01 - GSM Training Pack - Basics of the GSM[1]

8/3/2019 01 - GSM Training Pack - Basics of the GSM[1]

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Basics of the GSM technology

MRD/PSS/DSE


2/93

Basics of the GSM 2 All rights reserved 2003, Alcatel

Agenda

> Introduction

> Frequency Bands

> Network Architecture

> Radio Interface

> Logical Channels

> Radio Resource Management

> Erlang B Law

> Interference Reduction Techniques

> Densification Techniques


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IntroductionEarly Days

1978 Reservation of a 2 x 25 MHz spectrum block at 900 MHz.

1982 Foundation of Groupe Spcial Mobile within CEPT.

1985 The French and German PTTs become the major drivers fora new system due to spectrum shortage in their existing

analogue networks. They request proposals for a new mobilesystem based on an analogue FDMA system in the 900 MHzband.

All vendors proposed analogue systems except the Alcatelconsortium, which proposed a digital system called S900.

Following the Alcatel proposal, the first RfP was withdrawn, andre-issued explicitly asking for a digital system.


4/93


IntroductionGoing Digital

1987 After hot debates and a lot of political influence, theCEPT/GSM assembly finally decides on a digital narrow-bandsystem. The fundamental parameters of the projected system arefrozen.

1987 Foundation of Memorandum of Understanding Associationwith 13 member from 12 states.

1989 GSM becomes a Technical Committee within ETSI.

1990 GSM Phase 1 specification frozen.

1991 First networks in operation. DCS 1800 specification frozen.

1992 Commercial voice services in 13 networks in 7 countries.


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IntroductionConquering the World

1993 First roaming agreements are settled.

1994 First data services are offered.

1995 GSM Phase 2 including PCS 1900 are frozen.

2003 474 GSM networks on air in 172 countries. 70% of allmobile subscribers worldwide use GSM technology.

2006 2 billion GSM/W-CDMA subscribers. 690 networks on airin 213 countries (incl. 151 commercial EDGE networks). 81%

of all mobile subscribers worldwide use GSM family.

Standardization was a key driver for success


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IntroductionFrom ETSI to 3GPP

> GSM standardization was initiated in Europe by CEPT andthen by the European Telecommunications StandardsInstitute (ETSI) transferred to 3GPP in 2000 (latter createdin 1998 for UMTS standardization)

> GSM standardization in phases/releases:

Phase 1:Speech, SMS,

CSD

Phase 2

Phase 2+

Release 97:GPRS

Release 98:AMR, LCS

Release 99:EDGE,

Interworking withUMTS, QoS

Release 4:NACC, Extended

UL TBF mode

Release 5:O-TCH, WB-AMR,GERAN Iu mode

Release 6:MBMS, PShandovers

Release 7:GERAN

Evolutions


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Introduction

> GSM = Global System for Mobile Communication

>Digital technology, system architecture, common feature, openinterfaces

> Not only a radio technology but a complete system withstandard functional blocks and interfaces


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Frequency Bands

> Four Bands: 850, 900, 1800 and 1900 MHz (other bandsstandardized (450, 700, etc) but no handsets

880 890 915

P-GSMG1

925 935 960

P-GSMG1

1710 1785

DCS 1800

1805 1880

DCS 1800

E-GSM

45M

Bandwidth

Duplex95M

25M10M 75M

# Carriers 17450 374

> Americas: 850 MHz (824-849 and 869-894) and 1900 MHz(1850-1910 and 1930-1990)

> Guard band of 200kHz between bands

Downlink DownlinkUplink Uplink


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Network Architecture

> Overview

> Mobile Station (MS)

> Base Station Sub-System (BSS)

> Network Sub-System (NSS)

> Operation Sub-System (OSS)

> Interfaces

> Protocol Stack


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Network ArchitectureOverview

BSC

MSC/VLR

HLR

MSC/VLR

PSTN

ISDN

PDN

BTS

BTS

BTS

OMC

MS

BSS

NSS

OSS

Abis

Abis

A

C/D

E/G


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Network ArchitectureMobile Station

> MS = Terminal Equipment + SIM Card

> MS identity:

International Mobile Equipment Identity(IMEI)

> Terminal Classes:

Class Output Power (GSM900)

Output Power (GSM1800)

1 - 1W

2 8W 0,25W

3 5W 4W4 2W -

5 0,8W -

Majorityof terminals

> Min required sensitivity: -102dBm (-104dBm: aggressivefigure in 900MHz band)


