An All-Digital Up-Conversion Architecture for Low Power Transmitters

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Sirmayanti, Sirmayanti (2014) An All-Digital Up-Conversion Architecture for Low Power Transmitters. PhD thesis, Victoria University.


A sigma-delta (ΣΔ) based technique is used to embed a complex modulation scheme such as orthogonal frequency division multiplexing (OFDM) into a single ONOFF bit stream operating at the carrier frequency. The amplitude of the signal is related to the pulse widths and the phase is related to the pulse positions. The pulse edges are time quantised to align with the system clock. The work is based on a Cartesian ΣΔ scheme which uses Cartesian filters and a polar quantiser to move quantisation noise away from the band of interest. The first contribution of this thesis is the introduction of an improved odd quantisation and joint quantisation process. Odd quantisation uses pulse widths constrained to an odd number of clock periods compared to the more traditional even quantisation with an even number of clock periods. Joint quantisation uses pulses of unrestricted widths. It is shown that the odd quantisation scheme outperforms the even quantisation scheme by about 2 dB in terms of signal-to-noise ratio and the joint quantisation scheme enhances this by a further 3 dB. The improvement comes at the expense of complexity. A joint quantiser requires a two dimensional (2D) quantisation process, which for an oversampling ratio (OSRRF) of 8 involves 197 operations. A method that simplifies this to 44 operations is proposed. The second contribution involves the development of a new frequency tuning method. It involves phase rotators before and after the quantiser rotating at an intermediate frequency. Tuning spectrally shifts both the signal and noise null together, but the final phase conversion process generates an image and distortion products of both the noise and signal, resulting in a higher than expected noise floor and unwanted spurious signals. A deliberate choice of intermediate frequency (fIF) equal to or greater than the transmission bandwidth plus half of a channel bandwidth, will move all distortion products out of band. Unfortunately there is a deterioration in the in-band noise null compared to the non-shifted condition of about 8 dB and this worsens as the intermediate frequency increases. A mathematical derivation has been developed to predict the size and position of the unwanted images. The signal image can be removed by pre-distorting the input signal. A method for reducing noise enhancement caused by frequency tuning has also been developed. It involves distorting the feedback signal in the ΣΔ converter. The above improvements will enhance the use of all-digital ΣΔ based transmitters in future wireless communication system.

Item type Thesis (PhD thesis)
Subjects Historical > FOR Classification > 1005 Communications Technologies
Current > Division/Research > College of Science and Engineering
Keywords wireless transmitters, polar quantisation scheme, image cancellation, noise cancellation, baseband tuning, IF tuning
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