Giuseppe Bianchi

Wireless Cellular NetworksWireless Cellular Networks(basics) (basics)

Part 3 – GSM networks

Giuseppe Bianchi

History of Cellular systemsHistory of Cellular systems

1960's: Bell Labs developed cellular concept 1974-1978: First field Trial for Cellular System

AMPS (Advanced Mobile Phone System), Chicago 1981, Sweden, first European Systems

NMT-450 (Nordic Mobile Telephone) 1985, first italian cellular system

RTMS (Radio Telefono Mobile di Seconda Generazione), 450 MhZ 1990, TACS, first italian widespread systems

Total Access Communication System (TACS, 900 MHZ) Second generation system:

GSM in europe, D-AMPS & IS95 (CDMA) in USA, PDC JapanDigital, versus fist generation analog (frequency modulation)

GSM:Specification started in 1982; EU deployment since 1992; DCS-1800 since 1994

Generation 2 ½ : GPRS, EDGE (8PSK), HSCSD Generation 3: UMTS, HSDPA HSPA,

In ITA since 2004 Generation 4: LTE (2011/2012?)

Giuseppe Bianchi

GSM essential componentsGSM essential components

BTS

BTS

BTS

BTS

BTS

BSC

BSC

MSC

VLRHLRAUCEIRGMSC

To fixed network (PSTN, ISDN, PDN)

OMC

MS Mobile StationBTS Base Transceiver StationBSC Base Station ControllerMSC Mobile Switching Center GMSC Gateway MSCOMC Operation and Maintenance CenterEIR Equipment Identity RegisterAUC Authentication CenterHLR Home Location RegisterVLR Visitor Location Register

MS

Giuseppe Bianchi

GSM system hierarchyGSM system hierarchy

BTS

BSCLOCATION

AREA

MSC MSC region

Hierarchy: MSC region n x Location Areas m x BSC k x BTS

MSC: Mobile Switching CenterLA: Location AreaBSC: Base Station ControllerBTS: Base Transceiver Station

Giuseppe Bianchi

Mobile Station (MS)Mobile Station (MS) Key fact:

GSM separates user mobility from equipment mobility, by defining two distinct components

Mobile EquipmentThe cellular telephone itself (or the vehicular telephone)

Address / identifier: IMEI (International Mobile Equipment Identity)

Control: Equipment Identity Register (EIR): White list, black list, gray listStolen terminals, malfunctioning terminale

Subscriber Identity Module (SIM)Fixed installed chip (plug-in SIM) or exchangeable card (SIM

card)Addresses / identifiers:

IMSI (International Mobile Subscriber Identity)MSISDN (Mobile Subscriber ISDN number – the phone number)

Giuseppe Bianchi

Base Station Sub-SystemBase Station Sub-System

BTS

BTS

BTS

BSC

A-bisInterface

Um - RadioInterface

BSS

AInterface

OSS

Base Transceiver Station (BTS)Transmitter and receiver devices, voice coding & decoding, rate adaptation for dataProvides signaling channels on the radio interfaceLimited signal and protocol processing (error protection coding, link layer LAPDm)

Base Station Controller (BSC)performs most important radio interface management functions:Radio channels allocation and deallocation; handover management; …

Giuseppe Bianchi

Base Transceiver Station - Base Transceiver Station - BTSBTS

Outputfilter

InputFilter

HFTransmitter

HFReceiver

Slo

w f

req

.H

op

pin

g TRXDigital Signal

Processing

Tra

nsm

iss

ion

Sys

tem

Operation and Maintenance Functionality/clock distribution

Abi

s In

terf

ace

(to

BS

C)

Um Interface(to MS)

TRX radio interface functions:- GMSK modulation-demodulation- channel coding- encryption/decryption- burst formatting, interleaving- signal strength measurements- interference measurements

In essence, BTS is a complex modem!

