Modul 2 gsm air interface

GSM-GPRS Operation
GSM Air Interface
Module 2

kris.sujatmoko@gmail.com 2
Outline
 Introduction & Review
 GSM Frequency Band
 GSM Channel Numbering
 Physical Channel
 Logical Channel
 Mapping Logical Channel on to Physical
Channel

Air Interface = Um Interface
 Major problems at air interface  In GSM
network, it is about Um interface
Air interface

Arsitektur Jaringan GSM
Mobile
terminal device
BSS
Base Station
Subsystem
NSS
Network Switching
Subsystem
control/switching of
mobile services
BSS
Base Station
Subsystem
BSS
Base Station
Subsystem
PLMN
Public Land Mobile Network
PSTN
Public Switched
Telephone Network
ISDN
Integrated Services
Digital Network
PDN
Public Data
Network
MS
Mobile
Station
Mobile
components
Fixed network
components
Um
Air Interface
Fixed
network

GSM 900 MHz Frequency Band
200 kHz
CH 1 CH 2
CH 124
200 kHz
CH 1 CH 2
CH 124
890 915 MHz
Uplink
MS Transmit Band
25 MHz
935 960 MHz
Downlink
MS Transmit Band
25 MHz

Penomoran Kanal GSM
 GSM900
FU(n) = 890 + 0.2×n (MHz)
FD(n) = Fu(n) + 45 (MHz) 1 ≤ n ≤ 124
 E-GSM900
FU(n) = 890 + 0.2×(n-1024) (MHz) 974 ≤ n ≤ 1023
FD(n) = Fu(n) + 45 (MHz)
 GSM1800
Fu(n) = 1710.2 + 0.2×(n-512) (MHz)
FD(n) = FU(n) + 95 (MHz) 512 ≤ n ≤ 885
374 channels
n disebut Absolutely Radio Frequency Channel Number (ARFCN)

GSM Frequency Allocation In Indonesia
SATELINDO(INDOSAT) : 890 – 900 MHz (10 MHz)
TELKOMSEL : 900 – 907 MHz (7.5 MHz)
EXCELKOMINDO : 907.5 – 915 MHz (7.5 MHz)
GSM : [890+nx0.2] MHz up to 915MHz
DCS 1800 : [1710+nx0.2] MHz up to 1785MHz
890 915
Tx
Indosat Telkomsel Excelcomindo
900
Operators share
bandwidth

Frequency Allocation Per-Cell
1 2 3 4 5 6 7
1710 1732
f
Selanjutnya...bandwidth
displit di dalam sel-sel
1
1
2
3
2
4
5
6
7
3
4
5
6
7
1

Spesifikasi RF
F1 F2 F1' F2'
Frequency
1
2
3
4
5
6
7
8
45 MHz
BS Transmission
Band : 935 – 960 MHZ
MS Transmission
Band : 890 – 915 MHZ
Year Introduced 1990
Access method TDMA
Channel Bandwidth 200 kHz
Number of duplex
channels
125
Users per channel 8
Speech coding bit
rate
13 kbps
Data coding bit rate 12 kbps
Frame size 4.6 ms

GSM-GPRS Operation
Kanal Fisik

TDMA
frame
4.615
ms
time
TS4
TS5
TS6
TS7
TS0
TS1
TS2
TS3
Frequency [MHz]
••• •••
Duplex distance: 45 MHz
200 kHz
Example:
GSM900
890 915 935 960
UL DL
TS
577
µs
Physical channel (Um)Physical channel (Um)
Radio Interface (Layer 1)

Struktur Sinyal GSM
 GSM carrier selebar 200 kHz.
 Pada arah downlink BTS,
timeslot berbeda digunakan
untuk masing-masing MS – MS
mendengar hanya pada TS yg
dialokasikan untuknya
 Pada TS yg tidak digunakan,
MS dapat menggunakannya
utk pengukuran sinyal 
berguna utk Handover
 Pada arah uplink, MS transmit
hanya pada timeslot yg
dialokasikan untuknya
 Transmit MS terjadi berbeda 3
TS dari TS transmit BTS 
untuk mencegah transmit-
terima simultan pada MS
(membutuhkan duplekser)
3 timeslot

