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<h1>Discontinuous Transmission Downlink and Uplink (DTX)</h1>

<p>This description includes features the Tone Detection in
FR VAD and Training Sequence Change.</p>


<p>Discontinuous transmission (DTX) is a mechanism allowing
the radio transmitter to be switched off during speech pauses. This feature
reduces the power consumption of the transmitter, which is important for mobile
phones, and decreases the overall interference level on the radio channels
affecting the capacity of the network.</p>


<p>The BSS supports both downlink and uplink discontinuous
transmission.</p>


<p>The discontinuous transmission is implemented in the Transcoder
Submultiplexer (TCSM) by means of three main structural elements. On the transmitting
side, a Voice Activity Detection (VAD) is required for detecting whether the
signal in question contains speech or just background noise. The silence is
not transmitted, but the information about the background noise level is.
The VAD function is mainly based on analysing the energy of the signal and
spectral changes. In addition, a function for counting the parameters of the
background noise is required on the transmitting side.</p>


<p>On the receiving side, comfort noise is generated on the
basis of noise parameters received from the transmitting side in order to
avoid disturbing variations of complete silence and speech with background
noise for the listener on the receiving side. Background noise is generated
synthetically towards the trunk in the transcoder and for the mobile station
in the MS itself.</p>


<p>There is a system parameter in the BSC, which determines
the value of the downlink DTX flag if the MSC does not prevent the activation
of the DTX function. The default value of this parameter is "Downlink DTX
disabled". For data calls the value of the downlink DTX flag is always disabled
regardless of the parameter value.</p>


<p>The mode of uplink DTX is defined in the BTS parameters
in the BSC database.</p>


<p>When the downlink DTX is being used together with frequency
hopping, some mobiles have difficulties in separating good downlink speech
frames from bad ones if BCCH frequency is included in the mobile allocation
list. This can be avoided if the training sequence of the dummy burst being
transmitted is altered while the downlink DTX is active.This makes it easier
for the mobile to reject bad speech frames with the DTX.</p>


<p>ETSI has specified several alternative methods as the correction
of this problem. Nokia has implemented the option number 2.</p>
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