NTIA-REPORT-79-30

Signal Level Distributions and

Fade Event Analyses

for a 5 GHz Microwave Link

Across the English Channel

D.R. Wortendyke

A.P. Barsls

R.R. Larsen

u.s. DEPARTMENT OF COMMERCE

Juanita -M. Kreps, Secretary

Henry Geller, Assistant Secretary

for Communications and Information

October 1979

TABLE OF CONTENTS

ABSTRACT

Pag~

1

1.

2.

3.

4 •

5.

6 •

7.

8 •

INTRODUCTION

MICROWAVE SYSTEM DESCRIPTION AND

IMPLEMENTATION

DATA COLLECTION EQUIPMENT

MEASUREMENT RESULTS

4.1 Signal Level Distributions

4.2 Comparisons with Fading Models

4.3 Fade Event Durations

APPLICABILITY OF RESULTS TO SYSTEMS IN

OTHER AREAS

CONCLUSIONS

ACKNOWLEDGEMENTS

REFERENCES

1

2

4

8

8

11

13

20

22

23

23

Figure 1.

Figure 2.

Figure 3.

Figure 4.

Figu~e 5.

Figure 6.

Figure 7.

Figure 8.

Figure 9.

LIST OF FIGURES

Location map fb~ 5 GHz microwave link.

Terrain profile for 5 GHz microwave link.

Data flow diagram for RSL-2 rec6rding

system.·

Sample of digital strip chart recording

Received signal level distributions

(all data).

Comparison of measurement results with

CCIR signal level predictions.

Example of multiple fades within a

single fade event.

Fade event duration distributions for

winter time blocks.

Fade event duration distributions' for

summer time blocks.

2

3

5

7

9

12

15

16

17

Figure 10. Fade event duration distributions for

all hours. 18

iii

Table 1.

Table 2.

Table 3.

LIST OF TABLES

Sample Printout for Hourly "Quick-look"

Analysis

Path and Equipment Parameters

Fade Event Duration Statistics for Path~

A and B, Swingate - Houtem

v

Page

6

10

19

SIGNAL LEVEL DISTRIBUTIONS AND FADE EVENT

ANALYSES FOR A 5 GHz MICROWAVE LINK

ACROSS THE ENGLISH CHANNEL

D.R. Wortendyke*, A.P. Barsis*, and R.R. Larsen**

This report describes instrumentation for fading

studies and measurement results over an 88-km

multiple-diversity line-of-sight microwave link

across the English Channel operating in the

4 to 5 GHz frequency range. Signal level and

fade, duration statistics derived from the measurements are compared with CCIR models. It is found

that median fade duration tends to be less than

estimates from the CCIR models.

Key words: Microwave fading, multiple diversity

receivers, 5 GHz signal levels, automatic signal monitoring.

1. INTRODUCTION

The Ins,titute for Telecommunication Sciences (ITS) of the

National Telecommunications and Information Administration

u.s. Department of Commerce was tasked in 1972 by the U.S. Air I

Force Communications Service (AFCS) to develop automated test and

acceptance procedures and equipment to simplify and accelerate

the collection of test and performance data from Defense Communication Systems (DCS) microwave links. Emphasis was first on

measurement and analyses of received signal level data. Initially,

ITS developed a calculator-based instrument (RSL-l) in 1973,

which was used on the "SCOPE" conununication links in Germany and

as a measurement-analysis tool at AFCS Headquarters. Later, a

more sophisticated instrument for received signal level (RSL)

measurements and analysis was developed by ITS. The present

report deals with the utilization of this instrument in a longterm study and evaluation of propagation effects on a 'multidiversity microwav~ link across the English Channel between

*The authors are with the National Telecommunications and Information Administration, Institute for Telecommunication Sciences,

u.S. Dept. of Conunerce, Boulder, CO 80303.

**The author is with the' l842nd Electronics Engineering Group,

United States Air Force, Scott Air Force Base, IL 62225.
terminals near Dover, England, and Houtem, Belgium. This link is

88 km long (with 65 km over water), and operates in the 4 to 5

GHz frequency range. It is a portion of the DCS European network,

and has "been carrying traffic since mid-1974.

This report presents first a description of the path, its

implementation, and a brief out Li.ne of the RSL-2 instrumentation.

Next is a discussion and analysis of measurement results from a

I-year operating period. Statistics of signal level and fade

events are presented, and the results will also be evaluated for

applicability to similar links.

