Last week an IEEE Communications article on industrial wireless reminded me of when I first learned how well Wi-Fi radios can work over long distances. Today I would like to write about how multipath reflections present a problem for Wi-Fi in industrial settings.
Multipath Intersymbol Interference with Wi-Fi
One good feature of 802.11(a/g) and subsequent Wi-Fi standards is they use a symbol period of 4us. This is a symbol rate of 250k symbols/sec. Wi-Fi’s data rate is much higher than the symbol rate because the Wi-Fi signal contains many OFDM subcarriers. How many subcarriers depends on whether it’s a 20, 40, or 80 MHz data rate. The number of bits contained in each subcarrier symbol depends on the data rate. A good fallback algorithm will select a lower data rate (with few bits per symbol) if the signal strength is weak.
Its low symbol rate makes Wi-Fi robust against intersymbol interference (ISI) due to multipath. The duration of a Wi-Fi symbol is 3.2us. There is a 0.8us guard interval between symbols. It takes a radio wave 5us to propagate one mile [3us to propagate one km]. Supposed a Wi-Fi signal reflects off an object 0.1 mile [160 meters] away. It will take the signals 0.5us to reach the object and 0.5us more for the reflections to return. Since the guard interval is 0.8us, these reflections from the previous symbol will overlap part of the current symbol. As the amount this overlap increases, the ISI gets worse. Usually reflections from objects that far away are too weak to cause interference.
In a large metal factory building, however, I have seen ISI pose a problem with Wi-Fi links. If most large objects in the factory are stationary, the solution is to use directional antennas. The goal is to increase the ratio of the strength of the primary signal to that of the secondary signal.
For some reason, users often ask if reducing the output power will reduce ICI. They reason that the reflections will fall into the noise leaving only the primary signal. This approach does not work. Regardless of output power, for a given channel the relative strength of the reflections is the same. It’s this signal-to-noise ratio that matters.
Short Reflections
Reflections that are shorter than the guard interval do not cause ISI, but they do cause fading. Anyone who has listened to a weak FM radio station in an urban area has heard the effects of multipath fading. The reflections come together constructively or destructively in different locations, causing the signal to rise and fall. In the case of FM radio, the notes and phonemes of music and human speech last several milliseconds. The reflections would have to travel a thousand miles to be delayed enough to garble the music or speech due to ISI.
Wi-Fi is robust against multipath fading. In typical industrial environments at frequencies over 900MHz, whether interference at a particular point is constructive or destructive varies when the frequency changes by only a few MHz. A Wi-Fi signal is 20-80MHz in width, so some of the signal’s subcarriers will experience destructive interference while others experience constructive interference. The Wi-Fi physical layer is robust against fading knocking out a few subcarriers into the noise.
Mitigating Multipath
Antenna diversity can further reduce the effects of fading. This is why commercial routers and Wi-Fi cards have more than one antenna. The idea is if one antenna is in a bad location, chances are the other one will be in a better location. The quality of the algorithm to select which antenna to use varies greatly. The worst ones just pick one antenna, apparently for no good reason. The best ones, found in some (n) cards, have multiple signal chains. The (n) cards analyze the signals on both antennas simultaneously to get a few dB more (depending on the channel function) than it would get from either antenna alone.
Adapting Wi-Fi for Industrial Use
A good industrial Wi-Fi radio will allow the user to control things like diversity antenna selection, output power, and data rate, things that consumer equipment selects for you, often in clumsy way. Consumers may not notice if a wireless router does a poor job selecting antenna or data rate. Surprisingly if Wi-Fi is implemented right it is reliable enough to connect factory equipment, even environments where the ISM bands are crowded and multipath rich.
