The Base Station Subsystem

3
The Base Station Subsystem
Via the Air-interface, the BSS provides a connection between the MSs of a lim-
ited area and the network switching subsystem (NSS). The BSS consists of the
following elements:
•
One or more BTSs (base tranceiver station);
•
One BSC (base station controller);
•
One TRAU (transcoding rate and adaptation unit).
The tasks and the structure of those elements or modules are described in this
chapter.
3.1 Base Transceiver Station
The BTS provides the physical connection of an MS to the network in form of
the Air-interface. On the other side, toward the NSS, the BTS is connected to
the BSC via the Abis-interface.
The manufacturers of BTS equipment have been able to reduce its size
substantially. The typical size in 1991 was that of an armoire; today the size is
comparable to that of a mailbox. The basic structure of the BTS, however, has
not changed. The block diagram and the signal flow of a BTS with one TRX
are shown in Figure 3.1. The GSM Recommendations allow for one BTS to
host up to 16 TRXs. In the field, the majority of the BTSs host between one
and four TRXs.
19
3.1.1 Architecture and Functionality of a Base Transceiver Station
3.1.1.1 Transmitter/Receiver Module
The TRX module is, from the perspective of signal processing, the most impor-
tant part of a BTS. The TRX consists of a low-frequency part for digital signal
processing and a high-frequency part for GMSK modulation and demodula-
tion. Both parts are connected via a separate or an integrated frequency hop-

ping unit. All other parts of the BTS are more or less associated with the TRXs
and perform auxiliary or administrative tasks.
A TRX with integrated frequency hopping serves the tasks listed in
Table 3.1.
3.1.1.2 Operations and Maintenance Module
The operations and maintenance (O&M) module consists of at least one cen-
tral unit, which administers all other parts of the BTS. For those purposes, it is
connected directly to the BSC by means of a specifically assigned O&M chan-
nel. That allows the O&M module to process the commands from the BSC or
the MSC directly into the BTS and to report the results. Typically, the central
unit also contains the system and operations software of the TRXs. That allows
it to be reloaded when necessary, without the need to “consult” the BSC.
Furthermore, the O&M module provides a human-machine interface (HMI),
which allows for local control of the BTS.
20 GSM Networks: Protocols, Terminology, and Implementation
Output
filter
Diversity
Input
filter
HF transmitter
(HF-TX)
HF receiver
(HF-RX)
Slow frequency
hopping
TRX
Digital
signal processing
(NF functionality)

Transmission
system
O&M module Operation and maintenance functionality/clock distribution
Abis-
interface
Air-interface
Figure 3.1 Block diagram of a BTS with one TRX.
3.1.1.3 Clock Module
The modules for clock generation and distribution also are part of the O&M
area. Although the trend is to derive the reference clock from the PCM signal
on the Abis-interface, a BTS internal clock generation is mandatory. It is espe-
cially needed when a BTS has to be tested in a standalone environment, that is,
without a connection to a BSC or when the PCM clock is not available due to
link failure.
Still, there is a cost savings benefit in the approach of deriving the clock
from the PCM signal. By doing so, much cheaper internal clock generators can
be applied, because they do not require the same long-term stability as an inde-
pendent clock generator. Besides, there is no need for frequent maintenance
checks on the clock modules, since they synchronize themselves with the clock
coming from the PCM link.
When analyzing errors in call handling, particularly in the area of hando-
ver, even minor deviations from the clock have to be considered as possible
The Base Station Subsystem
21
Table 3.1
Tasks of a TRX With Integrated Frequency Hopping
Function LF HF
Channel coding and decoding ●
Interleaving and ordering again ●
Encryption and decryption (ciphering) ●

Slow frequency hopping ●
Burst formatting ●
TRAU frame formatting and conversion in direction to/from the BSC, setup
of the LAPD connection to the BSC
●
GMSK modulation of all downlink data ●●
GMSK demodulation of all received MS signals ●●
Creation and transmission of the broadcast common control channel (BCCH)
on channel 0 of the BCCH-TRX
●●
Measurement of signal strength and quality for active connections ●●
Provision of the results to the BSC (MEAS_RES message)
Interference measurements (idle channel measurements) on free channels
and forwarding of the results to the BSC in a RF_RES_IND message
●●
LF = low frequency part of the TRX; HF = high frequency part of the TRX.
causes for errors. GSM requires that all the TRXs of a BTS use the same clock
signal. The accuracy of the signal has to have a precision of at least 0.05 parts
per million (ppm). For example, a clock generator that derives the clock from a
10 MHz signal has to be able to provide a clock with a frequency accuracy of
10 MHz ±0.5 Hz (10 ⋅ 10
6
Hz ⋅ 0.05 ⋅ 10
−6
= 0.5 Hz).
3.1.1.4 Input and Output Filters
Both input and output filters are used to limit the bandwidth of the received
and the transmitted signals. The input filter typically is a nonadjustable wide-
band filter that lets pass all GSM 900, all DCS 1800, or all PCS 1900 frequen-
cies in the uplink direction. In contrast, remote-controllable filters or wideband

filters are used for the downlink direction that limits the bandwidth of the out-
put signal to 200 kHz. When necessary, the O&M center (OMC) controls the
settings of the filters, as in the case of a change in frequency.
3.1.2 Base Transceiver Station Configurations
Different BTS configurations, depending on load, subscriber behavior, and
morph structure, have to be considered to provide optimum radio coverage of
an area. The most important BTS configurations of a BTS are presented next.
3.1.2.1 Standard Configuration
All BTSs are assigned different cell identities (CIs). A number of BTSs (in some
cases, a single BTS) form a location area. Figure 3.2 shows three location areas
with one, three, and five BTSs. The systems are usually not fine-synchronized
(see synchronized handover in the Glossary), which prevents synchronized han-
dover between them. That method of implementing BTSs is the one most fre-
quently used. For urban areas with growing traffic density, that may change
soon. For this situation, the configurations described in Sections 3.1.2.2 and
3.1.2.3 are more appropriate.
3.1.2.2 Umbrella Cell Configuration
The umbrella cell configuration consists of one BTS with high transmission
power and an antenna installed high above the ground that serves as an
“umbrella” for a number of BTSs with low transmission power and small
diameters (Figure 3.3).
Such a configuration appears to make no sense at first, because the fre-
quency of the umbrella cell can not be reused in all the cells of that area due to
interference. Interference even over a large distance was one of the reasons why
the high radio and television towers were abandoned as sites for antennas
shortly after they were brought into service at the initial network startup.
22 GSM Networks: Protocols, Terminology, and Implementation
The Base Station Subsystem
23
300 m–35 km

BTS
TRX
BTS
TRX
BTS
TRX
BTS
TRX
BTS
TRX
BTS
TRX
BTS
TRX
BTS
TRX
Figure 3.2 BTSs in standard configuration.
BTS
TRX
BTS
TRX
BTS
TRX
BTS
TRX
BTS
TRX
BTS
TRX
Figure 3.3 Umbrella cell with five smaller cells.