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

Communications System Overview Transmitter Receiver Channel Message Source Modulation Demodulation Sink

Examples of Wireless Systems Terrestrial broadcast television and radio Mobile and cordless telephones Paging Satellite television Personal mobile radio Underwater and space-based communications

Frequency Allocation AM Radio Television (channels 2-13) Terrestrial Mobile WLAN MHz A portion of the radio frequency spectrum (not to scale).

Source Signals Types of signals Analog - continuous time and amplitude Digital - discrete time and amplitude, usually represented as a bitstream Conversion between the two types is possible Message Source Modulation Demodulation Sink

Modulation Modify a carrier wave (typically high-frequency) according to the message to be transmitted Modulation Techniques Analog - such as AM and FM Digital – such as on-off keying or phase-shift keying Message Source Modulation Demodulation Sink

Analog Modulation - AM Signal (Message)Modulated Carrier

Analog Modulation - FM Signal (Message)Modulated Carrier

Digital Modulation - Baseband Take one bit at a time from the bitstream. If the bit is a ‘1’ transmit a signal, otherwise transmit nothing. time

Digital Modulation - BPSK

Signal Transmission Degradation Types of Degradation power loss noise fading System Design Tradeoffs power quality data rate bandwidth Message Source Modulation Demodulation Sink

Power Loss

Noise

Multipath Fading

Multiple-Access Permit users to share channels Common types for cellular phones FDMA – frequency division multiple access TDMA – time division multiple access CDMA – code division multiple access User 1 Source Modulation Demodulation Modulation User 2 Source User 1 Sink Demodulation User 2 Sink

FDMA Each user is assigned a frequency band.

TDMA Each user is assigned a time slot.

Cellular System Overview

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

Providing Worldwide Coverage Multi-mode phones or systems Unified standards or overarching systems 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 Security will remain a significant concern

Summary There are many system components and considerations Signal representation and bandwidth Channel effects Modulation Multiple access Cells Communications system design involves tradeoffs of parameters in these components Wireless communications is a rapidly growing field with many challenges remaining