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GSM AIR INTERFACE
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Objectives:
At the end of this module, the student is able to:• Explain the difference between physical and logical channels• List and describe at least nine different types of logical channels and
their functions with the help of their abbreviations• Name two problems in the Air Interface and suggest one way of
decreasing each of these problems• Describe the main function of the transcoder
• List three Base Station Controller (BSC) / Base Transceiver Station(BTS) connections• List five steps in the radio network planning process• Explain how frequencies are reused in a GSM network• Name at least one advantage with a sectorised Base Transceiver
Station compared with an omnidirectional BTS• Name three sources of information that can be used when
monitoring the network's performance
GSM Air Interface
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GSM-900 Uplink Downlink
915 MHz890 MHz 935 MHz 960 MHz
GSM-1800 Uplink Downlink
1785 MHz1710 MHz 1805 MHz 1880 MHz
GSM frequency allocations
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GSM 900
Uplink: 890 - 915Mhz
Downlink: 935 - 960Mhz
Carrier pairs (in Mhz)
890.0 935.0
890.2 935.2
890.4 935.4
.... ....
.... ....
914.8 959.8915.0 960.0
Duplex frequency = 45 Mhz
GSM 1800
Uplink: 1710 - 1785 Mhz
Downlink: 1805 - 1880 Mhz
Carrier pairs (in Mhz)
1710.0 1805.0
1710.2 1805.2
1710.4 1805.4
.... ....
.... ....
1784.8 1879.8
1785.0 1880.0
Duplex frequency = 95 Mhz124 carriers
374 carriers
Carrier frequency range
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BTS
BTS
TimeSLot 0
TSL 1TSL 2TSL 3
TSL 4
TSL 5
TSL 6
TSL 7
Principle of Time Division Multiple Access (TDMA)
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GSM 450UL: 450,4 – 457,6 MHz and DL: 460,4 – 467,6 MHzGSM 480UL: 478,8 – 486 MHz and DL: 488,8 – 496 MHzGSM 700UL: 747 – 762 MHz and DL: 777 – 792 MHzGSM 850UL: 824 – 849 MHz and DL: 869 – 894 MHzGSM 900 (standard GSM, P-GSM, Primary GSM)UL: 890 – 915 MHz and DL: 935 – 960 MHzExtended GSM 900 (E-GSM; only in combination with GSM 900)UL: 880 – 915 MHz and DL: 925 – 960 MHzRailway GSM 900 (R-GSM; in combination with GSM)
UL: 876 – 880 MHz and DL: 921 – 925 MHzGSM 1800 (DCS 1800)UL: 1710 – 1785 MHz and DL: 1805 – 1880 MHzGSM 1900 (PCS 1900)UL: 1850 – 1910MHz and DL: 1930 – 1990 MHz
GSM frequency bands
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Digital signal
Amplitude modulation
Frequency modulation
0 1 0
Modulation techniques
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Digital signal
Phase modulation
Example: Binary Phase Shift Keying (BPSK)
duration of one bit duration of one bit
00 phase shift=> Interpretation
1800 phase shift=> Interpretation
1
1Digital signal1
0
Phase modulation
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Digital signal
Phase modulation
0 1 01
-900
-900
+900
+900
3.69µ s
Phase modulation
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• Phase changes of +/- 90 degrees for bit changes (new bit values)
• The phase change is done gradually over a small time period,
meaning that:
The change is not as abrupt as in BPSK
The Mobile Station causes less inter-frequency interference
• In GSM, one bit duration is 3.69 µ s (0.00000369 seconds).
GMSK – Gaussian Minimum Shift Keying
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8 seats in each vehicle
Logistical problem
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• Channel to transmit information to help the mobilestation to tune into the network.
• Channel to transmit synchronisation information.
• Channel to transmit information about the network to help the mobile know about the frequenciesbeing used in its cell as well as in surrounding cells.
BTS
BTS
TDMA FrameTDMA Frame
Sync.
Information
Sync.
Information
Number of channels required during call set-up (1)
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• Channel to transmit mobile station’s request to initiate
call set-up.• Channel to set up a call.
• Channel to transmit handover information.
BTS
TDMA FrameTDMA Frame
RequestRequest
ChannelallocationChannelallocation
TrafficTraffic
Number of channels required during call set-up (2)
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• Channel to page the called party.
• Channel to transmit measurements.Conclusion: No channel left for conversation!
Solution: We must send more than one type of information on a channel by sharing it.
