chapter4 air interface

8/10/2019 Chapter4 Air Interface

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GSM AIR INTERFACE


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INDEX

CONTENTS

10. GSM Air Interface

a.) TDMA

b.) GMSK

c.) Logical Channels

d.) Burst Formatione.) Multi Frames

f.) Multipath Propagation

g.) Shadowing

h.) BTS Configuration

i.) E1 Linkj.) Transcoder Position


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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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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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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 ms (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 mobile station

to tune into the network. Channel to transmit synchronisation information.

Channel to transmit information about the network tohelp the mobile know about the frequencies being used inits cell as well as in surrounding cells.

BTS

BTS

TDMA Frame

Sync.

Information

Number of channels required during call set-up (1)


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Channel to transmit mobile stations request to initiate call

set-up. Channel to set up a call.

Channel to transmit handover information.

BTS

TDMA Frame

Request

Channelallocation

Traffic



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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 ofinformation on a channel by sharing it.

TDMA Frame

Paging

Answer

TrafficBTS BTS



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COMMON

CHANNELS

BROADCAST

CHANNELSCOMMON

CONTROL

CHANNELS

DEDICATED

CONTROL

CHANNELS

TRAFFIC

CHANNELS

FCCH SCH BCCH SDCCH SACCH FACCH

PCH RACH AGCH TCH/F TCH/H TCH/EFR

DEDICATED

CHANNELS

LOGICAL

CHANNELS

Logical channels


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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 the TDMA

frame number (used in encryption).

Broadcast channels


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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


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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


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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

T ffi h l


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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 speech transmission. User data bit rate 12.2 kbit/s.

Traffic channels

B t d ti l t i th Ai I t f


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... ...

Bursts from Mobile Stations

BTS

2Mbit/s to BSC

TDMA Time Slot

TDMA Frame

Bursts and time slots in the Air Interface


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Burst Period

0 7

TDMA frame = 4.615 ms

= BURST PERIOD

0

0 0

f s

GSM b t t (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 s

GSM burst types (1)

GSM b t t (2)


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ext. tailbits

8

synchronisationsequence

41

encrypted bits36

tailbits

3

extended guard period68,25 bits

ACCESS BURST

tailbits

3

encrypted bits39

extended trainingsequence

64

encrypted bits39

tailbits

3

guardperiod8,25bits

SYNCHRONISATION BURST

tailbits

3

mixed bits142

tailbits

3

guardperiod

8,25 bits

DUMMY BURST

GSM burst types (2)

B rst t pes


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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


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GSM Logical Channels

COMMON

CHANNELSCOMMON

CHANNELS

BROADCAST

CHANNELSBROADCAST

CHANNELS

COMMON

CONTROL

CHANNELS

COMMON

CONTROL

CHANNELS

DEDICATED

CONTROL

CHANNELS

DEDICATED

CONTROL

CHANNELS

TRAFFIC

CHANNELSTRAFFIC

CHANNELS

FCCHFCCH

SCHSCH

BCCHBCCH

SDCCHSDCCH

SACCHSACCH

FACCHFACCH

PCHPCH

RACHRACH

AGCHAGCH

TCH/FTCH/F

TCH/HTCH/H

TCH/EFRTCH/EFR

DEDICATED

CHANNELSDEDICATED

CHANNELS

LOGICAL

CHANNELSLOGICAL

CHANNELS

Logical channels in TDMA frames


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FCCHSCH

BCCHBCCHBCCHBCCH

PCH

AGCH

012345

50 IDLE

0 1 2 3 4 5 6 7

51-FrameMult

iframe

TCH/FTCH/FTCH/FTCH/F

TCH/FSACCHTCH/F

IDLE

0123

11

26-FrameMultiframe

1213

25TCH/F24

FCCH

SCH

SDCCH

SACCH

...

...

...

...

...

...

...

...

...

...

...

...

..

.

...

Logical channels in TDMA frames


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FCCH- Frequency Correction CHannelDownlink channel used to enable MS in finding the slot alignment for demodulatingSCH burst (also allows for frequency correction).

SCH- Synchronisation CHannel

Downlink channel which provides MS will all the necessary information needed for

synchronisation (always follows FCCH burst, 8 BPs later, on same frequency). Alsocarries BTS identification information.

BCCH- Broadcast Control CHannel

Downlink channel used to transmit cell specific information to all MSs within a cellcoverage area, at regular intervals, e.g. frequencies used in a cell & ncells, channelcombination, paging groups, etc. (must be ubiquitous coverage).

PCH- Paging CHannel

Dowlink channel used to send paging messages to MSs (MTC)

AGCH- Access Grant CHannel

Downlink channel used by network to allocate a dedicated control channel to an MS.


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RACH- Random Access CHannel

Uplink Channel used by MS to request a dedicated control channel.

