ADDENDUM

A PRELIMINARY ESTIMATE OF THE EFFEC,]~S OF SPREAD SPECTRUM

INTERFERENCE ON TV

John R. Juroshe:k

The material on pages AD-I, AD-2, and AD-3, AD-4 (inside back

cov1er) is to be appended to Section 5, l~djacent Channel Interference.

It is possible to estimate the extent of interference from a signal in

an adjacent channel using standard methods if the emission spectrum of a

spread-spectrum system is known. The total i.nterfering power from a single

adjacent channel interferer is (CCIR, 1974)

I

f

2

f s(f) p(f) df

f

l

(AD-I)

where f is the radio frequency and p(f) is the power density (watts/hertz) of

the interfering emission at the receiver ante~nna terminals. The selectivity

function" s(f) t is the fraction of the power at frequency f that is admitted

to the r·eceiver. The limits of integration aLre chosen to include all frequencies with appreciable power.

Figure AD-Ion page AD-3 shows an example TV selectivity curve with an

interfering spread-spectrum emission in the uLpper adjacent channel. Notice

that the vertical scale is logarithmic. The in-channel portion of the TV

selectivity is the FCC cw protection criteria shown in Figure 8. The adjacent channel (6-12 MHz) portion of the curve was drawn to be above the measured CUl:;"ves in Figure 8. Comparison with da.ta in FCC Report LAB-74-0l

(Roberts and Middlekamp, 1974) shows that about 90 percent of the TV receivers

measured had adjacent channel rejection this good or better.

The spread-spectrum emission power density in Figure AD-I is a close

approximation to the emission of the Extended Area Test System (EATS) measured by Haakinson et ale (1977). The 4-MHz bandwidth of EATS is near the

maximum 1:hat could be assigned in a TV channel, and the emission shape has

been realized i.n the prototype hardware. Thus, the emission shown in Figure

AD-I is a realistic possibility.

AD-I
Figure AD-2 shows the product s{f)p(f). The area under this curve is

proportional to the total interfering power in the TV receiver. The large

area on the right is due to the main portion of the 55 emission, reduced by

the TV selectivity. The smaller area on the left is from the "skirt" of the

55 emission that overlaps the wanted TV channel. For this case, the area on

the right is 1.3 (10"4) and the area on the left is about one-fifth of

that--2.5 (10-5 ).

The emission spectrum of a conventional FM LMR is about 16 kHz wide. To

have the same total power as a 4-MHz 55 emission, the power density would

have to be 250 times as great. If plotted on Figure AD-I, the emission would

be about the width of a line and would extend off the top of the page. The

l6-kHz channel is so narrow that the TV selectivity is essentially constant

over it. If the FM emission is placed in the middle of the adjacent channel

-4 at 9 MHz, it produces 1(10 ) total interference power--about 2 dB less than

that of the 55 system.

The spread~spectrum system has greater interference power because some

of its main emission is very near the wanted TV channel and because some of

the skirt of the emission overlaps the channel. If the FM channel were

placed nearer the TV channel, it would produce more interference. For example,

if it were at 7 MHz, the interference would be 2.5 (10-4)_-about 2 dB more

than the 55 system. In either case, the FM LMR system and the 55 LMR system

with the same total power produce about the same interference.

Now consider the effect of varying some of the parameters. If the 55

bandwidth is reduced to 2 MHz with the same total power and emission shape

-4 and centered in the adjacent channel, it produces about 1.1 (IO ) interference. This is almost exactly the same interference as the FM system, because

now the 55 power density does not appreciably overlap the wanted TV channel.

On the other hand, retain the 4-MHz 55 emission and suppose that the TV

receiver has 20 dB more adjacent channel rejection. [Measurements show that

the best 10 percent of the receivers have 20 dB better adjacent channel

rejection than the worst 10 percent (Roberts and Middlekamp, 1974).] Then

-6 the FM interference would be 1(10 ) and the adjacent channel 55 interference

-6 would be 1.3 (10 ). But the 55 overlap into the desired channel would still

-5 be 2.5 (10 )--14 dB greater than that produced by anFM system centered in

the adjacent channel.

(continued on page AD-3, inside back cover)

AD-2
NTIA-REPORT-78-6

A Preliminary Estimate

of the Effects of

Spread-Spectrum

Interference on TV

J. R. Juroshek

u.s. DEPARTMENT OF COMMERCE

Juanita M. Kreps, Secretary

Henry Getler, Assistant Secretary -Designate

for Communications and Information

June 1978

PREFACE

This work was supported by the UHF Task Force of the

Federal Communications Commission.

iii

E:KPERIMENTAL MEASUREMENTS. •

A])JACENT CHANNEL INTERFERENCE.

INTERFERENCE TO TV •

PREFACE

LIST OF FIGURES •

ABSTRACT. • • • •

.1.

2.

3.

4.

5.

6.

7.

Bl\CKGROUND •

INTRODUCTION •

C()NCLUSIONS. •

Rl~FERENCES • •

Table of Contents

. . I. ....

'. . . . . . . .

v

iii

vi

1

1

1

7

17

24

26

26
Figure 1.

Figure 2.

Figure 3.

Figure 4.

Figure 5.

Figure 6.

Figure 7.

Figure 8.

Figure 9.

