Do You See What I See?
I want to write about line of sight (LOS) propagation in the context of wireless LANs. LOS propagation refers to the path that the photons must travel between the transmitting Wi-Fi antenna and the receiving station (STA). The wave of electromagnetic energy, radiating from the transmitter, interacts with the environment. As the wave travels through open air, it is subject to free-space path loss. That is to say that the further the receiver is from the transmitter, the lower the received signal will become. When the path is blocked by physical objects, other phenomena are observed. For example, the signal will reflect off of some objects more readily than others. A flat metal object will reflect the signal well. Other objects, like concrete or brick walls, will absorb the signal. A chain-link fence is mostly empty space. But with a spacing of 1” to 1 ½”, a chain-link fence affects the signal as if it were a standing wave and blocks transmissions in the 2.4 GHz band. The list of observable phenomena that affect a Wi-Fi signal includes, reflection, refraction, diffraction, scattering, and absorption.
With all of these environmental impacts on the Wi-Fi signal, it is easy to understand the desire for direct LOS between the access points and their associated client stations. Most Wi-Fi networks are indoors. And most interior walls are only constructed of drywall. A typical drywall has an attenuation value of 3 dB. This means that a normal interior wall will only degrade a signal by ½. Suppose a wireless client were in the same room as its AP and received a signal at -65 dBm. That same client could move to the room next door and still have a very good signal at -68 dBm.
When we do not have a clear line of sight between wireless devices, an initialism, NLOS, is often used to describe this situation. NLOS is used to mean two related but different situations, near line of sight and non-line of sight. The two situations, near LOS and non-LOS, are handled in two radically different ways. Near LOS describes a situation where the signal still follows an essentially straight path from the transmitter to the receiver by passing through any obstructions, like the aforementioned drywall. In some cases, the solution is to increase the transmit power on the stations to overcome obstructions. This can have a derogatory effect on the WLAN and should normally be avoided. The solution for the non-LOS problem is a bit more elegant but does present some challenges. In a plan for non-LOS, the engineer relies primarily upon the reflection of the signal. Suppose that a large metal object stands between an access point and a location that must receive Wi-Fi coverage. Although we cannot push the signal through the obstruction, we can bounce the signal around it. The signals traveling from the AP may, for example, travel over the obstruction, hit a far wall and reflect back into the space that would otherwise be in the RF shadow of the large metal object. Similarly, the signal could reflect off of walls on either side of the obstruction and still reach the client device.
In an earlier discussion, I was writing about using the RSSI values to locate client devices in a WLAN. We know and can predict how a signal will attenuate over a given distance of open space. But we now must consider that to rely upon the inverse square law as the only factor in how a signal degrades is folly. Suppose that a client is 20 meters away from an AP and is receiving the signal from the AP in a direct LOS. The client then moves into the RF shadow of delivery van. It is possible that the client could still remain connected if, for example, the signal bounces off of the exterior wall of a neighboring building creating a non-LOS connection. If we were to rely solely upon the inverse square law to determine the distance between the client and the AP we would calculate a distance that would be no less than the sum of the distance from the AP to the neighboring building added to the distance from the neighboring building to the client. And that calculation would assume a near perfect reflection from the exterior wall of that building. For near line of sight, we must understand the attenuation value of the interposing walls. Consider that two clients can both be 10 meters from an AP with one having direct LOS and the other having near LOS with an interposing drywall. The RSSI for the client behind the wall would be attenuated by 3 dB and would appear to be further away if the wall were not factored into our math.
If we are using triangulation of the RSSI values observed by three access points to locate a client, we must consider that as the client moves throughout the environment, the client will experience some combination of direct LOS, near LOS, and non-LOS with the monitoring APs. It is critical, therefore, that an engineer working on a real-time location tracking solution incorporate specific environmental details into the design.