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 ?