Non Line of Sight (NLOS)

5/23/2018 Non Line of Sight (NLOS)

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Overview of NLOS


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WHY NLOS

The evolution to denser radio-access networks

with small cells in cluttered urban

environments has introduced new challenges

for microwave backhaul. A direct line of sight

does not always exist between nodes, and this

creates a need for near- and non-line-of-sight

microwave backhaul.


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NLOS Working.

By using narrow beam antennas together with link-pathengineering skills enables very high performance point-to-point microwave links by only sending energy in theappropriate NLOS path between the terminals. This alsoprovides a very large power budget to cover up for lossimposed by the NLOS channel while still offering highthroughput and reliability.

A NLOS link is fundamentally different to a line-of-sight (LOS)link due to the radio energy interacts with obstacle that aredifficult to predict in detail or even obtain information about.Also obstacles in an urban environment tend to change overtime that makes performance predictions even more difficult.Thus, engineering skills and experience becomesindispensable when planning a reliable NLOS link.


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Small cell backhaul deployment using

NLOS

For general small-cell backhauling, line-of-sight

(LOS) propagation is always preferred but if a LOS

link is not achievable a communication channel

relying on non-line-of-sight (NLOS) propagation issometime possible. NLOS propagation scenarios

make use of one or more of the following effects:

Diffraction

Reflection

Penetration


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Diffraction

Roof before building corners

Reflection

Reflection on building walls

Penetration

Thin sheets or sparse obstacles, e.g.small greenery and flag poles

Diffraction

Reflection

Penetration


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Small cell backhaul deployment using

NLOS

Figure .Example of diffraction down from a roof edge and around the corner

of a building.


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Diffraction:Diffraction can be utilized by directing the

beam towards a roof edge in order to bring down

energy behind a building blocking the sight between

antennas as outlined in figure

Diffraction can also be used to bring energy behind a

tall building by sending energy towards a corner of a

building wall as shown in figure

Small cell backhaul deployment

using NLOS


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using NLOS


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The amount of energy behind the blocking

object depends highly on the angle from the

diffraction point. However, the diffraction process adds loss

from 6 dB at the initial blocking at 0

diffraction angle and down to virtually infiniteloss at 90diffraction angle


using NLOS


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using NLOS


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Reflection :

Reflection is more difficult to utilize since the

orientation of the reflecting area determinesthe angle of the incoming and outgoing beams

and thus the direction of the reflected beam


using NLOS


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Reflection alignment


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using NLOS

a. b. c.


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Penetration

Penetration in general is simply a loss added to

the channel due to penetration through ablocking obstacle,

penetration can only be utilized for thin sheets

or sparse obstacles e.g. flags and smallgreenery.


using NLOS


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Recommended planning approach


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Based on customer requirements determine the following:

- Distance

- Required throughput - Allowed transmitter power

- Allowed antenna size (gain)

- Required Availability

- Other constrains affecting the link budget as coveringsheds etc.

Requirements analysis


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With this input estimation on the available NLOS power budget canbe performed. A conventional tool such as ML PERF canaccommodate this by:

- Calculate total available power (PTX+ GTX+GRX in equation 1)

- Calculate free space path loss (Lfree space)

- calculate the required rain margin according to ITU standards.

- Look up required signal level for the specific radio for a statedchannel spacing and modulation format and calculate a minimum

required received signal strength (RSS) for the required performance - Calculate how much loss that can be allocated to NLOS

Recommended planning approach


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NLOS POWER budget


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Estimate total path loss

Estimate the total path loss for possible link paths

using the free space path loss added to the NLOS

obstruction bypass loss obtained from

diffraction/reflection/penetration Suggested additional NLOS margins due to difficulty:

1. High margin (20 dB) required for movable objects in path

2. Medium margin (10 dB ) required for many reflection areas

3. Low (0-5 dB) for clean diffraction

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Non Line of Sight (NLOS)