A Study of Propagation in a Difficult Environment Two Years in Mississippi George Kizer, Alcatel-Lucent NSMA Annual Conference May 19 & 20, 2015 Holiday

A Study of Propagation in a Difficult EnvironmentTwo Years in MississippiGeorge Kizer , Alcatel-Lucent

NSMA Annual ConferenceMay 19 & 20, 2015

Holiday Inn Rosslyn at Key BridgeArlington, Virginia

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Network Overview

142 Sites 488 Links150 Paths 1952 T-Rs

top of the state

near the gulf

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Maximum Path Length = 32 miles

Minimum Path Length = 3 miles

Median Path Length = 18 miles

Average Path Length = 19 miles

Six paths were at 11 GHzAll others were at lower or upper 6 GHz

17% of the paths were non-diversity83 % were space diversity

Architecture = rings and spurs

Network Path Varied

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System design was in accordance with standard Bell Labs criteria (below)

Path propagation performance of all 488 simplex paths was measured over one to two years (as network was created)

Path Performance

Vigants, “Space Diversity Engineering,” Bell System Technical Journal, pp. 103-142, January 1975Vigants, “Microwave Radio Obstruction Fading,” and Schiavone, “Prediction of Positive Refractivity Gradients for Line-of-Sight Microwave Radio Paths,“ Bell System Technical Journal, pp. 785 – 822, July –Aug 1981

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Overall the network measured path performance met customer specifications: 99.998% one way

Multipath Fading

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You can drown in a river with average depth of one foot – or a network with average satisfactory performance

Multipath Fading

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Path performance variations were primarily the following:

IP Network Characteristics

Path Multipath Fading

Path Obstruction Fading

Path Performance

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MW network supported a two layer IP network: a Multiprotocol Label Switching (MPLS) network which supported

another overlay proprietary LMR IP network

Each network used Open Shortest Path First (OSPF) for packet routing

Each network used Bidirectional Forwarding Detection (BFD) to monitor route reachability


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OSPF validity was tried to BFD

The lower level MPLS network went down after three consecutive 100 millisecond BFD probe failures

(e.g., after a MW path outage greater than 0.3 secs)

The higher level LMR network went down after three consecutive 300 millisecond BFD probe failures

(e.g., after a MW path outage greater than 0.9 secs)


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MPLS OSPF was restored 15 seconds after BFD came up

The LMR OSPF was restored 45 seconds after BFD came up


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For router networks, individual outage durations is not the only criteria

A single one second MW path outage can cause a 15 second outage in am MPLS protected network which can cause a 45

second outage in the customer LMR router protected network

Outage time extension in router protected network can be significant

The outage time experienced by the customer can be two to three orders of magnitude greater than the radio path outage

time

For router networks, number of outages can be more significant than individual outage duration

Individual Fading Events


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Ring and Mesh networks can provide significantly enhanced network performance with compared to the performance of

cascaded paths

In the State of Mississippi, the routers switched hundreds of times a week, but outages due to path propagation were much

less frequent

The weakness of rings is when multiple paths are affected simultaneously

Spurs without route protection are much more vulnerable to outage time magnification


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Spur performance was limited by cumulative outage duration but also by individual outage duration

Both durations were measured.

Path Multipath Performance

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Path Multipath PerformanceCumulative Outage Duration

obse

rved

estim

ated

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Individual path performance varied widely:

Worst Case Observed Outage / Predicted Outage (secs ratio) = 156

Best Case Observed Outage / Predicted Outage (secs ratio) = 0

Median (50%, typical) Observed / Predicted Ratio = 1.9

Average Observed / Predicted Ratio = 6.4

Ratio Not Exceeded for 33% of Paths = 1

Multipath Fading

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Average Total Path Outage Duration = 114 seconds

Minimum Number of Path Outages = 0

Maximum Number of Path Outages = 659

Median Number of Path Outages = 27

Average Number of Path Outages = 55

Individual Path Annual Performance Varied Widely


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Path Multipath PerformanceCumulative Outage Events

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Path Multipath PerformanceIndividual Outage Duration was relatively stable

Above statistics are for 6 GHz paths; 11 GHz paths experienced no outages.

Number of outage events Total outage seconds

One reason measure outage > predicted outage is that Vigant’s method predicts outage on an analog basis

while we measure it on an error-second basis

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Cumulative outage duration is directly related to Vigants C factor

Vigants, “Space Diversity Engineering,” Bell System Technical Journal, pp. 103-142, January 1975

Good: 0.5Average = 1Difficult = 2

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Vigants (1975) C Factor = 2

Measured C Factor = 7

This is consistent with Vigants’ and Barnett’s latter suggestions for the Gulf coast.

They suggested (1992) the C factor should be 10 for this area.

Loso, Inserra, Brockel, Barnett and Vigants, “U. S. Army Tactical LOS Radio Propagation Reliability,” Proceedings of the IEEE Tactical Communications Conference, pp. 109-117, 1992.

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Path Multipath PerformanceCumulative path outage varied significantly from the

average

R = measured cumulative individual path outage – average outage = cumulative path outage standard deviation

10 log () = 7 dB (measured)10 log () = 10 dB (reported by Bellcore*)

*Achariyapaopan, “A Model of Geographic Variation of Multipath Fading Probability,” Bellcore National Radio engineer’s Conference Proceedings, pp. TA1 – TA16, 1986.

