public safety radio system radio systems 101
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Public Safety Radio System Radio Systems 101. Public Safety Radio Bands. VHF-Low Band: 25 MHz to 50 MHz VHF-High: 138 MHz to 174 MHz UHF: 408 MHz to 512 MHz 700 MHz 800 MHz 4.9 GHz. Why is this a problem?. Radios only operate in one band! Multi-band radios exist but are expensive - PowerPoint PPT PresentationTRANSCRIPT
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Buchanan CountyCommunications
Public Safety Radio System
Radio Systems 101
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Public Safety Radio Bands
VHF-Low Band: 25 MHz to 50 MHz VHF-High: 138 MHz to 174 MHz UHF: 408 MHz to 512 MHz 700 MHz 800 MHz 4.9 GHz
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Why is this a problem?
Radios only operate in one band! Multi-band radios exist but are expensive
If Agency A uses VHF and Agency B uses 800, they can’t talk to each other UNLESS…
…They have planned ahead! Two radios in a rig, etc.
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Propagation & Band Characteristics VHF Low Band (30-50 MHz)
Best propagation in undeveloped and hilly terrain–Poor building penetration
VHF High Band (150-174 MHz) Very good propagation in undeveloped and hilly terrain–Moderate
building penetration UHF (450-512 MHz)
Good propagation in undeveloped and hilly terrain–Good building penetration
700/800 MHz Poor propagation in undeveloped and hilly terrain–Very good building
penetration– 700 currently subject to incumbent television stations in some areas 800 currently subject to interference from commercial carriers
4.9 GHz Microwave propagation used for short range (Wi-Fi type) or point-to-point links
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Frequencies vs. Channels A frequency is a point in the radio spectrum
part of what describes a channel A channel is a set of parameters that can include
one or more frequencies, CTCSS tones, name, etc. Example: VCALL is a channel with transmit and
receive frequency 155.7525 MHz, CTCSS tone of 156.7 Hz
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CTCSS (PL) Tones PL stands for Private Line, a Motorola trademark Other names include Code Guard, Tone Squelch,
Call Guard, Channel Guard, Quiet Channel, Privacy Code, Sub-audible Tone, etc.
“Generic” term is CTCSS – Continuous Tone Coded Squelch System
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What Are These Tones? A PL tone is a sub-audible (barely audible) tone that
is sent along with the transmitted audio A receiver that has CTCSS decode (a.k.a. a receive
PL tone) activated will only open its speaker if the correct tone is received
PL tones are different than tones used to set off pagers (two-tone sequential paging)
Remember…PL tones are sub-audible and continuous…they are being sent the entire time a radio is transmitting
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What Are They Used For PL Tones are used to MASK interference They DO NOT REMOVE INTERFERENCE Useful for masking interference from computers,
electronics, etc. Useful for masking interference from “skip” Should NOT be used to block out traffic from
neighboring (nearby) departments This is OK for taxis, etc., but not for public safety Creates “Hidden Interference” problem – missed calls
possible
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What Are They Used For (cont.)
Used to activate remote links Used to access repeaters
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DCS – Digital Coded Squelch
A.k.a. Digital Private Line (DPL) Similar to CTCSS, but uses a digital code
instead of an audio tone Used on analog radio systems, even though it
is a digital code
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Encode vs. Decode PL (or DPL, et.c) Encode means to transmit the tone Decode means that the receiver will listen for the tone
and not let anything through unless the correct tone is received
TX and RX tone can be different Radio can be set to TX tone but have no RX tone (all
traffic is received) If in doubt, don’t program RX tone “Monitor” function bypasses RX tone
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Simplex
Very Reliable Limited Range Radio Channel uses 1 frequency
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Duplex Radio Channel using 2 frequencies, Freq 1 to talk from radio
A to radio B, and Freq 2 to talk from radio B to radio A Each user must be line of sight with each other Examples: Cordless Telephone systems, which both parties
can talk at the same time and listen at the same time.
f1
f2
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Base Station – Height Improves Range
Unit 1
Dispatch CenterUnit 2
Unit 3
Unit 4
Some units don’t hear transmission because of obstructions
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Base Station – Height Improves Range
Unit 1
Dispatch CenterUnit 2
Unit 3
Unit 4
Dispatcher relays message – heard by all units
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Remote Base Operation
Unit 1
Unit 2Unit 3
Unit 4
Dispatch CenterRemote Link
Microwave, Phone Line, etc.
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Conventional Repeater Receives a signal on one frequency and retransmits
(repeats) it on another frequency Placed at a high location Increases range of portable and mobile radio
communications Allows communication around obstructions (hills,
valleys, etc.) User radios receive on the repeaters transmit
frequency and transmit on the repeater’s receive frequency (semi-duplex)
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f1f2 f2 f2
f2f1RXTX
Conventional RepeaterAll units within range of repeater hear all transmissions through the repeater
Unit 1
Unit 2Unit 3
Unit 4
Dispatch Centerf2
Repeater
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PD 2
PD 1
PD 3
PD 4
FD 1
FD 2
FD 3PW 1
PW 2PW 3
Conventional SystemsWhen one user is talking, other users on that channel are cannot talk, even though other repeaters in the area may be idle.