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Network ArchitectureBase Station Sub-System

> Base Transceiver Station (BTS) Set of Transmitter/Receivers (TRX)

Radio transmission/reception:

Modulation/de-modulation

Equalization

Channel coding and decoding incl. error correction

PHY layer functions:

TDMA multiplexing

Frequency synthesizer incl. hopping

Ciphering

Radio measurements sent to the BSC

Link layer management btw. MS and BSS (LAPDm)

Link layer management w/ BSC (LAPD)


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> BTS-BSC Configurations Star Configuration:

BTS

BSC

AbisBTS

BTS

Multi-drop Configuration:

Loop configuration:

BSC

AbisBTSBTSBTS

BSCAbis

BTS


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> Base Station Controller (BSC)

> Main Function: Radio Resource Management

Channel allocation

Power Control (using measurements retrieved through BTS)

Hand-over

> The BSC has PCM interfaces w/ the BTS and the MSC

BTS-BSC: LAPD

BSC-MSC: CCITT n7 signaling layers


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Network ArchitectureNetwork Sub-System

> Home Location Register (HLR)

> Data base managing the subscribers of a PLMN

> Subscriber identity

International Mobile Subscriber Identity(IMSI) (also in SIM card)

Mobile Station ISDN Number(MSISDN) (phone number)

Subscriber profile (e.g. authorized supplementary services)

> Localization Information

VLR number where each subscriber is registered even abroad

> Implementation of the HLR can be centralized or distributed

Each subscriber is associated to a single HLR

The network identifies the HLR w/ MSISDN or IMSI


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> Mobile-services Switching Center (MSC)

> Switching functions

Call establishment bw MS and other MSC

SMS transmission

Hand-over when required

> Mobility management

VLR interrogations

Localization information transfer

Check of subscriber profiles> Gateway MSC (GMSC)

For the communication w/ a PSTN subscriber

PSTN

MSC

GMSC


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> Visitor Location Register (VLR)

> Similar to HLR but for subscribers located in a givengeographical area

> VLR includes also

More precise localization information Temporary Mobile Subscriber Identity(TMSI)

> VLR and MSC are often co-locatedBSC

MSC

MSCA

E

VLR

VLR

HLR

B

B

D

D

C

G

AbisBTS


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Network ArchitectureOperation Sub-System

> Network Management: Commercial administration (subscriber, terminals declarations, billing,

statistics)

Security management

Performance management (traffic, quality,)

System configuration (SW upgrades, new HW, new features) Maintenance (fault detection, tests)

> Operations and Maintenance Center (OMC)

Local equipment supervision

> Network Management Center (NMC)

Global administration

> Equipment Identity Register (EIR)

> Authentication Center (AUC)


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Network ArchitectureInterfaces

Name Interface Use

Um MS-BTS Air interface

Abis BTS-BSC -

A BSC-MSC -

C GMSC-HLR HLR access for terminating callsD VLR-HLR Subscriber information mgt and

localization

E MSC-MSC Hand-over

F MSC-EIR Terminal identity check

G VLR-VLR Subscriber information management

B MSC-VLR -

H HLR-AUC Authentication


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Network ArchitectureProtocol Stack

> User Plane

relayPhysical

Link Layer

Um Abis / AterMS TCBTS A

A-law(or Mu-law)

G.711

TRAUframing

Physical

RF Layer

PhysicalLink Layer

Physical

RF Layer

TRAUframing

FR/EFR/HR/AMR

Speech codingGSM 06.xx

FR/EFR/HR/AMR

Speech codingGSM 06.xx

E1

G.703 / G.704E1

G.703

E1G.703

OR

T1T1.403

relay

Speech coding Trans-coding

PCM 64K Time Slots


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> Control Plane

network layer split in 3 sub-layers:

RR: for radio resource management and mobility management during a call

MM: for mobility management outside a call

CC: for call control (very similar to ISUP) + SMS + SS (supplementary serv.)

