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  1. 1. Unit 1 Introduction and System Fundamentals
  2. 2. Unit 1 Overview GSM System Standards GSM Services Basic System Elements and Principles
  3. 3. Unit 1 Objectives <ul><li>Identify the objectives for the Global System for Mobile Communication (GSM) standard </li></ul><ul><li>Define the basic terms relating to wireless and cellular communication </li></ul><ul><li>Compare and contrast GSM with other wireless services </li></ul><ul><li>Relate basic technical concepts to their use in cellular systems </li></ul><ul><li>Identify the components of a cellular system and their functions </li></ul><ul><li>Understand the phased release of GSM specifications </li></ul><ul><li>Recognise the types of services supported by GSM </li></ul>
  4. 4. Unit 1 Section 1 Basic System Elements and Principles
  5. 5. Global System for Mobile Communications (GSM) <ul><li>Definition </li></ul><ul><li>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 </li></ul><ul><li>Objectives </li></ul><ul><li>Integrated European system with international roaming </li></ul><ul><li>Increase available cellular system capacity </li></ul><ul><li>Take advantage of digital price/performance and economies of scale </li></ul><ul><li>Accommodate new technology and services </li></ul><ul><li>- ISDN services </li></ul><ul><li>- short messaging services </li></ul><ul><li>- user data and fax </li></ul><ul><li>- information privacy and secure access </li></ul><ul><li>- smart-card technology </li></ul><ul><li>- enhanced coding techniques </li></ul><ul><li>Apply to Cellular and Personal Communications Network services (GSM 900, GSM 1800, PCS 1900) </li></ul>
  6. 6. Major Worldwide Mobile System Standards <ul><li>First Generation - Analogue </li></ul><ul><li>Amps (US), TACS (UK), JTACS (Japan), NMT (Nordic) </li></ul><ul><li>- existing Analogue FM Standards </li></ul><ul><li>Second - Generation Digital </li></ul><ul><li>GSM (European Digital Standard) </li></ul><ul><li>- new 900 MHz Spectrum, TDMA, 271 kb/s </li></ul><ul><li>- new 1800 MHz Spectrum, TDMA, 271 kb/s </li></ul><ul><li>- PCS 1900, Air Interface Specification for 1.8 to 2.0 GHz Frequency Hopping Time Division </li></ul><ul><li> Multiple Access (TDMA) for Personal Communications Services, ANSI, J-STD-007 </li></ul><ul><li>IS-136 (North American TDMA Digital Standard) </li></ul><ul><li>- existing 850 MHz Bands, TDMA, 48 kb/s </li></ul><ul><li>- IS-136 Based, Air Interface Compatibility PCS 1900 MHz Standard, ANSI, J-STD-011 </li></ul><ul><li>IS-95 (North American CDMA Digital Standard) </li></ul><ul><li>- existing 850 MHz Bands, CDMA, 1.23 Mb/s </li></ul><ul><li>- Personal Station-Base Station Compatibility requirement for 1.8 to 2.0 GHz Code Division </li></ul><ul><li> Multiple Access (CDMA) Personal Communications, ANSI, J-STD-008 </li></ul><ul><li>PDC (Japanese Digital Cellular Standard) </li></ul><ul><li>- similar to IS-136 on the radio side and GSM on the network side </li></ul>
  7. 7. Layout of a Basic Cellular Network To Telephone Network Radio Link Land Links Mobile-services Switching Centre Mobile Station (Mobile Unit) Base Station System(Cell site) MSC MS BSS MSC BSS BSS BSS BSS BSS BSS BSS MS
  8. 8. Typical MSC Functions <ul><li>Provide switched connections between mobile and fixed (PSTN) phones </li></ul><ul><li>Provide switched connections between mobile subscribers </li></ul><ul><li>Provide coordination over signalling with mobiles </li></ul><ul><li>Coordinate the location and handover process </li></ul><ul><li>Provide custom services to mobile users </li></ul><ul><li>Collect billing data </li></ul><ul><li>Collect traffic data </li></ul><ul><li>Provisioning/service orders </li></ul><ul><li>Maintenance functions </li></ul>