TDMA FrameTDMA Frame
PagingPaging
Answer Answer
TrafficTrafficBTSBTS BTSBTS
Number of channels required during call set-up (3)
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COMMON
CHANNELSCOMMON
CHANNELS
BROADCAST
CHANNELSBROADCAST
CHANNELS
COMMON
CONTROL
CHANNELS
COMMON
CONTROL
CHANNELS
DEDICATED
CONTROL
CHANNELS
DEDICATED
CONTROL
CHANNELS
TRAFFIC
CHANNELSTRAFFIC
CHANNELS
FCCHFCCH SCHSCH BCCHBCCHSDCCHSDCCH SACCHSACCH
FACCHFACCH
PCHPCH RACHRACH AGCHAGCH TCH/FTCH/F TCH/HTCH/H TCH/EFRTCH/EFR
DEDICATED
CHANNELSDEDICATED
CHANNELS
LOGICAL
CHANNELS
LOGICAL
CHANNELS
Logical channels
L i l h l i TDMA f
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FCCHSCH
BCCHBCCHBCCHBCCH
PCH
AGCH
012345
50 IDLE
0 1 2 3 4 5 6 7
51-FrameMultiframe
51-Frame
Multiframe
TCH/FTCH/FTCH/FTCH/F
TCH/FSACCHTCH/F
IDLE
0123
11
26-Frame
Multiframe
26-FrameMultiframe
1213
25TCH/F24
FCCH
SCH
SDCCH
SACCH
...
...
...
...
...
...
...
...
...
...
...
...
..
.
...
Logical channels in TDMA frames
B d t h l
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Frequency Correction Channel
(FCCH)– Pure sine wave.
– The MS searches for this channels to switch on.
– Downlink.
Synchronisation Channel (SCH)– After locking to the frequency the MS
synchronises with the SCH.
– The SCH contains the BSIC of the BTS and theTDMA frame number (used in encryption).
Broadcast channels
B d t h l
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Broadcast Control Channel (BCCH)
– Common information about the BTS:– Used frequencies
– Frequency hopping sequence
– Channel combination
– Paging groups
– Surrounding cell information
Broadcast channels
C t l h l
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Paging Channel (PCH)– Used by BTS to page a mobile.
– A downlink channel only.
Random Access Channel (RACH)– Used by the MS to request a dedicated control
channel.
– Used for e.g. mobile originated calls.
– An uplink channel only.
Access Grant Channel (AGCH)– Used by the BTS to assign a dedicated control
channel.
– A downlink channel only.
Common control channels
D di t d h l
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Stand Alone Dedicated Control Channel (SDCCH)– Bi-directional channel.
– Used for call set-up procedures, e.g. authentication.
– The traffic channel (TCH) is assigned by using SDCCH.
Slow Associated Control Channel (SACCH)– Associated with SDCCH and TCH.
– Measurement reports.
– MS power control.
– Timing alignment.
Fast Associated Control Channel (FACCH)– Associated with TCH.
– For quick control communication, e.g. handover.
– Physically replaces 20 ms of speech, “stealing mode”
Dedicated channels
Traffic channels
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Full Rate
– Bi-directional channel.– Used for speech or data transmission.
– User data bit rate 13 kbit/s.
Half Rate– Bi-directional channel.
– Used for speech or data transmission.
– User data bit rate 5.6 kbit/s.
Enhanced Full Rate (EFR)– Bi-directional channel.
– Used for high quality speechtransmission.
– User data bit rate 12.2 kbit/s.
Traffic channels
Bursts and time slots in the Air Interface
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... ...
Bursts from Mobile StationsBursts from Mobile Stations
BTSBTS
2Mbit/s to BSC2Mbit/s to BSC
TDMA Time SlotTDMA Time Slot
TDMA FrameTDMA Frame
Bursts and time slots in the Air Interface
GSM burst types (1)
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tailbits
3
fixed bits ("0")142
tailbits
3
guardperiod
8,25 bits
FREQUENCY CORRECTION BURST
tailbits
3
encrypted bits57
SB1
trainingsequence
26
SB1
encrypted bits57
tailbits
3
guardperiod
8,25 bits
NORMAL BURST
TDMA FRAME ~ 4.615 ms
576.9 µs
GSM burst types (1)
GSM burst types (2)
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ext. tailbits
8
synchronisationsequence
41
encrypted bits36
tailbits
3
extended guard period68,25 bits
ACCESS BURST
tailbits3
encrypted bits39
extended trainingsequence64
encrypted bits39
tailbits3
guardperiod8,25bits
SYNCHRONISATION BURST
tailbits
3
mixed bits142
tailbits
3
guardperiod
8,25 bits
DUMMY BURST
GSM burst types (2)
Burst types
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• Frequency correction burstUsed to transmit the FCCH channel. No information.
• Synchronisation burstUsed to transmit synchronisation information.
• Access burstUsed to send RACH information.
RACH contains the first message from the MS to the BTS.
It has a long guard period to allow the BTS to calculate the MS distance from the BTS
and to provide timing advance information to the MS.
• Normal burstUsed to send all other logical channel information.
