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ECE DepartmentFlorida Institute of Technology
ECE 5221 Personal Communication Systems
Introduction to GSM
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Course Outline
Part 1: Introduction
o Historical overview
o Elements of network architecture
o Elements of air interface
Part 2: Signal processing and network features
o Voice processing
o GSM Network features
Part 3: Network design
o Coverage planning
o Capacity planning
o Migration towards 3G and beyond
The GSM logo used on numerous
handsets and by carries who wish to
identify a GSM product
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History
Driving Factors:
Incompatibility of the European analog cellular systems
Reaching of capacity limits
Costs of the equipment
1982, Conference of European Post and Telecommunications formed Group Speciale Mobile (GSM)
1987, 15 operators from 13 countries signed Memorandum of Understanding (MoU)
1991, Finlands operator Radiolinia launched first GSM network in July 1991
1992, Massive deployment of GSM started
By 2000 GSM became the most popular 2G technology worldwide
GSM standard still evolving and enriched with new features and services
GSM = Global System for Mobile communications
(GSM: originally from Groupe Spcial Mobile)
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Deployment worldwide
930 networks in 222 countries and regions
More than 3 billion subscribers worldwide
More than 80% worldwide market share
Worldwide map of GSM coverage (source www.gsmworld.com)
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GSM in the USA
1994, US FCC auctioned large blocks ofspectrum in 1900MHz
GSM started deployment in PCS band
1995, American PersonalCommunications launched first GSMnetwork
In 2002, 850 band opened for GSM
Currently there are ~ 95M GSMsubscribers
Largest GSM operators ATT
T-Mobile
ATT coverage map
T-Mobile coverage map
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GSM Standards
Divided into 12 series
Standardization efforts coordinatedby ETSI
www.etsi.org
Specifications available onlinefree of charge
Standardization and publicavailability of specification - one offundamental factors of GSMsuccess
Series Specifications area
01 General
02 Service aspects
03 Network aspects04 MS-BS interface and protocol
05 Physical layer and radio path
06 Speech coding specification
07 Terminal adapter for MS
08 BS-MSC interface
09 Network internetworking10 Service internetworking
11 Equipment and type approval specification
12 Operation and maintenance
GSM Standard
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GSM Network Layout
GSM system layout is standardized
o Standardization involves:
Elements of the network
Communication Interfaces
o Standard layout allows for the use of equipment from different suppliers
MSC
Area
HLR
MSC
Area
VLR
MSCTRAUBSC
BTS
BTS
BSS
MSC Area
BSS
BSSBTS
PSTN
PLMN - Public Land Mobile Network
Gateway
MSCNSS
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GSM Components and Interfaces
Network has many functional components
Components are integrated through a network protocolMAP
Standardized interfaces
Um (air interface)
AGERAN interface
A-Bis (somewhat standardized)
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Mobile Station (MS)
Two functional parts
o HW and SW specific forGSM radio interface
o Subscriber IdentityModule (SIM)
SIMdetaches user identityfrom the mobile
o Stores user information
o Without SIMonlyemergency calls
Functional diagram of GSM mobile
SIM card
Most popular GSM phone
Nokia 1100200M+ sold
Keyboard
Control
Display
Transmit Audio
Signal
Processing
Receive Audio
Signal
Processing
Channel
Decoding
Deinterleaving
Message
Regenerator
Channel
Encoding
Interleaving
Message
Generator
Ciphering
Ciphering
RF
Processing
RF
Processing
SIM
Duplexer
Antenna
ANTENNA
ASSEMBLY
TRANSMITTER
RECEIVER
TRANSCEIVER UNITCONTROL
SECTION
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Base Transceiver Station (BTS)
BTS is a set of transceivers (TX/RX).
GSM BTS can host up to 16 TX/RX.
In GSM one TX/RX is shared by 8 users.
The main role of TX/RX is to provide
conversion between traffic data on the
network side and RF communication on
the MS side.
Depending on the application, it can be
configured as macrocell, microcell, omni,
sectored, etc.
Typical BTS installation
BTS antenna system
Macrocell BTS radio
cabinet hosts TX/RX
Femto-cell
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BSC plays a role of a small digital exchange.
It can be connected to many BTSs and it offloads a great deal ofprocessing from MSC
One BSC connects to several tens to couple of hundred BTS
Some of BSC responsibilities:
o Handoff management
o MAHO management
o Power control
o Clock distribution
o Operation and maintenance TRAU is responsible for transcoding the user data from 16Kb/sec
to standard ISDN rates of 64Kb/sec.