PhysicalLink Layer

Um AbisMS BTSA

PhysicalRF Layer

PhysicalLink Layer

PhysicalRF Layer

LAPDmGSM 04.06

E1G.703

LAPDmGSM 04.06

LAPD

RSL/

OML

relay

RRGSM 04.08 RR

GSM 04.08

E1G.703

LAPD

RSL/

OML

BSSAPGSM 08.08

E1G.703

MTP

SCCP

BSSAPGSM 08.08

E1G.703

MTP

SCCP

relay

MMGSM 04.08

CC/SMS/SSGSM 04.08

CC/SMS/SS

GSM 04.08

MMGSM 04.08

BSC MSC

signalling over Ainterface uses SS7protocol stack

signalling over Abisinterface usesproprietary protocolover LAPD


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> Radio Resource (RR)

Mainly in MS and BSC

Establishment/maintenance/release of logical channels

In MS: cell selection, BCCH supervision

In BTS: some RR messages bw MS and BTS: RR layer

In BTS: commands from BSC handled by BTS Management (BTSM)> Mobility Management (MM)

Localization

Authentication

TMSI allocation

> Call Management (CM)

Call Control (CC): circuit connection management

Short Message Service (SMS)

Supplementary Services (SS)


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> In NSS, signaling is using SS7> Message Transfer Part (MTP)

Three layers dedicated to signaling

Datagram transfers

Implemented in MSC, VLR an HLR

> Mobile Application Part (MAP)

Specific signaling protocol above MTP for mobility

> Signaling Connection Control Part (SCCP)

Worldwide interconnection protocol for signaling

> ISDN User Part (ISUP)

Call management


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Radio Interface

> TDMA Frame Structure> Burst Format

> Duplexing

> Transmission Chain

> Performance


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Radio InterfaceTDMA Frame Structure

DL

UL 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 time

TDMA frame (4.615 ms)

Time Slot (577 s)

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

3 User active on TS#3 for each TDMA frame

3 TS time shift between UL/DLno duplexer in MS !


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Radio InterfaceBurst Format

Data (57) Training (26)1 13 3

Burst: 148 bits

Data (57) 8.25

156.25 bits: 577 s

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

Coded information For burst synchroand channel estimation

Guard time to avoidoverlapping of burst

due to mobility

For power rampingFor stealing frame e.g.

for HO command

> Data rate:

2x57 bits every 4.615 ms: 24.7 kbit/s


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Radio InterfaceBurst Format

> Training Sequence midamble with dirac auto-correlation function

Fine synchronization at burst level

Channel estimation

8 possible training sequences

> Stealing Flag

Speech vs. signaling

Radio resource can be used for signaling (e.g. FACCH)

> Dummy Burst BTS has no information to send but has to emit a signal


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Radio InterfaceDuplexing

> Duplexing = Frequency Division Duplex (FDD)

> Duplex interval is 45MHz in GSM 900, 95MHz in DCS 1800

> 3 slots shift bw Downlink and Uplink

> Number of duplex 200KHz channels

124 in GSM 900 174 in E-GSM

374 in DCS 1800

> Timing Advance

MS experience different propagation delays A guard interval of 30 ms

MS compensates for the timing advance (TA)

Max cell range in the standard is 35 Km

0 1 2 3 4 5 6 7

-

-Propagation

delay


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Radio InterfaceTransmission Chain

> Speech Coding:

Analogue to Digital

> Channel Coding:

error detection and correction

> Interleaving:

adjacent bits over several datablocks

to decorrelate error

> Ciphering

secret code

> Burst Formating

synchronisation and equalisation

> Modulation:

Binary signal to Analogue


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> Speech Coding Voice band = 300 3400 Hz

Voice packet of 20ms

5.6 Kbps HR

13 Kbps FR, 12.2 Kbps EFR

Speech codec

260 bits 13Kbps

Channel Coding

456 bits 22Kbps

Interleaving

0 1 2 3 4 5 6 7

0 1 2 3 4 5 6 7

0 1 2 3 4 5 6 7

8 TDMA Frames8 half bursts

Packet iinterleavedw/ packets i-1

and i+1


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> Error Control Automatic Repeat Request (ARQ): LAPDm

Forward Error Correction (FEC)

> Error Correction

Cyclic Redundant Check (CRC) Error detection only

Block code with polynomial of length 3 for TCH

Convolutional Code

Rate = (TCH)

Viterbi decoding (maximum likelihood)

> Ciphering

A5 algorithm based on time, frame number and session key Kc


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> Modulation Constant envelop

Trade-off between spectrumand co-channel resistance

Sinusoidal signal for all 1 or

all 0 sequences Used for frequency

synchronisation


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Radio InterfacePerformance

> Performance mainly depends on C and C/(I+N)> A GSM receiver measures the following parameters