Figure 10.

Figure 11.

Figure 12.

Figure 13.

Figure 14.

LIST OF· FIGURES

Examples of spread-spectrum systems.....

Spread-spectrum bandwidth versus system chip rate.

Example of minimum required signal-to-interference

ratio for VHF TV. The .interfering signal is a voice

modulated FM signal with a 5 kHz peak frequency

swing. . . . . . . . . . . . . . . . . . . . . . .

Example of minimum required signal-to-interference

ratio for VHF TV. The interfering signal is a voice

modulated FM signal plus a second cw interferer of

equal strength . .. . . . . . . . . . . . . . . . .

Example of minimum required signal-to-interference

ratio for VHF TV. The interfering signal is a voice

modulated FM signal plus two cw interferers all of

equal strength . .. . . . . . . . . . . . . . . . .

Example of minimum required signal-to-interference

ratio for UHF-TV channel 41. The interfering signal

is a single FM interfering source. . . .

Composite of Figures 3, 4, 5, and 6 plus the CCIR

recommended protection ratios for color TV interference to color TV . .. .

Composite of Figures 3, 4, 5, and 6 plus the FCC

recommended protection ratio for cw interference to

color TV •• ••••••••••••••

Estimates of the interference potential of a spreadspectrum signal as compared to a narrowband, FM landmobile radio interferer .

Block diagram of laboratory tests.

Results of laboratory tests showing median results

and variability .

Spectrum of direct sequence PRN signals with clocking

rates of (a) 0.1 Mbps and (b) 5 Mbps .•..

Expanded view of direct sequence PRN signals with

clocking rates of (a) '5 Mbps and (b) 0.1 Mbps.

Spectrum of a spread-spectrum signal operating in an

adjacent TV channel with clocking rates of (a) 0.1

Mbps "and (b) 1 Mbps.. .

vi

3

5

8

9

11

12

13

15

16

18

21

22

23

25
A PRELIMINARY ESTIMATE OF THE EFFECTS OF SPREADSPECTRUM INTERFERENCE ON TV

* John R. Juroshlek

The report examines the conditions under which spread-spectrum, land-mobile radio, and television can share spectrum. After

a preliminary assessment, the report concludes that the interference

caused by a constant amplitude, spread-spectrum system should be

comparable to that caused by a conventional, narrowband, FM, land

m()bi1e signal of same total power, provided the spread-spectrum

signal has an rf bandwidth less than 2 MHz. With spectrum spreads

greater than 6 MHz, the spread-spectrum system should have some

acivantage because of the out-of-band rejection capabilities of a

T\' receiver. A limited number of laboratory measurements are also

dE~scribed that support the conclusions of the report.

Keywords: Interference; land mobile radio; spread spectrum;

television

1. BACKGROUND

Tllis report is an initial assessment of the conditions under which

spread-spectrum, land-mobile radio (LMR) an.d television can share spectrum

in the 450-512 MHz frequency band. The rep~ort is intended as a quick preliminary study into the potential for interference and leaves many of the

more detailed system questions unanswered. It should be noted that this

.report considers only interference from LMF.. to television with the reverse

situation left to later studies.

2. INTRODUCTION

T]:le question of whether it is an economical use of the spectrum to

allow ~Nideband communication systems in a Inu1tiuser environment, has plagued

communication engineers for some time. The debate is also likely to continue

into tJhe future since new technological ad'vances often result in changes

that mlake systems, once thought to be wasteful, much more attractive in terms

* . The a'uthor is with the U. S. Department of Commerce, National Telecornmunlcations and Information Administration, Institute for Telecommunication

Sciences, Boulder, Colorado 80303.
of spectrum conservation. Spread-spectrum communications is an example of

one such area that has been affected by technological/ changes. Recent

advances in surface acoustic wave (SAW) and charge coupled devices (CCD)

have simplified spread-spectrum systems and made them much more attractive

for use in services such as land-mobile radio. This report will examine the

problems of using spread-spectrum systems in the TV frequency bands.

For this study, we shall arbitrarily assume a direct sequence type of

spread-spectrum system (Dixon, 1976). This does not necessarily mean that

a direct sequence spread spectrum is the b~st type of system for land-mobile

radio, but .only that it is to be used for this interference study. Other

types of spread-spectrum systems such as frequency hopping, time hopping, or

chirp systems could prove to be more beneficial in terms of the needs of the

450~512 MHz band. However, we do not anticipate a'significa~t difference in

the interference characteristics produced by the different types of systems

since the amplitude characteristics of a frequency hopping interferer are

similar to the amplitude characteristics of a direct sequence interferer.

Figure lea) gives one example of a oirect sequence, spread-spectrum

system. Basically the input to the system is a 3 kHz voice signal. The

voice signal is digitized and modulo 2 added to a binary bit stream generated

by ·a pseudorandom noise (PRN) generator. This summed binary waveform is

th~n sent to a balanced modulator where it is multiplied by a carrier of

frequency, f. This process produces a wideband, bi-phase, phase-shift- c

keyed (PSK) waveform. At the receiver, the revers~ process is implemented

which consists of collapsing the wideband PSK waveform back into the narrower

bandwidth, digitized, voice signal. The collapsed information bearing

signal is then demodulated back into the 3 kHz voice signal. Also, the

system could handle digitized information such as a teleprinter or cathode

ray display.

