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