Transcript How Radios Work - Lehigh University

Lehigh University Communications
Internship & Development Program
Summer Workshop
on Wireless Communications
Prof. Shalinee Kishore
Dept. of Electrical & Computer Engineering
Lehigh University
e-mail: [email protected]
July 26-August 3, 2004
This workshop was jointly supported by the National Science Foundation
under grant CCF-0346945, Lehigh University, and the Susquehanna County
Economic Development Office.
Welcome

This summer workshop will introduce you to the core
principles behind several important wireless
technologies.

First, we will discuss how wireless communications
occurs, how radio signals are generated, how they
move thru the air, etc.

Once, these basic ideas are understood, we will look
at how three important wireless networks operate.
Content of Workshop

The three wireless networks we focus on are:
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Cellular Telephony
Global Positioning Systems
WiFi
We will also briefly talk about WiMax, an emerging
fixed wireless application that may be relevant for
Susquehanna in the future.
Outline of Workshop

July 26th-27th: Intro to Wireless Communications

July 27th-28th: How do Cell Phones Work?
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July 29th-30th: How GPS Works
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July 30th: Basics of the Internet

August 2nd-3rd: How WiFi Works
How Radios Work
Radio Waves

Radio waves carry music, conversations, pictures,
and data invisibly through the air over millions of
miles.

Radios can transmit and/or receive radio waves.
They’re Everywhere
All wireless technologies use radio waves to
communicate.
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Some examples:
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AM/FM Radios
Cell Phones
GPS Receivers
Wi-Fi
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Some other examples:
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Cordless Phones
Garage Door Openers
Radio-Controlled Toys
Television Broadcasts
Ham Radio
Etc.
Some Other (not-so-obvious)
Examples
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Radar (police, air traffic control, military applications)
Microwave ovens
Navigation systems
Airplanes (contain dozen different radio systems)
Baby monitors
Simple, Cheap Radio
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Take a fresh 9V battery and a coin
Find AM radio and tune to an area of dial where there
is static
Hold battery near antenna
Quickly tap two terminals of battery using coin
Radio crackles due to connection/disconnection by
coin.
Battery/coin combo is a
radio transmitter!
Simple, Cheap Radio (Cont’d)
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Battery/coin radio transmits static.
Transmits only over short distance.
Could use static to tap Morse code messages and
communicate over several inches.
May not be practical but is a simple example of a
functional radio transmitter.
Why does it work? We’ll go over this next.
How Simple Transmitters Work

Battery: connect to ends (terminals) of a battery
with a piece of wire. Result: battery sends
electricity (stream of electrons) thru the wire. There
is voltage in the wire.

When start electrons moving (create current in wire),
a magnetic field is created around the wire.

Magnetic field is strong enough to affect a compass.
Simple Transmitter (Cont’d)
Result of Simple Transmitter
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Extend the experiment: take another wire, place it
parallel to the battery wire but a few centimeters
away from it.
Connect a sensitive voltmeter to this new wire.
Voltmeter will give a measure amount of electricity in
new wire.
When you connect/disconnect the battery wire, you
will read a small voltage and current in the second
wire.
Simple Transmitter (Cont’d)

Observation: by changing the magnetic field in one
wire, we can cause an change in the electric field in
the second wire.

Specifically,
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Battery creates electron flow in one wire
Moving electrons create magnetic field around one wire
Magnetic field stretches out to second wire
Electrons flow in second wire whenever magnetic field in first
wire changes.
Electrons flow in second wire only when you
connect/disconnect battery.
Simple Transmitter (Cont’d)
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We see then that a message can be converted to
Morse code and then tapped using first wire
(connect/disconnect).
This first wire is a simple transmitter.
The second wire is a receiver.
Simple Receiver