It can physically reside on either BSC side or MSC side. If it resides on the MSC side, it provides substantial changes in
the backhaul4 users over a single T-1/E-1 TDMA channel.
TRAU, BSC and BTSs form Base Station Subsystem (BSS
Base Station Controller (BSC) and TRAU
Typical BSC
TRAU = Transcoding and Rate Adaptation Unit
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Responsible for connecting the mobile to the
landline side
GSM MSC is commonly designed as a regular
ISDN switch with some added functionality for
mobility support
GSM Network can have more than one MSC
One of the MSC has an added functionality for
communication with public networkGateway
MSC (GMSC)
All calls from the outside networks are routedthrough GMSC
Mobile Switching Center (MSC)
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Registry HLR/VLR
HLRHome Location Registry
Database for permanent or semi-
permanent data associated with the user
Logically, there is only one HLR per
network
Typical information stored in HLR:International Mobile Service Identification
Number (IMSI), service subscription
information, supplementary services,
current location of the subscriber, etc.
HLR is usually implemented as an
integral part of MSC
VLRVisitor Location registry
Temporary database that keeps the
information about the users within the
service area of the MSC
Usually there is one VLR per MSC
The main task of the VLR is to reduce
the number of queries to HLR. When
the mobile, registers on the system its
information is copied from HLR to VLR
VLR is usually integrated with the switch
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AUC/EIR
AUCAuthentication center Integral part of HLR
GSM specifies elaborateencryption
Three levels
o A5/1 USA + Europe
o A5/2 COCOM country list
o No encryptionrest of theworld
EIREquipment Identity Registry
Responsible for tracking equipmentand eligibility for service
Maintains three lists
o White listapproved mobiletypes
o Black listbarred mobile types
o Gray listtracked mobiletypes
Over yearsmany other vendor specific features added to the system
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GSM Air Interface - Um
Interface between the MS and the GSM network
Subject to rigorous standardization process
We examine:
o Channelization
o Multiple access scheme
o Interface organization:
On the physical level
On the logical level
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Frequency allocation
For PCS-1900 band
o ARFCNul = (Fc-1850)/0.2+511; ARFCNdl = (Fc-1930)/0.2+511
For GSM-850
o ARFCNul = (Fc-824)/0.2+127; ARFCNdl = (Fc-969)/0.2+127
Mapping formulas
GSM is FDD technology
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TDMA Access Scheme
Multiple users operate on the same
frequency, but not at the same time.Advantages of TDMA:
o Relatively low complexity
o MAHO
o Different user rates can beaccommodated
o Easier integration with thelandline
Disadvantages:
o High sync overheado Guard times
o Heavily affected by themultipath propagation
Uplink ( From MS to BS)
Wireless Comm unication Channel
Downlink ( From BS to MS )
Base Station
fu0,
s1
fd0,
s1, s
2, ...,s
8
S1
S2
S3
.... S8 s1
s7 s8.... s1 s2 s3
fu0,
s2
fu0,
s8
TDMA = Time Division Multiple Access
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GSM as a TDMA system
GSM is acombination ofFDMA and TDMA
TDMA supports:
o Up to 8 full rateusers
o Up to 16 half rateusers
GSM uses
Frequency DivisionDuplexing
BTS
USER 1 USER 2 .... USER 8
U SER 6 U SER 7 U SER 8 U SER 1
USER 1,
ARFCN1
USER 2,
ARFCN1
USER 8,
ARFCN1
USER 9,
ARFCN2
USER 10,
ARFCN2
USER 16,
ARFCN2
ARFCN1
ARFCN2
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GSM bursts
Data sent over one time slot =burst
Five types: normal, frequencycorrection, synchronization,dummy, access
Format of a burst defied by itsfunction
DL: normal, frequency correction,synchronization, dummy
UL: normal, access
Time/Frequency/Amplitude diagram for GSM
normal burst
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Normal Burst
Used to carry information on both control and traffic channels
Mixture of data and overhead
GSM defines 8 training sequences assigned in color code mode
Both on the forward and reverse link
Total of 114 encoded user information bits
Total of 34 overhead bits
Tail Traffic/Signaling Flag Training Sequence Flag Traffic/Signaling Tail
3 57 1 26 1 57 3
Normal burst
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Frequency Correction Burst
Sometimes referred to as the F-burst
Provides mobile with precise reference to the frequency of the broadcast controlchannel
Inserting the F-bursts on the control channel produces spectral peak 67.7 KHzabove the central frequency of the carrier
Only on the forward link
Spectral characteristics of the control
channel.
