APR

jioulder laboratories

PB 161635

^o. 134

AIRBORNE TELEVISION COVERAGE

IN THE PRESENCE OF

CO-CHANNEL INTERFERENCE

BY

MARTIN T. DECKER

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

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NATIONAL BUREAU OF STANDARDS

technical <Vt©te

134

January, 1962

AIRBORNE TELEVISION COVERAGE

IN THE PRESENCE OF

CO-CHANNEL INTERFERENCE

by

Martin T. Decker

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide a means for making available scientific data that are of transient or limited interest. Technical Notes may be

listed or referred to in the open literature. They are for sale by the Office of Technical Services, U. S. Department of Commerce, Washington 25, D. C.

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Price $2.00

ABSTRACT

Predictions are made of the coverage to be expected from a network of airborne television transmitters operating in the UHF

television band. Various system performance and interference

conditions are assumed. The results are presented in a series of

graphs with probability of service as a function of receiving location

and in terms of the total effective area of a station or network of

stations. System requirements for a coverage approaching 100% of

a large area are indicated.

AIRBORNE TELEVISION COVERAGE

IN THE PRESENCE OF

CO-CHANNEL INTERFERENCE

by

Martin T. Decker

INTRODUCTION

A study of some of the technical factors involved in the planning

of an airborne television network has been undertaken by the National

Bureau of Standards under the sponsorship of the Ford Foundation. In

a previous report [ Decker, 1959] , the predicted coverage for a single

airborne station was described for a variety of operating conditions.

This report considers the problems of large area coverage, i.e., the

operation of a network of airborne stations in the presence of interfering

signals from co-channel airborne stations. Technical considerations

include expected coverage, equipment requirements for a specified

coverage, interference from co-channel stations, optimum geographical

spacing of stations, and number of stations and channels required for

a specified large area coverage. These factors are, of course, interrelated, and compromises between various requirements will be

necessary. In general, economic factors are also involved in the

decisions and compromises of the planning stages. This aspect has

not been considered in this report, except that a range of equipment

quality parameters has been chosen which is well within the capabilities

of current production techniques.
-2-

This study has resulted in a series of graphs which present a

statistical description of coverage in two forms . The first of these is

the probability that at a given location a specified picture quality, or

better, will be available for at least some minimum percentage of the

time. This probability is a function of the geographic location of the

receiving site with respect to the desired and interfering transmitting

stations. It is shown graphically as contours of location probability of

service for various operating conditions of the system. The second

form of the results of the calculations is a summation of "effective

area" of a station or of a network of stations and may be expressed in

square kilometers. For the case of a network of transmitters serving

a large area, it is also convenient to express this effective area as a percentage of the area receiving a specified service.

METHODS OF COMPUTATION

The method used to determine the effect of interfering signals

is basically that described by a special committee of the Federal

Communications Commission [ FCC 1950] . The method is also briefly

outlined by Norton, Staras and Blum [ 1952] . A simplified version has

recently been published by Livingston [ I960] . The method used is one

of a number of methods which have been proposed for calculating the

service area of broadcast type systems. The random nature of the

variations in signal strength with time, location, and some equipment

characteristics dictates that the method chosen must be capable of

handling these variations on a statistical basis. The methods which

have been proposed generally involve a compromise between 1) simplifying assumptions which decrease the accuracy of the results, and
-3-

2) complexity of calculation which increases time and expense of

computation. For example, in some methods, the problem of interference from more than one source is avoided by the simplifying

assumption that only one interfering source will be important and all

others may be neglected. This is evidently unsatisfactory when there

are a number of signals of approximately equal magnitude, as may well

be the case in some geographical configurations of stations. Other

methods do not consider the effects of system "quality" in terms of

transmitter power, receiver noise figure, or various system losses.

The method chosen for considering interfering signals is admittedly

complex, and yet it is felt to be necessary in order to adequately

assess the effects of various system parameters.

Without presenting in detail the steps involved in the computation

(for which the reader is referred to the FCC document), a few comments

on this method and some of the underlying assumptions are listed here.

As in the previous report [Decker, 1959, hereafter referred to

as Technical Note 35], the description of signal strength is in terms of

"basic transmission loss" and its variations. It is applicable to both

desired and interfering stations. Technical Note 35 contained a series

of curves showing basic transmission loss as a function of distance for

propagation over a smooth earth. These curves illustrated the effects

of energy reflected from a smooth surface, resulting in destructive

addition of signals at certain locations. While these minima of signal

strength, or "nulls", may or may not occur in practice, depending upon

the local terrain conditions, their possible adverse effects should not

be overlooked. In many cases where nulls exist, an appropriate antenna

height may be selected to take advantage of relatively broad lobe maxima.
-4-

In these computations it is assumed that if nulls do exist at a given

location, and satisfactory receiving antenna height cannot be found,

then diversity receiving antennas will be used. The maximum receiving

antenna height is taken to be 30 meters.

