Specular Direct Path (E,)

© To Receiver

Gaussian Noise (No)

Specular Reflected Path

Specular Direct Path (Ep)

> To Receiver

Gaussian Noise (No)

Diffuse Reflected Path

Figure 3. Channel models used to evaluate effects of multipath.


SS Base Interference to FM Base — Propagation Dependent
SS Base Interference to FM Base - Highly Probable
SS Base (Mobile) Interference to FM Mobile (Base) - Propagation Dependent

SS Base (Mobile) Interference to FM Mobile (Base) - Highly Probable

HME

100,

dgg, SS Separation (km)
<

[FEST rr ee a T T T
| 10

dey, FM Path Length (km)

Figure 14. Separation distance dog versus Gow that is required to insure

that S/I OUT > 20 dB with a 1 MHz SS system at 150 MHz.

33



dgg, SS Separation (km)

100

Ss

SS Base to FM Mobile
SS Mobile to FM Base
I50 MHz

bgg = 1OMHz

bgg= 10 MHz am

10 100
de, FM Path Length (km)

Figure 11. Separation distance dog versus dont to insure that

S/I OUT > 20 dB. SS base interference to an FM
mobile, or SS mobile interference to an FM base,
150 MHz.

29



7. +¥FM INTERFERENCE TO SPREAD-SPECTRUM
7.1 Single FM Interference

Interference curves similar to those shown in the last chapter can also
be produced for the reverse situation of FM interference to an SS system.
Tables 6 and 7 summarize the equations that are applicable in this instance
and were obtained by combining (13) and (30) with the appropriate path loss
equation in (23) through (26). As discussed previously, the performance of a
digital system in interference is determined by the E/N, out of the decor-
relator. For a binary SS system (k = 1), the requirements for E/N, were

previously determined to be

10 dB< E/N < 20 GB (35)

depending on channel and multipath conditions.

Graphs showing the separation distance dou that are required to protect
an SS system transmitting over a desired path deg are shown in Figures 18
through 21. The interferer's propagation path is now Gay’ Shading is again
used to’ denote the various propagation conditions, and areas labeled propa-
gation dependent occur when 10 dB < E/N, < 20 dB. Conversely the curves
labeled highly probable occur when E/N, < 10 dB. A summary of these results
is presented in Tables 8 and 9.

The RF bandwidth bop required by a S = 30 or 40 dB SS system depends on
the system design as well as the information being sent. Current state-of-
the-art in voice digitizing enables satisfactory voice recovery with digitizing
rates below 10 kHz. Thus, conceivably Dog for a 6, = 30 GB system could be
as low as 1 MHz while a &, = 40 dB system would probably require 10 MHz.

7.2 Multiple FM Interference

The analysis so far has only considered the separation requirements for
a single FM source interfering with a single SS receiver. However, since the
FM channels are 25 kHz wide, a 10 MHz SS receiver could conceivably receive
up to 10/.025 = 400 simultaneous interferers. Practically, one would expect

only a small fraction of this number to be on at any given instant in time.

38


(a)

harder to find. Also, the spectrum spreading process can
provide security, if designed properly, since the user must
know the spreading code, or process, before signal detection
can be attempted.

Ranging: The wide bandwidth of the transmitted signal enables
the resolution of time differences to a precision that is

necessary for ranging.

Some of the disadvantages are:

(a)

(b)

(c)

(a)

Complexity: The complexity of the system results in more
sophisticated and costly equipment.

Synchronization: Synchronization and acquisition problems
are often a major factor in system design and system per-
formance, particularly under conditions where multipath
exists.

Capture effects: A spread-spectrum system can always be
captured by a sufficiently strong interfering signal.
Bandwidth requirements: A spread-spectrum system requires
a considerable amount of spectrum. Implementation of sys-
tems with bandwidth expansion factors much below 100 is

generally considered impractical.

Although the discussions in this report are primarily directed toward an

SS system carrying voice traffic, the results are also generally applicable

to an SS system carrying data. In other words, the results are generally

independent of whether the information being transmitted is digitized voice

or data provided the bit rates involved are the same.

3.2  Spread-Spectrum Performance in Gaussian Noise

In estimating the performance of the DS and FH systems, we shall assume

that the receiver-decorrelators, as shown in Figure 1, are optimum matched

filters.

This will probably not be the case in actual practice since a sub-

optimum detection scheme, which is better adapted to the harsh land-mobile

environment will probably be used. Nevertheless, the errors involved are

minor considering the other unknowns and generalized nature of this report.

Given the assumption of matched filters, the ratio of signal power to

noise power at the output of the SS decorrelator is


fo

Frequency

+ mf;
'

2---

Time

Figure A-1. Typical frequency-hopped, time-encoded signal.

66


f, Fraction of SS Mobile Locations Denied Access

0.1

! ! ! l ! l | | ! i !
3 5 1 9 il 13 15
Nin» Number of Simultaneous FM Mobile interferers
Figure 23. Monte-Carlo simulation results for an SS system with

S = 30 dB and E/N, requirements of 20 dB at 150 MHz.

48


FM Base

Oo.

Oo

FM Mobiles

Figure 22. Geometry used in Monte-Carlo simulation.

46


8. FM SQUELCH THRESHOLD

Our definition of interference to FM so far has been based strictly on
output signal-to-noise ratio. However, other situations also exist that
should be examined. For example, consider the case of an FM mobile receiver
that breaks squelch every time an SS transmitter is keyed. The FM user might
consider this as interference even though he can still intelligibly receive
his FM base.

This situation was investigated with the results shown in Figures 26 and
27. Plotted here is the separation ay between an SS transmitter and FM
receiver that is necessary in order to avoid exceeding the FM squelch thres-
hold. The level of this threshold with a cw input is assumed to be 0.1 WV in
50 ohms or -157 dBW. Figure 26 is for a frequency of 150 MHz while Figure 27
shows the results for 900 MHz. It is assumed that the receivers are designed
so that the squelch is affected only by that portion of the SS signal that
passes through the IF filter. This means that the squelch threshold will be

exceeded when

b,
ss
- + - Se > -

ERP os L(4,,) AaM-R 10 log Dap 157 dBW , (38)
where ERPog is the effective radiated power of the SS transmitter, L(4,) is
the path loss over the distance aye and AaM-R is the FM receiver antenna
gain. The quantities bip and bes are the FM receiver bandwidth and SS RF
bandwidth, respectively. For these calculations, the values of AayeR = 0 dB

and bip = 25 kHz were used. As can be seen, considerable separation distances

are required to avoid exceeding squelch threshold even with Beg as large as
10 MHz.

9. ADDITIONAL FH SUPPRESSION

So far the possibility of spectrum sharing between FM and SS systems
appears to be bleak. As shown in Table 8, values of dog = 10 km imply that,
at best, dom > 2 km in order to avoid interference from FM mobiles. Inter-
ference from SS base stations is even worse as separations greater than 27 km

are required at 150 MHz.

52


_| Lom

(a)

_ | |_ 190 kHz
(b

)

Figure 28. Example of a frequency hopper with (a) all frequencies
present and (b) a missed frequency.

