Lecture 7n


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Lecture 7n

  1. 1. Code division Multiple Access <ul><li>CDMA (Code Division Multiple Access) </li></ul><ul><ul><li>all terminals send on the same frequency probably at the same time and can use the whole bandwidth of the transmission channel </li></ul></ul><ul><ul><li>each sender has a unique random number, the sender XORs the signal with this random number </li></ul></ul><ul><ul><li>the receiver can “tune” into this signal if it knows the pseudo random number, tuning is done via a correlation function </li></ul></ul><ul><li>Disadvantages: </li></ul><ul><ul><li>higher complexity of a receiver (receiver cannot just listen into the medium and start receiving if there is a signal) </li></ul></ul><ul><ul><li>all signals should have the same strength at a receiver </li></ul></ul><ul><li>Advantages: </li></ul><ul><ul><li>all terminals can use the same frequency, no planning needed </li></ul></ul><ul><ul><li>huge code space (e.g. 2 32 ) compared to frequency space </li></ul></ul><ul><ul><li>interferences (e.g. white noise) is not coded </li></ul></ul><ul><ul><li>forward error correction and encryption can be easily integrated </li></ul></ul>
  2. 2. Code division Multiple Access - Theory <ul><li>Sender A </li></ul><ul><ul><li>sends A d = 1, key A k = 010011 (assign: „0“= -1, „1“= +1) </li></ul></ul><ul><ul><li>sending signal A s = A d * A k = (-1, +1, -1, -1, +1, +1) </li></ul></ul><ul><li>Sender B </li></ul><ul><ul><li>sends B d = 0, key B k = 110101 (assign: „0“= -1, „1“= +1) </li></ul></ul><ul><ul><li>sending signal B s = B d * B k = (-1, -1, +1, -1, +1, -1) </li></ul></ul><ul><li>Both signals superimpose in space </li></ul><ul><ul><li>interference neglected (noise etc.) </li></ul></ul><ul><ul><li>A s + B s = (-2, 0, 0, -2, +2, 0) </li></ul></ul><ul><li>Receiver wants to receive signal from sender A </li></ul><ul><ul><li>apply key A k bitwise (inner product) </li></ul></ul><ul><ul><ul><li>A e = (-2, 0, 0, -2, +2, 0)  A k = 2 + 0 + 0 + 2 + 2 + 0 = 6 </li></ul></ul></ul><ul><ul><ul><li>result greater than 0, therefore, original bit was „1“ </li></ul></ul></ul><ul><ul><li>receiving B </li></ul></ul><ul><ul><ul><li>B e = (-2, 0, 0, -2, +2, 0)  B k = -2 + 0 + 0 - 2 - 2 + 0 = -6, i.e. „0“ </li></ul></ul></ul>
  3. 3. CDMA on signal level I data A key A signal A data  key key sequence A Real systems use much longer keys resulting in a larger distance between single code words in code space. 1 0 1 1 0 0 1 0 0 1 0 0 0 1 0 1 1 0 0 1 1 0 1 1 0 1 1 1 0 0 0 1 0 0 0 1 1 0 0 A d A k A s
  4. 4. CDMA on signal level II signal A data B key B key sequence B signal B A s + B s data  key 1 0 0 0 0 0 1 1 0 1 0 1 0 0 0 0 1 0 1 1 1 1 1 1 0 0 1 1 0 1 0 0 0 0 1 0 1 1 1 B d B k B s A s
  5. 5. CDMA on signal level III A k (A s + B s ) * A k integrator output comparator output A s + B s data A 1 0 1 1 0 1 A d
  6. 6. CDMA on signal level IV integrator output comparator output B k (A s + B s ) * B k A s + B s data B 1 0 0 1 0 0 B d
  7. 7. CDMA on signal level V comparator output wrong key K integrator output (A s + B s ) * K A s + B s (0) (0) ?