Giuseppe Bianchi

Base Station Controller - Base Station Controller - BSCBSC

FUNCTIONS: switch calls from MSC to

correct BTS and conversely

Protocol and coding conversion for traffic (voice) &

signaling (GSM-specific to ISDN-specific)

Manage MS mobility Enforce power control

Xswitchmatrix

BTS-1

BTS-2

BTS-K

1 BSC may controlup to 40 BTS

DB

From/to MSC

DB contains - state information for all BSS- BTS software

Giuseppe Bianchi

Transcoding and Rate Transcoding and Rate AdaptationAdaptation

BTS: -collects speech traffic-Deciphers and removes error protection-Result:

-13 kbps air-interface GSM speech-coded signalMSC:-A modified ISDN switch-Needs to receive ISDN-coded speech

-64 kbps PCM format (A-law)

Transcoding andRate Adaptation Unit (TRAU)needed!

Rationale: re-use existing ISDN switches & protocols

Giuseppe Bianchi

TRAU possible placementsTRAU possible placements

BTS BSC

64 kbit/s16 kbit/s13 kbit/sOn BSCTRAU MSC

BTS BSC

64 kbit/s(4x16 sub-mux)

16 kbit/s13 kbit/sOn MSCTRAU MSC

Why 16 kbps instead of 13? Inband signalling needed for BTS control of TRAU(TRAU needs to receive synchro & decoding information from BTS)

BTS TRAU BSC MSC

64 kbit/s64 kbit/sOn BTS

13 kbit/s

Giuseppe Bianchi

Network Switching Sub-Network Switching Sub-SystemSystem

Elements:Mobile Switching Center (MSC) / Gateway MSC (GMSC)

Enhanced telephone switching centers (digital, ISDN) With support for user mobility, registration, handover

Home Location Register (HLR ) / Authentication Center (AuC)Visitor Location Register (VLR)Equipment Identity Register (EIR)

Functions:Call controlUser management

Inter-component communicationVia SS7 signalling network with suitable extensions

(e.g. MAP – Mobile Application Part)

Giuseppe Bianchi

LocationLocation

LA-4 … LA-n

LA-1 LA-2 LA-3

MSC VLR

1 MSC 1 VLR Several Location Areas

Giuseppe Bianchi

Location Registration Location Registration (Update)(Update)

(very) basic idea(very) basic idea

MSC VLR

BTS

BTS

BTS

BSC

MS

HLR

1

1) MS switches ON; detects cell through BCCH carrier;Obtain Location Area Identifier (LAI)from BCCH

2

2) Register MS ID (IMSI) into local VLR; Authenticate; receive TMSI for local paging purposes

3

3) Update pointer at HLR, which now knows which LAI/VLR the user is located

Giuseppe Bianchi

Location Registration - Location Registration - detailsdetailsMS VLR HLR AUCBSS/MSC

Loc. Upd. RequestIMSI, LAI

Update Loc. AreaIMSI, LAI

Auth. Param. Req.IMSI

Auth. Info. Req.IMSI

Auth. Info(Auth. Parameters)

Auth. Info(Auth. Parameters)

authentication

Activateciphering

Update LocationIMSI, MSRN

Insert Subscrib. Data IMSI, additional data

Insert Subscrib. DataACK

Locat. Upd. AcceptIMSI

Start CipheringKc

Locat. Upd. Accept

Forward new TMSI TMSI

TMSI Realloc Cmd

Locat. Upd. Accept

TMSI Realloc ACKTMSI ACK

Giuseppe Bianchi

Changing MSC/VLRChanging MSC/VLR

Base Station

MSC Public switched telephone network

PSTN

Public switched telephone network

PSTN

Base Station

MSC

VLR

VLR

HLR

An MS always has a dedicated entry in the HLR Plus one entry in JUST 1 VLR

(related to the MSC the user is connected to)

Giuseppe Bianchi

Location Update: different Location Update: different VLRVLR

MS VLR-new HLR VLR-oldBSS/MSCLoc. Upd. RequestTMSI(+ old LAI), LAI

Update Loc. AreaTMSI(+ old LAI), LAI

authentication

Activateciphering

Update LocationIMSI, MSRN

Insert Subscrib. Data IMSI, additional data Ins. subs. data ACK

Locat. Upd. AcceptIMSI

Start CipheringKc

… …

Forward new TMSI

GenerateNew TMSI

Send parameters (TMSI, old LAI)

IMSI response (IMSI,RAND,SRES,Kc)

Cancel LocationIMSI

Cancel Locat. ACK

determineVLR-old

From old LAI

Giuseppe Bianchi

Call switchingCall switchingGateway MSC – GMSCGateway MSC – GMSC

X XX

XX

MSC

PLMNPublic Land

Mobile Network

MSCMSC

GMSC

Needed, as fixed networkswitches are not mobilecapable!!

