Modern Wireless Communication

Introduction of Wireless Communication.



Background.

• 1.2 Communication Systems
• 1.3 Physical Layer
• 1.4 The Data-Link Layer
1.4.1 FDMA
1.4.2 TDMA
1.4.3 CDMA
1.4.4 SDMA

In 1864, James Clerk Maxwell Formulated the electromagnetic theory of light and predicted the existence of radio waves
• In 1894, Maxwell and Hertz, Oliver Lodge demonstrated wireless communications
• In 1906, Reginald Fessenden conducted the first radio
broadcast using the technique that came to be known as amplitude modulation (AM) radio. Spread Spectrum techniques made their first
appearance before and during World War II.
• In 1946, the first public mobile telephone systems were introduced in five American cities.
• In 1947, the first microwave relay system consisting of
seven towers became operational.
• In 1958, a new era in wireless communications was
initiated with the launch of the SCORE (Signal Communication by Orbital Relay Equipment). In 1981, the first analog cellular system, “Nordic Mobile. Telephone (NMT)”, was introduced and this was soon followed by the Advanced Mobile Phone Service (AMPS) in North America in 1983.
• In 1988, the first digital cellular system “Global System for Mobile (GSM)” was introduced into Europe.

Communication Systems.



Physical Layer.


Provides the physical mechanism for transmitting bits between any pair of nodes.

Data-link Layer.


Layer for error correction or detection. Responsible for sharing the transmission medium for different
users.
Network Layer.


Determine routing of the information
Determine the quality of service
Flow control

Physical Layer.


Transmitter.

In wireless system. Transmitter shape the signal for efficient use of transmission medium resources.

Due to the power limitation, transmitter must use robust and power efficient modulation techniques.

As the medium is shared with other users, the design should minimize the interference.

Channel.

In wireless system, the channel impairments

include

Channel distortion in form of multipath.

Time-varying nature

Receiver noise

Receiver.

In wireless system:

Estimation of the time-varying nature of the channel is necessary for implementing compensation techniques.

Error-correction technique to improve the reliability.

Maintain synchronization.

The Data-Link Layer.

FDMA

Multiple access strategy:

Sharing the physical resources among the different users.

For wireless system, radio spectrum is the physical resource.

Four multiple access strategies for radio spectrum

FDMA (Frequency-division multiple access)

TDMA (Time-division multiple access)

CDMA (Code-division multiple access)

SDMA (Space-division multiple access)

Radio spectrum is divided into a number of channels.

Each pair of users is assigned a different channel.

Different channel is assigned for each direction of transmission.

The Data-Link Layer.

TDMA

Designed for point-tomultipoint architecture.

Multiple UTs communicate with single BS.

Using analog modulation with a simple push-to-talk protocol.

TDMA is suited only to data applications.

CDMA.

Cellular system

UTs in each cell communicate with a BS located at the center of the cell.

FDMA or TDMA can be used within a cell

SDMA.

Improvements of SDMA:

Reduce the total power needed to be transmitted.

Reduce the amount of interference.

Receiver would receive a stronger signal.

Multibeam antennas are used to separate radio signals by pointing them along different direction.