The peak in the spectrum allows for
easier MS network acquisition
Format of the F-burst
Fixed sequence consists of all zeros
fc fc+67.7 KHz frequency
Power Spectrum Density
BW = 200KHz
Tail Fixed Bit Sequence (All zeros) Tail
3 3142
Frequency correction burst
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Synchronization Burst
Facilitates the synchronization of the MS to the network at the base band
Commonly referred to as S-burst
Only on the forward link
The same sync sequence is used in all GSM networks
Tail Synchronization Training Sequence Synchronization Tail
3 33939 64
Synchronization burst
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Dummy Burst
Supports MAHO
Used to ensure constant power level of the broadcast
control channel
Only on the forward link
Tail Predefined Bit Sequence Tail
3 3142
Dummy burst
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Access Burst
Used when the MS is accessing the system
Shorter in lengthburst collision avoidance
Extended synchronization sequence
Used only on the reverse link
GSM mobiles use slotted ALOHA to access the system
In the case of collisiona hashing algorithm is provided
Tail Synchronization Access Bits Tail
8 41 36 3
Access burst
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GSM Time Division Duplex
Communication on the forward and reverse link does not
happen simultaneously
Delay of three slots between TX and RX
Time division duplexing avoids RF duplexer at the RF stage
o Reduces the cost of mobile
o Saves battery
0
1 2 3 4 5 6 7 00
1 2 3 4 5765
Forward Link - BTS Transmits
Reverse Link - MS T ransmits
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GSM Logical Channels
GSM Logical
Channels
TCH
TCH/F TCH/H
CCH
BCH CCCH DCCH
CBCH
ACCH SDCCH
FACCHSACCH
FCCH
SCH
BCCH
PCH
AGCH
RACH
TCH - Traffic Channel
TCH/F - Traffic Channel (Full Rate)
TCH/H - Traffic Channel (Half Rate)BCH - Broadcast Channels
FCCH - Frequency Correction Channel
SCH - Synchronization Channel
BCCH - Broadcast Control Channel
CCCH - Common Control Channels
PCH - Paging Channel
AGCH - Access Grant Channel
RACH - Random Access Channel
DCCH - Dedicated Control Channels
SDCCH - Stand-alone Dedicated Control Channel
ACCH - Associated Control Channels
SACCH - Slow Ass ociated Control
Channel
FACCH - Fast Associated Control
Channel
CCH - Control Channel
CBCH - Cell Broadcast Channel
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Traffic channel carries speech and user data in both directions
o Full rate ~ 33.85 Kb/sec
o Half rate ~ 16.93 Kb/sec
o Full rate uses 1 slot in every frame
o Half rate uses 1 slot in every other frame
Data rates differ due to differences in Error Control Coding
Traffic Channels (TCH)
Full Rate TCH can carry:
Voice (13 Kb/sec)
Date at rates:
-9.6 Kb/sec-4.8 Kb/sec
-2.4 Kb/sec
Half Rate TCH can carry:
Voice (6.5 Kb/sec)
Date at rates:
-4.8 Kb/sec-2.4 Kb/sec
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Control Channels
GSM Defines 3 types of Control Channels:1. Broadcast Channels (BCH)
Broadcast information that helps mobilesystem acquisition, frame synchronization,etc. They advertise properties andservices of the GSM network.
Forward link only
2. Common Control Channels (CCCH)
Facilitate establishment of the link betweenMS and system
Both forward and reverse link
3. Dedicated Control Channels (DCCH)
Provide for exchange the controlinformation when the call is in progress
Both forward and reversein bandsignaling
CCH
BCH
CCCH
DCCH
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Broadcast Channels (BCH)
Three types of BCH:
1. Synchronization channel (SCH)
Provides a known sequence that helps mobilesynchronization
at the baseband
Communicates with S-burst
Broadcasts Base Station Identity Code (BSIC)
2. Frequency Correction channel (FCH)
Helps mobile tune its RF oscillator
Communicates with F-burst
3. Broadcast Control Channel (BCCH) Provides mobile with various information
about network, its services, accessparameters, neighbor list, etc.
BCH
SCH
FCH
BCCH
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Broadcast Channels (BCH) contd.