GMSC task: query HLR forcurrent MS location

(if fixed network switcheswere able to query HLR,direct connection with local MSC would be available)

HLR

Giuseppe Bianchi

NotationNotation

A call involves two “Parties”Calling Party (caller)

user generating the callCalled Party (callee)

user receiving the callMobile Originating Call (MOC)

Call originated by an MSMobile Terminating Call (MTC)

Call directed to an MS

Giuseppe Bianchi

Call establishment basicsCall establishment basics

MS MSCFixedparty

setup

MS MSCFixedparty

setup

setup

Call confirmed

alertingalerting

connectconnect

DATA

setup

Mobile Terminated Call Mobile Originated Call

Call proceeding

alertingalerting

ConnectConnect

Connect Ack

DATA

In ISDN ISUP: - setup = IAM (Initial Address Message); - Alerting = ACM (Address Complete Message); - Connect = ANS (Answer)

Giuseppe Bianchi

Call establishment stepsCall establishment steps

Channel request

Paging request

Paging Response

Immediate Assignment

Authentication Response

Authentication Request

Ciphering Mode Complete

Ciphering mode command

Call Confirmed

Setup

Assignment Complete

Assignment Command

Alerting

Connect

Connect Acknowledge

MobileTerminated CallMS network

Channel request

Service Request

Immediate Assignment

Authentication Response

Authentication Request

Ciphering Mode Complete

Ciphering mode command

Call proceeding

Setup

Alerting

Connect

Connect Acknowledge

MobileOriginated CallMS network

Assignment Complete

Assignment Command

Giuseppe Bianchi

MSCAHLR

MSCC

MSCB

PLMN

ISDN

GMSC

VLRB

Routing an MTCRouting an MTC

1: M

SISDN

4: M

SRN

2: MSISDN

3: MSRN

5: MSRN

6: TMSI 7: paging

Giuseppe Bianchi

PLMN 1(ITA)

MSC

GMSC 1

HLR

PLMN 2(UK)

MSC

ISDN(ita)

TransitExchange

LocalExchange

InternationalSwitching

Center

MSISDN+39.335.1234567

335.1234567

InternationalSwitching

Center

ISDN(UK)

MSRN+44.NDC.8877665

Routing calls to Roaming MSRouting calls to Roaming MS

Giuseppe Bianchi

““tromboning”tromboning”

PLMN 1(ITA)

MSC

GMSC 1

HLR

PLMN 2(UK)

MSCISC(UK)

MSISDN+39.335.1234567

MSRN+44.NDC.9876543

Call to MSISDN+39.335.1234567

ISC(ITA)

Is the PRICE (!) to pay for

simple routing and billing

Call to MSISDN+39.335.3043125

Giuseppe Bianchi

Number portabilityNumber portability

Subscriber may switch operator without changing his number

First implemented in fixed networkmay 2002: extended to mobile networks

Essential for fair competition among network operatorsUK 2002 survey: 42% of corporate subscribers were willing

to change mobile operator; but 96% were, if number could be ported

Resistence from leading operatorsNumber portability helps newer operators to compete with

traditional ones

Giuseppe Bianchi

NotationNotation

Donor switchThe switch corresponding to a “ported” telephone

numberRecipient switch

The switch to which the ported number is attached

Giuseppe Bianchi

Technical solutionsTechnical solutionsa) call forwardinga) call forwarding

switch switch

switch

Originating network Donor network

Recipient network

Originating switch sets-up trunk to donor switchDonor switch sets-up trunk to recipient switchSimplest solution, as call forwarding is a feature available in virtually all switches

But extremely inefficient routing and trunking resource consumption!

