wireless communications

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

Police TechnologyPolice TechnologyChapter ThreeChapter Three

Wireless Wireless CommunicationCommunication

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Learning Objectives Learning Objectives Basic radio wave theoryBasic radio wave theory AnalogAnalog and and digitaldigital communications communications Basic parts of radio frequency Basic parts of radio frequency

communicationscommunications Digital communications; including Digital communications; including

Time Division Multiple Access (TDMA)Time Division Multiple Access (TDMA) Frequency Division Multiple Access (FDMA)Frequency Division Multiple Access (FDMA) Code Division Multiple Access (CDMA)Code Division Multiple Access (CDMA) Cellular Digit Packet Data (CDPD)Cellular Digit Packet Data (CDPD)

Cellular telephone systemsCellular telephone systems


Radio Basics Radio Basics

There are two medias for the There are two medias for the transmissiontransmission

of electronic information: of electronic information: Information can be transmitted over Information can be transmitted over

a a wirewire, like telephone conversations. , like telephone conversations. It can be transmitted through the It can be transmitted through the

atmosphere; atmosphere; wirelesswireless like the signals like the signals

your car radio receives.your car radio receives.



Wireless communication Wireless communication has has

an advantage over an advantage over hardwired communications hardwired communications Wireless can be Wireless can be

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

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


Radio BasicsRadio Basics

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

Voice and data information Voice and data information can be carried by either. can be carried by either. Both have advantages and Both have advantages and

disadvantages. disadvantages.


Analog radio system consists of a Analog radio system consists of a microphone that modulates (changes) microphone that modulates (changes) the human voice to an analog wave, the human voice to an analog wave, which is processed and sent to an which is processed and sent to an antennaantenna for transmission. The return for transmission. The return signal is passed through the antenna signal is passed through the antenna to be received, processed, and to be received, processed, and broadcast out a speakerbroadcast out a speaker




An analog radio system can carry An analog radio system can carry digital signals, but first the digital digital signals, but first the digital signal must be converted from its signal must be converted from its analog configuration to aanalog configuration to adigital configuration. digital configuration. This is accomplished by a This is accomplished by a

modulator-demodulator unit, a modulator-demodulator unit, a modemmodem. .

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


As you move further away As you move further away

from the antenna that is from the antenna that is

broadcasting an analog broadcasting an analog

signal, the signal becomes signal, the signal becomes

weaker until the strength of weaker until the strength of

the signal is lost in the signal is lost in

background static andbackground static and

the transmission cannotthe transmission cannot

be heard. be heard.




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

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



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

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

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


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



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

These ranges of These ranges of frequencies are frequencies are called called bandsbands. .

Different frequency Different frequency bands share bands share certain broadcast certain broadcast characteristics. characteristics.


Different groups of frequencies are called Different groups of frequencies are called channelschannels. .

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

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

A A guard bandguard band prevents the channels from prevents the channels from overlapping and confusing the overlapping and confusing the information exchange. information exchange.



Effective radiated powerEffective radiated power – how far – how far away from the source a signal can be away from the source a signal can be received and interpreted. received and interpreted.

Lower frequencyLower frequency = = Higher effective Higher effective radiated radiated

powerpower




Wireless signals sent in a Wireless signals sent in a Very High Very High Frequency (VHF)Frequency (VHF) - many wave cycles - many wave cycles per second - provide a wider area of per second - provide a wider area of coverage. coverage.

Shorter wavelengths of Shorter wavelengths of Ultra High Ultra High Frequency (UHF)Frequency (UHF) – even more cycles – even more cycles per second - go through building per second - go through building windows better than VHF. windows better than VHF.


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

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

The larger the frequency, the larger The larger the frequency, the larger the antenna required. the antenna required.



Communications Hardware Communications Hardware BasicsBasics

TransmitterTransmitter - generates radio wave - generates radio wave

(the signal)(the signal)

ReceiverReceiver - receives signal, changes - receives signal, changes it back into the original formit back into the original form

TransceiverTransceiver - combination of both - combination of both


Job of transmitter is to generate the Job of transmitter is to generate the signal. signal.

That frequency is a series of oscillating That frequency is a series of oscillating waves. waves. OscillationOscillation means the waves means the waves peak and valley in a predictable and peak and valley in a predictable and uniform manner. uniform manner.

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



To transmit data we use the To transmit data we use the

Frequency Shift Key (FSK)Frequency Shift Key (FSK) method – method –

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

To change voice into digital we use a To change voice into digital we use a voice coder (vocoder)voice coder (vocoder) – the reverse of – the reverse of a modem. a modem.



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

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




You can increase the You can increase the range of transmission range of transmission by adjusting the by adjusting the gain - gain - directing the antenna’s directing the antenna’s radiating energy in radiating energy in alternate directions.alternate directions.


Simple Voice (simplex system)Simple Voice (simplex system) - - Users transmit and receive Users transmit and receive information on a single channel or information on a single channel or frequency. frequency.

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

Trunked Radio System Trunked Radio System – locates an – locates an empty line and assigns it to the user empty line and assigns it to the user for the duration of the transmissionfor the duration of the transmission

Radio Frequency SystemsRadio Frequency Systems


Frequency Division Multiple Frequency Division Multiple AccessAccess

Increases spectrum efficiencyIncreases spectrum efficiency Typical frequency of 30 kHz is Typical frequency of 30 kHz is

divided into three channels of 10 divided into three channels of 10 kHz each.kHz each.

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


TDMA takes advantage of the space TDMA takes advantage of the space between words. between words.

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

Timed to not interfere with each other. Timed to not interfere with each other.

Time Division Multiple AccessTime Division Multiple Access


Code Division Multiple AccessCode Division Multiple Access

In CDMA, each transmission of digital In CDMA, each transmission of digital information is information is spread outspread out among all among all of the available channels. of the available channels.

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

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


Cellular TechnologyCellular Technology

Cell SiteCell Site – a – a physical location physical location that contains a that contains a cellular telephone cellular telephone antenna array and a antenna array and a base station base station controller. controller.

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


Cellular TechnologyCellular Technology

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

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

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


Cellular Digit Packet DataCellular Digit Packet Data

CDPD is a data networkCDPD is a data network, no voice., no voice. It has an IP address, not a It has an IP address, not a

telephone #telephone # Information from a CDPD system is Information from a CDPD system is

relayed like a cellular telephone call relayed like a cellular telephone call through a commercial carrier. through a commercial carrier.

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


Police TechnologyPolice Technology

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