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Wireless Communications

Wireless Communications. Outline. Introduction History System Overview Signals and Propagation Noise and Fading Modulation Multiple Access Design of Cellular Systems. History. Wireless communications pre-1800s 1897 Marconi develops long-distance ship-to-shore link

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Wireless Communications

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  1. Wireless Communications

  2. Outline • Introduction • History • System Overview • Signals and Propagation • Noise and Fading • Modulation • Multiple Access • Design of Cellular Systems

  3. History • Wireless communications pre-1800s • 1897 Marconi develops long-distance ship-to-shore link • 1906 Fessenden transmits analog signals laying the basis for radio stations • 1920 first radio station • 1954 color television • 1983 FCC allocates spectrum for AMPS system • 1991 USDC for digital cellular begins • 1996 Telecommunications Act • 1998 HDTV broadcasts begin

  4. System Overview

  5. Examples of Wireless Systems • Terrestrial broadcast television and radio • Mobile telephone • Paging • Satellite television • Personal mobile radio • Personal communications services • Underwater and space-based communications • Cordless telephone

  6. RF Spectrum

  7. Signals A sample speech signal

  8. Fourier Transform

  9. Sampling and Quantization

  10. Signal Reconstruction

  11. Signal Transmission Degradation • Power loss • Noise • Fading • Tradeoffs in power and quality, as well as data rate, bandwidth, power, and quality

  12. Power Loss

  13. Noise

  14. Multipath Fading

  15. Fading • Original signal is difficult to extract from sum of multipath signals • Doppler shift causes change in frequency • Mobile motion causes rapid change of channel • Requires sophisticated transmitters and receivers or extra bandwidth

  16. Modulation Techniques • Analog • AM • FM • Digital • baseband • binary • higher-order • BPSK • BFSK • Higher-order techniques

  17. Analog Modulation • AM • FM

  18. AM

  19. AM

  20. FM

  21. Frequency Translation

  22. Digital Modulation • Baseband Binary Signaling 1 1 1 1 0 0

  23. Digital Modulation • Baseband Binary Signaling 1 1 1 1 0 0

  24. Digital Modulation • Baseband Quarternary Signaling 00 11 00 10 00 01

  25. Digital Modulation • BPSK

  26. BPSK

  27. Digital Modulation • FSK

  28. Higher-Order Digital Modulation • QPSK

  29. Multiple-Access • Permit users to share a channel • Four common types • FDMA • TDMA • CDMA • create orthogonal signals and transmit simultaneously • separate at the receiver by making use of orthogonality • CSMA • sense the channel and transmit when empty • resolve collisions

  30. FDMA

  31. TDMA

  32. Cellular System Overview

  33. Cellular Systems • Frequency reuse • Basestations • linked to MTSO • uplink and downlink • Cell placement

  34. Cellular System Design Issues • Cell size • large cells desired to reduce number of basestations • Capacity vs. Grade of Service • trade off capacity versus the blocking probability • average cell traffic determined by measurements • Handoffs • switch between basestations as power fluctuates • seamless handoffs desired • Roaming • permit users to place calls outside their own networks

  35. Selected Current U.S.Standards • AMPS – analog cellular, FM with FDMA, 824-894 MHz • IS-95 – digital cellular, QPSK with CDMA, 824-894 MHz, 1.8-2.0 GHz • FLEX – paging, 4FSK, various • GSM – PCS, GMSK with TDMA, 1.85-1.99 GHz • cdma2000, W-CDMA 3rd generation standards proposed

  36. Providing Worldwide Coverage • Multi-mode phones or systems • Case study: Globalstar system • one standard – GSM • coverage via terrestrial basestations and satellite

  37. The Future of Wireless • Growth will continue in personal wireless system development with 3rd and 4th generation systems on their way • Expansion in PCS and other services • Integrated services • Worldwide standards and systems

  38. Conclusions • There are many system components and considerations • Signal representation and bandwidth • Channel effects • Modulation and coding • Multiple access • Cells and frequency re-use • Communications system design involves tradeoffs of parameters in these components • Wireless communications is a rapidly growing field with many challenges remaining

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