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

2( ) ( ) j ftV f v t e 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

2

24T R

R T

A A cP P

fd

Noise

( ) ( ) ( )y t v t n t

Multipath Fading 

 

  4

1

( ) iji i

i

y t a v t e

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

FM

( ) [1 ( )]cos(2 )c C cv t A v t f t

( ) cos(2 ( ) )t

c C cv t A f t v d

AM

AM

FM

Frequency Translation

Digital Modulation

Baseband Binary Signaling

1 0110 1

Digital Modulation

Baseband Binary Signaling

1 0110 1

Digital Modulation

Baseband Quarternary Signaling00 10001101 00

Digital Modulation

BPSK

1( ) cos 2 Cv t f t

2 ( ) cos 2 Cv t f t

BPSK

Digital Modulation

FSK

( ) cos(2 )i iv t f t

Higher-Order Digital Modulation

QPSK

( ) cos 2 ; 1 44i C

iv t f t i

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 3rd 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 3rd and 4th 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