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4 Radio Frequency Engineering (RF Engineering).
To understand the RF concepts it is essential to the implementation, expansion, maintenance, and troubleshooting of the wireless network. Without having proper knowledge of Radio Frequency it is not possible to implement, expend, maintain troubleshoot the wireless networks. Without this basic of knowledge, an administrator would be unable to locate proper installation locations of equipment and would not understand how to troubleshoot a problematic wireless network. In this chapter we explore RF in detail.
4.1 Radio Frequency (RF).
Radio frequencies are high frequency alternating current (AC) signals that are passed along a copper conductor and then radiated into the air via an antenna. An antenna transforms (converts) a wired signal to a wireless signal and vice versa. When the high frequency AC signal is radiated into the air, it forms radio waves. These radio waves propagate (move) away from the source (the antenna) in a straight line in all directions at once. As shown in the figure bellow.
If we can imagine dropping a rock into a still pond. And watching the concentric ripples flow away from the point where the rock hit the water, then this brings an idea of how RF behaves as it is propagated from an antenna. All electromagnetic waves, including radio waves, move at the speed of light, the frequency is related to the wavelength. Higher frequencies have shorter wavelengths, and lower frequencies have longer wavelengths. Understanding the behavior of these propagated RF waves is an important part of understanding why and how wireless networks (LANs) function.
4.2 RF Behaviors.
RF waves that have been modulated to contain information are called
RF signals. These RF signals have behaviors that can be predicted and detected. They become stronger, and they become weaker. They react to different materials differently, and they can interfere with other signals. RF is sometimes referred to as "smoke and mirrors" because RF seems to act erratically and inconsistently under given circumstances. Things as small as a connector not being tight enough or a slight impedance mismatch on the line can cause erratic behavior and undesirable results. The following sections introduce you to the major RF signal behaviors
4.2.1 RF Gain.
Gain is defined as the positive relative amplitude difference between two
RF wave signals. Gain is the term used to describe an increase in an RF signal's amplitude.
Amplification is an active process used to increase an RF signal's amplitude and, therefore, results in gain. There are two basic types of gain: active and passive. Both types can be intentional, and passive gain can also be unintentional. .
4.2.2 Active Gain.
Active gain is achieved by placing an amplifier in-line between the RF signal generator (such as an access point) and the propagating antenna. These amplifiers usually measure the gain they provide in decibel (dB).To determine the actual power of the signal after passing through the amplifier, we will have to know the original power of the signal from the RF generator and then perform the appropriate RF math.
4.2.3 Passive Gain.
Passive gain is not an actual increase in the amplitude of the signal delivered to the intentional radiator, but it is an increase in the amplitude of the signal, in a favored direction, by focusing or directing the output power. Passive gain can be either intentional or unintentional.
(a) Intentional Passive Gain.
Antennas are used to provide intentional passive gain in wireless networks using RF signals. The antenna propagates more of the RF signal's energy in a desired direction than in other directions. The RF signal is said to have gain in that direction. (Intentional passive gain is like cupping hands around mouth as we yell to someone at a distance. We are directing the sound waves, intentionally, toward that targeted location. You are not increasing your ability to yell louder.)
(b) Unintentional Passive Gain.
Unintentional passive gain happens because of reflection and scattering in a coverage area. When the RF signal leaves the transmitting antenna, the primary signal travels out from the antenna according to the propagation patterns for which the antenna is designed. However, this signal may encounter objects that cause reflection and scattering, resulting in multiple copies of the same signal arriving at the receiving antenna. If these signals arrive in phase, they can cause the signal strength to actually increase and this would be a form of unintentional passive gain.
4.3 RF Loss.
Loss is defined as the negative relative amplitude difference between two RF signals. Like gain, loss can be either intentional or unintentional.
4.3.1 Intentional Loss.
Due to FCC regulations and the regulations of other regulatory domains, we will have to ensure that the output powers of your wireless devices are within specified constraints. Depending on the radios, amplifiers, cables, and antennas you are using, you may have to intentionally cause loss in the RF signal. This means that you are reducing the RF signal's amplitude, and this is accomplished with an attenuator. Attenuation is the process that causes Loss.