Voltage changes in second wire can be used to
determine Morse code taps.
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Morse code message is then decoded to get the
message from the first wire.
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Result: communication of message occurs
“wirelessly” (over a couple of centimeters) from the
first wire to the second wire.
Creating Simple Transmitters
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When we change current in first wire in time, a
current is induced in second wire.
To create any radio transmitter, create a rapidly
changing electric current in a wire.
This can be done by connecting/disconnecting a
battery. When connected, voltage in wire is 9V.
When disconnected voltage in wire is 0V. Result:
square wave signal.
9V
0V
Time (s)
Sine Wave: Better than Square
Wave
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A better alternative to square wave is a continuously
varying electric current in a wire.
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Simplest and smoothest continuously varying wave
is a sine wave:
A simple radio transmitter created by running a sine
wave thru a wire.
Sine Waves
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By sending sine wave electric current to antenna,
you can transmit sine wave into space.
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All radios today, however, transmit continuous sine
waves to transmit information (audio, video, data).
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Why sine waves?
To allow many different people/devices to use radio
waves at the same time.
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Sine Waves: Frequency
One cycle of a sine wave is:
Sine wave can
be written as sin(2pt/T)
T seconds
When one cycle of a sine wave lasts T seconds, we
say that the sine wave as frequency 1/T Hertz (Hz).
1 Hz = 1 cycle/second.
More on Sine Waves
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If there was a way to see radio waves, we would find
there are literally thousands of different radio waves
(sine waves) traveling thru the air (TV broadcasts,
cell phone conversations, AM/FM broadcasts, etc.)
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Each different radio signal uses a different sine
wave frequency.
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Use of different frequencies help separate different
radio signals.
More on Frequency
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When you listen to AM broacast, your radio is tuning
into sine waves oscillating at a frequency around
1,000,000 cycles per second.
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For example, 880 on the AM dial corresponds to
listening to a radio (sine) wave that has frequency
880,000 Hz = 880 KHz.
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FM signals operate in range of 10,000,000 Hz. So,
90.9 on FM dial corresponds to 90,900,000 Hz =
90.9 MHz.
Kilo, Mega, Giga, etc.
1 Hz
1000 Hz = 1 KHz (kilohertz)
1,000,000 Hz = 1 MHz (megahertz)
1,000,000,000 Hz = 1 GHz (gigahertz)
More on Radio Basics
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Any radio setup has two parts: Transmitter and
Receiver
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Transmitter takes some form of message
(someone’s voice, pictures for TV set, etc.) encodes
it into a sine wave and transmits it with radio waves.
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Combination of encoded message on a radio wave
is commonly referred to as a signal.
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Receiver receives radio waves and decodes
messages from the sine waves.
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Both transmitter and receiver use antennas to
radiate and capture radio waves.
Transmitter Description
Radio Transmitter
Combine
Information
(voice message)
Radio Waves
Antenna
Sine
Wave
Transmitter generates its own sine wave using oscillators.
Receiver Description
Radio Transmitter
Antenna
Separate
Sine Wave
Information
(voice message)
Modulation
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If you have a sine wave and a transmitter that is
transmitting the sine wave into space using an
antenna (more antennas later), you have a radio
station.
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Problem with plain old sine wave: does not contain
information.
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Sine wave has to modulated in some way so that it
contains information, e.g., voice message.
3 Basic Modulation Methods
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Pulse Modulation (PM): turn sine wave on and off.
Easy way to send Morse code.
3 Basic Modulation Methods
(Cont’d)
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Amplitude Modulation (AM): Amplitude (peak-topeak voltage) of sine wave is changed so as to
contain information.
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AM radio stations and picture part of TV signals use
amplitude modulation to encode information signal.
Example of AM
carrier = sine wave with a given frequency
3 Basic Modulation Methods
(Cont’d)
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Frequency Modulation (FM): Radio transmitter
changes frequency of sine wave according to
information signal.
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Frequency modulation is most popular. Used by FM
radio stations, sound part of TV signal, cellular
phones, cordless phones, etc.
Frequency of Signal after
Modulation
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Radio wave transmitted after modulating sine wave
with information signal is not just a sine wave with
frequency f.
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For example, in FM, the frequency varies around
this frequency f. For example, it may increase up to
f+Df and be as small as f-Df.
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After modulating information signal, the radio wave
has some range of frequency, called the frequency
band, e.g., 2Df.
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The bandwidth, width of frequency band, depends
on the information signal (voice, data bit rate, etc.)
Summary of Modulation
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By modulating a sine wave at a transmitter,
information can be encoded into the radio wave.
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The resulting radio wave occupies a band of
frequency, centered on the frequency of the sine
wave.
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Receiver needs to demodulate the radio wave to
extract the information signal.
AM Modulation Example
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Car radio is tuned to radio station, say 880 AM.
Transmitter’s sine wave is transmitting at 880,000
Hz (sine wave repeats 880,000 times per second).
DJ’s voice is modulated onto sine wave, i.e.,
amplitude of sine wave is varied as DJ’s voice
varies.
A power amplifier magnifies power of modulated sin
wave, e.g., to 50,000 Watts for a large AM station.
Antenna then sends radio waves into space. High
power amplification helps waves travel large
distances.
How do we receive AM
signals?
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Unless you sit right next to the transmitter, you need
an antenna to pick out the radio waves from the air.
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An AM antenna is just a wire or a metal stick that
increases the amount of metal the transmitter’s
waves interact with.
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Radio receiver also needs a tuner. Antenna will
receive thousands of sine waves; tuner separates
out the radio wave that the listener desires, e.g., the
radio wave transmitted at 880 KHz.
AM Reception (Cont’d)
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Tuners operate using a principle called resonance.
That is, tuners resonate at and amplify one
particular frequency and ignore all other frequencies
in the air.
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After tuning in, radio receiver has to extract the DJ’s
voice signal from the sine waves.
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This is done using a demodulator (aka detector).
AM Reception (Cont’d)
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One type of a AM detector is something called an
envelope detector. Simply, it determines the
magnitude (amplitude) of the sine wave.
An amplify magnifies this amplitude signal and then
the receiver sends the output to the car radio
speakers.
What we hear is the DJ’s voice.
What about FM?
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FM reception is very similar.
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Difference: FM detector outputs changes in the sine
wave frequency as opposed to amplitude.
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Specifically, FM detector converts changes in sine
wave frequency into sound.
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Antenna, tuner, amplifier are largely the same in FM
as in AM.
Handout: AM Receiver
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Please refer to handout to develop a very simple AM
receiver.
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It works only when you are near the AM radio
station.
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If you are near an AM radio station and have the
basic ingredients (a diode, two pieces of wires, small
metal stake, and a crystal earphone), give this
receiver a try.
What about antennas?
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Almost every radio you see (cell phones, car radio,
etc.) has an antenna.
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Antennas come in all shapes and sizes. Shapes
and sizes depend on the frequency the antenna is
trying to receive.
Ranges from long stiff wire (as in car radios) to large
satellite dishes (as used by NASA).
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For satellites that are millions of miles away NASA
uses antenna dishes that 200 feet wide.
More on Antennas
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Often radio stations use extremely tall antenna
towers to transmit their signals.
Antenna at radio transmitter: launch radio signals
into space.
Antenna at radio receiver: pick up as
much of the transmitter’s power as
possible and feed it to the tuner.
Antennas (Cont’d)
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Size of optimum radio antenna is related to
frequency of the signal antenna is trying to transmit
and/or receive.
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Reason for this: speed of light and the distance
electrons can travel as a result.
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Speed of light is 186,000 miles/sec (300,000
meters/sec).
Determining Antenna Size
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Say you are building an antenna tower for radio
station 680 AM.
It is transmitting sine wave with frequency of
680,000 Hz.
In one cycle of sine wave, transmitter is going to
move electrons in the antenna in one direction,
switch and pull them back, switch push them out,
and switch and pull them back.
That is electrons change direction four times during
one cycle of the sine wave.
time
Antenna Size (Cont’d)
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When operating at 680,000 Hz, each cycle
completes in 1/680,000 = 0.00000147 seconds.
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One quarter of the cycle is 0.0000003675 seconds.
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At the speed of light, electrons can travel 0.0684
miles (361 feet) in 0.0000003675 seconds.
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Cell phones operate using 900,000,000 Hz; this
means that it needs antennas that are about 3
inches high.
Antenna Size (Cont’d)
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Question: why aren’t car radio antennas 300 feet
high?
It would be impractical for one.
If you made car radio antenna higher, reception
would be better.
AM radio stations transmit at high powers to
compensate for the suboptimal receive antenna
heights.
Some Questions
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Why do radio waves transmit away from antenna
into space at speed of light?
How can radio waves transmit millions of miles?
Doesn’t antenna only create magnetic field in its
vicinity?
How can the magnetic field variation be registered
millions of miles away?
Answer
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When current enters antenna, it creates a magnetic
field around the antenna. This magnetic field
creates an electric field (voltage and current) in
another wire placed close to the antenna.
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In space, magnetic field created by antenna induces
electric field in space.
This electric field induces another magnetic field in
space, which induces another electric field, …
These electric and magnetic fields (electromagnetic
fields) induce each other in space at the speed of
light in a direction away from the antenna.
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What is Radio
Spectrum?
Radio Frequencies
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A radio wave is an electromagnetic wave
propagated by an antenna.
Radio waves have different frequencies and by
tuning a radio receiver to a specific frequency, you
can pick up a specific signal.
Radio Frequencies (Cont’d)
Frequency
10 kHz to 30 kHz
30 kHz to 300 kHz
300 kHz to 3 MHz
3 MHz to 30 MHz
30 MHz to 328.6 MHz
328.6 MHz to 2.9 GHz
2.9 GHz to 30 GHz
30 GHz and above
Band
Very Low Frequency (VLF)
Low Frequency (LF)
Medium Frequency (MF)
High Frequency (HF)
Very High Frequency (VHF)
Ultra High Frequency (UHF)
Super High Frequency (SHF)
Extremely High Frequency (EHF)
FCC and Frequency Bands
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In the U.S., the Federal Communications
Commission (FCC) who is able to use which
frequencies for which purposes.
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It issues licenses to stations for specific frequencies.
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For example, AM radio stations must use
frequencies in 535 KHz to 1.7 MHz band.
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FM radio stations transmit in band of frequencies
from 88 MHz to 108 MHz.
Spectrum Licenses for Cellular

