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Wireless Communications Technology

Wireless Communications Technology. Lesson 5: GSM Bellevue Community College Bob Young, Instructor. GSM. G lobal S ystem for M obile Communications. New Features. Support for international roaming (this was the design driver) Distinction between user and device identification

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Wireless Communications Technology

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  1. Wireless Communications Technology Lesson 5: GSM Bellevue Community College Bob Young, Instructor

  2. GSM Global System for Mobile Communications

  3. New Features • Support for international roaming (this was the design driver) • Distinction between user and device identification • SIM Card (Subscriber Identification Module) • All of the interfaces are standardized • PCS 1900 MHz band (in North America)

  4. System Architecture • Mobile Switching Center (MSC) • Base Station Controller (BSC) • Base Transceiver Station (BTS) • Home Location Register (HLR) • Visitor Location Register (VLR) • Equipment Identity Register (EIR) • Authentication Center (AuC)

  5. SIM Contains: • International Mobile Subscriber Identity (IMSI) • Authentication Key (Ki) • Service Plan • Personal Directory (speed dial)

  6. Mobile Device Contains: • International Mobile Equipment Identifier (IMEI) • Temporary Mobile Subscriber Identity (TMSI) • Mobile Station Classmark (Mobile SCM)

  7. Base Station Contains: • Base Station Identity Code (BSIC) • Location Area Identity

  8. GMSK • Gaussian Minimum Shift Keying • "Slides" through the phase transitions • +90 degrees = 0 • -90 degrees = 1

  9. Frequencies • Downlink Frequency: 200 KHz bandwidth • Uplink Frequency: 200 KHz Bandwidth • Uplink and downlink are divided into a similar "slot" structure • Uplink delayed, referenced to downlink • Eliminates need for duplexer in mobile station

  10. Power Control • See chart on page 112 • Only the mobiles have power adjusted • Memory aid: top of chart: 2, 1, 39

  11. Two Channel Types Control channels Traffic channels

  12. Control Channels • Broadcast Control Channel • Common Control Channel • Dedicated Control Channels

  13. Broadcast Control Channel • Frequency Correction Channel (FCCH) • 148 zeros. In GMSK, this creates a sine wave. It is used for preliminary synchronization. • Synchronization Channel • Broadcast Control Channel (BCCH) • Contains info for setting up calls

  14. Common Control Channel • Paging Channel (PCH) • Alerts the mobile • Access Grant Channel • Directs the mobile to the dedicated control channel

  15. Random Access Channel • The mobile can use any time slot on the common control channel as the Random Access Channel (RACH) • Used by the mobile when not on a call: • To originate a call • To initiate registration • etc.

  16. Dedicated Control Channels • Stand Alone Dedicated Control Channel (SDCCH) • Two way logical channel: same time slot used on both the uplink and downlink for functions like registration and call setup • Slow Associated Control Channel (SACCH) • Different from SACCH on traffic channel

  17. Traffic Channels • 8 time slots (voice channels) per frame

  18. Vocoders • Linear Prediction Coding with Regular Pulse Excitation (LPC-RPE) • Enhanced Full Rate (EFR) • ACELP for GSM • Half Rate • ACELP for GSM

  19. Frequency Hopping • Frequency Hopping Spread Spectrum (FHSS) • Resistant to fading • Minimizes the effects of co-channel interference • 217 hops/sec, equal to 1200 bits/hop

  20. Handoffs • All handoffs in GSM are "hard" handoffs • Like TDMA, GSM supports Mobile Assisted Handoff (MAHO)

  21. Multipath Equalization • Receiver uses extensive synchronization information to select the best incoming signal (in a multipath environment)

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