  9. 9. Typical Base Station System Functions <ul><li>Provide RF transmission and reception </li></ul><ul><li>Provide data communications with the MSC and mobile stations </li></ul><ul><li>Locate mobiles </li></ul><ul><li>Perform routine maintenance testing </li></ul><ul><li>Perform equipment control and reconfiguration functions </li></ul><ul><li>Perform voice-processing functions </li></ul><ul><li>Perform set-up, supervision and termination functions </li></ul>
  10. 10. Typical Mobile Station Functions <ul><li>Provide a telecommunications interface to subscribers </li></ul><ul><li>Provide RF transmission and reception </li></ul><ul><li>Transmit and receive user information and control data </li></ul><ul><li>Perform voice-processing functions </li></ul><ul><li>Perform initialisation and self-test functions </li></ul>
  11. 11. Cellular Concepts <ul><li>The key ways in which a cellular system can meet its objectives are through: </li></ul><ul><ul><li>The architecture of the cellular system </li></ul></ul><ul><ul><li>Frequency re-use </li></ul></ul><ul><ul><li>Providing call handover capabilities </li></ul></ul><ul><ul><li>Roaming capabilities </li></ul></ul>Base Station Rural City
  12. 12. Frequency Assignment <ul><li>Available spectrum is limited </li></ul><ul><li>Need to support large number of users </li></ul><ul><li>The challenge is to assign the available frequencies across the network while minimising the co-channel reuse distances </li></ul><ul><li>The example shows a repeat pattern of 7 cells </li></ul>196 channels spread across cells gives 28 channels per cell 1-28 1-28 29-56 29-56 57-84 57-84 Brown gets channels 1-28 and these can be re-used 2 cells away, and so on
  13. 13. Frequency Re-use <ul><li>Depends on: </li></ul><ul><ul><li>Number and size of cells </li></ul></ul><ul><ul><ul><li>more smaller cells carry more total traffic </li></ul></ul></ul><ul><ul><ul><li>enables frequencies to be reused more </li></ul></ul></ul><ul><ul><ul><li>increased system cost </li></ul></ul></ul><ul><ul><li>Frequency re-use achieved </li></ul></ul><ul><ul><ul><li>tighter reuse gives increased capacity (more bandwidth per cell) </li></ul></ul></ul><ul><ul><ul><li>downside is increased interference </li></ul></ul></ul><ul><ul><li>Total available spectrum </li></ul></ul>4 cell repeat 3 cell repeat D R
  14. 14. Radio Frequency (RF) Channel Reuse 1,5,9,… 2,6,10,... 3,7,11,... 1,5,9,… 3,7,11,... 4,8,12,... 2,6,10,... Subscriber Set MSC Other PLMN Other MSCs
  15. 15. Interference and Re-use Distance <ul><li>The re-use distance D is directly related to the radius of the cell R </li></ul><ul><li>Clearly the re-use distance increases as the cell repeat number goes up (D=R  3N) </li></ul><ul><li>It is not too difficult to relate the carrier to interference ratio to re-use distance it is given approximately by C/I=1.5N 2 </li></ul><ul><li>The table summarises this for various re-use numbers </li></ul>
  16. 16. Cellular Architecture <ul><li>Coverage area of cell depends on traffic demand </li></ul><ul><li>National coverage achieved with mobile location continually monitored </li></ul><ul><li>Handover across cell boundaries </li></ul><ul><li>Small cells and lower transmit powers </li></ul><ul><li>High network capacity - frequency re-use </li></ul><ul><li>Radio channels are trunked </li></ul>To PSTN via Mobile Switching Centre
  17. 17. Omni/Sectored Base Stations <ul><li>Omni-directional Cells: </li></ul><ul><ul><li>360 degree coverage </li></ul></ul><ul><ul><li>low network capacity </li></ul></ul><ul><ul><li>cost-effective </li></ul></ul><ul><li>Sectored Cells: </li></ul><ul><ul><li>120 degree coverage </li></ul></ul><ul><ul><li>increases network capacity </li></ul></ul><ul><ul><ul><li>smaller coverage area </li></ul></ul></ul><ul><ul><ul><li>improved frequency reuse </li></ul></ul></ul><ul><ul><li>3 times as much equipment </li></ul></ul><ul><ul><li>improved antenna gain </li></ul></ul>