Cellular telephony has been a growing and
profitable tech industry.
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Service providers are keen to get licenses for this
use.
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For this reason, cellular licenses are obtained for
10’s to 100’s of millions of dollars.
Common Frequency Bands
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There are hundreds of frequency bands for different
wireless technologies.
Some examples:
 Cell phones: 824 to 849 MHz
 Global Positioning System: 1227 to 1575 MHz
 Garage Door Openers: Around 40 MHz
 Baby Monitors: 49 MHz
 MIR Space Station: 145 to 437 MHz
 Deep Space Communications: 2290 to 2300 MHz
Another Important Frequency
Band
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Human voice also has its own frequency band.
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Voice’s frequency band is from 0 Hz to 4000 Hz.
Why is AM radio at a lower band
than FM radio?
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Mostly, due to history.
AM was invented before FM.
Transmitting at higher frequencies means that
electronic equipment has to be faster.
Fast enough to oscillate and detect highly-changing
signals.
When AM radio invented electronic capabilities were
fairly limited (compared to nowadays).
Hence lower frequencies were allocated.
Later when FM radio was developed, it was
assigned unused frequencies at a higher band.
Duplexing
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In some wireless systems, a radio unit will have
capabilities to both transmit and receive (unlike a car
radio but like a cell phone) at the same time. These
radios are called full-duplex.
In other systems, a radio unit can either transmit or
receive at a given time. These radios care called
half-duplex.
Duplexing (Cont’d)
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Both CB radios and walkie-talkies are half-duplex
devices.
Two people communicating on a CB radio use same
frequency, so one person can talk at a time,
otherwise the signals would overlap and interfere
with each.
More on interference later.
In full-duplex systems, both radios can transmit and
receive at the same time. For example, they use
different frequencies to transmit and receive.
Duplexing (Cont’d)
CB Radio: Half-Duplex
Cellular: Full-Duplex
Beyond the Basics:
Analog vs Digital
Analog versus Digital
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AM and FM is analog technology.
Most new wireless systems are based on digital
technology.
What’s the difference?
Analog versus Digital (Cont’d)
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Analog signals take on a continuous range of values.
Digital signals are quantized to take on one of a set
of possible values.
Quantization
Quantization
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One way to convert an analog signal to digital is
quantization.
We assume an analog signal takes on a continuous
range of values from xmin to xmax (0 to 1 in previous
slide).
The digital signal is only allowed to take on values
{y1,y2,…yN}, (e.g., {0, 0.1, 0.2, 0.3,…0.9, 1.0}.
A quantizer takes the analog signal over an interval
and determines which of the possible values
{y1,y2,…yN} best represents the analog signal in this
range.
More on Digital Signals
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More specifically, digital signals used in
communications systems today are binary signals.
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Binary signals can only take on one of two values: 0
or 1, commonly known as bits.
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To understand bits, we first look at digits.
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A digit is a single place that can hold numerical
values in the range of 0 to 9.
Digits
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Digits are normally combined together to make
larger numbers.
For example, 6,957 has four digits. 7 is hold the 1s
place, 5 is holding the 10s, 9 is holding the 100’s
place, and 6 is holding the 1000’s place.
Each digit is placeholder for the next higher power
of ten.
Each place can have 10 different values; this implies
that digits use a base-10 number system.
Bits
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Bits use a base-2 (binary) number system.
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Reason computers and communication systems use
binary number system is that electronic components
are cheap.

