Wireless Communications - PowerPoint PPT Presentation

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Wireless Communications

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Wireless Communications – PowerPoint PPT presentation

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Title: Wireless Communications


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
10
00
11
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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