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7/27/2019 Radio Frequency Operations
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Radio Frequency Operations
and
TechnologyMobile Device Investigations Program
Senior Instructor
Technical Operations DivisionDHS - FLETC
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RF Operation and Technology
Radio propagation the eletromagnetic phenomenomdiscovered by Henrich Hertz in the late 1880s. Itshow energy travels through a given medium.
The medium can be air, water, aired cable, fiber opticsand the like. Radio signals travel at the speed oflight through the air 186,282 miles per second.
The only significant difference cellular systems andconventional landlines is the radio link that connectsthe wireless network to the cell base station.
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RF Operation and Technology
Different radio signals have different properties, i.e.
High frequencies and Low frequencies
Signal Refraction - signals bend through theatmosphere
Signal Diffraction - signals bend around obstructions
Signal Reflection - signals bounce off obstructions orsolid objects
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RF Operation and Technology
Examples of radio signal actions
Diffraction Air careens over a moving car andoff the back end
Reflection Light bouncing off a mirror
Radio Frequency propagation refers to howwell radio signals radiates or travels in freespace.
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RF Operation and Technology
Wireless signals are basically omnidiriectional
Omnidirectional; all directions, propagation isoften described as waves.
Pebble In Water - size of pebble, water andenegey
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RF Operation and Technology
Radio Frequency coverage from any basestation is determined by three factors;
1. The height of the antenna
2. The type of antenna used
3. The Radio Frequency Power Level emitted.
This is true no matter where the antenna isplaced
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There are a number of things that may interferwith the propagation of a radio signal.
Some of the ongoing issues wireless carriershave to deal with are;
1. Ducting
2. Signal Fading
Signal Fading and Ducting
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Ducting
Ducting is defined as the trapping of RF in theboundary area between two air masses.
Ducting of RF is an atmospheric anomaly knownas temperature inversion
Ducting is unavoidable because it is a naturalmeteorological phenomenon.
Downtilting or directing the RF toward the ground
may compensate for ducting.
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Absorption
Absorption describes how a radio signal isabsorbed by objects
The greater the amount of absorption the lessgeographical area can be covered.
Organic material absorbs more wireless signalsthan in non-organic.
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Absorption
Absorption can be compensated for in thefollowing ways:
1. By using higher-gain antennas
2. By using higher RF power levels in order
to cover the same geographic area3. By decreasing the distance between
Base Stations
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Free-space Loss or Path Loss describes the attenuationof a radio signal over a given distance.
The higher the frequency the greater the free space loss.PCS carriers operating at 1.9MHz will have greaterloss than wireless carriers operating at 850 MHz.
Power Levels of a Base Station signal and antenna gainwill determine when a wireless signal will fade
Free Space Loss
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Multipath or Rayleigh Fading; how thetransmitted base signal reacts to physicalfeatures or structures creating multiple signal
paths.
Using the pebble in the water analogy, think of
rocks, sticks or other objects in the water.
Mutlipath/Rayleigh fading will occur in bothdirections.
Multipath/Rayleigh Fading
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Multipath/Rayleigh Fading
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Obviously strongest signal will always be thesignal directly from the mobile unit to the
Base Station.
The indirect signals will arrive at the base either
in phase or out of phase with the directsignal.
Multipath/Rayleigh Fading
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Wireless Frequency Bands
The radio frequency assigned to the earlycellular industry was the mostly unused 850MHz UHF television spectrum.
The 850 MHz radio propagation is beneficial forthe following reasons;
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Wireless Frequency Bands
It has a very short wavelength, about 12 inches
It tends to be line of sight, unlike shortwave radio
signals
It is easily reflected off objects.
It is easily absorbed by foliage, which can be goodor bad. It can help with frequency reuse or incertain urban areas or highways it can degrade thesignal.
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Wireless Frequency Bands
The 1.9 MHz signal used by PCS carriers isgood for wideband services, but haspenetration problems as it relates to buildings
and the like.
The 1.9 MHz signal does not propagate as well
as the 850 MHz signal because it has aneven shorter wavelength.
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Wireless Frequency Bands
It has a very short wavelength, less than12inches
It tends to be line of sight, unlike shortwaveradio signals
It is easily reflected off objects.
