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Cellular Wireless Networks: Mostly Voice

Cellular Wireless Networks: Mostly Voice

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Cellular Wireless Networks: Mostly Voice

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  1. Cellular Wireless Networks: Mostly Voice • First Generation voice: Analog • AMPS: Advance Mobile Phone Systems • Residential cordless phones • Second Generation: Digital • IS-54: North American Standard - TDMA • IS-95: CDMA (Qualcomm) • GSM: Pan-European Digital Cellular • DECT: Digital European Cordless Telephone

  2. Cellular Wireless Networks: Mostly Data • Wide-area data networks • ARDIS • RAM Mobile Data • CDPD • Wireless LANs • InfraRed LANs • Radio LANs • Paging systems • Satellite systems (GEOs, MEOs, LEOs)

  3. Third Generation Wireless Networks PCN (Personal Communications Networks) • will merge: cellular, cordless, wireless LANs, paging etc. • will support multimedia services (data, voice, video, image) • several on going efforts: (a) GPRS (b) EDGE (c) UMTS

  4. BS BS BS Backbone Network BS BS BS Cellular Concept • Geographical separation • Capacity (frequency) reuse • Backbone connectivity

  5. 40 dB per decade Power Transmitter 20 dB per decade 10 100 1000 Characteristics of Radio Medium Path Loss • Attenuation increases with respect to frequency and distance • Free space loss = square of distance • Indoor mobile radio = quadrature of distance R1 Distance Distance

  6. Transmitter Characteristics of Radio Medium (cont’d) Object • Fading • Multipath fading • Shadowing • Delay spread • Intersymbol interference • Interference Receiver Power dB Deep fade - 40 dB

  7. B B B C C C G G G A A A D D D F F F E E E AMPS: Channel Assignment In each cell, 57 channels each for A-side carrier and B -side carrier Channels are divided into 4 categories: 1. Control (base to mobile) to manage the system. 2. Paging (base to mobile) to alert mobile users to incoming calls. 3. Access (bidirectional) for call set up and channel assignment. 4. Data (bidirectional) for voice, FAX, or data Frequencies are not reused in adjacent cells

  8. Handoff • Handoff: Transfer of a mobile from one cell to another • Each base station constantly monitors the received power from each mobile. • When power drops below given threshold, base station asks neighbor station (with stronger received power) to pick up the mobile, on a new channel. • The handoff process takes about 300 msec.

  9. Digital Cellular: IS-54 TDMA System • Second generation: digital • Same frequency as AMPS • Each 30 kHz RF channel is used at a rate of 48.6 kbps • 3 TDM slots/RF band • 16.2 kbps TDM digital channel • 8 kbps voice coding • 4 cell frequency reuse • Capacity increase per cell per carrier • 3 x 416 / 4 = 312 (instead of 57 in AMPS) • Additional factor of two with speech activity detection.

  10. Frame 1944 bits in 40 ms( 48600 b/s) SLOT 1 SLOT 2 SLOT 3 SLOT 4 SLOT 5 SLOT 6 DATA122 DATA122 DVCC 12 SYNC28 SACCH12 R6 G6 DATA16 MOBILE TO BASE G:GUARD TIME R:RAMP TIME DVCC: DIGITAL VERIFFICATION COLOR CODE RSVD: RESERVE FOR FUTURE USE RSVD 12 DATA130 DATA130 DVCC 12 SYNC28 SACCH 12 IS-54 slot and frame structure BASE TO MOBILE

  11. GSM (Group Special Mobile) Pan European Cellular Standard Second Generation: Digital Frequency Division duplex (890-915 MHz Upstream; 935-960 MHz Downstream) 125 frequency carriers Carrier spacing: 200 Khz 8 channels per carrier (Narrowband Time Division) Speech coder: linear predictive coding (Source rate = 13 Kbps) Modulation: phase shift keying (Gaussian minimum shift keying) Multilevel, time division frame structure Slow frequency hopping to overcome multipath fading

  12. CDMA (Code Division Multiple Access): IS-95QUALCOMM, San Diego • Based on DS spread spectrum • Two frequency bands (1.23 Mhz), one for forward channel (cell-site to subscriber) and one for reverse channel (sub to cell-site) • CDMA allows reuse of same spectrum over all cells. Net capacity improvement: • 4 to 6 over digital TDMA (eg. GSM) • 20 over analog FM/FDMA (AMPS)

  13. CDMA (cont’d) • One of 64 PS (Pseudo Random) codes assigned to subscriber at call set up time • RAKE receiver (to overcome fading) • Pilot tone inserted in forward link for: • power control • coherent reference • Speech activity detection • Voice compression to 8 kbps (16 kbps with FEC)

  14. Data transmissions in cellular networks • modems • CDPD (Cellular Digital Packet Data) • CDPD is built on top of AMPS • Packet switched digital datagram service • Any idle 30 kHz channel can be temporarily grabbed for sending data frames at 19.2 kbps • Base station broadcasts on a dedicated down link channel the data packets, along with info on idle uplink channels • Mobiles access the idle uplink channels in random access mode

  15. Limitations of cellular systems • wired backbone network infrastructure required. What if there is no infrastructure, e.g.: • search and rescue • disaster relief • battlefield • voice only (+ limited data, e.g. CDPD); not suitable for multimedia (data, voice, compressed video, image) • only one channel rate (up to 20-30 kbps). In multimedia, more flexible bandwidth allocation is required, with different data rates; mix of datagram and stream traffic; admission control; QoS support

  16. Packet Data: ARDIS • Nationwide packet radio • Partnership of IBM and Motorola • Coverage • 400 metropolitan areas • 1300 base station • Data rate • Original system • 4800 bps(raw), 50% overhead due to FEC • MAX packet length 240 bytes • New system • 19.2 kps • MAX packet length 512 bytes • Frequency range • 855-865 MHz • RF channel bandwidth, 25 MHz