4. Agenda
Introduction to wireless communications
Wireless link implications
Medium: the radio spectrum
The three main parameters that define radio–frequency:
Over-the-air data rate
Receive Sensitivity
Transmit power
Other Considerations
Barriers to Future Growth
5. Introduction
Wireless Communication System: Any electrical communication
system that uses a naturally occurring communication channel,
such as air, water, earth.
Examples:
Sonar
Broadcast: Radio, TV, pagers, satellite TV, etc.
Two Way: walkie talkie, cell phones, satellite phones,
Wireless Local Area Networks, etc.
6. Wireless link implications
Communications channel is natural (air)
Medium regulated by governments
frequency allocation, licensing, etc.
Security issues
7. Medium: the radio spectrum
Wireless communications use the electromagnetic
spectrum, which is regulated by government
institutions such as the Federal Communications
Commission (FCC) in USA.
10. RF CONSIDERATIONS
The three main parameters that define radio–
frequency:
Over-the-air data rate
Receive Sensitivity
Transmit power
Range is a result of these three RF parameters and can be used to define
them
11. RF CONSIDERATIONS
Transmit power
It is usually driven by regulatory and
power-consumptions considerations
For example, FCC allows up to 1 W of transmit
power in the United States in the 2.4 Ghz band
12. Other Considerations
Antenna Selection
Directionality
Omni (360 degree coverage) directional
Directional (limited range of coverage)
Gain
More gain means more coverage
Polarization
13. Other Considerations
Modulation Techniques
Amplitude Shift Keying (ASK):
very simple
low bandwidth requirements
very susceptible to interference
Frequency Shift Keying (FSK):
needs larger bandwidth
Phase Shift Keying (PSK):
more complex
robust against interference
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14. Range Depends On...
Frequency
Transmit power
Radio sensitivity
Processing gain from access technique and
redundancy
Interference effects
15. Barriers to Future Growth
Irreducible size of antennas
Rising level of RF emissions -
interference problems and safety concerns
Finite spectrum
Lack of standards and interoperability of hardware
17. Microwaves
microwaves - electromagnetic waves with a frequency
between 1GHz and 1000 GHz
microwaves frequency are further categorized into
frequency bands group.
receivers need an unobstructed view of the sender to
successfully receive microwaves
microwaves are ideal when large areas need to be
covered and there are no obstacles in the path
21. Advantages of microwaves over radio waves
because of high frequency, more data can be sent through
microwaves -> increased bandwidth, higher speeds
because of their short wave length, microwaves use smaller
antennas
smaller antennas produce a more focused beam which is
difficult to intercept
22. Disadvantages of microwave communication
they require no obstacle is present in the transmission path
the cost of implementing the communication infrastructure is
high
microwaves are susceptible to rain, snow, electromagnetic
interference
23. Microwaves usages
carrier waves in satellite communications
cellular communication
wireless local area network
GPS (Global Positioning System)
24. Microwave propagation modes
microwaves, one generated, propagate in a straight line in
all directions
there are 3 modes of propagation possible, and the mode
is decided based on distance and terrain:
27. Overview
Satellite is a microwave repeater in the space.
There are about 750 satellite in the space, most
of them are used for communication.
They are:
Wide area coverage of the earth’s surface.
Transmission delay is about 0.3 sec.
Transmission cost is independent of distance.
28. Satellite up links and down links can operate in different
frequency bands:
The up-link is a highly directional, point to point link
The down-link can have a footprint providing coverage for a substantial area "spot
beam“.
Band Up-Link (Ghz) Down-link (Ghz) ISSUES
C 4 6 Interference with ground links.
Ku 11 14 Attenuation due to rain
Ka 20 30 High Equipment cost
30. At the Geostationary orbit the satellite covers
42.2% of the earth’s surface.
Theoretically 3 geostaionary satellites provides
100% earth coverage
31. MAC(Media Access Control)
protocols for satellite links
ALOHA:
Every station can transmit any time
Very low efficiency 18- 36 %.
FDMA (Frequency Division Multiple Access)
It is the oldest and most common.
the available satellite channel bandwidth is broken
into frequency bands for different earth stations.
32. TDMA (Time Division Multiple Access)
channels are time multiplexed sequentially
Each earth station gets to transmit in a fixed time slot
only.
More than one time slot can be assigned to stations with
more bandwidth requirements.
Requires time synchronization between the Earth
Stations.
CDMA : (Code Division Multiple Access)
Combination of time/frequency multiplexing
( a form of spread spectrum modulation).
It provides a decentralized way of providing separate
channels without timing synchronization. It is a
relatively new scheme but is expected to be more
common in future satellites.
33. VSAT Network
At the Very Small Aperture Terminal a lower performance
microwave transceiver and lower gain dish antenna
(smaller size) is used.
VSAT networks are arranged in a star based topology.
Ideal for centralized networks with a central host (Banking
institutions with branches all over the country).
Use the S-ALOHA and TDMA
