Air Interface V2
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Transcript of Air Interface V2
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GSM Air Interface
YLI Sessions
VFE Radio Team
January 2008
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13 September 2013
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Idle mode behaviour
What is idle mode?The MS is said to be in Idle mode when it is powered on but not allocated any dedicatedchannel.
Idle mode activities :PLMN selection CELL selection & reselectionTune to the control channel of the cell, to listen to the system information messagestransmitted in that cell (Camping on the cell)4- It must also be able to register its current location to the network (Location Updating)
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Idle Mode: PLMN SelectionPLMN selection (Automatic/Manual mode) Automatic mode1- Last registered PLMN
2- Home PLMN3- According to PLMN list stored on SIM4- PLMNs with SS > -85 dBm in random order5- All other PLMNs in order of decreasing SS
Manual mode1- Last registered PLMN2- Home PLMN3- The MS will indicate the user all available PLMNs
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Idle mode: Cell Selection
Why MS must camp on a cell? The MS can receive system information The MS can initiate a call The system can reach the MSTypes of cell selection:Normal cell selection The MS will search all RF channels in its supported frequency band (124 channels inGSM 900)Stored list cell selection The MS will perform the measurements for only the last BA list BCCH carriers stored
on SIM before it was powered off.
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Idle mode: Cell Selection
Scan 124 channels and measure signal strength
Tune to the RF channel with the highest
received average signal strength
Determine if it is a BCCH frequency carrier by
searching for frequency correction bursts
Tune to the RF-channel with the
highest signal strength not already
tried
The MS shall attempt to synchronize to this
carrier and read BCCH info
Is it a BCCH carrier
Is it the wanted PLMN ?
Is the cell barred for access ?
Is C1>0 ?
Camp on this cell
No
Yes
No
Yes
No
Yes
Yes
No
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Idle mode: Cell reselection
Cell reselection:The MS detects that it is more suitable to camp on another cell.Why?The MS at all times try to camp on the cell which has the highest probability ofsuccessful communication on both up and downlinks when accessing the systemWhen?MS performs cell reselection if:
The serving cell becomes barred Downlink signaling failure C1C2(serving) (+CRH) more than 5sec
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Idle mode: Cell reselection
Cell reselection procedures: MS monitors Neighbors BCCH carriers as well as the serving one. MS measure server BCCH at least once every 30s MS measures neighbors BCCH carrier at least once every 5min. MS attempts to decode the BSIC parameter at least once every 30s (for each of the six strongest surrounding cells)
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Idle mode: location updating
Location Updating:The location updating is initiated by the MS to keep the network updated by its location.
Types of Location updating :1- Normal: MS listens to the broadcasted system information. It compares the broadcasted LAI with the stored one on the SIM. If it is different , a location updating type normal will be initiated.
2- Periodic registration:Reduces unnecessary paging of MSs which lost coverage or run out of battery. T3212 parameter (1 255) determines how often MS should perform periodic locationupdate.
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Idle mode: IMSI attach/detach
When a MS is powered on, an IMSI attach message is sent to the MSC/VLR When a MS is powered off, an IMSI detach message is sent to the MSC/VLR Implicit detach?
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Normal Location update within same MSC/VLR service area
MSC / VLR
BSC
1
1. The Mobile sends an SDCCH allocation
request message on the RACCH to the BTS
2
2. The BTS responds with the SDCCH allocation
message on the AGCH
3
3
3
3. The mobile sends a location update request
message with its IMSI on the allocated
SDCCH channel to the MSC/VLR
4
4
4
4. The MSC/VLR updates the location
information and sends a Location Update
confirmation message on the SDCCH
channel to the mobile then the SDCCH
channel is released
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Normal Location update between 2 MSC/VLR service areas
7
7
7
7. The new MSC/VLR sends a locatio
updating confirmation message to
the mobile
HLR
Old MSC
Old BSC
New MSC
New BSC
1
1
1
1. The mobile requests a location update just like the first case
IMSI MGT2
2. The new MSC/VLR receives the IMSI and conclude the MGT
3
3. The MSC/VLR sends a subscriber
information request with the IMSI
to the HLR
4. The HLR stores the address of
the new MSC/VLR
HLR
5
HLR
5. The HLR sends the data to the
new MSC/VLR and it is kept ther
New MSC
6
6. The HLR sends a location
cancellation message to the old
MSC/VLR to remove the data
Old MSC
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Dedicated Mode: Handover
Handover : Handover is essential to keep continuity of the call when MS roamsalong the network and moving from one cell to another betweendifferent nodes in the network. During a call, the MS is continuously measuring the signal levels ofneighboring cells in addition to servers and reports this results tothe BSC through the BTS. The BSC, being responsible on supervising the cells, is responsibleof handover initiation. Good neighbor relations between cells is an important factor inkeeping the network performance in the accepted level.Types of handover : Intra Cell Intra BSC Handover Inter BSC / Intra MSC Handover Inter MSC Handover
BSC
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Basic definitions
Basic definitions: Time Slot : TS is the basic radio resource during which the carrier serves one logic channel
MS receives TS by TS and also sends TS by TS. Burst : the contents of each time slot is called a burst. Physical channel : the recurrence of the same physical TS in successive frames. Logical channel : the recurrence of the same TS contents in successive frames.