A system such as the one shown in Figure lea) does have some disadvantages in that the digitizing or voice .encoding process also generally expands the bandwidth. The amount of expansion depends on the encoding process

used such as delta modulation, linear predictive coding, or pulse code

modulation. Fortunately spectrum spreadin9 at this stage is not entirely in

2
LV

Voice Digitized Voice

3KHz BW Voice 3KHz BW

ENCODER DEMOD J . DECODER \

BWRF g 2Rc

Clock I t

I

Clock

Rate = Rc f

o f·

I

IRate =Re

PRN 0PRN

GEN PRN Sequence GEN

of Rate Rc (a )

Voice

3KHz BW,r--- _

Voice

_----. 3 KHz BW

___....., AM OR FM

TRANS BWRF =2Rc

CONVENTIONAL

RECEIVER 1--1~~~

Clock

Rate= Rc PRN

GEN

Noise - Li ke Sequence

of BW =2Rc

( b )

Figure 1. Examples of spread-spectrum systems.

Clock

PR N I Rate=He

GEN
vain in that some degree of 'additional noise and interference immunity is

generally acquired in the digitizing process.

Figure l(b) gives another example of' a direct sequence, spread-spectrum system. In this system, the voice signal modulates a conventional

narrowband AM or.FM transmitter. The narrowband signal is then spread by

multiplying it with a "noise-like" waveform generated by a PRN generator.

The noise-like waveform may be either a conventional binary sequence as used

in the previous system or an amplitude varying sequence similar to Gaussian

noise. The receiver is nearly identical tOethe one described previously

except that now a conventional receiver can be used to demodulate the voice.

The theory of direct sequence spread-spectrum systems tells us that the

output power spectrum of this type of system is almost entirely d~termined

by the clocking rate of the PRN generator (Dixon, 1976). In particular, if

the PRN generator has a clocking rate of R , then the transmitted power c

spectrum will have a «sin x)/x)2 shape with an rf bandwidth of approximately

BW = 2 R

c

which is the bandwidth between the first two major nulls. Further, the

theory states that, in the despreading process, the desired narrowband

signal can enjoy a signal-to-noise ratio enhancement of g at the output of

p

the receiver as compared to its input signal-to-noise ratio where

BW

B. f J..n 0

and B. f is the bandwidth of the information, which in this case has been

ln 0 .

arbitrarily chosen as B. f = 3kHz. Figure 2 graphically shows the relaJ..n 0

tionship between BW, g , and R. For example, we see that, at a chip r~te p c

of 1 Mbps, the rf bandwidth required is. approximately 2 MHz,with a potential processing gain of 666 or 28 dB. We have lab~led the processing gain

as potential gain since system implementation losses will prevent full

realization of this amount.

Prior to examining the detailed interference calculations, it will perhaps be beneficial to briefly list a few comments on the advantages and disadvantages of spread-spectrum systems in general. Examples of some advantages that are generally'attributed to spread spectrum are:

4
104

0

c:

Ol

U5

N Q)

I u

10

.L: N

..- I "'0 -j

"'0 r<)

c:

0 0

(I)

lJ... J2

a:: 103 c:

Q)

-0 ..- <.9 0

E Ol

-;< c

0 "ti)

(f)

0- Q)

0- U

<1: 0

I 0...

(I) 0

:;:

·c

Q)

"0

0...

102

0.1 0.1 I 10

RC - Direct Sequence Chip Rate, Mbps

Figure 2. Spread-spectrum bandwidth versus system chip rate.

5
(a) Resistance to interference and jamming: Perhaps the greatest advantage of spread spectrum is its ability to reject

unwanted signals. Uncorrelated signals do not enjoy the

benefits of processing gain in the despreading process.

(b) Security: The system has security advantages in that a

the power-spectral density can be less than the normal

background noise. The system can also use conventional

digital encryption tech~iques.

(c) Graceful degradation: A multiuser spread-spectrum system

generally degrades gracefully as the number of users increases. Additional uncorrelatedusers appear as additional

wideband noise.

(d) Relatively low power density: Power spectral density ~s

generally lower than for a comparable narrowband system.

Thus, the potential for interference to a narrowband system

is also generally less.

(e)" Priority: A multiuser system can establish priority simply

by letting some users radiate more power than others.

(f) Surface acoustic wave devices: Attractive system implementation options exist with the use of surface acoustic wave

devices. Spr~ading and despreading circuits can be implemented

relatively easy_

(g) Resistance.to multipath: A wideband, spread-spectrum system

exhibits an inherent resistance to multipath. Sufficiently

delayedmultipath signals appear to be additional noise once

synchronization has been established.

Some of the disadvantag~s are:

(a) Complexity: The complexity of the system generally results

in more sophisticated and costly equipment.

(b) Synchronization: Synchronization and acquisition problems

are often a major factor in system design and system performance. Synchronization and timing problems are also

increased in a doppler environment where one or more of

the terminals are in motion and their locations unknown.

6
(c) capture by a strong signal: The system can be captured

by a strong nearby interfering signal. This could be particularly important to two collocated mobiles where one is

trying to transmit while .one is tl~ying to receive. A

pot~ntial for.catastrophic failure of an entire system

due to a single strong interfering source is always

possible.