56


Table 8. Summary of Separation Distance, Fou! that is Required to
Insure 10 dB < E/N, S°20 dB with &, = 30 4B

agg doy £
Interference (km) (km) (MHz)
*
FM Mobile to SS Base 10 2-5 150
30 6-14 150
FM Base to SS Base 10 27-70 150
30 >82 150
*
FM Mobile to SS Base 10 2-5 900
30 6-14 900
FM Base to SS Base 10 45-82 900
30 >100 900
Table 9. Summary of Separation Distance, , that is Required to
Insure 10 dB < E,/NS < 20 dB with &, = 40 dB
£
ss doy
Interference (km) (im) (MHz)
*
FM Mobile to SS Base 10 -2 150
30 -7 150
FM Base to SS Base 10 9-28 150
30 46-82 150
*
FM Mobile to SS Base 10 -2 900
30 - 900
FM Base to SS Base 10 14-45 900
30 66-100 900

*
Also applicable for an FM base interfering with an SS mobile.

44


Specular
Component
L |
o
2
o
Cc
Ww
a Scattered
Component

Time

Figure 6. Channel impulse model used in performance calculations.

16


100

dgg, SS Separation (km)
Ss

[[]]_ ssBose Interference to FM Base ~ Propagation Dependent

SS Base Interference to FM Base - Highly Probable

SS Base (Mobile) Interference to FM Mobile (Base) - Propagation Dependent
B=] SS Base (Mobile) Interference to FM Mobile (Base) - Highly Probable

Figure 15.

dem; FM Path Length (km)

Separation distance dog versus dom that is required to insure

that S/I OUT > 20 dB with a 10 MHz SS system at 150 MHz.

34


APPENDIX. COOPER-NETTLETON SYSTEM PARAMETERS
1. System Description

The purpose of this appendix is to review and summarize the system
design parameters that were used by Cooper and Nettleton in their proposal
for a spread-spectrum, land-mobile communication system (Cooper and Nettleton,
1976). The system they propose is basically a frequency hopped, time encoded
system that transmits a signal such as the one depicted in Figure A-l. As
can be seen, the signal is composed of m time slots of duration Vt, and m
frequency slots of width f Hz. The equation for the kth signal waveform
during the ith time slot (or ith chip) is

k,. _ k k
s(t) = of ¥28 cos 27 (£, + ay £, + 0,)

it, <t< (441) t

1 (1)

1 .
Here we use cx = + 1 to represent the transmitted digital Message, S the
signal power, and at the frequency-hopping pattern. It is important to note
that this system considers the possibility of "overlapping frequency slots."
Since the spectrum requirements are inversely proportional to the chip

period the overlapping frequency slots will occur if

1
fi<> . (2)
1 ty

The term 8 is a phase term that is added to the waveform to insure a con-
tinuous phase at the chip boundaries for the Overlapping case and is not
needed if f = V/t,- The spectrum of this signal is approximately uniform
with bandwidth

wo
i
|B

: (3)

In order to accommodate more users, the preceding code set is subdivided

by partitioning it into subsets of length n where n <m. The subdivided code
is the code that is being described in the remainder of this appendix. Note
that the spectrum required by the system still remains at m/t,- This is due

to the fact that even though a particular user ‘requires only n frequency

65



Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

1.

2.

4.

5.

1o.

11.

12.

13.

14.

15.

LIST OF FIGURES

Block diagram of direct sequence (a) and frequency
hopping (b) systems... - +e + ee eee ttt tt

Spectra of SS systems operating in cw interference
environments. Direct sequence system is shown in
(a) while frequency hopping is shown in (b). --. +--+ -

Channel models used to evaluate effects of multipath . -

Performance of a 2-phase, CPSK DS system over a chan-
nel composed of two equal strength specular paths. .. +

Performance of a 2-phase, CPSK DS system over a channel
composed of a direct specular path plus a diffuse,
Rayleigh fading, reflected path. . - +--+ + e+ + + eee

Channel impulse model used in performance calculations .

Expected performance of an n = 32 frequency hop system
for various channel conditions - --.. +++ ++ isee-s

Signal-to-noise ratio out at an FM discriminator as a
function of the signal-to-noise ratio into the dis-
ceriminator for nonfading and Rayleigh fading channels. .

Ge i i i se ee ee
eometry involved in measuring dg and dom

Separation distance d, versus to insure that
S/I OUT > 20 dB. SS base interference to an FM base
station, 150 MHz. - - / ee ee ee ee ee ee ee ee

Separation distance d versus to insure that
S/I OUT > 20 dB. SS BEse interference to an FM
mobile, or SS mobile interference to an FM base, 150 MHz

Separation distance d.. versus to insure that

S/I OUT > 20 dB. SS base interference to an FM base,
900 MHz. . 6 ew eee ee ee ee ee ee ee ee
Separation distance d,. versus to insure that

S/I OUT > 20 dB. SS Sse interference to an FM mobile,

or SS mobile interference to an FM base, 900 MHz... -

Separation distance a, versus d that is required to
insure that S/I OUT > 30 dB with a 1 MHz SS system at
150 MHz. 2. 2 ee ee ee ee ee ee ee

Separation distance dg versus a, that is required to
insure that S/I OUT >°3o dB with" 10 MHz SS system at
150 MHZ. 2. ee ee ee ee ee ee

Page

12

13

15

16

18

21

27

28

29

31

32

33

34



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68


slots, other users will be designed different slots; and hence, in general,
all m frequency slots will be used.

So far we have not discussed how the digitized voice is impressed on the
code set. In the proposed system, M-ary signalling is used where each user
encodes k = log, n information bits into the n chip waveform. This means
that n possible waveforms, n chips in length, must be decoded by each user
with each waveform yielding k decoded information bits. Details of the
decoder are shown in Figure A-2. The delay, T, shown on each of the taps
in Figure A-2, makes the detector differentially coherent. This technique
enables a comparison to be made of the phase of the ith chip with the phase
of its predecessor that arrived T=n ty seconds earlier.

Some of the other parameters and expressions that are pertinent are

shown in the following summary:
Voice Digitizing Rate R = 48,000 bits/sec.
Number of chips per code n = 32 or 64.
M-ary information bits per code k = log,n = 5 or 6.

Code period T = n t, = 104 usec (n = 32).

1
Ryn 1
Equivalent noise bandwidth of 1 chip B, = ———— = —
1 log, n ty

307.2 kHz (n = 32) .

Receiver equivalent noise bandwidth

|e

1

9.83 MHz (n = 32).

Available system bandwidth B = 7-= 40 Miz.
1

Total number of system codes N, =

Total number of frequency slots m =

67





then the equations in Tables 2 and 3 can be solved for ase as a function of
doy in order to produce the graphs shown in Figures 9 through 12. Here digg
corresponds to the SS separation distance that is necessary to insure that
there is no interference (S/I OUT > 20 dB) and is plotted as a function of
dau? the victim FM transmission distance. Figure 9 shows the geometry

involved in measuring ds and day:

SS Transmitter

FM Transmitter

dem

Victim Receiver

Figure 9. Geometry involved in measuring deg and Ga:

Figure 10 shows the results for a transmission frequency of 150 MHz with
interference from an SS base transmitter to an FM base receiver. These
curves are plotted for SS bandwidths of 1 and 10 MHz. As expected, the
separation requirements for the wider bandwidth 10 MHz system are less. The
zurves are also plotted for M, = 0 and 29 dB, which represents the two
extremes in propagation conditions when 8 = 2. Practically, most propagation
channels will probably be somewhere in-between these extremes. The discon-
tinuities that appear in the curves are due to the propagation models. and
occur when either the SS interferer or FM victim pass over the effective
radio horizon. The actual transitions are not abrupt as shown, but gradually
change as one crosses the radio horizon. Figure 11 is similar to 10 except

the interference is from an SS mobile to an FM base.