Signal variations with time at any fixed location are assumed

to be log-normally distributed, i.e., decibel values of hourly medians

are normally distributed. The standard deviation of this time distribution of signals is a function of distance from the transmitter and is

derived empirically from long-term measurements.

The signal level which is exceeded for a fixed percentage of

time will vary from location to location in the area surrounding a

station. This is a result of the irregularity of the terrain. This location variation is also assumed to be a log-normally distributed function,

with a standard deviation of 6 db.

A further assumption is that the desired signals are not correlated

with signals from interfering stations, either with respect to time or location variations. This appears to be a reasonable assumption when

the signals arrive at the receiver from different directions [ Kirby and

Capps, 1956] .

In this method, the power contained in interfering signals is

added, so that any number of interfering signals combine to produce

an effective interfering field. These various signals are modified by

the directivity of the receiving antenna, depending on their direction

of arrival, and by the appropriate desired -to-undesired signal ratios

which may be required for satisfactory television reception. The noise

power at the input to the receiver is also included as part of the unde sired

signal which must be overcome for satisfactory operation. The approximate method for adding log-normal probability distributions is described

in the FCC document [ 1950] and also by Fenton [ I960] .
-5-

To estimate time variations of multiple interfering fields, the

assumption is made that the resultant signal power from these sources,

exceeded for a specified percentage of time, is equal to the sum of all

the individual power levels exceeded for that same percentage of time.

This implies the further assumption that signals arriving from various

interfering sources are correlated with each other, but not with the

desired signal. It was demonstrated in the FCC document that this is

a good approximation when time availability is >90%, regardless of

true correlation coefficient.

"Service," as defined in this study, exists at a receiving location

during any hour for which the hourly median signal received from the

desired transmitting station exceeds the hourly median of the sum of

the receiver noise power and interfering signal power after specified

signal-to-noise and signal-to-signal ratios have been taken into account.

These ratios must therefore be adequate to provide the require picture

quality in the presence of any short-term or within-the -hour variations

of the signals. The percent of hours for which this service is available

is referred to as "time availability" of service. The required time

availability has been fixed in this study at 99%. Thus, a given location

either has this service, or it does not; a map showing the service area

of a given station could be similar to Figure 1 . In this hypothetical

picture, the black areas represent those locations in which the service

requirements have been met. The summation of all these areas in this

case is a useful quantity- -the effective service area.

The statistical nature of the variations with location and the

fact that we are not considering a specific geographical area enable

us to predict only the likelihood (or probability) that any location will

receive the specified 99% time availability of service. The predicted

effective area may then be obtained by summing the incremental areas,
-6-

multiplied by the probability of service in that area. The utility of

the effective area concept is that it provides a realistic basis upon

which to make determinations and comparisons of the effects of any

parameters on the coverage obtained. Where there is a probability

of service from more than one station, the probability of getting satisfactory service from at least one of these stations is computed from

p= 1 - (1 - Pl )(l -p

2

). . . (1 -p

n

)

where p, , p„, . . .p are the probabilities of service for the individual

1 2 n

stations

An alternative representation of the service provided by a

station consists of contours which indicate the probability that locations

will receive the specified signal quality for 99% of all hours. In a

specific case these contours might be quite irregular as a result of

the particular terrain involved. In this study, however, an "average"

station is considered, and the resulting contours are smooth and

symmetrical. In the absence of interference, they appear as circles

centered on the transmitting station, and departure from the circles

is the result of interfering fields from stations operating on the same

channel.

THE SYSTEM PARAMETERS

A measure of equipment performance is provided by "maximum

allowable basic transmission loss" as explained in Technical Note 35.

The formula for the computation of this quantity is
-7-

L % = P-L+G+G-L-10 1ogb-F -R+204

b(max) t t t r r where:

P = Transmitter power, db above 1 watt

L = Transmitter line losses, db

G = Transmitting antenna gain, db above

an isotropic radiator

G = Receiving antenna gain, db above an

r isotropic radiator

L = Receiver line losses, db

r b = Effective bandwidth in cycles per

sec ond

F = Receiver noise figure, db. For a discussion of effective noise figure

see Barsis, et al. [ 196 1] . R = Signal-to-noise ratio required at the

receiver input for satisfactory receiver

performance, db

204 db is a constant equal to - 10 log kT, where

k is Boltzmann's constant, and the

reference temperature, T, is 288.39°K.

