Does an outdoor wireless point to point bridge require Line-of-Sight ("LOS") or can a quality wireless Ethernet bridge perform under Non-line-of-Sight ("NLOS") conditions? LOS is when both antennas in a outdoor wireless bridge system must have clear visibility with one another and have no encroachments to the first Fresnel Zone. In a NLOS situation there is either limited visibility from one wireless antenna to the other (near-line-of-sight or" nLOS") caused by a Fresnel Zone encroachment or complete obstruction blocking the visibility between the two wireless antennas.
General Overview of Point to Point Wireless Backhaul:
A typical outdoor wireless backhaul is used to pass higher throughput over greater distances. Outdoor wireless bridges operate in the SHF (Super High Frequency) band in unlicensed wireless backhaul 5.3GHz, 4.9GHz, 5.4GHz, 5.8GHz, and 24GHz or licensed microwave backhaul 6GHz, 11GHz, 18GHz, and 23GHz. There is also unlicensed 60GHz and registered 80GHz millimeter wave in the EHF (Extreme High Frequency) band. The unlicensed wireless Ethernet bridges typically provide from 10Mbps to 300Mbps aggregate throughput. Unlicensed 24GHz and licensed microwave links offer up to 360+Mbps Full Duplex. 60GHz and 80GHz wireless bridge systems can provide up to GigE Full Duplex (gigabit wireless). The higher frequencies do not do well with penetrating obstructions.
For an outdoor wireless bridge to work the system gain must be greater that the total Path Loss. Historically, an outdoor wireless bridge required LOS providing first Fresnel Zone clearance. By having no obstructions in the first Fresnel Zone the receive signal are optimized and the out of phase signals are minimized.
General Overview of Non-Line-of Sight Wireless Bridges
When considering a point to point wireless backhaul, whether a licensed microwave link or an unlicensed wireless Ethernet bridge, one of the first questions asked is if there needs to be LOS to get a microwave link. Many don't understand the difference between wireless bridge technologies that they are use to (like cellular and cordless phones or Wi-Fi) compared to an outdoor point to point wireless Ethernet bridge.
Devices like cellular operate in a range from 800MHz to 1900MHz of the UHF (Ultra High Frequency) band. These frequencies do well with penetrating obstructions but have limited throughput capabilities. Most Wi-Fi operates in the 2.4GHz frequency of the UHF band and can provide higher bandwidth but is very limited in distance. Microwave communication signals are highly attenuated by an obstructed path. In a NLOS microwave link the RF signals will get to a destination by: diffraction around an object, reflection off objects, or by penetration through the obstruction.
For an outdoor wireless bridge, being used for high bandwidth, point to point backhaul to work in a NLOS application there are several requirements that need to be met. Proper power budget, fade mitigation, adaptive link characteristics, and proper demodulation in regards to dispersion. Because of obstructions in a NLOS situation there tends to be a large amount of multipath. Obstructions like trees add to multipath and add attenuation to the overall Path Loss of the microwave link. Trees can be tricky because they are not constant due to movement caused by wind, foliage changes during various seasons, moisture content of the foliage, etc. Constant obstructions like buildings or hills are easier to model and predict.
General Overview of NLOS Wireless Technology
Current wireless backhaul technologies can help in NLOS cases. MIMO (Multiple Input Multiple Output) antenna signaling and spatial diversity reduces the amount of fade margin required. OFDM (Orthogonal Frequency Division Multiplexing) which divides the data into several parallel data streams helping the fading that occurs with multipath. Adaptive rate modulation also helps by giving the wireless backhaul radio the ability to manage the modulation scheme and bandwidth according to the RSL (receive signal level) optimizing the microwave communication link. Outdoor wireless bridges that can take advantage of these wireless backhaul technologies are the unlicensed wireless systems. Unlicensed wireless backhaul using these technologies can provide up to 300Mbps aggregate throughput (depending again on the characteristics of the microwave link path).
A common question of why a licensed microwave link, which can provide higher, full duplex connectivity, doesn't use OFDM wireless or MIMO antenna solutions and why they can't be used in NLOS (non line of sight) applications. In a NLOS wireless link application point to point wireless Ethernet bridge radios that use OFDM or MIMO take advantage of multipath for their connectivity. Because a licensed microwave link is not to inject any interference on other licensed microwave backhaul operators in the area they must have LOS (line of sight) and not cause heavy multipath. If a licensed microwave radio was to cause a lot of wireless multipath it could potentially reflect into another existing licensed microwave communication radio belonging to another party.
Prior to considering a NLOS wireless backhaul, a wireless site survey and a proper wireless path calculation should be performed. Field test may need to be performed in order to verify if a NLOS microwave link will work or to gather accurate estimates on throughput performance. As with any point to point wireless backhaul, a certified expert should perform the wireless installation.
Add from Glenn De Haes
.Let me add to that some field experience. OFDM works by all means better in higher frequencies due to the higher reflective coefficiences of materials on those frequencies. This means a NLOS system with OFDM in 5GHz works better than in 2.4GHz. You also notice this even with wifi indoor systems using 11g or 11a.
A key thing with NLOS is the ability to catch as much signal as you can. It is thus easier to have a NLOS link running with two sector antennas instead of parabolic dishes.
MIMO is a nice addition to this since you will gather signals from multiple polarities and with a larger delay spread.
What most people also forget, is that in order to profit from reflections, you first of all need them. NLOS in a city envirnment is thus a bit easier then when there are other obstacles likes trees in the way. These things only absorb power and hardly reflect anything.
We see mostly examples from manufacturers with trees. Mainly to explain the difference between LOS, near LOS and non LOS. Take it from me, OFDM, MIMO whatever, if there are trees in the way, the only thing that can help you is power. We have had installations where we tried 2x2 versus 3x3 MIMO and since you have less gain in an antenna of the same size in 3x3 versus 2x2, the result in most circumstances was worse. 3x3 MIMO does give you a better path fading result so ideally for implementations in cities or amongst buildings; even in LOS.
Bottom line: ALWAYS test a NLOS up front and make sure that your reflections are trustworthy and not originating from something only temporarily available.
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