RXLEV: signal level (64 levels from110 to48dBm)

RXQUAL: signal quality (BER coded on 8 levels)

> RXLEV and RXQUAL are reported on the SACCH

> Two main parameters to assess the performance

Frame Erasure Rate (FER)

Bit Error Rate (BER)

> Receivers have to check that FER/BER are above thresholddefined by the standard


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Radio InterfacePerformance

> Voice Quality Mean Opinion Score (MOS): from 1(bad) to 5 (excellent),

subjective

Frame Erasure Rate (FER): highly correlated with MOS

Dropped Call Rate (DCR): percentage of connections lost

Call Success Rate (CSR) and Handover Success Rate


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Logical Channels

> Overview> Multi-frame Structure

> Dedicated Channels

> Beacon Channel

> Common Control Channels


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Radio InterfaceOverview

> Traffic Channels (TCH) Exchange of information between end-users after call

establishment

On dedicated channels

Voice or data

> Signalling

Exchange of information between the MS and the GSM networkequipment

In idle mode: authentication, location update During communication: handover, link control


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> One slot is a PHY channel

PHY-ChTCH

SACCH

> Control functions

Broadcast system information (broadcast channels)

Inform MS of incoming calls and allow access (common controlchannels)

Physical parameters control (FACCH, SCH and SACCH)

Transmission of telephone signaling (SDCCH)


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> Dedicated Channels Resource is reserved for a MS

A slot is allocated to the MS

In the cell, a single MS can transmit or receive in the slot

Dedicated channels are duplex

> Non Dedicated Channels

Shared among all MS in the cell

In downlink: information broadcast

In uplink: random access


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FCH* DL Frequencysynchronisation

Commonsignallingchannels

SCH* DL Time synchronisation

BCCH* DL System Information

PCH* DL Paging channel

RACH* UL Random access

AGCH* DL Access grant

SDCCH UL/DL Call establishmentDedicated

signallingchannelSACCH UL/DL In call signallingFACCH UL/DL Fast in call signalling

TCH UL/DL Traffic channel Traffic

* On TS0 of beacon frequency


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Radio InterfaceMulti-Frame Structure

> For more flexibility and to allocate less than one slot per frame

0 321 4 765

TDMA Frame

0 1 2 23 24 25 0 1 2 48 49 50

120 ms Multi-frame 235,8 ms Multi-frame

1 2 23 24 250

0 1 2 48 49 50

Super-frame

0 1 2 2045 2046 2047

Hyper-frame


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Radio InterfaceDedicated Channels

> Traffic Channel (TCH) TCH/FS 13kbps

TCH/HS 5,6kbps

Data 12kbps (9.6kbps services) or 14.4kbps

> Stand-alone Dedicated Control Channel (SDCCH) Dedicated signaling

800 bits/s

Information blocks of 184 useful bits/456 coded bits (8 half-bursts)

A PHY-Ch can transport

Either TCH and associated SACCH

Or 8 SDCCH and their associated SACCH


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> Slow Associated Control Channel (SACCH) Continuously controls the radio link

Timing advance information

Power control

Quality control

Measurements reports

380 bits/s

> Fast Associated Control Channel (FACCH)

Fast signaling in case of hand-over

TCH transmission is interrupted and resource is allocated tosignaling


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> TCH/SACCH Multiplexing

T T i

120 ms Multi-frame

TT T T A

12 25

> TCH/FACCH Multiplexing

Data (57) Training (26)1 13 3Data (57) 8.25

Bit=0 > even bits are TCHBit=1 > even bits are FACCH

Bit=0 > odd bits are TCHBit=1 > odd bits are FACCH


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> SDCCH/SACCH Multiplexing

D0 D1 D2 D3 D4 D5 D6 D7 A0/4 A1/5 A2/6 A3/7

SDCCH SACCH

0 4 32 50 DL

A1/5 A2/6 A3/7 D1 D2 D3 D4 D5 D6 D7D0 A0/4 UL

51 TDMA frames = 235,38 ms


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Radio InterfaceBeacon Channel

> In each cell, a frequency is dedicated to the beacon channel> Beacon channel is essential for mobility and hand-over

> Constant output power is ensured on the beacon frequency

> At power on, MS chooses the cell with the best beacon

received power> In idle mode and transfer modes, MS does signal

measurements on the beacon channel of its cell and ofneighboring cells

> Broadcast channels: Frequency Correction Channel (FCCH)