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Due to the nature of building construction and structure itis often difficult receive and maintain RF inside.
In-Building Coverage
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Frequency Coordination is the effort by carrierRF Engineers to place base stations in aorderly fashion to minimize interference.
This is done by adhering to the Distance toReuse ratio within their own system.
RF Engineers from the various carriers oftencoordinate with each to comply with theFCC mandates.
Frequency Coordination
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System Interference
Interference is defined as any adverseinteraction between two or more radio signalsthat cause noises or cancels out each other.
This generally occurs when transmitting RF toclose to each other or on the same
frequency.
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Cochannel Interference
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Cochannel Interference
The RF Engineer considers the following whenplanning cell placement
1. RF power levels at cochannel cell sites
2. Geographic distance between cochannel cells
3. Types of antennas used at cochannel cells
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Channel Interference
Other types of Channel Interference are;
Adjacent Channel Interference the inability of a
mobile unit to filter signals of adjacent channelsassigned to nearby cells
Intermodulation Interference or IM IM describes theeffect of several signals mixing together toproduce unwanted signals or no signals.
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Paired Channels
All wireless conversations require paired channels tofunction.
Mobile or portable phones conduct simultaneous two-way transmissions. This is known as Full Duplex.
One channel is used for transmitting and one channelis used to receive.
Depending on the technology, the frequency may bethe same or they maybe different.
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Control Channels
1. Setup wireless calls, mobile originated or terminated(destined), locating mobile phones to make contactwith them.
2. Collecting information for billing operations
3. Collecting traffic measurements on base stations
4. Autonomous mobile registration, registering thephones on the system (both home and roaming)
5. Initiating and assisting with call handoff
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Control Channels
Each channel set in a wireless network has a ControlChannel.
When a mobile unit is powered on it seeks out the
Control Channel.
The carrier sets one of its strongest frequencies as theControl Channel when they launch.
The mobile unit re-tunes to the Control Channelperiodically to maintain a strong connection with theControl Channel.
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RF Technologies
Ultra-Wideband Wireless or UWB UWB is adigital pulse technology designed to transmitlarge amounts of data over a wide spectrumof frequency bands.
UWB carries large amounts of data in distancesup to 230 feet at very low power levels.
The digital pulses produced by UWB are timedprecisely over the very wide spectrum at thesame time.
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RF Technologies
UWB can also be defined as wireless communicationstechnology that can transmit data at speedsbetween 40 megabits and 60 megabits persecond.
The two main difference between UWB and othernarrowband or wideband systems;
1. The bandwidth of UWB is greater than any other current
technology used for communication2. UWB is typically implemented in a carrierless fashion,
meaning it is not constricted by the RF allotmentschemes.
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RF Technologies
UWB advantages are;
1. High throughput
2. Effectiveness in environments with high levels ofmultipath fading
3. Short range high data rate
4. Carries a large amount of data with low powerconsumption
5. Impressive accuracy measuring positions of objectsand people
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RF Technologies
UWB pulse technology has proven effective in theareas of radar and sensor technology. It can beadapted for use in;
1. Motion Detection
2. Radar imaging for objects buried underground or behindwalls
3. Radar imaging in the medical field
4. 5. 6. Rescue application
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RF Technologies
Current wireless carrier protest the use of UWB basedon that argument that because they transmit onall frequencies, they may cause interference withintheir wireless networks.
UWB proponents argue because it transmit acrosssuch a wide spectrum and at such low power rates
it will be impossible for it to interfere with currentwireless networks
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RF Technologies
UWB seeks to replace Bluetooth as a short rangeconnection technology, as well as;
1.
Code Division Multiple Access (CDMA)
2. Global System for Mobile Communication (GSM)
3.
General Packet Radio Service (GPRS)
4. Enhanced Data GSM Environment (EDGE)
5.
Wideband-CDMA (W-CDMA)
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RF Technologies
Two other RF technologies are Software DefinedRadio (SDR) and Cognitive Radio (CR).
Software Defined Radio uses a software package toadminister the all the functions used in wirelesscommunication.
Everything from RF and channel selection, to handoffand logging is handled independently by thesoftware program.
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