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GSM Basic Principle
7/28/2019 GO NA01 E1 1 GSM Basic Principle-45
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Objective
At the end of this course, you will be able to: Understand GSM system architecture and function
State GSM common events
Describe basic calling process
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Content
GSM System Overview
Voice transmission process
Radio key technology
Radio channel
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AMPS
TACS
NMT
Others
1G
Analog
GSM
CDMAIS95
TDMAIS-136
PDC
2G
Digital
Market
Driving
3G
IMT-2000
UMTS
WCDMA
CDMA
2000
Market
Driving
TD-SCDMA
Mobile Telecommunication Technology Evolution
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GSM system architecture
BTS
BSC
MSC
VLR HLR AuC EIR
PSTN, ISDN...
OMC OMC
NMC X.25 links
GSM interfaces
Voicemail Serv
er
SM-SC
MS (Mobile Station)
BSS (Base Station System)
NSS(Network Switching
Subsystem)
OMS(Operations & MaintenanceManagement)
MS
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GSM network entities
MS (Mobile Station)：
Mobile Equipment
Subscriber Identity Module (SIM)
BTS (Base Transceiver Station)：
Provide radio channels
BSC (Base Station Controller)： Radio resource management
Transcoder
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GSM network entities
HLR (Home Location Register)：
Database：
Subscriber data
Subscriber location info.
VLR (Visitor Location Register)：
Database：
MSRN (Mobile Station Roaming Number)
TMSI (Temporary Mobile Subscriber Identification)
Location area code（LAC）
Subscriber data related to supplementary service
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GSM network entities
MSC (Mobile service Switching center)：
Circuit switching
AUC (Authenticate Center) :
Authenticate subscriber access
EIR (mobile station Equipment Identity Register) :
Identify terminal equipment
OMC (Operation and Maintenance Center)：
Provide MMI to control and monitor system
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GSM service area
GSM Service Area
PLMN Service Area
MSC Service Area
Location Area
Cell
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GSM interfaces
MS BTS BSC MSC
VLR VLR
HLR
MSCEIR
Sm
Um
Abis A B
D
C
E F
G
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GSM operation band
SYSTEM P-GSM 900 E-GSM 900 GSM 1800 GSM 1900
Frequencies
- Uplink- Downlink
890 - 915 MHz935 - 960 MHz
880 - 915 MHz925 - 960 MHz
1710 - 1785 MHz1805 - 1880 MHz
1850 - 1910 MHz1930 - 1990 MHz
Wavelength ~33 cm ~33 cm ~17 cm ~16 cm
Bandwidth 25 MHz 35 MHz 75 MHz 60 MHz
Duplex Distance 45 MHz 45 MHz 95 MHz 80 MHz
Carrier Separation 200 kHz 200 kHz 200 kHz 200 kHz
Radio Channels 125 175 375 300
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Absolute radio frequency channel Number （ ARFCN）
GSM900： Fu (n) = 890 + 0.2n MHz
Fd (n) = Fu(n) + 45 MHz，01 n 124
GSM1800：
Fu (n) = 1710.2 + 0.2(n-512) MHz
Fd (n) = Fu(n) + 95 MHz，512 n 885
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Multiple Access Technology (MAT)
Many subscribers share commontelecommunication lines without interference to
each other.
MAT
GSM
FDMA
TDMA
CDMA
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FDMA
FDMA： Identify by frequency
Time
Frequency
FDMA
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TDMA
TDMA： Identify by time
Time
Frequency
TDMA
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CDMA
CDMA： Identify by code
Time
Frequency
CDMA
Code
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Content
GSM System Overview
Voice transmission process
Radio key technology
Radio channel
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Voice Processing in the GSM System
A/D
D/A
Voice
codingChannel
coding
Interle
avingEncryption
Burst pulse
formingModulation
Voice
decoding
Channel
decodingDeinterleaving Decryption
Burst pulse
disassembleDemodulation
260bit/20ms 456bit/20ms 33.8kbit/s 270kbit/s
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Voice encoding
RPE-LTP (Rule Pulse Excitation-Long TermPrediction )
260bit/20ms=13Kbit/s voice block
Parameter encoding
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Channel coding
Channel coding serves to improve transmissionquality and overcome the negative impact of
interferences on signals.