A higher number for L, , 5. could indicate better equipment

b(max) ^ ^ performance resulting from improved receiver characteristics, greater

antenna gains, better transmission lines, or more transmitter power.

Values considered in this study range from 135 db to 150 db for single

station coverage, while values of 140 db and 145 db are used in the

interference computations. A typical set of values for the terms in

the L. ,. equation was given in Technical Note 35. It is repeated b(max) & r here for illustration:
-8-

p

t 30 dbw

L

t 1 db

G

t = 5 db

G = r 20 db

L

r

3 db

b 6 Mc/s

F 10 db

R 37 db

These values result in a maximum allowable transmission loss of

140.2 db. The same value of L, ,. could, of course, be obtained

b(max)

in other ways. For example, by increasing the transmitter power and

allowing a comparable decrease in antenna gains.

In addition to the gain value for the receiving antenna, it is

also necessary to consider its ability to reject undesired signals

arriving from directions other than that of the desired signal. Here

two directivity patterns have been used, as illustrated in Figure 2.

Antenna No. 2 represents the better antenna since it has a greater

rejection for signals outside its main beam than Antenna No. 1.

Acceptable picture quality is defined in terms of signal-to-noise

ratios or desired signal-to-interfering signal ratios at the input to the

receiver. The quality of pictures as a function of these ratios and the

"off-set" of co-channel picture carriers has been studied by a number

of authors and organizations [ RCA, 1950; Beherend, 1957; Chapin, et al.

1958; Middlekamp, 195 8; TASO, 1959; Dean, I960; Towlson and Young,

I960; Fine, 1961] . In evaluating these effects, subjective judgments

are made and not all viewers will agree as to an acceptable ratio.

Selection of the proper ratio then becomes a statistical process.

The required signal-to-noise ratio is included in the expression

for maximum allowable transmission loss, L, , ,. Fine [ 19611 gives b(max)

a value of 35 db for a "fine" picture and 30 db for a "passable" picture.
-9-

It is possible to reduce the adverse effects of interfering

co-channel signals by maintaining a carefully controlled frequency

difference, or "off-set," between co-channel stations. Further

improvement can be obtained by the operation of geographically adjacent

stations on different antenna polarizations [ Kuppelhoff, 1954; Peterson,

1958; C.C.I.R., 1959]. These two factors are combined here to obtain

the protection ratio, P, as indicated in the various curves and graphs.

Values of 27, 20, and 12 db have been used. It should be noted that

the doppler effects of the aircraft motion will make the use of very

precise (± a few c/s) off-set impractical, but that off-sets which require

a stability of ± 1000 c/s may be used.

A radio frequency of 785 Mc/s has been used to compute the

propagation effects. This frequency was chosen as the center of the

upper half of the UHF television band, and should be representative

of the performance in that portion of the frequency spectrum. Certain

comparisons of single station coverage were made at 575 and 785 Mc/s

in Technical Note 35.

Aircraft flight altitudes for the computations involving co-channel

interference are 7500 meters and 10,000 meters. The aircraft was

assumed to be flying in a circle of 15 kin radius. At any given receiving

location, the performance is computed as though the desired aircraft

were on the far side of its circle and all interfering aircraft on the near

side of their circles.

Two geographical configurations of co-channel stations have

been considered for the interference computations. It is realized that

an actual network of stations would probably not have a symmetrical

arrangement. However, it is likely that the stations, as they become
-10-

more numerous, would tend to approach a triangular lattice arrangement, as illustrated in Figure 3. This three -channel triangular lattice

is taken to be the most saturated arrangement of stations, while the

least saturated case (other than no interference) is the case for two

isolated co-channel stations.

In the absence of interference from co-channel stations, the

integrated "effective area, " as previously defined, may be shown

graphically with maximum allowable transmission loss and transmitting

antenna height as parameters. This is illustrated in Figure 4. The

advantages to be gained by improving system quality or increasing

aircraft height are evident from this figure, and could be compared

with the cost of providing these improvements. An experimental

system, currently planned (1961), operates in the range of 140 db to

145 db L, . . and at an aircraft height somewhat below 7, 500 m.

b(max)

As mentioned previously, the results of the study considering

the effects of interference are presented in two forms. The first of

these is a series of diagrams showing contours of constant probability

that locations will receive a specified signal quality for 99% of all

hours. The general effect (on the contours) of bringing stations

closer together in the simple two-station case is illustrated in Figure 5a.

For the conditions assumed and at a separation of 700 km, the contours

are nearly circular, indicating that very little mutual interference

exists. As the station separation is decreased to 600 kin and 500 km,

interfering effects are evident.

