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INTRODUCTION TO GSM
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HISTORY 1979-80 Came the first Cellular
Mobile system such as AMPS ( Advance Mobile Phone Service) NMT ( Nordic Mobile Telephony ) TACS ( Total Access Communication
System)These were analog in Nature and known as First Generation System
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HISTORY During 1990 many digital mobile
standards were introduced in different regions/ countries DAMPS ( Digital Advance Mobile Phone
Service) GSM ( Global System for Mobile
Communications) CDMA ( Code Division Multiple Access )These are known as 2nd Generation System
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HISTORY For introduction of data transfer
capability on the Mobile ,additional Hardware was introduced in GSM systems, known as GPRS ( General Packet Radio Service ) . These are being referred as 2.5 G System
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FUTURE Now a Global Standard IMT-2000
(International Mobile Telecom.-2000) is proposed which shall encapsulate all the existing networks and different access technologies to provide any time and any where type of communication known as 3rd Generation standard.
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GSM As early as 1982, the CEPT
( Confederation of European P&T ) formed a Group called Global system for Mobile Communications (GSM)
The objective was to develop a building mobile radio standard for Europe
GSM is an European digital mobile telephony Standard specified by the ETSI and provides a common standard.
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GSM The GSM Standard is the first
international that gives subscriber full access to the network of various operating companies in all participating countries.
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GSM Operational Requirements International Roaming High Spectral Efficiency Improved Transmission Quality as
compared to Analog Systems High Link Integrity Provision of Tele Services & Bearer
Services
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GSM Operational Requirements Economy in both sparsely and
heavily populated areas Better use of available frequencies
in order to improve the user capacity of Mobile Communications.
Large selection of interfaces to other services such as PSTN/ISDN, PSPDN etc.
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SPECIFICATIONS Trunked Radio system. Access Method - TDMA/FDMA Frequency Bands
Mobile to Cell - 890-915 MhzCell to Mobile - 935-960 Mhz
Therefore the frequencies for the uplink and the downlink are different ( Frequency Division Duplex) and the bandwidth of the GSM System is 25 Mhz..
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SPECIFICATIONS
Cell To Mobile – Down Link
Mobile to Cell – Up Link
935-960 Mhz
1805-1880 Mhz
890-915 Mhz
1710-1785 Mhz
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SPECIFICATIONS Channel Bandwidth – 200 Khz Number of Channels – 124 channels
pairs ( 25 Mhz/200 Khz )are available spaced at 200 Khz intervals
Due to interference to other systems, the very first carrier is not used.
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Freq
Mhz. 890.2
1
890.4
2
890.6
3
890.8
4
891.0
5 6
914.8
124
• FDMA Access along Frequency axis
• Total 124 RF Channels available. One or more carrier assigned to each base station
• Absolute Radio Freq Carrier Number (ARFCN) 1 & 124 not used untill it is co-ordinated with Non -GSM operators in adjacent freq. bands.
• In most cases 124 RF Channels are used
……...
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Specifications Therefore the actual frequency of
an ARFCN is Ful(n)= 890.0 Mhz +(0.2 Mhz)*n And Fdl(n)= Ful(n) + 45 Mhz Where n= 1<=n<=124
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FDMA/TDMA Each channel pair serves eight ( Full Rate ) or
Sixteen (half rate) channels successively in a Timed Cycle.
In FDMA system each channel requires a transmitter and receiver
8 Voice channels, staggered in Time are transmitted via one carrier frequency
Each of the 124 FDMA linked carrier frequency is used to transmit 8 TDMA voice channels
Discontinuous nature of TDMA makes the entire 200 Khz bandwidth available to all 8 Voice channels.
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GSMFDMA
25 MHz 25 MHz
Mobile to Base
0 1 2
890.2 890.4 890.6(MHz)
Base to Mobile
0 1 2
935.2 935.4 935.6
200 kHz45MHz
Channel layout and frequency bands of operation
890 935 960915
200 kHz
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GSMTDMA
8765
43
21
8765
43
21
45 MHz
FrequencyF2’F1’
(Cell transmit)F2F1
(Cell Rx)
Amplitude
Typical TDMA/ FDMA frame structure
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BP1BP2BP3BP4BP5BP6BP7BP8BP1BP2
TIME
890.0
890.2890.4
890.6890.8
891.0891.2 915.8
FREQ
MHz
BURST
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8 7 6 5 4 3 2 1
1 2 3 4 5 6 7 8
1
1
1
1
1
2
2
2
2
2
3
3
3
3
3
4
4
4
4
4
6
6
6
6
6
7
7
7
7
7
8
8
8
8
8
5
5
5
5
5
FRAME OF 8 TIME SLOTS
FRAME REPETITION
PHYSICAL CHANNELS
PHYSICAL CHANNELS
8 7 6 5 4 3 2 1 8 7 6 5 4 3 2 1 8 7 6 5 4 3 2 1
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BURSTING Therefore the mobile transmits
only in one time slot and stay idle for the remaining seven time slots with its transmitter off.