Synchronization Channel (SCH)

Broadcast Control Channel (BCCH)


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> Frequency Correction Channel (FCCH) One burst every 50 ms

Burst made of 148 bits at zero > pure sinusoidal signal

Fine tuning of the MS oscillator

FCCH is on slot 0 of the beacon channel

Frames 0, 10, 20, 30, and 40 of a 51 frames multi-frame

> Synchronization Channel (SCH)

Allows fine synchronization of MS and logical synchronization

Training sequence of the burst is 64 bits long iso 26 bits

SCH one slot 0 of the beacon channel

Always one frame after the FCCH burst

Transports RFN (frame number) and BSIC (color code)


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> Broadcast Control Channel (BCCH) Broadcast of system information

Cell selection parameters

Location area

RACH parameters

Organization of Common Control Channels

Description of neighbor cells

Cell identity

BCCH always on slot 0 of the beacon channel


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Radio InterfaceCommon Control Channels

> CCCH are RACH, AGCH, PCH, and CBCH> Random Access Channel (RACH)

Random access

Short request on a single burst w/ slotted ALOHA

Training sequence of 41 bits 8 useful bits+6bits CRC+6bits BSIC+4bits tail (code rate )

Information: requested service and random number

Training (41)8 3Data (36) 68.25


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Radio InterfaceCommon Control Channels

> Access Grant Channel (AGCH) After reception of a request, the network allocates a dedicated

signaling channel to the MS thanks to AGCH

Complete description of the signaling channel

Timing advance

Messages of 23 bytes coded in 8 half-bursts

> Paging Channel (PCH)

Broadcast of the identity of a MS on several cells

MS answers w/ an access on the RACH

Messages of 23 bytes including up to 4 paging messages

> PCH and AGCH are multiplexed on a 51 frame structure withBCCH


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Radio Resource Management

> Idle Mode> Management of Dedicated Channels

> Hand-over


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Radio Resource ManagementIdle Mode

> Cell Selection1. Scanning of beacon channels

MS does a list of beacon channels

Either scan of all GSM frequencies (124 in GSM900, 374 in1800)

Or scan of predefined beacon channels for the PLMN

2. Look for a suitable cell

Cell is part of the selected PLMN

Cell is not barred for access

Radio path loss MS-BTS is greater than a given threshold (C1)

3. PLMN selection

Automatic or manual mode


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> Camping> MS camps on the beacon channel of the selected cell

MS reads system information broadcasted on the BCCH

MS can establish a call on the RACH

MS monitors PCH to receive eventual paging messages> Measurements

MS receives the list of BCCH channels to measure

MS periodically measures signal strength on neighbor BCCH

MS establishes a list of 6 best cells

MS monitors path loss criteria C1 for the current cell

MS compares cells with criterion C2


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> Cell Re-selection> First cell selection is based on C1

> Then, MS computes C1 and C2 every 5s

> MS can re-select a cell if

C1, i.e., path loss is too high OR MS doesnt receive downlink signaling OR

Selected cell is barred OR

There is a better cell according to criterion C2 OR

Several RACH access have been unsuccessful

> C1 takes into account RXLEV_ACCESS_MIN andMS_TWPWR_MAX_CCH broadcasted on the BCCH

> C2 includes offsets to avoid ping-pong effect and to favor some cells


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Radio Resource ManagementManagement of Dedicated Channels

> Mobile Originating Call

MS BTS BSC MSC

RR Channel Request

Channel Required

Channel Activation

Channel Act. ack

Immediate Assignment Command

RR Immediate Assignment

SABM[Service Request]Establish Ind[Service Request]

SCCP Connection Req

SCCP Connection ConfUA[Service Request]

RACH

AGCH

SDCCH

SDCCH

Switch toSDCCH

Service required andPropagation delay

Choose aSDCCH or TCH

Channel description

(frequency, slot,)

ChannelReservation Channel description

Ref. byte used for accessFN of the requestTA

List of frequencies for SFHMS IDTerminal ClassService requested

Followed by Authentication, Ciphering, Call

Initiation, switching to TCH and Call Connection


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Radio Resource ManagementManagement of Dedicated Channels> Mobile Terminating Call

MS BTS BSC

Channel Required

Channel Activation

Channel Act. ack

Immediate Assignment Command

RR Immediate Assignment

SABM[paging response] Establish Ind[paging response]

UA[paging response]