Giuseppe Bianchi

Technical solutionsTechnical solutionsb) query on releaseb) query on release

switch switch

switch

Originating network Donor network

Recipient network

Donor switch “blocks” incoming call with a release message (REL)REL carries a QoR cause value, stating that called party number is ported Originating switch then queries Number Portability database

SS7 ISUP IAM

SS7 ISUP REL

NumberPortabilityDataBase

Giuseppe Bianchi

Technical solutionsTechnical solutionsc) all-call queryc) all-call query

switch switch

switch

Originating network Donor network

Recipient network

Originating switch queries Number Portability database for every call!!- best solution if majority of numbers are ported (no interaction with donor)- but very high DB load, as EVERY number must be looked-up!

NumberPortabilityDataBase

Giuseppe Bianchi

Mobile Number PortabilityMobile Number Portability Same ideas as fixed number portability

The donor switch is the GMSC of the donor network Donor GMSC Call forwarding (if more efficient

fixed number portability not supported)While porting number, may also get MSRN!

GMSC

Incoming call

Donor network

HLRSignaling relay

function

GMSC

Recipient network

HLRMSC Note: If path must cross GMSC:Use Intermediate Routing Number

MSRN(or IRN)

MSRN IRN

Clearly, still suffers of tromboning!

Giuseppe Bianchi

Return IRN

Mobile Number PortabilityMobile Number Portability(with all call query approach)(with all call query approach)

switch

Incoming call

GMSC

Recipient network

HLRMSC

IRN

NumberPortabilityDataBase

Query IRN

Return MSRN

Query MSRN

Giuseppe Bianchi

Mobile Number PortabilityMobile Number Portabilityimproved – (with all call query approach)improved – (with all call query approach)

Return MSRNswitch

Incoming call

GMSC

Recipient network

HLRMSC

MSRN

NumberPortabilityDataBase

Query MSRN

Signaling relayfunction

Giuseppe Bianchi

Wireless Cellular NetworksWireless Cellular Networks(basics) (basics)

Part 4 – GSM radio interface

Giuseppe Bianchi

GSM Radio SpectrumGSM Radio Spectrum

2 x 25 Mhz bandDuplex spacing: 45 MHz

124 carriers x band200 KHz channelsSuggested use: only 122

Use top & bottom as additional guard 8 TDMA slots x carrier

full rate calls – 13 Kbps If half-rate used, 16 calls at 6.5 kbps

Frequency [MHz]

890

915

935

960

UPLINKMS BS

DOWNLINKBS MS

890.2

890.4

“guard band”

1 2 3 4 5 6 7 8

MHz12.02.935

MHz12.02.890

nnF

nnF

dwlink

uplink

Giuseppe Bianchi

Adjacent channelsAdjacent channels(due to GMSK)(due to GMSK)

35dB60dB

Specification: 9dBIn practice, due to power control and shadowing, adjacent channelsCannot be used within the same cell…

Giuseppe Bianchi

Physical channel Physical channel

200 KHz bandwidth + GMSK modulation 1625/6 kbps gross channel rate (270.8333 kbps)

1 time slot = 625/4 bits 156.25 bits 15/26 ms = 576.9 s

time

timeslot0

577 s

timeslot7

1 frame = 60/13 ms = 4.615 ms26 frames = 120 ms (this is the key number)

Giuseppe Bianchi

Hybrid FDMA-TDMAHybrid FDMA-TDMAphysical channel = (time slot, frequency)physical channel = (time slot, frequency)

time

577us 577us 577us 577us 577us 577us 577us 577us

frequency

200 KHz

200 KHz

200 KHz

200 KHz

200 KHz

200 KHz

200 KHz

200 KHz

200 KHz

slot

Total n. of channels: 992

Giuseppe Bianchi

Slow Frequency hoppingSlow Frequency hopping(optional procedure within (optional procedure within individualindividual cell) cell)

f1f2f3f4f5f6f7

Hopping sequence (example):… f1 f2 f5 f6 f3 f7 f4 f1…

Slow = on a per-frame basis- 1 hop per frame (4.615 ms) = 217 hops/second

Physical motivation:- combat frequency-selective fading- combat Co-Channel Interference

next slot may not interferere with adjacent cell slot (different hopping sequence)- improvements: acceptable quality with 9 dB SNR versus 11 dB

Giuseppe Bianchi

DuplexingDuplexing

0 1 2 3 4 5 6 7UPLINK

0 1 2 3 4 5 6 7 DOWNLINK

- MS uses SAME slot number on uplink and downlink

- Uplink and downlink carriers always have a 45 MHz separation-I.e. if uplink carrier is 894.2 downlink is 919.2

-3 slot delay shift!!