(d) Bandwidth expaI).sion: A sp·read-spE~ctrum system requires a

reasonably large block of spectrurrl. Implementation of a

system with bandwidth expansion factors much below 100

are probably not practical due to implementation losses

and the relatively small processin,g' gain advantages.

3 • INTERFERENCE TO TV

So far.we have considered two types of direct sequence systems, as

shown. ill Figures I (a) and (b). Although both transmitted signals are spread

with a.PRN sequence, the characteristics of the radiated signals are entirely

differerlt. In Figure 1 (a), a system is shown that essentially transmits a

constant: amplitude, phase-shift-keyed signal. Figure l{b), however, considers

a system that can transmit an amplitude varying PRN signal. These two types

of signals will, certainly have a different effect in terms of interference

to TV. This report will consider only the constant amplitude type of

radiated signal since this form of spreading is simpler to implement and is

probably the type that is currently used mos·t often.

As a basis for comparison, we first exmnined the effects of conventional

narrowba.nd, constant amplitude interferers Oll TV. Figures 3 and 4 (FCC,

1968a) 9'ive two such examples. Both of these graphs show the minimum

signal-to-interference ratio. necessary to acllieve acceptable performance

with interference from conventional FM land-rnobil,etransmissions. Although

it is not the intent of this report to describe the results of the original

tests in any detail, it is important to know that both of these curves

reflect the minimum signal-to-interference ra.tio necessary to insure an

acceptable level of interference. AcceptablE~ interference is also described

as "perceptible" or a level of iI).terference t:hat "ultimately might be

adopted in the interest of maximum spectrum sharing." Signal-to-interference ratio, in these two figures, is defined as the ratio of the total

7
40

20

CD

-0 0

6

-"6

0:::

Q)

u -20 - c

Q)

L

(1)

4-

-- . (1)

C

.2

-40

0

c

0"'

(J) -60

Q)

-;:

(; 0- Q) u

..0 Ol ::J 1.L

-80 "0 (f) w Q)

-~ Q) Q)

"0

C (; U C 0 C Q)

CD U 0 U c

Q) -E "0 ::J e c 3 U ::J -100 0 0 ...J ([ U (f)

Frequency Offset, MHz

Figure 3. Example Of mlnlmum required signal-to-interference ratio for

VHF TV. The interfering signal is a voice modulated FM signal with a 5 kHz peak frequency swing.

8
m-0 0

cS

"-5

0:::

Cl)

-20 u

c

Cl)

!o....

Cl)'

'+-

Cl)

C

.2

-40

0

c::

en

(J) -60

Q3

"t:

0 0:- Q,) U Q) .0 01 :J Lt

-80 ""0 (J) w Q,)

""0 "\:: Q,)

u c c 0 c Q)

(l)

0 u 0 u c

"E ""0 Q,) :J e c

-100 u :J

.3 .s::- o ii: u (J)

I I I I

I

I

I

-120 -6 -4 -2 0 2 4 6 8 10

Frequency Offset, MHz

Figure 4. Example of ml.nl.mum required si9nal-to-interferenc'e ratio for

VHF TV. The interfering signal is a voice modulated FM signal

plus a second cw interferer of equal strength.

9
power in the visual signal relative to the total power in the interfering

signal in decibels. Frequency offset in these figures is the frequency

difference between the interferers center frequency and the lower band edge.

It should be noted that the curves in these figures, as well as in some of

the others to be described in this report, were obtained by replotting the

referenced data in a common format convenient for comparison purposes..

Figure 3 shows· the results of FCC laboratory measurements of interference tests conducted with a voice modulated FM signal. The tests in

Figure 4 are similar except that now a second cw interfering signal is added

to the FM signal at an equal level and with a 15 kHz frequency separation.

It is interesting to note that both Figures 3 and 4 show about the same

amount of interference whether the interferer is centered over the picture

carrier or the chrominance subcarrier.

A third example is shown in Figure 5 (FCC, 1968b) where similar tests

were conducted with the interference now composed of three constant amplitude interferers added together (one FM plus two cw signals ±30 kHz from

first). This figure essentially shows the minimum signal-to-interference

ratio that is necessary to achieve a level of interference that ~as judged

by a panel of observers to be a median of TASO 3, which means the picture

quality was judged to be passable (ITT, 1975). The figure is also important

since three interferers produce an interference environment that has amplitude variations. This is in cOntrast to the previous tests that all had

constant amplitude interference. Note that all of these examples are similar.

So far this report has examined only interferenqe to VHF TV. The frequency band of interest here is in the UHF frequency band, which might

behave differently because of the presence of the UHF converter. Figure 6

differs from the others in that it shows laboratory .measurements (Brandel,

1977) of interference to UHF channel 41. Although the frequency of this

measurement is above the frequency band of interest in this report, the

results do not appear to be drastically different from those reported previously. These curves show the minimum signal-to-interference ratios. necessary to insure acceptable TV performance with an FM land-mobile interference.