• Dummy burstUsed to fill up unused timeslots in the TRX, which transmits the BCCH channel.No real information.
Burst types
Multipath propagation
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Inter symbol interference
BTSBTS
RX sensitivity
Fading dips caused bymultipath propagation
Approx.17cm
Fading dips
Multipath propagation
Speech processing flow
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SpeechDigitising andsource coding
Channelcoding
Interleavingand ciphering
TDMA burstformatting
GMSKmodulation
22.8kbit/s
13kbit/s
33.8kbit/s
22.8kbit/sAir
Interface
Speech processing flow
Frequency hopping
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F2
F1
F3
F4
Time
Frequency hopping
Antenna receiver diversity
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Received signal
RXRX
Signal
Processing
Antennas
Approx. 6m (GSM-900)
Approx. 3m (GSM-1800)
Antenna receiver diversity
Shadowing
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Solution: Adaptive power control
BTSBTS
Shadowing
Propagation delay and TA
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allocated time slot
BTS
Solution using adaptive frame alignment
allocated time slot
Effect due to propagation delay
BTS
Propagation delay and TA
BTS configurations
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Omnidirectional BTS
f1,f2, f3
3 sectorised BTS
2 sectorised BTS
f2
f1, f2
f5, f6
f1
f3, f4
BTSBTS
BTS
BTS
BTS BTS
BTS configurations
BSC - BTS connections
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BSC
BTS BTS BTS
BTSBTSBTS
BTS BTS BTS
BTS
Point to point connection
Multi drop chain
Multi drop loop
BSC BTS connections
Pulse Code Modulation (PCM)
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o
oo
o
o
o
o
o
oo
oo
o
1
8000
Time(8000 samples / second)125 µ s
Amplitude300 - 3400 Hz 64000 bit / sec
8000 Hz x 8 Bit/s
¶
64,000 Bit/s
A/D-Converter
Pulse Code Modulation (PCM)
PCM30
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Exchange1
Exchange2
125 µs time frame
32 time slots
TS 0used for synchronisationand alarms
TS 16often used for commonchannel signalling
Transcoder positions
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BSCMSC TC
64 kbps 64 kbps 13 kbps
Transcoder is at BTS site
BTS
64 kbps 16 (13+3) kbps
13 kbps
16 (13+3) kbps
Transcoder is at MSC site
BSCMSC TC BTS
p
Transcoder positions
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BSCMSC TC
64 kbps 64 kbps 13 kbps
Transcoder is at BTS site
BTS
64 kbps 16 (13+3) kbps
13 kbps
16 (13+3) kbps
Transcoder is at MSC site
BSCMSC TC BTS
p
Transcoder and Submultiplexer
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TC
64 kbps
16 (13+3) kbps
13 kbps16 kbps
16 kbps
TCSM (Transcoder / Submultiplexer)
MSC
BTS
BSC
SMUX
TC
TC
p
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Network Planning
Factors affecting network planning
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• Intended coverage area
• Location of network elements (MSC, BSC, BTS)
• Quality of calls
• Maximum congestion allowed (grade of service)
• Capacity of the network
• Cost of the infrastructure
• Future development of the network
g g
Cellular radio network planning
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Network planning steps
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• Collection of all relevant information– Demography, penetration forecast, geographical extension forecast, services to be supported,
market segmentation
– Leased lines and microwave frequency availability,connections with other networks (PSTN, Internet)
– Regulations and laws
– Numbering, addressing and routing principles
– Topographical maps
– Existing infrastructure.
• Network dimensioning based on coverage and capacityrequirements.
• Selection of MSC, BSC and BTS sites.
• Survey of intended MSC, BSC and BTS sites.
• Use of a computer aided design system for coverage prediction,
interference analysis, microwave and frequency planning, etc.
Network planning steps
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• Collection of all relevant information– Demography, penetration forecast, geographical extension forecast, services to be supported,
market segmentation
– Leased lines and microwave frequency availability,connections with other networks (PSTN, Internet)
– Regulations and laws
– Numbering, addressing and routing principles
– Topographical maps
– Existing infrastructure.
• Network dimensioning based on coverage and capacityrequirements.
• Selection of MSC, BSC and BTS sites.
• Survey of intended MSC, BSC and BTS sites.
• Use of a computer aided design system for coverage prediction,
interference analysis, microwave and frequency planning, etc.
Switching network planning
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Detailed planning
• Network diagram set
• Detailed routing plan
• Digit analysis
• A detailed signalling plan
• A detailed numbering plan
• A detailed charging plan
• DCN settings
• Synchronisation plan
• Source data
MSC / LEOriginating Outgoing
Terminating Incoming
Network dimensioning
• The performance level
• Switch and signalling network
diagrams• Voice and signalling traffic
matrixes
• Routing planning
• Protection planning
• List of equipment types
• Synchronisation and managementprinciples
Cellular transmission network planning
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Network dimensioning• The general transmission network
diagram for base stations accessand core networks in phases.
