8/10/2019 1.8 Discontinuous Reception and Discontinuous
Transmission
1/2
, 2 2009 .
1.8 Discontinuous Reception and Discontinuous Transmission1.8.1
Discontinuous Reception and Paging ChannelIn idle mode, if MS
selects a cell as its service cell, it begins to receive the paging
information from thiscell. But in order to reduce power
consumption, discontinuous reception (DRX) is introduced in
GSM.Each user (IMSI) belongs to a paging group and each paging
group corresponds to a pagingsubchannel. MS can calculate which
group it belongs to based on the last three digits of its IMSI
andthe configuration of paging channel in this location area, and
then locate the paging subchannel of thispaging group. In fact, in
idle mode, MS just listens to the paging information from the
system on itssubchannel (MS also monitors the Relev of BCCH carrier
frequency in non-service area during thisperiod of time) and
ignores the information on other paging subchannels. Some of the
hardwareequipments are even switched off to save the power of MS.
But MS must complete the required task ofnetwork information
measurement within a specified time.Through DRX, MS can receive the
broadcast short messages that the users want to know with lesspower
consumption, thus extending the service time. BSC has to send
scheduling messages to supportDRX at MS. One scheduling message
contains lots of broadcast short messages to be sent soon. Thetime
that all broadcast short messages of a scheduling information takes
is a scheduling cycle.
Scheduling information contains the description of all short
messages to be broadcast in order and alsoindicates the position of
the messages in scheduling cycle. Through scheduling messages, MS
canfind the broadcast short messages it wants quickly so as to
reduce its power consumption.The number of paging subchannels of
each cell can be calculated based on the configuration type ofCCCH,
BS_AG_BLKS_RES (the number of blocks belonging to AGCH in 51
multiframe), andBS_PA_MFRMS (the number of 51 multiframes used as
one paging subchannel cycle).When there are three CCCHs in a 51
multiframe, the number of paging subchannels is (3-BS_AG_BLKS_RES)
BS_PA_MFRMSWhen there are nine CCCHs in a 51 multiframe, the number
of paging subchannels is (9-BS_AG_BLKS_RES)BS_PA_MFRMSIn addition,
the configuration of CCCH parameters has the following
principles:The greater the parameter BS_PA_MFRMS, the more the
paging subchannels, and the less the usersof each paging
subchannel, but the total capacity of the system remains the same,
because theaverage delay of the paging information on radio channel
increases. When the ratio of retransmissionwaiting is relatively
high, BS_PA_MFRMS should be improved to increase the paging
subchannels;when the ratio of retransmission waiting is relatively
low, BS_PA_MFRMS should be reduced to shorten
the paging delay.The capacities of paging subchannels of all
cells in a location area should be the same, because thepaging
message of a location area must be sent in all the cells of this
location area at the same time.The longer the cycle of paging
channel, the less power the MS in this service area takes. For
example,in cities, this cycle can be defined as 2, which means MS
listens to paging messages once for every102 frames. In rural
areas, this cycle can be defined as 4 or 6. The MS with the paging
channel cycle of6 consumes 18% less power than the MS with the
paging channel cycle of 2. After measuring thesystem information,
MS enters the rest state and listens to the paging information in
the specifiedpaging blocks only and measures the Relev of BCCH of
neighbor cells at the same time. After 30 s, MSwill listen to
system information again to judge the cell re-selection process.In
GSM, CCCH mainly includes AGCH and PCH. Its primary function is to
transmit immediateassignment messages and paging messages. CCCH can
be one or several physical channels and itcan also share a physical
channel with SDCCH. The combination mode of CCCH depends on
theparameter CCCH_CONF. The configuration of CCCH_CONF must be
consistent with the actualconfiguration. It is recommended that
when there is only one TRX in a cell, the configuration of CCCHcan
be a physical channel shared with SDCCH (3 CCCH information
blocks).When the traffic volume is extremely large, in case one
physical timeslot is not enough, GSMspecification allows the
configuration of multiple CCCH channels on the TRX besides BCCH,
but thesechannels must be used in timeslot 0, 2, 4, and 6.When
CCCH_CONF is confirmed, parameter BS_AG_BLKS_RES actually decides
the ratio of AGCHand PCH on CCCH. It is recommended that this
parameter be configured as little as possible in order toreduce the
response time of MS to paging.1.8.2 DTXI. DTXOverviewDuring
communication, only 40% time is used for conversation; no useful
information is transmittedduring the rest 60% time. If all the
information is transmitted to network, many of the system
resourceswill be wasted, in addition, the interference will
aggravate. In order to solve this problem, GSM adoptsDTX technology
to stop signal transmission when there is no voice signal.
