Intro to GSM - Slides (Rev 1)
Transcript of Intro to GSM - Slides (Rev 1)
ECE DepartmentFlorida Institute of Technology
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, Finland’s 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 Spécial 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 of spectrum in 1900MHz
GSM started deployment in PCS band
1995, American Personal Communications launched first GSM network
In 2002, 850 band opened for GSM
Currently there are ~ 95M GSM subscribers
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 coordinated by ETSI
www.etsi.org
Specifications available online – free of charge
Standardization and public availability of specification - one of fundamental factors of GSM success
Series Specifications area
01 General
02 Service aspects
03 Network aspects
04 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 internetworking
10 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
MSCArea
H LR
MSCArea
VLR
MSCTRAUBSC
BTS
BTS
BSS
MSC Area
BSS
BSSBTS
PSTN
PLMN - Public Land M obile Netw ork
Gatew ayM SC
NSS
•NSS: Network Switching subsystem.•BSS: Base station Subsystem.
•BTS: Base Transceiver Station. •TRAU :Transcoder and Adaptation Unit
•BSC: Base Station Controller.•EIR: Equipment Identity Register
•MSC: Mobile Switching Center.•HLR: Home Location Register
•VLR: Visiting Location Register.•AUC: Authentication Center.
•OMC: Operation & Maintenance Center.
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GSM Components and Interfaces
Network has many functional components
Components are integrated through a network protocol – MAP
Standardized interfaces
Um (air interface)
A – GERAN interface
A-Bis (somewhat standardized)
BTS
BSC MSC
VLR
EIR
VLR
Gatew ayMSCM S
H LR
E
FF
B B
C
D D
G
A -
In
terf
ace
Ab
is -
In
terf
ace
Air
- I
nte
rfac
e
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Mobile Station (MS)
Two functional parts
o HW and SW specific for GSM radio interface
o Subscriber Identity Module (SIM)
SIM – detaches user identity from the mobile
o Stores user information
o Without SIM – only emergency calls
Functional diagram of GSM mobile
SIM card
Keyboard
Control
Display
Transmit AudioSignal
Processing
Receive AudioSignal
Processing
ChannelDecoding
DeinterleavingM essage
Regenerator
ChannelEncoding
InterleavingM essage
Generator
Ciphering
Ciphering
RFProcessing
RFProcessing
SIM
Duplexer
Antenna
ANTENNAASSEM BLY
TRANSM ITTER
RECEIVER
TRANSCEIVER UNITCONTROLSECTION
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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 of processing from MSC
One BSC connects to several tens to couple of hundred BTS
Some of BSC responsibilities:o Handoff managemento MAHO managemento Power control o Clock distributiono 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 backhaul – 4 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 network – Gateway MSC
(GMSC)
All calls from the “outside networks” are routed
through GMSC
Mobile Switching Center (MSC)
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Registry HLR/VLR
HLR – Home 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
VLR – Visitor 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
AUC – Authentication center
Integral part of HLR
GSM specifies elaborate encryption
Three levels
o A5/1 USA + Europe
o A5/2 COCOM country list
o No encryption – rest of the world
EIR – Equipment Identity Registry
Responsible for tracking equipment and eligibility for service
Maintains three lists
o White list – approved mobile types
o Black list – barred mobile types
o Gray list – tracked mobile types
Over years – many 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 be accommodated
