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Understanding RF Site Propagation

As RF waves travel through the air, they also have resistance or opposition to movement known as path loss. As the frequency changes, so does the wavelength. These are inversely proportional and can actually be measured by the following formula:

As the frequency increases into the ultrahigh frequency (UHF) range and then into microwave frequencies (which are used for WLANs), the opposition offered by the atmosphere increases, which in turn reduces the energy being transferred. The end result is shorter radio range. This is the main reason that a 5-GHz WLAN signal utilizing the same transmitter power and antenna gain as a 2.4-GHz WLAN has less range.

As you progress through this book, you will learn about many issues that you should consider before the site survey portion of the WLAN project can begin. One such parameter is the frequency that will be used. Another factor that you need to determine is the minimum acceptable data rate for the users. Both of these parameters will affect the site survey and overall coverage capabilities.

Frequency Versus Coverage

Naturally, transmission range is an important consideration when judging wireless technology. With all other things being equal, as frequency increases, range decreases. First of all, the higher the frequency, the shorter the wavelength of the signal. The shorter the wavelength, the higher the attenuation caused by the atmosphere. Second, higher-frequency waves are more vulnerable to absorption by building materials, such as drywall and concrete. Other factors come into play as well, such as antenna selection, modulation schemes, data rates, transmitter power, and receiver sensitivity.

Material Absorption, Reflection, and Refraction

Other factors that drastically affect the range include signal absorption, signal reflection, and signal refraction. Many materials actually absorb RF energy. At 2.4 GHz, material that contains a high level of moisture (such as bulk paper and cardboard) absorbs the signal. Facilities that contain a significant number of metal objects (such as a steel warehouse) experience reflections that can either assist or hinder coverage based on the multipath created.

Many times, materials are totally out of your sight and beyond your knowledge, such as steel reinforcement in the concrete walls and flooring, certain types of tinting on windows that contain metal properties, or even some types of insulations used in walls. The only true way to discover how the material in a facility will affect the signal and coverage is to perform a site survey, which you learn more about in Part III, "Installing WLAN Components," of this book.

Reflection

Reflection is the signal bouncing back in the general direction from which it came. Consider a smooth metallic surface as an interface. As RF hits this surface, much of its energy is bounced or reflected.

Radio waves also reflect when entering different media. Radio waves can bounce off of different layers of the atmosphere. The reflecting properties of the area where the WLAN is to be installed are extremely important and can determine whether a WLAN works or fails. Furthermore, the connectors at both ends of the transmission line going to the antenna should be properly designed and installed so that no reflection of radio waves takes place. If the line and connectors are not properly matched, some energy will be thrown back as an echo and will constitute a loss in power from the system.

Signal Strength, Noise, and Signal-to-Noise Ratio

When performing a site survey, you will want to be concerned with several items to determine whether you have adequate coverage. Three of these things are signal strength, noise level, and the signal-to-noise ratio.

Signal strength, as defined earlier, is the value of the signal (usually expressed in dBm for receiver levels in WLAN systems) that is getting to the receiver. Most receivers have some method of displaying this value. Whereas some products only provide a level in general terms (percentage, or possibly good, fair, or poor), some actually provide a reading in dBm. It is important to understand and define the minimum signal strength that you want for your particular application, data rates, and radio devices being used. You will read more about this topic later in Part III of this book.

Ambient RF noise, referred to as the noise floor, occurs in the atmosphere. As you continue to add electronic devices to your environment (even computers now have bus speeds that run in the GHz range and give off unwanted RF signals), you will gradually increase the ambient RF noise levels. To properly receive a signal, the desired signal must have a signal strength greater than the noise floor by a defined amount, which will vary from one receiver type to another and from one data rate to another.

