GSM Overview

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					Unit 1




  Introduction and System Fundamentals
                LTW200L
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                  LTW200L
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Unit 1 Overview



                      Basic System
                  Elements and Principles




                   LTW200L
                  GSM System Standards
                   Version 8.1
                     LTW200L
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                      GSM Services
Unit 1 Objectives



 •   Identify the objectives for the Global System for Mobile Communication (GSM) standard

 •   Define the basic terms relating to wireless and cellular communication

 •   Compare and contrast GSM with other wireless services
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 •   Relate basic technical concepts to   their use
                                             Version 8.1   in cellular systems

 •   Identify the components of a cellular system and their functions

 •   Understand the phased release of GSM specifications

 •   Recognise the types of services supported by GSM
Unit 1 Section 1




         Basic System Elements and
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                 Principles
                     Version 8.1
Global System for Mobile
Communications (GSM)
 Definition
 The Global System for Mobile Communications (GSM) provides a common standard that enables
 users to roam from one country to another and obtain seamless telecommunications coverage and
 services

 Objectives
 •   Integrated European system with international roaming
 •   Increase available cellular system capacity
 •   Take advantage of digital price/performance and economies of scale
 •   Accommodate new technology and services LTW200L
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               - ISDN services                  Version 8.1
               - short messaging services
               - user data and fax
               - information privacy and secure access
               - smart-card technology
               - enhanced coding techniques

 • Apply to Cellular and Personal Communications Network services (GSM 900, GSM 1800, PCS 1900)
Major Worldwide Mobile
System Standards
First Generation - Analogue

• Amps (US), TACS (UK), JTACS (Japan), NMT (Nordic)
         - existing Analogue FM Standards

Second - Generation Digital

• GSM (European Digital Standard)
         - new 900 MHz Spectrum, TDMA, 271 kb/s
         - new 1800 MHz Spectrum, TDMA, 271 kb/s
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         - PCS 1900, Air Interface Specification for 1.8 to 2.0 GHz Frequency Hopping Time Division
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           Multiple Access (TDMA) for Personal8.1

• IS-136 (North American TDMA Digital Standard)
         - existing 850 MHz Bands, TDMA, 48 kb/s
         - IS-136 Based, Air Interface Compatibility PCS 1900 MHz Standard, ANSI, J-STD-011

• IS-95 (North American CDMA Digital Standard)
         - existing 850 MHz Bands, CDMA, 1.23 Mb/s
         - Personal Station-Base Station Compatibility requirement for 1.8 to 2.0 GHz Code Division
           Multiple Access (CDMA) Personal Communications, ANSI, J-STD-008

• PDC (Japanese Digital Cellular Standard)
         - similar to IS-136 on the radio side and GSM on the network side
Layout of a Basic Cellular Network

     BSS   Base Station System(Cell site)
     MS    Mobile Station (Mobile Unit)
     MSC   Mobile-services Switching Centre
           Radio Link                             To Telephone Network

           Land Links



                              MS
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                                                               MSC
                               BSS                                             BSS
                                                      BSS




                                               BSS                       BSS
Typical MSC Functions

 •   Provide switched connections between mobile and fixed (PSTN) phones

 •   Provide switched connections between mobile subscribers

 •   Provide coordination over signalling with mobiles

 •   Coordinate the location and handover process
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 •   Provide custom services to    mobile users 8.1
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 •   Collect billing data

 •   Collect traffic data

 •   Provisioning/service orders

 •   Maintenance functions
Typical Base Station System
Functions

 •   Provide RF transmission and reception

 •   Provide data communications with the MSC and mobile stations

 •   Locate mobiles

 •   Perform routine maintenance testing
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 •   Perform equipment control and   reconfiguration
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 •   Perform voice-processing functions

 •   Perform set-up, supervision and termination functions
Typical Mobile Station Functions

 •   Provide a telecommunications interface to subscribers

 •   Provide RF transmission and reception

 •   Transmit and receive user information and control data

 •   Perform voice-processing functions
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 •   Perform initialisation and self-test   functions
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Cellular Concepts

 • The key ways in
   which a cellular
   system can meet
                               Rural        Base Station
   its objectives are
   through:
    – The architecture   LTW200L
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      of the cellular
      system
    – Frequency re-use
    – Providing call
      handover
      capabilities
                                         City
    – Roaming
      capabilities
Frequency Assignment