2. MICROWAVE SYSTEM DESCRIPTION AND IMPLEMENTATION

As shown in Figure 1, the western terminal of the 88-km

link is at Swingate, near Dover, England, on top of the White

Cliffs. The eastern terminal is approximately 19 km inland from

the Channel coast near Houtem,Belgium. The path profile with

antenna positions, heights above mean sea level (in meters), and

a representation of the sea surface for two values of the

effective earth radius factor, k, is shown in Figure 2. Antennas A and B at each terminal are 4.6 m parabolic reflectors, and

antennas C and Dare 3.7 m parabolic reflectors.

ENGLAND

LONDON

-

FRANCE

-BRUSSELS

BELGIUM

PARIS

- Figure 1. Location map for 5 GHz microwave link.

2
V>

0::::

L.LJ

A 244

230 AH B 235 221 I

....J

L.LJ 184 >; d ----- ~-- ~K = 2/3 e:t: 14 1 C ---- -----

I..LJ ----- V> 1 /1 ---- -----

WHITE, /' ---- _

CLIFFS OF

DOVER

w \/ - ~HC 41 o

K=oo • db V> I I I I I I I I I I I I 3

0 10 20 30 40 50 60 70 80 90 (!' 3 74 88

SWINGATE DISTANCE IN km DUNKERQUE HOUTEM ::c ENGLAND FRANCE BELGIUM

Figure 2. Terrain profile for 5 GHz microwave link.
The design considerations for the multiple diversity system

wer~ described in detail by Zebrovitz (1975). For sub-refractive

and normal conditions (2/3 k 4/3), dual diversity operation

is provided by paths A and B. For the top path A, clearance

amounts to approximately the 11th or 12th Fresnel zone for

k = 4/3, but it is only a small fraction of the first Fresnel I

zone (less than 0.1) for k = 2/3. Under these -conditions, the

lower paths (C and D) contribute relatively little to the quadruple diversity combiner input since direct line of sight for

these lower paths is blocked by the sea surface. However, when

signals from the upper antennas are subject to severe multi~ath

fading because of superrefractive conditions (k > 4/3), the lower

paths C or D may be within the superrefractive layer with very

little attenuation. They would then be expected to provide the

principal contribution to the'combiner. The lower path C, as an

example, becomes grazing for k = 1.82.

Two of four transmitters are used as standbys at any

specific time, and up to four usable signals at a time are

provided to the combiner. It was anticipated that this design

would fulfill the performance requirements in terms of signal

level and noise power ratio for at least 99.99% of the time.

3. DATA COLLECTION EQUIPMENT

The RSL-2 measurement instrument is a minicomputer-controlled device designed to handle great quantities of timevariant data. The conceptual operation is shown in Figure 3; it

is controlled by software programs core-~esident in the 24-k word

memory. Fbur dc vo~tages, prop6rtional to the received signal

levels from each of four receivers, are measured every 0.1 s by a

12-bit A-D converter. The mini-computer sorts each of the four

measurement values against a stored calibration curve for the

particular receiver and then places the data in a histogram with

20 partitions that are equally spaced across the receiver dynamic

range. Once per hour the histograms are stored with a date-timegroup and the process begins again. A five-point cumulative distribution (signal levels in dBm exceeded for 10%, 50%, 90%, 99%,

4
F~~} R U

oT

ME

M

} } SWINGATE

LC-4 LC-4

RADIO RADIO

REC A REC C

REC B REC D

QUICK LOOK STATISTICS

" ONCE/HOUR

/

ON 1-1/2" TTY PAPER ,,-------1 FOR 4 RECEIVERS

AND COMBINED SIGNAL

TTY

RSL - 2

PROCESS, SORT,

AND STORE DATA

4

I------t~ REAL-TIME GRAPHS

i \ OF RSL .---..V DURING FADES

6

11 PAPER/MIN

MINICOMPUTER, PRINTER/PLOTTER.

C.I.u., AND

VOLTMETERS ",,/,

36,000 DATA POINTS EACH HOUR RECORDED FOR

EACH ICOHA.NNELO~ 100b

ONE MONTH'S DATA/CASSETTE FADE DATA

DUAL MAGNETIC TAPE CASSETTE

Figure 3. Data flow diagram for· RSL-2 recording

system.

5
and 99.9% of the hour) for each of the histograms is also printed

out each hour. This is called a quick-look analysis and an

example is shown in Table 1. Also, a simulated distribution for

all four channels is computed by additive combination of signal

power and listed in the right hand column of the printout; it

represents optimum system performance, provided all radio hardware

(particularly the combiner circuitry) is properly functioning and

adjusted. If the signal level'fades below a p~e-set threshold

for the normally line-af-sight upper two paths on receiver A or

B, a fUlly annotated, cal~brated, digital, 4-channel strip chart

is printed on a printer-plotter for the duration of the fade plus

a pre-set I-min interval.* Chart speed is 6 in/min. An example

is shown in Figure 4 that also illustrates the relatively high

and steady signals over the lower paths C and D occurring frequently when the signal over paths A and B is in deep fades.