The signal-to-noise ratio (SNR) can be compared to trying to listen to another person speak in a noisy environment. Based on the surrounding noise, the speaker will have to raise his voice to a level that is strong enough to be heard over the other surrounding noise. This would be the signal-to-noise ratio. SNR is just that, a ratio between the desired signal (signal strength) and the ambient RF noise (noise floor). It is expressed in dB, and the required SNR will vary based on modulation, data rate, and quality of the receiver.

Figure 2-17 shows an output screen from a WLAN device showing signal strength, noise floor, and SNR.

Coverage Versus Bandwidth

Chapter 4, "WLAN Applications and Services.") Because data rates affect range, selecting data rates during the design stage is extremely important. The client cards will automatically switch to the fastest possible rate of the AP; how this is done varies from vendor to vendor.

Because each data rate has a unique cell of coverage (the higher the data rate, the smaller the cell), the minimum data rate must be determined at the design stage. Cell sizes at given data rate can be thought of as concentric circles with higher data-rate circles nested within the coverage area of the immediately higher data rate. Selecting only the highest data rate will require a greater number of APs to cover a given area; therefore, care must be taken to develop a compromise between required aggregate data rate and overall system cost.

An example of data rate versus range is shown in Figure 2-18.

Modulation Versus Coverage

Another factor that can affect range and coverage is the modulation scheme. Certain modulation schemes such as OFDM have better performance in certain areas. Take a highly reflective environment, for instance, where there is a lot of multipath signal interference. OFDM offers a better performance in this type of environment due to its multicarrier format.

Under the 802.11 specifications, modulation techniques are defined and related to data rates. Therefore, a site survey should be done using the data rates intended for use in the specific environment.

Outdoor RF Issues

When using WLAN systems in an outdoor environment, many other factors come into play. Most WLAN devices are not geared to mount directly outdoors. Therefore, either a weatherproof enclosure must be used, or the AP will be placed indoors (and a cable will be used to attach the antenna). As stated earlier in this chapter, the use of cables can dramatically reduce the available power reaching the antenna and can affect overall ranges.

Another factor to consider with outdoor installations is lightning. Because you are now placing conductors outside, there is the possibility that the system may be exposed to lightning. Chapter 14 discusses lightning protection for both the antennas and the network.

Propagation and Losses

Outdoor RF links have different propagation characteristics than those indoors. Calculations can provide accurate information on possible performance and distance. The following are included in calculations for determining outdoor coverage performance:

Antenna gain

Transmitter power

Receiver performance

Cable losses

Environmental structures

All of these parameters are known values and are easily determined. However, environmental structures, such as buildings, trees, and so on, and basically anything in the line of sight between one antenna and the other, can cause major issues for outdoor RF links. For long-distance communications using WLAN frequencies, a line of sight between the antennas is necessary to maintain quality RF links (see Figure 2-19).

Earth Bulge and Fresnel Zone

Two other factors that affect outdoor links are the Fresnel zone and Earth bulge. Wireless links that carry data over long distances require additional care to ensure proper clearance. Christopher Columbus sailed the Atlantic Ocean and taught us that the world is not flat, but rather that the Earth has a curvature at the approximate rate of 12 feet for every 18 miles. It is important to make sure your antennas have proper height to maintain line of sight.

While observing these calculations, it's important to remember that this accounts only for Earth bulge. You must add the elevation of other objects (such as buildings, trees, hills, and so on) into this formula.

Another factor to consider at long distances is the Fresnel zone (pronounced "frennel"), which is an elliptical area immediately surrounding the visual path (see Figure 2-20). It varies depending on the length of the signal path and the frequency of the signal. The Fresnel zone can be calculated, and it must be taken into account when designing a wireless link. If the Fresnel zone is obstructed, required line of sight is not clear and the link may be unreliable.

The industry standard is to keep 60 percent of the first Fresnel zone clear from obstacles. Therefore, the result of this calculation can be reduced by up to 60 percent without appreciable interference. This calculation should be considered as a reference only and does not account for the phenomenon of refraction from highly reflective surfaces.

Chapter 14 covers this topic in more detail when outdoor RF links are covered in depth.