MS: no need to transmit and receive in the same time on two different frequencies!

Giuseppe Bianchi

GP8.25

Structure of a TDMA slotStructure of a TDMA slot

Symmetric structure DATA: 2 x 57 data bits

114 data bits per burst “gross” bits (error-protected; channel coded) “gross” rate: 24 traffic burst every 26 frames (120 ms)

22.8 kbps gross rate13 kbps net rate!

S: 2 x 1 stealing bit Also called stealing flags, toggle bitsNeeded to grab slot for FACCH (other signalling possible)

TB3

DATA57

S1

S1

Trainingsequence

26

Data57

TB3

148 bit burst

156.25 bit (15/26 ms = 0.577 ms)

Normal burst

Giuseppe Bianchi

Guard Period rationaleGuard Period rationale

Assume the following synchro mechanism:BTS transmits at time 0MS receives at time d/cMS transmits at time 3+d/cBTS receives at time 3+ 2d/c

Offset depending on d!

BTS

d

1 2 3 4BTS downlink tx

MS downlink rx 1 2 3 4

1

1

MS uplink rx

BTS uplink rx1

Expected RX time!

Giuseppe Bianchi

Guard period sizingGuard period sizing

BTS timeMS time

dwlink slot 1 dwlink slot 4…

dwlink slot 1 dwlink slot 4…

uplink slot 1

uplink slot 1

…

…

Maximum cell radius:

KmC

GTcd

c

d

C

GT

rate

bits

rate

bits 5.42708332

25.8300000

2

2

Is there something wrong? (GSM says that cells go up to 35 km)

Giuseppe Bianchi

Frame synchronizationFrame synchronization

Timing Advance (TA) Parameter periodically transmitted by

BTS during MS activity 6 bits = 0-63 Meaning: anticipate transmission of

TA bits TA=0: no advance

I.e. transmit after 468.75 bitsafter downlink slot

TA=63: Transmit after 405.75 bits time

BTS

TA (transmitted in the SACCH)

dwlink slot 4

uplink slot 1MS timeTA

dwlink slot 4

uplink slot 1uplink slot 1

BTS time

TA avoids collision!

Giuseppe Bianchi

Timing Advance analysisTiming Advance analysis

Downlink propagation delay:d/c

Uplink propagation delay:d/c

Uplink delay with TA:d/c-TA

Perfect resynchronization occurs whenTA = 2d/c

Maximum cell size for perfect resync:

kmskm

sbits

bitsc

TAd 89.34/300000

/270833

5.31

2

8.25 bits Guard time additionally available for imperfect sync (+/- error)

Giuseppe Bianchi

And when the user is not And when the user is not connected?connected?

But wants to connect…But wants to connect…

TB8

Trainingsequence

41

Data36

TB3

88 bit burst

156.25 bit (0.577 ms)

Access burst

Solution: USE A DIFFERENT BURST FORMATAccess Burst: much longer Guard Period availabledrawback: much less space for useful information

GP68.25

No collision with subsequent slot for distances up to 37.8 km

Giuseppe Bianchi

Logical vs Physical channelsLogical vs Physical channels

Physical channelsTime slots @ given frequencies Issues: modulation, slot synchronization, multiple access techniques,

duplexing, frequency hopping, etc Logical channels

Built on top of phy channels Issue: which information is exchanged between MS and BSS

Physical channels(FDMA/TDMA)

Logical channels(traffic channels, signalling (=control) channels)

Giuseppe Bianchi

Logical – physical mappingLogical – physical mapping

frequency

Physical Channel: data rate r, time slot i

frequency

Logical Channel Mapping: Different channels may share a same physical channel

Logical channel A: data rate r/3, time slot i, frame 3kLogical channel B: data rate 2r/3, time slot i, frame 3k+1, 3k+2