AGCH

SDCCH

Paging CommandRR Paging Request

RR Channel RequestPCH

RACH

Switch toSDCCH

Sent to all

BTS of the LAMay include severalpaging messages

Followed by Authentication, Ciphering, CallInitiation, switching to TCH and Call Connection


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Radio Resource ManagementManagement of Dedicated Channels

> Channel ReleaseMS BTS BSC MSC

BSSMAP Clear CommandRR Channel Release

Deactivate SACCH

DISC Release Indication

UA

RF Channel Release

BSSMAP Clear CompleteRF Channel Release ack

SCCP Released

SCCP Released Complete

SDCCH

SDCCH

Come-back

on beacon channel

RR connection released

BTS doesnt use SACCHany more

Radio resourcecompletely freed


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Radio Resource ManagementHand-over

> Intra-BSC Hand-overMS BTS BSC BTS MS

RR Measurements ReportMeasurements Result

Channel Activation

Channel Activation ack

RR Handover CommandRR Handover Access

HO Detection

RR PHY info

SABM

UA

RR Handover CompleteRF Channel Release

RF Channel Release ack

SACCH

FACCHTCH

FACCH

FACCH

FACCH

HO Decision

Switch to new cell

Access burstOn TCH

Channel reservation

New SDCCH/TCH channels

New cell characteristicsInitial powerIf possible: TACiphering mode

TA


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Erlang B Law

> Traffic Description> Queuing Systems

> Poisson Arrival Process

> Erlang B Formula


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Erlang B LawTraffic Description

> The unit that defines the traffic is the Erlang.

> The Erlang:

1 Erlang is one resource (e.g. one voice channel) which is usedpermanently.

> Traffic of one resource:

> Example: a subscriber who makes 2 phone calls of 90s per hour:

Traffic = (2 x 90) / 3600 = 0.05 Erlang

> Exercise: Compute the Traffic of one user with

BHCA= 3

Average Call Duration= 45s

Resource usage duration

Total durationT =


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Erlang B LawQueuing Systems

> A queuing system may be with or without loss.

> Example of queuing system with one server:

A one server queuing system without any loss is a serverwith an infinite queue size (theoretical only).

ErlangC

> We call loss systems systems that have the samenumber of servers as the queue length (no waiting time):

If all servers are used, process is rejected ErlangB

Arrival process Departure processService time

Queue


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Service time distribution : B

Arrival Process : A

System Capacity : C

Service Discipline : D

Queuing system

> Main Performance measure

Distribution of the Waiting Time probability Required for Erlang C

Distribution of the the Blocking probability

Required for Erlang B

Erlang B LawQueuing Systems


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Erlang B LawPoisson Arrival Process

> The arrival and departure process must be modeled, the mostcommon for real telecommunication systems is the Poisson process:

> Definition of Poisson process:

the arrivals between time t and t+are independent of the history of theprocess (memoryless process)

the arrivals between time t and t+are independent of the time t(stationary process)

> Probability of having i arrivals in T seconds:

> Mean interarrival time =1/, is the mean arrival rate

P TT e

i

i

i T

( )( )

!


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Erlang B LawFormula

> Erlang B:

n server loss system: when n servers are occupied, arriving customer isthrown (no call reattempt)

Arrival process is Poisson with rate

Service time is exponential, ie departure process is Poisson with rate m

Calls blocked

If overflow ( capacity isexceeded ) happens

then the calls aresimply blocked

k0 1 n-1 n

m (k+1)m

km

nm


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Erlang B LawFormula

k

km

k-1

> Steady state: number of departure=number of arrival

nmP(n) = P(n-1) ...

kmP(k) = P(k-1)

... mP(1) = P(0)

(/m)kP(0)k!

P(k) =

P(0) + P(1) + ... + P(n) = 1 P

i

i

i

i n( )

( / )

!

0

0

S

m1


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Erlang B LawFormula

> Notation:

m = 1/T, T is the mean inter-departure time, ie the mean holdingtime

/m= T is the offered traffic to the system

> Probability of arriving customer being blocked = probability of

n customers in the system, ie P(n):

Pblock T n

T

n

T

i

n

i

i

i n

( , , )

( )

!

( )!