When considering the contour diagram of a single station in a network of stations, it is likely that more than one interfering station

will have to be considered. An idealized "triangular lattice" arrangement
-11-

of stations has been selected to represent this more nearly "saturated"

arrangement of stations. The arrangement of stations in this case is

illustrated in Figure 3. Note that any station in the network will be

surrounded by six co -channel stations at 60° intervals at a uniform

separation distance. This is the "co-channel separation." Other

co-channel stations which may contribute interfering fields are located

at greater distances as illustrated. In this study, interference is

assumed to be produced by co -channel stations only, and the stations

labeled B and C in Figure 3 do not interfere with the A stations. All

channels are considered, however, when the area served by a threechannel network is computed. Figure 5b is an example showing the

effect of changing the co-channel separation in a triangular lattice.

For simplicity, only the 90% contours are shown. While a qualitative

idea of the change in service area of a single station may be obtained

from this type of representation, it does not show the change in

effective area per channel, an important parameter which will be

discussed in detail later.

The contour diagrams for the various conditions considered

are arranged in a "catalog," Figures 9 through 56. The table of contents may be consulted to determine the figure number illustrating any

specific combination of 1) maximum allowable basic transmission loss,

LL/ ., 2) receiving antenna, 3) protection ratio, 4) aircraft height, b(max) r 6

and 5) geographical configuration. The diagrams are arranged so that

the effect of changing co-channel separation only may be seen in a

single figure. That is, for a given combination of the first four parameters listed above, all diagrams for different separation distances

are included in one figure. In some cases, the contours for the shorter
-12-

separation distances have been omitted since the resulting service

areas were too small to be of interest. The contour diagrams for the

triangular lattice also show for reference the interference -free service

contours as dashed lines.

The second method of presenting the results of this studyinvolves the use of the "effective area" concept as explained in the

section on methods of computation. When considering the area served

by more than one station, it is important to recall that the area served

by a single station, as illustrated by Figure 1, is not really bounded

by a single fixed contour, but service is available to decreasing percentages of the area out to great distances. It follows, that in order to

cover a high percentage of a given area, a certain amount of overlapping

and interference will inevitably occur. Hence, the area served by n

stations is not necessarily n times the area served by one isolated

station. However, in a network of stations and for the application

under consideration, the effective area of a single station is not the

most valuable criterion. A better method for examining the performance of a network of stations will show the percentage of a large area

which will receive a specified service. Here we have considered the

network of stations to be arranged in a triangular lattice, and the area

to be large enough so that the departures from uniformity at the edges

of this area may be neglected. The actual minimum area required to

make this approximation valid will depend on the separation distance of

stations in the triangular lattice. For the largest separations considered,

this area would be on the order of half of the United States.

The resulting integrations are shown graphically in Figures 6

and 7. Information in Figure 6 is for an aircraft height of 7, 500 meters,

while Figure 7 is for 10,000 meters. In each of the figures, the percent
-13-

of a large area receiving a specified service from at least one station

is plotted versus the separation of co-channel stations. The percent

of area that could be served by a single channel or by three channels

is indicated in each case. The relatively high percentage in the single

channel case is a result of the nature of the airborne system as well

as the use of directional receiving antennas. However, it is clear

that the single channel arrangement will not provide service to more

than about three-quarters of a large area. In order to achieve a

"blanket" coverage, additional channels are required. Area coverage

approaching 100% can be achieved with the three -channel systems.

Some estimate of the total number of stations required to serve

an area such as the continental United States may be triad e using

Figures 6 and 7. With the assumption that stations operating on

channels A, B and C are arranged in the ideal triangular lattice of

Figure 3, the approximate number of stations in the United States is

as shown in the following table:

TABLE I

Co-Channel Total Number

Separation of Stations

km

600 75

700 55

800 42

900 33

1000 27

The percentage of this area which has satisfactory service

available may be read from the various curves in Figures 6 and 7. It

is very likely that in any firm plan for large area coverage there

would be departures from the ideal triangular lattice of stations. Hence,

numbers such as those given here are only approximations.
-14-

In the calculations for the better receiving antenna (No. 2), it

is assumed that this antenna is used at all locations. It is clear that

this very good antenna would not be required at all locations in order

to achieve the coverage shown in the curves labeled "antenna No. 2."

Figure 8 is included to give an indication of the percent of receiving

locations which would actually require an antenna with better performance than that of antenna No. 1 in order to achieve the coverage of

the "antenna No. 2" curves. These locations would require antenna

performance better than No. 1 but in no case better than No. 2. The

figure shows that the number of locations requiring a better antenna

is a relatively small part of the total.

