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."