Commonly used channel coding methods are:
Convolution coding
Fire coding
Parity check coding.
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Channel coding
50 bit (Ia) 132 bit
(Ib)
78 bit (Ic)
260 bits /20ms
50 132 783 4
Fire
Code
Protection
bits
136 bit53 bit
189
bit
CONVOLUTIONAL
CODER (1:2)
378
bit
78 bit
456
bit
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Channel Interleving
Target: Change continuous bit errors into
discontinuous bit errors which can be corrected by
channel coding
3 41 2 3 41 2 3 41 2 3 41 2
3 41 3 41 2 3 41 2 3 41 2
3 41 2 3 41 2 3 41 2 3 41 2
2
Message block
Interleving
After
interleving
Error bits
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Two stage interleving
456 bit
0 1 2 3 4 5 6 7
8 9 10 11 12 13 14 15
1 2 3 4 5 6 7 8
456 bit
0 1 2 3 4 5 6 7
8 9 10 11 12 13 14 15
1 2 3 4 5 6 7 8
456 bit
0 1 2 3 4 5 6 7
8 9 10 11 12 13 14 15
1 2 3 4 5 6 7 8
456 bit
0 1 2 3 4 5 6 7
8 9 10 11 12 13 14 15
1 2 3 4 5 6 7 8
57 1 57 1 57 1 57 1 57 1 57 1 57 1 57 1
Block A Block B Block A+1 Block B+1
116-bit block 116-bit block 116-bit block 116-bit block
Odd N+4 Odd N+5Odd N+6
Odd N+7Even N Even N+1 Even N+2 Even N+3
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Encryption/Decryption
Encryption is accomplished by exclusive or operation of an encryption sequence (computed
by A5 encryption algorithm via key Kc and frame
number) and 114 information bits on a normal
burst.
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Modulation and Demodulation
GMSK is a special digital FM modulation mode The modulation rate is 270.833 kilobauds.
The Frequency Shift Keying (FSK) modulation
with bit rate four times of frequency offset (67.708
kHz )is called MSK (Minimum Shift-frequency
Keying).
Gaussian demodulation filter is used to further
reduce the modulation spectrum.
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The GMSK can be expressed by a I/Q diagram
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Content
GSM System Overview Voice transmission process
Radio key technology
Radio channel
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Discontinuous Transmission（DTX）
Two aims can be achieved by adopting DTX mode. Lower the total interference level in the air
Save transmitter power.
The DTX mode and the normal mode are optional,
since the former will slightly lower the transmissionquality.
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Frequency Hopping (FH)
Type Baseband
Synthesized
Function Frequency diversity
Interference
diversity
Parameters HSN
MAIO
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Diversity receiving
The diversity reception technology is usually introduced tothe GSM system to receive on several tributaries the
signals with little relativity but carrying the same
information and then output the signals after they are
combined.
In this way, the impact of fading on the stability of receiving
signals can be decreased.
Types:
Space Diversity
Frequency Diversity
Time Diversity
Polarization Diversity
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Dynamic power control
Target: On the basis of certain service quality Lower the total interference level in the air
Save transmitter power
Power control command sent by SACCH
MS: Dedicated mode
Static 6 level, Dynamic 15 level
Step: 2dBm
A B
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Timing Advance（TA）
In the GSM, because TDMA is adopted in the air interface, the MS must employ the TSs allocated
to it only, and remain inactive in other TSs.
Otherwise, it may affect the MSs using other TSs
on the same carrier.