4.3.2 Natural Loss.
In addition to the intentional loss that is imposed on an RF signal. To comply with regulatory demands, Unintentional or Natural losses can occur. This kind of loss happens because of the natural process of RF propagation. Many things can cause RF signal loss, both while the signal is still in the cable as a high frequency AC electrical signal and when the signal is propagated as radio waves through the air by the antenna. Resistance of cables and connectors causes loss due to the converting of the AC signal to heat. Impedance mismatches in the cables and connectors can cause power to be reflected back toward the source
When an RF signal bounces off of a smooth, nonabsorptive surface, changing the direction of the signal, it is said to reflect and the process is known as reflection.
For example, we can shine a light on a mirror at an angle and see that it reflects off that mirror. In fact, when you look in the mirror, you are experiencing the concept of electromagnetic reflection, which is the same as RF reflection. As you can see, the light waves. Which are electromagnetic waves similar to RF signals, first reflect off the object and travel toward the mirror. Next, the light waves reflect off the mirror and travel toward our eye.
It is important to remember that reflected signals are usually weaker after Reflection. This is because some of the RF energy is usually absorbed by the reflecting material.
Refractionoccurs when an RF signal changes speed and is bent while moving between media of different densities. The light reflection analogy helps us understanding Refraction. If we wear Glasses, we are wearing a refraction device. The lens refracts, or bends, the light to make up for the imperfect lens in your eye. This allows us to See clearly again because the lacking focus of the eye is corrected by the Refraction caused on the lens of the glasses.
When refraction occurs with RF signals, some of the signal is reflected and some are refracted as it passes through the medium. Of course, as with all mediums, some of the signal will be absorbed as well. RF signal refraction is usually the result of a change in atmospheric conditions. For this reason, refraction is not usually an issue within a building, but it may introduce problems in wireless site-to-site links outdoors. Common causes of refraction include changes in temperature, changes in air pressure, or the existence of water vapor. The issue here is simple: if the RF signal changes from the intended direction as it's traveling from the transmitter to the receiver, the receiver may not be able to detect and process the signal. This can result in a broken connection or in increased error rate. The figure 4.4 below shows refractions phenomena.
Diffractionis defined as a change in the direction and/or intensity of a wave as it passes by the edge of an obstacle. This can cause the signal's direction to change, and it can also result in areas of RF shadow. Instead of bending as it passes into or out of an obstacle, as in the case of refraction, light is diffracted as it travels around the obstacle. Diffraction occurs because the RF signal slows down as it encounters the obstacle and this causes the wave front to change directions. This could be understood by an analogy of a rock dropped into a pool and the ripples it creates. Think of the ripples as analogous to RF signals. (looking at figure 4.5) 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. Diffraction is often caused by buildings, small hills, and other larger objects in the path of the propagating RF signal.
Scatteringhappens when an RF signal strikes an uneven surface causing the signal to be scattered. So that the resulting signal are less significant than the original signal. In other 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 can cause scattering. When RF signals encounter things like rocky terrain, leafy trees, or Rain and dust can cause scattering as well. As shown in figure bellow.
4.8 Voltage Standing Wave Ratio (VSWR).
Voltage standing wave ratio (VSWR) is a measurement of mismatched impedance in an RF system and is stated as an X: 1.Before the RF signal is radiated through space by the antenna; it existsas an alternating current (AC) within the transmission system. Withinthis hardware, RF signal degradation occurs. All cables, connectors, and devices have some level of inherent loss. In a properly designed system, this loss by attenuation is unavoidable. However, the situation can be even worse if all the cables and connectors do not share the same impedance level. If all cables, connectors, and devices in the chain from the RF signal generator to the antenna do not have the same impedance rating, there is said to be an impedance mismatch. For example, you would not want to use cables rated at 50 ohms with connectors rated at 75 ohms. This would cause an impedance mismatch. Maximum power output and transfer can only be achieved when the impedance of all devices is exactly the same.
4.8.1 Return loss.
There is some level of power loss due to backward reflection of the RF signal within the system. This energy that is reflected back toward the RF generator or transmitter results in return loss. Return loss is a measurement, usually expressed in decibels, of the ratio between the forward current (incident wave) and the reflected current (reflected wave). The results of this return loss will be, the RF transmitter may be destroyed, to minimize VSWR and return loss, you must avoid impedance mismatches. This means you will want to use all equipment (RF transmitters, cables, and connectors) with the same ohm rating.