Table 1. Sample Printout for Hourly "Quick-look"

Analysis - Distribution Levels in dBm

% Time That Recvr A Recvr B Recvr C Recvr D Combined

Level is

Exceeded

10%

50%

90%

99%

99.9%

-43 -40 -34 -37 -31

-46 -43 -37 -40 -34

-55 -58 -40 -43 -37

-70 -70 -52 -46 -37

-79 -82 -55 -49 -40

The measured data are stored in a buffer for 10 s, and the

output to the printer-plotter is taken from the oldest of the

buffered data. Thus, the displays start prior to the leading

edge of the fade. At the end of the fade interval (fade

duration plus 1 min) the histograms, with starting and ending

times, are recorded in a file on a magnetic tape cassette.

Hourly data histograms and calibrations are recorded periodically on tape for later evaluation. Each of these cassette

tapes can store one month of data. The fade tape capacity is

340 "events" regardless of duration.

*The effect of this delay on fade duration statistics will be

discussed in Section 4.3.

6
..,.,

:>

c::;, I 05 f'T'1

--f

::I:

:::c

rT'1

en I --I 10 ::I:

c::>

rc::::;:)

I I

15

20

25

1121949 r

II l

30

35·

40

35

30

10

15

25

05

20

.40

RECEIVER C RECEIVER B RECEIVER A

-25 -85 -55 -25 1121948 -~5i -55 -25 -85 -55 -25 30 , i i i i i i i i i Ii i , i C i i i i i i i i , 30

35 I..,., - 35 ..,.,

:> :> c::;,

c::;, f'T'1

f'T'1 --f

40 t- I~ 40 ::I:

:::c :;::ID f'T'1

rTI en en ::I:

::I: c::> c::> r45 r- 45 c::;,

t::::'

50 50

55 55

1121949

>o

rTI

--f

::x:

:::c

,.."

en

:x:

c::>

rt::::'

RECEIVER 0

-85 -55

....J

Figure 4. Sample of digital strip chart recording.
Data acquisition and sorting is done in about 10 ms, leaving 90 ms between clock ticks available for other tasks such as

further processing when the teleprinter is not actually printing

any data. Processed data include a histogram and cumulative

distributions of received signal levels for any hour, day, month,

or fade period. Any set of calibration voltages maybe read

from the monthly tape for quality assurance checks of system

performance.

4. MEASUREMENT RESULTS

Signal level distributions and fade event statistics were

obtained over the period from mid-July 1974 to the beginning of

June 1975. During this time, the radio system and the RSL-2

equipment were operating for approximately 3650 hrs distributed

over the entire year so that all seasons are represented. This

constitutes the data base for the analyses of signal level

'distributions and fade duration.

4.1 Signal Level Distributions

Figur~ 5 is a plot of instantaneous signal level distributions for all four receivers obtained from the RSL-2 equipment

at the rate of 10 data points per second. Approx.lrnat.eLy

130 x 106 samples for each receiver are represented here. The

data are p~otted on Rayleigh paper and compared with the

theoretical Rayleigh distribution. Qualitatively, the approximation appears to be good for the data from the line-of-sight

paths (receivers A and B) suggesting mUlti-component multipath as

the principal cause of fading. The data from receivers C and D

deviate substantia~ly from the Rayleigh distribution, and the

effect of super refraction during small percentages of time is

quite evident from the steep rise of the curves toward the freespace level of -32.9 dBm for small percentages of time. The graph

also shows the calculated distribution for all signal powers combined additively (see the, discussion in Section 3 in conjunction

with Table l)~ The apparent system performance improvement is

about 12 dB for 99.9% of the time over the performance of a single

8
6

""",

"0,

RECEIVER B

'<,

~""l:i

"~RAYLEIGH SQUARED

"....,'....

........_--

RAYLEIGH

99.8 99.999.95 99.98 99.99 99.995 99.998 99.999 99.9995

-- ....

j "' "'~"' "

98 99

DATA TAKEN BETWEEN MID 1974 AND MID 1975

DATA RATE = 10/SECOND

I I I I

,

x

"

-,

RECEIVER D~'x-,,

,

5 :20 40 50 60 70 80 90 95

\

x

\

\

x

\

\ x,

-80 I I JIlt, ), > J

COMBINER OUTPUT COMPUTED

FROM INSTANTANEOUS SAMPLES

I I .... , I

':::...::.......

