advancing information transport systems

Transcription

1 BICSInews advancing information transport systems January/February 2007 PRESIDENT S MESSAGE 3 EXECUTIVE DIRECTOR MESSAGE 4 BICSI UPDATE COURSE SCHEDULE STANDARDS REPORT Volume 28, Number 1 Building a Redundant and Resilient College Network 14 How to Work with the Authority Having Jurisdiction 22 The Wi-Fi Path Loss Equation and Antenna Specifications 26 Increasing Power Margin with High-Performance Optical Fiber 34 Testing Multimode Optical Fiber: Importance of Controlling Launch Conditions 38

2 Feature The Wi-Fi Path Loss Equation and Antenna Specifications Making sense of manufacturer specifications. BY JOE BARDWELL Wi-Fi is the designation for a relatively broad family of wireless devices that communicate in accordance with the standards set by the IEEE committee and a variety of working groups. The antenna that is used with a Wi-Fi radio (e.g., access point, repeater, mesh router, bridge, or client device) makes a big difference when it comes to the radio s ability to transmit and receive. Understanding the choices available and their strengths and weaknesses requires some background. This article provides information that can be applied to selection of indoor and outdoor antennas for Wi-Fi systems. Over the past year, the type of Wi-Fi devices found in the marketplace has dramatically evolved from the basic notebook computer used to check and surf the Web, to multi-mode cell phones with Wi-Fi wireless VoIP, wireless security systems and location tracking, backhaul for radio frequency identification (RFID) product tag readers, licensed public safety applications, and much more. As the sophistication and capabilities of the applications and devices that communicate over Wi-Fi networks has increased, so has the need for best practice designs and equipment specifications. While a consumergrade access point may provide minimal levels of service for simple and Web access, only commercial-grade equipment, coupled with proper RF engineering in the design, will support the levels of service that will be required over the course of 2007 and beyond. The Antenna as Part of a System A Wi-Fi system design consists of a number of access points spread across the indoor or outdoor coverage area. The design of the network determines where the access points will be installed. Requirements for a correct design are simple: 1. Transmitters must propagate signal energy that is powerful enough to propagate throughout the intended coverage area. 2 Receivers must be sensitive enough to recover the data bits out of the signal energy present in the installation environment. 3. The ratio of signal energy to background noise and interference must be large enough to allow the receiver to identify the desired transmission. The requirements may be simple, but meeting them can be complex and confusing. While this article focuses on the first point, transmitted power, none of these points can stand completely alone. The power that must be radiated from a transmitter is required to meet the sensitivity requirements of the receiver. The signal-tonoise ratio (SNR) is a requirement specified by the radio manufacturer and is based on the capability of the receiver s circuitry. In essence, receiver sensitivity and required SNR may be considered fixed values, varying only from one model of radio to the next. In addition, the maximum power output of the transmitter may also be considered a fixed value, limited by the manufacturer, often in compliance with legal requirements. The variable that remains, and the choice which must be made by the designer of a wireless network system, is the antenna to be used by the radio. We will discuss antenna selection on the basis of the underlying physics of wave propagation. Antenna Basics Selecting one antenna over another is always a matter of trade-off. The antenna is simply a radiating device that receives power from the access point and causes that power to propagate outwards as electromagnetic waves. An antenna designer can do many things to shape, direct and focus the propagating signal, but they cannot create more signal than was input into the antenna by the access point. In this lies the focus of the tradeoffs in antenna design. The design is based on the laws of physics of electromagnetic wave propagation and there is no free lunch in the physics department. Data bits are sent from a device s operating system software to an Wi-Fi chipset. The chipset and supporting circuitry modulates a carrier signal to represent bits using specific variations of the carrier. This modulated carrier is the electrical signal that is input into an antenna. The result is the propagation of an electromagnetic field that conveys the data bits in a known series of variations. There are three basic concepts that are the foundation for antenna specification: 26 Advancing Information Transport Systems