In general, the information sent over BCCH can be grouped into four categories:
1) Information about the network
2) Information describing control channel structure
3) Information defining the options available at the particular cell
4) Access parameters
Some BCCH messages
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Common Control Channel (CCCH)
Three types of CCCH:
1. Random Access Channel (RACH)
Used by mobile to initialize communication
Mobiles use slotted ALOHA
Reverse link only
2. Paging Channel (PCH)
Used by the system to inform the mobile
about an incoming call
Forward link only
GSM Supports DRX
3. Access Grant Channel (AGC)
Used to send the response to the mobiles
request for DCCH
Forward link only
CCCH
RACH
PCH
AGC
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Dedicated Control Channels (DCCH)
Three types of DCCH:
1. Stand Alone Dedicated Control Channel(SDCCH)
Used to exchange overhead informationwhen
the call is not in progress
2. Slow Associated Control Channel (SACCH)
Used to exchange time delay tolerantoverhead
information when the call is in progress
3. Fast Associated Control Channel (FACCH)
Used to exchange time critical information
when the call is in progress
DCCH
SDCCH
SACCH
FACCH
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Logical Channels - Summary
UL - Uplink DL - Downlink
Channel UL only DL only UL/DL Point topoint
Broadcast Dedicated Shared
BCCH X X X
FCCH X X X
SCH X X X
RACH X X X
PCH X X X
AGCH X X X
SDDCH X X X
SACCH X X X
FACCH X X X
TCH X X X
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Timing Advance
Mobiles randomlydistributed in space
Timing advanceprevents burst collisionon the reverse link
Maximum
advancement is 63bits
BTS
SLOT 0 SLOT 1 SLOT 2 SLOT 3 SLOT 4 SLOT 5
MS2
MS1
d2, Slot 2
d1, Slot 1
d1> d2MS2
MS1
T1
T2
Collision
T1- Delay of MS 1
Signal
T2- Delay of MS
2
Signal
SLOT 7SLOT 6
km35bit
s10693.3bit63
s
m103
2
1max 68
D
Si l P i
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Signal Processing
From Voice to Radio Waves
Sampling,Quantization and
source encoding
ChannelEncoding
(Error Correction
Coding)
InterleavingBurst
Formating
Mapping
De-
Ciphering
Modulation
De-Modulation
Ciphering
Burst
Formating
Mapping
De
-Interleaving
Channel
Decoding
(Error Correction )
Source Decoding
and Waveform
Generation
Um
Interface
VoiceSignal
Voice
Signal
Transmit Side
Receive Side
As a digital TDMA technology GSM implements extensivesignal processing
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Sampling and Quantization
Sampling
o Sampling theoremspecifies conditions fordiscretization of bandlimited analog signals
o Voice needs to besampled at the samplingrate greater then
8000Hz
Quantization
o Discrete valuesassigned to continuoussamples
o Quantization noise
o In GSM, voice issampled at 8 Ksamples/sec andquantized with 8192levels (13 bit words)
111 +3V
110 +2V
101 +1V
0V
001 -1V
010 -2V
011 -3V
111 +3V
110 +2V
101 +1V
0V
001 -1V
010 -2V
011 -3V
Analog Signal
Sampling Pulse
PAM
101 110 101 100 010 010 010 100 111 111PCM
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Speech Source Encoding
Speech coder reduces the data rateneeded for voice signal representation
GSM specifies operation of:
o Full rate vocoder
13Kb/sec
o Half rate vocoder 5.6Kb/sec
o Enhanced Full Rate (EFR)
12.2Kb/sec
o AMR (Adaptive multi rate)
AMR-FR (4.75-12.2Kb/sec)
AMR-HR (4.75-7.95Kb/sec)
AMR rate - function of C/I
BPF A/D
converter
SPEECH
ENCODER
CHANNEL
CODING
TO
MODULATOR
MICROPHONE
BAND-PASS
300 Hz-3.4 kHz
SPEECH
DECODER
CHANNEL
DECODERLP
LOW-PASS
4 kHz
D/Aconverter
Vocoders enable efficient channel
utilization
P f i f
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Performance comparison of some
commercial vocoders
Mean Opinion Scores (MOS) - Voice Quality
source IIR. The First Annual CDMA Congress
London, Oct. 29-30, 1997
0
0.5
1
1.5
2
2.5
33.5
4
4.5
Clean Speech 20dB SNR
Babble
20dB SNR
Car
15dB SNR
Street
Mu-PCM
8Kb/s EVRC(CDMA)
13Kb/s CELP
(CDMA)
IS-136 ACELP
GSM EFR
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Error control coding (ECC) increases the robustness of thesignal
ECC increases the overhead and reduces the efficiency of thecommunication
In GSM, the ECC increases the overhead per user by 57%
Channel Encoding
TYPE Ia
BITS
TYPE II
BITS
TYPE Ib
BITS
CONVOLUTIONAL
ENCODER
r=1/2
K=5
MUX
ERROR DETECTING CODE50
132
78
3
4
189
189
378
456
0TO
INTERLEAVER
FROM
VOCODER
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Interleaving
In mobile
communications, the
errors are bursty
Optimal performance
from ECC is obtained for
uniform error
distribution
Interleaving increases
the performance of ECC
in mobile environment
252015105
24191494
23181383
22171272
21161161
bbbbb
bbbbb
bbbbb
bbbbb
bbbbb
Data is written
column-wise
Data is read
row-wise
Interleaver
b1b
2b
3b
4b
5b
6b
7b
8b
9b
10b
11b
12b
13b
14b
25b
16b
17b
18b
19b
20...
