Download - Lecture GSM
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GSM System Standard
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Introduction
Year Introduced:1990
Global System for Mobile (GSM) - a 2-G cellular system standard
Originally developed to serve as the pan-European cellular service
Aimed at specifying a common mobile communication system for Europe
Originally in the 900 MHz band
GSM (Groupe spe'cial mobile) : a committee of working group of the CEPT.
Later, GSM changed its name to the Global System for Mobile Communications
Standards by European Technical Standards Institute (ETSI)
Adopted by several non-European countries by the end of 1993
Technically equivalent offshoot - DCS 1800
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GSM Variants
Variant Uplink
(MHz)
Downlink
(MHz)
Total
Bandwidth
GSM-400 451-458 and
479-486
461-468 and
489-496
Twice 14 MHz
GSM-900
(primary
band)
890-915 935-960 Twice 25 MHz
Extended
GSM-900
880-915 925-960 Twice 35 MHz
GSM-R 876-880 921-925 Twice 4 MHz
DCS-1800 1710-1785 1805-1880 Twice 75 MHz
PCS-1900 1850-1910 1930-1990 Twice 60 MHz
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GSM System Architecture
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Mobile Station (MS)
MS consists of following two components Mobile Equipment (ME)
Subscriber Identity Module (SIM)
Removable plastic card
Stores Network Specific Data such as list of carrier frequencies.
Stores International Mobile Subscriber Identity (IMSI) + ISDN
Stores Personal Identification Number (PIN) & Authentication Keys.
Also stores short messages, charging information, telephone book etc.
Allows separation of user mobility from equipment mobility
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Base Transceiver Station (BTS)
One per cell
Consists of high speed transmitter and receiver
BTS antennas
Function of BTS
Provides two channels
Signalling and Data Channel
Performs error protection coding for the radio channel
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Base Station Controller (BSC)
Controls multiple BTS
Functions:
Performs radio resource management Assigns and releases frequencies and time slots for all the MSs in
its area
Reallocation of frequencies among cells
Hand over protocol is executed here
Time and frequency synchronization signals to BTSs
Time Delay Measurement and notification of an MS to BTS
Power Management of BTS and MS
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Mobile Switching Center (MSC)
Switching node of a PLMN
Allocation of radio resource (RR) Handover
Mobility of subscribers Location registration of subscriber
There can be several MSCs in a PLMN
Gateway MSC (GMSC) Connects mobile network to a fixed network
Entry point to a PLMN
Usually one per PLMN
Request routing information from the HLR and routes the connection to the local MSC
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HLR/VLR
HLR - Home Location Register
For all users registered with the network, HLR keeps user profile
MSCs exchange information with HLR
When MS registers with a new GMSC, the HLR sends the user profile to the new MSC
VLR - Visitor Location Register
VLR is responsible for a group of location areas, typically associated with an MSC
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Air Interface: MS to BTS
Uplink/Downlink of 25MHz
890 -915 MHz for Uplink (Reverse), 935 - 960 MHz for Downlink (Forward)
Combination of frequency division and time division multiplexing
FDMA No. of carriers = 25 MHz / 200 kHz = 125
Total 125 RF channels.
124 channels of 200 kHz +1 Guard band occupying RF channel BW.
TDMA Burst (8 time slots per RF channel)
Modulation used
Gaussian Minimum Shift Keying (GMSK), BT=0.3
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Air Interface: Logical Channel
Two Types
Traffic Channels (TCHs)
Speech Channel
Data Channel
Signalling Channel (Control Channels)
Broadcast Channel (BCH)
Common Control Channel (CCCH)
Dedicated/Associated Control Channel
(DCCH/ACCH)
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1. Broadcast Channel (BCH)- BTS to MS
Broadcast Control Channel (BCCH)
Frequency Correction Channel (FCCH)
Synchronization Channel (SCH)
2. Common Control Channel (CCCH)
Paging Channel (PCH)
Random Access Channel (RACH)
MS to BTS
Uses Slotted Aloha
Access Grant Channel (AGCH)
Control Channels
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3. DCCH (dedicated control channel)- Bidirectional point-to-point - main signaling channels
3 types 1. SDCCH (stand-alone dedicated control channel):
for service request, subscriber authentication, equipment validation, assignment to a traffic channel
2. Slow Associated Control Channel (SACCH) Always associated with either TCH or SDCCH Information
Optimal radio operation; Commands for synchronization, Transmitter power
control; Channel measurement Should always be active; as proof of existence of physical radio
connection
3. Fast Associated Control Channel (FACCH) for urgent signaling on a traffic channel, e.g, for handoff messages
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Traffic Channels (TCHs)
Speech Channel
Full-Rate Speech Channel
User speech digitized at a raw data rate of 13 kbps.
With GSM channel coding added to the digitized speech, the full-rate speech channel carries 22.8 kbps.
Half-Rate Speech Channel
Designed for data rate of about 6.5 kbps.
With GSM channel coding added to the digitized speech, the half-rate speech channel will carry 11.4 kbps.
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Data Channel
Full-Rate data Channel (TCHIFS)
Full-Rate Data Channel for 9600 bps (TCH/F9.6)
Full-Rate Data Channel for 4800 bps (TCH/F4.8)
Full-Rate Data Channel for 2400 bps (TCH/F2.4)
All produce 22.8 kbps after forward error correction coding
Half-Rate data Channel (TCHIFS)
Half-Rate Data Channel for 4800 bps (TCH/H4.8)
Half-Rate Data Channel for 2400 bps (TCH/H2.4)
Both produce 11.4 kbps after forward error correction coding
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GSM Signal Processing
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Frequency Hopping
Optionally, TDMA is combined with frequency
hopping to address problem of channel fading
TDMA bursts are transmitted in a pre-calculated sequence of
different frequencies (algorithm programmed in mobile
station)
If a TDMA burst happens to be in a deep fade, then next
burst most probably will not be so
Helps to make transmission quality more uniform among all
subscribers
Max. hopping rate:217.6 hops per second
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Roaming
VLR registers users roaming in its area
Recognizes mobile station is from another PLMN
If roaming is allowed, VLR finds the mobiles HLR in its
home PLMN
VLR constructs a global title from IMSI to allow signaling
from VLR to mobiles HLR via public telephone network
VLR generates a mobile subscriber roaming number
(MSRN) used to route incoming calls to mobile station
MSRN is sent to mobiles HLR
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GSM handoffs
Intra-BSS: if old and new BTSs are attached to same
base station
MSC is not involved
Intra-MSC: if old and new BTSs are attached to
different base stations but within same MSC
Inter-MSC: if MSCs are changed
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Upgrade in GSM: 2.5 G Standards
HSCSD - High Speed Circuit Switched Data GPRS - General Packet Radio Service EDGE - Enhanced Data Rates for GSM Evolution Provide solutions to increase the data rates over existing
GSM and IS-136 Networks.
HSCSD, GPRS & EDGE are collectively called 2.5G, However, EDGE is regarded by many as a 3G
technology.
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