The multi-level and binary systems both provided for four channels, each operating at a signalling speed of 1 200 bits per second. Data input signals to each channel modulate a Walsh carrier and the modulator outputs are combined to produce a multi-level signal. In binary systems of 8DM this multi-level signal must be converted to a binary signal that switches between logic 1 and logic O. Both systems of 8DM have an inherent delay of 1 data bit. All the necessary circuits were designed and built to permit testing of both systems of SDM. Performance tests were conducted on both systems to determine the bit error rate for each channel for different values of signal to noise ratio. These tests were repeated using low pass filters with cut-off frequencies of 22,5kHz, 27kHz and 58,9kHz in the line simulator. The results show that sequency multiplexing of digital data is a viable alternative to other methods of multiplexing. However, as SDM requires a larger bandwidth than conventional methods it's applications would be better suited to communications links such as co-axial cable or single wire links such as in telemetry systems for the control of irrigation equipment.