Harmonic Feedback and Predistortion Techniques for Wideband Amplifier Linearisation
Tez Türü: Bütünleşik Doktora
Tezin Yürütüldüğü Kurum: University of Bristol, Engineering, Electrical and Electronics, İngiltere
Tez Danışmanı: Nishan Canagarajah, Joe Mcgeehan
Tezin Onay Tarihi: 2002
Tezin Dili: İngilizce
Desteklendiği Program: San-Tez Programı
Özet:
The efficient utilisation of allocated spectrum for mobile and wireless communication systems has become an important issue due to the increasing number of users and services. Distortion and interference in a channel limits high bit rate transmission and degrades spectrum efficiency by allowing fewer channels in the available bandwidth. Increasing the talk time of mobile equipment requires extending the battery life. The power amplifier is one of the most non-linear and power consuming parts in a transceiver. Extensive research has been carried out on several amplifier linearisation schemes to force these devices to operate at their saturation points for maximum efficiency while generating less distortion. Some of these schemes have provided satisfactory performance with 2nd-generation systems. The importance of linearity has increased with the introduction of 3rd-generation systems and Software Defined (or reconfigurable) Radio concept. These applications require excellent linearity together with broadband operation, which are contradictory properties and achieving one without compromising the other is a challenging task for the RF designer. Previous linearisation techniques have failed to fully satisfy these requirements, thus new techniques would be beneficial for future communication systems. This research has investigated harmonic feedback and analogue polynomial predistortion techniques as potential solutions to these new challenges in the RF amplifier design. These techniques were mathematically analysed with concentrating on the points ignored by other researchers. For practical investigations, several prototypes were built and measured results are presented to characterise the performance of the proposed techniques. The harmonic feedback technique is ideal for mobile handsets due to its simplicity and small circuitry. Also, it has not been as thoroughly investigated as other linearisation techniques and further research on this technique would be both exciting and a considerable contribution to RF engineering. The theoretical analysis has shown that linearisation can be achieved with harmonic feedback without compromising the linear amplifier gain, unlike traditional feedback. A new feedback configuration was proposed by combining harmonic and active RF feedback in order to minimise linear gain reduction while maximising distortion suppression. Computer simulation was used for testing this new network and simulated results show 27dB distortion suppression. Four feedback amplifier prototypes were built in different forms, with the last one using the new feedback configuration. The harmonic feedback prototypes gave up to 24dB distortion suppression where 10dB improvement was achieved within 30MHz of linearisation bandwidth. The prototype using the new feedback configuration achieved a wider linearisation bandwidth compared to the others. This showed that the new configuration has provided considerable improvement in the performance of a feedback amplifier. This prototype gave up to 26.5dB distortion suppression with 10dB improvement for a bandwidth much greater than 30MHz. It also showed promising performance both with narrowband and wideband modulated carriers by providing up to 15dB adjacent channel interference (ACI) suppression at one centre frequency. Analogue polynomial predistortion was investigated due to its excellent broadband operation capability, which is ideal for a reconfigurable radio application. The theoretical analysis has shown that the performance of this technique is mainly limited by the generation of additional high-order terms and not by the gain and phase matching requirement of the predistorter. This was practically demonstrated by the prototypes, which also explains the importance of generating correct predistorting signals. The practical measurements at several centre frequencies with different signal bandwidths have shown that this RF predistorter has an impressive linearisation bandwidth and multiband operation capability. The predistorter can provide about 15dB distortion suppression on average from 250MHz to 1GHz. Similarly, it has been tested both with narrowband and wideband modulation schemes at different centre frequencies where up to 14dB ACI suppression was obtained.