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WiFi Application
RF Planning
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1.   Planning
1.   Planning

Understanding what needs to be covered is an important part of the planning process, so do some quick calculations to better understand the situation. For indoor coverage, understand the effects of the attenuating and reflecting materials where the coverage is desired. A link budget will tell you what is practical given the environment and how to plan cells. With a link budget, one can have an estimate of how many cells will be required for the project. Tradeoffs take place between more cells and running more power.

First and Second Fresnel Zones

For an outdoor application, also consider checking the Fresnel Zone. The tradeoff between working on one long-distance shot versus two back-to-back links can be discovered by working out a few things on paper first.

Fresnel provided a means to calculate where the zones are where obstacles will cause mostly in phase and mostly out of phase reflections between the transmitter and the receiver.

Obstacles in the first Fresnel will create signals that will be 0 to 90 degrees out of phase, in the second zone they will be 90 to 270 degrees out of phase, in third zone, they will be 270 to 450 degrees out of phase and so on. Odd numbered zones are constructive and even numbered zones are destructive.

Therefore, on long-distance shots, it is necessary to take into account ground/water reflections and vertical surfaces like tall buildings.

If unobstructed, radio waves will travel in a straight line from the transmitter to the receiver. But if there are obstacles near the path, the radio waves reflecting off those objects may arrive out of phase with the signals that travel directly and reduce the power of the received signal. On the other hand, the reflection can enhance the power of the received signal if the reflection and the direct signals arrive in phase. Sometimes this results in the counterintuitive finding that reducing the height of an antenna increases the S+N/N ratio.

In fact, the contributions from adjacent zones may act to cancel each other because of their relative phase relationships. The practical situation is made even more complex because, due to the obliquity factor, higher-order zones contribute less energy than lower-order zones. The overall picture is that at the receiver the total field from all other zones is about 50% of that from the first zone alone. Thus clearance of the radiated field to the first Fresnel zone is very critical if an unobstructed transmission path is to be approximated at least at 60% of the zone radius.

To give you an example, since the majority of the transmitted power is in the first Fresnel Zone, any time the path clearance between the terrain and the line-of-sight path is less than 0.6 of the first Fresnel Zone distance, some knife-edge diffraction loss will occur. On the other hand, it is possible to gain in the signal strength at the receiver up to 3dB by having a flat surface such as a lake, a highway, or a smooth desert area at the second Fresnel Zone in such a way that the signals get reinforced at the receiver.

The combination of power output at the antenna and the gain of the antenna itself are legally limited by the Office of Telecom Authority (OFTA). In other countries, similar regulations exist but will differ. For WiFi, the maximum power output and antenna gain is limited based on what frequency band is used and whether the application is point-to-point or point-to-multipoint.

The manufacturer of an access point or client adapter card will typically specify the output power in their spec sheet in milliwatts and dBm - decibels over a one milliwatt reference.

For antennae, the gains is usally specified in dBi, or decibels over isotropic. The "i" in dBi stands for the reference of an isotropic antenna. So the effective radiated power of an antenna (dBi) fed with the output source of an access point (dBm) should not exceed regulated power limit.

Indoor propagation tools are available, such as WinProp , campus-wide propagation tools like SitePlanner, and ray-tracing tools like CINDOOR that can be used to model buildings and campuses given a computer-aided-design (CAD) floor plan.

One of the shortcomings is that the floor plan does not tell you much about the construction materials used in the structure. For instance, is the wall made of bricks, Sheetrock, or reinforced concrete? Is the floor totally RF isolated from the one above or below it? Only a walkthrough will tell you these things accurately.

Once things work on paper with an adequate link budget and the Fresnel Zone, you can go out to the site and see if the paper plan works.

Outdoor Site Survey - All the data on paper may indicated that everything will work for a particular link. The Fresnel Zone can be checked against a topographical view of the point-to-point shot. You may even use expensive ray-tracing programs to predict the path, but only one way will tell you if the installation will work.

To perform an outdoor site survey for a point-to-point shot, take along binoculars, two-way radios and cell phones, topology maps, a a Global Positioning System (GPS), a spectrum analyzer, a 5 and 10 dB attenuator, and your radio equipment to take the trial shot. Have a colleague go to another hill and talk to you on the radio.

Drive out to the proposed site and see with the binoculars if the shot is clear. Check for trees or buildings that might block the path. Keep in mind that trees will grow and block the line-of-sight.

Using the location of both endpoints, one can calculate a bearing and tilt angle to point the antenna. Most GPSs have a function to do this built in. High-gain dishes are more difficult to aim the farther out you go. Take both dishes and roughly point them toward each other. Transmit a signal into one dish. With 802.11 gear, link test software enables you to send a series of management frames.

You can take the output of the other dish and feed it into the spectrum analyzer. You will see a display of frequency (across) versus amplitude (up and down). Pick the channel that has the least amount of noise.

Once you get the antennas close, you will see a spike on the frequency that the transmitting dish is tuned to surrounded by noise. Sweep the antenna on each end one at a time and lock down the antenna at the point where the signal is the strongest. At this point, you should have a sufficient SNR ratio to receive the signal with a sufficient margin.

The 5 and 10 dB attenuators can be used in line to check to see if the link margin is adequate. With 15 dB of attenuation in line, a link should last easily for a few hours. If not, you need to plan on larger dishes and amplifiers.

To perform an indoor site survey, prepare by getting the floor plans of the structure. During an initial meeting, find out which areas need to be covered and which ones don't. Also obtain information about where existing wiring closets are located and if the wiring closets or hub rooms will be used to connect the APs to the wired network.

Then walk through the structure while looking for existing RF sources. If RF sources exist, note which channels the interference is on and the relative signal's strength. Typical cells can cover closed-off areas such as four classrooms or a large area like a basketball gym or a bowling alley.

Starting at the most complicated area, place a potential AP that has to be within 300 feet of the nearest wiring closet, with a cabling path between the two. Then using a laptop with a site survey tool, find the points where 20 dB SNR is observed. This becomes the cell boundary. Place the trial AP so that its 20 dB SNR cell boundary overlaps the one identified by the first trial location. Continue to lay out cells until the whole structure is covered.

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