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Gsm air interface

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GSM system working

GSM system working

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Transcript

  • 1. Radio Transmission Techniques FDMA TDMA CDMAChannels Physical channels Logical channels
  • 2. FrequencyChannel Time
  • 3. Frequency Time SlotChannel Time 0 1 2 3 4 5 6 7
  • 4. Frequency Code Time Code 1 Code 2 Code 3
  • 5. ChannelsPhysical Channels Associated with frequency bands, time slots, codes Physical channels transfer bits from one network element to anotherLogical Channels Distinguished by the nature of carried information and the way to assemble bits into data units Three types  one-to-one: traffic channels between a BTS and a MS  one-to-many: synchronization signals from BTS to MSs in a cell  many-to-one: from MSs to the same BTS
  • 6. Air Interface Layers Layer 3 Messages Messages Messages Logical Layer 2 Logical Channels Packets Channels Radio Layer 1 Radio Transmission Bits Transmission Terminal Base Station
  • 7. GSM Frame Structure Superframe ... 51 Multiframe 6.12 sec 120 msec ... 26 Frames 4.615 msec 0 7 8 Multiframe Coded data Midamble0.57692 msec 3 57 1 26 1 57 3 8.25156.25 bits Tail bit Stealing Flag Guard period
  • 8. Frame Types • There are two types of multiframe – 26 TDMA-frame multiframe is used to carry TCH, SACCH and FACCH – 51 TDMA-frame multiframe is used to carry BCCH, CCH, SDCCH and SACCH
  • 9. Burst and Frames• The information contained in one time slot on the TDMA frame is call a burst.• Five types of burst – Normal Burst (NB) – Frequency Correction Burst (FB) – Synchronization Burst (SB) – Access Burst (AB) – Dummy Burst
  • 10. Traffic TCH/F: Full-rate Traffic Channel Two-waychannels TCH/H: Half-rate Traffic Channel (TCH) FCCH: Frequency correction BCH SCH: Synchronization BCCH: Broadcast control Base-to-Signaling PCH: Paging mobile CCCHchannel AGCH: Access grant RACH: Random access SDCCH: Stand-alone dedicated control DCCH SACCH: Slow associated control Two-way FACCH: Fast associated control
  • 11. Logical Channels• Control Channels – Broadcast Channels (BCH) – Common Control Channels (CCCH) – Dedicated Control Channels (DCCH)• Traffic Channels (TCH) – Full Rate (TCH/F) – Half Rate (TCH/H)
  • 12. UP / Down-LinkDown-link: the transmission path from Base Station to Mobile StationUp-link: the transmission path from Mobile Station to Base Station
  • 13. Control Channels• Broadcast Channels (BCH) – Frequency Correction Channel (FCCH) – Synchronization Channel (SCH) – Broadcast Control Channel (BCCH)• Common Control Channels (CCCH) – Paging Channel (PCH) – Random Access Channel (RACH) – Access Grant Channel (AGCH)• Dedicated Control Channels (DCCH) – Stand alone Dedicated Control Channel (SDCCH) – Cell Broadcast Channel (CBCH) – Slow Associated Control Channel (SACCH) – Fast Associated Control Channel (FACCH)
  • 14. Broadcast Channels (BCH)To help the MH (Mobile Handset) to turn to a BTS to listen for the cell information  to start roaming, waiting for calls to arrive, making calls Because BTSs are not synchronized with each other, every time a MH decides to camp to another cell, its FCCH, SCH, and BCCH must be read.
  • 15. Frequency Correction Channel (FCCH)• Provide MH with the frequency reference of the system – To enable the Mobile Handset (MH) to synchronize with the frequency• Transmission properties – Transmit on the down-link – Point to multi-point.
  • 16. Synchronization Channel (SCH)• MH synchronize with the structure within the locative cell – MH can receive information from the proper time slots on the TDMA structure• To ensure a GSM BTS is chose – The Base Station Identity Code (BSIC) can only be decoded by a GSM BTS• Transmission properties – Transmit on down-link – Point to multi-point.
  • 17. Broadcast Control Channel (BCCH)• BTS broadcast cell information to MH – LAI ( Location Area Identity), to start roaming, waiting for calls to arrive, making calls – maximum output power allowed in the cell – information about BCCH carriers for the neighboring cells • MH will perform measurement to BTS• Transmission properties – Transmit on down-link – Point to multi-point
  • 18. Common Control Channels (CCCH)CCCH support the establishment of a dedicated communication path (dedicated channel) between the MH and the BTSThree types of CCCH Paging Channel (PCH) Random Access Channel (RACH) Access Grant Channel (AGCH)
  • 19. Paging Channel (PCH)• Used by BTS to page particular MH in the cell – MH actively listen to PCH to check contact info within certain time – Contact could be incoming call or short message• Contact info on PCH include – IMSI (MH’s identity number), or – TMSI (temporary number)• Transmission properties – Transmit on down-link – point to point
  • 20. Random Access Channel (RACH)• Used by MH to request a dedicated channel for call setup – Shared by any MH attempts to access the network – Channel request message contains the reason for the access attempt• Transmission properties – Transmit on up-link – Point to pint.
  • 21. Access Grant Channel (AGCH)• The network assigns a signaling channel via AGCH – A Stand alone Dedicated Control Channel (SDCCH) is assigned• Transmission properties – Transmit on down-link – Point to point
  • 22. Dedicated Control Channels(DCCH) DCCH are used for transferring nonuser information between the network and the MH Messages on DCCH Including  channel maintenance  mobility management  radio resource management Four kinds of DCCH Stand alone Dedicated Control Channel (SDCCH) Cell Broadcast Channel (CBCH) Slow Associated Control Channel (SACCH) Fast Associated Control Channel (FACCH)
  • 23. Stand alone Dedicated Control Channel (SDCCH)• Transfer signaling information between the BTS and the MH• Typically used for location updating prior to use of a traffic channel• Transmission properties – Bidirectional channel, transmit on both up and down-link – Point to point.
  • 24. Cell Broadcast Channel (CBCH)• To carry Short Message Service Cell Broadcast (SMSCB)• Use the same physical channel as SDCCH• Transmission properties – Transmit on down-link – Point to multi-point
  • 25. Slow Associated Control Channel (SACCH)• Carries control and measurement parameters along with routine data necessary to maintain a radio link between the MH and the BTS – On the uplink, MS sends averaged measurements (signal strength and quality) of current and neighboring BCCH – On downlink, MS receives information about transmitting power to use and an instruction with time advance/retard• Transmission properties – Bidirection channel, transmit on both up and down link – Point to point
  • 26. Fast Associated Control Channel (FACCH)• An FACCH is used over a TCH where it steals time slots from a TCH – a 20 ms segment of speech is stolen to carry handover signaling information• Appears on demand
  • 27. Traffic Channels (TCH)• TCH transport user information (speech/data)• TCH are bidirectional dedicated channels between the network and the MH
  • 28. Burstinformation contained in one time slot is a The burst Five types of burst Normal Burst (NB)  To carry information on traffic and control channels Frequency Correction Burst (FB)  To synchronize the frequency of the mobile Synchronization Burst (SB)  To synchronize the frames of the mobile Access Burst (AB)  For random and handover access Dummy Burst  For padding the frame