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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. |
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发表于 2008-12-21 12:12:39
