Vandana_PhD_2011 - A Digital Up-Conversion Architecture for...

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A Digital Up-Conversion Architecture for Future High Efficiency Wireless Base Stations Vandana Bassoo B.Eng. (Hons) CENTRE FOR TELECOMMUNICATIONS AND MICRO-ELECTRONICS FACULTY OF HEALTH, ENGINEERING AND SCIENCE VICTORIA UNIVERSITY SUBMITTED IN FULFILLMENT OF THE REQUIREMENTS OF THE DEGREE OF DOCTOR OF PHILOSOPHY JULY 2010
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VICTORIA UNIVERSITY “I, Vandana Bassoo, declare that the PhD thesis entitled ‘A digital up-conversion architecture for future high efficiency wireless base stations’ is no more than 100,000 words in length including quotes and exclusive of tables, figures, appendices, bibliography, references and footnotes. This thesis contains no material that has been submitted previously, in whole or in part, for the award of any other academic degree or diploma. Except where otherwise indicated, this thesis is my own work.” Signature: Date: ii
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Abstract Over the past few years, there has been a growing need for wireless communications with higher data rates and ubiquitous coverage, and these must be achieved at reduced cost and with a lower carbon footprint. This evolution in wireless demand places a big burden on transmitter architectures. The need for higher efficiency has stimulated research into the potential replacement of current linear power amplifiers (PAs) by switch mode power amplifiers (SMPAs) at cellular frequencies. The radio frequency (RF) PA currently accounts for a significant part of the cost, and most of the power requirements of a typical wireless base station. This research is focused on the modulation and up-conversion circuits for gener- ating the SMPA drive signals. The switched (‘on’/‘off’) nature of the amplifier drive signal creates an opportunity for an all-digital solution removing traditional analog components such as the digital to analog converters, reconstruction filters, quadra- ture modulator and local oscillators. Digital signal processing techniques used for signal modulation are extended to digital upconversion to generate suitable drive signals for the SMPA. In this thesis, a sigma-delta (Σ∆) based technique is used to embed a complex modulation scheme such as OFDM into a single ‘on’-‘off’ bit stream. iii
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The first contribution of this thesis is the proposal of an all-digital modulation and up-conversion architecture that combines polar quantisation with Cartesian Σ∆ filtering. The drive waveform has a maximum of one pulse per half period thus limiting the number of switchings and hence increasing efficiency. It is shown that the proposed architecture provides an adjacent channel power (ACP) of lower than -50 dB for a bandwidth of over 100 MHz. An oversampling ratio of eight is required to meet the ACP specifications of wireless local area network (WLAN) and an oversampling of 32 is required to meet the third generation partnership project (3GPP) mobile phone standards. The all-digital approach described in this thesis is currently unfeasible for these frequency bands due to the limitations of today’s digital circuit technology.
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