0


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Erlang B LawFormula

> 3 parameters are used in the Erlang formulas:

Offered Traffic (T) Number of circuits (n)

Blocking probability (Pblock)

> With 2 of these parameters, one can calculate the third:

On the air interface: (n,Pblock)->(T) On the A interface: (T,Pblock)->n

Erlang law: Offered Traffic=f(n) with 2% blocking rate

0

10

20

30

40

50

60

0 10 20 30 40 50 60

Number of channels

OfferedT

raffic

Channel Efficiency=Offered Traffic / n

0

20

40

60

80

100

0 10 20 30 40 50 60

Number of channels

Efficienc

y(%)

The Erlang law is

not linear !!!4TRX (21.9Erl) >2x2TRX (16.4Erl)


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Interference Reduction Techniques

> Slow Frequency Hopping> VAD/DTX

> Power Control

I t f R d ti T h i


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Interference Reduction TechniquesSlow Frequency Hopping

> Definition: change randomly and regularly (each TDMA frame)the frequency used by a channel

> Consequences:

Frequency diversity: spreads in time lost bursts due toselective frequency signal fading

Interference diversity: changes in the interference positionfrom TDMA frame to TDMA frame

reduction in the standard deviation of the co-channelinterference level

increasing the number of receivers having a SINR above acertain threshold

I t f R d ti T h i


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> BBH: BasebandHopping

Number of hoppingfrequency = nb ofTRX

TS0 of beaconfrequency does nothop

0 1 2 3 4 5 6 7

TRX1

TRX2

TRX3

TRX4

BCCH

> SFH: Synthesized Frequency Hopping

Number of hopping frequency is higher than the number of TRX Beacon frequency does not hop

I t f R d ti T h i


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TSi TSi+1

Availablefrequencies:

Interfering cells position change from a TS to TS

F0:

F1:F2:

F3:

I t f R d ti T h i


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Without SFH With SFHC/I

Receiver

C/I

ThresholdThreshold

SFH reduces the distribution of average C/I across the cell howeverit increases the variations for a given user (interference less stable)

overall the average C/I over the cell is increased

C/I meanC/I mean

Receiver

I t f R d ti T h i


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> Each channel has a different SFH sequence defined by: N: Number of hopping frequencies

HSN: Number of a hopping sequence (0 to 63)

MAIO: Initial offset in the sequence (0 to N-1)

Time-slot number

> 64xN hopping sequences are then available with a radiospectrum of N frequencies

> MSs belonging to different cells use different HSNs and

statistically interfere 1/N of the time (pseudo-orthogonality)

> MSs belonging to the same cell use the same HSN but adifferent MAIO and never interfere (SFH laws are orthogonal)



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> More benefit for slow moving MS> Highest improvements when hopping is on more than 4

carriers

TU3

TU50

6

7

8

9

10

11

12

1314

15

1 2 3 4 5 6 7 8 9 10 11 12

number of frequencies in hopping sequence

re

quiredC/I(dB)

TU3

TU50



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Interference Reduction TechniquesVAD/DTX

Inhibiting transmission on air interface when no relevantinformation has to be transmitted

> DTX: Discontinuous Transmission

user is speaking: speech coded at 13 kbit/s (FR)

silence: transmission of SID (Silence Descriptor for comfortnoise) frames every 480ms (500bits/s)

> VAD: Voice Activity Detection

Distinguish a speech signal from background noise.

Algorithm based on a comparison between the filtered signaland a threshold (both are continuously adjusted)



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Interference Reduction TechniquesVAD/DTX

> Typically 40% to 50% of silence in a speech communication

> Reduction of average interference level:

possible reduction of reuse cluster size

increase in network capacity

> Increase of MS battery life time



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Interference Reduction TechniquesPower Control

Modification of Tx power according to signal quality andsignal level

> Independently applied for uplink and downlink

> Managed by the BSC

> Optional (operator choice)

> Proprietary algorithm

> Beacon frequency is not subject to PC



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> At initial access, MS transmits at MS_TXPWR_MAX_CCHbroadcasted on the BCCH

> PC is managed in both directions by the BSC:

Uplink: required MS transmission level is computedthrough reception level and quality measurementsperformed by the BTS

Downlink: for each connection, BTS transmissionpower is based on measurements performed by theMS and reported to the BTS every 480ms (5 bits onthe SACCH)



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> PC command is received by MS every 480 ms on SACCH.