b1b
6b
11b
16b
21b
2b
7b
12b
17b
22b
3b
8b
13b
18b
23b
4b
9b
14b
19b
24..
Burst Error
Caused by
Rayleigh Fading
Errors are spread over the bit stream
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Modulation: GMSK (Gaussian MSK)
GMSK has excellent spectralcharacteristics
o Low sidelobes
o Robust to non- linearities
Price paid is in the increasedInter Symbol Interference (ISI)
Simplified GMSK block diagram
MSK
Filtered MSKGMSK
(f-fo) / Rb0 1 2 3
-80
-60
-40
-20
0
POWER SPECTRALDENSITY
dBSpectral
characteristics
of GMSK
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Tail Traff ic /Si gnali ng F lag Tra in ing Sequence F lag Traffi c/Si gnali ng Tai l
3 57 1 26 1 57 3
Sequence used for equalizer training
Equalization
Necessary due to the multipathpropagation
Needs to have :
o Fast convergence
o Low complexity
Two modes of operation
1. Training
2. Equalization
GSM equalizer capable of equalizing fortwo equal multi paths separated by 16microseconds
Introduces overhead of about 18%
RF
Processing
Adaptive
Equalizer
Equalization
AlgorithmExtraction of
Synchronization
Bits
Unequalized
Data
Equalized
Data
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GSM Network Features
Mobile Assist Handoff (MAHO)
Discontinuous Transmission (DTX)
Dynamic Power Control (DPC)
Frequency Hopping (FH)
Intercell Handoff
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Mobile Assisted Handoff (MAHO)
GSM ImplementsMAHO
In the process ofevaluating handoffcandidates, GSMsystems evaluatemeasurementsperformed by both the
MS and BTS
There are three typesof measurements:
1. Signal StrengthMeasurements
2. Signal Quality
Measurements
3. Timing Advance
Measurements
Measurement
type
Link Cell DTX Measurement
Source
RSL Downlink Serving Cell Full Set Mobile
RSL Downlink Serving Cell Subset Mobile
RSL Downlink Neighbors N/A Mobile
Quality Downlink Serving Cell Full Set Mobile
Quality Downlink Serving Cell Subset Mobile
RSL Uplink Serving Cell Full Set BTS
RSL Uplink Serving Cell Subset BTS
RSL Uplink Neighbors Full Set BTS
RSL Uplink Neighbors Subset BTS
Quality Uplink Serving Cell Full Set BTS
Quality Uplink Serving Cell Subset BTS
Timing Advance Uplink Serving Cell N/A BTS
Measurement
type
Link Cell DTX Measurement
Source
RSL Downlink Serving Cell Full Set Mobile
RSL Downlink Serving Cell Subset Mobile
RSL Downlink Neighbors N/A Mobile
Quality Downlink Serving Cell Full Set Mobile
Quality Downlink Serving Cell Subset Mobile
RSL Uplink Serving Cell Full Set BTS
RSL Uplink Serving Cell Subset BTS
RSL Uplink Neighbors Full Set BTS
RSL Uplink Neighbors Subset BTS
Quality Uplink Serving Cell Full Set BTS
Quality Uplink Serving Cell Subset BTS
Timing Advance Uplink Serving Cell N/A BTS
Measurement
type
Measurement
type
LinkLink CellCell DTXDTX Measurement
Source
Measurement
Source
RSLRSL Downlink Downlink Serving CellServing Cell Full SetFull Set MobileMobile
RSLRSL Downlink Downlink Serving CellServing Cell SubsetSubset MobileMobile
RSLRSL Downlink Downlink NeighborsNeighbors N/AN/A MobileMobile
QualityQuality Downlink Downlink Serving CellServing Cell Full SetFull Set MobileMobile
QualityQuality Downlink Downlink Serving CellServing Cell SubsetSubset MobileMobile
RSLRSL Uplink Uplink Serving CellServing Cell Full SetFull Set BTSBTS
RSLRSL Uplink Uplink Serving CellServing Cell SubsetSubset BTSBTS
RSLRSL Uplink Uplink NeighborsNeighbors Full SetFull Set BTSBTS
RSLRSL UplinkUplink NeighborsNeighbors SubsetSubset BTSBTS
QualityQuality Uplink Uplink Serving CellServing Cell Full SetFull Set BTSBTS
QualityQuality Uplink Uplink Serving CellServing Cell SubsetSubset BTSBTS
Timing AdvanceTiming Advance UplinkUplink Serving CellServing Cell N/AN/A BTSBTS
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MAHO - Signal Strength Measurements