This periodic switching (ON&OFF) is called BURSTING ( 216.6 Hz=1/4.615ms)
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MODULATION METHOD Modulation Method
GMSK ( Gaussian Minimum Shift Keying )
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Duplex Operations Frequency Division Duplex
Frequency for uplink and downlink are different.
Time Division DuplexThe mobile station transmits 3 timeslots latter than the BTS but the Time slot number stays same as if both are using the same time slot at the same time.
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Duplex Operation
0 1 2 3 4 5 6 7 0 1 2 3
5 6 7 0 1 2 3 4 5 6 7 0
BTS Transmits
MS Transmits
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Timing Advance & Power Control In a cell MS can be at different
distances, hence different delay and diff. Attenuations
Due to above the collisions of signals from mobiles assigned to adjacent time slots
Mobiles farthest from the base station are given a head start.
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Timing Advance & Power Control To compensate for the attenuation
over different distances within the cell, the BTS commands the MS to use different power levels in such a way that the power arriving at the base station receiver is approx same for each time slot.
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Timing Advance & Power Control
A Long signal delayHigh signal Attenuation
BShort Signal DelaySmall signal Attenuation
BTS
AB
TSn TSn+1
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Logical channels Traffic Channel :Used for
Transmission of Speech Full Rate Half Rate
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Logical channels Control Channel : Intended to carry
signalling and synchronisation for network management messages and channel maintenance task. Broad Cast Channel BCH Common Control Channel CCCH Dedicated Control Channel DCCH
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Broadcast Channel Transmitted by the BTS and
are intended to provide sufficient information to the mobile station for it to synchronize with the network.
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Broadcast Channel Broadcast Control Channel
BCCHInforms the mobile station about specific parameters it need to identify the network or to gain access to the network
( LAC, MNC, Freq. Of neighboring cells etc.)
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Broadcast Channel Frequency correction Channel
FCCH Informs the mobile station about the frequency reference of the system. This logical channel is mapped onto the freq. Correction burst. It appears on frequency correction burst and freq. Corr. Burst contains only FCCH.
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Broadcast Channel Synchronization Channel SCH
Informs the mobile station about the key it needs to be able to demodulate the information coming from the base station.It is mapped on to synchronization burst.
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Common Control ChannelThey support the
establishment of a dedicated link between a mobile and base station .
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Common Control Channel Random Access Channel
It is used by the MS to request a dedicated channel from the network. It is mapped into the random access burst and contains the first message sent to the BTS.
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Common Control Channel Paging Channel PCH
BTS calls individual MSs within its cell on this channel.
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Common Control Channel Access Grant Channel AGCH
MS gets information from the BTS on which dedicated channel it should use for its immediate needs from the access grant channel. The information about the timing advance also comes on this channel.
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Dedicated Control Channel They are used for message
transfer between the network and the MS, not the traffic. The messages are needed for registration, authentication and call set up. They are also used for low level signalling messages for channel maintenance.
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Dedicated Control Channel Standalone Dedicated Control
Channel : SDCCH It is intended for the transfer of signaling information between a mobile and a BTS
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Dedicated Control Channel Slow Associated Control Channel :
SACCH It is used in association with either traffic channel or SDCCH. The purpose is channel maintenance to maintain a link between mobile and BTS. In the downlink – latest information about the system ( Timing advance and power control ) and in the uplink– measurement performed on the neighboring cells.
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Dedicated Control Channel Fast Associated Control
Channel : FACCH If during a call there is a need for some heavy duty signalling, then FACCH appears in place of the traffic channel.
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ADVANCED LEVEL TELECOM TRAINING CENTREOPERATIONAL CONCEPTS• Subscribers are not allocated dedicated channels
• TCH Allocated to users only when needed
• Hence IDLE MODE & DEDICATED MODE
• DEDICATED MODE -- When a full Bi -directional P to P CHL has been allocated during an established call • IDLE MODE
-- When MS is powered on (active) without being in dedicated mode
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IDLE MODE -- When MS is powered on (active) without being in dedicated mode
• MS stays continuously in touch with BS
• Listens to transmissions from BS to intercept Paging Messages ( for incoming calls)
•
• Monitors Radio Environment in order to evaluate Chl Quality & choose the most suitable BS • Listens to BS to avail short message broadcast service
Yuri Sherman
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