3 mw and dbm Defined The mw and dbm scales are both used to represent signal power. Conversion between the units involves a scientific calculator, but there are a number of rules of thumb that can be easily applied. To convert from mw to dbm: Calculate the base 10 logarithm of the mw value, and multiply the result by 10. dbm = log10(dbm) * 10 To convert from dbm to mw: Divide the dbm value by 10, then raise 10 to that exponential power. mw = 10^(dBm/10) Here are some rules of thumb to remember: 100 mw = 20 dbm (power consistent with commercialgrade access points) 1000 mw (1 watt) = 30 dbm 30 mw = 15 dbm (power consistent with many notebook computers) Adding 3 dbm results in doubling of the mw value (18 dbm = 60 mw, start with 30 mw = 15 dbm and double the mw because you added 3 dbm) Subtracting 3 dbm results in halving the mw value (17 dbm = 50 mw, start with 100 mw = 20 dbm and divide mw by 2 because you subtracted 3 dbm) 1. The isotropic radiator. 2. The inverse square law. 3. The decibel unit of measurement (dbm, db, dbi). The Isotropic Radiator To begin considering how antennas work, imagine a single tiny point in the vacuum of outer space. Imagine that signal energy has somehow been input into the point without any wires or connections that would distort it and a resulting electromagnetic field is radiating outward. This field radiates out as a perfect sphere, with equal power at equal distance from the point, in all directions. This theoretical point source is called an isotropic radiator (from the Greek isos, meaning equal and tropos meaning turn ). It produces a consistent, equal electromagnetic field in all three-dimensional directions. The Inverse Square Law The details of how the power that is applied to the antenna (from the access point radio s antenna output) and how the energy ends up being propagated into space as electromagnetic waves is beyond the scope of this article. Nonetheless, it is important to realize that signal strength initially drops off quickly in the area very close to an antenna roughly one to two wavelengths or five to 10 in for a 2.4 GHz b/g transmitter. After that, the expanding electromagnetic field decreases in strength in accordance with the inverse square law. This law of electromagnetic wave propagation holds that when the distance from the antenna doubles, the signal strength drops to 1/4 of its original value. If a measurement is made 20 ft away from an antenna, and another measurement is made 60 ft away from the antenna, the signal power decreases by a factor of nine because the distance is three times greater hence, the signal power is 1/9 of its original value. Power Represented as Milliwatts or dbm (db Milliwatts) RF engineers represent signal power in a variety of ways. Two common representations encountered in wireless LAN design are the milliwatt (mw) and the dbm. These, somewhat like miles-per-hour and kilometers-perhour, both represent identical quantities, simply using different numeric scales. The mw scale is linear and the dbm scale is logarithmic, as discussed in the mw and dbm sidebar on the left. A typical Wi-Fi access point has a maximum transmit power output (TPO) of 100 mw, which is the same as saying 20 dbm. A typical client device may only have a 30 mw (14 dbm) power output. A typical Wi-Fi device may be able to receive signals that have propagated outwards and fallen from 100 mw to a low power level of mw. This is why the dbm scale is helpful. This mw power level is represented as -95 dbm, where the negative exponent indicates the value is a fraction less than 1. The term Received Signal Strength Indicator (RSSI) is often used to refer to this receiver sensitivity value. It is common to see specifications of transmitter power output represented as mw, such as 100 mw TPO. However, receiver sensitivity values are always shown as a logarithmic, dbm value, such as -95 dbm RSSI. 28 Advancing Information Transport Systems

4 Figure 2. Coverage Model Showing Elevation Plane Signal Power at Various Angles Measured at a Fixed Distance Figure 1. The Rubber Duck with a Surrounding Electromagnetic Field The Dipole Antenna There is no perfect isotropic radiator in the real world. The simplest antenna is a pair of radiating elements typically encased in plastic or fiberglass called a dipole antenna, or more commonly known as a rubber duck. Early versions of this antenna were flexible and covered in rubber, hence the nickname. Like a bar magnet with lines of force circling outwards from the north and south poles, electromagnetic waves propagate horizontally outwards from a vertically oriented dipole antenna with very little signal energy present straight out the top and bottom of the antenna, shown in Figure 1. The radiation pattern coming out of a dipole antenna, following the electromagnetic lines of force, does not radiate in a perfectly spherical pattern. Rather, the sphere is flattened to form a shape that might be described as a doughnut. More correctly, this shape is called a toroidal pattern. Antenna Gain Consider a situation in which a transmitter has an 18 dbm TPO. As such, 18 dbm of power is being input into the dipole antenna. The output power, however, does not radiate equally in all directions. Consequently, the power density is not equal in all directions. The 18 dbm has to go somewhere, and it doesn t go out equally in all directions, which makes the effective power density on the horizontal plane (of the vertical antenna) greater than 18 dbm. This is because very little power goes out the ends of the dipole. The power is concentrated to the sides. This effect is called antenna gain. It is the degree to which the signal power is concentrated more in some directions than in others. One way to represent gain is as the ratio of the actual signal power density and that which would be present if the antenna were a perfect isotropic radiator. This logarithmic ratio is called db relative to isotropic, dbi. A simple dipole antenna has a gain of 2.15 dbi and, therefore, adds 2.15 db to the TPO. Therefore, the 18 dbm transmitter has an effective power of dbm. When TPO and antenna gain are added, the resulting value is called the Equivalent Isotropic Radiated Power (EIRP.) Occasionally you may see gain represented as dbd (db relative to dipole). The dbd metric tells you how much better, in the real world, the measured antenna is relative to the simplest possible antenna (the dipole). An antenna with a 5 dbi gain would be rated with a 2.85 dbd gain BICSINEWS January/February