It is interesting to view a composi~e of the experimental data along

with other current TV interference protection criteria. In Figure 7 we show

a composite of Figures 3, _4, 5, and 6 along with the current CCIR recommended

10
40

20

m"0 0

cf

"5

0:::

Q)

u

c --20 Q)

Q)

Q)

--40

.E

0

c

CJ'l

en --60

--80

-100

:v-i::

0 c;r

a> u

Ol .D LL -0 lI- (f) W .~ lI-

"0 lI- a> a>

c:: lI- u C 0 0 c:: a> (1) U 0 U

a> -~ lI- ll- E -0 a> e c:: 3 13 0 a: .c 0 -.J U (J)

I

'I

I

I

I

0 2 4 6 8 10

Freque ncy Offset, MHz

F~9ure 5. Example of minimum required signal-to-interference ratio for

VHF TV. The interfering signal is a voice modulated FM

signal plus two cw interferE~rs all of equal strength.

11
40

20

IT) 0 -0

_Q

-+--

0

0:::

Q)

u -20 c

Q)

Q)

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Q)

C

.2

-40

0

c

O'l

(J5 -60 Q)

"E I 0 cr

-80 I

Q) u Q)

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W (/) Q)

-0 -E Q) Q)

c u C 0 c Q) 0 u 0 u CD c

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-100 u l: :J

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- I20 '-----_6~~-_-1....4 ~~__--L.2_--L-_.L.------!..-.L.---l----L_----L---LL--...L..L-_-L-----L_-L-----JL..,-.---J

o 2 . 4 6 8 10

Frequ ency Offset, MHz

Figure 6. Example of minimum required ~ignal-to-interferenceratio for

UHF-TV channel 41. The interfering signal is a single FM

interfering source.

12
:~o

CCIR

m-0 0 ( \

a \ 1\

"-5 \..J \ I\.

,0::

(])

u

c -~~O (])

(])

4-

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-40 -

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\0-

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CJl ..0

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-80 ""0

Q> (/) W \0-

""0 .~ Q) Q)

c u C 0 0 c Q)

CD U 0 U c

Q> ~. "E ""0 ::J e c

-100 3 u ::J 0 0::: ..c: 0 U (f)

I

I

I

I

I

- 120 -6 -4 -2 0 2 4 6 8 10

Frequency Offset, MHz

Figure 7. Composite of Figures 3, 4, 5, and 6 plus the CCIR recommended

protection ratios for color TV interference to color TV.

13
protection ratios (CCIR, 1974a) for color TV interference to color TV.

Figure 8 is a similar example where we now compare the experimental measurements with the FCC recommended protection ratios for interference to color

TV from cw interference (Waldo and Daniel, 1963). Although the results are

generally similar, the FCC curve does differ slightly in that it recommends

approximately 5 dB more protection be given to the ~hrominance signal than

to the lower frequency video signal. The reason for this needs to be explored in more detail.

It would appear from these tests that the interference mechanism of

interest here is a simple interaction between an amplitude-modulated picture

or chrominance signal and the interfering signal. Since the rf spectral

bandwidths of both the chrominance and picture signals are of the order of

2 MHz, a spread-spectrum signal with a lesser rf bandwidth would be largely

unfiltered in the TV receiver and, therefore, should produce interference

similar to a conventional FM land-mobile signal. While it is acknowledged

that the spectral shape of an interferer can have an effect on the level of

interference, the magnitude of the effect is generally felt to be second

order in view of the preliminary nature of this study. For example, interference from a noise source with a triangular spectral shape can be approximately 5 dB greater than from a noise source with a flat noise spectrum and

still produce the same subjective level of interference (CCIR, 1974b). In

other words, the interference produced by a constant envelope, spreadspectrum signal (2 MHz spread or less) should be nearly identical to that

produced by a'narrowband FM modulated signal of equal power. For spectrum

spreads greater than 2 MHz, some additional advantage can be expected since

some of the interferers power can be rejected because of filtering in the

video and IF circuits. It is difficult to tell exactly where this filtering

advantage occurs because ofa lack of detailed information on the effects of

wideband spread spectrum interference to TV and a lack of detailed information on the filtering characteristics of a typical TV receiver.

Figure 9 has been prepared as a summary of our estimates of the potential for' interference from a spread-spectrum signal. In this figure a plot

of the estimated interference effect relative to a narrowband FM landmobile signal is shown as a function of the direct sequence chip rate (rf

bandwidth is also shown at the 'top of the plot). For chip rates up to 1 Mbps,

14
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c

(J'l

(J)

40

20

o

-20 -

-40

-60

\ ," \.....J \

\ 1\

V '-,\

\.-

.- I20 L---I_

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----1.----...l.

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en

Frequency Offset, MHz

15

-80

-100

Figure 8. Composite of Figures 3, 4, 5, and 6 plus the FCC recommended

protection ratio for cw int:erference to color TV.
BW-Approximate RF Bandwidth, MHz

10 100 I0....----,--r---~--.,..-..,...--r-...,...----....,._.--r----r-~_r___"'r___r_..,._r_--___,_-___r_____;,___,r__T"___.__T_r_.,.__-_____,

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W

Assumes Spread Spectrum System

and TV Are Occupying Same

Frequency Band

I 10

Direct Sequence Chip Rate, Mbps

Figure 9. Estimates of the interference potential of a spread-spectrum

signal as compared to a narrowband FM, land-mobile radio

interferer.

