The Design and Investigation of an Extremely Low Frequency (ELF) Transmission Data Link

The telecommunication engineering field is playing a big role in the technology revolution, with most new technologies, smart phones and computers etc requiring 3G internet connection to be constantly in communication worldwide. A high spectral efficiency spectrum and maximising the number of users and transmission rate within a frequency band are the most critical design objectives and challenge to improve the reliability and performance of wireless communications systems.

The use of the Extremely Low Frequency (ELF) band has largely been confined to submerged submarine communications and its application to civilian data transmission has been neglected. ELF signals propagate through the ground itself using earth rods for long distances without regeneration compared to Radio Frequency (RF). There are two major noise sources in the ELF band, namely flicker noise (1/f) and 50Hz radiation of power feed cables underground. This thesis has described the theoretical aspects of adopting the ELF band and the ground itself as transmission medium in a communication link based on Orthogonal Frequency Division Multiplex (OFDM scheme. The use of 16-Quadrature Amplitude Modulation (16-QAM) and OFDM in ELF link enhances the transmission capacity at such low frequencies and reduces the multi-path interference. The link could handle at least 1000 bit/s (bps) and replace the complexity and reduces the power needed for RF link in some applications.

This thesis investigated the practical implementations of Binary Amplitude Shift Keying(BASK), 8-Phase Shift Keying (8-PSK) and 16-QAM with 20 Hz carrier and a data rate of 4 symbols/s from context to practice using Field Programmable Gate Array (FPGA). The highly flexible nature of FPGAs allows the systems designers to integrate any signal processing function. FPGAs replace the use of Digital Signal Processing (DSP) processors in many applications. Very high speed circuit Hardware Description Language (VHDL) language, Matlab, Altera DSP Builder and Altera Quartus II were used to design the ELF link.

A thorough literature review showed that demodulating phase data is generally performed using carrier recovery. The current technology generally incorporates a complex mathematics ‘tan’ function and Costas loop to detect the phase changes of the 8-PSK and 16-QAM signals. The 8-PSK and 16-QAM are widely used in Wide-band Code Division Multiple Access (WCDMA), High-Speed Download Packet Access (HSDPA), World wide Inter-operability for Microwave Access (WiMAX) and Wireless Local Area Network (WLAN) Broadband wireless access technologies. Alternative method has been proposed to use cross correlation to detect the phase modulated data. Patent literature review showed that there is no such patent or known prior art related to the proposed technique. This novel technique does not extract the carrier for demodulation purposes and offer improved phase detection performance as well as superior resistance to Additive-White-Gaussian-Noise (AWGN) compared to a carrier recovery scheme operating at low Signal-to-Noise Ratio (SNR).

The proposed demodulation scheme significantly reduces the implementation resources to demodulate 8-PSK and 16-QAM. As a consequent, this technique will yield cost reduction in products using 8-PSK and 16-QAM demodulators, namely Broadband internet and Digital TV. Furthermore, the cross correlations would have potential impacts on the development of ELF link using OFDM scheme that would require significantly less resources compared to recent OFDM systems.