Wireless Networking Chapter 2 -Radio Frequency Fundamentals
Analog Signals
Continuous
Loss Amplitude
decrease in amplitude , is a phenomena that is common to both wired and wireless networks. Negative difference in amplitude between signals
Spectrum analyzer
functions in the frequency domain - measures amplitude
Oscilloscope
functions in the time domain- measures amplitude
VSWR
is a measure of how well the components of the RF system are matched in impedance. Impedance is the resistance to current flow, measured in Ohms)
attenuation
RF signals lose their signal strength as they travel through space and matter.
Radio wave
When electric current passes through a wire it creates a magnetic field around the wire. As magnetic field radiates, creates an electromagnetic radio wave.
RF Properties
Can move through matter or space. i.e. Gamma waves,X rays, Visible light, Radio waves
EIRP
Equivalent Isotropically Radiated Power
Frequency
Rate at which an event occurs
Time-Domain Concepts - Frequency (f )
Rate, in cycles per second, or Hertz (Hz) at which the signal repeats
out of phase
Signals out of alignment
Sound wave
air pressure changes in path
Antenna
an electrical conductor or system of conductors - Transmission - radiates electromagnetic energy into space - Reception - collects electromagnetic energy from space
Refraction
bending of the RF signal as it passes through a medium or material with a different density. Most often due to atmospheric conditions. For long-range outdoor links, we use the kfactor to examine refraction. Typical atmospheric value is 4/3. deflection from a straight path, (bending) as the wave travels through a medium.
multipaths Data corruption
multiple reflected signals and the delay spread result in difficulty demodulating the information from the RF signal. The end result is corrupted data.
Microwave Reflection
occurs for microwave signals (1 GHz - 300 GHz). Higher frequency-Smaller wavelength signals. This type of reflection is very commonplace both indoors and out and with a variety of materials.
Wavelength
the distance between crests or valleys of a waveform.
Direct Current (DC)
the flow of electric charge is only in one direction.
Alternating Current (AC)
the flow of electric charge periodically reverses direction. An electric charge move forward, then backward, then forward, then backward, over and over again.
Wave Propagation
the way in which RF waves move i.e. ripple in a pond. Can vary considerably depending on material (e.g. metal, wood, drywall, air, foliage, fog, etc.)
Active gain
usually the result of an amplifier between the transceiver and the antenna.
Sine Wave
wave form of AC circuit power
VSWR
Voltage Standing Wave Ratio
Radio Wave Properties
- Spreads out through space in all directions - Can travel long distances -Can penetrate non-metallic objects
Things that can happen when RF signal as is passes (propagates) through space and matter
Absorption . Attenuation Reflection . Gain Scattering Refraction Diffraction Free Space Path Loss Multipath
EM WAVE lifecycle
An RF signal starts as an electrical alternating current (AC), generated by a transmitter. THE AC signal is sent through a copper conductor and radiated out an antenna as an electromagnetic wave - this EM wave is our wireless signal!
RF shadow
Area behind obstruction is referred to as _______, which can become a dead zone of suffered from a degraded signal
Advantages of Reflection
Can use reflection to go around obstruction.
Scattering - Uneven or rough surfaces
Caused when an RF signal encounters an uneven surface (rocky terrain, foliage, chain link fence). Can cause significant degradation of the signal even loss of the signal.
Amplitude modulation (AM)
Changes amplitude so that highest peaks of carrier wave represent 1 bit while lower waves represent 0 bit
Frequency modulation (FM)
Changes number of waves representing one cycle Number of waves to represent 1 bit more than number of waves to represent 0 bit
Phase modulation (PM)
Changes starting point of cycle When bits change from 1 to 0 bit or vice versa
Cycle
Changing event that creates different radio frequencies. When wave completes trip and returns back to starting point it has finished one __________
Hertz (Hz)
Cycles per second
How to reduce multipath
Directional antennas Antenna diversity Reducing transmit power Lower-gain antenna
Digital Signals
Discreet
Three types of modulations enable carrier signals to carry information
Height of signal Frequency of signal Relative starting point
carrier signal
Increasing voltage will change frequency of ________ sent out by radio transmissions
Disadvantages of Reflection
Multipath reflection - occurs when reflections cause more than one copy of the same transmission to arrive at the receiver at slightly different times.