Performed on uplink and downlink
Reported as a quantized value RXLEV:
RXLEV = RSL[dBm] + 110
Minimum RXLEV:
-110, MAX RXLEV = -47
On the downlink, measurementperformed for both serving cell and up
to 32 neighbors
Up to 6 strongest neighbors are
reported back to BTS through SACHH
Example measurement report
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MAHO - Signal Strength Measurements
Measurements of theneighbors are performed onthe BCCH channelsnotaffected by the DTX
Measurements on the servingchannelaffected by theDTX.
Perform over a subset ofSACCH that guaranteestransmission even in the caseof active DTX
Before processing, the RXLEVmeasurements are filtered to
prevent unnecessary handoffs
-100
-90
-80
-70
-60
-50
-40
0 500 1000 1500 2000
Measurement
RXLEV(dBm)
510
520
530
540
550
560
570
580
BCCHARFCN
RX LEV (dBm) BCCH
Example RSL measurement
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MAHOSignal Quality Measurements
Performed on uplink and downlink
Only on the serving channel Reported as a quantized value RXQUAL
For a good quality call RXQUAL < 3
Measurements are averaged before thehandoff processing
If DTX is active, the measurements areperformed over the subset of SACCH that
guarantees transmission
RXQUAL BER
0 Less than 0.1
1 0.26 to 0.30
2 0.51 to 0.64
3 1.0 to 1.3
4 1.9 to 2.7
5 3.8 to 5.4
6 7.6 to 11.0
7 Above 15
RXQUAL BER
0 Less than 0.1
1 0.26 to 0.30
2 0.51 to 0.64
3 1.0 to 1.3
4 1.9 to 2.7
5 3.8 to 5.4
6 7.6 to 11.0
7 Above 15
RXQUALRXQUAL BER BER
00 Less than 0.1Less than 0.1
11 0.26 to 0.300.26 to 0.30
22 0.51 to 0.640.51 to 0.64
33 1.0 to 1.31.0 to 1.3
44 1.9 to 2.71.9 to 2.7
55 3.8 to 5.43.8 to 5.4
66 7.6 to 11.07.6 to 11.0
77 Above 15Above 15
RXQUAL mapping table
RXQUAL
measurements
Measurement report
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Performed on uplink (BTS)
Only on the serving channel
Used by the BTS to estimatedistance to the MS
Expressed in number of bitsof TX advancement
Can be between 0 and 63
TA
MAHOTime Alignment Measurement
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Discontinuous Transmission (DTX)
Typical voice activity is around 60%
DTX discontinues transmission duringsilent periods
Benefits of DTX
o Uplink:
System interference reduction
Lower battery consumption
o Downlink
System interference reduction
Reduction of the intermodulationproducts
Lower power consumptions
Downsides of DTX usage:
o MAHO measurements are less accurate
o Voice quality is degraded due to
slowness of VAD
Mobile station Environment Typicalvoice
activityHandset Quiet location 55%
Handset Moderate office noisewith voice interference
60%
Handset Strong voiceinterference (ex. airport,
railway station)
65-70%
Hands free /handset
Variable vehicle noise 60%
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Dynamic Power Control (DPC)
There are three reasons for DPC:
1. Reduction of battery consumption2. Elimination of near-far problem
3. Reduction of system interference
Power Class GSM (900MHz)
[W]
PCS-1900 / GSM1800
[W]
1 20(1) 1
2 8 0.24
3 5 Not Defined
4 2 Not Defined
5 0.8 Not Defined
Power Class GSM (900MHz)
[W]
PCS-1900 / GSM1800
[W]
1 20(1) 1
2 8 0.24
3 5 Not Defined
4 2 Not Defined
5 0.8 Not Defined
Power ClassPower Class GSM (900MHz)
[W]
GSM (900MHz)
[W]
PCS-1900 / GSM1800
[W]
PCS-1900 / GSM1800
[W]
11 20(1)20(1) 11
22 88 0.240.24
33 55 Not DefinedNot Defined
44 22 Not DefinedNot Defined
55 0.80.8 Not DefinedNot Defined
(1) Not available commercially
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Dynamic Power Control (DPC)
DPC for MS
o Depending on its power class, MS can adjust its power between the max and min
value in 2dB steps
o MS can perform 13 adjustments every SACCH period, i.e., 480ms
