High Temperature Quadrature Amplitude Modulation over Orthogonal Frequency Division Multiplexing
Authors: Cashion Avery , Cieslewki Grzegorz 
Published: July 2017

As downhole tools advance, they require increasingly higher data rates in communication with the surface. Current data rates over HT single-conductor wireline are limited to approximately 200 kbps. Long cables and harsh environments create a limiting challenge to standard serial bit rates due to high cable impedance and component availability. Keying techniques are often used to improve the robustness and speed of data links over long cables. Frequency shift keying (FSK) is a common implementation where data is encoded by specific frequencies on the data line. Amplitude shift keying (ASK) and phase shift keying (PSK) work on similar principles. In this project we apply quadrature amplitude modulation (QAM), a combination of PSK and ASK, and carry it over multiple frequency channels simultaneously using orthogonal frequency division multiplexing (OFDM). QAM over OFDM is a digital communications technique similar to those used in the telecommunications industry for digital subscriber line (DSL) connections and cellular networks. Using our data link, laboratory tests have demonstrated data rates of up to 3.8 Mbps over a 5000 ft (1524 m) single conductor wireline cable with less than 1e-8 bit error rate. Higher data rates are possible if error mitigation and correction coding is used. Gray encoding is an error mitigation technique where nearby points on the QAM constellation are similar in value so the errors are off only by least significant bits.
A prototype point to point link protocol was developed in which the cable bandwidth is assessed empirically to optimize the OFDM by allowing for correction of cable distortion at each frequency independently. The protocol can enable use of the developed data link for a variety of tools with little to no changes by hiding all the complexity from the tool. Future versions of the protocol will automatically detect and correct transmission errors, allow for data buffering/retransmission and provide feedback as to the quality of the link. However, this new approach to the problem requires more complex computation to encode the data and signal generation electronics to produce the required waveform.
One of the key challenges to full implementation of this concept on high temperature electronics is that a downhole fast Fourier transform (FFT) is required to decode the QAM information for downlink and an inverse FFT (IFFT) is required to efficiently encode data for uplink. The IFFT allows combination of multiple QAM data streams into a time series which can be converted to analog waveforms. High temperature testing of FFT and IFFT performance on the RelChip RC10001 microcontroller is included here. Also included here are the details of development of a high temperature digital to analog converter (DAC) to create composite signals and a high temperature line driver to boost analog signal transmission. This paper describes the communication architecture, the systems developed for high temperature implementation, and evaluations/implementations on high temperature computation devices.