~o -50 I ,\ "' . "

-40 I >5t

-60 I '.... ',,= \: "" '" .... c

E

aJ

"C

.:

:::J

en

a:

--' LU

>

LU

--'

--' «

2:

t!J

en

0

LU

>

W

U

LU

a:

% OF TIME ORDINATE IS EXCEEDED

Figure 5. Received signal level distributions (all data).
line~of-sight path. This combined distribution follows the

"Rayleigh squared" curve as postulated by White for uncorrelated

fading (White, 1970).

Note that this graph represents the distribution of

instantaneous signal level values and should not be comp~red

with calculated distributions of hourly medians that would tend

toward a normal distribution.

From the system parameters (Zebrovitz, 1975), the received

signal level corresponding to line-of-sight transmission through

the atmosphere without the effects of reflections or multipath

is -32.9 dBm. The parameters are listed in Table 2 below.

Figure 5 shows that the median ~eceivedsignal levels· over the

measurement period differ from this value: they are -38.7 dBm

for receiver A, and -41.3 dBm for receiver B. This discrepancy

as well as the difference between the results from the two

receivers may be caused by uncertainties in equipment parameters, antenna alignment, and possibly by path or atmospheric

parameters not considered in the calculations.

Table 2. Path and Equipment Parameters

Pathlength: 87.84 km

Carrier frequency (mean for

receivers A and B):

Free-space basic transmission loss:

Antennas:

Antenna gain:

Transmitter power:

Estimated wave guide and

related losses:

Average atmospheric absorption:

Estimated average annual mean

temperature:

10

4.908 GHz

145.1 dB

4.57 m (15 ft)

paraboloids

44.8 dB (relative to

isotropic)

6.3 W (+38 dBm)

14.5 dB

0.8 dB

10 0 C
4.2 Comparisons with Fading Models.

Prediction models for fade ~epth distributions over microwave line-of-sight links have been developed by a number of

workers (Barnett, 1972; Morita, 1970). At the XIV Plenary

Assembly of the CCIR, use£ul formulations of these models were

adopted and included in Report 338-3 (CCIR, 1978). This formulation is based on a general expression with specific constants

for Japan, Northwest Europe, and the united States, and is intended to be valid for the "worst fading month". Consequently,

the probabilities of fade occurrence calculated in this manner

should be converted from "worst month" to "all year" for comparison with the data described here. Vigants (1975) suggests a

procedure for doing this ca~culation as a funct.i-on of the

annual mean temperature over the path.

Using the applicable path parameters from Table 2, the

model distributions were calculated in accordance with the

CCIR formulation for Northwest Europe and plotted together

with data points for receivers A and B as shown in Figure 6,

but only those for deep fades (at least 20 dB below the yearly

measured median) where the Rayleigh d~stribution postulated

by the models (CCIR, 1978) is most applicable. Agreement between

the data and the model distributions is reasonable but illustrates

the uncertainty in extrapolating from "worst month" to "all year"

on the basis of an annual mean temperature only. In fact, the

data tabulations do not suggest a particular "worst month";

fading is~distributed rather uniformly over the year.

Zero reference for the ordinate in Figure 6 is the mean

of the yearly median signal levels derived from the receiver

A and receiver B measurements (see Figure 5).

Additional calculations have shown that the model given

in CCIR Report 338-3 (1978) is not applicable to this path

since it greatly overestimates fading occurrence. This is

not surprising since this model was derived primarily from data I»

for much shorter overland paths in the midwestern united States.

11
-10

.. :z: 10

es

LU

:E:

LU

0:::

20

<C

LU

:IE

:-

c:>

--J

30

40

o ENGLISH CHANNEL DATA (RECEIVER A)

o ENG LISH CHA'N NELOATA (REC EIVERB)

CCIR worst month"

~D,

o 0

c

0.99 0.9 0.5 0.1 0.01 0.001 0.0001 0.00001

FRACTION OF TIME, P, BELOW LEVEL

Figure 6. Comparison of measurement results with

CCIR signal level predictions.

12
4.3 Fade Event Durations.

As explained in Section 3, the RSL-2 equipment automatically acquires, stores, and calculates statistics of fade

event duration at pre-set received signal levels. Datawere

obtained throughout the measurement period for receivers A

and B; i.e., for the line-of-sight paths between the high

antennas (see Figure 2). The pre-set fade threshold for all

data was -75 dBm which is 35 dB below the average of the

all-year signal level medians for the two receivers.

