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MobileComm Technologies India Pvt. Ltd
Dallas . Atlanta . Washington . LA . Sao Paulo . New Delhi . Toronto . M
GSM Overview
Ritesh Karan
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Copyright 2010 Mobile Comm Technologies India Pvt. Ltd.
All rights reserved
Mobile Comm is committed to providing our customers with quality instructor
Telecommunications Training.This documentation is protected by copyright. No part of the contents of thisdocumentation may be reproduced in any form, or by any means, without thMobile Comm Technologies .
Document Number: RK/CT/1/2010 This manual prepared by: Mobile Comm Technologies
MobileComm Technologies(India)Pvt. Ltd.424, First Floor, Udyog Vihar Phase -4,
Gurgaon-122002 Headquarter:
MobileComm Professionals Inc.1255 West 15th Street, Suite 440 Plano, TX, 75075 Tel: (972) 633-5100 Fax: (972) 633-5106
www.mcpsinc.com
Ritesh Karan
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Module Objectives
Give an overview about GSM Channel
Describe the basic configuration i.e combined mode and no
Demonstrate speech coding and channel coding.
Give an overview about the Burst structure.
Ritesh Karan
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Speech Flow in BSSSpeech,64 kbps
compression ChC
= red
Inteand
TDMAforma
GMSKmodulation
13kbit/s
33.8kbit/sAirInterface
MSC
TRAU TRAU BTS
CODING
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S
Sourcdeco
Chandecod
De-in
Burst
Decip
Dem
equa
Digitizing andsource coding
Channel
coding
Modulation
Ciphering
Burst formatting
Interleaving
Step 1
Step 2
Step 3
Step 4
Transmission
Step 5
Step 6 Diver
From Speech to Radio TransmisP D F C r eat ed wi t h d es k P D F P D F
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Speech Coding
BP A/D SPEECHENCODERCHCO
LP D/A SPEECHDECODERCHDE
BANDPASS
300 Hz -
3.4 kHZ
Every 125µ s value issampled from analog signaland quantised by 13 bitwordData rate = 13/125*10 -6
= 104 kbps
Every 20ms 160 samplestakenData rate = 160 * 13/20ms
= 104 kbps
Linear Predictive Coding & RegularPulse Excitation Analysis1. Generates 160 filter coeff2. These blocks sorted in 4 sequence
1,5,9,…37 / 2,6,10----38/3,7,11…39/8,12,16…40
3. Selects the sequence with most energy
So data rate = 104/4 = 26 kbps
Long term prediction analysis1. Previous sequences stored in memory2. Find out the correlation between the presentseq. And previous sequences3. Select the highest correlation sequence4. Find a value representing the differencebetween the two sequences.
Reduces data rate = 26 kbps/2 = 13 kbps
ie 260 bits in 20ms
50
1A
50 3
3 crc bits
378 code
Conv cod
456 57 x
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C20 ms
A
A8
A7
A6
A5
B4
B3
B2
B1
B8
B7
B6
B5
57 bits
Information
1 1
CRL CRL
3
Tail
26 bits
Training
8 Bursts
8 Sub blocof 57 bits
Source coding
Channel coding
Interleaving
Normalburst
20 msB
456 bitsA 456 bitsB C
In
A8B4A7B3A6B2A5B1 B8C4B7C3B6C2B5C1
Speech blocks
Codec dependent Codec dependent
Channel Processing in GSMOverview for Full Rate
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0 1 2 3 4 5 6 7 8 ... ... 452 453
0 1 2 3 4 5 6 78 9 10 11 12 13 14 15• • • • • • • •• • • • • • • •• • • • • • • •
448 449 450 451 452 453 454 455
Divide 456 bits in 8 sub-blocks
765432107654 0 1 2
b0 b1 b56 b1b0
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Bursts and time slots in the Air Interface
...
Bursts from Mobile StationsBursts from Mobile Stations
TDMA Time TDMA Time
TDMA FrameTDMA Frame
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Burst
• The information format transmitted during one timframe is called a burst.
