Chapter Three - Wireless Communications

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Nov 21, 2013 (3 years and 7 months ago)

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Copyright 2005
-

2009: Hi Tech Criminal
Justice, Raymond E. Foster

Police Technology

Chapter Three

Wireless
Communications

Copyright 2005
-

2009: Hi Tech Criminal
Justice, Raymond E. Foster

Learning Objectives


Basic radio wave theory


Analog

and
digital

communications


Basic parts of radio frequency communications


Digital communications; including


Time Division Multiple Access (TDMA)


Frequency Division Multiple Access (FDMA)


Code Division Multiple Access (CDMA)


Cellular Digit Packet Data (CDPD)


Cellular telephone systems



Copyright 2005
-

2009: Hi Tech Criminal
Justice, Raymond E. Foster

Radio Basics

There are two medias for the transmission

of electronic information:


Information can be transmitted over a
wire
, like telephone conversations.


It can be transmitted through the
atmosphere;
wireless

like the signals your
car radio receives.


Copyright 2005
-

2009: Hi Tech Criminal
Justice, Raymond E. Foster

Radio Basics

Wireless communication has

an advantage over

hardwired communications


Wireless can be
broadcast over a wide
area and to a large
number of users.


Because wireless
communication uses radio
frequencies, it is often
referred to as
RF
communication


Copyright 2005
-

2009: Hi Tech Criminal
Justice, Raymond E. Foster

Radio Basics


Radio waves are the way in
which both analog and digital
signals are carried as
wireless Communications.


Voice and data information
can be carried by either.


Both have advantages and
disadvantages.

Copyright 2005
-

2009: Hi Tech Criminal
Justice, Raymond E. Foster


Analog radio system consists of a
microphone that modulates (changes) the
human voice to an analog wave, which is
processed and sent to an
antenna

for
transmission. The return signal is passed
through the antenna to be received,
processed, and broadcast out a speaker



Radio Basics

Copyright 2005
-

2009: Hi Tech Criminal
Justice, Raymond E. Foster

Radio Basics

An analog radio system can carry

digital signals, but first the digital

signal must be converted from its

analog configuration to a

digital configuration.


This is accomplished by a
modulator
-
demodulator unit, a
modem
.


For this transmission scheme to
work, both the sender and receiver
must have a modem.

Copyright 2005
-

2009: Hi Tech Criminal
Justice, Raymond E. Foster

As you move further away

from the antenna that is

broadcasting an analog

signal, the signal becomes

weaker until the strength of

the signal is lost in

background static and

the transmission cannot

be heard.

Radio Basics

Copyright 2005
-

2009: Hi Tech Criminal
Justice, Raymond E. Foster

Radio Basics


A digital signal can be broadcast only up
to a certain distance.


Once that distance is reached, the signal
for transmission is gone

Copyright 2005
-

2009: Hi Tech Criminal
Justice, Raymond E. Foster

Radio Basics


Radio signals travel in waves (like sound
and light). Radio waves are like ripples
in a pond with two important
differences:


You cannot see radio waves. They are
invisible; and,


Radio waves stay constant. They do not
distant themselves (grow further apart).


Copyright 2005
-

2009: Hi Tech Criminal
Justice, Raymond E. Foster

Radio Basics

Radio waves travel in a constant
pattern. The number of wave cycles
per second is called a
Hertz
, or more
commonly the radio

frequency.

Copyright 2005
-

2009: Hi Tech Criminal
Justice, Raymond E. Foster

Radio Basics


AM/FM radios work in
ranges that are
different from each
other.


These ranges of
frequencies are called
bands
.


Different frequency
bands share certain
broadcast
characteristics.

Copyright 2005
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2009: Hi Tech Criminal
Justice, Raymond E. Foster


Different groups of frequencies are called
channels
.


The number of frequencies in a channel is
referred to as the channel’s
bandwidth
.


The more bandwidth, the more information a
channel can handle.


A
guard band

prevents the channels from
overlapping and confusing the information
exchange.

Radio Basics

Copyright 2005
-

2009: Hi Tech Criminal
Justice, Raymond E. Foster


Effective radiated power



how far away
from the source a signal can be received
and interpreted.


Lower frequency

=
Higher effective






radiated power


Radio Basics

Copyright 2005
-

2009: Hi Tech Criminal
Justice, Raymond E. Foster

Radio Basics


Wireless signals sent in a
Very High
Frequency (VHF)

-

many wave cycles per
second
-

provide a wider area of coverage.