RF
Radio Frequency
Frequency
The number of repetitions per unit time of a complete waveform The number of complete oscillations (cycles) per second of electromagnetic radiation The number of times per second that a signal's amplitude "peaks".
Electromagnetic wave
Travels freely through space in all directions at speed of light
in phase
Two waveforms in exact alignment
Modem (MOdulator/DEModulator)
Used when digital signals must be transmitted over analog medium
Radio Frequency Characteristics
Wavelength Frequency Amplitude Phase
Scattering - Small particles
When RF signal moves through a substance and individual waves are reflected off tiny particles in the medium (e.g. fog). Usually small scale and less impactful on signal strength and quality.
Diffraction
also bending of the RF signal, but around an object instead of passing through it. Diffraction depends on shape, size, and material of obstructing object, as well as phase, amplitude, and polarization of the RF signal. Usually caused by a small or partial blockage. Waves take a longer, different path
Time-Domain Concepts - Period (T )
amount of time it takes for one repetition of the signal T = 1/f
Gain/Amplification
an increase in amplitude or signal strength, measure of amplification. Can be active or passive .
Frequency of radio wave can be changed
by modifying voltage
Scattering
can be thought of as multiple reflections, which occurs when the signals wavelength is larger than the pieces of material the signal is passing through (e.g. foliage).
RF wave
can travel through any medium (including vacuum)
multipaths Nulling
complete signal cancellation. Signals arrive at same time and 180 degrees out of phase.
Modem originating End
converts distinct digital signals into continuous analog signal for transmission
distance between any two successive identical points on the wave
cycle
Time-Domain Concepts - Wavelength (λ):
distance occupied by a single cycle of the signal Or, the distance between two points of corresponding phase of two consecutive cycles
Phase
is a relationship between two or more signals of the same frequency -is a measurement of the change in position of the crest or valley of two waveforms
Voltage Standing Wave Ratio
is the ratio of the maximum voltage to the minimum voltage in a standing wave. For maximum power transfer the ideal is 1:1.
OHMS
is the resistance between two points of conductor
EIRP
is used to estimate the service area of the transmitter, and to co-ordinate transmitters on the same frequency so that their coverage areas do not overlap.
Attenuation
loss of signal strength,is usually undesirable, although on rare occasion - it can be added to a system to conform to power regulations or to address capacity issues
Time-Domain Concepts - Peak amplitude (A)
maximum value or strength of the signal over time; typically measured in volts
amplitude
measure change in RF energy caused by a passing RF wave (volts) Usually correspond to signal strength Directly related to power output of the transmitter
Time-Domain Concepts -Phase (φ):
measure of the relative position in time within a single period of a signal
Sky Wave Reflection
occurs in frequencies below 1 GHz (very large wavelength). An example is a signal bouncing off the Earth's ionosphere. This is why you can hear AM radio stations over long distance - especially on a clear night.
Multipath
phenomenon where a signal takes 2 or more path to get from transmitting antenna to receiving antenna. All of the various types of RF behavior we have discussed can contribute to multipath, although reflection is the main culprit. Because of the distance traveled, the reflected signals usually take longer to arrive. The time differential between signals is called the delay spread and is typically on the order of a nanosecond (10-9 seconds).
Passive gain
result of focusing the RF signal with a highly directional antenna. Much like we focus the beam of a flashlight.
Modem Receiving End
reverse process performed WLANs use digital transmissions
multipaths Downfade
signals arrive simultaneously, but out of phase from original. Results is decreased signal strength. Phase difference between 121 and 179 degrees result in downfade.
multipaths Upfade
signals arrive simultaneously, in phase of partially out of phase. Results in increased signal strength. Phase differences between 0 and 120 degrees. [Cannot be great amplitude than original].
Absorption
the ability of some materials to absorb some are all of the energy of the RF signal i.e. Water (Humans are 70% water), Brick/concrete, Drywall
Free Space Path Loss (FSPL)
the laws of physics dictate that any electromagnetic signal will attenuate or die out as it travel. this is the natural loss of signal strength due to this expansion of the waves.
Reflection
wave hits a smooth object larger than the wave and bounces in another direction
When does diffraction occur
when the wave encounters an edge. The wave has the ability to turn the corner of the edge. It is markedly dependent on frequency -- the higher the frequency, the less ____. Very high frequencies (light) hardly ____ at all; "light travels in straight lines."