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Basic definitions
Physical Channels
CH 1 54321 6 70 0 1Time
CH 3 54321 6 70 0 1
CH 2 54321 6 70 0 1
CH 12454321 6 70 0 1
GSM band is divided into 124 RF channels, and each channel is divided into 8 time
slots using TDMA. These time slots are called physical channels.
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Logical Channels
Synchronization ChannelBroadcast Control Channel
Frequency Correction Control Channelaging Channel
Random Access ChannelAccess Grant Channel
Standalone Dedicated Control ChSlow Associated Control Ch
Fast Associated Control ChCell Broadcast Control Ch
BroadcastommonDedicated
Logical Channels
Control ChannelsTraffic Channels
Half Rate Full Rate
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Traffic Channels
TCH is allocated to the MS when it is in dedicated mode. TCH is intended to carry two types of bi-directional user information : Encoded speech
and data . Two types of traffic channels are defined
TCH/F: Full rate uses one physical channel TCH/H: Half rate shares one physical channel with another TCH/H
Channel rate:Full Rate (22,8 kbps gross bit rate on air interface)Half Rate (11,4 kbps gross bit rate)
Speech versionsFR SPV 1 = normal Full Rate (FR) 13 Kbps (speech coding rateFR SPV 2 = enhanced Full Rate (EFR) 12.2 Kbps (speech coding rate)FR SPV 3 = AMR Full Rate (AMR-FR) ACS (Active Codec Set)HR SPV 1 = normal Half Rate (HR) 5.6 Kbps (speech coding rHR SPV 3 = AMR Half Rate (AMR-HR) ACS (Active Codec Set)
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Broadcast Channels
Frequency Correction Channel (FCCH): The 1st Burst that MS looks for just after switching on It is the repetition of frequency correction bursts Bursts containing only zeros are transmitted Make sure that this is the BCCH carrier Allow the MS to synchronize to the frequency Only transmitted DL , TS0
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Broadcast Channels
Synchronization Channel (SCH): Used to provide synchronization information between MS & BTS in the time domain. Repetition of synchronization bursts Transmitted DL only. Base Station Identity Code (BSIC) TDMA Frame Number (RFN)
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Broadcast Channels
Broadcast Control Channel (BCCH): When a mobile is turned on, it scans the GSM spectrum to find an available BTS. It is a permanent channel to be scanned, providing network information (called
system information) Transmitted DL only. Contains; cell description, cell identity, location area identity, network identity, BCCH
of neighboring cells.
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Common Control Channels
Paging Channel (PCH) : When the network wants to initiate a communication with the MS, it looks for the
mobile by sending a paging request. Incoming call or incoming short message. Mobile Identity parameter (IMSI) or (TMSI). Transmitted DL only.
Access Grant Channel (AGCH) : Inform about the SDCCH or TCH allocated. Transmitted DL only. Channel description, Timing advance.
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Common Control Channels
Random Access Channel (RACH) : When the MS requests allocation of signaling channel by sending a Channel Request Answering to paging, network contact Only transmitted UL
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Dedicated Control Channels
Slow Associated Control Channel (SACCH) : When an SDCCH or a TCH is allocated, an associated channel is used by MS &
Network in Order to exchange general information Transmitted both UL and DL Up-Link: Measurement report Down-Link: System information, MS power level, timing advance
Fast Associated Control Channel (FACCH) : During a hand over, both MS and Network need to send a fast message to the other Works in stealing mode Transmitted both UL and DL
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Dedicated Control Channels
Standalone Dedicated Control Channel (SDCCH) : MS and network need to exchange information before initializing communication. Used in authentication, ciphering, setup and assignment procedures Transmitted both UL and DL Transmission of SMS in idle mode.
Cell Broadcast Control Channel (CBCCH) It displays general information (weather, addresses, traffic..) . It uses one of the SDCCH channels MS must be configured to receive this channel
.
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Normal Burst Structure
.
GSM Overview
Technical Session
TailBits
Encrypted Bits Training Sequence Encrypted BitsTailBits
GuardPeriod
3 57 1 26 1 57 3 8.25
0 1 2 3 4 5 6 7
TDMA Frame
156.25 bits in 577 m Sec
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Mapping of logical channels (DL)
CBBBBSF
F S F S F S F S F S IB C C C C C C C C C
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
Frame 1 Frame 2 Frame 3 Frame 4 Frame 5 Frame 6 Frame 7
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Mapping of logical channels (UL)
RRRRRRR
R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
Frame 1 Frame 2 Frame 3 Frame 4 Frame 5 Frame 6 Frame 7
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Post installation cycle
Site integration
Site testing
Site optimization
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Drive-test Sample
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Questions
Thanks