The recording and analysis routine inherent in the RSL-2

equipment emphasize&what we choose to call "fade events"

rather than individual fades. The start of a fade event is

defined as that instant when the received signal level from

either receiver A or receiver B drops below the -75 dBm

threshold. The computer then sorts and stores signal level

histograms for all four receivers until one minute after signals from receivers A and B have both risen above the threshold levels and have remained there. It records the total

elapsed time for receivers A and B signals at and below the

threshold level, and the total length of the fade event correcting for the one-minute time lag. A fade event may include

individual fades on both channels.

The time lag was introduced to allow for limitations. in

the peripheral hardware (particularly the tape drive); the

recording mechanism cannot completely follow a nurnberof short

fades in rapid succession. This limitation introduces a

significant bias (against short fades) in the statistical

evaluation of the p~esent data.

Thus, any additional deep fa~e below the threshold level

that occurs within the one-minute period after the first rise

above the threshold is not evaluated separately, but recorded

as part of the original fade. Consequently, any number of

short fades separated by less than one minute are lumped into

one single longer fade event. Processing methods inherent in

the computer and used for the original data recording methods

13
make it impossible to recover the number and durations of the

individual fades when separated by less than one minute.

Inspection of available graphic printouts suggest that many

of the fades w~re indeed isolated; i.e., separated by at

least one minute. However, Figure 7 shows an example of

multiple fades of short duration (about 1 s or less) within

a single fade event lasting approximately 45 seconds.

Figures 8, 9, and 10 show cumulative distributions of fade

event durations obtained in the manner described above. These

distributions are plotted on a log-hormalgrid; i.e. the absissas are linear in terms of 10 times the logarithm (to the base

10) of the fade event durations in seconds. Because of the

bias introduced by the recording method, the total number of

individual fades is greater than the number of fade events and

and their duration may be appreciably shorter as will be shown

later. The distributions are categorized into "time blocks" for

winter (Figure 8) and summer (Figure 9) which were introduced

originally by Norton, et ale (1955) to evaluate tropospheric

propagation data.* These categories provide an indication of

diurnal and seasonal changes in the fade event duration distributions. Summary statistics for fade event durations are given

in Table 3 below. Similarly, overall cumulative distributions

for all hours - winter, all hours - summer, and all hours of the

year are shown in Figure 10 with the statistics also listed in

Table 3. It is quite evident that the median fade event durations

are substantially greater in winter than in summer. However,

there are almost twice the number of fade events in summer than

in winter (781 versus 414). Nevertheless, the total time in

fades of 35 or more dB below the yearly median signal level is

still greater in winter than in summer.

*Thewinter time block comprises November through April, and the

summer time block May through October.

14
-25

RECEIVER A

-55

55

50

-25 -85

15

Received Sig no I Leve I (dBm)

Houtem to Swingote

RECEIVER B

-55

05

10

15

20 ---t

::::I:

-n ::x:J

::J> rT1

c::::J cr» rT1 25 :::c

c:::>

--f r-

::::I: c::::J

::::tJ

rT1 30 cr»

::c

c:::>

rc::::J

35

40

45

Figure 7. Example of multiple fades within a single

fade event.

2141719

-85

2141718
...... , .... ...... ......

MEDIANS

Sec

0- 6 6.0

6-13 12.0

13-18 6.9

18-24 9.0

"~~.

.~:~

ll.. r- ,

"-. . ,

~, <.

<, ." ••••

" " . " " . " "-

-, \".

" \ ".

. " '. "'.~ -. ",. -. .,~ ...

\~:- ..

,-.-.'.

\ \ ".

\ \'.

',".

~~'-. ' ..

.18'?t1

",.~

."" 0'8

WINTER MONTHS

1000 , iii(. iii iii iii I I I i

,"",100

fn

-a

c::

0

c.,)

CI,)

tn

...........

c::::>

t-

< 10

0:::

:::::>

t-

:IE

LU

:>

LU

I-' LLJ

O't ce

La0.1

0.01 0.1 2 10 20 30 40 50 60 70 80 90 98 99

PERCENT OF EVENTS FOR WHICH ORDINATE IS EXCEEDED

99.9 99.99

Figure 8. Fade event duration distributions for winter time blocks.
1000 i I I Iii iii iiiIii i ,

100 .....-..-.

tn

-a

C

0

U

CI)

tn

.........,

:z

c::::>

t= 10

<C

c:::

::::>

t--

z:

I--J

LLJ

-......]