5 Figure 4. Dipole Elevation Graph Detail View Figure 3. Typical Dipole Elevation Pattern Graph (because = 5). Most manufacturers don t use dbd metrics because, all other things being equal, the numbers are smaller than the dbi values used by their competitors and the marketplace would likely be confused. Using RF CAD modeling and simulation software, a coverage model showing signal power was developed for a vertical 2.15 dbi antenna with a 30 mw TPO. See Figure 2. The display is a side view, as seen by someone standing on the ground, looking at the antenna that is pointing straight up in the middle. This coverage model represents a distance of left-to-right horizontal distance of 7500 feet. Signal power measurements for three different angles are shown. The term elevation plane is used to refer to a side view. A top-down view is called the azimuth plane. In the elevation graph, red and yellow hues are hot (higher power) and cooler (lower power) signal levels are represented by varying blue hues. Notice in the elevation plane coverage model that the maximum power measured is -85 dbm. At another angle the power is 5 db less, or -90 dbm, and another measurement was 10 db less at -95 dbm. This relationship between angle and power reduction is consistent no matter how far away a measurement is made. A special graph, called an antenna pattern graph, is provided by manufacturers to show how their antenna will operate. The Antenna Pattern Graph Manufacturers and distributors provide antenna pattern graphs to show the performance of their antennas. There are two graphs typically presented: the azimuth graph showing the top-down view and the elevation graph showing the side view. Pattern graphs are presented on a polar coordinate plane marked from 0- to 360-degrees relative to the antenna (in the middle.) It s important to confirm exactly how the manufacturer has oriented their antenna for measurement. For example, an antenna intended for ceiling mounting is oriented 90-degrees to one that s intended for wall mounting. Interpreting the azimuth and elevation pattern graphs depends on knowing what was intended by the manufacturer. A typical mast mount dipole antenna is assumed to be mounted vertically, with the base of the antenna towards the bottom. The elevation pattern graph for a typical dipole is presented in Figure 3. Notice that the circumference is marked from 0 to 360 degrees and the horizontal line across the middle is marked from 5 db to 30 db, going from the outside to the center (note that there is no 5 for the +5 db gain point to the right of the 0 point.) A detailed view of the graph s markings is shown in Figure 4 for clarity. To understand the meaning of the graph, consider the angle of elevation. If you are at the same height as the antenna you receive the maximum signal. Hence, at 0 degrees there is 5 db of gain. If you are elevated to a height 40 degrees above the horizontal (320 degrees on the graph) then the signal has been reduced by 7 db (half-way between the 5 and 10 db marking across the center of the graph.) When you are directly above the antenna, the signal is reduced by 27 db. Note that the angles do not imply distance, and the shape of the pattern does not imply some imaginary three-dimensional shape in space. The signal doesn t form a three-dimensional volume with the shape of the pattern graph. The graph is a tool to determine the degree of attenuation at a particular angle relative to the antenna. It is true, from a purely visual perspective, that the shape of the pattern gives a good indication of where the signal will be strong or weak, as if it actually were a three-dimensional volume (e.g., expanded in some directions, shrunk inwards in other directions.) Do not be confused, though, into thinking that the graph is intended to show you a three-dimensional shape. 30 Advancing Information Transport Systems

6 Antenna Beamwidth The term beamwidth is confusing because it assumes you know how the width part is being measured. Beamwidth is an angle, measured on an elevation pattern graph that intersects the pattern at the points where the signal power has been reduced by 3 db. Because a 3 db reduction is equal to a 50 percent reduction you ll often hear beamwidth referred to as half-power beamwidth (HPBW). If you examine an elevation pattern graph you can determine the points where the pattern has been reduced by 3 db. These are shown as green dots on the 40-degree Half-Power Beamwidth diagram in Figure 5. The angle formed between these points (40 degrees in the diagram) is the HPBW angle. It is very important to realize that the HPBW angle is not an absolute barrier, beyond which no signal is transmitted. As can be seen by studying the example, even at double the HPBW angle, the signal has dropped -7 db from the maximum; it has not disappeared completely. Here is why this is important. Consider a dipole on a mast, 30 feet in the air. At first thought it might be a concern that the pattern graph shows what looks like a dead spot directly below the antenna. It is not dead; it is just down by greater than roughly 50 db. Over 30 feet, the signal reduction through the air (free space path loss) is roughly 60 db. The receiver on the ground, directly Figure Degree Half-Power Beamwidth underneath the antenna, suffers 50 db loss from the antenna s pattern and the 60 db loss from the signal propagation. If the transmitter were operating at 100 mw (20 dbm) then: 20 dbm 50 db 60 db = -90 dbm. That is still enough for a 1 Mbps or 6 Mbps connection enough, but not optimal. As the user walks away from the point directly underneath the antenna, there is more path loss but the degree of loss from the pattern of the antenna diminishes even more quickly so the weaker coverage area remains very small. Conclusion: Applying Manufacturer s Specifications To design an antenna system you first must know the TPO for your transmitter and the required RSSI for your receiver. You then calculate the loss across the path between the two. Now you select antennas with sufficient gain in the appropriate direction to allow the transmitter to reach the receiver at or above the minimum required signal strength level. The relationship between these metrics is: TPO + TransmitterGain Path Loss + ReceiverGain => RequiredRSSI This is called the Path Loss Equation, and it s the basis for any Wi-Fi or other wireless network system.. Joe Bardwell Joe Bardwell is chief scientist with Connect802 Corporation, a systems integrator and wireless network design consulting firm based in California. Joe can be reached at or at joe@connect802.com. 32 Advancing Information Transport Systems