Communicating
Idle
IdlePD 3 cannot talk to PD 4 because PD 1 is using the repeater
Public works repeater may be idle 90% of the time, which means that frequency is largely wasted
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Trunking Trunking is a method of combining repeaters at
the same site to “share” frequencies among users Spectrally efficient Allows many more “virtual” channels (called
talkgroups) than there actually are frequencies Computer controlled
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f1
f2f2
f2f1RX TX
Trunked System
f4f3RX TX
f6f5RX TX
PD 1
PD 2
Shared Repeater Bank
PD 3
FD 1
FD 2
f3
f4
•Frequencies are dynamically assigned by system controller
•User radio may be on a different frequency every time it transmits
•Talkgroups are “virtual” channels
•Possible to have many more talkgroups than actual frequencies
•Statistically, not all talkgroups will be active at the same time
System Controller
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Trunked System Operation User radios continuously monitor a dedicated “control channel” When a user wants to transmit, the user’s radio makes a request to the
system controller If a repeater is available, the system controller temporarily assigns that
repeater channel to the talkgroup making the request Transmitting user’s radio will give a “talk beep”, indicating that a repeater
has successfully been assigned…user can talk All user radios monitoring that talkgroup automatically switch to the
frequency of the assigned repeater and hear the transmission When the transmission is complete, all radios return to monitoring the
control channel
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Multi-Site Systems
Conventional Repeaters on same output, different input Linked repeaters on different frequencies Remote Receive Sites
Voting
Simulcasting Trunking
Roaming Simulcasting
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Repeaters on same output frequency, different input frequency (or PL tone)
Users must manually change channel to different repeater depending on their physical location
Only one repeater active at a time
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Repeaters on same output frequency, different input frequency (or PL tone)
Users must manually change channel to different repeater depending on their physical location
Only one repeater active at a time
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Linked repeaters on different frequencies
Users must manually change channel to different repeater depending on their physical location
Link (microwave, phone line, etc.)
Both repeaters active at the same time with same traffic, but on different frequencies
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Voting Receivers
Users do not need to change channel depending on location. System (voter) automatically picks best receive tower site.
Link (microwave, phone line, etc.)
RX Only Site
Voter
Central Transmitter
Voter (comparator) chooses best received signal and sends that signal to the transmitter
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Simulcasting
Transmitters must be carefully synchronized to prevent interference in overlap areas
Both repeaters transmit at the same frequency at the same time
Link (microwave, phone line, etc.)
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Analog vs. Digital Modulation
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Common Analog Modulation Schemes FM – Frequency Modulation AM – Amplitude Modulation SSB – Single Sideband AM Almost all analog public safety communications
use FM AM is used for CB radio, aircraft communication
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Frequency Modulation (FM)
To modulate means “to change” or “to vary” Frequency Modulation means changing the frequency
of the transmitter in proportion to the audio being picked up by the microphone
The receiver detects the change in transmitter frequency and uses it to reproduce the audio signal at the speaker
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FM Radio Block Diagrams(simplified)
Modulator RF AmplifierMicSoundWaves
Audio Signal RF SignalAmplfied RF
Signal
AudioAmplifier
Demodulator Spkr
AmplifiedAudio Signal
Audio SignalRF Signal
Transmitter
Receiver
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Digital Modulation Signal from microphone is converted from a
voltage into numbers through a process called sampling
Those numbers are processed by a computer Binary information (ones and zeros) is sent over
the air instead of analog (continuous voltage) information
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Vocoding Vocoding is used to reduce the amount of data
that needs to be sent over the air Used to reduce necessary bandwidth – conserves
spectrum “Compresses” digital audio – analogous to .mp3
versus .wav audio files Uses known human speech characteristics to “fill
in gaps” of data that is removed
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Digital Radio Block Diagrams(simplified)
Sampler(A/D Converter)
Modulator RF AmplifierMicSoundWaves
Raw AudioSignal
Raw DigitalSignal
RF SignalAmplified RF
Signal
100111010011
Vocoder
1100101
EncodedDigital Signal
D/AConverter
Demodulator SpkrSoundWaves
ReconstructedAudio Signal
DecodedDigital SignalRF Signal
100111010011
Vocoder
1100101
EncodedDigital Signal
Transmitter
Receiver
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The Digital Radio “Problem” Parametric vocoder uses known human voice
characteristics to encode and decode data When background noise (non-human noise) is
present, vocoder doesn’t always know how to respond Unpredictable results (garble, loss of communication,
etc.) In a similar situation, an analog radio would transmit
the background noise right along with the intended audio (background noise might overpower voice, but some audio is still received)
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