Chapter 2: RF Characteristics and Behavior

Sine Waves Properties

-Period -Wavelength -Frequency

Wavelenght Calculation

2.4 GHz = 12.5 cm 5 GHz = 6 cm λ (cm) = 30/Frequency (GHz)

Frequency Hopping Spread Spectrum (FHSS)

A spread-spectrum broadcasting method defined in the 802.11 standard that sends data on one frequency at a time, constantly shifting (or hopping) frequencies. Commonly used in Bluetooth. So if there is interference in one of the frequencies used by FHSS, it doesn't keep you from getting any data through.

Sine Wave Amplitude

Amount of signal energy. It is the strength or power of the signal. measured in Watts, Milliwatts (mW), Decibels to milliwatt (dBm).

Basic Types of Modulation

Baseband Signal is the Original data bits in the radio's baseband, which we have to modulate someway. There are 3 kinds: -Amplitude Modulation: Varies Amplitude Shift Keying (ASK), varies the amplitude of the carrier signal to encode data -Frequency Modulation: Frequency Shift Keying (FSK), varies the frequency of the carrier signal to encode data. We don't use it in 802.11 Modulation. -Phase Modulation: Phase Shift Keying (PSK), We shift the phase in order to represent the change in data. So as long as the phase stays the same in a time period, we keep the same bit value.

Newton's Inverse Square Law

Can describe the change in the power of a wave. The change in the power of the signal is equal to 1 divided by the square of the change in distance. So if the distance tripled, the change in power would be: (1/3) ^ 2 = 1/9 This decrease in power is know as Free Space Path Loss (FSPL).

Diffraction

Change in the direction (bending) and/or intensity of RF waves after passing by or through an obstacle. Instead of bending as it passes into or out of an obstacle, like refraction, diffraction describes what happens as light travels around the obstacle. Consider the analogy of a rock dropped into a pool and the ripples it creates. Think of the ripples as analogous to RF signals. Now, imagine there is a stick being held upright in the water. When the ripples encounter the stick, they will bend around it, since they cannot pass through it. A larger stick has a greater visible impact on the ripples and a smaller stick has a lesser impact.

RF Loss

Decrease or weaking in signal amplitude often due to external interference or path loss. Caused by attenuation, absorption, reflection, interference.

Absorption

Dense materials absorb RF energy as the signal passes through, decreasing amplitude. Causes weaker signal results from absorption from Bricks and concrete, drywall, windows, doors, people, water, etc... Both manmade structures such and buildings and natural obstacles such as trees and other foliage can cause absorption. Absorption is the conversion of the RF signal energy into heat. This conversion happens because the molecules in the medium through which the RF signal is passing cannot move fast enough to "keep up" with the RF waves. Many materials absorb RF signals in the 2.4 GHz ISM spectrum. These include water, drywall, wood and even humans.

Delay Spread

Difference between the time of arrival of the earliest multipath signal, which is usually the line of sight signal, and the arrival time of the last multipath signal from the same transmission.

Modulation Constellations

Helpful visual tool for understanding modulation, showing an in-phase and quadrature component of a carrier wave. Rotation around the center axis represents the phase component of a wave.

RF Gain

Increase of RF signal strength or amplitude. There is Active and Passive Gain. Gain of a signal can be viewed with either an oscilloscope or spectrum analyzer.

Active RF Gain

Increasing the power from the transmitter. Accomplished by increasing the transmit power of the RF radio.

Sine Wave Wavelenght

Indicated by the Greek lambda symbol λ. It is the distance between one value to the same value on the next cycle. The wavelengths of the RF signals used in WLAN technologies are usually measured in centimeters

Wavelength and frequency are

Inversely related. If there is a Low Frequency, there is a Long Wavelenght. As if there is a High Frequency, there is a Short Wavelenght

Waveform

Is a representation of how alternating current (AC) varies with time.

Modulation

Is manipulating the waves so they represent information. Modulation is a process by which some property of a carrier signal is modified to represent digital bits. Amplitude, frequency, and phase are the three basic elements of a waveform that can be varied.