Bit stands for Binary digIT. A binary digit is a
sequence of 0’s and 1’s that represent numbers.
More on Bits
0=0
1=1
2 = 10
3 = 11
4 = 100
5 = 101
6 = 110
7 = 111
8 = 1000
9 = 1001
10 = 1010
11 = 1011
12 = 1100
13 = 1101
14 = 1110
15 = 1111
16 = 10000
17 = 10001
18 = 10010
19 = 10011
20 = 10100
There are simple rules that have been developed to
convert a digit to a bit.
Bytes
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Bits are rarely seen alone in computers.
They are typically grouped together into 8 bit
collections, called bytes.
With 8 bits in a byte, a byte can represent values
ranging from 0 to 255:
0 = 00000000
1 = 00000001
2 = 00000010
.
.
.
254 = 11111110
255 = 11111111
How is a Quantized Digital
Signal converted into Bits?
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After quantization, a digital signal takes on one of a
discrete set of values {y1,y2,….yN}.
Each possible discrete value can be represented in
binary format.
Whenever the digital signal takes on particular value
yi, the value yi is converted to binary format, i.e., a
sequence of 0’s and 1’s.
Result is a binary (digital) signal.
How do we transmit binary
signals using radio waves?
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Binary signals are converted to radio signals using
sine waves.
There are several techniques (modulation)
schemes.
We described the most basic one, called binary
phase shift keying (BPSK).
BPSK
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Rule for transmitting binary signals using BPSK is
pretty simple.
When the radio wants to transmit a 1, it just sends
the normal sine wave.
When the radio wants to transmit a 1, it multiplies the
sine wave by -1 and sends that.
An Example of BPSK
Modulation
Radio
Waves
Voice
(analog signal)
Sine wave
Analog to
Digital
Converter
Antenna
11
0  -1
X
1
1
0
-1
Beyond Basics:
Radio Propagation
Radio Channel