 • Available spectrum is
   limited
 • Need to support large                196 channels spread across cells
                                        gives 28 channels per cell
   number of users
                                            29-56
 • The challenge is to
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                                     1-28           57-84          29-56
   assign the available
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                                                            1-28           57-84

   frequencies across the
   network while
   minimising the co-
   channel reuse distances
                                      Brown gets channels 1-28 and
 • The example shows a                these can be re-used 2 cells away,
                                      and so on
   repeat pattern of 7 cells
Frequency Re-use

  • Depends on:
    – Number and size of cells
                                                     R
       • more smaller cells carry more
                                         D
         total traffic
       • enables frequencies to be
         reused more  LTW200L
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       • increased system cost
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    – Frequency re-use achieved              4 cell repeat

       • tighter reuse gives increased
         capacity (more bandwidth per
         cell)
       • downside is increased
         interference                        3 cell repeat
    – Total available spectrum
Radio Frequency (RF)
Channel Reuse
                       Other                   Other MSCs
                       PLMN
                                        MSC




                                                            3,7,11,...
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                                              4,8,12,...                 2,6,10,...
          Subscriber
             Set


                        3,7,11,...                          1,5,9,…



                                              2,6,10,...



                        1,5,9,…
Interference and Re-use
Distance
 • The re-use distance D is directly related to the
   radius of the cell R
 • Clearly the re-use distance increases as the
   cell repeat number goes up (D=R3N)
 • It is not too difficult to relate the carrier to
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   interference ratio to re-use distance it is given
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   approximately by C/I=1.5N2
 • The table summarises this for various re-use
                 N       D/R    C/I
   numbers       4        3.5   13.8dB
               7          4.6        18.7dB
               12          6         23.3dB
Cellular Architecture

  • Coverage area of cell
    depends on traffic demand
  • National coverage achieved
    with mobile location
    continually monitored
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  • Handover across cellVersion 8.1
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    boundaries
  • Small cells and lower
    transmit powers
                                        To PSTN via
  • High network capacity -             Mobile Switching
    frequency re-use                    Centre
  • Radio channels are trunked
Omni/Sectored Base Stations
  • Omni-directional Cells:
     – 360 degree coverage
     – low network capacity
     – cost-effective
  • Sectored Cells:
     – 120 degree coverage
     – increases network LTW200L
                          capacity
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         • smaller coverage area
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         • improved frequency
           reuse
     – 3 times as much equipment
     – improved antenna gain
Why Sectorise?


  • Effectively creates a                                       C/I=4.5N2
    number of smaller cells,
    increasing capacity without
    needing extra sites                              A2        C1        A3        A2

  • Less interference because
                           LTW200L              B1        C3        C2
                                                                              B1        C3
    sector antenna are     Version 8.1
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    directional
                                           B3        B2                  B3        B2        A1
                                                               A1
  • Can also increase range
  • Gain limited by antenna                     C1        A3        A2        C1        A3
    leakage and handover
    problems
  • Typical GSM deployment
    has some omni-cells and                          The old omnis
    some 3-sectored cells
Network Capacity & Frequency
Reuse
 • There are many
   combinations of
   sectorisation and re-
   use patterns
                                                                           A2
                                                      LTW200L                        C3        C2        D3        B3        B2
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            A2        C1         A3                                   A2        A2        B1        D1        D2
                                           A2                                                                           C1        A3


       B1        C3        C2                                              D3        B3        B2                  C3        C2        D3
                                      B1        C3                                                       A1


  B3        B2                   B3        B2        A1               D1        D2        C1        A3        A2        B1        D1
                      A1


       C1        A3        A2         C1        A3                                   C3        C2        D1        B3        B2        A1



                                3/9 Re-use                                            4/12 Re-use
Frequency Allocation




  Frequency Allocation for a Total of 27 Carriers

  A1     B1       C1      A2        B2         C2   A3     B3      C3
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  1      2        3       4         5
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                                 Version 8.1   6    7      8       9

  10     11       12      13        14         15   16     17      18

  19     20       21      22        23         24   25     26      27

 For sectored cells the frequencies must be allocated so that cells do not
 use adjacent frequencies
Mobile Control
 • Mobiles need a general channel to
    – Log on, initiate calls, accept calls,
       etc
 • This is called the control channel                       Control Channel
    – Each base station has at least one
 • When a call is established the mobile
   is re-tuned to a traffic channel
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    – During the call all signalling takes
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                                                               Idle
       place over the traffic channel
                                              Idle

