introduction to mobile communication
DESCRIPTION
ÂTRANSCRIPT
Introduction to Mobile communications
Basic Communications System Elements
● Source
● Destination
● Transmission Medium
Source Destination
Transmission Medium
Why Wireless?
Network
Transmission Medium
Wire Wireless
Transition medium
Twisted-pair cable Fiber-optic cableCoaxial cable LOS Non-LOS
What Wireless?
Why Wireless?
Drawbacks of wireless communication
Types of communication systems
TX RX
TX+RX TX +RX
TX+RX TX +RX
Duplex Transmission & Multiple Access
Duplex Transmission
Multiple Access
Multiple-access for Digital Communication Systems
• The frequency spectrum must be shared by all the users in the system.• Three method for sharing spectrum:
– FDMA• Frequency-division multiple-access.
– TDMA• Time-division multiple-access.
– CDMA• Code-division multiple-access.
• Most modern systems use combinations:– TDMA/FDMA– CDMA/FDMA
Multiple-Access
• Three ways to separate signals.– Frequency– Time– Code
Frequency
Time
Code
FDMA
– Frequency-division multiple-access.– Each user is assigned one frequency
frequency
Channel 1 32 4
30 kHzguardband
FDMA
• Frequency-Division Multiple-Access
• Examples:– AMPS
Frequency
Time
Code
TDMA
• Time-division multiple-access• All users transmit at same frequency.• Each user transmits at a different time.
User 1
User 2
User 3
User 1
User 2
User 3guardtime
20 msectime slot
time
TDMA
Frequency
Time
Code
• Time-Division Multiple-Access
• Examples:– USDC/IS-136
CDMA
Frequency
Time
Code
• Code-Division Multiple-Access
• Examples:– IS-95– Bluetooth
Wave Properties
Fading dip point
nλ
Lower frequencies, with longer wavelengths, are better suited to transmission over large distances,
Higher frequencies, with shorter wavelengths, are better suited to transmission over small distances,
Questions ?Questions ?
archaic mobile communication
• optical transmission (smoke/light signals,..)• acoustic transmission (drums, alpine horns,...)
Early telecommunications involved smoke, flags, drums, and other such methodsto relay messages and information.
History of wireless History of wireless communication
History of wireless History of wireless communication
The Wireless Telegraph
• Wireless (electrical) telegraph (Marconi).– 1899, Transmission across English channel.– 1901, 1st transatlantic communication.– 1907, Commercial ship-to-shore service.
Mobile Telephone Service (MTS)
• MTS: – High power transmitter, at high elevation.
Calldropped
Washington Baltimore
Same Frequecies
Improved Mobile Telephone Service (IMTS)
• Improvements to MTS:– Full-duplex signaling, 1965.
• No need for push-to-talk.– Improved capacity
• 120 kHz channels in 1946.• 60 kHz channels in 1950 (2x capacity).• 30 kHz channels in 1965 (4x capacity).
– Automatic trunking, 1965.• Allows more subscribers.
The Birth of Cellular
• Problem with IMTS: – Not enough channels for the demand.
• Solution:1) Release more bandwidth.
• 1974-1975, FCC released 80 MHz of bandwidth previously used by UHF television.
2) Break the coverage region into cells• The cellular concept.
Questions ?Questions ?