The BTS or Base Transceiver Station is the GSM network
element that provides for the basic connectivity towards the mobile stations over the air
interface.
Note that GSM is a cellular network to provide for a high frequency reuse factor.
Therefore, the network structure of the several BTS's that together form the coverage area
of a PLMN is somewhat as
shown in figure 1 underneath.
Figure 1: The Various Cells (<=>
BTS's) of a GSM Network
As figure 2 illustrates, a BTS can be divided into the transmit and
receive path. The receive path is also known as uplink direction (from the mobile station
to the network) while the transmit path is also known as downlink direction (from the
network towards the mobile station). One transmitter / receiver pair in GSM is denoted
TRX. Each TRX supports one ARFCN, whose
frequency depends on the respective standard (P-GSM, DCS 1800, PCS 1900, ... (see ARFCN)). The BTS in figure 2 comes with one TRX,
but GSM provides for BTS's with only 1 TRX up to 16 TRX's. However, usually BTS's contain
1 to 4 TRX's. Towards the network, the BTS is connected to the BSC.
Please note the diversity function of the
BTS in figure 2, that is, there are two uplink paths from the antenna up to the channel
decoder to allow the BTS to select the better uplink signal.

Figure 2: Illustration of a BTS with one
Transmitter / Receiver
For GPRS, no hardware upgrade of the BTS is required. However, to
support the new channel coding schemes that GPRS introduces a software upgrade is needed.
Important Functions of the BTS:
- Lighthouse:
Each BTS transmits on one TRX with a constant output power to allow the
surrounding mobile stations to find and select a suitable serving cell (see FCCH, SCH,
BCCH). This TRX is the BCCH frequency.
- Channel Coding / Decoding:
The BTS is the network element that performs channel coding to make the
data or speech signals more robust against bit errors during transmission over the air
interface.
- Ciphering / Deciphering:
In GSM, the BTS can cipher / de-cipher the information that is sent to / received
from the mobile station. Note that ciphering
in plain GSM is a layer 1 function that is initiated by the MSC/VLR (opposed
to ciphering in GPRS).
- Frequency Hopping:
Slow frequency hopping is a functionality that averages the signal quality on the
different frequencies that a BTS supports. Slow frequency hopping will switch the sender /
receiver frequency from one burst to the next according to a pre-defined hopping sequence.
- Interleaving / De-Interleaving:
Another method to provide for data protection is interleaving (figure 3).
Interleaving means that the information of one channel coded radio block is spread over
multiple bursts. As a consequence, if one
burst is lost due to bad radio conditions, only a small part of many radio blocks is lost
instead of a big part of one radio block. Therefore, the probability that the channel
decoder is able to recover the transmission error is higher.
The spreading rate of the interleaving process is different for the different TCH's that GSM supports. For instance, speech TCH's
are interleaved over 8 bursts while data TCH's are interleaved over 19 bursts.
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 Figure 3: The Process of
Interleaving for Full Rate Speech TCH's |
- Measurements:
While involved in an active transaction, the BTS will constantly measure a set of
parameters:
=> The propagation delay between itself and the mobile station (see also TA).
=> The estimated bit error rate based on the corruption of the TSC (RXQUAL).
=> The reception level of the uplink signal (RXLEV)
The measurement results are appended to those that have been received from the mobile
station (<=> MEAS_REP-message)
and are forwarded to the BSC (<=> MEAS_RES-message).
The BSC is then in charge to decide over possible power control adjustments or handovers
to a different TRX or to a different BTS.
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