RF Propagation

Like sound waves, light waves, or even waves in calm lake, RF waves move away from the source of the wave. As a wave moves further away from it's source, the wave becomes broader and looses power, becoming weaker. If the encounters objects in it's path, these objects will effect the path that the wave travels.

Free Space Path Loss (FSPL)

Measure of how much signal remains with increasing distance in free space (approximately same as air). As signal moves away from the source, it naturally decreases in power due to the broadening of the wave. This decrease in power is know as Free Space Path Loss (FSPL). Mathematically, the a 2.4 GHz signal would travel twice as far as a 5 GHz, and provide the same signal loss. Due to other variables like noise and device configuration, this is not actually the case. 2.4 GHz signals will typically travel further than 5 GHz signals, however due to many other factors the differential is not 2:1.

Spread Spectrum

Method of communications we use 802.11 Physical Layers, spreading the bits we need to transmit over multiple frequencies (20/22/40/80/160 MHz)

Reflection

Occurs when an RF signal bounces off a smooth, non-absorptive surface, changing the direction of the signal. Reflective environments cause multipath. Causes: Steel and metal beams, shelving, roofing, walls, furniture.

Refraction

Occurs when the RF signal passes through a medium of different density than the original medium. Causing the primary wave to change direction. In most cases there is some level of reflection too.

Quadrature Amplitude Modulation (QAM)

Phase and amplitude shifts applied to carrier to represent bit sequences. 16-QAM — 16 possible phases — 4 data bits 64-QAM — 64 possible phases — 6 data bits

Sine Wave Frequency

The number of repetitions or cycles per unit time is the frequency, typically expressed in cycles per second, or Hz. (1 Hz = 1 cycle/second) 1 thousand = 1 KHZ 1 million = 1 MHz 1 billion = 1 GHz The inverse relationship between time (t), the period in seconds, and frequency (f), in Hz, is indicated by the following formulas: t = 1/f f = 1/t

What is the constant that results in the interrelationship between frequency and wavelength?

The speed of light, which is the speed at which electromagnetic waves travel, is the constant in the relationship between frequency and wavelength.

Sine Wave Period

The time it takes for a sine wave to complete one cycle is defined as the period of the waveform. The distance traveled by the sine wave during this period is referred to as its wavelength. The 802.11 standard and other WLAN documentation may refer to a certain number of things occurring within a period or within each period

Phase is a relational radio concept in that it compares

Two waves to each other as either in phase or out of phase. Phase is a comparison of the cycle or period of different waves. A single wave does not have a phase. However, two waves will have a phase relationship with one another. This is true for reflected copies of the same wave, which can have different phase relationships. An example of phase would be if there are two roads that intersect. The roads can be parallel, perpendicular or at any angle to one another but you can not have an angle between just one road. This is similar to phase.

Scattering

Type of reflection caused by uneven surfaces and increases echoes and multipath. Another way to define scattering is to say that it is multiple reflections. Scattering can happen in a minor, almost undetectable way, when an RF signal passes through a medium that contains small particles. These small particles cause the scattering. Smog is an example of such a medium. The more common and more impacting occurrence is that caused when RF signals encounter things like rocky terrain, leafy trees or chain link fencing. Rain and dust can cause scattering as well.

Modulations & Coding Schemes (MCS) used in 802.11 WLANs

Types of Modulations such as DBPSK, DQPSK and QAM. There are various types of coding schemes as well, including Barker, CCK, PBCC, and FEC (convolutional).

As the frequency becomes higher,

Wavelength becomes shorter

We say that an RF signal [blanket] as it travels through [free blanket]. Additionally, materials such as [blanket] can attenuate the signal more than free space itself.

We say that an RF signal [attenuates] as it travels through [free space]. Additionally, materials such as [walls] can attenuate the signal more than free space itself. Attenuation occurs in free space and as signals travel through materials such as walls and doors. The attenuation is the weakening of the signal as it travels farther away from the point of origination.

Attenuation (RF Loss)

Weakening of the RF signal as it reacts to the environment. Propagation behaviors that cause attenuation include: Reflection, Refraction, Diffraction, Scattering and Absorption

RF in the Frequency Domain

What we see in WLAN spectrum analyzers, like watching the wave coming at you. We are seeing each frequency the spikes of energy happening, as the wave is coming to the antenna of the spectrum analyzer.

