01 - GSM Training Pack - Basics of The GSM

Basics of the GSM technology
MRD/PSS/DSE
Agenda
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Introduction Frequency Bands Network Architecture
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> > > > >
Radio Interface
Logical Channels Radio Resource Management
Erlang B Law
Interference Reduction Techniques Densification Techniques
Basics of the GSM 2
All rights reserved 2003, Alcatel
Introduction
Early 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 for a new system due to spectrum shortage in their existing analogue networks. They request proposals for a new mobile system based on an analogue FDMA system in the 900 MHz band. All vendors proposed analogue systems except the Alcatel consortium, which proposed a digital system called S900. Following the Alcatel proposal, the first RfP was withdrawn, and re-issued explicitly asking for a digital system.
Basics of the GSM 3
Introduction
Going Digital
1987 After hot debates and a lot of political influence, the CEPT/GSM assembly finally decides on a digital narrow-band system. The fundamental parameters of the projected system are frozen.
1987 Foundation of Memorandum of Understanding Association with 13 member from 12 states.

1989 1990 1991 1992 GSM becomes a Technical Committee within ETSI. GSM Phase 1 specification frozen. First networks in operation. DCS 1800 specification frozen. Commercial voice services in 13 networks in 7 countries.
Basics of the GSM 4
Introduction
Conquering the World
1993 1994 1995 First roaming agreements are settled. First data services are offered. GSM Phase 2 including PCS 1900 are frozen.
2003 474 GSM networks on air in 172 countries. 70% of all mobile subscribers worldwide use GSM technology.
2006 2 billion GSM/W-CDMA subscribers. 690 networks on air in 213 countries (incl. 151 commercial EDGE networks). 81% of all mobile subscribers worldwide use GSM family.
Standardization was a key driver for success
Basics of the GSM 5
Introduction
From ETSI to 3GPP
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GSM standardization was initiated in Europe by CEPT and then by the European Telecommunications Standards Institute (ETSI) transferred to 3GPP in 2000 (latter created in 1998 for UMTS standardization)
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GSM standardization in phases/releases:

Phase 2+ Release 98: AMR, LCS Release 4: NACC, Extended UL TBF mode
Phase 1: Speech, SMS, CSD
Release 6: MBMS, PS handovers
Phase 2
Release 97: GPRS
Release 99: EDGE, Interworking with UMTS, QoS

Release 5: O-TCH, WB-AMR, GERAN Iu mode
Release 7: GERAN Evolutions
Basics of the GSM 6
Introduction
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GSM = Global System for Mobile Communication
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Digital technology, system architecture, common feature, open interfaces Not only a radio technology but a complete system with standard functional blocks and interfaces
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Basics of the GSM 7
Frequency Bands
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Four Bands: 850, 900, 1800 and 1900 MHz (other bands standardized (450, 700, etc) but no handsets Downlink
880 G1 890 P-GSM 915 E-GSM
Uplink
935 960 P-GSM
Downlink
1710 1785 1805
Uplink
1880
925
G1
DCS 1800
DCS 1800
Bandwidth # Carriers Duplex
10M 50
25M 174 45M
75M 374 95M
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Americas: 850 MHz (824-849 and 869-894) and 1900 MHz (1850-1910 and 1930-1990) Guard band of 200kHz between bands
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Basics of the GSM 8
Network Architecture
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Overview Mobile Station (MS) Base Station Sub-System (BSS)
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Network Sub-System (NSS)

Operation Sub-System (OSS) Interfaces
Protocol Stack
Basics of the GSM 9
Overview
OMC
OSS
HLR
B T S
C/D Abis A
PSTN ISDN
MS
B T S BSC
NSS
MSC/VLR
BSS
E/G
PDN
Abis
B T S MSC/VLR
Basics of the GSM 10
Mobile Station
> >
MS = Terminal Equipment + SIM Card MS identity:
International Mobile Equipment Identity (IMEI)
Majority of terminals
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Terminal Classes:
Class 1 2 3 4 5 Output Power (GSM 900) 8W 5W 2W 0,8W Output Power (GSM 1800) 1W 0,25W 4W -
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Min required sensitivity: -102dBm (-104dBm: aggressive figure in 900MHz band)