  8. 8. SAMA - Spread Aloha Multiple Access <ul><li>Aloha has only a very low efficiency, CDMA needs complex receivers to be able to receive different senders with individual codes at the same time </li></ul><ul><li>Idea : use spread spectrum with only one single code (chipping sequence) for spreading for all senders accessing according to aloha </li></ul>1 sender A 0 sender B 0 1 t narrow band send for a shorter period with higher power spread the signal e.g. using the chipping sequence 110101 („ CDMA without CD “) Problem: find a chipping sequence with good characteristics 1 1 collision
  9. 9. SAMA <ul><li>Here, the chipping sequence used is </li></ul><ul><li>[1 1 0 1 0 1] </li></ul><ul><li>Very important: </li></ul><ul><li>All the signals are sent with the same chipping sequence </li></ul><ul><li>BUT the chipping phase differs slightly . </li></ul><ul><li>How do we separate the signals? </li></ul><ul><li>One receiver is synchronized to signal A and the other to signal B. </li></ul><ul><li>THROUGHPUT IS THE SAME AS CLASSICAL ALOHA </li></ul><ul><li>BUT IT IS MORE ROBUST BECAUSE OF SPREAD SPECTRUM </li></ul>
  10. 10. Comparison SDMA/TDMA/FDMA/CDMA
  11. 11. Telecommunications Systems year Subscribers [million] 0 200 400 600 800 1000 1200 1400 1600 1996 1997 1998 1999 2000 2001 2002 2003 2004 approx. 1.7 bn GSM total TDMA total CDMA total PDC total Analogue total W-CDMA Total wireless Prediction (1998) Mobile Phone Subscribers world-wide
  12. 12. Development of mobile telecommunication systems 1G 2G 3G 2.5G IS-95 cdmaOne IS-136 TDMA D-AMPS GSM PDC GPRS IMT-DS UTRA FDD / W-CDMA EDGE IMT-TC UTRA TDD / TD-CDMA cdma2000 1X 1X EV-DV (3X) AMPS NMT IMT-SC IS-136HS UWC-136 IMT-TC TD-SCDMA CT0/1 CT2 IMT-FT DECT CDMA TDMA FDMA IMT-MC cdma2000 1X EV-DO HSDPA
  13. 13. GSM <ul><li>GSM </li></ul><ul><ul><li>formerly: Groupe Spéciale Mobile (founded 1982) </li></ul></ul><ul><ul><li>now: Global System for Mobile Communication </li></ul></ul><ul><ul><li>Pan-European standard (ETSI, European Telecommunications Standardisation Institute) </li></ul></ul><ul><ul><li>simultaneous introduction of essential services in three phases (1991, 1994, 1996) by the European telecommunication administrations (Germany: D1 and D2)  seamless roaming within Europe possible </li></ul></ul><ul><ul><li>today many providers all over the world use GSM (more than 200 countries in Asia, Africa, Europe, Australia, America) </li></ul></ul><ul><ul><li>more than 1.2 billion subscribers in more than 630 networks </li></ul></ul><ul><ul><li>more than 75% of all digital mobile phones use GSM (74% total) </li></ul></ul><ul><ul><li>over 200 million SMS per month in Germany, > 550 billion/year worldwide (> 10% of the revenues for many operators) [be aware: these are only rough numbers…] </li></ul></ul>
  14. 14. Additional features of GSM <ul><li>GSM-Rail </li></ul><ul><ul><li>19 available channels exclusively for the railway data and voice </li></ul></ul><ul><ul><li>Emergency calls with ACK </li></ul></ul><ul><ul><li>Voice group call service </li></ul></ul><ul><ul><li>Voice broadcast service </li></ul></ul><ul><ul><li>Control of railway switches, gates and signals </li></ul></ul>
  15. 15. Performance characteristics of GSM (wrt. analog sys.) <ul><li>Communication </li></ul><ul><ul><li>mobile, wireless communication; support for voice and data services </li></ul></ul><ul><li>Total mobility </li></ul><ul><ul><li>international access, chip-card enables use of access points of different providers </li></ul></ul><ul><li>Worldwide connectivity </li></ul><ul><ul><li>one number, the network handles localization </li></ul></ul><ul><li>High capacity </li></ul><ul><ul><li>better frequency efficiency, smaller cells, more customers per cell </li></ul></ul><ul><li>High transmission quality </li></ul><ul><ul><li>high audio quality and reliability for wireless, uninterrupted phone calls at higher speeds (e.g., from cars, trains) </li></ul></ul><ul><li>Security functions </li></ul><ul><ul><li>access control, authentication via chip-card and PIN </li></ul></ul>
  16. 16. Disadvantages of GSM <ul><li>There is no perfect system!! </li></ul><ul><li>no end-to-end encryption of user data </li></ul><ul><li>no full ISDN bandwidth of 64 kbit/s to the user, no transparent B-channel </li></ul><ul><li>reduced concentration while driving </li></ul><ul><li>electromagnetic radiation </li></ul><ul><li>abuse of private data possible </li></ul><ul><li>roaming profiles accessible </li></ul><ul><li>high complexity of the system </li></ul><ul><li>several incompatibilities within the GSM standards </li></ul>