o Large adjustments > 24 dB will not be completed before the arrival of new
command
o Commonly implemented as BSC feature. Many vendors are moving it at the BTS
level DPC for BTS
o Vendor specific
o Based on MAHO reports
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Hierarchical Cell Structure (HCS)
Incorporates various cell sizes into
layers of RF coverage
Three common layers:
1. Umbrella cells (HL = 0)
2. Macrocells (HL = 1)
3. Microcell (HL = 2)
HCS provides a way to assignpreference levels between the cells
Very effective way for capacity andinterference management
SignalStrength
ReselectionPoints
Select Micro-Cell
SS_SUFF
Macrocel
Preferred
Micro-Cell
Distance
HL = 1
HL = 2
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Handling of Fast Moving Mobiles
If the mobile is moving at a high speed, it will
spend a short time in the coverage area of the
microcell
To prevent excessive handoffs, a temporal
GSM introduces temporal penaltypreventsimmediate handoff initialization
If the duration of mobile stay within the
coverage area is shorter than the temporal
penalty, it will never initialize handoff
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Frequency Hopping (FH)
FH - multiple carriers used over the course of radio transmissiono There are two kinds of FH:
1. Slow Hoppingchange of carrier frequency happens at the rateslower than the symbol rate
2. Fast Hopingcarrier frequency changes faster than the symbol
rate
o GSM implements slow FH Scheme
o Carrier frequency is changed once per time slot
o There are two reasons for frequency hopping
1. Frequency Diversity
2. Interference avoidance
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I t f A id f FH
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Interference Avoidance of FH
FH averages interference
Allows for tighter reuse of frequencies
Increases the capacity of the system
User 1
User 2
User 3
User 4
User 5
f1
f4
f1
f1
f1
f1
f1
f2
f2
f2
f2
f3
f4
f1
f1
f2
f3
f3
f4
f1
f4
f3
f1
f3
f4
4TT 2T 3T 5T
4TT 2T 3T 5T
4TT 2T 3T 5T
4TT 2T 3T 5T
4TT 2T 3T 5T
B b d FH i GSM
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Baseband FH in GSM
Each radio operates on a
fixed frequency The bursts are routed to
individual radios inaccordance to their hoppingsequence
Advantages of baseband hopping
No need to real time retune simpler
radios
More efficient combiners
Disadvantage of baseband hopping
Number of hopping frequencies limited
by the number of radios
TX/RX
TX/RX
TX/RX
Carrier
Freuqnacy
f1
CombinerCarrier
Freuqnacy
f2
Carrier
Freuqnacy
fn
1
2
n
Bus for Routingand Switchning
S th i d FH i GSM
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Synthesized FH in GSM
Each radio is hopping in anindependent way
Radios retunereal time
Advantages of synthesized hopping:
Set of the hopping frequencies can be assigned in
an arbitrary way
Disadvantage of synthesized hopping:
Need for expensive and lossy combiners
TX/RX
TX/RX
TX/RX
Carrier
Freuqnacyf0,f
1,...,f
m
Broadband
Combiner
1
2
n
CarrierFreuqnacyf0,f
1,...,f
m
CarrierFreuqnacy
f0,f
1,...,f
m
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I t ll H d ff
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Intracell Handoff
High Interference
Measurement indicates:
o Poor RXQUAL
o Good RXLEV
There is high probability that the call will improve with the handoff to
different carrier within the same cell
To avoid unnecessary handoffs, system introduces maximum number
of intercell handoffs
GSM RF Pl i / D i
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GSM RF Planning / Design
Link Budget and Nominal Cell Radius Calculation
Receiver Sensitivity
Required C/I ratio
Mobile Transmit Power
Examples of Link Budget
Calculation of a Nominal Cell Radius
Frequency Planning and Reuse Strategies
