Pulse Modulation Techniques for UMTS Handset Transmitters

UMTS transmitters in handsets, based on WCDMA, have to fulfill contradicting requirements. On the one hand, they should operate in a highly power efficient way because battery lifetime is a major consideration for handsets. For this purpose, the application of switchedmode RF power amplifiers seems to be advantageous. On the other hand, the spectral efficiency of the transmitted signal has to be very high. In order to fulfill these spectral requirements, non-constant envelope modulation schemes are necessary, for which linear RF power amplifiers seem to be optimal to meet the stringent Adjacent Channel Leakage Power Ratio (ACLR) specification. Switched-mode power amplifiers can be applied to a linear transmitter, using a pulse modulated signal. Since the non-constant envelope signal then has been converted into a pulse sequence in time domain, the amplification of the pulse signal will not necessarily be linear and the amplified pulse signal can be demodulated with a bandpass filter. This concept is however hard to be realized for frequencies in the GHz range. We propose a technique to drive a switched-mode power amplifier such that the desired linearly amplified signal can be obtained by an RF band-selection filter. The usefulness of this approach is investigated by high-level simulations. Results will be applied to optimize a UMTS transmitter architecture in order to combine high linearity and maximum power-addedefficiency (PAE). Keywords— UMTS, WCDMA, transmitter, switchedmode power amplifier, pulse modulation