27



Probability of a Bit Error,

TTTTT

2 Equal Specular Paths

10"!

0°?

03

At >ty

This curve is also applicable
for a single specular path

4

TTT

T

T

pst

!

|

titi

l

|

0 5 10 5
Ep/No, 4B

Figure 4. Performance of a 2-phase, CPSK DS system over a channel
composed of two equal strength specular paths.

13



10 4B < E/N, <20 dB , (18)

where the lower 10 dB limit would provide satisfactory performance in urban
channels with a dominate specular path, while the 20 dB limit would be
required in channels that are predominately diffuse. While it is admitted
that these values are largely based on conjecture, they are supported to
some extent by the previous discussion. Measurements of spread-spectrum
systems in an urban environment would be needed to more accurately define
the E/N, requirements. The Ny here is Gaussian noise out of decorrelator

due to non-Gaussian interference at the input to the decorrelator.

4. FM PERFORMANCE CALCULATIONS

The interaction between an FM receiver and an interfering, DS, spread-
spectrum signal is reasonably straightforward. This is due to the fact that
the IF filter in the FM receiver essentially transforms the wideband SS signal
into Gaussian noise. Since the FM receiver is a filter with bandwidth bige
the ratio of average signal power to average noise ‘power out of the FM
receiver's IF and therefore at the input of the FM discriminator is
Pes s

DIS = 10 log > + | IN aBo, (19)
‘IF

Zla

Ss. A . A
where Tr is the ratio of average signal power to average noise power at the
input. Here bop is the IF bandwidth of the FM receiver and Deg is the band-
width of the SS signal as previously defined. This means that the signal-

to-noise ratio after demodulation is given by

b
ss. Ss
b +7 IN aB,

& our = 10 log[382(8 + 1)] + 10 log
IF

(20)
where the first term is the classical FM improvement factor and B is the
modulation index (Stein and Jones, 1967). This equation assumes that inter-
ference is the dominate source of noise and that the receiver noise is
negligible.

Equations (19) and (20) are valid only if the desired signal is specular

and the value of S/N rg is sufficiently large that the receiver is operating

19



4

TTTTT
Litt)

2 Equal Paths
| Specular Plus | Diffuse
At st,

|

0"!

Diffuse Only (xe =0)

Fe

T

13

Probability of a Bit Error,

Specular Only (ze = o)

4

TT TTT

T

a
a 5 10 5
Ep/No» dB

Figure 5. Performance of a 2-phase, CPSK DS system over a channel composed
of a direct specular path plus a diffuse, Rayleigh fading, reflected
path.

15



100

dgg, SS Separation (km)

bgg = |IOMHz

Mp =294B

bsg = |MHz

bgs =10 MHz qT

SS Base to FM Base
150 MHz

dey, FM Path Length (km)

Figure 10. Separation distance dog versus dom to insure that

S/I OUT > 20 dB. SS base interference to an FM
base station, 150 MHz.

28


approximately equal to Dep? These signals are then filtered to a bandwidth

Ding! which means that the interference is transformed into Gaussian noise

and reduced in power by the factor

b.
10 log SE = - a , (11)
RF P

Since the signal power is essentially unchanged by the decorrelator, the
output signal-to-interference radio is

s _s

2 = 2iIn+ 12

y OUT = 7 IN+G oB (12)
as originally described in (1). However, in order to remove the data rate
parameter, most technical literature avoids using £ ovr, but instead measures

performance in terms of E/N, or EL/N,: For the binary case (k = 1), expres-
sions (8), (10), and (12) can be combined to yield

HM

IN + S& db . (13)

7 |"

The effect of cw interference on a frequency hopper is somewhat dif-
ferent as shown in Figure 2(b). As can be seen, the FH decorrelator "maps"
the cw interferer to a new frequency at each hop of the local reference sig-
nal. Exactly what frequency and how many frequencies the interfering signal
can be mapped to depends on the design of the PRN frequency hopping sequence.
In fact, one can visualize the case where the frequency increments are suf-
ficiently close to each other that the interference spectra would be continuous.
In other words, the FH system would remap the cw interference into a continuous
spectra spread through a frequency band of bop: The interference spectra out of
the decorrelator would thus be identical for the FH and DS systems, which
means that (12) and (13) apply to both types of systems.

_ So far the discussion on the performance of SS systems in interference
has been idealized, and has concluded that the cw interference power out of
the decorrelator will be reduced by the factor Ss: Unfortunately, there are
practical design considerations that prevent one from achieving this limit.
This is particularly true for the FH system where numerous articles have
appeared describing why the actual improvement can be less than predicted

(Pettit, 1977; Kullstam, 1977; and Davies, 1973). The article by Pettit shows

10



Typical values for Nor m, and fit) are

No n fit
49,764 1276 -1020
49,842 1278 -1019
51,320 1282 -1016

2. References
Cooper, G. R., and R. W. Nettleton (1976), Efficient spectrum utilization

with wideband signals, Final Report TR-EE 77-12, School of Electrical
Engineering, Purdue University, W. Lafayette, Indiana.

69



1000 Por

[— TTT TY T T TT TT

F=900 MHz

100

SS Base to FM Base

SS Mobileto FM Base or
SS Base to FM Mobile

dth » Separation Required to Avoid Exceeding - 157 dBW Threshold (km)

100 W

ERPgg = |OW

1 10
bgg (MHz)

Figure 27. Separation den that is required to avoid exceeding a

-157 dBW squelch threshold on a bip = 25 kHz receiver.
Frequency is 900 MHz.

54

100


This report will attempt to shed some light on at least one facet of the
problem; namely, the possibility of spectrum sharing between a spread-spectrum
land mobile system and a conventional frequency-modulated (FM), land-mobile
system. Specific attention will be given to the problems involved in the
overlaying of a new spread-spectrum system into the frequency bands already
occupied by existing FM systems. Overlaying here is interpreted as the
unrestricted operation of SS and FM land-mobile systems in the same service
area and on the same frequency. The scope of this report has been limited

to only voice transmission. However, some of the results are applicable to

both data and voice as will be shown later.

2. BACKGROUND

The basic principle behind the success of a spread-spectrum system is
that the transmitted signal is spread over a frequency band much wider than
the information bandwidth of the signal being sent. This increased bandwidth
buys a number of advantages as well as, unfortunately, some disadvantages.
The major advantage, and probably the most compelling reason behind the
development of spread-spectrum systems, is a reduced vulnerability to jamming
and interference. It can be easily shown (Dixon, 1976) that the output
signal-to-noise ratio of a spread-spectrum system in an interference or

jammer dominated environment is

s s
yOT=TIN+G, 4B, (1)

where (S/I) IN is the ratio in decibels of signal-to-interference power at

the input to the receiver and S is the processing gain given by

b.
by

G, = 10 log ap. (2)

NF

Throughout this report a convention will be used where quantities such as G
will be shown with upper case letters if it is in decibels and lower case if
it is in numeric form. Here bap! Piyp represents the ratio of the system's
transmission bandwidth to information bandwidth. Thus, with bop = 1 MHz and

Dine = 10 kHz, the value of the processing gain will be S = 20 dB, which


6. SPREAD-SPECTRUM INTERFERENCE TO FM
The definition of interference throughout this report will be based on
output signal-to-noise ratio. From the previous discussions in Section 4 on
threshold, it is evident that with B = 2, the threshold level occurs when
S/I OUT = 20 dB in a specular propagation channel (M, = 0 dB). Although the
sharp threshold no longer exists on a Rayleigh fading channel, a value of
S/I OUT = 20 dB would be considered a minimal level. Thus this report will

define interference as
s
" ouT < 20 dB . (31)

Tables 2 and 3 list the equations that are used in this report for calcu-
lating S/N OUT due to interference from a single SS source. These equations
are obtained by combining equations (21) and (30) with the appropriate path
loss equation given in (23) through (26). The expressions therefore represent
the median S/I OUT since they are based on median propagation losses.