::::-

LLJ

LU

<C

La-

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. I.>

'V,/

" /& /8.... ~A" ' •

. " ................. <,

~~.~. ", ...........

-~"".:.:--'. '. , \

8~ij""" . '<\

\ . . '"

\ \ . "' '\

0. ~'.\ .... '-0

.. ,. '. ,~

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'. v

'. \\

...\'"

".','" ....\ '\

0\ . 00' \

00",<•."" .

SUMMER MONTHS ,.."":':~ "'. ..

'~..

MEDIANS

Sec

0-6 3.16

6-13 1.46

13-18 - 2.0

18- 24 . 2.57

0.01 0.1 2 10 20 30 40 50 60 70 80 90 98 99

PERCENT OF EVENTS FOR WHICH ORDINATE IS EXCEEDED

99.9 99.99

Figul~e 9. Fade event duration distributions for summer time blocks.
1000 I I · I K I I I I I I I I I I I I I

Sec

8.12

2.24

3.16

MEDIANS

WINTER

SUMMER

ALL YEAR

'.

ALL YEAR

S'(/~ '~Ep

<, <,rr ~#; <, , ' ••-1« ~J: <, ' .. {s q, "" , . ·f4>. " " ....... .e. " . " ' . " . <, ", ".

<, -, '.

<, "'. <, ". <, \'.

<. \ .... ~""

"'.,,,, . -, ')-..

, '.~ " .

\.... -. CC/tf \<..· "Ot/,l" ,. ~J (

-, ". ,/81..., -, '. <, a) -, '.

-, '.

-, '. ,e.-.'."".".'e.

100

..--...

en

-a

c::

<::)

u

Q)

tn

............

:z10

c:::>

t--

<C

et:::

I-' t--

co :iii!:

LU

::::-

LU

LU

c::::.

<

LL..

0.1

0.01 0.1 2 10 20 30 40 50 60 70 80 90 98 99

PERCENT OF EVENTS FOR WHICH OROI NATE IS EXCEEDED

99.9 99.99

Figure 10. Fade event duration distributions for all hours.
Table 3. F.ade Event Duration Statistics for Paths A and B,

Swingate - Houtem

Fade Event Durations

Median Mean Total Number

Hours seconds seconds St. Dev. ** of Fade Events

Winter: 00 - 06 6.8 3.8 9.1 91

06 - 13 12.0 6.7 11.7 123

13 - 18 6.9 6.1 10.1 91

18 - 24 9.0 5.0 9.7 109

Summ~r: 00 - 06 3.2 2.3 7.3 148

06 - 13 1.5 1.6 8.4 162

13 - 18 2.0 2.0 9.4 235

18 - 24 2.6 2.5 9.3 236

Winter: all hours 8.1 5.4 10.3 414

Summer: all hours 3.5 2.1 8.8 781

All Year: all hours 3.2. 2.9 9.5 1195

In terms of diurnal variations, the winter data show the

longest median fade event durations for morning hours (0600 -

1300), while in summer the longest median fade event durations

occur during night and early morning hours (0000 - 0600). This

may be related to atmospheric effects during and after sunrise

which generally occurs after 0600 in winter and before 0600 in

summer at the latitude where the path is located. Data are not

available for diurnal variations in the refractivity gradients

that could be associated with the fade event statistics.

The summary cumulative distributions of fade event duration

in Figure 10 are compared with a theoretical curve based on a

(long-term) median individual fade duration of 4.26 s derived

from (11) in CCIR Report 338-3 (1978). The same report cites a

standard deviation of 10 logI 0 (3.6 s) = 5.6 in accordance with

Bullington's work (1971). Note that the all-year median fade

event duration from Table 3 is less than the CCIR estimate for

the median duration of individual fades.

**Standard deviations are expressed in terms of 10 times the

logarithm (to the base 10) of the duration ratio.

19
We have already noted that individual fades for the

Swingate - Houtem path are frequently shorter in duration than

fade events. Although it is not possible to recover the

statistics of individual fades from the cassette records, the

comparison of fade event durations with signal level distributions for the -75 dBm threshold level permits an estimation of

the relation between the number and the duration of individual

fades and fade events. A sample analysis was conducted for

typical summer and winter data; also, some of the fade event

strip charts are available for inspection. The example shown in

Figure 7 is somewhat extreme since there are 8 individual fades,

each lasting about 1 s or less, for the single 47.3 s fade

event on receiver A.