o Easier integration with the landline
Disadvantages:
o High sync overhead
o Guard times
o Heavily affected by the multipath propagation
Uplink ( From MS to BS)
Wireless Communication Channel
Downlink ( From BS to MS )
Base Station
fu0, s1
fd0, s1, s 2, ...,s 8
S 1 S 2 S 3 .... S 8 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 a combination of FDMA and TDMA
TDMA supports:
o Up to 8 full rate users
o Up to 16 half rate users
GSM uses Frequency Division Duplexing BTS
USER 1 USER 2 .... USER 8
USER 6 USER 7 USER 8 USER 1
USER 1,ARFCN 1
USER 2,ARFCN 1
USER 8,ARFCN 1
USER 9,ARFCN 2
USER 10,ARFCN 2
USER 16,ARFCN 2
ARFCN 1
ARFCN 2
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GSM bursts
Data sent over one time slot = burst
Five types: normal, frequency correction, synchronization, dummy, access
Format of a burst defied by its function
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 control channel
Inserting the F-bursts on the control channel produces spectral peak 67.7 KHz above 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 length – burst collision avoidance
Extended synchronization sequence
Used only on the reverse link
GSM mobiles use slotted ALOHA to access the system
In the case of collision – a hashing algorithm is provided
Tail Synchronization Access Bits Tail
8 41 36 3
Access burst
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GSM TDMA Hierarchical Organization
0 1 2 3 4 5 6 7 21 22 23 24 25
1 TDM A Frame4.615 ms
26 M ultiframe120 ms
51 M ultiframe235.4 ms
51 x 26 Superframe or 26 x 51 Superframe6s 120 ms
Hyperframe3 h 28 min 53 s 760 ms
0 1 2 3 4 48 49 50
0 1 2 3 4 5 6 7 2043 2044 2045 2046 2047
0 1 2 3 4 5 6 7 46 47 48 49 50
0 1 2 3 4 23 24 25
0 1 2 3 4 5 6 7
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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
F o rw a rd L ink - B T S T ra nsm its
R eve rse L in k - M S T ra nsmits
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GSM Logical Channels
G S M Logic a l Channels
TCH
TCH /F TCH /H
CC H
B C H CC CH DC CH
CB CH
A C CH S D CCH
F A C CHS A CCH
F CC H
S C H
B C CH
P C H
A G CH
RA CH
TCH - Traf f ic Channel
TCH/F - Traf f ic Channel (Full Rate)
TCH/H - Traf f ic Channel (Half Rate)BCH - Broadc as t Channels
FCCH - Frequenc y Correc tion Channel
SCH - Sy nc hroniz ation Channel
BCCH - Broadc as t Contro l Channel
CCCH - Common Contro l Channels
PCH - Paging Channel
A GCH - A c c es s Grant Channel
RA CH - Random A c c es s Channel
DCCH - Dedic ated Contro l Channels
SDCCH - Stand-a lone Dedic ated Contro l ChannelA CCH - A s s oc iated Contro l Channels
SA CCH - Slow A s s oc iated Contro l Channel
FA CCH - Fas t A s s oc iated Contro l Channel
CCH - Contro l Channel
CBCH - Cell Broadc as t 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 mobile system acquisition, frame synchronization, etc. They advertise properties and services of the GSM network.
Forward link only
2. Common Control Channels (CCCH)
Facilitate establishment of the link between MS and system
Both forward and reverse link
3. Dedicated Control Channels (DCCH)
Provide for exchange the control information when the call is in progress
Both forward and reverse – in band signaling
C C H
BC H
C C C H
D C C H
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Broadcast Channels (BCH)
Three types of BCH:
1. Synchronization channel (SCH)
Provides a known sequence that helps mobile synchronization
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, access parameters, neighbor list, etc.
BC H
SC H
FC H
BC C H
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Broadcast Channels (BCH) cont’d.