• The block diagrams showingcapacity requirements, mediaselection and protection.
• General management network principles with area definitions.
• Gateway and switching network connections.
• Synchronisation principles for the
main network.• Expansion plans includingguidelines for elementswitchovers.
Detailed planning
• Network layout diagram.
• Connections and capacitiesbetween sites.
• Timeslot allocation of each link.
• Routing diagram.
• Branching and cross-connecttables.
• Equipment availability calculation.• The exact synchronisation plan
with sources and hierarchy.
• The settings in the nodes.
• Management network diagramwith defined buses and addresses.
BTS configurations
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Omnidirectional BTS
f1,f2, f3
3 sectorised BTS
2 sectorised BTS
f2
f1, f2
f5, f6
f1
f3, f4
BTS
BTS
BTS
BTS
BTS BTS
Partial Erlang table
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Chs 1% 2% 3% 5%... ..... .... .... .....15 8.11 9.01 9.65 10.60
16 8.88 9.83 10.50 11.5017 9.65 10.70 11.40 12.50
18 10.40 11.50 12.20 13.40
19 11.20 12.30 13.10 14.3020 12.00 13.20 14.00 15.2021 12.80 14.00 14.90 16.20
22 13.70 14.90 15.80 17.10.... ...... ...... ..... .....
.... ...... ...... ..... .....
xcalls per hour average conversation time
ErlangsSeconds
=
×( ) ( )
3600
Frequency reuse exercise
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•
•
•
•
••
• •
•
•
•
•
•
• •
•
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Network optimization and monitoring
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• Grade of service
• Quality of service
• Present and future demand
• Cost efficiency
Need for continuous
monitoring, optimisation and
development of the network
Customer feedback
NMS / Performancemanagement
Field tests
Advantages /
Disadvantages
with each of these
Drive survey tool
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Review Questions to
„GSM Air Interface & Network
Planning“
Review
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a) frequency modulation.
b) amplitude modulation.c) phase modulation.
d) None of the above.
1. Duplex frequency means:
2. The modulation scheme used in GSM is predominantlybased on:
a) the difference between the uplink anddownlink frequency pair.
b) the uplink and downlink frequency pair.
c) twice the uplink or downlink frequency band.
d) GSM 900 and GSM 1800 frequency bands.
Review
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a) inform the mobile station of the frequency
hopping sequence.
b) provide the mobile station the handoverinformation.
c) inform the mobile station of a dedicated
signalling channel.
d) transmit adaptive frame alignment information
to the mobile station.
3. Which of the following are dedicated channels?
4. The function of the AGCH is to:
a) FCCH, SCH, AGCH.b) SDCCH, TCH, SACCH.
c) RACH, FACCH, TCH.
d) BCCH, SDCCH, SACCH.
Review
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a) BCCH.
b) FCCH.c) RACH.
d) AGCH.
5. Short message service is transmitted in:
6. Information about the frequency hopping sequence can befound in the:
a) the SDCCH.b) the SACCH.
c) both the SDCCH and the SACCH.
d) neither the SDCCH nor the SACCH.
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a) eliminates the problem of fading dips.
b) eliminates the problem of inter symbolinterference.
c) is part of channel coding.
d) spreads the problem of fading dips to many
mobile stations.
7. Inter symbol interference is caused by:
8. Frequency hopping:
a) fading dips.b) the Viterbi equaliser.
c) reflection.
d) interleaving.
Review
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a) Intended coverage area.
b) Intended grade of service.c) Cost of the network elements.
d) All of the above.
9. Speech transcoding from 13 to 64 Kbits/s and vice versa is done bya transcoder between which two points?
10. Which of the following are factors in network planning?
a) The BTS and the BSC at the BTS site.
b) The BTS and the BSC at the BSC site.
c) The BSC and the MSC at the MSC site.
d) All above are possible.
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a) the number of available frequencies is lowerthan the number of carrier channels needed for
a financially viable GSM network.
b) the spacing of 200 kHz between carriers
instead of 25 kHz (like in analogue networks)reduces the number of frequencies.
c) it increases the number of subscribers.
d) None of the above is quite correct.
12. Frequency reuse is done in GSM networks, because:
It can be argued that,in special cases,
this wouldbe a valid answer
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13. In a certain PLMN, an average subscriber makes five callsduring office hours (8 AM - 6 PM). It is known that in a certaincell area, there are going be 1000 subscribers, at any givenhour, during these office hours. Assuming that a subscriber’sconversation lasts for 100 seconds, how many TRXs are
needed in this cell to provide a grade of service of 2%?a) 2
b) 3
c) 4
d) There is not enough information given for anexact answer.