16

100
the plot shows 0 dB, which means we expect the degradation produced to be

nearly identical to a conventional FM larld-mobile signal of the same total

power. For bandwidth spreads beyond 2 :MFlz, a linearly decreasing curve is

shown, indicating that the spread-spectrl~ system is expected to be less

detrimental than an FM,signal of the same total power since the TV receiver

should be able to reject some of the out-of-band portion of the 'interfering

signal. The curve assumes the amount of rejection to be directly proportional to the spreading. A shaded area is shown to acknowledge that it is

not clear whether this advantage will begin to occur with chip rates near 1

Mbps (2 MHz rf spectrum) or at chip rates greater than 3 Mbps (6 MHz rf spectrum). The exact values for these estima.tes depend, of course, on a number

of factors such as TV receiver design and can be determined only after

experimental.measurements. The exact sha.pe of the interfering spectrum will

also influence the results.

In view of these conclusions, the rules for protecting TV from spreadspectrum systems operating within a TV band could be as follows:

s/r min = 50 dB BW < 6 MHz

S/1 min 50 - 10 log (BW/6) dB BW ..2: 6 MHz

These equations are estimates based on the current protection ratios from cw

interferers, and the exact numbers might be modified slightly with additional

experimental evidence. For the adjacent channel case, which is taken to be

the case where the TV and the spread spectrum's spectral energy does not

overlap, the protection ratio would probcilily be

S/! min = -6 dB

which is based on current minimum adjacent channel rejection capabilities of

a TV receiver. Spread-spectrum signals separated more than one channel away

may be able to operate at even smaller signal-to-interference ratios.

4. EXPERIMENTAL MEASUREMENTS

A limited set of laboratory measurements were made in order to check

the assumptions made in the report. The laboratory setup, as shown in

Figure 10, uses a commercial generator to produce a binary PRN waveform.

This binary waveform is adjusted so that it has a zero dc component. Next

17
CLOCK PRN *

GENERATOR

TV Antenna

.....

00

SIGNAL

GENERATOR

f SS CO LOR

TV

FM SIGHAL I f FM r I

GENERATOR

* Tests were also conducted with a random generator

in place of thepseudora.ndom generator.

Figure 10. Block diagram of laboratory tests.
it is multiplied by a sine wave of frequency f in a balanced modulator

ss

producing a conventional bi-phase, PSK, direct sequence, spread-spectrum

signal. A conventional FM generator is used to simulate an FM land-mobile

signal. For simplicity, the FM generator is modulated with a 1000 Hz tone

with the peak frequency deviation adjusted to 5 kHz. Both of the simulated

signals are routed through adjustable attenuators where they can be adjusted

in magnitude. After the attenuators, one or the other of these signals is

selected .by a coaxial switch, and the selected signal is then added to the

TV signal that is received on the laboratory antenna. The result is then

viewed on a 19-inch, portable, color TV.,

The laboratory tests were conducted by sUbjecting various personnel to

a mOl~e-or-less random selection of interference conditions. A comparison

method was used where a given level of FM interference was set, and the

viewE~r was then asked to ,adjust the spread-spectrum attenuator to produce' a

ncOmI)arabl~n level of interference. The viewer could readily switch between

the FM and spread-spectrum interference until he was satisfied that he had

adjusted the degradatiop to be the same. The attenuator settings were then

recol~ded so that the difference between s'pread-spectrum and FM interference

power could be determined. It should be noted that the frequency of both

the spread-spectrum and FM signals were set prior to each series of tests by

adjusting the attenuators for a moderate level of interference and then

askirlg each viewer to adjust the two frequencies to produce the worst interference conditions. Once these frequencies were set, they were not changed

thrOtlghout that viewer's sequence of tests.

The main portion of the tests were made with a PRN generator that produced a maximum length sequence of 2l5_lbits. Clock rates of 0.1, 0.5, 1,

and 5 Mbps were used, which produces a spread-spectrum signal with an rf

bandwidth of 0.2, 1.0, 2, and 10 MHz, respectively. A limited series of

tests were also conducted using a binary generator that produced a random,

nonrepeating, binary sequence. ,Unfortunately, this generator could not be

clocked at rates above 0.1 Mbps. In addition to varying the direct sequence

cloc}~ rate, the ratio of TV signal to FM interference {TV/FM} was also set

at either 34 or 39 dB. The power level of the TV signal was measured on a

spectrum analyzer as the maximum observed power of video portion of the

signal.

19
The tests with the PRN generator were conducted using three observers.

Each observer was subjected to the same combination of clock rate and TV/FM

ratio three different times. The tests with the random generator were

conducted similarly e~cept that three different observers were used. Also,

the PRN tests were conducted using TV channel 6, while the random generator

tests used channel 4.

Results of the laboratory tests are shown in Figure 11 with squares and

circles connected' by vertical lines. The ~quares and circles denote the

median values of nine observations from three ·observers. The median is used

since the observations are the difference between the spread-spectrum and FM

interference levels expressed in decibels. The lines on the symbols show

the spread of the data. with the highest and lowest value of the nine observations removed. A positive number on the figure means that less spreadspe,ctrum power is required to achieve the same level of interference as the

FM. ~egative numbers are, of course, the reverse situation. As can be

seen, the measurements generally agree with the predicted curve shown in

Figure 9.

