Chapter 2 Radio Frequency Fundamentals

Exam Essentials Understand wavelength, frequency, amplitude, and phase. Know the definition of each RF characteristic and how it can affect wireless LAN design. Remember all the RF propagation behaviors. Be able to explain the differences between each RF behavior (such as refection, diffraction, scattering, and so on) and the various mediums that are associated with each behavior. Understand what causes attenuation. Loss can occur either on the wire or in the air. Absorption, free space path loss, and multipath downfade are all causes of attenuation. Define free space path loss. Despite the lack of any obstructions, electromagnetic waves attenuate in a logarithmic manner as they travel away from the transmitter.

Exam Essentials Explain the difference between active and passive gain. RF amplifiers are active devices, whereas antennas are passive devices. Explain the difference between transmit and received amplitude. Transmit amplitude is typically defined as the amount of initial amplitude that leaves the radio transmitter. When a radio receives an RF signal, the received signal strength is most often referred to as received amplitude. Remember the four possible results of multipath and their relationship to phase. Multipath may cause downfade, upfade, nulling, and data corruption. Know the results of intersymbol interference and delay spread. The time differential between a primary signal and reflected signals may cause corrupted bits and affect throughput and latency due to layer 2 retransmissions.

Understanding Wireless Need to understand how wireless works at the physical layer of the OSI model RF Signals move through the air in an unpredictable manner Unbounded Medium

What is Radio Frequency (RF) Part of the Electromagnetic Spectrum Starts as Alternating Current (AC) generated from a transmitter Radiated out of an antenna element Changes in current produce changes in Electromagnetic Field Pg 31

Alternating Current Current where the magnitude and direction varies in a cycle over time Produces a sine wave Fluctuation is the oscillation Movement of the wave through air is the propagation behaviors Absorption Reflection Scattering Refraction Diffraction Amplification attenuation hấp thụ Reflection tán xạ Khúc xạ nhiễu xạ khuếch đại suy giảm Pg 32

RF Characteristics Wavelength Frequency Amplitude Phase Pg 32

Wavelength Distance between the peaks of the waveform Distance traveled in a single cycle Pg 32

Wavelength Represented by Greek lambda λ Inverse relationship between wavelength and frequency Pg 33

Wavelength The higher the frequency, the shorter the wavelength. The longer the wavelength, the shorter the frequency. Pg 33

Wavelength As RF travels though space, signal attenuate, or lose signal strength Shorter Wavelengths will attenuate FASTER Signals keep traveling, but may be below the sensitivity threshold of receiver. Pg 34

Comparing 5 Ghz and 2.4 Ghz Higher Frequencies (shorter wavelength) attenuate faster Attenuation through the air is Free Space Path Loss Higher Frequencies also don’t penetrate objects as well. Pg 34

Frequency Measurement of how many times something happens in a second 1 hertz (Hz) = 1 cycle per second 1 kilohertz (KHz) = 1,000 cycles per second 1 megahertz (MHz) = 1,000,000 (million) cycles per second 1 gigahertz (GHz) = 1,000,000,000 (billion) cycles per second Pg 36

Amplitude Measure of signal strength or power λ is for wavelength y is for amplitude Loss of amplitude is attenuation or loss Transmit Amplitude Initial amplitude at transmitter Received Amplitude Received signal strength Pg 37

Amplitude Different RF technologies use different transmit amplitudes AM Radio may use 50,000 Watts 802.11 Access Poitns from 1mW an 100 mW Pg 38

Phase Difference in degrees separating two overlapping sine waves Out of phase Measured from 0-360 0-in phase 90-quarter out of phase 180-cancels out original Etc. Pg 100

Phase Pg 100

Phase Differences in phase are important to understanding multipath Can cause interference in 802.11 signals Pg 38

RF Behaviors Wave Propagation How waves move through the air and obstacles Absorption Reflection Scattering Refraction Diffraction Loss-Attenuation FSPL Multipath Amplification hấp thụ Reflection tán xạ Khúc xạ nhiễu xạ Sự suy giảm thua lỗ FSPL đa khuếch đại Pg 39

Absorption If a signal does not bounce off, move around, or pass through an object, then 100 % absorption has occurred. Significant cause of Loss Most materials absorb some level of RF signal Brick, Concrete, Water all absorb a lot Even things with lots of water in them Drywall absorbs less Pg 40

Reflection If a wave hits a smooth object larger than itself it may bounce off Sky wave reflection Lower Frequencies bouncing off charged particles in the ionosphere Microwave reflection Higher frequencies (1 Ghz to 300 Ghz) that bounce off smaller objects like a metal door Important for WLAN Buildings, roads, water, earth’s surface Pg 41

Reflection Can be a problem in WLAN as reflected signals will arrive out of phase with original signal Multipath Pg 41

Scattering Multiple reflections If the wavelength is longer than the medium that the wave is passing through Two types If the particles are smaller than the wavelength, minor scattering of the signal If the signal encounters an uneven surface and is reflected in multiple directions Fences, trees, etc Pg 43

Refraction Signals can also be BENT by refraction Bending of a signal as it passes through a medium with a different density Causes the direction of the wave to change. Issue for long distance bridging Pg 44

Refraction Pg 44

Diffraction Bending of a signal AROUND an object Also spreads the signal Usually caused by a partial blockage If you are behind the blockage, you are in the RF shadow! Pg 44

Loss (Attenuation) Decrease of amplitude or signal strength On a wire due to impedance Over the air-Free Space Path Loss Loss and gain are measured in Deibels Pg 46

Multipath When multiple signals arrive at receiver due to different obstructions/effects on RF signal Difference in arrival is delay spread. Pg 49

Multipath Pg 51