• Different Types of Bursts
– Normal Burst
– Random Access Burst
– Frequency Correction Burst – Synchronization Burst
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NORMAL BURSTNORMAL BURST
0 1 2 3 4 5 6 7 0 1 2 3 4
57 bits 57 bits26 bits 11 3
FRAME1(4.615ms) FRAM
Training
sequenceData Data
Tail
Bits
Stealing
Bit
Guard
Period
0.577ms
Carries traffic channel and control channels BCCH, PCH,
SACCH and FACCH.
CHANNEL CONCEPTCHANNEL CONCEPT
Stealing
Bit
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Data Data - Two blocks of 57 bits each. Carries speech, data or control info.
Tail bits - Used to indicate the start and end of each burst. Three bits always 0
Guard period Guard period - 8.25 bits long. The receiver can only receive and decode if the
timeslot designated for it .Since the MS are moving. Exact synchronizatio
practically. Hence 8.25bits corresponding to about 30us is available as guard p
error.
Stealing bits Stealing bits - This bit is used to indicate if the 57 bits data block is used as FA
Training SequenceTraining Sequence - This is a set sequence of bits known by both the transm
of BSIC). When a burst of information is received the equalizer searches for
The receiver measures and then mimics the distortion which the signal has bee
then compares the received data with the distorted possible transmitted sequ
likely one.
NORMAL BURSTNORMAL BURST
CHANNEL CONCEPTCHANNEL CONCEPT
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FREQUENCY CORRECTION BURSTFREQUENCY CORRECTION BURST
0 1 2 3 4 5 6 7 0 1 2 3 4
FRAME1(4.615ms) FRAM
Fixed DataTail
Bits
0.577ms
Carries FCCH channel.
Made up of 142 consecutive zeros.
Enables MS to correct its local oscillator locking it to that
CHANNEL CONCEPTCHANNEL CONCEPT
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SYNCHRONISATION BURSTSYNCHRONISATION BURST
0 1 2 3 4 5 6 7 0 1 2 3 4
FRAME1(4.615ms) FRAM
Synchronisation
Sequence
Tail
Bits
Encrypted
Bits Encrypte
BitsCarries SCH channel.Enables MS to synchronise its timings with the BTS.
Contains BSIC and TDMA Frame number.
CHANNEL CONCEPTCHANNEL CONCEPT
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DUMMY BURSTDUMMY BURST
0 1 2 3 4 5 6 7 0 1 2 3 4
FRAME1(4.615ms) FRAM
Training
sequenceData Data
Tail
Bits
Ta
Bi
0.577ms
Transmitted on the unused timeslots of the BCCH
downlink.
CHANNEL CONCEPTCHANNEL CONCEPT
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AACCESS BURSTCCESS BURST
0 1 2 3 4 5 6 7 0 1 2 3 4
FRAME1(4.615ms) FRAM
Tail
Bits
Tail
Bits
Guard
Period
0.577ms
Synchronisation
Sequence
Encrypted
Bits
Carries RACH.
Has a bigger guard period since it is used during initial
MS does not know how far it is actually from the BT
CHANNEL CONCEPTCHANNEL CONCEPT
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Time
TSL 1TSL 2TSL 3
TSL 4
TSL 5
TSL 6
TSL 7
Principle of Time Division Multiple Access (TDMA)
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1 9
8 s e a t s
i n e a c h v e h i c l e
L o g i s t i c a l p r o b l e m
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Physical channel Physical channel - Each timeslot on a carrier is referred to as a phy
there are 8 physical channels.
Logical channel Logical channel - Variety of information is transmitted between the
different logical channels depending on the information sent. The logtypes
• Traffic channel
• Control channel
Downlink
Uplink
CHANNELSCHANNELS
CHANNEL CONCEPT
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FDMA
UPLINK DOWN
GSM900: 890 MHz - 915 MHz 935 M
GSM1800: 1710 MHz - 1785 MHz 1805
1 2 3 ...
