gsm-power control ,h-o, dtx

8/10/2019 GSM-Power Control ,H-O, DTX

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For circulation to Trainees only

PROCEDURE IN GSM

-POWER CONTROL

-HANDOVER

-DISCONTINUOUS TRANSMISSION-DTX

RTTC Guwahati


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Procedure In GSM

Role of Different Procedure in GSM as follow:-

Establish stable connections between the mobile stations

and the BSC (Base Station Controller)

Maintain them despite user movement for the duration

of a call.

Release the connections between the mobile stations

and the BSC.


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The maximum nominal output power of a

GSM 900 Class 4 MS is 33 dbm (2W)

Simple calculation:-

0 dbm =1 mw, 3 dbm=2 mw, 6 dbm= 4 mw, 9 dbm=8mw,

12 dbm=16mw, 15 dbm=32mw, 18 dbm=64mw,

21 dbm=128mw, 24 dbm=256mw, 27 dbm=512mw,

30 dbm=1024mw, 33 dbm=2W.


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Procedure In GSM-Power control

PERIODICAL MEASURING

The mobile station measures periodically the performance of

the downlink, and sends the Measurements in the SACCH

(Slow Associated Control Channel) via the serving BTS to the

BSC.

it transfers the measurements in the SACCH to the BSC every

480ms i.e.2 Hz. This is also the power control rate in GSMNetwork.


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MEASUREMENTS-mobile station is active on a call

DOWNLINK/UPLINK MEASUREMENTS- Strength of the

signal being receiving from its serving BTS & MS (in

dBm)

Quality of the signal being received from its serving BTS

& MS (in bit error rate).


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REASONS FOR POWER CONTROL

Power Control is to save mobile station battery power.

Power Control is improving the carrier-to-interference

ratio within the cellular network

It also decrease interference caused on the other calls in

the surrounding area.


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Procedure In GSM-Handover

HANDOVER- NEUTRALISE MS MOBILITY EFFECT

It is process of automatically switching a call in progress

from one traffic channel to another to neutralize the

adverse effects of user movements.

Handover process will normally only be started if power

control is not helpful anymore.

Handover measurements done on SAACH but final trigger

of Handover done on FACCH.


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HANDOVER TRIGGERS OBJECTIVE1. Maintain a good speech quality

2. Minimize the number of calls dropped

3. Maximize the amount of time the mobile station is in

the best cell

4. Minimize the number of handovers.

5. Neutralize the effect of user mobility


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HANDOVER PROCESS

As a mobile station moves away from its serving BTS towards the

coverage area of neighboring BTSs(candidate )

The mobile station measurement reports will show a gradual

decrease in signal strength from its serving BTS while showing an

increase in measured signal strength from one or more neighboring

BTSs.

serving BSC to analyze the measurement reports from the mobile

station and to decide when a handover should be performed as per

defined Handover Margin (Default HO Margin 5 dbm, can be

increase or decrease)


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The hand-over process may be of as consisting of three

phases-

Recognition that handover is required.

Decision of a target cell.

Execution of the handover.

Allocation and connection of new channel.

Reallocation and release of old channel.


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BSC receives measurements data about the performance of

the air interface from its serving BTSs and mobile stations and

than decide when to trigger the handover.

DOWNLINK MEASUREMENTS

Quality of the signal being received from its serving BTS (in

bit error rate)

Signal strength of the 6 best neighboring BTS downlinkcontrol channels (candidate list). In 7 cell cluster N/W.


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UPLINK MEASUREMENTS

The BTS measures and reports the following measurements

to the BSC regarding the performance of the uplink:

Strength of the signal being receiving from the mobile station

Quality of the signal being received from the mobile station.

Distance between the serving BTS and the mobile station (in

meters)


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Conditions that trigger the Handovers:-1. Distance (or propagation delay) between the mobile

station and the BTS becomes too big:-If the timing advance

for the mobile station becomes too big, because it is too far away

from the base station, handover has to take place to a cell that iscloser to the mobile station

2. Received signal level (or signal strength) on the downlink or uplink

becomes too bad

3. Received signal quality on the downlink or uplink becomes too bad4. Path loss situation for the mobile station to another cell is better

i.e. less path loss in neighbour cell


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HANDOVER TYPES: four types of handovers: -

1. Internal or intra-BSS handover, which can be:

IntraBTS handover

Inter-BTS handover.

2. External or inter-BSS handover, which can be:

Intra-MSC handover.

Inter-MSC handover.