This sample analysis resulted in median values of 2.3 individual fades per fade event for the summer example, and 2.1 for

the winter example. Consequently, on the average there will be

at least twice as many individual fades as fade events. Also,

the median duration of individual fades is likely to be not more

than about one-half of the median of the fade events. Even in

the absence of information permitting more complete statistical

analyses, it would appear that the CCIR model tends to overestimate median fade duration for this path, and possibly for

other paths of this type; i.e., long over-water links in the I

4 to 5 GHz frequency range. A comparison of the slopes for the

theoretical and the measured distributions shown in Figure 10

is not appropriate because of the bias introduced by the measurement methods.

5 . APPLICABILITY OF ~ESULTS TO SYSTEMS IN OTHE'R AREAS

The microwave communication link evaluated in this report

is somewhat atypical because of its length (88 km) and environment (across water). It is, however, possible to apply the

results to other over-water and coastal areas since fading

characterisitcs are related to terrain and atmospheric stratification. A measure of the latter are statistics o£ refractivity gradients. Detailed, gradient statistics for the exact path

location during the measurement period could not be obtained.

20
The points closest to the path for which long-term cumulative

distributions of the refractivity gradients are available are

Cardington, England (north of London), and Brussels, Belgium

(Samson, 1975). At these two locations, ground-based superrefractive layers (N-units/km < -40) at least 100 m thick occur

for 60 to 70% of the observations (twice daily at Brussels and

four times daily at Cardington). Also, gradients more negative

than -250 N-units/krn were observed for approximately 0.1% of the

occurrences at Cardington in the groundbased layer up to 500 m

above ground.

The compilations of gradient statistics prepared by

Samson (1975, 1976) illustrate the preponderance of superrefractive gradients in most coastal locations in the United

States. Examples of locations for which such gradients in the

ground-based IOO-m layer occur during at least 25% of the

observations are Anchorage and Barrow, Alaska; Long Beach,

Oakland, and San Diego, California; Key West and Miami, Florida;

Washington, D.C.; Hila, Hawaii; Caribou, Maine; New York, N.Y.;

Cape Hatteras, ,North Carolina; Seattle, Washington; and Brownsville, Texas. Even ducting gradients are relatively frequent

at these locations with the highest incidence found at Hilo and

Cape Hatteras (>9%).

It may therefore 'be concluded that the results of the

measurement program described here may be applicable to most

coastal areas in North America and to links in Pacific Island

chains. We must remember, however, that the path discussed

here is unusually long for a line-of-sight link, and that the

antennas for ,the line-of-sight situation are by necessity high

above the surface.

The results could also be applied to some inland locations

in continental climates where the path extends over relatively

smooth terrain and super-refractive gradients exist for appreciable fractions of the observations. Examples, during the

summer months, are 'Denver, Colorado; Bismarck, North Dakota;

21
and Dayton, Ohio. However, terrain influences the homogeneity

of the gradient structure, and a careful study of the terrain

is essential in these cases.

6. CONCLUSIONS

The.purpose of this study is to check the measurement

results against available prediction models, particularly those

recommended by the CCIR. Agreement between predicted signal

level distributions and those derived from the measurement~ is

fair in the deep fading region where the CCIR model applies.

The results, if representative of a full year, do not confirm

the- validity of extrapolating from worst month to all year on

the basis of annual mean temperature as suggested by Vigants

(1975). However, the maritime climate characteristics of the

path environment tend to lessen seasonal changes and make the

concept of a "worst month" less applicable.

Because of the limitations in the measurement techniques,

precise statistics of individual fades could not be obtained.

From the analyses and reasoning described in Section 4, it appears

that the median duration of individual fades is not more than

approximately one-half of that expected from available fade event

models. Total fade event durations are longer in winter than in

summer while the number of fade events is larger in summer.

Assuming that fading characteristics are primarily a function of refractivity gradient statistics, the results of the

measurement program are applicable almost universally over water

or smooth coastal terrain since the gradient distributions for

the area of the path are not unique. Because of its length,

however, the path is not typical of overland communication

links. Since the super-refractive gradients that favor fade

events occur frequently even inland in continental climates,

allowance for substantial fading must be made for long links

unless the terrain can be expected to inhibit homogeneous

stratification over the path area.

Finally, the measurement results confirm the usefulness of

the original design concept for this path. Because of the link

22
length and inadequate clearance (under generally accepted

standards), extensive fading exists on the nominally line-ofsight paths between the upper antennas. The quadruple diversity system utilizing paths between lower antennas that are

normally blocked, but can become active during'ducting conditions, appears to work well, and_insures adequate communication

reliability. It is reasonable to expect that similar designs

will work equally well in most coastal or maritime environments where super-refractive gradients exist for large fractions

of the time.