  17. 17. GSM: Mobile Services <ul><li>GSM offers </li></ul><ul><ul><li>several types of connections </li></ul></ul><ul><ul><ul><li>voice connections, data connections, short message service </li></ul></ul></ul><ul><ul><li>multi-service options (combination of basic services) </li></ul></ul><ul><li>Three service domains </li></ul><ul><ul><li>Bearer Services </li></ul></ul><ul><ul><li>Telematic Services </li></ul></ul><ul><ul><li>Supplementary Services </li></ul></ul>GSM-PLMN transit network (PSTN, ISDN) source/ destination network TE TE bearer services tele services R, S (U, S, R) U m MT MS
  18. 18. Bearer Services <ul><li>Telecommunication services to transfer data between access points </li></ul><ul><li>Specification of services up to the terminal interface (OSI layers 1-3) </li></ul><ul><li>Different data rates for voice and data (original standard) </li></ul><ul><ul><li>data service (circuit switched) </li></ul></ul><ul><ul><ul><li>synchronous: 2.4, 4.8 or 9.6 kbit/s </li></ul></ul></ul><ul><ul><ul><li>asynchronous: 300 - 1200 bit/s </li></ul></ul></ul><ul><ul><li>data service (packet switched) </li></ul></ul><ul><ul><ul><li>synchronous: 2.4, 4.8 or 9.6 kbit/s </li></ul></ul></ul><ul><ul><ul><li>asynchronous: 300 - 9600 bit/s </li></ul></ul></ul><ul><li>Today: data rates of approx. 50 kbit/s possible – will be covered later! </li></ul>
  19. 19. Bearer Services <ul><li>Bearer services permit transparent and non-transparent, synchronous and asynchronous data services </li></ul><ul><li>Transparent Bearer services only use the functions of physical layer for data transmission. </li></ul><ul><ul><li>Forward error correction is possible </li></ul></ul><ul><ul><li>Data rates : 2.4, 4.8 & 9.6 kbps </li></ul></ul>
  20. 20. Non-Transparent Bearer Services <ul><li>Use layers 2 & 3 to introduce error correction and flow control </li></ul><ul><li>Special selective-reject mechanisms to trigger re-transmission of erroneous data. </li></ul><ul><li>Bit error date is < 10 -7 </li></ul>
  21. 21. Tele Services I <ul><li>Telecommunication services that enable voice communication via mobile phones </li></ul><ul><li>All these basic services have to obey cellular functions, security measurements etc. </li></ul><ul><li>Offered services </li></ul><ul><ul><li>mobile telephony primary goal of GSM was to enable mobile telephony offering the traditional bandwidth of 3.1 kHz </li></ul></ul><ul><ul><li>Emergency number common number throughout Europe (112); mandatory for all service providers; free of charge; connection with the highest priority (preemption of other connections possible) </li></ul></ul><ul><ul><li>Multinumbering several ISDN phone numbers per user possible </li></ul></ul>
  22. 22. Tele Services II <ul><li>Additional services </li></ul><ul><ul><li>Non-Voice-Teleservices </li></ul></ul><ul><ul><ul><li>group 3 fax </li></ul></ul></ul><ul><ul><ul><li>voice mailbox (implemented in the fixed network supporting the mobile terminals) </li></ul></ul></ul><ul><ul><ul><li>electronic mail (MHS, Message Handling System, implemented in the fixed network) </li></ul></ul></ul><ul><ul><ul><li>... </li></ul></ul></ul><ul><ul><ul><li>Short Message Service (SMS) alphanumeric data transmission to/from the mobile terminal (160 characters) using the signaling channel, thus allowing simultaneous use of basic services and SMS (almost ignored in the beginning now the most successful add-on!) </li></ul></ul></ul>
  23. 23. Supplementary services <ul><li>Services in addition to the basic services, cannot be offered stand-alone </li></ul><ul><li>Similar to ISDN services besides lower bandwidth due to the radio link </li></ul><ul><li>May differ between different service providers, countries and protocol versions </li></ul><ul><li>Important services </li></ul><ul><ul><li>identification: forwarding of caller number </li></ul></ul><ul><ul><li>suppression of number forwarding </li></ul></ul><ul><ul><li>automatic call-back </li></ul></ul><ul><ul><li>conferencing with up to 7 participants </li></ul></ul><ul><ul><li>locking of the mobile terminal (incoming or outgoing calls) </li></ul></ul><ul><ul><li>... </li></ul></ul>
