RF Planning | RF Primer

RF planning is designed to address the two problems of multicellular networks. The first problem is to ensure that the coverage levels within the network are high enough that the expected data rates, based on the minimum required signal to noise ration, can be achieved at every useful square foot of the building or campus environment. The second problem is to avoid the intercell interference which results from multiple devices transmitting on the air without mitigation.

Proper RF planning is an expensive, time-consuming process. The basics of RF planning are for the installers to predict what the signal propagation properties will be in the expected environment. This sort of activity always requires using sophisticated RF prediction tools. RF prediction tools operate by requiring the operator to designate the locations and RF properties—attenuation, mostly—of each physical element in the building, the furniture, the walls, the floors, and the heavy machinery. Clearly a laborious process, the operator must copy in the location of these elements one at a time. Some tools are intelligent enough to take CAD drawings or floor-plan maps and estimate where the walls are, but an operator is required to verify that the guesses are not far from reality. RF planning tools then use RF calculations, based on electromagnetic principles, to determine how much the signal is diminished or attenuated by the environment. The planning tools need to know the transmit power capabilities and antenna gains of all of the access points that will be deployed in the network.

RF planning can be used this way to assist in determining where access points ought to be located, to maximize coverage given the particular SNR requirements. Because RF planning uses exact equations to predict the effects of the environment, it can be only as good as the information it is given. Operators must enter the exact RF and physical properties of the building to have a high likelihood of getting an accurate answer. For this reason, RF planning suffers from the garbage-in-garbage-out problem. If the operator has uncertainty about the makeup of the materials in the building, then the results of the RF plan share the same uncertainty.

Furthermore, RF planning cannot predict the effects of multipath. Multipath is more crucial than ever in wireless networking, because the latest Wi-Fi radios take advantage of that multipath to provide services and increase the data rate. Not being able to predict multipath places a burden on RF planning exercises, and requires RF planners to look for the worst-case scenarios.

Using RF planning tools to determine what power levels or channel settings each access point takes, then, is not likely to be a successful proposition as the network usage increases. Unfortunately, Wi-Fi self noise is a problem that does not show itself until the network is being heavily used, at which point it shows with vigor. Until then, as the network is just getting going, self noise will not be present at high levels and will not occupy 100% of the airtime. Thus, network administrators will see early successes with almost any positioning of Wi-Fi equipment, and can gain a false sense of security. (It is important to note that this is a property of trying to predict how RF propagates. Tools or infrastructure that constantly monitor and self-tune suffer the same problems, but with the added wrinkle that the self-tuning is disruptive, and yet will be triggered when the noise increases and the network needs to be disrupted the least.)

The one place where RF planning shows strength is in determining a rough approximation of the number and position of access points that are needed to cover a building. This does not require the sort of accuracy as complete RF plan, and tends to work well because of the fact that Wi-Fi networks are planned for a much higher minimum SNR than is necessary to cover the building. That higher SNR is required, however, to establish a solid data rate, and so what appears to be padding or overprovisioning from a coverage point of view can be lost capacity from a data rate point of view. Nonetheless, determining the rough number of access points needed for large deployments is a task that can do with some automation, and RF planning tools used only to plan for coverage (and not for interference), can be reasonably effective—even more so if the infrastructure that is deployed is able to tolerate the co-channel interference that is generated.