RF 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.

Transmitter Power and Antenna Gain

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.

Site Plan Prediction Tools

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.

Site Survey - Outdoor

My home wireless router can go up to a speed of 600Mbps, why do I always get much below that?

Signal strength and quality affect the connection speed. You'll get a slower speed if you are far away from your access point, or if walls are blocking the wireless transmission path. In such a situation, you'll need high power antenna to improve the wireless coverage.

I am losing signal strength in parts of my house, particularly to a room at a remote corner. Can the CnSR WiFi extender bridge the gap and how can I maximize its effect?

Signal strength decreases as you move away from the wireless router. Try moving your access point and TurboTenna 11N PRO Booster in different directions, even away from the area that you are trying to reach. Sometimes rotating the TurboTenna 11N PRO Booster on its axis so that the connection point sideways or up will help to boost the signal. Finally, in your remote room try moving your laptop toward the middle of the room, away from the walls or metal. You can also try tilting your laptop (or WiFi receiver if you have a desktop computer) on its horizontal axis.

I do not have an access point or Internet connection, but I was told that my apartment complex has a WiFi network. Can I get free Internet access with the TurboTenna 11N PRO Booster?

You will need a 802.11b or 802.11g wireless device that connects to your laptop or desktop computer. There are few 802.11b/g PCMCIA wireless adapter in the market today equipped with and external RF antenna port, such as the newly launched Buffalo WLI-CB-G54S at 125*/54 Mbps High Speed Mode. Normally the build-in antenna would barely pickup any signal next door at all, that is why you need your wireless adapter to be able to connect to the TurboTenna 11N PRO Booster which scans and picks up the signal(s) at the far end.

Whether or not you will be able to pick up a signal depends on the proximity of a broadcasting signal, obstructions, and signal strength. If the person who broadcasts the signal uses encryption and MAC address screening, you have to request a username and password or submit the MAC address of your wireless adapter to gain access.

Site Survery - Indoor

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.

hen 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.

Making a Frequency Plan

After completing an RF site survey, you'll have a good idea of the number and location of APs necessary to provide adequate coverage and performance for users.

2.4GHz has three non-overlapping channels

2.4GHz Frequency Reuse

The 2.4 GHz band has 11 22-MHz-wide channels defined, starting at 2.412 GHz every 5 MHz through 2.463 GHz. Three non-overlapping channels are available: 1, 6 and 11, as shown in Figure rf-1. These non-overlapping channels can be used in a three-to-one reuse pattern, as shown in Figure rf-2.

Three-to-one reuse pattern

5GHz Frequency Reuse

The operating channel center frequencies are defined at every integral multiple of 5 MHz above 5 GHz. The valid operating channel numbers are 36, 40, 44, 48, 52, 56, 60, 64, 149, 153, 157, and 161. The lower and middle U-NII sub-bands accommodate eight channels in a total bandwidth of 200 MHz. The upper U-Nii band accommodates four channels in a 100 MHz bandwidth. The centers of the outermost channels are 30 MHz from the bands' edges for the lower and middle U-NII bands, and 20 MHz for the upper U-NII band (see Figure rf-3).

5 GHz Channels

Point-to-point links operates on the other four channels: 149, 153, 157, and 161. This allows four channels to be used in the same area. 802.11a APs and client adapter cards operates on eight channels: 36, 40, 44, 48, 52, 56, 60, and 64. This allows two four-to-one reuse patterns to be used (see Figure rf-4).

Four-to-one reuse pattern

Using both the low- and mid-frequency ranges together allows a seven-to-one reuse pattern with a spare. The spare can be added for a fill to extend coverage or to add capacity in areas like conference rooms where more capacity is needed (see Figure rf-5).

Seven-to-one reuse pattern with a spare

Frequency Allocation

For a simple project like one or two APs, simply assign the least used frequencies from the site survey. For more complex projects involving three or more APs, pick a frequency reuse pattern for the frequencies that are used for the project; start with the most complicated part of your site survey and start assigning frequencies. Plan the location of APs initially for coverage, not capacity. Avoid overlapping channels if possible. However, if an area has to be overlapped, plan it such that it is naturally an area where the most capacity would be required, such as in a library, conference room, or lecture hall (see Figure rf-6).

Overlap in a central library

For multiple-floor installations, if more than 30 dB of isolation is used between floors (such as concrete and rebar floors), try not to use the same frequency directly above or below a cell that has already been allocated. Where less than 30 dB of isolation is used between floors (such as a two-by-four framed apartment building), the plan needs to take the three-dimensionality of the cell into account (see Figure rf-7).

Cells are 3-D in buildings

Some of the most complex problems are areas where not enough channels are available to plan out the space. In a two-dimensional space, this can happen in areas where a central room has to be covered with surrounding classrooms or offices, such as a library or lab. In a three-dimensional space, this can happen in a tri-level portion of a building (see Figure rf-8).

A case where RF planning is difficult

The signals from difference floors can overlap and intrude on another. In some cases when you have no way around two cells that use the same frequency being bordered against each other, plan the seam to be in areas where no coverage is necessary, such as equipment rooms, restrooms, wiring closets, stairwells, and janitorial supply rooms.

:: Copyright ® 2020 DataCom Network Limited
The registered trademarks and logos belong to the respective companies.
:: Copyright ® 2021 DataCom Network Limited. The registered trademarks and logos belong to the respective companies.