chapter4 air interface
TRANSCRIPT
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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
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 ofinformation on a channel by sharing it.
TDMA Frame
Paging
Answer
TrafficBTS BTS
Number of channels required during call set-up (3)
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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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GSM Logical Channels
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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GSM Logical Channels
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
51 TDMA frames = 235 ms
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
51 TDMA frames = 235 ms
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
26 TDMA frames = 120 ms
0 25
i
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Half Rate Traffic Channel
(TCH) Multiframe
Downlink and Uplink
CHANNELS: t = TCH s = SACCH i=idle
26 TDMA frames = 120 ms
0 25
Downlink and Uplink
26 TDMA frames = 120 ms
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.
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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.
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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.
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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.
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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
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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.