These equations can be further reduced provided one is willing to make
some assumptions. The first assumption is that the transmitter power in an
SS system is going to be comparable to those currently used in FM. Although
this is strictly conjecture, it has already been shown that the E/N, require-
ments for a DS system and a narrowband CPSK system are the same in a non-
fading, Gaussian, additive-noise situation. The only advantages occur on
propagation channels with severe multipath or channels with narrowband inter-

ference. This report thus assumes that

Pog = Pam * (32)

Since dip will probably vary somewhat depending on receiver quality, this

report will. use the value

bin = 25 kHz (33)

and

B=2 ; (34)

25



(b) SS system with & = 40 4B
= 1, N, = 0, 0.010 < £; < 0.025
= 5, N, = 0, 0.020 < f, < 0.140

N= 5, N, = 5, 0.026 < f, < 0.210.
m B

»

W

A
be

b

The range of fy corresponds to the requirements that 10 < E,/N, < 20 4B

to account for the various multipath conditions that might be expected in an
urban channel. These results lead to the conclusion that the SS system will
also receive significant levels of interference with unrestricted operation
in frequency bands already occupied by existing FM systems.

One type of SS system that appears to have significant promise is an FH
system that is programmed to avoid frequency channels already in use in a
given locality. With proper design, this type of system could essentially
achieve the signal suppression necessary for unrestricted operation. The
frequency avoidance method is essentially a method of reclaiming unused spec-
trum or. spectrum used outside of the immediate service area.

It is also interesting to compare the conclusions of this report with
those achieved in an independent study (Dvorak, 1978). This study which also
examines the compatibility of spread-spectrum signals with narrowband FM

receivers in VHF mobile networks concludes the following:

"It follows from the preceding that the compatibility of
even a single SS transmitter with a power comparable to the levels
currently used in present VHF mobile communication would be dif-
ficult to achieve. Although the SS interference may remain
unidentified, because of its noise-like character, receiver thres-
hold sensitivity will be reduced up to relatively large distances
around the SS transmitter. With diminishing separation, the 2.5
amplitude of interference will increase approximately to a 1/d“"~-
law so that reception of all but the strongest signals would soon
become impossible. Especially in mountainous terrain the linear
dependence of the interfering signal on the effective height of
the interferer's antenna may contribute to an accelerated onset
of these effects."

Numerous assumptions have been used throughout this report. Although
arguments can undcubtedly be made for and against these assumptions, it is
doubtful that the conclusions would be significantly altered. Some of the
major assumptions are that the transmitter powers of the SS and FM systems
are comparable. The only instance where the transmitter powers are apt to be

drastically different is if the ss system were implemented using the cellular

61



Dvorak, T. (1978), Compatibility of spread-spectrum signals with narrowband
FM receivers in VHF mobile networks, IEEE Symposium on EMC, Atlanta,
Georgia, pp. 19-23.

Hernandez, Arecio A. (1975), Investigation of pulsed interference to narrow-
band FM receivers, ESD-TR-75-023, U.S. Dept. of Defense, Electromagnetic
Compatibility Analysis Center, Annapolis, Md.

Kullstam, P. A. (1977), Spread spectrum performance analysis in arbitrary
interference, IEEE Trans. on Communications, COM-25, No. 8, pp. 848-
853.

Park, J. H., Jr., and C. Chayaradhanangkur (1977), Effect of fading on FM
reception with cochannel interference, IEEE Trans. on Aerospace and
Electronic Systems, AES-13, No. 2, March, pp. 127-132.

Pettit, Ray H. (1977), On the use of "processing-gain" as a performance-
measure for spread spectrum systems, IEEE Southeast Conference Record,

Atlanta, Georgia, pp. 185-188.

Stein, Seymour, and J. J. Jones (1967), Modern Communication Principles
(McGraw-Hill, New York), p. 180.

63





nearly Gaussian. This is due to the fact that the impulse response time of
the IF is significantly greater than the average duration between hits. Thus
integration of the hits occurs and the output approaches Gaussian statistics.
This means all of the assumptions made previously for DS also apply for FH in
this instance.

As the hit rate is decreased to a value less than the IF bandwidth, the
Gaussian assumption is no longer true since the IF now has sufficient time to
fully respond to the interferer that has suddenly hopped into its passband.
The output of the IF in this instance will become a pulse whose duration ir
determined by the hopping rate.

The effects of frequency hopping interference on the performance of an
FM receiver remain largely unexplored and are beyond the scope of this
limited report. A report by Hernandez (1975) which studies the performance
of FM receivers in pulsed interference shows that the performance remains
relatively constant with decreasing pulse rate, at least until rates of
around 200 Hz. Therefore, one can summarize by saying that equation (21) can
be expected to be valid for both DS and FH interference provided the hit rate
from the FH system is greater than the IF bandwidth of 10 to 25 kHz. There

is also some evidence to suggest that the equations are also valid with hit
rates as low as 200 Hz.

5. PROPAGATION MODELS

One additional area that needs discussion before interference studies
can be completed is in propagation. Antenna heights for base stations will
be assumed to be at 200 m, which means that the median path loss between
two base stations will be essentially line-of-sight as given by

(a) base-to-base d < 64 km

L(f,d) = 32.5 + 20 log £(MHz) + 20 log d(km) dB , (23)

The distance 64 km corresponds to the effective radio horizon between two
such base stations. If the separation is-greater than 64 km, the median path

loss will be assumed as

(b) base-to-base d > 64 km

L(£,d) = 32.5 + 20 log £(MHz) + 20 log d(km) + C, + Hy log d(km) dB,

(24)

4

22


Figure 30.

Figure A-1.

Figure A-2.

Table 1.

Table 2.

Table 3.

Table 4.

Table 5,

Table 6.

Table 7.

Table 8.

Table 9.

Table 10.

Separation distance d, versus that is required to
insure S/I OUT > 20 a8°with.a 10°Muz ss system at

150 MHz. Assumes additional 30 dB suppression due to
FH pattern. 2 6. 1 6 6 ee ee ee ee ee ee eee ee

Typical frequency-hopped, time-encoded signal ... ...