Frequency Planning Using Regular Schemes
Automatic Frequency Planning
Capacity of GSM Networks
GSM Mi ti T d 3G
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Migration:
1. High speed circuits
switched data
(HSCSD)
2. Packet switched data
(GPRS,EDGE)
3. Integrated packet
servicespossibly
under different access
scheme (UMTS)
GSM Migration Towards 3G
GSM 2+
9.6 Kb/sec
HSCSD
64 Kb/sec
GPRS
114 Kb/sec
EDGE
384 Kb/sec
UMTS
2Mb/sec
1999 1Q2000
2Q2000
3Q2001
4Q2002
Timeline
Data Rate
HSCSD - High Speed Circuit Switched Data
GPRS - General Packet Radio System
EDGE - Enhanced Data GSM Environment
UMTS - Universal Mobile Telephone Service
GSM 2+ D t S i
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GSM 2+ Data Services
GSMs traffic channel can support the data transfer of a bit rate up to9.6Kb/sec
o This data rate can be used for:
Short messages
Fax services
E-mail, etc.
o Circuit switched data services
o Not suitable for Internet
Too slow
Too costly (user would pay for the circuit even if there is notraffic exchanged
High Speed Circuit Switched Data (HSCSD)
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High Speed Circuit Switched Data (HSCSD)
HSCSD is using existing GSM organization to provide dataservices of a somewhat higher data rates
It can combine several existing traffic channels into a singleconnection, i.e., it allows for mobiles multislot operation
HSCSD can be implemented through software upgrades onexisting networks and no hardware upgrades are needed
Seems to be less accepted by the service providers
General Packed Radio Data (GPRS)
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GPRS is another new transmission capability for GSM that will be especiallydeveloped to accommodate for high-bandwidth data traffic
GPRS will handle rates from 14.4Kbps using just one TDMA slot, and up to
115Kbps and higher using all eight time slots
It introduces packet switching - can accommodate the data traffic
characteristics
General Packed Radio Data (GPRS)
GPRS Network architecture
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GPRS Network architecture
New type ofnode:
GPRS ServiceNode (GSN)
BSC
BSC
MSC
VLR
HLR
AUC
EIR
BTS
BTS
BTS
BTS
BTS - Base Station
BSC - Base Station Contoller
MSC - Mobile Switching Center
VLR - Visitor Location Register
HLR - Home Location Register
AUC - Authentification Center
EIR - Equipment Identity Register
Um
Interface
A-Bis
Interface
A
Interface
D
C
PSTN
B
B,C,D,E,F - MAP
Interfaces
SGSN
GGSN
SGSN - Service GPRS Support Node
GGSN - Gateway GPRS Support Node
Gn
Interface
Gr
Outside
Packet
Network
GPRS Call routing
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GPRS Call routing
SGSN
GGSN
GGSN
SGSN
BTS
BTS
GPRS - PDN
GPRS - PDN
Routing is performed parallel to the GSM network
Enhanced Data GSM Environment (EDGE)
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Packet switched
Upgrades the modulation schemeo From GMSK to 8-PSK
o Maximum speed ~59 Kb/sec per time slot, ~473.6 Kb/sec for all 8 time slots
o Variable data ratedepending on the channel conditions
Defines several different classes of service and mobile terminals
Enhanced Data GSM Environment (EDGE)
EDGE enabled data mobile
Practically achievable data rates
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Practically achievable data rates
Theoretical rates are constrained by mobilepower and processing capabilities
Most mobiles support less than the maximumallowed by standard
Practically achievable data rates
Universal Mobile Telephone Service (UMTS)
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UMTS3G cellular service
Provides data rates up to 2Mb/sec
Possibly standardized as W-CDMA
Universal Mobile Telephone Service (UMTS)
Outline of UMTS(WCDMA) network
Recommended