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
C C C H
R AC H
PC H
AG C
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Dedicated Control Channels (DCCH)
Three types of DCCH:
1. Stand Alone Dedicated Control Channel (SDCCH)
Used to exchange overhead information when
the call is not in progress
2. Slow Associated Control Channel (SACCH)
Used to exchange time delay tolerant overhead
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 to point
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 randomly distributed in space
Timing advance prevents burst collision on the reverse link
Maximum advancement is 63 bits
BTS
SLOT 0 SLOT 1 SLOT 2 SLOT 3 SLOT 4 SLOT 5
MS 2
MS 1
d2, Slot 2
d1, Slot 1
d1 > d 2MS 2
MS 1
T 1
T 2
Collision
T 1 - Delay of MS 1
Signal
T 2 - Delay of MS 2
Signal
SLOT 7SLOT 6
km35bit
s10693.3bit63
s
m103
2
1max 68
D
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Signal Processing – From Voice to Radio Waves
Sampling,Quantization andsource encoding
ChannelEncoding
(Error CorrectionCoding)
InterleavingBurst
FormatingMapping
De-Ciphering
Modulation
De-Modulation
Ciphering
BurstFormatingMapping
De-Interleaving
ChannelDecoding
(Error Correction )
Source Decodingand Waveform
Generation
UmInterface
VoiceSignal
VoiceSignal
Transmit Side
Receive Side
As a digital TDMA technology GSM implements extensive signal processing
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Sampling and Quantization
Sampling
o Sampling theorem specifies conditions for discretization of band limited analog signals
o Voice needs to be sampled at the sampling rate greater then 8000Hz
Quantization
o Discrete values assigned to continuous samples
o Quantization noise
o In GSM, voice is sampled at 8 K samples/sec and quantized with 8192 levels (13 bit words)
111 +3V110 +2V101 +1V
0V001 -1V010 -2V011 -3V
111 +3V110 +2V101 +1V
0V001 -1V010 -2V011 -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 rate needed 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/Dconverter
SPEECH
ENCODER
CHANNEL
CODING
TO
MODULATORMICROPHONE
BAND-PASS300 Hz-3.4 kHz
SPEECH
DECODER
CHANNEL
DECODERLP
LOW-PASS4 kHz
D/Aconverter
Vocoders enable efficient channel utilization
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Performance comparison of some commercial vocoders
Mean Opinion Scores (MOS) - Voice Qualitysource IIR. The First Annual CDMA Congress
London, Oct. 29-30, 1997
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
Clean Speech 20dB SNRBabble
20dB SNRCar
15dB SNRStreet
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 the signal
ECC increases the overhead and reduces the efficiency of the communication
In GSM, the ECC increases the overhead per user by 57%
Channel Encoding
TYPE IaBITS
TYPE IIBITS
TYPE IbBITS
CONVOLUTIONAL
ENCODER
r=1/2
K=5
MU
XERROR DETECTING CODE
50
132
78
3
4
189
189
378
456
0TO
INTERLEAVER
FRO
M V
OC
OD
ER
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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 writtencolumn-wise
Data is readrow-wise
Interleaver
b1 b 2 b 3 b 4 b5 b 6 b 7 b 8 b 9 b 10 b 11 b 12 b 13 b 14 b 25 b 16 b 17 b 18 b 19 b 20 ...
b1 b 6 b 11 b 16 b21 b 2 b 7 b 12 b 17 b 22 b3 b 8 b 13 b 18 b 23 b 4 b 9 b 14 b 19 b 24 ..
Burst ErrorCaused by
Rayleigh Fading
Errors are spread over the bit stream
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Modulation: GMSK (Gaussian MSK)
GMSK has excellent spectral characteristics
o Low sidelobes
o Robust to non- linearities
Price paid is in the increased Inter Symbol Interference (ISI)
Simplified GMSK block diagram
MSK
Filtered MSKGMSK
(f-f o) / Rb0 1 2 3
-80
-60
-40
-20
0
POWER SPECTRALDENSITY
dBSpectral characteristics of GMSK
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Tail Traffic/Signaling Flag Training Sequence Flag Traffic/Signaling Tail
3 57 1 26 1 57 3
Sequence used for equalizer training
Equalization
Necessary due to the multipath propagation
Needs to have :
o Fast convergence
o Low complexity
Two modes of operation
1. Training
2. Equalization
GSM equalizer capable of equalizing for two equal multi paths separated by 16 microseconds
Introduces overhead of about 18%
RFProcessing
AdaptiveEqualizer
EqualizationAlgorithmExtraction of
SynchronizationBits
UnequalizedData
EqualizedData
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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 Implements MAHO
In the process of evaluating handoff candidates, GSM systems evaluate measurements performed by both the MS and BTS
There are three types of measurements:
1. Signal Strength Measurements
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 typeMeasurement type
LinkLink CellCell DTXDTX Measurement SourceMeasurement Source
RSLRSL DownlinkDownlink Serving CellServing Cell Full SetFull Set MobileMobile
RSLRSL DownlinkDownlink Serving CellServing Cell SubsetSubset MobileMobile
RSLRSL DownlinkDownlink NeighborsNeighbors N/AN/A MobileMobile
QualityQuality DownlinkDownlink Serving CellServing Cell Full SetFull Set MobileMobile
QualityQuality DownlinkDownlink Serving CellServing Cell SubsetSubset MobileMobile
RSLRSL UplinkUplink Serving CellServing Cell Full SetFull Set BTSBTS
RSLRSL UplinkUplink Serving CellServing Cell SubsetSubset BTSBTS
RSLRSL UplinkUplink NeighborsNeighbors Full SetFull Set BTSBTS
RSL RSL Uplink Uplink NeighborsNeighbors SubsetSubset BTSBTS
QualityQuality UplinkUplink Serving CellServing Cell Full SetFull Set BTSBTS
QualityQuality UplinkUplink 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, measurement performed 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 the neighbors are performed on the BCCH channels – not affected by the DTX
Measurements on the serving channel – affected by the DTX.