It does not work,
Let MS send me
By one TA in advance
Time delayTA
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Timing Advance（TA）
BTS calculate TA Send by SACCH to MS
MS Dedicated mode
Range: 0-63Bits
Distance: 0-35Km
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Content
GSM System Overview Voice transmission process
Radio key technology
Radio channel
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Frame1
1 2 3 4 5 760RF
Frame0
1 2 3 4 5 760
Frame N
1 2 3 4 5 760
Channel 0
Channel 1
Channel 7
Time Division Multiple Access (TDMA)
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GSM Frame structure
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Physical and logical channel
Physical Channels –
the path used to carryinformation between an MS and a BTS
Logical Channels – different information carried on
the physical channels
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FCCH
SCH TDMA#
BCCH
Hey. I’m a GSM xmitter !
GSM?
GSM!!!
MCC...MNC...Cl…
Ok…ok
Broadcast Channel (BCH)
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PCH downlink only
Hello! You have a call.
RACH uplink only
Hello! I have to set
up a call.
I need SDCCH.
AGCH downlink only
Ok. Use SDCCH.
Common Control Channel (CCCH)
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SDCCH
SACCH/TCH
MS power
FACCH/TCHhandover
On SDCCH:-call set up signaling
-SMS
-etc…..
On SACCH
-SMS
-MS power
Don’t shoutat me.
Dedicated Control Channel (DCCH)
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Channel Mapping
Item Configuration Combination
1 TCHFull TCH/F + FACCH/F + SACCH/TF
2 TCHHalf TCH/H)+FACCH/H+ SACCH/TH
3 TCHHalf2 TCH/H + FACCH/H+ SACCH/TH+ TCH/H
4 MainBCCH FCCH + SCH + BCCH + CCCH
5BCCHCombined
FCCH+SCH+BCCH+CCCH+SDCCH/4+
SACCH/C4
6 BCH BCCH + CCCH
7 SDCCH SDCCH/8+ SACCH/C8
8 BCCHwithCBCHFCCH+SCH+BCCH+CCCH+ SDCCH/4+
SACCH/C4 + CBCH
9 SDCCHwithCBCH SDCCH/8+SACCH/C8+CBCH
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Structure of 26-Frame Traffic Channel
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Structure of 51-Frame Control Channel
SF B C
R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R
51 Ö¡
SF C C SF C C SF C C I
R R R R R R R R R R
D0 D1 D2 D3 D4 D5 D6 D7 A0 A1 A2 A3
SF C C
R R R R R R R R R R
III
D0 D1 D2 D3 D4 D5 D6 D7 A4 A5 A6 A7 III
A1 A2 A3 III
A5 A6 A7 III
D0 D1 D2 D3 D4 D5 D6 D7 A0
D0 D1 D2 D3 D4 D5 D6 D7 A4
SF B C SF C C SF D0 D1 SF D2 D3 ISF A0 A1
SF B C SF C C SF D0 D1 SF D2 D3 ISF A2 A3
D3
D3
R R
R R
A2 A3
A0 A1
D2
D2
SF
SF
D0 D1
D0 D1
R R R R R R R R R R R R R R R R R R R R R R R
R R R R R R R R R R R R R R R R R R R R R R R
F£ºFCCH S£ºSCHB£ºBCCH C£ºCCCH£¨CCCH=PCH+AGCH+RACH£©R£ºRACH D£ºSDCCH
A£ºSACCH/C I£ºidle
BCCH+CCCH£¨ÏÂÐУ©
BCCH+CCCH£¨ÉÏÐУ©
8 SDCCH/8£¨ÏÂÐУ©
8 SDCCH/8£¨ÉÏÐУ©
BCCH+CCCH+4SDCCH/4
£¨ÏÂÐУ©
BCCH+CCCH+4SDCCH/4£¨ÉÏÐУ©
(a) FCCH+SCH+BCCH+CCCH
(b) SDCCH/8(0,...,7)+SACCH/C8(0,...,7)
(c) FCCH+SCH+CCCH+SDCCH/4(0,...,3)+SACCH/C4(0,...,3)
R R
R R
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