ECE DepartmentFlorida Institute of Technology
Wireless Data Communication Networks
Lecture 20: Introduction to GSM
ECE DepartmentFlorida Institute of Technology
Outline
GSM logical channelsRadio resource management
Page 2
ECE DepartmentFlorida Institute of Technology
GSM Logical Channels
Logical channels – provide services to upper layers of the protocol stack
They belong to different layers (1-3) of the OSI model
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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 ChannelACCH - Associated Control Channels
SACCH - Slow Associated Control Channel
FACCH - Fast Associated Control Channel
CCH - Control Channel
CBCH - Cell Broadcast Channel
ECE DepartmentFlorida Institute of Technology
Traffic Channel (TCH)
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Traffic channel carries speech and user data in both directions
o Full rate ~ 33.85 Kb/seco Half rate ~ 16.93 Kb/seco Full rate uses 1 slot in every frameo Half rate uses 1 slot in every other frame
Data rates differ due to differences in Error Control Coding
Most contemporary GSM networks operate AMR vo-coders
Full Rate TCH can carry:• Voice (13 Kb/sec)• Data 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/secAdaptive Multi-Rate Coder (AMR)
• Voice 12.2, 10.2, 7.95 and 7.40 in FR• Voice 6.7, 5.9, 5.15, 4.75 in HR
ECE DepartmentFlorida Institute of Technology
Control Channels
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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 only2. Common Control Channels (CCCH)
Facilitate establishment of the link between MS and system
Both forward and reverse link3. Dedicated Control Channels (DCCH)
Provide for exchange the control information when the call is in progress
Both forward and reverse – in band signaling
CC H
BC H
C C C H
D C C H
ECE DepartmentFlorida Institute of Technology
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.
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BC H
SC H
FC H
BC C H
ECE DepartmentFlorida Institute of Technology
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
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C C C H
R AC H
PC H
AG C
ECE DepartmentFlorida Institute of Technology
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
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DCCH
SDCCH
SACCH
FACCH
ECE DepartmentFlorida Institute of Technology
Logical Channels - Summary
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Channel UL Only
DLOnly
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
SDCCH X X X
SACCH X X X
FACCH X X X
TCH X X X
Channel UL Only
DLOnly
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
SDCCH X X X
SACCH X X X
FACCH X X X
TCH X X X
ChannelChannel UL OnlyUL Only
DLOnlyDLOnly
UL/DL UL/DL Point to Point
Point to Point
BroadcastBroadcast DedicatedDedicated SharedShared
BCCHBCCH XX XX XX
FCCHFCCH XX XX XX
SCHSCH XX XX XX
RACHRACH XX XX XX
PCHPCH XX XX XX
AGCHAGCH XX XX XX
SDCCHSDCCH XX XX XX
SACCHSACCH XX XX XX
FACCHFACCH XX XX XX
TCHTCH XX XX XX
UL - Uplink DL - Downlink
ECE DepartmentFlorida Institute of Technology
GSM Radio Resource Management (RRM)
Time Advancement (TA)Mobile Assisted Handoff (MAHO)Dynamic Power Control (DPC)Discontinuous Transmission (DTX)Frequency Hopping (FH)
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ECE DepartmentFlorida Institute of Technology
Time Advancement (TA)
Mobiles randomly distributed in space
Timing advance prevents burst collision on the reverse link
Maximum advancement is 63 bits
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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 1Signal
T 2 - Delay of MS 2Signal
SLOT 7SLOT 6
km35bits10693.3bit63
sm103
21max 68
D
Maximum cell radius
ECE DepartmentFlorida Institute of Technology
Mobile Assisted Handoff (MAHO)
GSM Implements MAHO
In the process of evaluating handoff candidates, GSM systems evaluate measurements
Evaluation done at BSC Three types of
measurements Signal strength Signal quality Timing advance
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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
ECE DepartmentFlorida Institute of Technology
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 SACCH
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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
Example MAHO measurements
ECE DepartmentFlorida Institute of Technology
MAHO – Signal Quality Measurements
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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
Mapping between RxQUAL and BER
ECE DepartmentFlorida Institute of Technology
Discontinuous Transmission (DTX) Idea: No voice – no need for transmission Benefits of DTX
o Uplink: System interference reduction Lower battery consumption
o Downlink System interference reduction Reduction of the inter-modulation products Lower power consumptions
Downsides of DTX usage:o MAHO measurements are less accurateo Voice quality is degraded due to slowness of VAD
Implemented both for BTS and MS Uses Voice Activity Detection (VAD) to detect periods of silence
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ECE DepartmentFlorida Institute of Technology
Dynamic Power Control (DPC) Three reasons
o Elimination of near-far problemo Reduction of system interferenceo Improvement of MS battery life
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 specifico Based on MAHO reports
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GSM power classes for some popular bands
ECE DepartmentFlorida Institute of Technology
Frequency hopping (FH)
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FH - multiple carriers used over the course of radio transmission
Two kinds of FH:o Slow Hopping – change of carrier
frequency happens at the rate slower than the symbol rate
o Fast Hoping – carrier frequency changes faster than the symbol rate
GSM implements slow FH Scheme Carrier frequency is changed once per
time slot Two reasons for FH
o Frequency diversityo Interference avoidance
ECE DepartmentFlorida Institute of Technology
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
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ECE DepartmentFlorida Institute of Technology
Interference avoidance of FH
FH averages interferenceAllows for tighter reuse of frequencies Increases the capacity of the system
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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
Synthesized FH in GSM
Synthesized hoppingo The hopping frequencies assigned in an arbitrary wayo Needs relatively expensive broadband combiners
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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
Each radio is hopping in an independent way
Radios retune – “real time”
ECE DepartmentFlorida Institute of Technology
FH Algorithms
Cyclic hoppingo Frequencies are used in consecutive ordero If the radio is performing cyclic FH the order of frequencies in the
sequence goes from the lowest ARFCN to the highest ARFCN
Random hoppingo Implemented in a pseudo-random wayo Uses one of 63 available PN sequenceso The actual frequency obtained through module operation with the total
number of frequencies in the mobile allocation list
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,,,,,,, 3214321 fffffff
,,,,,,, 3234421 fffffff
ECE DepartmentFlorida Institute of Technology
Frequency Planning and Reuse Strategies BCCH Channels – fixed assignment TCH Channels
o Fixed, or o Frequency hopping
Frequency plan is critical for GSM performance
Used plans: 7/21, 4/12 or 3/9 Frequency plan determines the
amount of interference in the system GSM requires at least 9dB of C/I Modern GSM systems implement “ad-
hock” frequency planning obtained from AFP tools
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1
10
100
7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25Required C/I [dB]
Size
of t
he C
lust
er (N
)
n = 3
n = 3.5
n = 4
n = 4.5
n = 5
Relationship between cluster size and C/I
ECE DepartmentFlorida Institute of Technology
Review questions
What is the difference between TCH/FR and TCH/HR?
What is the BCCH channel?What is TCH?Why is time advancing necessary
in GSM?What is MAHO?What is RxLev?What is RxQual?What are advantages of DTX?Can DTX be used on the BCCH?
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How often does GSM mobile adjust its power?
What is the power adjustment step for the mobile?
What is frequency hopping?Can frequency hopping be used on
BCHH?What are the advantages of FH?What are typical reuse schemes
for BCCH in GSM?