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Chapter 4: Fade Margin and Rain Fade

“Rain fade” is a particularly common phenomenon, and needs to be accounted for in link calculations. Droplets of water in the air can attenuate signal, to differing degrees depending upon frequency. The lower frequencies such as 900 mhz and 2.4 ghz barely see a change for rain. 5.8 ghz sees a minimal change. Higher frequencies such as 24 ghz can easily be reduced to worthless by a torrential downpour.

After predicting that “for this link, I will require -75 dB”, it is worth considering that radio conditions are dynamic. Rain, thermal ducting, humidity changes, and even solar flares can lead to the conditions of your link changing, sometimes seemingly at random. Therefore, you should always try to aim for better link quality than the minimum required for the throughput you need. There are many factors that can lead to a sudden change in radio performance; some you can plan for (such as rain), some you can't (such as somebody putting up a radio that interferes with your link). At the very least, you want to allow for 5 dB of “fade margin” (that is, margin-of-error set aside for links fading) – ideally, you'd like more than that.

“Rain fade” is a particularly common phenomenon, and needs to be accounted for in link calculations. Droplets of water in the air can attenuate signal, to differing degrees depending upon frequency. The lower frequencies such as 900 mhz and 2.4 ghz barely see a change for rain. 5.8 ghz sees a minimal change. Higher frequencies such as 24 ghz can easily be reduced to worthless by a torrential downpour.

The International Telecommunications Union has produced a series of documents for calculating rain fade effects. Rainfall is calculated in millimeters per hour, with percentage likelihood. These are calculated for various “rainfall zones”. Note that these are very general figures; if your network is in a micro-climate with above average or below-average rainfall levels, then your network may exhibit different climate zone characteristics. To discover the rainfall zone in which your network operates, consult the following map or use Google to search for an “ITU rain zone calculator”:

ITU Rain Zone Map

For example, the author’s network operates in Missouri USA, which falls within ITU Zone “K” – for a peak rainfall of 42 mm/h, 0.01% of the time. This means that we can expect to experience less than 42 mm/h or fain 99.99% of the time.

The following table provides world climate zones and their precipitation characteristics (reproduced from the ITU):

Percentage of time (%)

A

B

C

D

E

F

G

H

J

K

L

M

N

P

Q

1.0

<0.1

0.5

0.7

2.1

0.6

1.7

3

2

8

1.5

2

4

5

12

24

0.3

0.8

2

2.8

4.5

2.4

4.5

7

4

13

4.2

7

11

15

34

49

0.1

2

3

5

8

6

8

12

10

20

12

15

22

35

65

72

0.03

5

6

9

13

12

15

20

18

28

23

33

40

65

105

96

0.01

8

12

15

19

22

28

30

32

35

42

60

63

95

145

115

0.003

14

21

26

29

41

54

45

55

45

70

105

95

140

200

142

0.001

22

32

42

42

70

78

65

83

55

100

150

120

180

250

170

From this table, we can determine that Missouri (“K”) generally receives 1.5mm/hour of rain in a day, but very rarely (0.001% of the time) receives as much as 100 mm/hour of rain.

The following formula is then applied:

LRAIN = YRDRAIN

DRAIN is the distance in kilometers through which the path remains beneath cloud level. For most WISPs, this is the entire length of the link (for satellite calculations, it can be calculated from angle to the horizon – but that doesn’t apply to most WISPs).

γR is derived from the mm/hour rainfall found in the above table. Two additional variables are required from the following table:

Frequency (Ghz)

Horizontal Polarization

Vertical Polarization

K

K

1

0.000387

0.912

0.0000352

0.880

2

0.000154

0.963

0.000138

0.923

4

0.000650

1.121

0.000591

1.075

6

0.00175

1.308

0.00155

1.265

8

0.00454

1.327

0.00395

1.1310

10

0.0101

1.276

0.00887

1.264

12

0.0188

1.217

0.0168

1.200

15

0.0367

1.154

0.0335

1.128

20

0.0751

1.099

0.0601

1.065

25

0.124

1.061

0.113

1.030

30

0.187

1.021

0.167

1.000

35

0.263

0.979

0.233

0.963

40

0.350

0.939

0.310

0.929

Taking the values from this table, YR can be calculated as follows:

γR=kR

For example, for a 5.8 Ghz link (we’ll call it 6 Ghz), K has a value of 0.00175 and ∝ has a value of 1.308. Assuming a 10 kilometer link:

DRAIN = 10 * (0.00175 * 42)1.308
DRAIN = 0.32893 dB

For our 10km link, we can count on losing 0.32 dB of signal 0.01% of the time. That isn’t going to impact our link very much.

As another example, let’s try a 24 Ghz AirFiber pair at the same distance. K has a value of 0.124, and ∝ has a value of 1.061.

DRAIN = 10 * (0.124 * 42)1.061.
DRAIN = 57.591 dB

Our 10 km link can expect to drop by as much as 57 dB during heavy rainfall, 0.01% of the time! This is a good example of why 24 ghz can be unsuitable for longer links – rain can completely drop the link.

« Chapter 4: Introduction - Non-Ideal Conditions Up To Contents Chapter 4: Trees and Other Obstacles »

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