Shorter wavelengths of
Ultra High
Frequency (UHF)



even more cycles per
second
-

go through building windows
better than VHF.


Copyright 2005
-

2009: Hi Tech Criminal
Justice, Raymond E. Foster


Frequencies within a band share a
propensity to
skip
. Skip occurs when a
broadcast reflects off the Earth’s
ionosphere.


Noise and interference more of a problem
for lower frequencies (like AM radio)


The larger the frequency, the larger the
antenna required.

Radio Basics

Copyright 2005
-

2009: Hi Tech Criminal
Justice, Raymond E. Foster

Communications Hardware
Basics


Transmitter

-

generates radio wave


(the signal)



Receiver

-

receives signal, changes it
back into the original form



Transceiver

-

combination of both

Copyright 2005
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2009: Hi Tech Criminal
Justice, Raymond E. Foster


Job of transmitter is to generate the
signal.


That frequency is a series of oscillating
waves.
Oscillation

means the waves peak
and valley in a predictable and uniform
manner.


The part of the transmitter that generates
these waves is the
oscillator
.

Communications Hardware
Basics

Copyright 2005
-

2009: Hi Tech Criminal
Justice, Raymond E. Foster


To transmit data we use the


Frequency Shift Key (FSK)

method




two separate frequencies are used to
represent ones and zeros of binary data.



To change voice into digital we use a
voice coder (vocoder)



the reverse of a
modem.

Communications Hardware
Basics

Copyright 2005
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2009: Hi Tech Criminal
Justice, Raymond E. Foster


Different antennas have different
propagation characteristics (the way
the antenna sends out the signal.



Basic antenna is a dipole (straight wire
or metal).



Communications Hardware
Basics

Copyright 2005
-

2009: Hi Tech Criminal
Justice, Raymond E. Foster

Communications Hardware
Basics


You can increase the
range of transmission by
adjusting the
gain
-

directing the antenna’s
radiating energy in
alternate directions.

Copyright 2005
-

2009: Hi Tech Criminal
Justice, Raymond E. Foster


Simple Voice (simplex system)

-

Users
transmit and receive information on a
single channel or frequency.


Repeater System



The signal is picked up
by a repeating antenna and repeated at a
higher point and on another frequency to
base.


Trunked Radio System


locates an empty
line and assigns it to the user for the
duration of the transmission

Radio Frequency Systems

Copyright 2005
-

2009: Hi Tech Criminal
Justice, Raymond E. Foster

Frequency Division Multiple
Access


Increases spectrum efficiency


Typical frequency of 30 kHz is divided
into three channels of 10 kHz each.


Increases the number of frequencies
that can be used in conjunction with a
trunked radio system.

Copyright 2005
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2009: Hi Tech Criminal
Justice, Raymond E. Foster




TDMA takes advantage of the space
between words.


Each user is allowed a small portion of the
broadcast cycle.


Timed to not interfere with each other.

Time Division Multiple Access

Copyright 2005
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2009: Hi Tech Criminal
Justice, Raymond E. Foster

Code Division Multiple Access



In CDMA, each transmission of digital
information is
spread out

among all of the
available channels.


The digital frames move from one
frequency to the next in milliseconds.


The frames must be put back together
with a unique code (direct sequence) to
each call.

Copyright 2005
-

2009: Hi Tech Criminal
Justice, Raymond E. Foster

Cellular Technology


Cell Site



a physical
location that contains
a cellular telephone
antenna array and a
base station
controller.


Cell

is the area of
coverage provided by
the cell site antenna
array.



Copyright 2005
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2009: Hi Tech Criminal
Justice, Raymond E. Foster

Cellular Technology


The configuration of cell sites has some of
the same characteristics of the repeater
antenna configuration.


A primary characteristic that identifies a
cellular system is
frequency reuse
.


A primary characteristic of cellular
technology is the ability hand signals off
from cell to cell.


Copyright 2005
-

2009: Hi Tech Criminal
Justice, Raymond E. Foster

Cellular Digit Packet Data


CDPD is a data network
, no voice.


It has an IP address, not a telephone #


Information from a CDPD system is
relayed like a cellular telephone call
through a commercial carrier.


The subscriber pays only for the
packets of data sent over the network

Copyright 2005
-

2009: Hi Tech Criminal
Justice, Raymond E. Foster

Police Technology

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