Discontinuous Transmission
Encyclopedia
Discontinuous transmission (DTX) is a means by which a mobile telephone is temporarily shut off or muted while the phone lacks a voice input.
and NMT
. While older network types theoretically allocated two 25–30 kHz channels per conversation, in practice some radios would cause interference on neighbouring channels making them unusable, and a single radio may broadcast too strong a oval to let nearby cells reuse the same channel.
GSM combines short packet sizes, frequency hopping
, redundancy, power control, digital encoding, and DTX to minimize interference and the effects of interference on a conversation. In this respect, DTX indirectly improves the over-all capacity of a network.
, it is possible to combine DTX with capacity increase when VoIP is used for telephony. In such cases, resources freed up when one user is in silence can be used to serve another user. The increase of the number of users will contribute to the interference level. Systems that use voice codecs such as AMR can reduce vocoder rate adaptively to better combat interference.
Systems based upon CDMA air interfaces such as IS-95
/CDMA2000
, and most forms of UMTS, can use a form of implied DTX by usage of a variable rate codec such as AMR. As with the packet radio systems above, when one side of the conversion is silent, the amount of transmitted data is minimized. Again, the effect is reduce interference.
In wireless transmitters, VAD is sometimes called voice-operated transmission (VOX).
SP flag = 1 indicates speech frame
Speech frame = 260 bit
Transmit side:
TX DTX handle performs speech encoding, comfort noise computation, voice activity detection
TX Radio Subsystem (RSS):
Performs SP flag monitoring and Channel coding
Hangover period:
After the transition from VAD=1 to VAD=0 it take N+1 consecutive frames to make a new updated SID frame available the bursts are directly passed to RSS with SP=1 this period is defined as Hangover period.
Noise Background spikes can often be confused with the speech frame and hence in order to nullify this issue a check list for SID computation is Nelapsed >23 , old SID is utilized with VAD=0.
Once after the end of speech SID is computed it is continuously passed to the RSS marked with SP=0 as long as VAD=0.
If a SID (SP=0) is chosen for transmission is stolen for FACCH signaling than the subsequent frame is scheduled for transmission.
Receive side:
BFI=0 Meaningful information bit
BFI=1 Not Meaningful information bit
Interestingly a FACCH frame in not considered as a meaningful information and should be transmitted with BFI=1
Traffic frames aligned with SACCH multi frame have TAF(Time alignment flag) =1
RX DTX handler performs speech decoding and comfort noise computation.
RX Radio subsystem:
Performs Error Correction and Detection and SID frame detection
Whenever a good speech frame is detected the RX DTX handler shall pass directly to speech decoder
Whenever a lost speech or lost SID frames are detected the substitution or mutation shall be applied.
Whenever a valid SID frame result in comfort noise generation.
In case of invalid SID frame after consecutive Speech frames the last valid SID frame will be applicable
Misconception
A common misconception is that DTX improves capacity by freeing up TDMA time slots for use by other conversations. In practice, the unpredictable availability of time slots makes this difficult to implement. However, reducing interference is a significant component in how GSM and other TDMA based mobile phone systems make better use of the available spectrum compared to older analog systems such as AMPSAdvanced Mobile Phone System
Advanced Mobile Phone System was an analog mobile phone system standard developed by Bell Labs, and officially introduced in the Americas in 1983, Israel in 1986, and Australia in 1987. It was the primary analog mobile phone system in North America through the 1980s and into the 2000s...
and NMT
Nordic Mobile Telephone
NMT is the first fully automatic cellular phone system...
. While older network types theoretically allocated two 25–30 kHz channels per conversation, in practice some radios would cause interference on neighbouring channels making them unusable, and a single radio may broadcast too strong a oval to let nearby cells reuse the same channel.
GSM combines short packet sizes, frequency hopping
Frequency-hopping spread spectrum
Frequency-hopping spread spectrum is a method of transmitting radio signals by rapidly switching a carrier among many frequency channels, using a pseudorandom sequence known to both transmitter and receiver...
, redundancy, power control, digital encoding, and DTX to minimize interference and the effects of interference on a conversation. In this respect, DTX indirectly improves the over-all capacity of a network.
Packet radio systems
In packet radio systems such as GPRS/EDGEEnhanced Data Rates for GSM Evolution
Enhanced Data rates for GSM Evolution is a digital mobile phone technology that allows improved data transmission rates as a backward-compatible extension of GSM...
, it is possible to combine DTX with capacity increase when VoIP is used for telephony. In such cases, resources freed up when one user is in silence can be used to serve another user. The increase of the number of users will contribute to the interference level. Systems that use voice codecs such as AMR can reduce vocoder rate adaptively to better combat interference.
Systems based upon CDMA air interfaces such as IS-95
IS-95
Interim Standard 95 is the first CDMA-based digital cellular standard by Qualcomm. The brand name for IS-95 is cdmaOne. IS-95 is also known as TIA-EIA-95....
/CDMA2000
CDMA2000
CDMA2000 is a family of 3G mobile technology standards, which use CDMA channel access, to send voice, data, and signaling data between mobile phones and cell sites. The set of standards includes: CDMA2000 1X, CDMA2000 EV-DO Rev. 0, CDMA2000 EV-DO Rev. A, and CDMA2000 EV-DO Rev. B...
, and most forms of UMTS, can use a form of implied DTX by usage of a variable rate codec such as AMR. As with the packet radio systems above, when one side of the conversion is silent, the amount of transmitted data is minimized. Again, the effect is reduce interference.
In wireless transmitters, VAD is sometimes called voice-operated transmission (VOX).
Technical details
SP flag = 0 indicates SID frameSP flag = 1 indicates speech frame
Speech frame = 260 bit
Transmit side:
TX DTX handle performs speech encoding, comfort noise computation, voice activity detection
TX Radio Subsystem (RSS):
Performs SP flag monitoring and Channel coding
Hangover period:
After the transition from VAD=1 to VAD=0 it take N+1 consecutive frames to make a new updated SID frame available the bursts are directly passed to RSS with SP=1 this period is defined as Hangover period.
Noise Background spikes can often be confused with the speech frame and hence in order to nullify this issue a check list for SID computation is Nelapsed >23 , old SID is utilized with VAD=0.
Once after the end of speech SID is computed it is continuously passed to the RSS marked with SP=0 as long as VAD=0.
If a SID (SP=0) is chosen for transmission is stolen for FACCH signaling than the subsequent frame is scheduled for transmission.
Receive side:
BFI=0 Meaningful information bit
BFI=1 Not Meaningful information bit
Interestingly a FACCH frame in not considered as a meaningful information and should be transmitted with BFI=1
Traffic frames aligned with SACCH multi frame have TAF(Time alignment flag) =1
RX DTX handler performs speech decoding and comfort noise computation.
RX Radio subsystem:
Performs Error Correction and Detection and SID frame detection
Whenever a good speech frame is detected the RX DTX handler shall pass directly to speech decoder
Whenever a lost speech or lost SID frames are detected the substitution or mutation shall be applied.
Whenever a valid SID frame result in comfort noise generation.
In case of invalid SID frame after consecutive Speech frames the last valid SID frame will be applicable