What You Need to Know about Line of Sight

August 25, 2016 at 8:00 AM


When designing a wireless network, one of the most significant factors to consider is Line of Sight (LOS) - the path between two antennas. Obstructions in the LOS path can wreak havoc on a Wi-Fi signal so determining what, if anything, is between the antennas is crucial to your network working properly. Here, we’ll examine the main concepts you need to know in order to clear the path for Line of Sight and make sure your wireless network is a success.

 

The first step in navigating Line of Sight is to determine the LOS conditions. Once the conditions are defined, the correct type of wireless system can be chosen for the network area. There are three main Line of Sight conditions:

 

1.       Full Line of Sight (LOS) – no obstacles between the two antennas

2.       Near Line of Sight (nLOS) – partial obstructions between the two antennas, such as tree tops

3.       Non Line of Sight (NLOS) – full obstructions between the two antennas, such as an entire tree  

 

Outdoor networks may encounter the largest obstacles, but Line of Sight is also important for indoor wireless networks. Obstacles like walls, ceilings and furniture have to be considered because they will also affect the wireless signal reception.                                                                                                                         

In addition to obstructions, there are three other factors to consider that can affect Line of Sight:

 

1.       Multipath and Reflections

2.       Fresnel Zone

3.       Path Loss

 

In wireless transmissions, multipath and reflections are as important as signal strength because they too can degrade the performance of the network. Multipath is when wireless signals travel in multiple paths and arrive at the receiver at different times. Reflections occur when wireless signals "bounce" off of objects. When signals are transmitted through walls and ceilings and are reflected off of metallic objects, they will also have peaks and nulls in amplitude and changes in polarization (vertical or horizontal).

 

Fresnel Zone is an electromagnetic phenomenon where light waves or radio signals get diffracted or bent by solid objects near their path. The reflected waves/signals become out-of-sync with those that traveled directly to the receiving antenna, this delay reduces the power of the received signal.

 

Path Loss is another area of concern when determining Line of Sight. Some radio frequencies travel well through certain objects while other frequencies are not able to pass through, resulting in path loss. For example, 2.4 GHz radio waves easily pass through walls but experience path loss when going through trees and leaves. This is because walls are very dry, trees contain high levels of moisture and 2.4 GHz radio waves are easily absorbed into water. On the other hand, 900 MHz radio waves are not as easily absorbed by water. In cases like this, when trees cause nLOS or NLOS conditions, 900 MHz is a better frequency to use than 2.4 GHz to avoid path loss.

 

There are many factors to consider when designing a wireless network, but with proper site evaluation and planning you can correctly navigate Line of Sight obstacles to achieve peak performance.

 

 Comments on this post? Other topics you’d like us to cover? Email us at engineeringhub@l-com.com

 

How Line of Sight (LoS) Can Affect Your Wireless Installation

September 18, 2014 at 10:00 AM

 

Though the term Line of Sight seems self explanatory, there’s actually more than meets the eye when it comes to LoS and installing a wireless network.

 

As you might already know, Line of Sight is the path between two antennas. One of the first questions you’ll want to ask yourself when designing an outdoor wireless network is what is between point A (antenna 1) and point B (antenna 2)?

 

These details are important since Line of Sight does not only apply to a straight line. Wireless signals being sent from point A to point B can and will, most likely, run into to some obstacles that will alter the path they take.

 

When light waves or radio signals get diffracted or bent due to solid objects near their path, it’s an electromagnetic phenomenon referred to as The Fresnel Zone (referenced in the diagram below). The radio waves reflecting off the objects may arrive out of phase with the signals that traveled directly to the receiving antenna, thus reducing the power of the received signal.

 

It is important to also note that the line of site broadens with wavelength, which means that for low frequency, high wavelength signals, you need to have a larger Fresnel radius free of obstructions.  

 

 

 

 

As you can see, there are three main categories of Line of Sight to use as guidelines:

1.       Full Line of Sight (LOS), where no obstacles reside between the two antennas.

2.       Near Line of Sight (nLOS) which includes partial obstructions, such as tree tops between the two antennas.

3.        Non Line of Sight (NLOS), where full obstructions exist between the two antennas.

 

By determining the specific line of sight conditions in the WiFi network area, you can then determine the correct type of wireless system to install.

 

For example, most WiFi systems typically run on the 2.4 GHz and 5.8 GHz frequencies. Both of these frequencies are very dependent on a clear line of sight to obtain good performance, so clear LoS is very important.

 

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