Channel 1 - 124
1 - 374
200 kHz
1 2 3
Duplex frequency 45 MHz / 95 MHz
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01
23 4
5
76
01
23
4
5
76
01
23
4
200 kHz
Physical channel,e.g. allocated to one
subscriber with FR voice &no frequency hopping
frequen
t i m e
TDMA fram
TDMA
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Physical channel:One timeslot of a TDMA-frame on one carrier is referred to as a phy
8 physical channels per carrier in GSM, channel 0-7(timeslot 0-7)
Logical channel:A great variety of information must be transmitted between BTS anddata and control signaling. Depending on the kind of information tradifferent logical channels. These logical channels are mapped on p
Channel concept
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Logical Channels on Air interface
LOGICAL
CHANNELS
COMMON
CHANNELS
DEDICAT
CHANNE
BROADCAST
CHANNELS
COMMON
CONTROL
CHANNELS
DEDICATED
CONTROL
CHANNELS
FCCH BCCHSCH SDCCH SACCH FAC
PCH AGCHRACH TCH/F
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FACCH
BTS
0 1 2 3 4 5 6 7TS
FCCH
SCH
BCCH
PCH
AGCH
CBCH
SDCCH
SACCH
TCH
TCH
FACCH
SDCCHSACCH
FCCH
SCH
BCCH
RACH
PCH
AGCH
RACH
CBCH
Synchronization
Frequency correction
Broadcast control
Access request
Subscriber paging
Answer to Access request
Broadcast info
Dedicated Signaling
Sys InFo 5, 6 + SMS
Traffic (speech data)
Associated Signaling
A
T
R
D
B
M
A
S
A
The Logical Channels on Radio Interface
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Full Rate - Downlink & Uplink
T : TCH A : SACCHTi: TCH
sub-channel no. i
Ai
: SACCH
sub-channel no. i
26 frames = 120 ms
T0
A0
T0
T0 T0
T0
T0
T0
T0
T0
T0
T 1
T 1
T 1
T 1
T 1
T 1
T 1
T 1
T 1
T 1
Half Rate - Downlink & Uplink
T AT T T T T T T T TT T T T T T T T T T
T
26 frames = 120 ms
Logical Channel Mapping-TCH
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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 + agc
1
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BCCH/CCCH Multiframe
f s bb bb c fc fc sc cc c cc c c fc fsc cc c cc cc f fc ccc 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
Downlink
Uplink
CHANNELS: f = FCCH b = BCCH r = RACHs = SCH c = CCCH = PCH/AGC
51 TDMA frames = 235 ms
0
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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
ss ss s ss fs ss s t tt t t ts ft t t t t tt t t t ft t t t tt t tt
Downlink
Uplink
CHANNELS: t = SDCCH/8 s = SACC
t
51 TDMA frames = 235 ms
1. 2. 3. 5. 6. 7. 8.4.
0
iii
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Combined BCCCH + SDCCH/4 Multiframe
f s bb bbc fc fc scc cc cc cc fc fs t t t t tt t t f ft t t t tt t ts
t t tt r r s fs ss s s sr r rr r r rs fr r r r r rr r r r fr r r r tr t tr
Downlink
Uplink
CHANNELS: f then s = FCCH then SCH b = BCCH
ssss = SACCH c = CCCH t = SDCC
51 TDMA frames = 235 ms
0
1. 2. 3. 4.
BTS Physical ChannelARFCN (n) TS (s)FCCH
SCH BCCH
PCH/AGCH
Combined Mode
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00
11
00
11
22
20452045
1 Hyperframe = 2048 superframes = 2,715,648 TDM3h 28min 53s 760ms
1 Superframe = 1326 TDMAframes = 51(26 fr) 0r 26(51 fr) multiframes
11 2 33 4
00
11
0
0
1
1
2
2
23
23
24
24
25
25
0
0
1
1
2
2
22
33
44
55
66
77
6.12s
0
235.3120ms
Control 51 -Traffic 26 - Frame Multiframe
4.615ms
TDMA Fr
HYPERFRAME AND SUPERFRAME STRUCTUREHYPERFRAME AND SUPERFRAME STRUCTURE
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•FCCH•SCH
•BCCH•BCCH•BCCH•BCCH
•PCH
•AGCH
•0•1•2•3•4•5
•50 •IDLE
•0 •1 •2 •3 •4 •5
• 5 1 - F r a m e M
u l t i f r a m e
• 5 1 - F r a m e M
u l t i f r a m e
•TCH/F•TCH/F•TCH/F•TCH/F
•TCH/F•SACCH•TCH/F
•IDLE
•0•1•2•3
•11
• 2 6 - F r a m e M
u l t i f r a m e
• 2 6 - F r a m e M
u l t i f r a m e
•12•13
•25•TCH/F•24
•FCCH
•SCH
•SDCCH
•SACCH
• . . .