Adaptive frame alignment:
Mencegah pengiriman/penerimaan
secara bersamaan/simultan
UL/DL di delay 3 TS
76543210
76543210 DL
UL
Timing Advance TA:
Kompensasi delay propagasi
BTS memerintahkan MS mengirim lebih lambat:
2 x waktu propagasi MS - BTS
Adaptive Frame Alignment / Timing Advance (TA)

TDMA
frame
Frames
RFC
3
RFC
2
RFC
1
0
1
2
3
4
5
6
7
0
1
2
3
4
5
20
21
22
23
24
25
43
44
45
46
47
48
49
50
Time
RFC
124
Frequency
0
1
2
3
4
5
6
7
Time
FDMA
User
Traffic
Signaling
cyclical repetition
of certain contents
cyclical repetition
of certain contents
• TDMA-
• Multi-
• Super-
• Hyper-
Multi-
Frames
Konsep Frame

1 Signalling Multiframe =
51 TDMA frames ≈ 235,4 ms
1 TCH Multiframe =
26 TDMA frames = 120 ms
Time
Structure
Hyperframe =
2048 Superframes ≈ 3h 29 min
0 1 2 3 4 5 6 7
0 1 2 3 24 25••• 0 1 2 3 49 50•••
0 1 2 3 ••• 4950
0 1 2 3 24 25
1 Superframe =
51 x 26
TDMA frames
≈ 6.12 ms
Numbering Period
e.g. repetition of
• frequency hopping
• ciphering
Channel organisation
scheme
Repetition scheme
for TCH / Signalling
BURST = TS content
1 TDMA frame
= 8 TS = 4,615 ms
1 Burst = 156,25 bit = 576,88 us
(1 bit = 3,6923 us)

GSM-GPRS Operation
Kanal Logik

Logical Channel

GSM-GPRS Operation
Mapping Kanal Logik ke Kanal
Fisik

Kanal logik GSM harus dipetakan ke kanal fisik, artinya : informasi pada
kanal logik harus ditempatkan pada kanal fisik dengan format burst
tertentu untuk ditransmisikan
Analogi pemetaan kanal logika pada kanal fisik...
Tipe burst pada kanal fisik dapat dianalogikan sebagai ‘tipe gerbong’ pada
rangkaian kereta api. Orang makan malam berada pada gerbong restorasi.
Demikian pula pada kanal logik, penempatan kanal logika pada ‘gerbong’ burst
tergantung dari fungsi kanal logik tersebut beserta sifat-sifat fisisnya.
FCCH Burst
SCH Burst
Synchronization
Channel
Frequency
Correction
ChannelDedicated
Control
Channels
Traffic
Channel
Normal Burst
Mapping

FCCH Burst
SCH Burst
Synchronization
Channel
Frequency
Correction
ChannelDedicated
Control
Channels
Traffic
Channel
Normal Burst
Kanal RF selebar 200 kHz  Rel kereta
Kanal logik  Isi gerbong kereta
Kanal fisik (timeslot)  Gerbong kereta
Burst  Tipe/jenis gerbong
Kasus analogi di GSM
Mapping

512 513 514 808 809 810
299 200kHz Radio Channels
FDMA
FDD
0 1 2 5 6 7
8 0.57ms Time Slots
3 4TDMA {
Burst - Transmission Quantum in GSM
1 Frame
Burst Burst Burst
BurstBurstBurst Burst Burst
Frame Train on
Frequency Track…an analogy
For GSM 1900...
U D
60 MHz 60 MHz
ARFCN
Numbers
Multiplexing
Membutuhkan sinkronisasi yg sangat tepat!!

TB
3
Information
57
S
1
Training Seq.
26
S
1
TB
3
GP
8.25
Information
57
0 1 2 3 4 5 6 7
RFC
1
RFC
2
RFC
3
RFC
i
RFC
123
RFC
124
••• •••
UL: 890 MHz 915 MHz
FDMA
GMSK
Modulation
200 kHz
270.833
kbit/s
TDMA
1 TDMA Frame: 4.615 ms / 1250 bit
1 TS: 33.85 kbit/s
1 Normal Burst: 576.9 µs / 156.25 bit
1 Bit = 3.6923 µs
24.7 kbit/s = 22.8 kbit/s TCH data (incl. redundancy)
+ 0.95 kbit/s SACCH + 0.95 kbit/s “Idle”
Physical Channel / GSM Data Rates

GSM-GPRS Operation
End of Section 2
GSM Air Interface

Modul 2 gsm air interface

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