Perform over a subset of SACCH that guarantees transmission even in the case of active DTX
Before processing, the RXLEV measurements are filtered to prevent unnecessary handoffs
-100
-90
-80
-70
-60
-50
-40
0 500 1000 1500 2000
Measurement
RX
LE
V (
dB
m)
510
520
530
540
550
560
570
580
BC
CH
AR
FC
N
RX LEV (dBm) BCCH
Example RSL measurement
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MAHO – Signal 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 the
handoff processing If DTX is active, the measurements are
performed 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 BERBER
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 estimate distance to the MS
Expressed in number of bits of TX advancement
Can be between 0 and 63
TA
MAHO – Time Alignment Measurement
ECE DepartmentFlorida Institute of Technology Page 50
Discontinuous Transmission (DTX)
Typical voice activity is around 60% DTX discontinues transmission during
silent periods Benefits of DTX
o Uplink: System interference reduction Lower battery consumption
o Downlink System interference reduction Reduction of the intermodulation
products 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 Typical voice
activityHandset Quiet location 55%
Handset Moderate office noise with voice interference
60%
Handset Strong voice interference (ex. airport,
railway station)
65-70%
Hands free / handset
Variable vehicle noise 60%
ECE DepartmentFlorida Institute of Technology Page 51
Dynamic Power Control (DPC)
There are three reasons for DPC:1. Reduction of battery consumption
2. Elimination of “near-far” problem
3. Reduction of system interference
Power Class GSM (900MHz)[W]
PCS-1900 / GSM – 1800[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 / GSM – 1800[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 / GSM – 1800[W]
PCS-1900 / GSM – 1800[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
ECE DepartmentFlorida Institute of Technology Page 52
Dynamic Power Control (DPC)
DPC for MSo 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 BTSo Vendor specific
o Based on MAHO reports
ECE DepartmentFlorida Institute of Technology Page 53
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 assign preference levels between the cells
Very effective way for capacity and interference management
SignalS trength
ReselectionPoints
Select M icro-Cell
SS_SUFF
M acrocel
PreferredM icro-Cell
D istance
HL = 1
HL = 2
ECE DepartmentFlorida Institute of Technology Page 54
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 penalty – prevents
immediate handoff initialization
If the duration of mobile stay within the
coverage area is shorter than the temporal
penalty, it will never initialize handoff
ECE DepartmentFlorida Institute of Technology Page 55
Frequency Hopping (FH)
FH - multiple carriers used over the course of radio transmission
o There are two kinds of FH:
1. Slow Hopping – change of carrier frequency happens at the rate slower than the symbol rate
2. Fast Hoping – carrier 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
ECE DepartmentFlorida Institute of Technology Page 56
Frequency Diversity of FH
Mobile environment is characterized with small scale fading
The depth of signal fade is a function frequency
If two signals are sufficiently separated in frequency domain they fade independently
Frequency diversity gain diminishes for fast moving mobiles
ECE DepartmentFlorida Institute of Technology Page 57
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
f3f4
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
ECE DepartmentFlorida Institute of Technology Page 58
Baseband FH in GSM
Each radio operates on a fixed frequency
The bursts are routed to individual radios in accordance to their hopping sequence
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
CarrierFreuqnacy
f1
CombinerCarrier
Freuqnacyf2
CarrierFreuqnacy
fn
1
2
n
Bus for Routingand Switchning
ECE DepartmentFlorida Institute of Technology Page 59
Synthesized FH in GSM