ACKNOWLEDGMENTS

The author wishes to acknowledge the contributions of the

persons who performed the many calculations required. They included

L. G. Hause, G. D. Gierhart, J. E. Farrow, H. R. Dahms,

M. J. Miles, J. S. Miller, and K. A. McElfresh. The manuscript

was typed by Miss S. J. Bush.
-15-

REFERENCES

Barsis, A. P., K. A. Norton, P, L. Rice, and P. H. Elder,

Performance predictions for single tropospheric communication

links and for several links in tandem, NBS Technical Note 102,

PB 161603 (Aug. 1961).

Behrend, W. L. , Reduction of co-channel television interference byprecise frequency control of television picture carriers, IRE

Trans, on Broadcast Transmission Systems, BTS-7 , 6-14

(Feb. 1957).

C.C.I.R., Documents of the IXth Plenary Assembly, III, 155-157,

Los Angeles (1959).

Chapin, E. W. , L. C. Middlekamp and W. K. Roberts, Co-channel

television interference and its reduction, IRE Trans, on Broadcast

Transmission Systems, BTS-10, 3-24 (June 1958).

Dean, C. E. , Measurements of the subjective effects of interference

in television reception, Proc. IRE 48, No. 6, 1035 -1049 (June I960)

Decker, M. T., Service area of an airborne television station, NBS

Technical Note 35, PB 151394 (Oct. 1959).

Federal Communications Commission, Report of the Ad Hoc Committee

for the evaluation of the radio propagation factors concerning the

television and frequency modulation broadcasting services in the

frequency range between 50 and 250 Mc . , Vol. II (July 7, 1950).

Fenton, L. F. , The sum of log-normal probability distributions in

scatter transmission systems, IRE Trans, on Commun. Systems,

CS-8, No. 1, 57-67 (Mar. I960).

Fine, H. , A further analysis of TASO Panel 6 data on signal to interference ratios and their application to description of television

service, IRE Trans, on Broadcasting, BC-7, No. 1, 22-38

(Jan. 1961).

Kirby, R. S., Measurement of service area for television broadcasting,

IRE Trans, on Broadcast Transmission Systems, BTS-7, 23-30

(Feb. 1957).
-16-

Kirby, R. S., and F. M. Capps, Correlation in VHF propagation over

irregular terrain, IRE Trans, on Antennas and Propagation,

AP-4 No. 1, 77-85 (Jan. 1956).

Kuppelhoff, F. H. , Advantages gained by considering polarization in

the planning of VHF and UHF broadcasting services,

Fernmeldetechnisches Zentralant der Deutchen Bundespost,

Technical Report No. 5501 (Sept. 15, 1954).

Livingston, D. C, Presentation of coverage information, Proc. IRE

48, No. 6, 1102-1112 (June I960).

Middlekamp, L. C, Reduction of co-channel television interference

by very precise offset carrier frequency, IRE Trans, on Broadcast

Transmission Systems, BTS-12, 5-10 (Dec. 1958).

Norton, K. A., H. Staras and M. Blum, A statistical approach to the

problem of multiple radio interference to FM and television

service, IRE Trans, on Antennas and Propagation, PGAP-1,

43-49 (Feb. 1952).

Peterson, D. W., The use of vertical polarization to solve UHF

television "ghosting" problems in a shadowed valley, RCA Review

19, No. 2, 208-215 (June 1958).

RCA Laboratories Division, A study of co-channel and adjacent-channel

interference of television signals, Pt. I, RCA Review 11, 99-120

(Mar. 1950).

Television Allocations Study Organization (TASO), Engineering aspects

of television allocation, Report to the Federal Communications

Commission (March 16, 1959).

Towlson, H. G., and J. E. Young, Findings of TASO Panel I on

television transmitting equipment, Proc. IRE 48, No. 6,

1081-1087 (June I960).
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V. S. DEPARTMENT OF COMMERCE

Luther H. Hodges, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director

THE NATIONAL BUREAU OF STANDARDS

The scope of activities of the National Bureau of Standards at its major laboratories in Washington, D.C., and

Boulder, Colorado, is suggested in the following listing of the divisions and sections engaged in technical work.

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its title. A brief description of the activities, and of the resultant publications, appears on the inside of the

front cover.

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Prediction Services. Sun-Earth Relationships. Field Engineering. Radio Warning Services. Vertical Soundings Research. Radio Propagation Engineering. Data Reduction Instrumentation. Radio Noise. Tropospheric Measurements.

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Research. Antenna Research. Navigation Systems.

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Scatter. Airglow and Aurora. Ionospheric Radio Astronomy.