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How Your Cellphone Works…

How Your Cellphone Works…. Matthew C. Valenti Lane Department of CSEE. And Why Sometimes it Doesn’t…. How many cellular subscribers are there worldwide? In the US?. Wireline access lines. 176.4 million Mar. 05. Wireless subscribers (cellular and PCS). 3 rd generation. 2 nd generation.

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How Your Cellphone Works…

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  1. How Your Cellphone Works… Matthew C. Valenti Lane Department of CSEE

  2. And Why Sometimes it Doesn’t…

  3. How many cellular subscribers are there worldwide? In the US?

  4. Wireline access lines 176.4 million Mar. 05 Wireless subscribers (cellular and PCS) 3rd generation 2nd generation 1st generation 1000 The Wireless Revolution 100 Millions of Subscribers 10 1 0.1 2004 1983 1986 1989 1992 1995 1998 2001 wireless growth in US alone Source: www.ctia.org

  5. Largest Carriers in US • Cingular, 49.1 million (GSM) • Verizon, 43.8 million (CDMA) • Sprint, 24.8 million (CDMA) • T-mobile, 17.3 million (GSM) • Nextel, 16.2 million (iDen) In the US, the wireless industry brings in $100B/yr.

  6. Wireless Worldwide • Over 1 billion subscibers. • Penetration Rates for Select Countries • Taiwan: 101% • U.K: 100% • Italy: 96% • Netherlands: 94% • Germany: 84% • France: 76% • Japan: 65% • United States: 59% • Russia: 50% • India: 9% • China: 38% Growing Fast

  7. PCS: 1.85-2.0 GHz Cellular: 824-894 MHz

  8. How Crowded is the spectrum • 176.4 million subscribers • 220 MHz available for Wireless • So just give each subscriber 0.8 Hz of dedicated bandwidth (0.4 Hz each direction), right? • Problem: cellular signal occupies: • 200 kHz (GSM) • 550 distinct channel pairs. • Time division multiple access divides channels into 8 subchannels. • So 4400 “conversations” • 1.25 MHz (CDMA) • 88 distinct channel pairs. • Code division multiple access divides each channel into 64 subchannels • So 5632 “conversations”

  9. Something’s Got to Give…

  10. The Cellular Concept • Transmit power drops off with distance. • When you are far-enough away you can re-use the channel. • Similar concept to frequency re-use for radio and television stations. Ch #1 Ch #2 Ch #3 Ch #1 Low power transmitter, Frequency is re-used

  11. The Cellular Concept Set #1 Set #2 Set #3 Set #2 Set #3 Ch #1 Set #4 Set #1 Lower power transmitters provide coverage to a small portion of the service area. Frequency is reused

  12. 175,368 cell towers in the US

  13. Cell Patterns Idealized Cells Idealized Coverage Footprint Reality!

  14. Break the metropolitan area into small areas Each area is approximated with a hexagonal cell. A base station is located at the center of each cell. Each cell is assigned only a fraction of the total number of channels. Cells that are sufficiently far apart can reuse the same frequency. The Cellular Concept Cluster #1 Cluster #2 A F B A B G F E C G E D C A D F B G E C D Cluster #3

  15. Frequency Reuse 2 1 2 5 1 5 4 4 3 7 3 7 6 6 2 1 5 4 3 7 N = 7 6

  16. Cell Pattern N = 4 2 1 3 2 2 4 1 3 3 1 4 2 4 3 1 4

  17. Sectorized Antennas • Further interference reduction by using sectorized antennas.

  18. Hand Off Ch #2 Ch #1 • Mobile must be transferred between cells as it moves • Hard handoff • Soft handoff (CDMA) • Softer handoff (sectorized antennas)

  19. Wireless Propagation • Fading • Due to relative motion between TX and RX. • Multipath • Due to signal reflections. • Diffraction • Signal bending around objects (mountain, buildings) • Shadowing • Obstructions that attenuate signal (foliage) • Interference • Other signals • Noise • Thermal excitement of electrons in receiver. • Background noise in space.

  20. But all is not lost… • Source Coding • Companding: Reduces BW needed by voice. • Channel Coding • Forward Error Correction Coding. • By adding parity bits to transmitted data, errors can be corrected. • Advanced Receiver Processing • Equalizer: Undoes multipath • This is the type of stuff that EE’s working in the communications industry work on!

  21. To Learn More (A Lot More)… • Wireless Networking • CPE 493g • 9-10 AM, M-W-F • ESB 801 • Valenti • EE 327 and STAT 215 prereq’s. • Wireless Communication Systems • EE 562 • Reynolds • 12 PM – 1 PM M-W-F • ESB 201 • Reynolds • EE 461 and/or EE 513 prereq’s.

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