Radio Frequency Fundamentals PDF

Handout 2 Radio Frequency Fundamentals Course Name: Wireless Networks Course Code: CSN 405 Notes appended and modified to those accompanying “CWNA Certified Wireless Network Administrator: Official Study Guide”, D. Coleman & D. Westcott, John Wiley & Sons - Sybex, 6th Ed., 2021, Ch. 3 Radio Frequ...

Handout 2 Radio Frequency Fundamentals Course Name: Wireless Networks Course Code: CSN 405 Notes appended and modified to those accompanying “CWNA Certified Wireless Network Administrator: Official Study Guide”, D. Coleman & D. Westcott, John Wiley & Sons - Sybex, 6th Ed., 2021, Ch. 3 Radio Frequency Fundamentals • What is an RF signal? • Radio frequency properties • Radio frequency behaviors 2 Spectrum  The electromagnetic (EM) spectrum, which is usually simply referred to as spectrum, is the range of all possible electromagnetic radiation.  This radiation exists as self-propagating electromagnetic waves that can move through matter or space. 3 RF Signal  An RF signal starts out as an electrical alternating current (AC) signal that is originally generated by a transmitter.  This AC signal is sent through a copper conductor (typically a coaxial cable) and radiated out of an antenna element in the form of an electromagnetic wave. 4 RF Signal Radio Transceiver  This electromagnetic wave is the RF signal.  Changes of electron flow in an antenna, otherwise known as current, produce changes in the electromagnetic field around the antenna. 5 RF Signal  An alternating current is an electrical current with a magnitude and direction that varies cyclically, as opposed to direct current, the direction of which stays in a constant form.  The shape and form of the AC signal (waveform) is known as a sine wave. 6 RF Signal  Sine wave patterns can also be seen in light, sound, and the ocean.  The fluctuation of voltage in an AC current is known as cycling, or oscillation. 7 Sine Wave A B  An oscillation, or cycle, of this alternating current is defined as a single change from up to down to up, or as a change from positive to negative to positive. 8 Sine Wave A B  A cycle is a complete wave oscillation from point A to point B.  The period is the time it takes for a sine wave to complete one cycle. 9 RF Properties  Wavelength  Frequency  Amplitude  Phase 100 Wavelength  A wavelength is the distance between the two successive crests (peaks) or two successive troughs (valleys) of a wave pattern.  In simpler words, a wavelength is the distance that a single cycle of an RF signal actually travels. 111 Wavelength Length of 1 Cycle Length of 1 Cycle Length of 1 Cycle λ 122 Wavelength  2.4 GHz = 12.5 cm (4.92 inches)  5 GHz = 6 cm (2.36 inches)  Formulas to calculate wavelength: λ (in.) = 11.811/Frequency (GHz) λ (cm) = 30/Frequency (GHz) 133 Wavelength  It is very important to understand that there is an inverse relationship between wavelength and frequency.  The three components of this inverse relationship are frequency (f, measured in hertz, or Hz), wavelength (λ, measured in meters, or m), and the speed of light (c, which is a constant value of 300,000,000 m/sec). 144 Wavelength The following reference formulas illustrate the relationship: λ = c/f f = c/λ 155 Wavelength  The perception is that the higher frequency signal with smaller wavelength will not travel as far as the lower frequency signal with larger wavelength.  The reality is that the amount of energy that can be captured by the aperture of a high frequency antenna is smaller than the amount of RF energy that can be captured by a low frequency antenna. 166 Frequency  Frequency is the number of times a specified event occurs within a specified time interval.  A standard measurement of frequency is hertz (Hz), which was named after the German physicist Heinrich Rudolf Hertz.  An event that occurs once in 1 second has a frequency of 1 Hz. 177 Frequency  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 188 Inverse Relationship The following reference formulas illustrate the relationship: λ = c/f f = c/λ 199 Amplitude  Amplitude is characterized simply as the signal’s strength, or power.  When speaking about wireless transmissions, this is often referenced as how loud or strong the signal is.  Amplitude can be defined as the maximum displacement of a continuous wave. 20 Amplitude  With RF signals, the amplitude corresponds to the electrical field of the wave.  When you look at an RF signal using an oscilloscope, the amplitude is represented by the positive crests and negative trough of the sine. 21 Amplitude Transmit Amplitude +15 dBm Received Amplitude -70 dBm  Transmit amplitude is typically defined as the amount of initial amplitude that leaves the radio transmitter.  For example, if you configure an access point to transmit at 50 milliwatts (mW), that is the transmit amplitude. 