Therefore, the interferencelevel is reduced and the system
efficiency is improved.There are two kinds of transmission modes in
GSM: normal mode and discontinuous transmission(DTX) mode. In
normal mode, noise and voice have the same transmission quality. In
DTX mode, thetransmission of unuseful messages is prohibited. MS
only sends man-made noise signals that aretolerable, which means
this noise will not annoy the listeners nor affect the
conversation. This kind ofnoise is called comfort noise. In DTX
mode, 260-bit code is transmitted in every 480 ms; in normalmode,
260-bit code is transmitted in every 20 ms.Whether the downlink DTX
is adopted or not is controlled by network operators of the
exchange part.This kind of control is based on BSC. The control
information is transmitted to baseband processingpart through
dedicated signaling channel, and then arrives at TC through the
inband signaling of TRAUframe to indicate whether downlink DTX is
adopted. For some vendors, the downlink DTX can beconfigured on the
basis of cell.
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8/10/2019 1.8 Discontinuous Reception and Discontinuous
Transmission
2/2
Uplink DTX is configured by network operators of the radio part.
The parameter DTX in systeminformation consists of 2 bits.Parameter
DTX is contained in cell option of information unit and transmitted
periodically in the systeminformation of each cell broadcast. MS
decides whether to start DTX function based on this information.DTX
can be used for voice signal transmission and nontransparent data
transmission. BCCH TRX doesnot use this technology. The benefits of
DTX are listed below:Uplink DTX can save MS batteries and reduce
interference.Downlink DTX can save BTS power consumption and reduce
interference and intra-BTS intermodulation.Uplink DTX and downlink
DTX used together can improve the intra-frequency ratio of the
system. Thiskind of improvement, when used in
aggressive-frequency-reuse cell planning, especially when used
withfrequency hopping, can greatly expand the system capacity.II.
Voice Activity DetectionFor voice activity detection (VAD), the
source must indicate when the transmission is required. WhenDTX
mode is activated, the encoder must detect the signal is voice or
noise. Therefore, the VAD isrequired. VAD can differentiate voice
from noise through calculating some signal parameters andthreshold
values. This kind of differentiation is based on an energy rule:
the energy of noise is alwayslower than that of voice.VAD generates
a group of threshold value in every 20 ms to judge whether the next
20ms block is voiceor noise. When the background noise is too loud,
the noise signal will be regarded as voice signal totransmit.III.
Silence IndicatorThe coding procedure of noise is the same as that
of voice. After sampling and quantification, a noiseblock will be
produce by encoder in every 20ms. Like voice block, the coded noise
block also contains260 bits, which forms a SID frame. The SID frame
will go through channel coding, interleaving,encryption and
modulation and finally be sent by eight continuous bursts.On TCH, a
complete SACCH information block has four 26 muliframe cycles (480
ms). In order todifferentiate voice frame and SID frame, these
eight continuous bursts are arranged at the beginning ofthe third
multiframe. During other time of the 480 ms, no information is
transmitted except SACCHtimeslot. The SID frame made from the 20 ms
noise block is interleaved with the preceding frame andthe
following frame; the first SID frame is interleaved with the
preceding voice frame and the followingSID frame.IV.
MeasurementUplink DTX and downlink DTX are two irrelevant
procedures that are activated by system parametersrespectively.
There are two kinds of measurement in GSM: full measurement and sub
measurement.Global measurement is the average of the level and
quality of the 104 timeslots in a measurement cycle(four 26
multiframes); local measurement is the average of level and quality
of 12 timeslots, includingeight continuous TCH bursts (for TCH/F,
0-103 TDMA frames as a cycle. The frame numbers of theseeight
bursts are 52, 53, 54, 55, 56, 57, 58, and 59. when no voice or
signaling is transmitted, thedescriptor of comfort noise they
contain is called SID) and four SACCH bursts (0-103 TDMA frames asa
cycle, for timeslot 0, the frame numbers of these four bursts are
12, 38, 64, and 90; for timeslot 1, theframe number is that of
timeslot 0 plus 13. similarly, the frame numbers that the eight
timeslotscorrespond to can be obtained in this way). In order to
achieve uniformity, no matter the uplink DTX ordownlink DTX is
activated or not, BTS and MS must complete these two kinds of
measurement. EachSACCH measurement report of BTS and MS indicates
whether DTX is used in last measurement reporttime. BSC choose one
of the two kinds of measurement based on this indication.: ourdot
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