Base Station Sub-System
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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
BTS
BSC
Star Configuration:
BTS
Abis
BTS
Multi-drop Configuration:
BTS
BTS
BTS
BSC
Abis
Loop configuration:
BTS
Abis
BSC
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Base Station Controller (BSC) Main Function: Radio Resource Management

Channel allocation Power Control (using measurements retrieved through BTS) Hand-over BTS-BSC: LAPD BSC-MSC: CCITT n7 signaling layers
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The BSC has PCM interfaces w/ the BTS and the MSC
Network Sub-System
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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) VLR number where each subscriber is registered even abroad
Each subscriber is associated to a single HLR The network identifies the HLR w/ MSISDN or IMSI
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Localization Information
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Implementation of the HLR can be centralized or distributed

Network Sub-System
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Mobile-services Switching Center (MSC) Switching functions

Call establishment bw MS and other MSC SMS transmission Hand-over when required VLR interrogations Localization information transfer Check of subscriber profiles
GMSC
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Mobility management
PSTN
MSC
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Gateway MSC (GMSC)
For the communication w/ a PSTN subscriber
Network Sub-System
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Visitor Location Register (VLR) Similar to HLR but for subscribers located in a given geographical area VLR includes also

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More precise localization information Temporary Mobile Subscriber Identity (TMSI)

BTS BSC
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VLR and MSC are often co-located

B A
MSC VLR
D C
HLR
Abis
G E B
MSC VLR
Operation Sub-System
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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) Local equipment supervision Global administration
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Operations and Maintenance Center (OMC)
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Network Management Center (NMC)
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Equipment Identity Register (EIR) Authentication Center (AUC)

Interfaces
Name Um Abis A C D
E F G B H
Interface MS-BTS BTS-BSC BSC-MSC GMSC-HLR VLR-HLR

MSC-MSC MSC-EIR VLR-VLR MSC-VLR HLR-AUC
Use
Air interface HLR access for terminating calls Subscriber information mgt and localization Hand-over Terminal identity check Subscriber information management Authentication
Protocol Stack
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User Plane
Speech coding
Trans-coding
relay
FR/EFR/HR/ AMR
Speech coding GSM 06.xx
FR/EFR/HR/ AMR
Speech coding GSM 06.xx
A-law (or Mu-law)

G.711
Physical Link Layer Physical RF Layer
relay
Physical Link Layer Physical RF Layer Um
TRAU framing E1
G.703
TRAU framing E1
G.703 / G.704
E1
G.703
OR
T1
T1.403
MS
BTS
Abis / Ater
TC
PCM 64K Time Slots

Protocol Stack
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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.)
signalling over A interface uses SS7 protocol stack signalling over Abis interface uses proprietary protocol over LAPD
CC/SMS /SS
GSM 04.08
CC/SMS /SS
GSM 04.08
MM
GSM 04.08
MM
GSM 04.08
RR
GSM 04.08
relay
RR
GSM 04.08
BSSAP
GSM 08.08
BSSAP
GSM 08.08
relay
LAPDm
GSM 04.06
LAPDm
GSM 04.06
RSL/ OML LAPD
RSL/ OML LAPD E1

G.703
SCCP MTP E1
G.703
SCCP MTP E1
G.703

Um
E1
G.703
MS
BTS
Abis
BSC
MSC
Protocol Stack
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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)
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Mobility Management (MM)
Localization Authentication TMSI allocation

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Call Management (CM)
Call Control (CC): circuit connection management Short Message Service (SMS) Supplementary Services (SS)
Protocol Stack
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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 Specific signaling protocol above MTP for mobility Worldwide interconnection protocol for signaling Call management
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Mobile Application Part (MAP)
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Signaling Connection Control Part (SCCP)
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ISDN User Part (ISUP)
Radio Interface
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TDMA Frame Structure Burst Format Duplexing
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Transmission Chain
Performance
Radio Interface
TDMA Frame Structure
TDMA frame (4.615 ms)
DL UL
0 1 2 34 5 6 7 0 1 2 34 5 6 7 0 1 2 34 5 6 7 0 1 2 34 5 6 7
0 1 2 34 5 6 7 0 1 2 34 5 6 7 0 1 2 34 5 6 7 0 1 2 34 5 6 7
time
Time Slot (577 s)
3 TS time shift between UL/DL no duplexer in MS !
User active on TS#3 for each TDMA frame
Radio Interface
Burst Format
0 1 2 34 5 6 7 0 1 2 34 5 6 7 0 1 2 34 5 6 7 0 1 2 34 5 6 7
For stealing frame e.g. for HO command
For power ramping
Data (57)
Training (26)
Data (57)
8.25
Coded information
For burst synchro and channel estimation

