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 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  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     

j

2 

ft dt

Sampling and Quantization

Signal Reconstruction

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

Power Loss

P R



P T



A A c T R

4 

fd

2  2

Noise  

Multipath Fading 

i

4   1

i

  

i



e j



i

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

Modulation Techniques   Analog   AM FM Digital   baseband  binary  higher-order BPSK   BFSK Higher-order techniques

Analog Modulation  AM

c



A C

[1    FM

c



f t c

) 

A C

cos(2 

f t c

 

t

 

v d

)

AM

AM

FM

Frequency Translation

Digital Modulation  Baseband Binary Signaling 1 1 1 0 0 1

Digital Modulation  Baseband Binary Signaling 1 1 1 0 0 1

Digital Modulation  Baseband Quarternary Signaling 00 01 11 00 10 00

Digital Modulation  BPSK

v t

1 ( ) 

v t

2   

f t C

  

f t C

  

BPSK

Digital Modulation  FSK

i

 cos(2 

f t i

)

Higher-Order Digital Modulation  QPSK

i

    cos 2 

f t C

 

i

4    ;

i

 1 4

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

FDMA

TDMA

Cellular System Overview

Cellular Systems  Frequency reuse  Basestations  linked to MTSO  uplink and downlink  Cell placement

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

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 3 rd generation standards proposed

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

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

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