• . . .
• . . . •
. . .
• . . .
• . . .
• . . .
• . . .
• . . .
• . . .
• . . .
• . . .
•...
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List of the frequencies of the
selected PLMN
IMSI Attach
Look for the cell with the best C1 in the
suitable cells list
Eligible cell?
YesNo
C1
for eligible cell
Computation s
Suitable cell?
YesNo
Initial Cell Selection
PLMN
End of Cell Selection
No
Eligible cell
Suitable cell:
- cell of the selected PLMN
- cell not barred- C1 > 0
Rejected?
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IDLE MODE--Cell Selection
Path loss criterion parameter C1 used for cell selection and reselection
is defined by:
C1 = (A - Max(B,0))
where
A = Received Level Average - rxLevAccessMin
B = msTxPwrMaxCCH P
Where,
rxLevAccessMin = Minimum received level at the MS required for
access to the system.
msTxPwrMaxCCH = Maximum TX power level an MS may use when
accessing the system until otherwise commanded.
P = Maximum RF output power of the MS.
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Example:
C1(cell_A) =AV_RXLEV rxLevAccessMin-Max(0, msTxPwrMaxCCH max outpu
MS)
C1(cell_A) = -80dBm (-100dBm) max(0, 36dBm 33dBm)
C1(cell_A) = 17 > 0
C1(cell_B) = -82dBm (-105dBm) max(0, 33dBm 33dBm)
C1(cell_B) = 23 > C1(cell_A)
Thus MS camps on cell_B
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Cell Reselect Hysteresis
• Cell reselection on the border of two location areas result in a lan MS moves on the border of two location areas lots of locatioTo avoid these ping pong location updates, the reselect hystere
Cell1
LA1
RX LevelCell1
RX Cell
A B C
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Idle Mode--Cell Re-selection
Why C2?Cell Prioritisation
As a means of encouraging MSs to select some suitable
cells in preference to others Example:
"In dualband network--to give different priorities for different ban
"In multilayer--to give priority to microcell for slow moving traff
"Any other special case where specific cell required higher priorit
rest
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Pathloss Criterion C2
C1 + CELL_RESELECT_OFFSET – TEMPORARY OFFSET * H(PENAL
for PC1 - CELL_RESELECT_OFFSET for P
C2 =
0 .. 126 dBstep size: 2 dB
0, 10, 20, 30,40, 50, 60, ∞
dB
PENALTY_TIME
TEMPORAR
CELL_
C2
new candidate= formerly non-
serving cell
CELL_
C2no TEMPORARY OFFSET
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Cell Reselection Events
When shall Iselect a new cell?
Pathloss Criterion C1C1 ≤ 0 for a period of 5 s
Pathloss Criterion C2• neighbour‘s C2 value >
current cell‘s C2 value (sameLA)
• neighbour‘s C2 value + CRH >current cell‘s C2 value (new LA)
for a period of 5 s. Downlink SigDS
RACH FailureMAX RETRANS + 1 times
RACH FailureMAX RETRANS + 1 times
Authennotificatio
Cell has bCELL_BAR_ACCESS &
Cell has bCELL_BAR_ACCESS &
P D F C r eat ed wi t h d es k P D F P D F
Wr i t er -T r i al : : h t t p: / / www.d oc ud es k .c om
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Downlink Signalling Failure
B C C H
( ..., B S_ PA
MS
Downlink radio link
failure detected!
DSCinit
:= roundBS_PA_MFRMS
90
paging
message successfully
decoded?
DSC ≤≤≤≤ 0
DSC := min (DSC+1, DSCinit
) DSC := DSC - 4
yes no
yes
no
DSC = Downlink Signalling failure Counter
DSC := DSCinit
P D F C r eat ed wi t h d es k P D F P D F
Wr i t er -T r i al : : h t t p: / / www.d oc ud es k .c om
8/15/2019 1.GSM Overview.pdf
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4 1