6 Air Interface

8/9/2019 6 Air Interface

http://slidepdf.com/reader/full/6-air-interface 1/59

1

GSM AIR INTERFACE



2

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



3

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



4

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



5

BTS

BTS

TimeSLot 0

TSL 1TSL 2TSL 3

TSL 4

TSL 5

TSL 6

TSL 7

Principle of Time Division Multiple Access (TDMA)



6

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



7

Digital signal

Amplitude modulation

Frequency modulation

0 1 0

Modulation techniques



8

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



9

Digital signal

Phase modulation

0 1 01

-900

-900

+900

+900

3.69µ s

Phase modulation



10

• 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



11

8 seats in each vehicle

Logistical problem



12

• 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)



13

• Channel to transmit mobile station’s request to initiate

call set-up.• Channel to set up a call.

• Channel to transmit handover information.

BTS


RequestRequest

ChannelallocationChannelallocation

TrafficTraffic




14

• 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.


PagingPaging

Answer Answer

TrafficTrafficBTSBTS BTSBTS




15

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



16

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



17

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



18

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



19

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



20

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



21

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



22

... ...

Bursts from Mobile StationsBursts from Mobile Stations

BTSBTS

2Mbit/s to BSC2Mbit/s to BSC

TDMA Time SlotTDMA Time Slot


Bursts and time slots in the Air Interface

GSM burst types (1)



23

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)



24

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



25

• 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



26

Inter symbol interference

BTSBTS

RX sensitivity

Fading dips caused bymultipath propagation

Approx.17cm

Fading dips

Multipath propagation

Speech processing flow



27

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



28

F2

F1

F3

F4

Time

Frequency hopping

Antenna receiver diversity



29

Received signal

RXRX

Signal

Processing

Antennas

Approx. 6m (GSM-900)

Approx. 3m (GSM-1800)

Antenna receiver diversity

Shadowing



30

Solution: Adaptive power control

BTSBTS

Shadowing

Propagation delay and TA



31

allocated time slot

BTS

Solution using adaptive frame alignment

allocated time slot

Effect due to propagation delay

BTS

Propagation delay and TA

BTS configurations



32

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



33

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)



34

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



35

Exchange1

Exchange2

125 µs time frame

32 time slots

TS 0used for synchronisationand alarms

TS 16often used for commonchannel signalling

Transcoder positions



36

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



37

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



38

TC

64 kbps

16 (13+3) kbps

13 kbps16 kbps

16 kbps

TCSM (Transcoder / Submultiplexer)

MSC

BTS

BSC

SMUX

TC

TC

p



39

Network Planning

Factors affecting network planning



40

• 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



41

Network planning steps



42

• 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



43

• 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



44

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



45

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



46

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



47

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



48

•

•

•

•

••

• •

•

•

•

•

•

• •

•



Network optimization and monitoring



50

• 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



51



52

Review Questions to

„GSM Air Interface & Network

Planning“

Review



53

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



54

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



55

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.

Review



56

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



57

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.

Review



58

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

Review



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.

6 Air Interface

Documents

Transcript of 6 Air Interface