Proposed decoder .---.-.-.-+.+-+-+-+--

LIST OF TABLES

Constants Used in Propagation Formulas. -..-.-.-.-

Equations of Output Signal-to-Noise Ratio for an FM
Base Station Receiving Interference from an SS Base
Station . 6... eee ee ee ee

Equations of Output Signal-to-Noise Ratio for an FM
Base Station Receiving Interference from an SS Mobile
or an FM Mobile Receiving Interference from an SS Base
Station . 6 6 1 ee ee ee ee ee ee

Summary of Separation Distance, d,,, that is Required
to Insure S/I OUT > 20 dB (SS System Bandwidth is
Summary of Separation Distance, d.., that is Required

to Insure S/I OUT > 20 dB (SS System Bandwidth is
10 MHz)... 2... eee

Equations of Output Signal-to-Noise Ratio, E, JN + for

an SS Base Station Receiving Interference from &n FM
Base Station. . . .

Equations of Output Signal-to-Noise Ratio, E. /N_, for
an SS Base Station Receiving Interference from Sn FM
Mobile or an SS Mobile Receiving Interference from an
FM Base Station... .

Summary of Separation Distance, qa , that is Required
to Insure 10 < E/N, < 20 4B with > = 30 dB. ...--

Summary of Separation Distance, , that is Required
to Insure 10 < B/N, < 20 4B with u, OdB......

Summary of Monte Carlo Simulation Program where f, is

the Fraction of SS Mobiles Denied Access in the
Circular Urban Model... . - 6-6 ee eee ee eee

vii

58

66

68

23

26

26

37

37

39

39

44

44

51



Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

16.

17.

18.

19.

20.

21.

22.

23.

24.

25.

26.

27.

28.

29.

Separation distance dy, versus qa, that is required to
insure that S/I our >°So ap withTMY 1 maz ss system at
900 MHZ. 6 6 ee ee we we ee ee ee ee ee

Separation distance dy, versus that is required to
insure that S/I our >°3o ap withTMS 10 MHz SS system at
900 MHz. 2. - ee ee ee ee ee

Separation distance versus des that is required to
insure 10 < E/N, < 20°dB with SS = 30 dB, 150 MHz. .

Separation distance versus d, S that is required to
insure 10 < B/N, < 26“ap, with & = 40 4B, 150 MHz. .

Separation distance d. versus d Ss that is required to
insure 10 < E/u, < 26"ap, with & = 30 4B, 900 MHz. .

Separation distance d. M versus d, S that is required to
insure 10 < E/N, < 26"ap, with & = 40 dB, 900 MHz. .

Geometry used in Monte-Carlo simulation. .......

Monte-Carlo simulation results for an SS system with
S = 30 dB and E/N, requirements of 20 dB at 150 MHz.
Monte-Carlo simulation results for an SS system with
G = 30 dB and E./N = 10 dB. The results also apply
fBr ac. = 40 dB syStem with E/N requirements of

20 4B ak 150 Maz. 2 2 OO,

Monte-Carlo simulation results for an SS system with
G, = 40 dB and E/N, requirements of 10 dB at 150’ MHz.

Separation 4, that is required to avoid exceeding a
-157 dBW squéich threshold on a bip = 25 kHz receiver.
Frequency is 150 MHz ......7..........
Separation 4, that is required to avoid exceeding a

-157 dBW squéich threshold on a bip = 25 kHz receiver.
Frequency is 900 MHz... ... 0... 2. ee eee
Example of a frequency hopper with (a) all frequencies
Present and (b) a missed frequency ..........

Separation distance d., sg Versus that is required to
insure S/I our > 20 dBSwith a 1 Milz ss system at 150

MHz. Assumes additional 30 dB suppression due to FH
pattern,

vi

35

36

40

41

42

43

46

48

49

50

53

54

56

57


that the actual processing gain for a &, = 27 dB FH system can range anywhere
from 6 to 27 dB. One of the major problems that must be overcome in an FH
system is that, if there are m available frequencies for hopping to, a cw
interferer effectively destroys 1/m of the available spectrum. Thus, ideally |
one would like m to be as large as possible. Digital encoding is also recom-
mended to insure that correct reception will be achieved even though the 1
out of N frequencies has been disabled.

Equations (12) and (13) also apply if there are multiple independent
interferers on different frequencies. The value for (S/I) IN now becomes the

ratio of signal power to total received power from all interferers.

3.4 Performance in Multipath

The performance of an SS system in multipath can probably best be under-
stood by looking at specific examples. Unfortunately, examples dedicated to
the urban multipath environment are almost nonexistent. Calculations do
exist, however, for some elementary channels shown in Figure 3.

The first example is the DS system operating in a channel composed of
two specular paths. The signal power received over each path is asstined to
be equal, with a difference in time of arrival between the two signals At.

It ‘is also assumed that each of the paths adds a random phase perturbation to
the signal and that the phase perturbation is uncorrelated between paths.
This means that fading will still occur even as AT approaches zero.

Estimates of the probability of a bit error Po for this- situation have been
derived by Cahn (1973) and are shown in,part in Figure 4. Four curves are
shown in this figure for different values of differential time delay AT,
where At is expressed relative to the PRN chip element duration ty (a 1 MHz
SS system would have t. = 1s). As can be seen, the worst case occurs when
the time delay between two paths is such that At = 0, which is an unlikely
condition in nature. Next note that, unlike conventional narrowband systems,
the performance improves with increasing time delay until eventually, when
At > tyr the system performs as though there were no multipath at all. The
reason for this is that a signal delayed by more than te is essentially
uncorrelated from the direct signal. Therefore, this signal is treated as
uncorrelated interference in the decorrelation process, which means that its

effective power is reduced by the factor S,- Thus the performance of a DS

11



A COMPATIBILITY ANALYSIS OF SPREAD-SPECTRUM AND FM LAND MOBILE
RADIO SYSTEMS

*
John R. Juroshek

The possibility of overlaying a spread-spectrum system into
frequency bands containing conventional FM land-mobile systems
is examined. Overlaying here is interpreted as meaning the
unrestricted operation of spread-spectrum and FM mobiles through-
out the same service area and on the same frequency. The report
assumes conventional spread-spectrum and FM systems where a sin-
gle base serves a large urban area. The small cell narrow
coverage concept is not discussed. A theoretical compatibility
study is described that concludes that significant interference
would result to existing FM systems. This conclusion assumes
spread-spectrum transmitter powers comparable to existing FM
systems. The report considers land-mobile operating frequencies
of 150 and 900 MHz.

The study also examines the reverse problem of interference
from FM to spread spectrum. Curves are prepared showing separa-
tion requirement for various channel multipath conditions. A
computer simulation program is also described that simulates the
operation of a spread-spectrum system in a multiple FM interferer
environment. The conclusions are that an overlayed spread-
spectrum system also would receive significant interference.

The report also describes a frequency hopping, spread-spec-
trum system that is programmed to miss those FM channels in use
at a given locality. The advantages obtained with this technique
are briefly discussed.

Key words: frequency modulation; interference; land mobile
radio; multipath; propagation; spread spectrum

1. INTRODUCTION

The issues of whether or not to allow the use of wideband, spread-spec-
trum (SS) systems in an already crowded frequency spectrum are numerous and
complex. One can undoubtly find a number of arguments both for and against
the wisdom of such a decision. To compound the situation, the arguments
become more confusing when one is considering the urban land-mobile

environment where spectrum is scarce and propagation conditions are poor.