F,igure 12 shows the spectrum of the PRN generated signal at clock rates

of 0.1 and 5 Mbps. The spectrum using the random generator is similar

except that the PRN generated signal contains discrete line spectra while

the random signal is continuous. This can be seen by examining expanded

views of the spectra. Figure l3(a) shows an expanded view of the PRN gene~ated

spectrum at a 5 Mbps clocking rate. As can be seen, discrete line spectra

exist at approximately every 100 Hz. If the clocking rate is reduced to 0.1

Mbps, then the frequency separation between components decreases (repetition

period of PRN sequence increases) 'as shown in Figure l3(b). The spectrum

analyzer could not resolve the individual co~ponents at the lower clocking

rate.

It is interesting to note the frequencies that were chosen by the

observers as being the "worst" in regards to producing interferenc;::e. Tab~e 1

lists the frequencies selected by the observers where the first three entries

are the frequencies chosen during the PRN interference tests to channel 6,

and the last three entries are for the interference tests to channel 4.

Note that the frequencies selected are near the video carrier frequency

whi.ch for channel 6 is 83.25 MHz and for channel 4 is 67.25 MHz. None of'

20
BW -Approximate RF Bandwidth .. MHz

10 100 10 , iii I t i , iii Iii iii iii iii iii i i

51--1 0 Random Sequence, Tv/FM =34dB

X Random Sequence, Tv/FM = 39 dB CD Ljtl

"U • PRN Sequence, TV/FM = 34d-S

/ Predicted [] PRN Sequence, TV/FM = 39 dB """- LL

"U

C

0 0

E

Ell t ::J

UCl>

Co

Cf)

"U

0 -5 Cl>

Co

f'V Cf)

f-I c

Cl>

Cl>

....

Cl> -10 CD

Cl>

U

C

Q)

Cl>

0 -15

-20' , r", , , , , , t_ ( , , I , , , I I'(" I , I ,

0.1 10 100

Direct Sequence Chip Rate, Mb ps

Figure 11. Results of laboratory tests showing median results and variability.
10 dS I (0)

I~' 0.05 kHz

IOdS f

2 MHz

Figure 12. Spectrum of direct sequence PRN signals with clocking

rates' of (a) 0.1 Mbps and (b) 5 Mbps.

22
IOdS 1

lOdS f

(0)

(b)

I~ ~OOHz

Figure 13.- Expanded view of direct sequence PRN signals with

clocking rates of (a) 5 Mbpsand (b) 0.1 Mbps.

23
the observers selected frequencies near the chrominance subcarrier frequencies

of 86.8295 and 70.8295 MHz.

Table 1. Frequencies Selected by Observers as Producing

Worst Interference

Spread

Spectrum FM

83.409 83.934

83.772 84.090

83.118 83.431

68.185 67.472

67.869 68.013

67.475 67.463

5 • ADJACENT CHANNEL INTERFERENCE

During the course of the study, we were also asked to consider. the case

where' a spread-spectrum system was operating in a frequency assignment next

to an· operational TV station. The object being, of course, to compare adjacent channel interference from spread-spectrum systems with adjacent channel

interference from conventional, narrowband, FM signals. Although measurements were attempted, they were abandoned because we felt the results were

negatively i'nfluenced by the experimental setup being used. The reason for

this conclusion can be seen by examining Figure 14 where the spectrum of a

spread-spectrum signal operating in an adjacent channel is shown. The clock

rate of the signals is 0.1 and 1 Mbps, which means the approximate rf bandwidths of the signals is 0.2 and 2 MHZ, respectively. The laboratory signals

have very poor spectral characteristics since they radiate a significant

amount of out-of-band energy. This is due to the fact that nO filtering was

included in the experimental setup since out-of-band spectral characteristics are not significant to the cochannel tests. The tests were abandoned

since the amount of side lobe filtering that can be added involves system

tradeoffs that are beyond the sco.pe of this study. These tradeoffs, for

example, involve the cost of filters and the.amount of degradation to the

24
-----~·~I-----------.~I

Adjacent Operational TV

Channel Channel

'1 MHz

--+i 'I--

(a )

,OdsI

Adjacent

Channel

Opera tional TV

Channel

( b )

Figure 14. Spectrum ofa spread-spectrum signal operating in an

adjacent ·TV channel with clocking rates of (a) 0.1

Mbps and (b) 1 Mbps.

25
spread-spectrum system, as well as the amount of adjacent channel protection

that is desired. For these reasons it was felt that any results obtained

·with the current experimental setup would not be meaningful.

6. CONCLUSIONS

The amount of interference caused by a constant-amplitude, spreadspectrum system should be about the same'as that caused by a conventional

narrowband FM land mobile signal. This should be true as long as the rf

bandwidths are less than 2 MHz. With spectrums greater than 6 MHz, the

spread-spectrum system should 'have some advantage over conventional FM landmobile signals because of the out-of-band rejection capabilities of the TV

receiver.

7 . REFERENCES

Brandel, R. W. (1977)', Television interference associated with the proposed

class E 220 MHz citizen radio service, 27th Annual Conference IEEE

Vehicular Technology Group, Orlando, Florida, March 1618.

CCIR (l974a), Broadcasting Service (Televi?ion) , Vol. XI, Xlllth Plenary

Assembly, Geneva, 1974, pp. 106.

