TLCOM 612 Advanced Telecommunications Engineering II

Report
Wireless Communications
Outline
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Introduction
History
System Overview
Signals and Propagation
Noise and Fading
Modulation
Multiple Access
Design of Cellular Systems
History
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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
System Overview
Examples of Wireless Systems
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Terrestrial broadcast television and radio
Mobile telephone
Paging
Satellite television
Personal mobile radio
Personal communications services
Underwater and space-based communications
Cordless telephone
RF Spectrum
Signals
A sample speech signal
Fourier Transform

V ( f )   v(t )e

 j 2 ft
dt
Sampling and Quantization
Signal Reconstruction
Signal Transmission Degradation
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Power loss
Noise
Fading
Tradeoffs in power and quality, as well as
data rate, bandwidth, power, and quality
Power Loss
PR  PT
AT AR c 2
 4 fd 
2
Noise
y(t )  v(t )  n(t )
Multipath Fading
4
y(t )   ai v  t   i  e
i 1
ji
Fading
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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
Modulation Techniques
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Analog
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AM
FM
Digital
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baseband
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binary
higher-order
BPSK
BFSK
Higher-order techniques
Analog Modulation
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AM
vc (t )  AC [1  v(t )]cos(2 fct )
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FM
t
vc (t )  AC cos(2 fct    v( )d  )

AM
AM
FM
Frequency Translation
Digital Modulation
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Baseband Binary Signaling
1
0
1
1
0
1
Digital Modulation
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Baseband Binary Signaling
1
0
1
1
0
1
Digital Modulation
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Baseband Quarternary Signaling
00
01
11
00
10
00
Digital Modulation
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BPSK
v1 (t )  cos  2 fCt 
v2 (t )  cos  2 fCt   
BPSK
Digital Modulation
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FSK
vi (t )  cos(2 fit )
Higher-Order Digital Modulation
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QPSK
i 

vi (t )  cos  2 fC t   ; i  1
4

4
Multiple-Access
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Permit users to share a channel
Four common types
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FDMA
TDMA
CDMA
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create orthogonal signals and transmit simultaneously
separate at the receiver by making use of orthogonality
CSMA
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sense the channel and transmit when empty
resolve collisions
FDMA
TDMA
Cellular System Overview
Cellular Systems
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Frequency reuse
Basestations
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linked to MTSO
uplink and downlink
Cell placement
Cellular System Design Issues
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Cell size
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Capacity vs. Grade of Service
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trade off capacity versus the blocking probability
average cell traffic determined by measurements
Handoffs
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large cells desired to reduce number of basestations
switch between basestations as power fluctuates
seamless handoffs desired
Roaming
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permit users to place calls outside their own networks
Selected Current U.S.Standards
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AMPS – analog cellular, FM with FDMA, 824894 MHz
IS-95 – digital cellular, QPSK with CDMA, 824894 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
Providing Worldwide Coverage
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Multi-mode phones or systems
Case study: Globalstar system
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one standard – GSM
coverage via terrestrial basestations and
satellite
The Future of Wireless
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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
Conclusions
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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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