The most common use of the Global System for Mobile telecommunications (GSM) mobile communication system is to transmit voice. It is well known that during a normal conversation, the participants alternate so that, on the average, each direction of transmission is occupied only about 50% of the time. Discontinuous transmission (DTX) is a mode of operation in the GSM system where the base station and mobile transmitters are switched on only for those frames which contain useful information. This is done in order to prolong battery life at the mobile station and to reduce the average interference level over the “air”, leading to better spectrum efficiency.
When DTX mode is enabled in communication, the received frames (one frame contains eight bursts and only one burst is allocated to one user in most conditions for GSM system) can be classified by the mobile station as either useful or silent. The useful frames carry data bits and the silent frames contain only noise and interference.
It is important to determine which frames are useful frames and which frames are silent frames. If the type of the received frame is not known, certain measurement processes in the mobile station will be erroneously based on the noise and interference of a silent frame. For example, if silent frames are not discarded, processes such as the measurement of received signal strength information (RSSI) will be nonsensical. This error will cascade into other processes such as the automatic gain control (AGC). Therefore, it is important to determine whether a received frame is a useful frame or a silent frame at the mobile station.