Burst: 148 bits 156.25 bits: 577 s
Guard time to avoid overlapping of burst due to mobility
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Data rate:
2x57 bits every 4.615 ms: 24.7 kbit/s

Radio Interface
Burst Format
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Training Sequence

midamble with dirac auto-correlation function Fine synchronization at burst level Channel estimation 8 possible training sequences Speech vs. signaling Radio resource can be used for signaling (e.g. FACCH) BTS has no information to send but has to emit a signal
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Stealing Flag

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Dummy Burst
Radio Interface
Duplexing
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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

0 1 2 34 5 6 7
Propagation delay
124 in GSM 900 174 in E-GSM 374 in DCS 1800
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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
Radio Interface
Transmission Chain
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Speech Coding:
Analogue to Digital error detection and correction
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Channel Coding:
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Interleaving:
adjacent bits over several data blocks to decorrelate error

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Ciphering
secret code synchronisation and equalisation Binary signal to Analogue

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Burst Formating
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Modulation:
Radio Interface
Transmission Chain
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Speech codec
260 bits 13Kbps
Speech Coding

Voice band = 300 3400 Hz Voice packet of 20ms 5.6 Kbps HR 13 Kbps FR, 12.2 Kbps EFR
Channel Coding
456 bits 22Kbps
0 1 2 3 4 5 6 7
Packet i interleaved w/ packets i-1 and i+1
0 1 2 34 5 6 7
Interleaving
8 TDMA Frames
0 1 2 3 4 5 6 7
8 half bursts
Radio Interface
Transmission Chain
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Error Control

Automatic Repeat Request (ARQ): LAPDm Forward Error Correction (FEC) Cyclic Redundant Check (CRC)
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Error Correction
Error detection only Block code with polynomial of length 3 for TCH
Convolutional Code
Rate = (TCH) Viterbi decoding (maximum likelihood)
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Ciphering
A5 algorithm based on time, frame number and session key Kc
Radio Interface
Transmission Chain
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Modulation

Constant envelop Trade-off between spectrum and co-channel resistance Sinusoidal signal for all 1 or all 0 sequences
Used for frequency synchronisation
Radio Interface
Performance
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Performance mainly depends on C and C/(I+N) A GSM receiver measures the following parameters

RXLEV: signal level (64 levels from 110 to 48dBm) RXQUAL: signal quality (BER coded on 8 levels)
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RXLEV and RXQUAL are reported on the SACCH Two main parameters to assess the performance
Frame Erasure Rate (FER) Bit Error Rate (BER)
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Receivers have to check that FER/BER are above threshold defined by the standard
Radio Interface
Performance
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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
Logical Channels
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Overview Multi-frame Structure Dedicated Channels
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Beacon Channel
Common Control Channels
Radio Interface
Overview
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Traffic Channels (TCH)

Exchange of information between end-users after call establishment On dedicated channels Voice or data
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Signalling

Exchange of information between the MS and the GSM network equipment In idle mode: authentication, location update During communication: handover, link control
Radio Interface
Overview
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One slot is a PHY channel TCH

SACCH
PHY-Ch
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Control functions

Broadcast system information (broadcast channels) Inform MS of incoming calls and allow access (common control channels) Physical parameters control (FACCH, SCH and SACCH) Transmission of telephone signaling (SDCCH)
Radio Interface
Overview
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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 Shared among all MS in the cell In downlink: information broadcast In uplink: random access
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Non Dedicated Channels

Radio Interface
Overview
FCH* SCH* BCCH* PCH* RACH* DL DL DL DL UL Frequency synchronisation Time synchronisation System Information Paging channel Random access Common signalling channels
AGCH*
SDCCH SACCH FACCH TCH
DL
UL/DL UL/DL UL/DL UL/DL
Access grant
Call establishment In call signalling Fast in call signalling Traffic channel
Dedicated signalling channel

