The doctoral dissertations of the former Helsinki University of Technology (TKK) and Aalto University Schools of Technology (CHEM, ELEC, ENG, SCI) published in electronic format are available in the electronic publications archive of Aalto University - Aaltodoc.

Digital Modulators with Crest Factor Reduction Techniques

Olli Väänänen

Dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Department of Electrical and Communications Engineering for public examination and debate in Auditorium S4 at Helsinki University of Technology (Espoo, Finland) on the 17th of March, 2006, at 12 noon.

Dissertation in PDF format (ISBN 951-22-8081-7)   [947 KB]
Dissertation is also available in print (ISBN 951-22-8080-9)


Many of the modulation methods currently in use suffer from a high Peak-to-Average power Ratio (PAR), also known as the Crest Factor (CF). The Global System for Mobile communication (GSM) is a widespread second-generation (2G) system that uses constant envelope Gaussian minimum shift keying modulation. The advantage achieved by constant envelope modulation is the possibility of using power-efficient power amplifiers (PAs). However, it might be beneficial to combine the carriers in a digital intermediate frequency in order to reduce the number of analogue components. The drawback with this is that the signal is no more a constant envelope signal, but it has a strongly fluctuating envelope with a high CF. Enhanced Data rates for GSM Evolution (EDGE) is an enhancement to the GSM system with the primary objective of tripling the on-air data rate while meeting essentially the same bandwidth occupancy of the original GSM signal. Also in the case of EDGE, if the carriers are combined prior to amplification we would end up with the same high CF problem.

Wideband Code Division Multiple Access (WCDMA) has been selected by the European Telecommunications Standards Institute for wideband wireless access to support third-generation (3G) services. Orthogonal Frequency Division Multiplexing (OFDM) is commonly considered to be a technical solution for fourth-generation (4G) services. In both cases, the transmitted signal is generated by adding together a large number of statistically independent signals, which leads to a signal with a high CF.

The high CF sets strict requirements for the linearity of the PA. In order to limit the adjacent channel leakage, it is desirable for the PA to operate in its linear region. High linearity requirements for the PA leads to low power efficiency and therefore to high power consumption. An alternative to the expense of a wide-dynamic-range PA is the use of deliberate clipping to digitally distort the signal so that the signal quality is still maintained at a sufficient level. As an extra advantage, the decreased CF gives rise to the possibility of utilizing the dynamic range of the digital circuitry and digital-to-analog converter efficiently.

This thesis discusses digital modulator design, concentrating on CF reduction algorithms. Two modulators, one capable of generating GSM, EDGE and WCDMA signals and one a very wideband OFDM modulator for 4G, are implemented. Several CF reduction algorithms are presented in the literature. Those most essential to this thesis are studied, and their applicability for the above mentioned transmission schemes is tested. The windowing method is developed further, concentrating on the implementational issues. Also, a new method for CDMA-based systems is presented and analysed. The method presented exploits the properties of the CDMA modulation in a way that, despite the high error measured by using error vector magnitude and peak code domain error, the receiving user does not experience any error. A specialised method to compensate the sinc distortion in the OFDM system is also presented.

Keywords: WCDMA, OFDM, clipping

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© 2006 Helsinki University of Technology

Last update 2011-05-26