There is another very important player in the wireless
game: the physical environment over which radio
waves travel.
Radio waves can take many different paths to get
from transmitter to receiver.
Transmitter
Receiver
Radio Channel



Essentially, the radio waves interact with the
physical environment along each of these paths.
There are typically (unless you are in free-space)
many paths from the transmitter to the receiver.
Each path is called a multipath.
Multipaths
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The lengths of multipaths are different.
As a result, sine waves along one path reach the
receiver at different times than the same signal
along a different path.
Transmitter
Receiver
Worst Case Example of 2
Multipaths
Received radio wave along multipath 1
Received radio wave along multipath 2
The antenna combines (sums) these two multipaths.
In the example above, the output of the antenna will be:
No
Signal !!
+
=
Another aspect of multipaths


Whenever a radio wave bounces off or passes
through a physical obstruction, the amplitude of the
sine wave changes.
Also amplitude of sine wave shrinks the further the
radio wave travels, regardless of whether there are
obstructions or not.
Reflection
A
-A
Originally
transmitted
radio wave
aA
-aA
Received
radio wave, a < 1
Impact of Multipaths
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
When all the radio waves on the multiple paths
reach the receiver’s antenna, they combine
together.
Some multipaths cancel each other out, some add
up together constructively, some partially cancel
Signal
each other, etc.
A
fades in
and out
and is
distorted
Radio Channel:
Impact of Physical
Environment
-A
Transmitted
radio wave
Overall combined
received signal at
receive antenna
Fading

Fading, which is both signal attenuation and
distortion, is a major challenge in wireless
communications.

We have all experienced it, e.g., fading of radio
station in a car radio.

Fading varies in frequency: assuming physical
conditions are fixed, if a signal transmitted at one
frequency fades, it may not if transmitted at a
different frequency.
Problem with Distortion

Distortion not only impacts the strength of the
received signal but also changes the “shape” of the
received signal.

In this digital communications, this can be especially
detrimental because bits can be inverted at the
receiver due to multipath. Example is shown in next
slide.
Fading (Cont’d)
1
0 0
1 1 0
Signal along Multipath 1
Transmitted
Signal
Overall Received
Signal
Signal along Multipath 2
This type of distortion occurs anytime there is long
time spread in the multipaths.
Fading Challenges

Time spread of multipaths = delay due to echoes.

These “echoes” are particularly problematic in urban
areas, due to reflections from buildings.

Also problematic in hilly, mountainous areas, like
Susquehanna County.

