Modulation and Upconversion

Because the handset operates in a very power restrictive environment, all stages must
be optimized not only for signal performance but also power efficiency. Following the
RRC filtering, the signal must be modulated onto an IF and up-converted to the final
transmission frequency. It is here the Node B and handset processes differ. The signal
could continue to be processed digitally to generate a digitally sampled modulated IF
to be converted in a fast DAC for analog up-conversion for final transmission. However,
the power (DC) required for these stages prohibits this digital technique in the
handset. (We will return to this process in Node B discussions.) Following the RRC filtering,
the I and Q streams will be processed by matched DACs and the resulting analog
signal applied to an analog vector modulator (see Figure 3.11).
Aprime challenge in the design of a W-CDMAhandset is to achieve the modulation
and power amplification within a defined (low) power budget but with a minimum
component count. This objective has been pursued aggressively in the design and
implementation of later GSM handsets. Sufficient performance for a single-band (900
MHz) GSM phone was achieved in early-generation designs, but the inclusion of a second
and third (and later fourth—800 MHz) band has driven the research toward minimum
component architectures—especially filters. Chapter 2 introduced the offset
loop transmitter architecture, which is successfully used for low-cost, low component
count multiband GSM applications.

This architecture is very suitable for the GMSK modulation of GSM, as it has a constant
amplitude envelope. The PLL configuration is only required to respond to the
phase component of the carrier. The QPSK and HPSK modulation used in W-CDMAis
non-constant envelope—that is, the modulated carrier contains both phase and amplitude
components. Because the offset loop is unable to reproduce the amplitude components,
it is unsuitable in its simple form. However, since it is particularly economic
in components, there is considerable research directed toward using this technique for
W-CDMA. To use the technique, the offset loop is used, with the amplitude components
being removed by the loop function, but an amplitude modulator is used on the
PAoutput to reproduce the amplitude components. This method of processing the carrier
separately from its amplitude components is referred to as Envelope Elimination and
Restoration (EER).

Other methods of producing sufficient PA linearity include adaptive predistortion
and possibly the Cartesian loop technique although the latter is unlikely to stretch
across the bandwidth/linearity requirement. 74