* . .
The author is with the U.S. Department of Commerce, National Telecommunica-
tions and Information Administration, Institute for Telecommunication
Sciences, Boulder, Colorado 80303.



where > is the parameter that determines the correlation bandwidth and a is
the parameter that controls the ratio of specular to scattered energy. A
value of a = 1 means complete specular propagation while a = 0 means only
scatter propagation. Correlation bandwidth B, is defined here as that Af

where
c(B,) =0.5 . (15)

Figure 6 shows the performance of this system for the case where there

are n = 32 frequencies in the hopping sequence. The curves in this figure
are parametric in a and B/Bapt which is the ratio of the correlation band-

width relative to the rf bandwidth. The curves labeled B/Bpp = © are the
limiting case where differential time delay is allowed to go to zero.
Practically, BL/Bap = © also represents the case where the time delay of all

significant components are such that

1
Ars. (16)
Bop

The second case shown in the curves is BL/Bap = ra which denotes a multi-
path situation where a significant fraction of the energy is arriving with
time delays

At > _— . (17)

PRP

One should note that the error rate in Figure 7 is measured in terms of Pat
the symbol error probability. The reason for this is the system uses M-ary
encoding where more than one bit is transmitted for each frequency-hopping
pattern. The M-ary encoding also accounts for the fact that the system
requires slightly less E,/N, than the DS system to achieve a given level of
performance.

The important conclusions in regards to spread spectrum in multipath is

that unlike narrowband digital systems the performance improves with increasing

multipath time delays. Another fact that will be used throughout this report
is that an SS system will probably require an E/N, such that

17



NTIA-REPORT-79-23

A COMPATIBILITY ANALYSIS
OF SPREAD-SPECTRUM AND
FM LAND MOBILE RADIO SYSTEMS

J. R. Juroshek

At OF
nee Oy,
ff %,
é %
8 o
On iF
Sr,
vares OF

U.S. DEPARTMENT OF COMMERCE
Juanita M. Kreps, Secretary

Henry Geller, Assistant Secretary
for Communications and Information

August 1979



ina FM Base Interference to SS Base - Propagation Dependent

FM Base Interference to SS Base - Highly Probable

FM Base (Mobile) Interference to SS Mobile (Base) - Propagation Dependent
FM Base (Mobile) Interference to SS Mobile (Base) - Highly Probable

dem, FM Separation (km)

dgg, SS Path Length (km)

Figure 18. Separation distance Fou versus dog that is required to insure
10 dB< EL /N < 20 dB with G = 30 dB, 150 MHz.
° P

40



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*T eanbta

peziibig

Y3TIOULNOD
yoo|9 aud
4300030 YOLYINGOWIG
<—____
| Bd1IOA, JOIOA R03u
N39 0
490|9 NUd $
4300030 YOLYINGOWIC
<«__]
BIO, JOIOA MEL)
Baio’

(9)
YITIOULNOD
03u4 Tee) a)
¥3ZIL1910
910A Bd10A,
<=
(0)
N39
Nud y90|9
%y
X + YAZILI9IG
CON 010A O10
Bd10A,
2. peziyibiq


means that the spread-spectrum system can have an output signal-to-noise
ratio greater than 0 dB even though S/I at the input to the receiver is less
than O dB.

One must not assume, however, that a spread spectrum system offers any
improvement over a narrowband system operating in white Gaussian noise. The
output signal-to-noise ratio for an SS system in Gaussian noise can be shown

to be

£ ovr = 10 log Fatal 4B , (3)
{INF “o
where s/n, is the ratio of input signal power to noise power density and Dine
is the information bandwidth. This means that the output signal-to-noise
ratio for white Gaussian noise is independent of the rf bandwidth or pro-
cessing gain.

These concepts can be generalized to some extent. Generally speaking,
a wideband, spread-spectrum system will perform better than a narrowband
system in an interference environment where the spectral bandwidth of the
interference is much less than bor’ If the spectral bandwidth of the inter-
ference is greater than bop the performance advantage of a wideband system
over a narrowband system is generally minimal since the increase in per-
formance due to processing gain is offset by the increase in interference
power due to the wider bandwidth receiver. The word "generally" must be
emphasized here since secondary considerations do exist. For example, the
performance of most spread-spectrum systems is determined by the average
power in the interference signal and thus is fairly insensitive to the inter-
ferer's peak power characteristics. ‘This means a spread-spectrum system can
potentially offer an advantage in a wideband pulsed type of interference

where the average power is much less than the peak.
3. SPREAD-SPECTRUM PERFORMANCE CALCULATIONS

3.1 General Description

Spectrum spreading can be accomplished by many different methods.
Figure 1 shows block diagrams of two common methods that will be considered
in this report. These are spectrum spreading by direct sequence (DS) tech-
niques as shown in Figure 1(a) and frequency hopping (FH) as shown in Figure 1(b).



dth , Separation Required to Avoid Exceeding -157 dBW Threshold (km)

1000 T TTT TT TTY T T TOT TT TT

. F=150 MHz

100

SS Base to FM Base

SS Mobile to FM Base or
SS Base to FM Mobile

Litt
] 10 100

bgg (MHz)

Figure 26. Separation qn that is required to avoid exceeding a

-157 dBW squelch threshold on a dip = 25 kHz receiver.
Frequency is 150 MHz.

53



SS Base Interference to FM Base — Propagation Dependent

SS Base Interference to FM Base - Highly Probable

SS Base (Mobile) Interference to FM Mobile (Base) - Propagation Dependent
SS Base (Mobile) Interference to FM Mobile (Base) - Highly Probable

dsg, SS Separation (km)

dew, FM Path Length (km)

Figure 17. Separation distance deg versus Fou that is required to insure
that S/I OUT > 20 4B with a 10 MHz SS system at 900 MHz.

36



The short-term fading around the median can be assumed to be Rayleigh distri-
buted, while the long-term fading is log normal.

The discussion of propagation models also leads to the question of
antenna gain. This report will assume omnidirectional antennas which means
that the signal-to-interference calculations are independent of antenna gain.
For example, consider the case where the signal level at the input terminals

of an FM receiver is

Stw > Pam * Arwen * Apyer 7 B(Er doy) OB + (27)

where Pam denotes FM transmitter power, A denotes antenna gain relative to
isotropic, and L(£ diy) denotes the propagation loss encountered by the
victim FM signal as described previously. The subscripts FM-T and FM-R have
been added to denote transmitter and receiver, respectively. Similarly, the

interference power at this point is

Tin = Pos + Ago in + AoM-R - L(f 1d.) ap. (28)

The subscripts SS have now been added to denote the spread-spectrum system.

This means that the signal-to-interference ratio at this point is

s
TIN = Paw 7 Psg + Apyen ~ Aggon 7 L(£,4,4) + L(f£,d,,) 4B. (29)

If one further assumes that the antenna gain of the FM and SS transmitters

are equal, then
s :
I IN = Pom - Pss - L(£/doy) + L(£,dg.) ap. (30)

The assumption that Faun = Aggup is probably a good assumption particularly

if both antennas are omnidirectional.

24


FM Base Interference to SS Base - Propagation Dependent

FM Base Interference to SS Base - Highly Probable

FM Base (Mobile) Interference to SS Mobile (Base) - Propagation Dependent
FM Base (Mobile) Interference to SS Mobile (Base) - Highly Probable

100

dem, FM Separation (km)
Ss

dgg, SS Path Length (km)

Figure 19. Separation distance dont versus agg that is required to insure
< < i =
10 4B < EL/N, < 20 4B, with &, 40 dB, 150 MHz.