4.9 Wave Propagation.
The way RF waves move through an environment is known as wave propagation. Attenuation occurs as RF signals propagate through an environment. When the RF signal leaves the transmitting antenna, itwill begin propagation through the local environment and continue on, theoretically, forever. The signal cannot be detected after a certain distance, and this becomes the usable range of the signal. Attenuation occurs as the signal propagates through the environment. Some of the signal strength is lost through absorption by materials encountered by the RF signal; however, even without any materials in the path of the signal, the amplitude will be lessened. This is due to a phenomenon known as “Free Space Path Loss''.
Attenuation is the process of reducing an RF signal's amplitude. This is occasionally done intentionally with attenuators to reduce a signal's strength to fall within a regulatory domain's imposed constraints. Loss is the result of attenuation, and gain is the result of amplification. RF cables, connectors, and devices may have some level of imposed attenuation and this attenuation is usually stated in decibels and is often stated as loss in decibels per foot this is also known as insertion loss. Insertion loss is the loss incurred by simply inserting the object (cable, connector, etc.) into the path of the RF signal between the source and the intentional radiator.
4.11 Free space path loss.
Free space path loss, sometimes called free space loss (FSL) or just path loss, is a weakening of the RF signal due to a broadening of the wave front. This broadening of the wave front is known as signal dispersion. As the wave moves out from the antenna this broadening of the wave front causes a loss in amplitude of the signal at a specified point in space. This broadening of the wave is also called beam divergence. Beam divergence can be calculated by subtracting the beam diameter (D1) at a greater distance from the beam diameter (D2) closer to the antenna and then dividing by the distance between these two points (L).
The following formula illustrates this Divergence = (D1 − D2)/L.
For example when blowing bubbles with bubble gum, the outer shell that forms the bubble boundary becomes thinner as the bubble grows larger. Similarly, RF signals grow weaker as the cell grows larger or the distance becomes greater.
4.12 Basic RF Math's.
It is interesting we have been able to implement wired networks so many times with very little math other than counting the number of Ethernet ports needed for no of users. Wireless is different. Because the wireless network uses an RF signal, one must understand the basics of RF math in order to determine if the output power of an RF transmitter is strong enough to get a detectable and usable signal to the RF receiver. We had to deal with similar issues with cabling in that you could only use a CAT 5 cable of a particular maximum length (100m). we simply knew we could not span a greater distance than that which was supported by the cabling type. In order to understand and perform RF math, there are a few basic things that will need to know. First is to understand the units of power that are measured in RF systems, second to understand how to measure power gains and losses, third and finally is to understand how to determine the output power you will need at a transmitter in order to get an acceptable signal to a receiver. If we are creating a point-to-point connection using wireless bridge, or if we are installing an access point in an access role, in both cases, a sufficient signal must reach the receiver listening on the other end of the connection. This is the point RF mathematics become necessary.
4.12.1 Watt (W).
The basic unit of power is a watt. A watt is defined as one ampere (A) of current at one volt (V). As an example of what these units mean, think of a garden hose that has water flowing through it. The pressure on the water line would represent the voltage in an electrical circuit. The water flow would represent the amperes (current) flowing through the garden hose. Think of a watt as the result of a given amount of pressure and a given amount of water in the garden hose. One watt is equal to an Ampere multiplied times a Volt.
WLANs do not need a tremendous amount of power to transmit a signal over an acceptable distance. For example, we can see a 7-watt light bulb from more than 50 miles away on a clear night with line of sight. This gives us an idea of just how far an electromagnetic signal can be detected. This is why many WLAN devices use a measurement of power that is 1/1000 of a watt. This unit of power is known as a milliwatt. 1 W, then, would be 1000 milliwatts (mW). Enterprise-class devices will often have output power levels of 1-100 mW, while SOHO wireless devices may only offer up to 30 mW of output power.
4.12.3 Decibel dB.
Decibels are based on a logarithmic relationship to the linear measurement of power “watts”. For example ,When a receiver is very sensitive to RF signals, it may be able to pick up signals as small as 0.000000001 Watts. Other than its obvious numerical meaning, this tiny number has little intuitive meaning to the layperson and will likely be ignored or misread. Decibels allow us to represent these numbers by making them more manageable and understandable. Let we explain why it is necessary these tiny number and why not to round them?