22 Amplitude Transmit Amplitude +15 dBm Received Amplitude -70 dBm  Cables and connectors will attenuate the transmit amplitude, while most antennas will amplify the transmit amplitude. 23 Amplitude Transmit Amplitude +15 dBm Received Amplitude -70 dBm  When a radio receives an RF signal, the received signal strength is most often referred to as received amplitude.  RF signal strength measurements taken during a validation site survey is an example of received amplitude. 24 Phase  Phase is not a property of just one RF signal but instead involves the relationship between two or more signals that share the same frequency.  The phase involves the relationship between the positions of the amplitude crests and troughs of two waveforms. 25 RF Propagation Behaviors  Absorption  Reflection  Diffraction  Scattering 26 Propagation  The way in which the RF waves move— known as wave propagation—can vary drastically depending on the materials in the signal’s path.  For example, drywall will have a much different effect on an RF signal than metal or concrete. 27 Absorption  Absorption refers to the absorption of RF Waves by a material.  The absorption is the "missing piece" when comparing the total reflected and transmitted energy to the incident energy. 28 Demo: Absorption 29 Reflection 30 Refraction 31 Diffraction The bending of an RF signal as it propagates around an object is called diffraction. Much like sunlight bending around a building. 32 Scattering 33 Gain (Amplification) Peak amplitude after gain Peak amplitude before gain FREQUENCY TIME * As viewed by oscilloscope * As viewed by spectrum analyzer 34 Loss (Attenuation) Peak amplitude before loss Peak amplitude after loss TIME * As viewed by oscilloscope FREQUENCY * As viewed by spectrum analyzer 35 Loss - Environment 36 Free Space Path Loss (FSPL) FSPL = 36.6 + (20log10(f)) + (20log10(D)) FSPL = path loss in dB f = frequency in MHz D = distance in miles between antennas FSPL = 32.44+ (20log10(f)) + (20log10(D)) FSPL = path loss in dB f = frequency in MHz D = distance in kilometers between antennas 37 Free Space Path Loss (FSPL) 38 Multipath  Propagation phenomenon that results in two or more paths of a signal arriving at a receiving antenna at the same time or within nanoseconds of each other.  Because of the natural broadening of the waves, the propagation behaviors of reflection, scattering, diffraction, and refraction will occur differently in dissimilar environments. 39 Multipath  It usually takes a bit longer for reflected signals to arrive at the receiving antenna because they must travel a longer distance than the principal signal.  The time differential between these signals can be measured in billionths of a second (nanoseconds).  The time differential between these multiple paths is known as the delay spread. 40 Multipath - Upfade  Upfade is increased signal strength.  When multiple RF signal paths arrive at the receiver at the same time and are in phase or partially out of phase with the primary wave, the result is an increase in signal strength (amplitude).  Smaller phase differences of between 0 and 120 degrees will cause upfade. 41 Multipath - Upfade  Please understand, however, that the final received signal can never be stronger than the original transmitted signal because of free space path loss.  Upfade is an example of constructive multipath. 42 Multipath - Downfade  Downfade is decreased signal strength.  When the multiple RF signal paths arrive at the receiver at the same time and are out of phase with the primary wave, the result is a decrease in signal strength (amplitude). 43 Multipath - Downfade  Phase differences of between 121 and 179 degrees will cause downfade.  Decreased amplitude as a result of multipath would be considered destructive multipath. 44 Multipath - Nulling  Nulling is signal cancellation.  When the multiple RF signal paths arrive at the receiver at the same time and are 180 degrees out of phase with the primary wave, the result will be nulling.  Nulling is the complete cancellation of the RF signal. A complete cancellation of the signal is obviously destructive. 45 Multipath – Data Corruption  Because of the difference in time between the primary signal and the reflected signals known as the delay spread, along with the fact that there may be multiple reflected signals, the receiver can have problems demodulating the RF signal’s information. 46 Multipath – Data Corruption  The delay spread time differential can cause bits to overlap with each other, and the end result is corrupted data.  This type of multipath interference is often known as intersymbol interference (ISI).  Data corruption is the most common occurrence of destructive multipath. 47 Demo: Upfade, Downfade, Nulling 48 Questions Home Work 1. Open your book and go through all the review questions at the end of the chapter. 2. Review the answers by using Appendix A. 49