  18. 18. Why Sectorise? <ul><li>Effectively creates a number of smaller cells, increasing capacity without needing extra sites </li></ul><ul><li>Less interference because sector antenna are directional </li></ul><ul><li>Can also increase range </li></ul><ul><li>Gain limited by antenna leakage and handover problems </li></ul><ul><li>Typical GSM deployment has some omni-cells and some 3-sectored cells </li></ul>The old omnis C/I=4.5N 2
  19. 19. Network Capacity & Frequency Reuse <ul><li>There are many combinations of sectorisation and re-use patterns </li></ul>3/9 Re-use 4/12 Re-use
  20. 20. Frequency Allocation For sectored cells the frequencies must be allocated so that cells do not use adjacent frequencies
  21. 21. Mobile Control <ul><li>Mobiles need a general channel to </li></ul><ul><ul><li>Log on, initiate calls, accept calls, etc </li></ul></ul><ul><li>This is called the control channel </li></ul><ul><ul><li>Each base station has at least one </li></ul></ul><ul><li>When a call is established the mobile is re-tuned to a traffic channel </li></ul><ul><ul><li>During the call all signalling takes place over the traffic channel </li></ul></ul>Idle Idle Idle Call Control Channel Traffic Channel
  22. 22. Call Handover <ul><li>An essential part of any cellular radio system </li></ul><ul><li>Enables conversations to continue as mobiles move between base station coverage areas </li></ul><ul><li>Process controlled by the system </li></ul><ul><li>Decision mainly based on measurements by the mobile of the “best” available servers </li></ul><ul><li>A “margin” is allowed before the decision is made - this prevents “ping-ponging” </li></ul>Base 1 Base 2 Handover Point Received Power Base 1 Base 2 Distance Decision Margin
  23. 23. GSM Handover From Frequency 6 Time Slot 3 To Frequency 9 Time Slot 7 MSC Subscriber Set <ul><li>Lanline switched at MSC </li></ul><ul><li>Frequency and time slot changed at MS </li></ul>MS BSS BSS
  24. 24. Intra-network Roaming <ul><li>This is simply the normal process whereby a MS can move about within the coverage area of its home network </li></ul><ul><ul><li>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 </li></ul></ul>Location Areas
  25. 25. Inter - network Roaming <ul><ul><li>Here the mobile is moving between two different networks - usually in different countries </li></ul></ul><ul><ul><li>When the mobile arrives in the foreign network the network determines its identity and seeks information from its home network to authenticate its request for service </li></ul></ul><ul><ul><li>Any calls made to the mobile first arrive at its home network before being forwarded to the foreign network in which it is roaming </li></ul></ul>
  26. 26. Network Coverage <ul><li>Depends on: </li></ul><ul><ul><li>system characteristics (e.g. antenna gains etc) </li></ul></ul><ul><ul><li>type of service required (i.e. on-street, in-building etc) </li></ul></ul><ul><ul><li>terrain characteristics </li></ul></ul><ul><ul><li>surroundings (i.e. “clutter” - trees, buildings etc) </li></ul></ul><ul><li>Typically use smaller cells in urban areas </li></ul><ul><ul><li>high traffic </li></ul></ul><ul><ul><li>dense clutter </li></ul></ul><ul><li>Larger cells in rural areas </li></ul><ul><ul><li>lower traffic </li></ul></ul><ul><ul><li>less clutter </li></ul></ul>40 km radius 1 km radius
  27. 27. Microcells <ul><li>In city centres, all the spectrum saving measures are not enough </li></ul><ul><li>Capacity is measured in channels per unit area </li></ul><ul><li>The smaller the cell, the higher the capacity per given area </li></ul><ul><li>Main way to make cells small is to bring antennas below the rooftop level </li></ul><ul><li>Result is signal constrained to up to 1km of street </li></ul>
  28. 28. Microcells <ul><li>More capacity but more cost </li></ul><ul><li>Make handover difficult </li></ul><ul><li>But allow massive capacity increase </li></ul><ul><li>Save mobile battery power </li></ul><ul><li>Product now available </li></ul>Cell size is determined by the power of the base station and the mobile There are many other ways of increasing capacity at hot spots - cell splitting and overlaid cells