Frame 8 Frame 9 Frame 10 Frame 11 Frame 12

Giuseppe Bianchi

GSM logical channelsGSM logical channels

Traffic channel (TCH) TCH/F TCH full rate MSBSS

TCH/H TCH half Rate MSBSS

Broadcast channel BCCH Broadcast control BSSMS

(same information to all MS in a cell) FCCH Frequency Correction BSSMS

SCH Synchronization BSSMS

Common Control channel (CCCH) RACH Random Access MSBSS

(point to multipoint channels) AGCH Access Grant BSSMS

(used for access management) PCH Paging BSSMS

Dedicated Control channel (DCCH) SDCCH Stand-alone Dedicated control

MSBSS

(point-to-point signalling channels) SACCH Slow associated control MSBSS

(dedicated to a specific MS) FACCH Fast associated control MSBSS

Additional logical channels available for special purposes(SMS, group calls, …)

Giuseppe Bianchi

Traffic channels (TCH/F)Traffic channels (TCH/F)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Periodic pattern of 26 frames (120 ms = 15/26 ms/TS * 8 TS/frame* 26 frame)

24 TCH frames over 26

20 1 3 4 5 6 7 20 1 3 4 5 6 7 20 1 3 4 5 6 7 20 1 3 4 5 6 7Same TS in every frame

GP8.25

TB3

DATA57

S1

S1

TrainingSeq. (26)

Data57

TB3

148 bit burst156.25 bit (0.577 ms)

Theoretical rate: 1/8 channel rate: r=33.85 kbps

2 signalling frames: r 31.25 kbps

Burst overhead (114 bits over 156.25): r 22.8 kbps

Giuseppe Bianchi

Slow Associated Control Slow Associated Control ChannelChannel

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

TCH/F(0…7) TCH/F(0…7)

SACCH(0…7) IDLE frame

SACCH-0 SACCH-1 SACCH-2 SACCH-3 SACCH-4 SACCH-5 SACCH-6 SACCH-7

1 SACCH burst (per TCH) every 26 frames (120 ms)

Always associated to instaurated call on TCH (TCH + SACCH = TACH)On the same Time Slot

Periodic (order of ½ second) time-scale information for radio link control

Giuseppe Bianchi

SF BB BB PP PP SF PP PPPP PP SF PP PPPP PP SF PP PPPP PP SF PP PPPP PP

Broadcast Channel (usual) Broadcast Channel (usual) organizationorganization

51 frame structure vs 26235.38 ms period (vs 120 ms)

Sub-blocks with 10 framesStarting with Frequency Correction Channel (FCCH) Immediately followed by Synchronization Channel (SCH)

Other frames (numbered from #0 to #50):#50 idle#2,3,4,5 BCCHRemaining: Paging (PCH) / Access Grant (AGCH) [=PAGCH]

51 frame structure - downlink

10 frame sub-block

Giuseppe Bianchi

BCCH contentsBCCH contents 184 bits

Coded in 456 bits and interleafed in 4 bursts same coding and interleaving as SACCHBCCH capacity

184 bits / (51*8*15/26 ms) ~ 782 bps Information provided

Details of the control channel configurationParameters to be used in the cell

Random access backoff values Maximum power an MS may access (MS_TXPWR_MAX_CCCH)Minimum received power at MS (RXLEV_ACCESS_MIN)Is cell allowed? (CELL_BAR_ACCESS)Etc.

List of carriers used in the cellNeeded if frequency hopping is applied

List of BCCH carriers and BSIC of neighboring cells

Giuseppe Bianchi

BCCH carrier placementBCCH carrier placement

On DownlinkCorresponding uplink dedicated to Random Access Channel

RR RR RR RR RR R

51 frame structure - uplink

On one frequency per cell (beacon)MUST BE on Time Slot #0Other Time slots may be used by TCH

Provided that:• All empty slots are filled with DUMMY bursts

• Downlink power control must be disabled

RR RR RR RR RR RR RR RR RR RR RR RR RR RR RRRR RR RR RR RR

Giuseppe Bianchi

Fast Associated Control Fast Associated Control ChannelChannel

FACCH: urgent signallingUsed when several signalling information needs to be transmitted

Call setupHandover

FACCH block = 184456 after coding

Interleaved as voice blockSpreaded on 8 bursts

Replaces a voice block (20 ms) on the TCHVia stealing bitsVoice block(s) discarded

Maximum FACCH bit rate184*6/120 [bits/ms] = 9.2 kbps (vs 383 bps of SACCH!)