7. ACKNOWLEDGEMENTS

The authors wish to thank D.D. Crombie and H.T. Dougherty

for their careful review and pertinent suggestions.

8 . REFERENCES

Barnett, W.T. (1972), Multipath propagation at 4, 6, and

11 GHz, Bell System Tech. J., 51, No.2, 321 - 361.

Bullington, K. (1971), Phase and amplitude variations in

multipath fading of microwave signals, Bell System

Tech. J., 50, No.6, 2039 - 2053.

CCIR (1978), Propagation data required, for line-af-sight

radio-relay systems, Report 338-3, Documents of the

XIV Plenary Assembly (ITU, Geneva).

Morita, K. (1970), Prediction of Rayleigh fading occurrence

probability of line-of-sight microwave links, Rev.

Elect. Comm , Lab,., NTT, Japan, 18, 11 -12.

Norton, K.A., P.L. Rice, and L.E. Vogler (1955)" The use of

angular distance in estimating transmission loss and

fading range for propagation through a turbulent

atmosphere over irregular terrain, Proc. IRE 43,

No. 10, 1488 - 1526.

Samson, C.A. (1975) ," Refractivity gradients in the northern

hemisphere, OT Report 75-59, Office of Telecommunications (Supt. of Documents" U.S. Government Printing

Office, Washingtpn, D.C. 20402).

23
Samson, C.A. (1976), Refractivity and rainfall data for

radio systems engineering, OT Report 76-105, Office of

Telecommunications (Supt. of Documents, U.S. Government Printing Office, Washington, D.C. 20402).

Vigants, A. (1975), Space diversity engineering, Bell System

Tech. J., 54, No.1, 103 - 142.

White, R.F. (1970), Engineering. considerations for microwave

communications systems, Lenkurt Electric Co., Inc.,

San Carlos, California.

Zebrovitz, S. (1975), The use of multiple diversity to

minimize the effects of ducting on a long microwave

path, Nat. Teleconun. Conf. 1975, Conference Record,

10-23 - 10-28, (IEEE, New York, N.Y.).

24
FORM OT-29 (3-73)

u.s. DE PARTMENT OF COMMERCE

OFFICE OF TELECOMMUNICATIONS

BIBLIOGRAPHIC DATA SHEET

1. PUBLICATION OR REPORT NO~ 2. Govt Accession No.3. Recipient's Accession No.

NTIA Report 79-30

4. TITLE AND SUBTITLE

SIGNAL LEVEL DISTRIBUTIONS AND FADE EVENT

ANALYSES OF A 5 GHz MICROWAVE LINK ACROSS

THE ENGLISH CHANNEL

7. AUTHOR(S)

D. R. Wortendyke, A.P. Barsis and R.R. Larsen

8. PERFORMING ORGANIZATION NAME AND ADDRESS •

U.S. Department of Commerce, Natlonal Telecommunications and Information Administration,

Institute for Telecommunication Sciences,

Boulder, CO 80303

5. Publication Date

October 1979

6. Performing Organization Code

NTIA/ITS

9. Project/Task/Work Unit No.

9108240/9103484

10. Contract/Grant No.

l:i:.TSSonsoring Organization Name and Address

U.S. Dept. of Conunerce

Boulder, CO 80303

14. SUPPLEMENTARY NOTES

1842nd E.E.G.

USAF

Scott AFB, IL

12. Type of Report and Period Covered

NTIA - T.R.

13.

15. ABSTRACT (A 200-word or less factual summary of most significant information. if document in cludes a significant

bibliography at literature survey, mention it h ere,')

This report describes instrumentation for fading studies and

measurement results over an 88-kmmultiple-diversity line-of~sight

microwave link across the English Channel operating in the

4 to 5 GHz frequency range. Signal level and fade duration statistics derived from the measurements are compared with CCIR models.

It is found that median fade duration tends to be less than

estimates from the CCIR models.

16. Key Words (AlphabetIcal order, s epera t ed by s etni col ons )

Automaticsign~l monitoring; 5 GHz signal levels; microwave fading;

multiple diversity receivers.

17. ;\ VAILABILITY STi\ TE~lENT

lxJ UN LIMITED.

:-~=.J FOR OFFiCIAL DISTRIBCTION.

18. Se c ur i tv Class (111is report)

Unclassified

!

Unclassified

I

20. Nurn be r of pages

27

21. P~lce.

'ttU.S. GOVERNMENT PR INTING OFFICE: 1979-0-682-175/162 l' 5C OMM· DC ,29716- P7.3