                                                                 Call

                                                             Traffic
                                                     Idle    Channel
Call Handover
 • An essential part of any cellular
   radio system
 • Enables conversations to                          Base 1
   continue as mobiles move
   between base station coverage
                                                                               Base 2
   areas
 • Process controlled by the
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 • Decision mainly based on                 Received
                                            Power
                                                                    Decision
   measurements by the mobile of                                     Margin

   the “best” available servers             Base 1

 • A “margin” is allowed before the
   decision is made - this prevents         Base 2

   “ping-ponging”                                                                       Distance
                                                              Handover
                                                              Point
GSM Handover


                                                           To
                                                       Frequency 9
                 MSC                                   Time Slot 7

                          From
                       Frequency 6
                       Time Slot 3
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                                    LTW200L                      BSS
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                                                  MS



    Subscriber
       Set                  BSS




                       •Lanline switched at MSC
                       •Frequency and time slot changed at
                       MS
Intra-network Roaming

 • This is simply the normal process whereby a
   MS can move about within the coverage area
   of its home network
   – The home network tracks and records which base
     stations the mobile is served by at any given point
     so that calls can be routed to and from it
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                                           Location Areas
Inter-network Roaming

   – Here the mobile is moving
     between two different networks -
     usually in different countries
   – When the mobile arrives in the
     foreign network the network
     determines its identity and seeks
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     information from its home network
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     to authenticate its request for
     service
   – Any calls made to the mobile first
     arrive at its home network before
     being forwarded to the foreign
     network in which it is roaming
Network Coverage

 • Depends on:
   – system characteristics (e.g. antenna gains etc)
   – type of service required (i.e. on-street, in-building
     etc)
   – terrain characteristics
   – surroundings (i.e. “clutter” - trees, buildings etc)
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 • Typically use smaller cells in urban areas
                                                  1 km radius
   – high traffic
   – dense clutter
 • Larger cells in rural areas
   – lower traffic                       40 km radius
   – less clutter
Microcells
  • In city centres, all the spectrum
    saving measures are not enough
  • Capacity is measured in channels
    per unit area
  • The smaller the cell, the higher the
    capacity per given area
  • Main way to make cells small is to
                              LTW200L
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    bring antennas below the 8.1 8.1
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    level
  • Result is signal constrained to up to
    1km of street
Microcells

 •   More capacity but more cost
 •   Make handover difficult
 •   But allow massive capacity increase
 •   Save mobile battery power
 •   Product now available     LTW200L
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                                               Cell size is
                                               determined by
     There are many other ways
                                               the power of the
     of increasing capacity at
                                               base station and
     hot spots - cell splitting and
                                               the mobile
     overlaid cells
The Need for Medium Access
Control
 • The earlier example showed that
   for a 10 MHz bandwidth and a
   repeat factor of 7 the maximum
   number of calls was 28 per cell
 • But in a cell there might be
   thousands of people with a phone
 • Therefore, each person only gets a
   channel when they need LTW200L
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 • Access to the medium needs to be
   controlled
Multiple Access Methods


              Frequency Division Multiple Access (FDMA)
                              Frequency 1   ch
                              Frequency 2   ch



                              Frequency N   ch


                Time Division Multiple Access (TDMA)
                         LTW200L
                Time       Version 8.1
                              Time
                             LTW200L                  Time
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                Slot 1       Slot 2                   Slot N
                  ch           ch                       ch



                Code Division Multiple Access (CDMA)
                         Code Sequence 1         ch
                         Code Sequence 2         ch



                         Code Sequence N         ch
MAC Alternatives - FDMA


                                   Each Carrier Carries One
                                   Traffic Channel
     Power




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             Ch 1     Ch 2      Ch 3            Ch 4      Ch 5       Ch 6




                                                       Centre Freq          Frequency
                    Bandwidth
MAC Alternatives - TDMA


       Freq
                       TDMA “Frame” Length

     Carrier 6    1   2    3   4   5      6       7    8   1   2   3   4   5   6

     Carrier 5    1   2    3   4   5      6       7    8   1   2   3   4   5   6
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     Carrier 4    1   2    3   4   5Version 8.1 8.17
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                                      Version
                                                       8   1   2   3   4   5   6