Giuseppe Bianchi

FACCH insertion in TCHFACCH insertion in TCH

Via Stealing bits- upper bit = odd bits stolen- lower bit = even bits stolen- both bits = all burst stolen

time

Figure: shows example of 2 FACCH blocks stealing a TCH (note begin and end behavior due to interleaving)

Giuseppe Bianchi

Why pagingWhy pagingChannel assignment:

only upon explicit request from MSPaging

needed to “wake-up” MS from IDLE state when incoming call arrives to MS

MS accesses on RACH to ask for a channel Generally SDCCH (but immediate TCH assignment is possible)

BSS/MSCMS

1) paging

3) Channel assignment

2) Random access

Paging channel: PCHAccess Grant Channel: AGCHRandom Access Channel: RACH

PAGCHCCCHCommon ControlCHannel

Giuseppe Bianchi

PagingPaging

Paging message generated by MSCWhich receives incoming call

Transferred to subset of BSCPaging limited to user’s location areaPaging message contains:

List of cells where paging should be performedIdentity of paged user (IMSI or TMSI)

Paging message coded in 4 consecutive bursts over the air interfaceSame coding/interleaving structure of SACCH (184456 bits)

Paging for more MSs may be joined in one unique paging message

Giuseppe Bianchi

An example procedure involving signallingAn example procedure involving signalling

Setup for an incoming call (call arriving from fixed network part - MS responds to a call)

Steps:- paging for MS- MS responds on RACH- MS granted an SDCCH- authentication & ciphering on SDCCH- MS granted a TS (TCH/FACCH)- connection completed on FACCH- Data transmitted on TCH

Giuseppe Bianchi

Radio Resource allocationRadio Resource allocationthree standardized solutionsthree standardized solutions

Non-Off Air Call Set-Up (Non-OACSU)Normally used (previous description)

Off Air Call Set-Up (OACSU)TCH assigned only when the called party actually responds!

Best utilization of radio resource (avoids allocation if callee not available)Call drop if no TCH is available at this point

Very Early Assignment (VEA) Immediate assignment of TCH

Fastest signalling processWaste of resources

RACH

RACH

RACH

VEA TCH (FACCH)

TCH (FACCH)Non-OACSU

OACSU

SDCCH

TCH (DATA)

SDCCH

TCH (DATA)

TCH (DATA)

Connection established Callee responds

Giuseppe Bianchi

handoverhandover Procedure in which an MS releases a connection with

a BTS, and establishes a connection with a new BTS, while ensuring that the ongoing call is maintainedThe MS remains in dedicated state (unlike cell reselection, where MS is in idle

state) Handoff: synonymous of handover Needs two mechanisms

Handover preparation: detection of cell-border crossingBased on radio link quality measurements

Handover execution: setup of a new channel in a cell, and tear-down of a previous channel

Improved handover mechanisms:Seamless handover: when active call performance is not impaired

Not possible in GSM: for about 100-200ms, communication is interruptedSoft Handover: when two channels are simultaneously set-up (old and new)

Not possible in GSM; possible in UMTS

Giuseppe Bianchi

Hard, Seamless, Soft Hard, Seamless, Soft handoverhandover

MSC

BSS 1 BSS 2

MSf1

MSC

BSS 1 BSS 2

MSf1

MSC

BSS 1 BSS 2

MSf1

MSC

BSS 1 BSS 2

MSf1

MSC

BSS 1 BSS 2

MSf1

MSC

BSS 1 BSS 2

MSf1

MSC

BSS 1 BSS 2

MSf2

MSC

BSS 1 BSS 2

MS

MSC

BSS 1 BSS 2

MS

f2

f1

f2

f1f1

before during after

Hardhandover

(GSM)

Seamless(DECT)

Softhandover(UMTS)

Giuseppe Bianchi

Handover classificationHandover classification

Rescue handover (mandatory handover)Driven by radio channel quality

degradation Confinement handover

(network-directed handover)Target: minimize radio interferenceAssign new channel when old

channel results critical for total interference

Traffic handover (network-directed handover)Driven by traffic congestion

conditionsAlso called load-balancing

Internal handover Intra-BTS

New radio channel in the same cell

Not termed as “handover” but as“subsequent assignment”