41


where the constants C, and H, have been added to account for propagation over

4 4
the radio horizon. Path loss for over the horizon propagation will be assumed
to have a variation of distance of a®°. For base-to-mobile propagation, the

corresponding formulas for median path loss are

(c) base-to-mobile d < 30 km

L(£,d) = 32.5 + 20 log £(MHz) + 20 log d(km) + C, + H, log d(km) dB,

1 1
(25)

and

(d) base-to-mobile d > 30 km

L(£,d) = 32.5 + 20 log £(MHz) + 20 log d(km) + cy + Hy log d(km) dB

(26)

These formulas are representative of the urban environment at frequencies
between 150 and 900 MHz (Berry, 1978). The antenna height for the mobile is
chosen as 1.5 m which accounts for the reduction in effective radio horizon

to 30 km. The various constants used in these formulas are given in Table 1.

Table 1. Constants Used in Propagation Formulas

Frequency Cy Cy cy Hy Hy Hy
(Miz) (ap) (4B) (4B)
150 19 -25.6 -72.2 10 40.3 40
450 20 -31.7 -72.2 10 45.4 40
900 23 -37.8 -72.2 10 50.4 40

23



The reverse situation of interference from FM to SS systems is only

marginally better at best. For this situation, dom

separation distance while agg is the victim SS transmission path distance.

now becomes the required

The corresponding separation requirements at 150 MHz are:

(a) FM mobile interfering with a & = 30 dB SS agg = 10 kn,
base station dom = 2-5 km;
(b) FM mobile interfering with a eq = 40 dB SS dg = 10 kn,
base station Fay =1- 2 kn;
(c) FM base interfering with a S = 30 dB SS ds. = 10 km,
base station dom = 27- 70 km;
(ad) FM base interfering with a Ss, = 40 dB SS dog = 10 kn,
base station dom = 9 - 28 km.

These estimates are for the case of a single FM interferer.

An SS receiver, however, can be expected to see multiple FM interferers
because of its wide bandwidth. The multiple interference situation is
examined with a Monte-Carlo, computer simulation program. The parameter used
to measure interference in this case is fi, which is the fraction of SS
mobile locations denied access within a circular service area. These SS
Mobile and interfering FM mobiles are assumed to be randomly located through-
out the circular service area. Interfering FM base stations are also included
in the simulation program, but are restricted to operation at the outer
perimeter of the SS service area. This restriction was imposed since exces-
sive levels of interference from an FM base to SS base were encountered
unless steps were taken to insure that the FM base station was outside of the
SS service area.

Results of the simulation program are shown in the following where Ny
represents the number of simultaneous interfering FM mobiles and Np repre-

sents the number of simultaneously interfering FM base stations:

(a) SS system with S = 30 dB

A
a
A
°
B
N
3

= 1, N, = 0, 0.025 <
N= 5) Ny = 0, 0.140 < £,
= 5, N, = 5, 0.210 < £, < 0.940

A
ia
IA
°
'S
ior)
f=}




system is generally unaffected by multipath components delayed by more than
to: This, of course, is a simplified explanation as other considerations
exist, particularly in the area of maintaining synchronization.

A similar channel, for which the performance of the DS system is known,
is.one in which the reflected path is a diffuse or scatter type of reflection
(Cline, 1973). The probability of a bit error as a function of E/N, in
this situation is shown in Figure 5. The curves in this figure are parametric,
in the ratio PQ/Ppe which is the power in the direct path relative to the
power in the indirect path. For this case, the worst performance occurs when
the channel is all diffuse; however, the performance rapidly improves with
the addition of a specular-direct path. These calculations assume that all
multipath components have differential time delays such that ATS ty

It should be pointed out that the curves in Figures 4 and 5 that are
shown ‘for specular-only propagation are also applicable for conventional
narrowband CPSK digital systems. In other words, the E/N, requirements for
a DS system are identical to those for conventional. CPSK on a nonfading,
Gaussian, additive-noise, propagation channel. The performance however is
significantly different on fading channels with multipath or any propagation
channel with narrowband interference.

The last example is an FH system that is designed specifically for the
land-mobile environment. This system, as proposed by Cooper and Nettleton
(1976), uses a M-ary time encoded waveform that frequency hops over m dif-
ferent frequencies. A detailed description of this system is given in
the Appendix.

The performance of this system has been computed for the channel containing
both a specular and fading component as shown in Figure 3(b). However,
unlike the previous calculations, the multipath is no longer restricted to
the case AT < ty Figure 6 shows the channel impulse model that is used in
these calculations. It contains a specular component due to the direct path
plus a diffuse component whose energy versus time-of-arrival is exponentially
decaying. The correlation function of this channel as a function of frequency
offset Af is given as

Q_-_a)

* c(A£) at Tou ' " (14)

a

14


These results can be summarized by stating that, with a marginal SS sys-
tem (ft > 0.1 with N= 1 and N, = 0), the addition of a single base station
can have a significant effect. This is not true however with the nonmarginal
system where the increase in fy due to a single interfering base is negli-
gible. Table 10 summarizes these results by listing fi for various Ni, and
N.. It is important to remember that the different multipath conditions

B
accounts for the 10 dB < E,/NG < 20 dB range.

Table 10.. Summary of Monte Carlo Simulation Program where f, is the
Fraction of SS Mobiles Denied Access in the *
Circular Urban Model

S, fy

(4B) Ny NB E/N, = 10 dB E/N, = 20 GB
30. L 0 025 .120
30 5 0 140 480
30 5 5 210 -940
30 10 5 -760 -960
30 10 10 -780 v1
40 1 10) <.010 .025
40 5 0 020 .140
40 5 5 «026 210
40 10 5 -037 -760
40 10 10 .078 .780

51



SS Base Interference to FM Base - Propagation Dependent

SS Base Interference to FM Base - Highly Probable

SS Base (Mobile) Interference to FM Mobile (Base) - Propagation Dependent
SS Base (Mobile) Interference to FM Mobile (Base) - Highly Probable

100 T TTT TT TTY

dgg, SS Separation (km)
Ss
T

dey, FM Path Length (km)

Figure 30. Separation distance dog versus Gon that is required to insure

S/I OUT > 20 dB with a 10 MHz SS system at 150 MHz. Assumes
additional 30 dB suppression due to FH pattern.

58


dgg, SS Separation (km)

100 T TTT

T

T

| Mr = 29dB
(off scale)

bsg = 10 MHz

SS Base to FM Base
900 MHz

dem, FM PathLength (km)

Figure 12. Separation distance dog versus doy to insure that

S/I OUT > 20 dB. SS base interference to an FM
base, 900 MHz.

31

100



: US. DEPARTMENT OF COMMERCE
Bent O78 vse e oe Tet ecommUMreA TIONS

BIBLIOGRAPHIC DATA SHEET

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

NTIA Report 79-23

4, TITLE AND SUBTITLE 5, Publication Date
August 1979

A COMPATIBILITY ANALYSIS OF SPREAD-SPECTRUM AND FM LAND 6. Performing Organization Code
MOBILE RADIO SYSTEMS NITA/TTS

7. AUTHOR(S) 9 Project/Task/Work Unit No.
John R. Juroshek

8. PERFORMING ORGANIZATION NAME AND ADDRESS
National Telecommunications & Information Administration | 9101115

Institute for Telecommunication Sciences 10. Contract/Grant Now
325 Broadway
Boulder, CO 80303
11, Sponsoring Organization Name and Address 12, Type of Report and Period Covered
National Telecommunications & Information Administration
Institute for Telecommunication Sciences

325 Broadway 13.
Boulder, CO 80303
14, SUPPLEMENTARY NOTES

Ts. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant
bibliography of literature survey, mention it here.)

