GPRS RF PA

There are two particular issues with GPRS RF PA design:
 The duty cycle can change from 1/8 to 8/8.
 Power levels can change between time slots.
The need to improve power efficiency focuses a substantial amount of R&D effort on
RF materials. Table 2.3 compares present options on the basis of maturity, cost per
Watt, whether or not the processes need a negative voltage, and power-added efficiencies
at 900/1800 MHz.
Given that power-added efficiencies better than 55 percent are very hard to achieve
in practice (even with Class C amplifiers), heat dissipation is a critical issue and has led
to the increased use of copper substrates to improve conductivity. Recently, silicon germanium
has grown in popularity as a material for use at 1800 MHz/2 GHz, giving efficient
gain and noise performance at relatively low cost.
Implementing multislot GPRS with modulation techniques that contain amplitude
components will be substantially harder to achieve—for example, the (4DQPSK modulation
used in IS54TDMA or the eight-level PSK used in EDGE.

In a practical phone design, linearity is always traded against DC power consumption.
Factors that decide the final position on this linearity/efficiency trade-off include
the following:
 The semiconductor material, Si, GaAs, SiGe, and so on
 The transistor construction, packaging technique, bond wire inductances, and
so on
 The number of components used in and around the power amplifiers
 The expertise and capability of the design engineer
Designers have the option of using discrete devices, an integrated PA, or a module.
To operate at a practical power efficiency/linearity level, there will inevitably be a
degree of nonlinearity in the PA stage. This in turn makes the characterization of the
parameters that influence efficiency and linearity difficult to measure and difficult to
model. Most optimization of efficiency is carried out by a number of empirical
processes (for example, load line characterization, load pull analysis, and harmonic
shorting methods).
Results are not always obvious—for instance, networks matching the PA output to
50 ohms are frequently configured as a lowpass filter that attenuates the nonlinearities
generated in the output device. The PAappears to have a good—that is, low—harmonic
performance. The nonlinearity becomes evident when a non-constant envelope
(modulated) signal is applied to the PA as intermodulation products and spectral
spreading are seen.
Until recently the RF PAwas the only function in a 2 GHz mobile phone where efficiency
and linearity arguments favored GaAs over silicon. The situation is changing.
Advances in both silicon (Si) and silicon germanium (SiGe) processes, especially in 3G
phone development, make these materials strong contenders in new designs.
Higher performance is only obtained through attention to careful design. Advanced
design techniques require advanced modeling/simulation to obtain the potential benefits.
Design implementation is still the major cause of disappointing performance. In
Chapter 11 we examine GPRS base station and 3G Node B design, including linearization
techniques, power matching, and related performance optimization techniques.