  29. 29. The Need for Medium Access Control <ul><li>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 </li></ul><ul><li>But in a cell there might be thousands of people with a phone </li></ul><ul><li>Therefore, each person only gets a channel when they need it </li></ul><ul><li>Access to the medium needs to be controlled </li></ul>
  30. 30. Multiple Access Methods Frequency Division Multiple Access (FDMA) Frequency 1 ch Frequency 2 ch Frequency N ch Time Division Multiple Access (TDMA) Time Time Time 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
  31. 31. MAC Alternatives - FDMA
  32. 32. MAC Alternatives - TDMA
  33. 33. MAC Alternatives - CDMA Code 1 Code 2 Code 3 Code 4 All Channels Share Same RF Band Ch 1 Ch 2 Ch 3 Ch 4 Power Freq
  34. 34. MAC Summary <ul><li>Take an example of 2 MHz of bandwidth </li></ul><ul><ul><li>Frequency division multiple access could divide this into 40 bands, each 50 kHz wide </li></ul></ul><ul><ul><li>Time DMA could divide this into 40 time slots, each 25ms wide </li></ul></ul><ul><ul><li>Code DMA could divide this into 40 codes, each causing the information to be spread by 40 </li></ul></ul><ul><li>GSM uses a mixture - FDMA/TDMA </li></ul>
  35. 35. CDMA <ul><li>Not easy to understand </li></ul><ul><li>Easier to hear your colleague at a cocktail party when everyone else is speaking a different language </li></ul><ul><li>Digital signal generated by the speech encoder </li></ul><ul><li>Signal multiplied by the code allocated and transmitted </li></ul><ul><li>Received signal multiplied by the same code </li></ul><ul><li>Result passed to the speech decoder </li></ul><ul><li>Spreading the signal by 40 means it takes up 40 times the bandwidth but can tolerate 40 times the interference </li></ul>Spread Spectrum One cell repeat pattern
  36. 36. TDMA vs CDMA <ul><li>An impassioned debate over the last few years </li></ul><ul><ul><li>GSM capacity easy to calculate </li></ul></ul><ul><ul><li>CDMA much more difficult - softer </li></ul></ul><ul><li>Practical deployments suggests that CDMA may be around 30% better than GSM </li></ul><ul><li>But </li></ul><ul><ul><li>GSM hardware cheaper </li></ul></ul><ul><ul><li>world-wide roaming </li></ul></ul><ul><ul><li>lower risk and can be deployed now </li></ul></ul><ul><li>CDMA is the chosen basis for the next generation </li></ul>
  37. 37. Channels for Two-Way Communications Frequency Division Duplex Uplink Downlink 1 2 3 Uplink RF carrier channels 1 2 3 Downlink RF carrier channels frequency Frequency separation between uplink and downlink channel pairs
  38. 38. Digital Radio System Information Transmit Processing Modulation Processing Information Input Information Receive Processing Demodulation Processing Information Output Receiver Transmitter Received RF Signal Transmitted RF Signal
  39. 39. Physical Channel Structure Used in P-GSM900 Frequency 1 ch ch Slot 0 Slot 1 ch ch Slot 7 Frequency 2 ch ch ch ch Frequency 124 ch ch ch ch Downlink Frequency 1 ch ch Slot 0 Slot 1 ch ch Slot 7 Frequency 2 ch ch ch ch Frequency 124 ch ch ch ch Uplink 992 Duplex Physical Channels Available Time Domain ARFCN 1 Frequency Domain
  40. 40. TDMA Operation in GSM Full Rate 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 Frame (Count) Frame (Count + 1) MS7 MS1 MS5 MS0 BS UPLINK DOWNLINK 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 Frame (Count) Frame (Count + 1)
  41. 41. Simplified Digital TDMA Implementation Downlink Timeslot detection and Baseband processing Mod Time MUX Baseband Processing 1 Baseband Processing M M time slots d1 f Mod Time MUX Baseband Processing 1 Baseband Processing M M time slots dN f Demod d1 f Combiner Timeslot detection and Baseband processing Demod dN f MS MxN MS 1 Mobile Receivers Base Transmitters N frequency carriers M time slots