The
Many Forms Of NLOS
Copyright © 2002 By Steven
K
Stroh
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 severe 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 GH 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.
Steve Stroh is Editor of Focus
On Broadband Wireless Internet Access, an independent newsletter covering
the rapid emergence of the Broadband Wireless Internet Access industry. More
information on Focus is available at http://www.strohpub.com/focus.htm.
Steve can be contacted at steve@strohpub.com
and by phone at 425-481-0600. This article appears in Broadband Wireless
Exchange by special arrangement.
End
of Document
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