
more and more digital channel selection (‘‘software
defined radio’’). Transparent transponders with sig-
nal processing exclusively in the microwave regime
represent an exception from this trend. In contrast to
channel selection, other filter functions need to be
performed in the microwave regime. These functions
are the suppression of image reception in superhet-
erodyne receivers and the rejection of strong out-of-
band interferers. The second function stems from the
necessity to prevent strong interfering signals from
entering the LNAs and mixers, where they would
cause saturation, desensitisation, and intermodula-
tion effects due to the nonlinear response of these
active devices.
Cryogenic units for base-transceiver stations (BTS)
of wireless communication networks became com-
mercially available in the late 1990s. These units
comprise HTS bandpass filters followed by cryogenic
LNAs, both integrated in an enclosed vacuum which
incorporates the cryocooler. In order to meet the
different requirements associated with frequency al-
location in a first-generation analogue and second-
and third-generation digital systems, these systems
are available for different center frequencies and
bandwidths, as well as with multiple passbands and
with narrowband stopbands embedded in wider pass-
bands. The number of filter-LNA chains in one cry-
ogenic unit equals the number of antenna ports
(sector antennas and diversity antennas) implemented
in a one BTS.
For the evaluation of system benefits offered by
cryogenic front-ends, the issue of a significantly im-
proved interference suppression due to steeper filter
skirts and the issue of an improved sensitivity due to
the reduced receiver noise figure should be clearly
distinguished. The first feature becomes significant in
the case of wireless systems which have to operate in
areas with strongly interfering systems with unwanted
signals at frequencies closely spaced to the edge fre-
quencies of the desired signal. Here, the improved
selectivity is transformed into a lower intermodula-
tion noise level and hence into improving the quality
of service.
Alternatively, this advantage can be transformed
into reduced guard-band widths and hence in an en-
hanced spectral efficiency and capacity.
In case of spread-spectrum systems, like the third-
generation W-CDMA standard, HTS stopband filters
with extremely narrow bandwidths can be used to
reject narrowband interferers falling into the wide-
band operational frequency band.
The reduced receiver noise figure of cryogenic filter-
LNA configurations originates from three different
effects, namely the reduced noise temperature of the
cooled LNA, the reduced thermal noise of the filter
due to the lower physical temperature, and the reduced
contribution of the amplifier noise due to the de-
creased passband insertion loss of the filter. Reduced
receiver noise becomes important in noise-limited
situations (e.g., rural areas), where it transforms into
a lower number of required BTSs or an improved
coverage for a fixed number of BTSs. Typically, the
noise figure of a BTS with the cryogenic front end at
the ground (noise figure degeneration due to lossy ca-
ble between antenna and front-end) resembles the
noise figure of a conventional, but mast-mounted
front-end.
See also: High-temperature Superconductors: Thin
Films and Multilayers; Superconducting Radiation
Sensors
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