As from our course of “Algorithms analysis” we know that If we are given the number 1000 and asked to find the logarithm (log), we find that log 1000 = 3 because 103 = 1000. Notice that our logarithm, 3, is the exponent. A logarithm is the exponent to which the base number must be raised to reach some given value. An important thing to note about logarithms is that the logarithm of a negative number or of zero does not exist. This kind of knowledge is very helpful in “RF math” and understanding dB.
Using the rules of 10s and 3s. This system will usually allow us to calculate RF signal power levels without ever having to resort to logarithmic math. Here are the basic rules
- 1: A gain of 3 dB magnifies the output power by two.
- 2: A loss of 3 dB equals one half of the output power.
- 3: A gain of 10 dB magnifies the output power by 10.
- 4: A loss of 10 dB equals one-tenth of the output power.
- 5: dB gains and losses are cumulative. (They can be add and subtract).
Now let evaluate these rules of 10s and 3s. First 3 dB of gain doubles the output power. This means that 100 mW plus 3 dB of gain equals 200 mW of power. And the gain or loss of 10dB results in a gain of 10 times or a loss of 10 times. The 10 dB of gain twice causes a 40 mW signal to become a 4000 mW signal. Simply in gain we multiply and in loss we divide by that 10 or by 2 (in case of double -3dB).
The following is the general equation for converting mW to dBm:
This equation can be manipulated to reverse the conversion.
Let's take a simple example here to understand these five rules.
For example the power level of a 12 mW signal increases by an amplifier with 16 dB of gain. The power in watt would be. Here is the math:
The answer is very simple: I added 10 dB and then I added 3 dB twice. Here it is in longhand:
12 mW + 10 dB + 3 dB + 3 dB = 480 mW
12 mW × 10 × 2 × 2 = 480 mW
(Similar for loss -16dB)
Sometimes you are dealing with both gains and losses of unusual amounts.
Suppose a signal of 30mW gain power of 8dB. One my ask how this could be calculate by rules of 10s and 3s. The answer is so simple. We will have to find the combination of positive and negative 10s and 3s that add up to 8.
Now using these numbers to perform RF dB based math.
30mW+10 dB + 10 dB -3 dB - 3 dB -3 dB -3 dB= 187.5 mW
30 mW × 10 = 300 mW
300 mW × 10 = 3000 mW
3000 mW/2 = 1500 mW
1500 mW/2 = 750 mW
750 mW/2 = 375 mW
375 mW/2 = 187.5 mW
Let we assume that you have a transmitter that transmits at the 14.77 dBm and we are passing its signal through an amplifier that adds 6 dB of gain. One can quickly calculate that the 14.77 dBm of original output power becomes 20.77 dBm of power after passing through the amplifier. Now, remember that 14.77 dBm is 30 mW. With the 10s and 3s of RF math, which we learned about earlier, we can calculate that 30 mW plus 6 dB is equal to 120 mW. The interesting thing to note is that 20.77 dBm is equal to 119.4 mW (converting 20.77 to mW). As you can see, the numbers are very close indeed. While we have been using a lot of more exact figures in this section, we found that rounded values are often used to be disastrous for calculation. That is why using Decibel.
4.13 Signal to Noise Ratio (SNR).
Background RF noise, which can be caused by all the various systems and natural phenomena that generate energy in the electromagnetic spectrum, is known as the noise floor. “The power level of the RF signal relative to the power level of the noise floor is known as the signal-to-noise ratio or SNR.”
If we imagine in a large conference room, there are hundreds of people having conversations at normal conversation sound levels. Now, if one wants to say something so that everyone will hear, therefore cupping hands around mouth and yell loudly. Say that the conversations of everyone else in the conference room constitute a noise floor and that yelling is the important signal or information. Furthermore, we could say that the loudness of yelling relative to the loudness of all other discussions is the SNR for our communication. In WLAN networks, the SNR becomes a very important measurement. If the noise floor power levels are too close to the received signal strength, the signal may be corrupted or may not even be even detected. It's almost as if the received signal strength is weaker than it actually is when there is more electromagnetic noise in the environment.
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