Discontinuous Transmission (DTx) in GSM | Global System for Mobile

Definition:

Discontinuous Transmission (DTX) is a mechanism that allows the radio transmitter to be switched off most of the time during speech pauses.

DTX may be applied independently to each direction, so that the control of DTX must take into account two components:

  • The uplink mode
  • The downlink mode

DTX can be enabled or disabled for the uplink and/or downlink mode on a per-cell basis.

The following figure shows Discontinuous Transmission (DTX):

Discontinuous Transmission Process:

DTX inhibits the transmission of the radio signal when not required from an information point of view. In the DTX mode, speech is encoded at 13kbit/s when the user is effectively speaking, but in a speech pause information is transmitted at a bit rate around 500 bit/s. this low rate flow is sufficient to encode the background noise, which is re-generated to ensure that the listener does not think that the connection is broken (comfort noise).

At the transmission side, the voice activity detection function detects whether speech will be transmitted on a particular radio link or not. When it detects that no speech has to be transmitted, transmission will cease after a defined period of time after speech activity has stopped.

The transmitter will periodically send a signal called a Silence Indicator Block every certain period of time. The Silence Indicator Block provides the comfort noise level information to the mobile station or BTS.

Reasons For DTX:

When DTX is applied, actual transmission on the radio path is reduced. This will cause a decrease of the interference level in co-channel cells (using the same frequency). Another advantage will appear when using DTX in the uplink mode: it saves battery power for the mobile station. However, a disadvantage of the DTX mode is that it slightly deteriorates the quality of transmission.

# Note that transmitting in DTX mode does not save timeslots on the air-interface.

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