Issues to Resolve

One problem with the architectures considered so far is that the final frequency is
generated at the start of the transmitter chain and then gained up through relatively
(tens of MHz) wideband stages. The consequence of this is to cause wideband noise to
be present at the transmitter output stage�"unless filters are inserted to remove it. The
noise will be a particular problem out at the receive frequency, a duplex spacing away.
(This noise will radiate and desensitize adjacent handsets ).
To attenuate the far out noise, filters must be added before and after the PA, and so
the duplex filter has been retained. Again, in a multiband design this can increase the
number of filters considerably.
To remove the need for these filters, an architecture called the offset loop or translational
loop transmitter has been developed. This relies on using the noise-reducing bandwidth properties of the PLL. Essentially the PLL function is moved from the start
of the transmitter to the output end, where the noise bandwidth becomes directly a
function of the PLL bandwidth.
In a well-designed, well-characterized PLL, the wideband noise output is low. If the
PLL can be implemented without a large divider ratio (N) in the loop, then the noise
output can be reduced further. If such a loop is used directly in the back end of the
transmitter, then the filters are not required.
A typical configuration will have a VCO running at the final frequency within a
PLL. To translate the output frequency down to a reference frequency, a second PLL is
mixed into the primary loop. Tuning is accomplished by tuning the secondary loop,
and in this example, modulation is applied to the sampling frequency process. If there
are no dividers, modulation transfer is transparent. A number of critical RF components
are still needed, however. For example, the tuning and modulation oscillators
require resonators, and 1800 MHz channels need to be produced by a doubler or bandswitched
resonators.
Figure 2.9 shows a similar configuration, but using dividers, for a multiband implementation
(single-band, dual-band, or tri-band GSM). Modulation is applied to a PLL
with the VCO running at final frequency. Again, this reduces wideband noise sufficiently
to allow the duplex filter to be replaced with a switch. Because of the lack of upconversion,
there are no image products, so no output bandpass filters are required.
The advantage of this implementation is that it reduces losses between the transmit
power amplifier and the antenna and allows the RF power amplifier to be driven into
saturation without signal degradation. The loop attempts to track out the modulation,
which is introduced as a phase error and so transfers the modulation onto the final frequency
Tx VCO. Channel selection is achieved by tuning the offset oscillator, which
doubles as the first local oscillator in receive mode.

There are a number of implementation challenges in an OPLL design:
 The noise transmitted in the receive band and modulation accuracy is determined
by the closed-loop performance of the OPLL. If the loop bandwidth is
too narrow, modulation accuracy is degraded; if the loop bandwidth is too
wide, the receive band noise floor rises.
 Because the OPLL processes phase and can only respond to phase lock functions
for modulation, an OPLL design is unable to handle modulation types
that have amplitude components. Thus, it has only been applied to constant
envelope modulations (for example, FM, FSK, and GMSK).
 Design work is proceeding to apply the benefits of the OPLL to non-constant
envelope modulation to make the architecture suitable for EDGE and QPSK.
Approaches depend mainly on using the amplitude limiting characteristics of the
PLL to remove the amplitude changes but to modulate correctly the phase components
and then to remodulate the AM components back onto the PAoutput.
With the above options, the advantage of having a simple output duplex
switch (usually a GaAs device) is only available when nonsimultaneous transmit/
receive is used. When higher-level GPRS classes are used, the duplex filter
must be reinstated.
A number of vendors are looking at alternative ways to manage the amplitude and
phase components in the signal path. For illustration purposes, we’ll look at an example
from Tropian (www.tropian.com). The Tropian implementation uses a core modulator
in which the amplitude and phase paths are synchronized digitally to control
timing alignment and modulation accuracy (see Figure 2.10). The digital phase and
amplitude modulator is implemented in CMOS and the RF PA in GaAs MOSFET.

We revisit linearization and adaptive predistortion techniques again when we study
base station hardware implementation in Chapter 11. In the last part of this chapter we
focus on the remaining design brief areas for achieving a multiband, multislot, multimode
handset.