     Carrier 3    1   2    3   4   5      6       7    8   1   2   3   4   5   6

     Carrier 2    1   2    3   4   5      6       7    8   1   2   3   4   5   6

     Carrier 1    1   2    3   4   5      6       7    8   1   2   3   4   5   6

                                                                           Time
                 Each Time-slot Carries One Traffic
                 Channel
MAC Alternatives - CDMA



    Power




                                     All Channels Share
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                                        Same RF Band
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                                                                             Freq




            Code 1          Code 2          Code 3           Code 4

                     Ch 1            Ch 2             Ch 3            Ch 4
MAC Summary

• Take an example of 2 MHz of bandwidth
   – Frequency division multiple access could
     divide this into 40 bands, each 50 kHz wide
   – Time DMA could divide this into 40 time slots,
     each 25ms wide
   – Code DMA could divide this into 40 codes,
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     each causing the information to be spread by
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     40
• GSM uses a mixture - FDMA/TDMA
CDMA
• Not easy to understand
• Easier to hear your colleague at a cocktail party
  when everyone else is speaking a different language
• Digital signal generated by the speech encoder One cell repeat pattern
• Signal multiplied by the code allocated and
  transmitted
• Received signal multiplied by the same code
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• Result passed to the speech decoder
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• Spreading the signal by 40 means it takes up 40Spread Spectrum
  times the bandwidth but can tolerate 40 times the
  interference
TDMA vs CDMA
• An impassioned debate over the last few years
   – GSM capacity easy to calculate
   – CDMA much more difficult - softer
• Practical deployments suggests that CDMA may be around 30%
  better than GSM
• But
   – GSM hardware cheaper LTW200L
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   – world-wide roaming Version 8.1
   – lower risk and can be deployed now
• CDMA is the chosen basis for the next generation
Channels for Two-Way
Communications

                                  Frequency Division Duplex

                                    Frequency separation
                                     between uplink and
                                    downlink channel pairs


             1 2 3                                                1 2 3

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         Uplink RF carrier   channels Version 8.1            Downlink RF carrier channels




                                    Downlink



                                              Uplink
Digital Radio System



                                                                  Transmitted
                                   Transmitter                     RF Signal

                     Information
       Information                                  Modulation
                       Transmit
          Input                                     Processing
                     Processing

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                                                                      Received
                                    Receiver                          RF Signal

                     Information
       Information                                 Demodulation
                       Receive
          Output                                    Processing
                     Processing
Physical Channel Structure
Used in P-GSM900
                                            Time      Domain

                        Uplink
                                     Slot 0 Slot 1            Slot 7

                      Frequency 1     ch     ch          ch    ch

                      Frequency 2     ch     ch          ch    ch




                                                                       ARFCN 1
                     Frequency 124 ch        ch          ch    ch
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                                    LTW200L
         Frequency                  Version 8.1
          Domain       Downlink
                                     Slot 0 Slot 1            Slot 7

                      Frequency 1     ch     ch          ch    ch

                      Frequency 2     ch     ch          ch    ch




                     Frequency 124 ch        ch          ch    ch

                             992 Duplex Physical Channels Available
TDMA Operation in GSM

                                                 Full Rate

                  Frame (Count)                    Frame (Count + 1)

          0   1   2   3   4   5   6   7      0    1       2   3   4     5   6   7
                                                                                                  DOWNLINK

                              Frame (Count)                           Frame (Count + 1)

                      0   1   2   3   4      5    6       7   0   1     2   3   4   5     6   7

                                          LTW200L                                                 UPLINK
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     BS



                                                   MS1                              MS7
                              MS0                                       MS5
Simplified Digital TDMA
Implementation
                                                     Downlink


                         Base Transmitters                                            Mobile Receivers
            Baseband
    1                                                                       MS 1
            Processing

                            Time
                                        Mod
                            MUX                                                               Timeslot detection
                                                                             Demod              and Baseband
            Baseband       M time                                                                processing
    M                       slots            f d1
            Processing
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            Baseband                                                       MS MxN
    1
            Processing                                          Combiner

                            Time
                                        Mod                                                   Timeslot detection
                            MUX
                                                                             Demod              and Baseband
            Baseband       M time                                                                processing
    M                       slots            fdN
            Processing
                                                                                    fdN


        M time slots            N frequency carriers

				
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