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