CCIR (1974b), Transmission of sound broadcasting and television signals over

long distances, Vol. XII, Xlllth Plenary Assembly, Geneva, 1974, p. 62.

Dixon, R. C. (1976.), Spread Spectrum Systems (John Wiley & Sons, New York) ,

pp. 13-26.

FCC (1968a-), Interference to TV by other services, Pt. I, Project 2229-45,

February 8.

FCC (1968b), Interference to TV by other services, Pt. II, Project 2229-45,

May 8.

ITT (1975), Reference Data for Radio Engineers, 6th edition, Howard W. Sams

& Company, Inc·., Indianapoles, Indiana, pp. 30-38.

Waldo l G. V., and W.A. Daniel (1963), A method of estimating the extent of

interference to TV broadcast service caused by low-power transmitters'

operating on VHF TV channels, FCC Report R-6306, December 31.

26
FORM 0;--29

(3-73)

4. TITLE AND SUBTITLE

U.S. DEPARTMENT OF COMMERCE OFFICE OF TELECOMMUNICATIONS

BIBLIOGRAPHIC DATA SHEET

1. PUBLICATION OR'REPORT NO.2. Gov't Accession No.3. Recipient's Accession No.

NTIA-Report-78-6

s. Publication Date

A PRELIMINARY ESTIMATE OF THE EFFECTS OF SPREADSPECTRUM INT:ERFERENCE ON TV

7. AUTHOR(S)

John R. Juroshek

8. PERFORMING ORGANIZATION NAME AND ADDRESS

U.S. Department of Commerce

.National Tel1ecommunications & Information Admin.

Institute for Telecommunication Sciences

Boulder, CO 80303

11. Sponsoring Organization Name and Address

Federal Communications Commission

Washington, DC 20554

14. SUPPLEMENTARY NOTES

June 1978

6. Performing Organization Code

NTIA/ITS, Div. 1

9. Project/Task/Work Unit No.

9103517

10. Contract/Grant No.

12. Type of Report and Period Covered

NTIA Technical Report

13.

1S. ABSTRACT (A 200-word or less factual summary of most sign~ficant information. If document includes a significant

bibliography ot' literature survey, mention it here.)

The repc:>rt examines the conditions under whi.ch spread-spectrum, land-mobile

radio, and tlelevisio~ can share spectrum. After a preliminary assessment, the

report concludes that the interference caused by a constant amplitude, spreadspectrum sys·tem should be comparable to that caused by a conventional, narrowband,

FM, land mobile signal of same total pow,er, provi.ded the spread-spectrum signal

has an rf bandwidth less than 2 MHz. With spectrum spreads greater than 6 MHz,

the spread-spectrum system should have some advantage because of the out-of-band

rejection capabilities of a TV receiver. A limit:ed 'number of laboratory measurements arce also described that support the conLclusions of the report.

16. Key Words (Alphabetical order, separated by semicolons)

Interference; ,land mobile radio; spread spectrum; television.

17. AVAILABILITY STATEMENT

[Xl UNLIMITED.

D FOR OFFICIAL DISTRIBUTION.

18. Securi ty Class (Th.is report)

Unclassified

.19. Security Class (This page)

Unclassified

20. Number of pages

26

21. Price:

*U.S. Government Printing Office:: 1978 -7 7 8 -19 2/376 Region 8 l'SCOMM-DC 29716-P73

\

\

\

\

,

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

r--

I--

/

Spread Spectrum

Emission Power Density

, (Normalized)

,

/

TV Receiver Selectivity

10-5

10-6

~_oIooo--~-~-....LIl4~---&.._---L_---J._---J_---J~_L...-_.L..-_.L-.J

5 6 7 8 9 10 11 12

Frequency Above Lower Edge of TV Channel, MHz

Figure AD-I. TV selectivity function and SS power density in an adjacent

channel.

It)

0....

)(

6 ....

tJ)

c:

Q)

Q 5 '-

Q,)

4

Q)

(.)

c:

Q) 3 '-

Q)

'-

0)

....

c: 2 ......

"'C

Q)

N 1 -

C

E

0

'- 0

z 0 2 3 4 5 6 7 8 9 10 11 12

Frequency Above Lower Edge of TV Channel, MHz

Figure AD-2. The product of the two functions in Figure AD-I.

AD-3
Equation AD-I can be used to compare S5 and FM interference for other

examples. In each case, it will be found that interference to TV from an SS

LMR system in an adjacent channel is about equal to, or somewhat greater

than, interference from an FMLMR system with the same total power, location,

and traffic.

ADDEMDUM REFERENCES

CCIR Report 523 (1974), System Models for the Evaluation of Interference

(International Telecommunications Union, Geneva).

Haakinson, E. J., A. C. Stewart, J. E. Adams, and L. E. Vogler (19,77), Extended area test system in the l36~148 MHz band--an EMC assessment, OT

Report 77-122 (Institute for Telecommunication Sciences, Boulder, CO

80303) .

Rob·erts, W. K., and L. C. Middlekamp (1974), A study of the characteristics

of typical television receivers relative to the UHF taboos, FCC Report

LAB-74-01(National Technical Information Service, PB-235 057).

AD-4