SDCCH- Standalone Dedicated Control CHannel

Bi-directional channel used for system signalling, e.g. call set-up, location updates, SMS.

SACCH- Slow Associated Control CHannel

Bi-directional (low rate) channel used to transport signalling data (two messages per sec~ 1 every 480ms) such as measurement reports for handover process. RLT is alsobased on decoding SACCH.

FACCH- Fast Associated Control CHannel

Bi-directional channel used to transport urgent signalling messages, e.g. command ahandover, authenticate a subscriber, etc.

TCH (F/H)- Traffic CHannel

Bi-directional channel used to carry user speech or data - can be either full rate or halfrate.

CBCH- Cell Broadcast CHannel

Downlink channel used only for general (non point-to-point) short message information.


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Configuration of Signalling Channels

0 7

Non-combined Configuration

Combined Configuration

0 7

ts0=bcch + pch + agch ts1=sdcch/8

ts0=bcch + sdcch/4 + pch + agch

1


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BCCH/CCCH Multiframe

f s bb b b c fc fc s c c c c cc c c fc fs c c c c cc c c f fc c c c cc c c fs fc c c c cc c cs

r r rr r r r fr rr r r r r r rr r r rr fr r r r r rr r r r fr r r r rr r r rr fr r r r rr r rr

Downlink

Uplink

CHANNELS: f = FCCH b = BCCH r = RACH i=idle

s = SCH c = CCCH = PCH/AGCH

51 TDMA frames = 235 ms

r

0 50

i


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SDCCH/8 Multiframe

t t tt t t t ft tt t t t t t tt t t tt ft t t t t tt t t t fs s s s ss s s s fs s s s ss

s s ss s s s fs ss s t tt t t ts ft t t t t tt t t t ft t t t tt t t tt ft t t t st s st

Downlink

Uplink

CHANNELS: t = SDCCH/8 s = SACCH/8 i=idle

s

t


s

1. 2. 3. 5. 6. 7. 8.4.

0 50

iii

iii


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Combined CCCH + SDCCH/4

Multiframe

f s bb b b c fc fc s c c c c cc c c fc fs t t t t tt t t f ft t t t tt t t fs fs s s s ss s ss

t t tt r r s fs ss s s s r r rr r r rs fr r r r r rr r r r fr r r r tr t t tr ft t t r tr t tt

Downlink

Uplink

CHANNELS: f then s = FCCH then SCH b = BCCH r = RACH

ssss = SACCH c = CCCH t = SDCCH i=idle


t

0 50

1. 2. 3. 4.

i


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Full Rate Traffic Channel

(TCH) Multiframe

t t tt t t t ft tt t t tt t t tt ft t t tt

Downlink and Uplink

CHANNELS: t = TCH s = SACCH i=idle

ts


0 25

i


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Half Rate Traffic Channel

(TCH) Multiframe

Downlink and Uplink

CHANNELS: t = TCH s = SACCH i=idle


0 25

Downlink and Uplink


0 25

t t t t t t t t t t tt

t t t t t t t tt t tt

s i


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Radio Timeslots & Frames

TCH

0 1 2 24 25 0 1 2 49 50

0 7

Hyperframe = 2048 Superframes

Superframe = 26x51 or 51x26 Multiframes

26 Multiframe = 120 ms 51 Multiframe = 235 ms

TDMA frame = 4.615

ms

SIGN.

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


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Radio Wave Characteristics

Multipath propagation


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Inter symbol interference

BTS

RX sensitivity

Fading dips caused bymultipath propagation

Approx.17cm

Fading dips

u pa p opaga o

Frequency hopping


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F2

F1

F3

F4

Time

q y pp g

Antenna receiver diversity


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Received signal

RXRX

Signal

Processing

Antennas

Approx. 6m (GSM-900)

Approx. 3m (GSM-1800)

y

Shadowing


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BTS

g

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

g

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

PCM30


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Exchange1

Exchange2

125 s time frame

32 time slots

TS 0used for synchronisationand alarms

TS 16often used for commonchannel signalling

E1

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


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Review Questions to

GSM Air Interface

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 predominantly based on:

a) the difference between the uplink and downlinkfrequency 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 handover information.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 be found inthe:

a) the SDCCH.b) the SACCH.

c) both the SDCCH and the SACCH.

d) neither the SDCCH nor the SACCH.

Review


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a) eliminates the problem of fading dips.

b) eliminates the problem of inter symbol interference.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 by atranscoder 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


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a) the number of available frequencies is lower than thenumber 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 numberof 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


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13. In a certain PLMN, an average subscriber makes five calls during officehours (8 AM - 6 PM). It is known that in a certain cell area, there aregoing be 1000 subscribers, at any given hour, during these office hours.Assuming that a subscribers conversation lasts for 100 seconds, howmany 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 an exactanswer.

chapter4 air interface

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