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96 MICROWAVE JOURNAL APRIL 2011
Special RepoRt
more base stations, often contain a
multi-carrier power amplifier embed-
ded in a DPD loop. The main goals of
digital pre-distortion are to meet the
stringent adjacent channel power ra-
tio (ACPR) requirements by reducing
odd order intermodulation distortion
and to maximize power amplifier ef-
ficiency.
In wideband, multicarrier systems
such as W-CDMA, there is also the
concept of UTRAN sharing where
two network operators actively share
the same network infrastructure to re-
duce capital and operating expenses.
3
For the systems engineer, that can
translate to a more complex receive
front-end that has to meet the sensi-
tivity requirements of two co-located
receivers operating simultaneously
within the same band.
Sorting through the standard speci-
fications can be challenging enough
besides having to select components
that enable the designer to meet
the stringent system requirements.
In most cases, simply meeting the
technical requirements may not be
enough; it has to be done at a low cost
and with as little power consumption
as possible.
UltraCMOS teChnOlOgy
addreSSeS Market deMandS
Systems engineers are offered an
extensive menu of amplifiers, switch-
es, mixers and attenuators in various
competing technologies from CMOS
to GaAs to Silicon-On-Insulator
(SOI). UltraCMOS technology, a
patented Silicon-On-Sapphire (SOS)
technology, has demonstrated the ca-
pability to provide broadband, highly-
integrated and highly linear RF com-
ponents. One of the main advantages
of UltraCMOS technology is that by
using an insulating sapphire substrate,
there are no nonlinear, voltage depen-
dent parasitic bulk capacitances that
commonly plague other technologies.
Coupled with HaRP technology
enhancements, UltraCMOS compo-
nents have consistently demonstrated
the capability to meet the market re-
quirements for broadband, high lin-
earity at a low power consumption,
size and cost.
With the progression of mainly
voice communication (2G networks)
to data intense communication requir-
ing different multiplexing and modu-
system design. The use of an adap-
tive modulation scheme that varies
from QPSK to 64 QAM necessitates
reducing the amplitude and phase
distortion of the modulated RF signal.
The transmitter is required to meet
specific spurious emissions require-
ments and minimize adjacent channel
leakage. On the flip side, the receiver
has to account for a worst-case sen-
sitivity degradation from the mixing
of an out-of-band interferer with its
own transmit signal or a transmit sig-
nal from a separate antenna. Coupled
with the use of closely spaced (15 kHz
wide) orthogonal subcarriers and the
limited transmit band to receive band
spacing, LTE raises the complexity of
the overall system linearity require-
ments.
For most components such as
switches and digital step attenuators
used in mobile wireless and wireless
infrastructure systems, the out-of-
band distortion products are quanti-
fied in terms of Harmonic Distortion
(HD) and Intermodulation Distortion
(IMD). To better analyze the impact
of IMD with respect to the modulated
RF carrier, second- and third-order
distortion terms IMD2 and IMD3 are
specified as second- and third-order
intercept points IP2 and IP3. In ad-
dition, Cross Modulation Distortion
(CMD) is a critical in-band consider-
ation when multiple transmitters and
receivers co-exist in the same geo-
graphic area.
Examining one scenario in LTE,
the second harmonic of the ‘C’ Block
(777 to 787 MHz) uplink signal will
fall into the GPS ‘L1’ band of 1575.2
MHz, making HD2 an important
consideration. In another scenario in
W-CDMA, the operating band I (1950
MHz) uplink signal will intermodulate
with a GSM1800 out-of-band inter-
ferer at 1760 MHz, causing a de-sense
of its own receiver.
2
In this case the
component IIP3 would be the critical
specification.
Designing highly efficient, highly
linear systems has always been a chal-
lenge. Even with current 3G net-
works, the systems used in the back-
end infrastructure typically use some
form of linearization, be it analog
pre-distortion or digital pre-distortion
(DPD). For example, remote-radio
heads, a cost-effective way to extend
cell coverage without the addition of
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