Inter-BTS (Intra-BSC)Under control of same BSC

External handover Inter-BSC (Intra-MSC)

Change reference BSC; may imply a location area update

Inter-MSCMost complex: need to change

MSC

Classification by motivation Classification by typology

Giuseppe Bianchi

Types of handoverTypes of handover

A-MSC

BSC

BTS BTS BTS BTS

BSC BSC

R-MSC

A

A-bis

radiointerface

Anchor MSC: the MSC that first

managed the current call

Relay MSC: the MSC that currently

manages the call

Switchingpoint forinternal

handover

Switchingpoint for

all inter-MSChandover

Switchingpoint for

inter-BSChandover

Giuseppe Bianchi

Handover taxonomyHandover taxonomy BCHO: Base station Controlled Handover

Handover detection: BSHandover Execution: BS

MCHO: Mobile Controlled HandoverHandover detection: MSHandover Execution: MS

MAHO: Mobile Assisted HandoverHandover detection: MSHandover Execution: BS

GSM: somehow a BCHO with a flavor of MAHOHandover decision always taken by BSCBased on measures taken at both BTS and MSNew channel selection decision taken at BSC or R-MSC or A-MSC

(depending on handover type) based on traffic consideration

Giuseppe Bianchi

Handover preparationHandover preparation Measurements performed at BTS

Up-link signal level received from MS lower than thresholdRXLEV_UL < L_RXLEV_UL_H

Up-link signal quality (BER) received from MSRXQUAL_UL < L_RXQUAL_UL_H

Distance between MS and BTS adaptive timing advance parameter > MAX_MS_RANGE

Interference level in unallocated time slots. Measurements performed at MS.

Down-link signal level received from serving cellRXLEV_DL < L_RXLEV_DL_H

Down-link signal quality (BER) received from serving cellRXQUAL_DL < L_RXQUAL_DL_H

Down-link signal level received from n-th neighbor cell RXLEV_NCELL(n) > RXLEV_MIN(n)

RX signal

level

From

(dBm)

To

(dBm)

RXLEV_0 - -110

RXLEV_1 -110 -109

RXLEV_2 -109 -108

RXLEV_3 -108 -107

… … …

… … …

RXLEV_62 -49 -48

RXLEV_63 -48 -

Bit error

Ratio

From

(%)

To

(%)

RXQUAL_0 - 0.2

RXQUAL_1 0.2 0.4

RXQUAL_2 0.4 0.8

RXQUAL_3 0.8 1.6

RXQUAL_4 1.6 3.2

RXQUAL_5 3.2 6.4

RXQUAL_6 6.4 12.8

RXQUAL_7 12.8 -

Giuseppe Bianchi

Handover preparation – Handover preparation – additional metricsadditional metrics

Transmission power Maximum MS transmission power Maximum serving BTS transmission power Maximum neighboring BTSs transmission

power congestion status

of serving BTS of neighboring BTSs

provided they can support the MS. Handover Margin

To avoid ping-pong handover effect 5-10 dB in normal operation; up to 30dB in

urban operation (to fight shadowing)

RXLEV(cell A)

RXLEV(cell B)Handover

RXLEV(cell A)

RXLEV(cell B)Handover

hysteresis

HANDOVER ALGORITHM: operator-dependent!GSM standard SUGGESTS a simple referencealgorithm, but implementation left to operator

Giuseppe Bianchi

handover procedure handover procedure skeletonskeleton

2) Switching point prepares new path on fixed net

2

1) Handover request goes up to switching point

1

MSC

BTSBTS

BSCBSC

3) Switching point sends HO command to MS

3

4) MS accesses new channel

4

5) Old channel/path torn down

5

Giuseppe Bianchi

Inter-MSC handoverInter-MSC handover More complex, as an ISDN circuit must be set

between MSCsWe’ll not enter into details (just the basic ideas)

Two cases

MSC-A MSC-R1

First MSC change (basic handover)

MSC-A MSC-R1

Second MSC change (subsequent handover)

MSC-R2

X

X

XNote the role of the

Anchor MSC!