General Architecture of the BTS.

A BTS in general has the following units:
Transceiver (TRX)
Quite widely referred to as the driver receiver (DRX). Basically does transmission and reception of signals. Also does sending and reception of signals to/from higher network entities (like the base station controller in mobile telephony)
Power amplifier (PA)
Amplifies the signal from DRX for transmission through antenna; may be integrated with DRX.
Combiner
Combines feeds from several DRXs so that they could be sent out through a single antenna. Allows for a reduction in the number of antenna used.
Duplexer
For separating sending and receiving signals to/from antenna. Does sending and receiving signals through the same antenna ports (cables to antenna).
Antenna
This is also considered a part of the BTS.
Alarm extension system
Collects working status alarms of various units in the BTS and extends them to operations and maintenance (O&M) monitoring stations.
Control function
Control and manages the various units of BTS including any software. On-the-spot configurations, status changes, software upgrades, etc. are done through the control function.
Baseband receiver unit (BBxx)
Frequency hopping, signal DSP, etc..