Traffic
* On TS0 of beacon frequency

Radio Interface
Multi-Frame Structure
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For more flexibility and to allocate less than one slot per frame TDMA Frame 0 1 2 3 4 5 6 7 120 ms Multi-frame 0 1 2 23 24 25 235,8 ms Multi-frame 0 1 2 48 49 50
Super-frame 0 1 2 23 24 25 0 1 2 48 49 50 0 1 2045 2046 2047 2 Hyper-frame
Radio Interface
Dedicated Channels
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Traffic Channel (TCH)

TCH/FS 13kbps TCH/HS 5,6kbps Data 12kbps (9.6kbps services) or 14.4kbps Dedicated signaling 800 bits/s Information blocks of 184 useful bits/456 coded bits (8 half-bursts) A PHY-Ch can transport
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Stand-alone Dedicated Control Channel (SDCCH)

Either TCH and associated SACCH Or 8 SDCCH and their associated SACCH
Radio Interface
Dedicated Channels
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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 signaling in case of hand-over TCH transmission is interrupted and resource is allocated to signaling
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Fast Associated Control Channel (FACCH)

Radio Interface
Dedicated Channels
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TCH/SACCH Multiplexing 120 ms Multi-frame T T T T A T T i 12 25
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TCH/FACCH Multiplexing
Data (57)
Training (26)
Data (57)
8.25
Bit=0 > even bits are TCH Bit=1 > even bits are FACCH
Bit=0 > odd bits are TCH Bit=1 > odd bits are FACCH
Radio Interface
Dedicated Channels
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SDCCH/SACCH Multiplexing
SDCCH SACCH D0 D1 D2 D3 D4 D5 D6 D7 A0/4 A1/5 A2/6 A3/7

0
DL
50
32
A1/5 A2/6 A3/7
D0 D1 D2 D3 D4 D5 D6 D7 A0/4 UL
51 TDMA frames = 235,38 ms
Radio Interface
Beacon Channel
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In each cell, a frequency is dedicated to the beacon channel
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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 of neighboring cells Broadcast channels:

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>
Frequency Correction Channel (FCCH) Synchronization Channel (SCH) Broadcast Control Channel (BCCH)
Radio Interface
Beacon Channel
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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 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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Synchronization Channel (SCH)

Radio Interface
Beacon Channel
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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
Radio Interface
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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)
Data (36)
68.25
Radio Interface
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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 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
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Paging Channel (PCH)

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PCH and AGCH are multiplexed on a 51 frame structure with BCCH

Radio Resource Management

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Idle Mode Management of Dedicated Channels Hand-over

Idle Mode
> 1.
Cell Selection Scanning of beacon channels

MS does a list of beacon channels Either scan of all GSM frequencies (124 in GSM900, 374 in 1800) Or scan of predefined beacon channels for the PLMN 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)
2.
Look for a suitable cell

3.
PLMN selection
Automatic or manual mode


Idle 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 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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Measurements

Idle Mode
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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
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C1 takes into account RXLEV_ACCESS_MIN and MS_TWPWR_MAX_CCH broadcasted on the BCCH C2 includes offsets to avoid ping-pong effect and to favor some cells
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Management of Dedicated Channels
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Mobile Originating Call

MS
RACH
Channel description (frequency, slot,)
BTS
BSC
MSC
Service required and Propagation delay
RR Channel Request Channel Required Channel Activation Channel Act. ack Immediate Assignment Command
Switch to SDCCH
Channel Reservation
AGCH SDCCH
Choose a SDCCH or TCH

Channel description Ref. byte used for access FN of the request TA List of frequencies for SFH SCCP Connection Conf
RR Immediate Assignment
MS ID Terminal Class Service requested
SABM[Service Request] Establish Ind[Service Request]SCCP Connection Req UA[Service Request]
SDCCH
Followed by Authentication, Ciphering, Call All Basics of the GSM 54 Initiation, switching rightsTCH and Call Connection to reserved 2003, Alcatel