Each radio is hopping in an independent way
Radios retune – “real 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
CarrierFreuqnacyf0,f 1,...,f m
BroadbandCombiner
1
2
n
CarrierFreuqnacyf0,f 1,...,f m
CarrierFreuqnacyf0,f 1,...,f m
ECE DepartmentFlorida Institute of Technology Page 60
FH Algorithms
• Random Hopping• Implemented in a pseudo – random way• Uses one of 63 available pseudorandom sequences• The actual frequency is obtained as a modulo
operation with number of available frequencies in allocation list (FH group)
,,,,,,, 3214321 fffffff
,,,,,,, 3234421 fffffff
Cyclic Hopping
o Frequencies are used in the consecutive order
o If the radio is performing cyclic FH the order of frequencies in the sequence goes from the lowest ARFCN to the highest ARFCN
ECE DepartmentFlorida Institute of Technology Page 61
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
ECE DepartmentFlorida Institute of Technology Page 62
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
ECE DepartmentFlorida Institute of Technology Page 63
Migration:
1. High speed circuits switched data (HSCSD)
2. Packet switched data (GPRS,EDGE)
3. Integrated packet services – possibly under different access scheme (UMTS)
GSM Migration Towards 3G
G SM 2+9.6 Kb/sec
H S C S D64 K b/sec
G P R S114 K b/sec
E D G E384 K b/sec
U M TS2M b/sec
19991Q
20002Q
20003Q
20014Q
2002
T im eline
D ata R ate
H S C S D - H igh S peed C ircu it S w itched D ataG P R S - G enera l P acket R ad io S ystemE D G E - E nhanced D ata G S M E nvironm entU M TS - U n iversa l M ob ile T e lephone S ervice
ECE DepartmentFlorida Institute of Technology Page 64
GSM 2+ Data Services
GSM’s traffic channel can support the data transfer of a bit rate up to 9.6Kb/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 no traffic exchanged
ECE DepartmentFlorida Institute of Technology Page 65
High Speed Circuit Switched Data (HSCSD)
HSCSD is using existing GSM organization to provide data services of a somewhat higher data rates
It can combine several existing traffic channels into a single connection, i.e., it allows for mobiles multislot operation
HSCSD can be implemented through software upgrades on existing networks and no hardware upgrades are needed
Seems to be less accepted by the service providers
ECE DepartmentFlorida Institute of Technology Page 66
GPRS is another new transmission capability for GSM that will be especially developed 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)
ECE DepartmentFlorida Institute of Technology Page 67
GPRS Network architecture
New type of node:
GPRS Service Node (GSN)
BSC
BSC
MSC
VLR
HLR
AUC
EIR
BTS
BTS
BTS
BTS
BTS - Base StationBSC - Base Station ContollerMSC - Mobile Switching CenterVLR - Visitor Location RegisterHLR - Home Location RegisterAUC - Authentification CenterEIR - Equipment Identity Register
Um Interface
A-Bis Interface
AInterface
D
C
PSTNB
B,C,D,E,F - MAPInterfaces
SGSN
GGSN
SGSN - Service GPRS Support Node
GGSN - Gateway GPRS Support Node
GnInterface
Gr
OutsidePacket
Network
ECE DepartmentFlorida Institute of Technology Page 68
GPRS Call routing
SGSN
GGSN
GGSN
SGSN
BTS
BTS
GPRS - PDN
GPRS - PDN
Routing is performed “parallel” to the GSM network
ECE DepartmentFlorida Institute of Technology Page 69
Packet switched
Upgrades the modulation scheme
o 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 rate – depending on the channel conditions
Defines several different classes of service and mobile terminals
Enhanced Data GSM Environment (EDGE)
EDGE enabled data mobile
ECE DepartmentFlorida Institute of Technology Page 70
Practically achievable data rates
Theoretical rates are constrained by mobile power and processing capabilities
Most mobiles support less than the maximum allowed by standard
Practically achievable data rates
ECE DepartmentFlorida Institute of Technology Page 71
UMTS – 3G cellular service
Provides data rates up to 2Mb/sec
Possibly standardized as W-CDMA
Universal Mobile Telephone Service (UMTS)
Outline of UMTS (WCDMA) network