Ever wonder why moving your computer or changing the direction of your Wi-Fi antenna can make such a difference in signal strength? In this Wi-Fi 101 lesson, I explain how RF signals-especially Wi-Fi ones-react to real world conditions.
Before I jump into a discussion on multipath environments and RF propagation, here are a few basic terms you should be familiar with:RF propagation: The movement of RF signals through a medium. In theory the simplest example would be a signal traveling directly from the transmitter to the receiver. Under real-world conditions RF signals do not emanate as a single wave, but as an expanding wave front similar to ripples on a pond. Multipath environment: A term used to describe real world conditions encountered by the expanding RF wave front. RF Line of Sight (LoS): A measurable parameter where anything less than 20 percent blockage of the Fresnel Clearance Zone is considered RF LoS conditions-- RF LoS and visual LoS are not one and the same. RF Non-Line of Sight (NLoS): Defining RF NLoS is slightly more complicated. Two distinctly different RF propagation models need to be considered. One model-essentially the compliment of RF LoS-is when physical obstructions block the Fresnel Zone by more than 20 percent. The second less intuitive model focuses on situations where the original RF wave front has been altered into multiple RF signal components that may or may not reach the implied destination. In theory, virtually every WLAN is affected by RF NLoS to some degree.
RF propagation basics
Initially a uniform RF wave front leaves the transmitting antenna. As the wave front traverses space, it may encounter obstacles that alter the original wave front or create new RF signals. One or more components of the original RF wave front may continue traveling straight to the receiving antenna, other components may diffract, scatter, or reflect off of obstructions.
- Diffraction: The phenomena where radio waves are bent around sharp objects creating a new wave front.
- Scattering: Is where RF energy is reflected off of a non-uniform surface in multiple directions.
- Reflection: Occurs when the wave front contacts a uniformly flat surface such as sheet metal siding and is reflected at a predictable angle.
These elements can have either a positive or negative impact on WLAN performance. For example, without diffraction, scattering, and reflection, WLAN's would not work in the typical office environment, which has less than optimal RF LoS conditions. Negative impact elements or anomalies are also very important to understand. Having that insight may help explain why a WLAN is working poorly or help decipher location specific problems.
RF propagation anomalies
RF propagation anomalies are the results of the original RF wave front being altered, broken into separate RF components, and the resultant RF components reacting to each other when they meet at a given physical location-receiver's antenna. Since each RF component is following its own distinct path, there will be differences in travel times and RF wave geometry. Signal strength, timing, phase and angle of arrival are just a few of the affected parameters. It now becomes the receiver's job to decipher what is the original signal and what are distorted copies. Described below are some of the most frequently encountered propagation anomalies.
- Multipath fading: Is a phenomena associated with wave propagation and occurs when the receiver sees the superposition of multiple copies of the transmitted signal, each traversing a different path. The results can either be constructive (amplified) or destructive (attenuated) interference at the receiver. The moving of computers and/or fiddling with antennas mentioned in the first paragraph are methods of combating destructive interference.
- Time delay: An anomaly that goes hand in hand with multipath fading. Time delay is the amount of timing variation between different RF signals. Time delay can cause phase and polarization changes as well as multipath fading. Unlike multipath fading which affects signal amplitude, time delay adversely affects the receiver's ability to decode signals due to distortion.
- Doppler effect: RF Doppler effects are created in two different ways, relative motion between the transmitter and receiver and RF NLoS conditions that alter the relative motion of the RF signals themselves. Both of which especially affect 802.11a/g and the use of OFDM technology.
RF NLoS should be considered a double-edged sword and any knowledge gained about how RF signals disseminate in a specific environment will be helpful when deciding what type of technology and equipment is needed. In a future post, I'll examine technologies that reduce or even use propagation anomalies to improve RF signal transmission.
Michael Kassner is currently a systems manager for an international company. Together with his son, he runs MKassner Net, a small IT publication consultancy.