Management of Dedicated Channels > Mobile Terminating Call MS
May include several paging messages RR Paging Request RR Channel Request
BTS
Paging Command
BSC
Sent to all BTS of the LA
PCH
RACH
Channel Required Channel Activation Channel Act. ack Immediate Assignment Command
Switch to SDCCH
RR Immediate Assignment
AGCH
SABM[paging response]
SDCCH
UA[paging response]
Establish Ind[paging response]
Followed by Authentication, Ciphering, Call Initiation, switching to TCH and Call Connection

Management of Dedicated Channels
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Channel Release MS
BTS
RR Channel Release
BSC
MSC
BTS doesnt use SACCH any more
RR connection released
BSSMAP Clear Command Deactivate SACCH DISC
SDCCH
UA
Release Indication RF Channel Release BSSMAP Clear Complete RF Channel Release ack SCCP Released
SDCCH
Come-back on beacon channel
SCCP Released Complete
Radio resource completely freed


Hand-over
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Intra-BSC Hand-over BTS

SACCH
RR Measurements Report Measurements Result
HO Decision
Channel reservation
MS
BSC
BTS
MS
Access burst On TCH
Channel Activation Channel Activation ack
RR Handover Command
FACCH
RR Handover Access
Switch to new cell

New SDCCH/TCH channels New cell characteristics Initial power If possible: TA Ciphering mode
HO Detection
TCH
RR PHY info TA
FACCH
SABM
FACCH
UA RF Channel Release RF Channel Release ack
RR Handover Complete
FACCH
Erlang B Law
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Traffic Description Queuing Systems Poisson Arrival Process
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Erlang B Formula
Erlang B Law
Traffic Description
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The unit that defines the traffic is the Erlang. The Erlang:
1 Erlang is one resource (e.g. one voice channel) which is used permanently. Resource usage duration Total duration
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Traffic of one resource:
T=
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Example: a subscriber who makes 2 phone calls of 90s per hour:
Traffic = (2 x 90) / 3600 = 0.05 Erlang
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Exercise: Compute the Traffic of one user with
BHCA= 3 Average Call Duration= 45s
Erlang B Law
Queuing Systems > A queuing system may be with or without loss.
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Example of queuing system with one server:

Queue
Arrival process Service time Departure process
A one server queuing system without any loss is a server with an infinite queue size (theoretical only).
ErlangC
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We call loss systems systems that have the same number of servers as the queue length (no waiting time):
If all servers are used, process is rejected
ErlangB
Erlang B Law
Queuing Systems
Service time distribution : B
Arrival Process : A
Service Discipline : D
Queuing system
System Capacity : C
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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 Law
Poisson Arrival Process
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The arrival and departure process must be modeled, the most common for real telecommunication systems is the Poisson process: Definition of Poisson process:
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the arrivals between time t and t+are independent of the history of the process (memoryless process) the arrivals between time t and t+are independent of the time t (stationary process)
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Probability of having i arrivals in T seconds:
( T ) e P (T ) i!
i i
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Mean interarrival time =1/, is the mean arrival rate
Erlang B Law
Formula > Erlang B:
n server loss system: when n servers are occupied, arriving customer is thrown (no call reattempt) Arrival process is Poisson with rate Service time is exponential, ie departure process is Poisson with rate m

Calls blocked
0 1
k n-1 n
If overflow ( capacity is exceeded ) happens then the calls are simply blocked
km
(k+1)m
nm
Erlang B Law
Formula
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Steady state: number of departure=number of arrival
nmP(n) = P(n-1) ... kmP(k) = P(k-1)
k-1
...
km
mP(1) = P(0) P(k) = (/m)kP(0)

k!
P(0) + P(1) + ... + P(n) = 1
P(0)
( /m ) S i!
in i0
Erlang B Law
Formula > Notation:

m = 1/T, T is the mean inter-departure time, ie the mean holding time /m= T is the offered traffic to the system
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Probability of arriving customer being blocked = probability of n customers in the system, ie P(n):
(T ) n! Pblock ( , T , n) (T ) i!
n in i0
Erlang B Law
Formula > 3 parameters are used in the Erlang formulas:

Offered Traffic (T) Number of circuits (n) Blocking probability (Pblock) On the air interface: (n,Pblock)->(T) On the A interface: (T,Pblock)->n
Channel Efficiency=Offered Traffic / n
100 80
Efficiency (%)
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With 2 of these parameters, one can calculate the third:

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

60 50
40 30 20 10 0 0 10 20 30 40 50 60
60 40 20 0 0 10 20 30 40 50 60 Number of channels
The Erlang law is not linear !!! 4TRX (21.9Erl) > 2x2TRX (16.4Erl)
Offered Traffic
Num be r of channe ls
Interference Reduction Techniques

> > >
Slow Frequency Hopping VAD/DTX Power Control

Slow Frequency Hopping
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Definition: change randomly and regularly (each TDMA frame)

the frequency used by a channel
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Consequences:

Frequency diversity: spreads in time lost bursts due to selective frequency signal fading Interference diversity: changes in the interference position from TDMA frame to TDMA frame
reduction in the standard deviation of the co-channel interference level increasing the number of receivers having a SINR above a certain threshold

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BBH: Baseband Hopping
0 TRX1 TRX2 TRX3 TRX4

BCCH
Number of hopping frequency = nb of TRX TS0 of beacon frequency does not hop
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SFH: Synthesized Frequency Hopping

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

Slow Frequency Hopping Interfering cells position change from a TS to TS
Available frequencies:
F0: F1: F2: F3: TSi TSi+1

Without SFH
C/I C/I
With SFH
C/I mean Threshold
C/I mean Threshold
Receiver
Receiver
SFH reduces the distribution of average C/I across the cell however it increases the variations for a given user (interference less stable) overall the average C/I over the cell is increased

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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
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64xN hopping sequences are then available with a radio spectrum 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 a different MAIO and never interfere (SFH laws are orthogonal)
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>

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More benefit for slow moving MS Highest improvements when hopping is on more than 4 carriers
15 14 13 12 11 10 9 8 7 6 1 2 3 4 5
TU3 TU50
required C/I (dB)
TU3
TU50
6 7 8 9 10 11 12
number of frequencies in hopping sequence

VAD/DTX Inhibiting transmission on air interface when no relevant information has to be transmitted
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DTX: Discontinuous Transmission

user is speaking : speech coded at 13 kbit/s (FR) silence: transmission of SID (Silence Descriptor for comfort noise) frames every 480ms (500bits/s)
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VAD: Voice Activity Detection

Distinguish a speech signal from background noise. Algorithm based on a comparison between the filtered signal and a threshold (both are continuously adjusted)

VAD/DTX
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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

Power Control Modification of Tx power according to signal quality and signal level
> > > > >
Independently applied for uplink and downlink
Managed by the BSC

Optional (operator choice) Proprietary algorithm Beacon frequency is not subject to PC

Power Control
>
At initial access, MS transmits at MS_TXPWR_MAX_CCH broadcasted on the BCCH

PC is managed in both directions by the BSC:
>
Uplink: required MS transmission level is computed through reception level and quality measurements performed by the BTS
Downlink: for each connection, BTS transmission power is based on measurements performed by the MS and reported to the BTS every 480ms (5 bits on the SACCH)

Power Control
>
PC command is received by MS every 480 ms on SACCH. Requested value is reached by step of 2 dB every 60 ms
Transmission level (dBm) 33 29 25
5 1 Commands: 5 dBm 29 dBm 25 dBm 23 dBm Time (60ms intervals)
>
An immediate power transition is performed in case of channel connection and Hand-Over procedure (new serving cell command)

Power Control
>
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
Densification Techniques
> > >
Hierarchical Networks Concentric Cells Multi-band Cells
Hierarchical Networks
>
3 advantages using micro cells:

Traffic increase Provision of localized coverage
complete overlap with existing coverage partial overlap
Quality of service Hot spot Continuous layer Outdoor Indoor

>
Different implementations
>
Different Coverage objectives:

> >
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 free TCH
>
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 to another TRX
> >
Directed Retry Assign at call establishment a TCH in a neighbor cell in case of 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 incoming call to be served in the serving cell
> > > >
> >
Alcatel Integrated Multi-layer Solution (AIMS) 3 frequency groups are used in macrocell layer with fractional re-use 1/3 Two of the 3 frequency groups are reused in the microcells with fractional re-use 1/1.
>
Concentric Cells
> > >
Realize two concentric zones within one cell MS1 can use F1 or F2 and MS2 can use F2. BCCH on outer TRX
F2 F1
MS1 BS1
MS2
Concentric Cells
>
Two ways of using concentric cells :
capacity oriented : by using it on an interfered cell and guaranteeing a high received level in the inner zone. This allows an additional TRX in the inner zone with a reduced reuse cluster size. F2 I1 F1 C1 C2 MS1 MS2 I2 F1
F2
BS1
Concentric Cells
QoS oriented :
by using it on an interfering cell to bring down the level of interference by powering down the inner zone carriers. if a frequency is interfered, it is possible to convert it in an inner zone frequency.
F1 F1 F1
MS1 BS1 MS2 Reduced power less interference F1 less interfered

BS2
Concentric Cells
> > >
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 the SDCCH
High level on UL and DL > inner zone
>
Outgoing Hand-over
Same HO criteria and strategies as for non-concentric cells MS is handed over in the zone corresponding to its location
>
Incoming Hand-over
Concentric Cells
>
Intra-cell Hand-over

Two types: inter or intra zone hand-over Two triggers: emergency or better zone hand-over 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) Too low level on DL or UL in the inner zone Hand-over towards outer zone is triggered The other zone is more suitable to handle the MS
>
Emergency intra-cell HO due to interference
>
Emergency intra-cell HO due to level

>
Better zone intra-cell hand-over
Multi-band Cells
>
Mixture of GSM 900 and 1800 channels in a single band cell with all CCCH in a single band (single BCCH concept)
Single band Dual band Dual band
BS
1800 MHz TCH
900 MHz BCCH and TCH
Multi-band Cells
>
Advantages of Multi-band cells:

less cells to operate : the operator only adds TRX working in the second frequency band where the traffic demand is high, dual band mobiles have less cells to monitor leading to more reliable measurements reports (each BCCH is measured more often), reduced number of inter-cell handovers, only one BCCH frequency plan, higher traffic efficiency of the second frequency band not limited by a BCCH frequency plan which requires a lot of frequencies for only 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

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01 - GSM Training Pack - Basics of The GSM

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Basics of the GSM technology

Introduction Frequency Bands Network Architecture

Basics of the GSM 2

All rights reserved 2003, Alcatel

Basics of the GSM 3

All rights reserved 2003, Alcatel

1987 Foundation of Memorandum of Understanding Association with 13 member from 12 states.

Basics of the GSM 4

All rights reserved 2003, Alcatel

Standardization was a key driver for success

Basics of the GSM 5

All rights reserved 2003, Alcatel

GSM standardization in phases/releases:

Phase 1: Speech, SMS, CSD

Release 6: MBMS, PS handovers

Release 97: GPRS

Release 99: EDGE, Interworking with UMTS, QoS

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

Release 7: GERAN Evolutions

Basics of the GSM 6

GSM = Global System for Mobile Communication

Basics of the GSM 7

All rights reserved 2003, Alcatel

Bandwidth # Carriers Duplex

25M 174 45M

75M 374 95M

Basics of the GSM 8

Overview Mobile Station (MS) Base Station Sub-System (BSS)

Network Sub-System (NSS)

Basics of the GSM 9

All rights reserved 2003, Alcatel

Basics of the GSM 10

All rights reserved 2003, Alcatel

MS = Terminal Equipment + SIM Card MS identity:

International Mobile Equipment Identity (IMEI)

Min required sensitivity: -102dBm (-104dBm: aggressive figure in 900MHz band)

Basics of the GSM 11

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

Basics of the GSM 12

All rights reserved 2003, Alcatel

Basics of the GSM 13

All rights reserved 2003, Alcatel

Base Station Controller (BSC) Main Function: Radio Resource Management

Basics of the GSM 14

All rights reserved 2003, Alcatel

Implementation of the HLR can be centralized or distributed

Basics of the GSM 15

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Mobile-services Switching Center (MSC) Switching functions

Gateway MSC (GMSC)

For the communication w/ a PSTN subscriber

Basics of the GSM 16

All rights reserved 2003, Alcatel

More precise localization information Temporary Mobile Subscriber Identity (TMSI)