From The Archives: The Many Forms Of NLOS

This article was originally posted on Broadband Wireless Exchange in March, 2002.

The Many Forms Of NLOS

In the last eighteen months, the term “NLOS” has come into semi-common usage in the Broadband Wireless Internet Access (BWIA) industry. NLOS stands for Non or Near Line Of Sight. The term is intended to convey that a particular wireless system has certain characteristics enhanced to make it possible to deliver usable signal where “conventional” BWIA systems cannot.

There is general agreement on the term “Line Of Sight” (LOS)– literally two points that that (weather permitting) have an unobstructed optical path (sight) between them, including sufficient “Fresnel Zone” clearance (the line of sight cannot be too close to ground, or a the radio signal will be severely attenuated).

In contrast to LOS, there is no specific or widely agreed-upon definition for the phrase NLOS, and thus the phrase can be construed to mean whatever is most advantageous to the vendor. In my research on the BWIA industry, I’ve encountered four distinct “meanings” for NLOS.

NLOS Type 1A – “Better Than Average” 2.4 GHz Foliage Penetration
Foliage represents a severe challenge to outdoor / Metropolitan Area Network (MAN) BWIA systems that use license-exempt spectrum. The 2.4 GHz band, by far the most popular choice of spectrum and equipment for BWIA, is almost completely blocked by foliage. A few companies have developed BWIA systems that make use of the North American 902-928 MHz license-exempt band. 902-928 MHz is also attenuated by foliage, but the attenuation is not as sever as with 2.4 GHz signals. Thus, 902-928 MHz systems, in comparison to 2.4 GHz systems, can be claimed to be “NLOS”; in a side-by-side comparison of a 2.4 GHz system and a 902-928 MHz system trying to operate through foliage, all other factors being equal, the 2.4 GHz system will be LOS (doesn’t work through the trees) and the 902-928 MHz system will be “NLOS” (does work through the trees). As with nearly everything in BWIA, such a comparison is not absolute; some types of trees will attenuate 902-928 MHz, and the penetration claimed for 902-928 MHz is a matter of degree. An example of 902-928 MHz systems is WaveRider Communications’ LMS system.

Several vendors claim that, all things being equal, certain “advanced” modulation techniques such as Orthogonal Frequency Division Multiplexing (OFDM) and Code Division Multiple Access (CDMA) will penetrate foliage better than “conventional” modulation techniques. Although the physics of foliage penetration would seem to argue against such claims, something I was told by a WISP owner with a scientific background gave me pause. The owner told me that he had actually measured the attenuation of several kinds of foliage. He found that, in the aggregate, attenuation effects of 2.4 GHz signals were not nearly so severe as he had been led to believe. However, he found that the attenuation effects of 2.4 GHz signals were highly variable in frequency and degree of attenuation. One part of the 2.4 GHz signal might be attenuated only a little, and another part of the signal attenuated severely, and that the attenuation effects changed rapidly. If this is the case, then perhaps “advanced” modulation techniques can deal with foliage attenuation effects somewhat better than “conventional” modulation techniques. Various product lines from Wi-LAN, Inc. make extensive use of OFDM.

What if the paltry power limitations in effect for the 2.4 GHz band were not in effect? That’s somewhat the case for licensed spectrum, where much higher transmit power levels (than in the 2.4 GHz band) are allowed, resulting in noticeably better foliage penetration.

NLOS Type 1B – Through Wall Penetration
Several vendors claim that by using “higher” (in comparison to license-exempt 2.4 GHz power limits) power and advanced modulation techniques, that their Customer Premise Equipment (CPE) does not require an outdoor antenna (thus there is no “optical” LOS- walls are in the way). That is, their CPE can be placed by the customer on a desk, or a bookshelf and with minimal orientation, receive a BWIA signal of sufficient strength. Generally in such systems, the “base station” or other “mesh node” (see NLOS Type 3) must be located relatively close to achieve adequate signal strength. An example of through wall penetration is NextNet’s Expedience system.

NLOS Type 2 – Making Use Of Multipath
Multipath is best described as “stray signal bounce”. After being transmitted, Radio Frequency (RF) signals inevitably reflect from objects, and if the reflected signals are sufficiently strong, can interfere with the effective processing of the “direct” signal.

Multipath can theoretically be useful. Perhaps in between a BWIA base station and a potential customer there is a large hill. Because of the hill, there is no LOS… however, there is a water tower on a nearby hill that is “visible” to both the BWIA base station and the potential customer. The reflections from the water tower could possibly allow the potential customer to receive signal from the base station. An example of “make use of multipath” is Aperto Networks’ PacketWave system.

NLOS Type 3 – Mesh Networking
If making use of Multipath can be thought of using “passive” signal repeaters to work around LOS obstructions, mesh networking can be thought of using active signal repeaters to work around LOS obstructions. Typically, a mesh networks consist of nodes that are simultaneously repeat signals for other nodes as necessary and deliver service to the customer. In general, the customer is unaware of the “repeater” function of his node.

Mesh networking not only routes around LOS obstacles, but can also effectively extend networks beyond the range of individual nodes. An example of mesh networking is the Nokia Rooftop system.

NLOS Type 4 – Highly Focused Dynamic Beam Forming
Dynamic Beam Forming transmits a highly directive, tightly focused RF signal to each customer in turn. This has the effect of raising the apparent transmitted power and receiving only the individual customer’s signal (in turn). With the highly focused beam, the apparent transmit power and received signal is much stronger than with conventional antenna systems, and thus may be able to overcome some LOS obstructions such as foliage. An example of highly focused dynamic beam forming is BeamReach Networks’ BeamPlex system.

Used In Combination
In reality, most Broadband Wireless Internet Access systems that claim NLOS capabilities actually use a combination of the above techniques.

Space didn’t permit more of an in-depth technical explanation of NLOS, nor a comprehensive list of implementations of NLOS techniques. Hopefully this article clears up a bit of the confusion when you see claims by one vendor for “NLOS” that don’t seem to be comparable to another vendor’s claims of NLOS.

(End of article)

Author update, March 31, 2007:

• Outdated author information at the end of the article was removed;
• Several minor typos in this article were (finally :-) corrected;
• Overall, I think the information in this article has held up fairly well, even though the Rooftop mesh system has been discontinued by Nokia, Wi-LAN no longer makes actual radio equipment, NextNet Wireless was acquired by Clearwire and then Motorola, and BeamReach Networks and WaveRider Communications are out of business;
• Regarding "Advanced Modulation Techniques" - OFDM, I've learned that what I was attempting to describe is called "Subchannelization" and its effectiveness in dealing with path obstructions and multipath are two of the primary reasons OFDM has become widely adopted.

by Steve Stroh

Copyright 2002 © by Steven K. Stroh