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Path Multipath PerformanceIndividual Outage Duration was relatively stable

95 % of the paths have cumulative outage durations of the following:

8.2 x network average duration (measured 10 log () = 7)16.5 x network average duration (Bellcore 10 log () = 10)

Since the typical outage duration one second, 95% of the number of outage events average the following:

8.2 x average number of outages (measured 10 log () = 7)16.5 x network average number of outages(Bellcore 10 log () = 10)

For router networks, the number of outage events is more significant than the actual outage duration

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Path Obstruction Fading

Obstruction fading was relatively infrequent but could be quite troublesome when it occurred

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Typical Multipath Fading

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Classical Obstruction Fading

RSLs for Both Receivers(Space Diversity)

Threshold RSL

Nominal RSLTransmit Power

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Slow Speed Obstruction Fading

RSLs for Both Receivers(Hot Standby Non-diversity)

RSLs for Both Receivers(Hot Standby Non-diversity)

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RSLs for Both Receivers (hot standby)

Moderate Speed Obstruction Fading

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ReceiverOverload

ReceiverOverload

ReceiverOverload

RSLs for Both Receivers (space diversity)

High Speed Obstruction Fading

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Unexpected diversity effectObstruction Fading

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Unexpected path diversity effectDifferent Paths from the Same Node

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Path diversity effectPath Pairs from Same Node

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Obstruction Fading Predicted vs Measured Outages

Currently airport refractivity measurements are taken at different times (typically at sun up and sun down) than when obstruction fading typically occurs (from about midnight to

just before sun rise)

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Obstruction Fading Lessons Learned

Obstruction fading comes in two forms:

Relatively short duration amenable to space diversityRelatively long duration indifferent to space diversity

Obstruction fading very localized:

Geographically close paths can be de-correlated

Obstruction Fading estimation unreliable:

Results are unpredictableModels under estimate outages on some paths

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Radio Wave Propagation is a Function of Atmospheric Refractivity

n = index of refraction 1.000319N = refractivity = (n – 1) 106

= dry component + wet componentdry component = [ 77.6 p ] / [ 273 + T ]wet component = [3.73 x 105 eS HR ] / [ 273 + T ]2

p = atmospheric pressure in millibars= 1.33 (pressure in mm of mercury)= 33.9 (pressure in inches of mercury)

T = temperature in degrees CentigradeHR = relative humidity (percent) / 100

N is a weak function of pRefractivity increases as HR increases or T decreases

See G. Kizer, Digital Microwave Communication, Chapter 12, pages 461 to 463.

What Causes Obstruction Fading

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The electromagnetic wave is reflected or refracted depending

upon the angle of incidence

If refracted, the electromagnetic wave bends toward region of

higher refractivity

ReflectionRefraction

Optical Wave Propagation

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Radio Wave Propagation is a Function of Atmospheric Refractivity

Without wind or rain atmospheric refractivity a function of weather

When radio wave is inside a slab of high refractivity air, wave bends down (moves toward region of higher refractivity)

When radio wave encounters a nearby slab of significantly different refractivity air, wave is reflected if angle of incidence is less than Brewster’s angle (<< 10 degrees)

Radio Wave Propagation

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Obstruction fading for a

path inside a high

refractivity layer

Layer acts like a large lake with slowly moving waves

Above a layer of different refractivity air, the radio wave is subjected to reflective (interference) fading

Within a layer of different refractivity air, the radio wave is subjected significant bending (earth bulge) or trapping (ducting)

Reflective fading for a path above a high refractivity layer

Obstruction Fading as a Function of Height

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October 24th Early Morning Outages

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October 27th Early Morning Outages

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October 24, 2013Outages Occurred at Times of No Wind, Low Temperature and High Humidity

These conditions are conducive to formation of a dense atmospheric layer

Example of Jackson, Mississippi for day of October 24th

(see www.wunderground.com/history)

See local airport for more exact weather.

Low TemperatureHigh Relative Humidity

No Wind(Static Atmosphere)

Weather History

http://www.wunderground.com/history

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October 25, 2013A typical day had no path propagation outages




Weather History


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October 27, 2013Outages Occurred at Times of No Wind, Low Temperature and High Humidity

These conditions are conducive to formation of a dense atmospheric layer




Low TemperatureHigh Relative Humidity

No Wind(Static Atmosphere)

Weather History


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For a graphical depiction of unusual air flow, go to www.wunderground.com/history. Select Jackson, MS, Oct 27, View, and then select View Animated Radar Loop. Watch the cold moist Gulf air come inland in early morning and then blown out later that day.

Weather History


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If reflective fading occurs during periods of unusual weather, we are dealing with “abnormal propagation” that

cannot be predicted accurately.

This fading can significantly expand the fading season (summer for normal multipath, fall and winter for

abnormal weather layering).

Expect path to have history of outages significantly longer than one second.

Obstruction Fading

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Typical Remedies for Obstruction Fading:

Decrease the distance between sitesThe effect of distance reduction cannot be predicted accurately

Increase the antenna heightsThe placement of antennas cannot be predicted accurately without

radiosonde measurements from nearby airports

Convert to space diversityImprovement typically moderate

Convert to adaptive modulationImprovement can be dramatic

Turn linear routes into ringsWeather anomalies must be localized

Powerful with tall antennas (moderate uncorrelated outages)

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Linear router based paths will experience longer outages than the underlying microwave paths

Error extensive can be as much as two orders of magnitude

Actual path performance may be significantly different than estimates

Use architecture (rings or meshes) to tame path performance.

System performance can significantly exceed the performance of cascaded paths.

Lessons Learned

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RememberAvailability estimates and clearance guidelines

are no guarantee of path performance !

> Use architecture to overcome propagation surprises <

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Documents

A Study of Propagation in a Difficult Environment Two Years in Mississippi George Kizer, Alcatel-Lucent NSMA Annual Conference May 19 & 20, 2015 Holiday