Typical 2G Base Station Product Specifications

Gives some typical sizes and weights for presently installed GSM base stations
supplied by Motorola. Although there are 195 × 200 kHz RF carriers available at
900 MHz and 375 × 200 kHz RF carriers available at 1800 MHz, it is unusual to find
base stations with more than 24 RF carriers and typically 2 or 6 RF carrier base stations
would be the norm. This is usually because 2 or 4 or 6 RF carriers subdivided by 8 to
give 16, 32, or 48 channels usually provides adequate voice capacity for a small reasonably
loaded cell or a large lightly loaded cell. Having a small number of RF carriers
simplifies the RF plumbing in the base station—for example, combiners and isolators,
the mechanics of keeping the RF signals apart from one another. Table 11.3 shows that
hardware is preloaded with network software prior to shipment.

Products from Nokia have a similar hardware form factor. This has
the option of a remote RF head, putting the LNA (Low-Noise receive Amplifier) close
to the antenna to avoid feeder losses. There is also the choice of weather protection
(IP54/IP55; IP here stands for “intrinsic protection”).

ANokia PrimeSite product weighs 25 kg in a volume of 35 liters. This is a single RF carrier
base station with two integrated antennas to provide uplink and downlink diversity.
Similar products are available from Ericsson, also including mast-mounted LNAs to
improve uplink sensitivity. Table 11.5 gives key specifications, including number of RF
carriers, and highlights additional features such as the inclusion of automatic hardware
and software revision tracking.
The GSM specification stated that different vendor BTS products should be capable
of working with different vendor BSCs. As you would expect, all BTSs have to be compatible
with all handsets. Because GSM is a constant envelope modulation technique,
it has been possible to deliver good power efficiency (typically >50 percent) from the
BTS power amplifiers and hence reduce the hardware form factor. This has been
harder to achieve with IS95 CDMA or IS136 TDMA base stations because of the need to
provide more linearity (to support the QPSK modulation used in IS95 and the
π/4DQPSK modulation used in IS136 TDMA).
Table 11.6 shows the specification for a Motorola base station capable of supporting
AMPS, CDMA, and TDMA. The CDMA modem provides 1.25 MHz of RF channel
bandwidth (equivalent to a GSM 6 RF carrier base station) for each RF transceiver with
a total of 16 transceivers able to be placed in one very large cabinet to access 20 MHz of
RF bandwidth—the big-is-beautiful principle. The linear power amplifier weighs 400
kg! The products are differentiated by their capacity—their ability to support highdensity,
medium-density, or very localized user populations (microcells).

Shows a parallel product range from Ericsson for AMPS/IS136. These
are typically 10 W or 30 W base stations (though a 1.5 W base station is available for the
indoor microcell). The size is measured in terms of number of voice paths per square
meter of floor space. Again, the product range is specified in terms of its capacity capabilities
(ability to support densely populated or less densely populated areas).

As with IS95 CDMA, here there is a need to support legacy 30 kHz AMPS channels
(833 × 30 kHz channels within a 25 MHz allocation). This implies quite complex combining.
The higher the transmitter power, the more channel spacing needed between
RF carriers in the combiner. Note also that if any frequency changes are made in the
network plan, the combiner needs to be retuned. In this example, the cavity resonators and combiners can be remotely retuned (mechanically activated devices). If there is a
power mismatch with the antenna because of a problem, for example, with the feeder,
then this is reflected (literally) in the voltage standing wave ratio (VSWR) reading and
an alarm is raised.
The receiver sensitivity is specified both for the analog radio channels and the
digital channels. 12-dB SINAD is theoretically equivalent to 3 percent BER. This shows
that these are really quite complex hardware platforms with fans (and usually air conditioning
in the hut), motors (to drive the autotune combiners), and resonators—RF
plumbing overhead. These products absorb what is called in the United States
windshield time—time taken by engineers to drive out to remote sites to investigate RF
performance problems.
In the late 1980s in the United Kingdom, Cellnet used to regularly need to change
the frequency plan of the E-TACS cellular network (similar to AMPS) to accommodate
additional capacity. This could involve hundreds of engineers making site visits to
retune or replace base station RF hardware—the cost of needing to manage lots of
narrowband RF channels. 241