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INTRA-BSS INTRA BTS HANDOVER

mobile station is handed over to a different radio

channel within the same BTS area i.e. handover in sector

of same BTS.

A probably cause for this type of handover would be

poor signal strength, possibly due to co-channel

interference.


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For circulation to Trainees only 18

Intra BSS, Inter cell Handover process:-

The MS is continuously listening to the broadcastchannel, measuring the strength of currently usedradio channel and neighboring channels andreporting the result to the BSS.

The BSC allocates a new radio channel and instructsthe MS to re-tune to it

The MS tunes to the new radio channel and sends

Handover Complete to the BSS

The BSS informs the MSC about the new location ofthe MS



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For circulation to Trainees only 19

Intra BSS, Inter cell Handover

MSMSC/VLR

BSS

1

2

3

4

Signal Strength Measurements

SACCH

Handover

Command FACCH

Handover Access

Handover

Complete

Handover

Performed

FACCH

FACCH


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INTRA-BSS, INTERBTS HANDOVER

The mobile station moves from one BTS towards another

BTS area both are connected to same BSC.

BSC will determine from the signal strength

measurement reports to trigger the handover ,without

the involvement of MSC .

BSC is responsible for such type of Handover.


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Procedure In GSM-RRM

INTER-BSS INTRA- MSC HANDOVER The mobile station moves from one cell area to another cell &

both cell belong to different BSC ,but both BSC connected to

same MSC.

MSC will trigger the handover to the candidate base station

from serving base station.

MSC will switch voice paths from the old BSC to the new

BSC.

The decision of Handover is taken by MSC.


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INTER BSS, INTER-MSC HANDOVER

The mobile station moves from two different cell ,both

cell connected to different BSC & both BSC are

connected to different MSC.

Different MSCs are controlled by GMSC of same PLMN.

The decision of handover based on signal strength is

here taken by GMSC.


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DISCONTINUOUS TRANSMISSION PROCESS

In the DTX mode:- speech is encoded at 13kbit/s when the user iseffectively speaking, but in a speech pause information is

transmitted at a bit rate around 500 bit/s.

REASONS FOR DTX

When DTX is applied, actual transmission on the radio path is

reduced i.e. less bit transmission on air interface.

This will cause a decrease of the interference level in co-channel

cells (using the same frequency) i.e. improve C/I (Carrier to

interference)

when using DTX in the uplink mode:- it saves battery power for the

mobile station.

d


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Procedure In GSM-DTX

DTX can be enabled or disabled for the uplink and/or

downlink mode on a per-cell basis.

DTX mode does not save the radio channel, but

definitely , improve the networks quality of service.

P d I GSM DTX


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figure shows Discontinuous Transmission (DTX).


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P d I GSM DTX


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VAD (Voice Activity Detector):-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

Bit rate reduced from 13 Kbps to 500 kbps during pause orsilence condition.

P d I GSM DTX


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SID(Silence Indicator Descriptor ):-The transmitter

will periodically send a signal called a Silence Indicator

Descriptor frame every certain period of time.

The Silence Indicator Block provides the comfort noise

level information to the mobile station or BTS.

Note that transmitting in DTX mode does not save

timeslots on the air-interface.

P d I GSM C ll R bli h


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Procedure In GSM-Call Re-establishment

CALL RE-ESTABLISHMENT Call re-establishment enables the mobile station to resume

the contact with the cellular network when the connection to

a particular BTS is suddenly broken.

This may happen because of a brutal propagation loss, due

to obstacles such as bridges and tunnels big high rise building

etc.

Call re-establishment is a GSM feature that can be enabled or

disabled on a per cell basis.

P d I GSM C ll R t bli h t


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CALL RE-ESTABLISHMENT PROCESS

After the communication has been lost, the mobile

station selects the cell with the highest signal strength

from the neighbor cell list.

The neighbor cell list contains the cell identifiers to

which a handover is allowed. It is kept in the BSC of a

particular cell. The list is transferred to the mobile

station in the BCCH during the registration phase of a

wireless call

P d I GSM C ll R t bli h t


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The mobile station uses the neighbor cell list by only

measuring the signals from the BTSs located in the cells thatare on the list.

The selected cell identifier is used to re-establish the

connection to that particular cell by following the normal

access procedures.

At the moment of time the communication is lost, a timer is

initiated in the serving MSC.

When the timer expires, it is not possible anymore for the

mobile station to re-establish the call.

A typical value for the timer is 4 seconds.


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gsm-power control ,h-o, dtx

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