Designers of radio systems have spent a lot of time
and effort trying to overcome this fading challenge.
One Solution: Multiple
Antennas


One way to overcome fading problem is to design
receivers with multiple, say 2, antennas.
Both antennas can receive the desired radio wave.
Transmitter
Receiver
Multiple Antennas (Cont’d)

If receive antennas are adequately separated, then
paths followed by radio waves to the first antenna
are different from paths followed by radio waves to
the second antenna.

Result: fading of received signal on first antenna is
different from fading of received signal on second
antenna. Chances are if one antenna experiences a
deep fade, the other does not.

Receiver can adaptively choose the “stronger”
antenna to determine the received radio wave at
any given time.
Multiple Antennas on Cell
Towers
Cellular base station
tower (antenna tower
that cell phones “talk”
to) use multiple
antenna to improve the
quality of voice signal
received from cell
phone users.
Another Solution: Adaptive
Antenna Array

Of all the multipath, if there is a line-of-sight (LOS)
path between the transmitter and receiver, it is the
strongest.

If we can design antennas receive patter, we would
like to make it in the direction of the strongest
multipath.

This is what adaptive antenna array do.
Adaptive Antenna Array
(Cont’d)

Adaptive antenna array is a group of receive
antennas that work together to form a desirable
radiation pattern.

For example, if the LOS path is in a particular
direction, the antennas work together to pick out as
many radio waves in that direction as possible.

In doing so, the antennas ignore radio waves in nonsignificant directions. These waves do not
contribute to the overall fading.
Cellular Tower Example
Assume that a cellular base station tower is receiving signal
from four cell phone users. The tower can use its antennas to
form the following radiation power (bird’s eye view).
No signals
received from
this direction
User 1
User 2
User 3
No signals
received from
this direction
Cellular
Base Station
User 4
No signals
received from
this direction
Other Fading
Countermeasures




Both previous schemes are antenna techniques to
counter fading.
Designers have developed other methods, e.g.,
coding.
One simple coding method: repetition.
Repetition: transmit the same signal a few times.
Chances are one of the copies will be received
without deep fades.
Beyond Basics:
Interference
Limited Radio Spectrum




Assume one transmitter is sending a signal using a
sine wave of frequency f1.
Ideally, we would like there to be only one
transmitter sending a signal at this frequency.
Problem with this: since there is only limited
frequency (see radio spectrum), only a limited
number of transmitters can send radio waves at a
given time.
Because of limited radio spectrum, it becomes
necessary to reuse frequencies.
Frequency Reuse

Regulators (FCC) and system designers allow
multiple transmitters to use the same frequency to
transmit radio waves.

Problem with this: receive antenna may receive
signals from the wrong (undesirable) transmitter.

Worse yet, the two sets of radio waves from the two
transmitters may combine together at receiver to
produce interference, an undecipherable received
signal, e.g., loud static on a car radio.
Methods to Reduce Multiple
Access Interference


This multiple access interference is typically avoided
by making sure that transmitters that use the same
frequency are separated by large distances.
Large distance ensures that interference power is
small (recall: amplitude of radiated sine wave
shrinks with distance).
Interference
Desired
Signal
Transmitter
Desired
Signal
Receiver
Interference
Receiver
Transmitter
Both transmitters use same frequency
Reuse Distance

If the frequency reuse distance is large, then
interference power is low compared to desired
signal power.

If desired signal power is much higher than the
interference power, then, chances are, the receiver
will be able to determine the (desired) transmitted
signal correctly.
Frequency Reuse




This concept of frequency reuse is used in cellular
systems (explained in more detail) next time.
Frequency reuse also used in broadcast TV.
In NYC, ABC, CBS, and NBC use channels 7,2, and
4, respectively. Channel number corresponds to
specific frequency band assigned to the broadcaster
in NYC.
In Philadelphia, these channels are not used at all.
These frequencies are not reused over a large
region surrounding NYC. This is to ensure that
broadcasters do not interfere.
Other Interference Avoidance
Techniques

Because of limited spectrum, interference is
unavoidable in wireless systems. Reuse distance is
one way to reduce the interference.

Engineers have developed (and continue to
develop) other interference avoidance techniques:
 Receiver structures
 Antenna methods
 Etc.
Summary

In these slides, we have learned the basics of
wireless communication systems.

In the next few days, we will look at how particular
systems work.

Next time, we focus on the cellular phone system.