Requested value is reached by step of 2 dB every 60 ms

> An immediate power transition is performed in case of channelconnection and Hand-Over procedure (new serving cellcommand)

Transmission level (dBm)

Commands: 5 dBm 25 dBm29 dBm1

33

Time(60ms intervals)

23 dBm

5

2925



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> Improves the spectral efficiency by reducing the interference

caused on other calls

> Decreases energy required for transmission

extends the battery life for the mobile station


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Densification Techniques

>Hierarchical Networks

> Concentric Cells

> Multi-band Cells



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Densification TechniquesHierarchical Networks

>3 advantages using micro cells:

Traffic increase

Provision of localized coverage

complete overlap with existing coverage

partial overlap

Quality of service

> Different implementations

Hot spot

Continuous layer

> Different Coverage objectives: Outdoor

Indoor



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>Micro-cellular hand-over

> IDLE MODE

Cell selection (C1) at switch on

and reselection (C2) after switch on.

Direct a MS to the micro layer

> CALL ATTEMPT

Forced directed retry

if during queuing, the serving cell is congested and a neighboring cell

is reported with a sufficient level and has a sufficient number of freeTCH



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>DURING CALL

Better cell condition handovers

Speed discrimination handover

Emergency causes:

consecutively missing SACCH frames

too low quality (based on Rx_Qual level) UL & DL too low received signal UL & DL

too high interference level (high level & low quality): intra-cell HO toanother TRX



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>Directed Retry

> Assign at call establishment a TCH in a neighbor cell in caseof lack of traffic resource in the serving cell

> Internal directed retry: cells are managed by the same BSC

> External directed retry: cells are managed by different BSC> Fast Traffic Hand-Over

> Push out of a cell a MS in dedicated mode to allow a incomingcall to be served in the serving cell



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>Alcatel Integrated Multi-layer Solution (AIMS)

> 3 frequency groups are used in macrocell layer with fractionalre-use 1/3

> Two of the 3 frequency groups are reused in the microcellswith fractional re-use 1/1.



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Densification TechniquesConcentric Cells

> Realize two concentric zones within one cell

> MS1 can use F1 or F2 and MS2 can use F2.

> BCCH on outer TRX

F1

F2

MS1

BS1

MS2



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> Two ways of using concentric cells :

capacity oriented : by using it on an interfered cell andguaranteeing a high received level in the inner zone. This allowsan additional TRX in the inner zone with a reduced reuse clustersize.

BS1

F1

F1F2

MS1MS2

C1

I1

C2

F2

I2



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QoS oriented :

by using it on an interfering cell to bring down the level ofinterference by powering down the inner zone carriers.

if a frequency is interfered, it is possible to convert it in an inner zonefrequency.

F1

MS1

BS1 BS2MS2

F1

Reduced power less interferenceF1 less interfered

F1



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> Use of Concentric Cells

> Idle Mode: MS camps on the outer zone

> Call Establishment

An SDCCH connection is always allocated in the outer zone

TCH is allocated in a zone according to the signal level on theSDCCH

High level on UL and DL > inner zone

> Outgoing Hand-over

Same HO criteria and strategies as for non-concentric cells

> Incoming Hand-over

MS is handed over in the zone corresponding to its location



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> Intra-cell Hand-over

Two types: inter or intra zone hand-over

Two triggers: emergency or better zone hand-over

> Emergency intra-cell HO due to interference

Too high interference either on DL or on UL

Assign another less interfered channel in the same cell

The may or may not change of zone (inter vs. intra zone)

> Emergency intra-cell HO due to level

Too low level on DL or UL in the inner zone

Hand-over towards outer zone is triggered

> Better zone intra-cell hand-over

The other zone is more suitable to handle the MS



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Densification TechniquesMulti-band Cells

> Mixture of GSM 900 and 1800 channels in a single band cellwith all CCCH in a single band (single BCCH concept)

900 MHz BCCH and TCH

BS

Single bandDual band

Dual band

1800 MHz TCH



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Densification TechniquesMulti-band Cells

> Advantages of Multi-band cells:

less cells to operate : the operator only adds TRX working in thesecond frequency band where the traffic demand is high,

dual band mobiles have less cells to monitor leading to morereliable measurements reports (each BCCH is measured moreoften),

reduced number of inter-cell handovers,

only one BCCH frequency plan,

higher traffic efficiency of the second frequency band not limitedby a BCCH frequency plan which requires a lot of frequencies foronly one TRX per cell,

Optimum TCH allocation at call set-up:

SDCCH phase in 900 MHz band

Then, possible direct TCH allocation in the 1800 MHz band


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01 - GSM Training Pack - Basics of the GSM[1]

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