Direct Sequence Spread Spectrum (DSSS)

A spread-spectrum broadcasting method defined in the 802.11 standard that sends data out on different frequencies at the same time. So if there is interference on just a small portion of the frequencies space used by Spread Spectrum used by DHSS or OFDM, it doesn't keep you from getting some of the data through.

Modulations Types can be

Absolute or Differential. In other words, with BPSK, the phase shifts are relative to an absolute phase value. The receiver must know the absolute phase value against which it is comparing received data, but that point is fixed. Conversely, differential BPSK (DBPSK) uses differential phase values. In DBPSK (or DQPSK), the phase shifts are compared with the phase setting of the previous signal measurement. In other words, if the phase is 0 on the first wave and the phase of the next wave is 180, then we know the phase shift was 180 degrees (180 degree phase differential). If the following wave is 180 degrees, we know that the phase shift was 0 degrees because the prior measurement was also 180. Then if the following wave is 270 degrees, we know that it shifted by 90 degrees from previous. In this way differential PSK modulation changes the encoded data only when the phase changes. DSSS and HR/DSSS utilize DBPSK and DQPSK, whereas OFDM (802.11a/g) utilizes BPSK and QPSK.

What characteristic of an RF wave defines its strength or power?

Amplitude. A higher amplitude is a stronger signal. A lower amplitude is a weaker signal. Therefore, an amplitude of 3 dB is stronger than 1 dB.

Interference

Anything that corrupts or modifes the original signal (usually destructive) such as non-Wi-Fi transmitters, Multipath fading, co-channel and adjacent channel Wi-Fi devices. Interference may occur between waves of identical, similar, or harmonically related frequencies, may be caused by multipath components, and is usually destructive to the primary signal. Multiple Input, Multiple Output (MIMO) technology used in the new 802.11n amendment (rolled up into the latest 802.11 standard) and takes advantage of multipath rather than being harmed by it.

Sine wave

Commonly referenced AC waveform which derives its name from the fact that the current or voltage varies with the mathematical sine function of the elapsed time. An ideal wireless signal has a sine waveform The sine wave is unique in that it represents energy entirely concentrated at a single frequency.

Hertz (Hz)

Cycles or waves per second. -A million cycles per second is represented by megahertz (MHz) -A billion cycles per second represented by gigahertz (GHz)

Electromagnetic Spectrum

Is the complete range of all electromagnetic radiation. It is often references simply as spectrum without the reference to electromagnetics. Many technologies use electromagnetic spectrum frequency ranges, such as: AM radio, FM radio, Wi-Fi, Radar, Light bulbs.

Multipath

Multipath occurs when two or more of these signals arrive at the receiving antenna at the same time. As these signals are term-28combined at the receiver they create a distorted version of the transmitted signal.

Diffusion (RF Loss)

Natural spreading of the RF wave as it travels away from the signal source

Phase

Relational or comparison radio concept. A Wave has a phase in relation to another one. When two sine waves differ by exactly zero° or 360°, the two waves are said to be in phase. Two sine waves that differ in phase by any other value are out of phase, with respect to each other. 90 out of phase (destructive) 180 out of phase (cancels) In-phase (gain) Out of Phase (destructive)

Passive RF Gain

Shaping or focusing power from antenna. Antenna gain will also increase the signal and this is known as passive gain.

Heatmap Showing Absorption

Signal is absorbed by the walls

Higher frequencies have shorter

Wavelengths and lower frequencies have longer wavelengths.

RF in the Time Domain

Way to look at it similar to a oscilloscope, like the wave going by and in front of you. The wave traveling through space.

Phase Shift Keying (PSK)

We have Binary PSK (BPSK) with two possible phases represent 1 data bit and Quadrature PSK (QPSK) with four possible phases representing 2 data bits. With BPSK, the receiver has a wider margin of error (180 degrees), whereas with QPSK, the margin for error is less (90) because there are more possible data points. However, QPSK is also twice as efficient as BPSK because the same carrier signal can represent 2 bits instead of 1.