ENP Study Group
Principles of
TelecommunicationsRadio
BROUGHT TO YOU BY: THE FLORIDA NENA EDUCATION COMMIT TEE
Radio Spectrum3kHz- 300GHz
FCC- Federal Communications Commission
Licensing
Radio Wave
Simulcast
Wavelength
Analog signal
Band
Data
Channels
GPS & DGPS
Trunking
Conventional
Repeater
Simplex
Voting
Digital signal
Radio Waves
Electromagnetic waves that travel through the atmosphere.
To better understand properties of Radio Waves, consider waves in the ocean.
Similar to radio waves, they are a series of peaks and valleys.
Images and descriptions courtesy of: Understanding Wireless Communications in Public Safety- A guidebook to Technology, Issues, Planning and
Management. Published by the National Law Enforcement and Technology Center. http://transition.fcc.gov/pshs/docs-best/imel-wireless03.pdf
Wavelength & Frequency
The length of a wave is measured from one point on a wave to the next similar point.
◦ i.e. from peak to peak, or valley to valley
◦ A wave with a short wavelength would have peaks that are relatively close to each other.
◦ A wave with a long wavelength would have peaks that are further apart.
Properties of Radio Waves
In a public safety radio system carrying, a person’s voice is converted to electrical energy that
rides a wave.
The size or length of the wave is dependent on the frequency used by the radio system.
The distance that a wave can travel is dependent on factors such as the frequency, wavelength,
humidity, obstacles interference, etc.
Waves are similar to a long distance runner. The further the runner goes, the less energy he has.
Public Safety BAND plans
VHF-low (Very High Frequency)
◦ 39-72 MHz
◦ Travel greater distances, often used for intercity or statewide
communications.
◦ Prone to problems with atmospheric skip.
VHF-high
◦ 145-159 MHz
◦ Used by Police, Fire and EMS for day-to-day communications.
UHF (Ultra High Frequency)
◦ 400, 500, 700, 800, 900 MHz
◦ Used by Police, Fire and EMS for day-to-day communications.
Channels
The FCC groups frequencies within a BAND into individual CHANNELS which are licensed to
public safety radio systems.
Simplex radio systems
As might be inferred by the name, SIMPLEX radio systems are the simplest of systems.
SIMPLEX radio systems utilize portable, mobile and possibly base station radios that ALL
transmit and receive transmissions on the SAME channel.
◦ Think: walkie-talkie, CB, FRS (Family Radio Service) or GMRS (General Mobile Radios Service)
radios
SIMPLEX is used when units do not need to communicate over great distances.
◦ E.g. Firefighter in a building talking to the Incident Commander or truck pump operator.
All modern public safety radios have the ability to operate in a SIMPLEX mode.
◦ This is usually referred to as ‘direct’ or ‘talk-around’.
Repeater based radio systems
If communications is needed over a greater distance (larger coverage area), a system of
repeaters is used.
A repeater system requires two channels. One for “talk-in”, the other to retransmit.
Transmit antenna at greater height
Simplex vs. Repeater
Frequency B
repeat
Frequency A
“talk-in”
Receive antenna 60’+ above ground
Power limitations
Portable radios in the public safety bands generally transmit 3-5 Watts
Mobile radios in the public safety bands generally transmit 35-50 Watts
Base stations and repeaters generally transmit 5-100 Watts
To increase spectrum efficiency and quality, the FCC imposes power restrictions on radio
systems.
◦ In the past a jurisdiction would put an antenna in a central location, with an antenna as high as
possible, with a large amount of power to push the signal as far as possible.
◦ Radio systems are now engineered so that frequencies may be re-used by other jurisdictions.
(e.g. 75 miles between systems).
Power systems (battery packs) and antenna’s must be considered as well.
Trunking (Trunked Radio Systems)
Consider a repeater based radio system for a county that utilized the following in their system:
◦
◦
◦
◦
◦
EMS
FIRE
Law Enf.
City Gov’t.
County Gov’t.
1 radio channel
3 radio channels (dispatch, tactical, command)
3 radio channels (PD, SO, car-to-car)
1 radio channel
1 radio channel
This small county maintains a total of eight radio channels.
The county EMS normally runs two ambulances. One radio channel is more than sufficient.
Now, imagine a large Mass Casualty Incident (MCI) requiring EMS coordination with surrounding
EMS providers, hospitals and trauma centers.
How TRUNKING works
CONVENTIONAL RADIO SYSTEM
TRUNKED RADIO SYSTEM
Voting Receivers
Since Portable and Mobile radios have significantly less power than do repeaters, there are often
areas where users may be able to hear radio traffic, but are not be able to get a transmission
into the system.
Satellite (remotely located) receivers may be located strategically throughout the required
coverage area.
These receivers receive the signal from the portable and mobile radios and send the signal, over
the microwave or landline to the central controller, which ‘votes’ (or selects) the best audio
signal. This signal is passed to the system and that is what the other users hear.
Simulcast
A simulcast system transmits radio traffic across multiple synchronized
transmitters (sites).
Simulcast systems are often engineered with voted receiver sites allowing for
robust and wide spread coverage systems.
Digital vs. Analog
Traditional radio systems carried analog
signals through the system. An analog
system uses the voice’s vibration.
Newer radio systems (including P25) use digital
signaling to send the voice through system. A device
in the radio called a voice encoder or ‘vocoder’
transmits the analog signal into a digital signal which
is transmitted through the system and then
converted back to an audible signal.
Data
Data transmissions may also be sent across a radio system.
Simple data transmissions may be the radio call sign transmitted at regular intervals, or to
control remote equipment.
Many systems utilize a special signal to indicate an emergency.
Mobile Data systems may use radio systems for data transport.
GPS location information can also be sent across a radio system.
Some systems have been used to transport live video feeds.
GPS
The Global Positioning Service was developed by the DOD as a worldwide navigation and
positioning tool.
GPS relies on (24) satellites orbiting the earth to serve as reference points using time
measurements from at least four satellites. These can be translated into lat/long, altitude,
course and speed.
Differential Global Positioning
System (DGPS) relies on a
calculation from a second
receiver at a fixed location to
determine the error in the
signal.
Questions