  24. 24. Architecture of the GSM system <ul><li>GSM is a PLMN (Public Land Mobile Network) </li></ul><ul><ul><li>several providers setup mobile networks following the GSM standard within each country </li></ul></ul><ul><ul><li>components </li></ul></ul><ul><ul><ul><li>MS (mobile station) </li></ul></ul></ul><ul><ul><ul><li>BS (base station) </li></ul></ul></ul><ul><ul><ul><li>MSC (mobile switching center) </li></ul></ul></ul><ul><ul><ul><li>LR (location register) </li></ul></ul></ul><ul><ul><li>subsystems </li></ul></ul><ul><ul><ul><li>RSS (radio subsystem): covers all radio aspects </li></ul></ul></ul><ul><ul><ul><li>NSS (network and switching subsystem): call forwarding, handover, switching </li></ul></ul></ul><ul><ul><ul><li>OSS (operation subsystem): management of the network </li></ul></ul></ul>
  25. 25. GSM: overview fixed network BSC BSC MSC MSC GMSC OMC, EIR, AUC VLR HLR NSS with OSS RSS VLR
  26. 26. Nomenclature <ul><li>OMC : Operations and Maintenance centre </li></ul><ul><li>MSC : Gateway MSC </li></ul><ul><li>EIR : Equipment Identity Register </li></ul><ul><li>AUC : Authentication Centre </li></ul><ul><li>PDN : Public Data Network </li></ul>
  27. 27. GSM: elements and interfaces NSS MS MS BTS BSC GMSC IWF OMC BTS BSC MSC MSC A bis U m EIR HLR VLR VLR A BSS PDN ISDN, PSTN RSS radio cell radio cell MS AUC OSS signaling O
  28. 28. GSM: system architecture U m A bis A BSS radio subsystem MS MS BTS BSC BTS BTS BSC BTS network and switching subsystem MSC MSC fixed partner networks IWF ISDN PSTN PSPDN CSPDN SS7 EIR HLR VLR ISDN PSTN
  29. 29. System architecture: radio subsystem U m A bis A BSS radio subsystem network and switching subsystem MS MS BTS BSC MSC BTS BTS BSC BTS MSC
  30. 30. System architecture: network and switching subsystem network subsystem MSC MSC fixed partner networks IWF ISDN PSTN PSPDN CSPDN SS7 EIR HLR VLR ISDN PSTN <ul><li>Components </li></ul><ul><ul><li>MSC (Mobile Services Switching Center): </li></ul></ul><ul><ul><li>IWF (Interworking Functions) </li></ul></ul><ul><ul><li>ISDN (Integrated Services Digital Network) </li></ul></ul><ul><ul><li>PSTN (Public Switched Telephone Network) </li></ul></ul><ul><ul><li>PSPDN (Packet Switched Public Data Net.) </li></ul></ul><ul><ul><li>CSPDN (Circuit Switched Public Data Net.) </li></ul></ul><ul><li>Databases </li></ul><ul><ul><li>HLR (Home Location R egister) </li></ul></ul><ul><ul><li>VLR (Visitor Location R egister) </li></ul></ul><ul><ul><li>EIR (Equipment Identity Register) </li></ul></ul>
  31. 31. Radio Subsystem <ul><li>Consists of mobile stations and Base station subsystems </li></ul><ul><li>A Interface : Between RSS and NSS </li></ul><ul><li>O interface between RSS & OSS – uses X25 standard for data transfer with OMC </li></ul>
  32. 32. BSS <ul><li>GSM network conprises many BSSs each controlled by a base station controller </li></ul><ul><ul><li>Performs all radio functions </li></ul></ul><ul><ul><li>Coding-decoding of voice </li></ul></ul><ul><ul><li>Data rate adoption </li></ul></ul><ul><ul><li>Contains a BSC and several BTSs. </li></ul></ul>
  33. 33. BTS <ul><li>Comprises radio equipment (antennas, signal processing entities, amplifiers etc) </li></ul><ul><li>Can form a radio cell or several cells using sectored antennas. </li></ul><ul><li>Um interface is used to connect a BTS to Mobile station. </li></ul><ul><li>Um interface contains all the mechanisms for wireless transmission namely TDMA, FDMA, CDMA etc. </li></ul>
  34. 34. Base Station Controller <ul><li>Manages BTSs </li></ul><ul><li>Allocates frequencies </li></ul><ul><li>Manages handover from BTS to another BTS within the same BSS. </li></ul><ul><li>Performs paging functions </li></ul><ul><li>Multiplexes the radio channel onto a fixed network. </li></ul>
  35. 35. Mobile Station : MS <ul><li>Consists of </li></ul><ul><li>user independent hardware and software </li></ul><ul><li>Subscriber Identification Module : SIM </li></ul><ul><li>MS can be identified via a IMEI (International Mobile equipment identity) </li></ul><ul><li>SIM can be used to personalize user preferences </li></ul><ul><li>IMEI can be used for device specific features such as theft protection etc. </li></ul>
  36. 36. Mobile Station : MS <ul><li>SIM card has identifiers and tables to store: </li></ul><ul><li>Card type </li></ul><ul><li>Serial no. </li></ul><ul><li>List of subscriber services </li></ul><ul><li>Personal Identity No.(PIN) </li></ul><ul><li>Pin Unblocking Key(PUK) </li></ul><ul><li>Authentication key </li></ul><ul><li>International Mobile Subscriber Identity (IMSI) </li></ul>
  37. 37. Mobile Station : MS <ul><li>Dynamic Information Stored on MS </li></ul><ul><li>Cipher key Kc </li></ul><ul><li>Location information (Temporary Mobile Subscriber Identity) </li></ul><ul><li>Location Area Identification </li></ul>