The possibility of overlaying a spread-spectrum system into frequency bands
containing conventional FM land-mobile systems is examined. Overlaying here is
interpreted as meaning the unrestricted operation of spread-spectrum and FM mobiles
throughout the same service area and on the same frequency. The report assumes
conventional spread-spectrum and FM systems where a single base serves a large urbar
area. The small cell narrow coverage concept is not discussed. A theoretical
compatibility study is described that concludes that significant interference would
result to existing FM systems. This conclusion assumes spread-spectrum transmitter
powers comparable to existing FM systems. The report considers land-mobile oper-
ating frequencies of 150 and 900 MHz.

The study also examines the reverse problem of interference from FM to spread
spectrum. Curves are prepared showing separation requirement for various channel
multipath conditions. A computer simulation program is also described that

(continued)

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

Frequency modulation; interference; land mobile radio; multipath; propagation;
spread spectrum.

17. AVAILABILITY STATEMENT 18. Security Class (This report) 20, Number of pages
C4 unumitep. Unclassified 70
19. Security Class (This page) 21. Price:

(FoR OFFICIAL DISTRIBUTION.
Unclassified

USCOMM-DC 20716-P73
sco = EL



dgg, SS Separation (km)

100

SS Base to FM Mobile
SS Mobile to FM Base
900 MHz

bsg =!MHz

bsg = 10 MHz

bgg= 10 MHz 4

Figure 13.

10 100
dey, FM Path Length (km)

Separation distance deg versus Fay to insure that

S/I OUT > 20 dB. SS base interference to an FM
mobile, or SS mobile interference to an FM base,
900 MHz.

32


(c) simulate Ny base stations along the perimeter,

(d) calculate the total interference power Tom at the victim
SS base from the Ny interfering FM mobiles,

(e) calculate the total interference power Top at the SS
base from the NB interfering FM base locations,

(£) the total interference power at the input to the SS
base receiver is then Tay = Tig + Inge

(g) calculate the resulting E/N: and

(h) if E/N, is less than required (curves are shown for
both 10 and 20 dB), then consider the SS mobile as having

been interfered with.

In order to keep the problem as simple as possible, it was decided that these
preliminary estimates would be made w?th R < 30 km, which means that the dis-
tances are all within the radio horizon. Thus propagation equations (23) and
(25) apply. The computer programs, ‘however, can be easily extended to the
case of over-the-horizon propagation.

ane main output from the simulation program is fi, which is the number of
SS mobile transmissions that were interfered with divided by the total number
examined. Figure 23 shows a plot of fy for a &, = 30 dB SS system with channel
E/N, requirements of 20 dB at 150 MHz. The symbols on the graph denote the
actual simulation locations. One can see that in this instance the presence
of one base station causes significant interference and, in fact, is worse
than the interference encountered from 13 mobiles with no base stations.
Results for the same system with reduced E/N, requirements of 10 dB are shown
in Figure 24. Here the addition of a single base station is not as severe.
In fact, the reverse situation is true where increasing the number of simul-
taneous mobiles from 1 to 2 causes more interference than changing the number
of base stations from 0 to 1. Figure 24 also applies for the & = 40 dB,
E,/NG = 20 dB system, since the increased E/N requirements offset the addi-
tional performance obtained by the increase in processing gain. The last
case, shown in Figure 25, is for the S = 40 GB, E,/N, = 10 dB case. With
this situation, the effect of the interfering base stations is minimal. In

fact one can add five base stations with only minor increases in interference.

47



above threshold. The value of this threshold depends, of course, on whether
or not the demodulator uses threshold extension techniques as well as the
modulation index 8. Figure 8 shows the threshold performance of an FM |
receiver both with and without threshold extension. As can be seen with
6 = 2, an S/Nyrg greater than 8 dB is required to be above threshold without
threshold extension and greater than 4.5 dB with threshold extension. Thus
with 8 = 2, the threshold occurs when S/N OUT is 20 4B if threshold
extension is used and 24 dB with conventional demodulation.
With fading, the output signal-to-noise ratio can be substantially less.
Figure 8 shows what can happen when the desired signal is transmitted over a
Rayleigh fading channel (Park and Chayaradhanangkur, 1977). The sharp thres-
hold due to capture is no longer apparent. Also, the loss in signal-to-noise
ratio for 8 = 2 is approximately 29 dB. This means that if a S/N OUT = 20 4B
is desired with 8 = 2, S/I DIS must be approximately 33 dB. This loss is the
result of additional noise from random phase modulation that is impressed |
upon the signal by the fading and the "click noise" that occurs when the FM
signal is below threshold. Thus, throughout the report we will assume that

b
OUT = 10 log[387(B + 1)] + 10 log 2 + Say - M, 4B , (21)
Bip I

Zln

where M, has been added to account for fading. Since 8 for FM systems is

typically about 2, this report will use a value of M,

OSM, < 29 eB , (22)

where the lower limit is typical of those urban propagation paths that are
largely line-of-sight and the 29 dB limit is encountered on those (probably
rare) paths that are largely diffuse (Rayleigh fading).

So far the discussion has avoided the effects of FH interference on FM.
This has been intentional because of the different characteristics that can
be encountered with FH interference. In order to observe these character-
istics, we first define the hit rate as the number of times per second that
the frequency hopper's frequency falls within the passband of the victim FM.
If the hit rate is significantly greater than the IF bandwidth (nominally 10
kHz to 25 kHz) of the FM receiver, then the output of the IF will be very

20


f,, Fraction of SS Mobile Locations Denied Access

0.01

| | 1 i | ! L ! !

| 3 5 7 9 i 13 15
Nay Number of Simultaneous FM Mobile Interferers

Figure 25. Monte-Carlo simulation results for an SS system with
S = 40 dB and E/N, requirements of 10 dB at 150 MHz.

50


expected in an LMR environment. The problem is only compounded when one
considers the possibility of interference from an SS base to an FM base where
free space propagation conditions can exist.

The report has estimated the separation distance, d.., that is required

ss
to protect an FM system transmitting over a path of distance dom’ Some

examples of these estimates for a beg = 1 MHz system at 150 MHz are as

follows:
(a) SS mobile interfering with an FM base diy = 10 kn,
agg = 4 - 22 km;
(b) SS base interfering with an FM base aout = 10 km,
>
dog 64 km.

These estimates are only slightly reduced for a Beg = 10 MHz system where

(a) SS mobile interfering with an FM base dong = 10 km,
dog = 2 - 15 km;

(b) SS base interfering with an FM base dow = 10 km,
dog > 22 km.

The variability in agg corresponds to different fading margins for different
propagation channels. The lower figure is for a fading margin of M, = 0 4B,
while the larger is for M, = 29 GB.

A second problem that is encountered is the effect that an SS trans-
mitter would have on the squelch of an FM receiver. For example, it is
estimated that the separation distance qe between an SS mobile and FM base

that would be necessary to avoid exceeding a -157 dBW FM squelch threshold
(0.1 LV in 50 ohms) is:

a) SS mobil i = =

(a) obile with bo. = 1 MHz di, = 45 km
ERP. = 10 W

Db Ss i i = =

(b) 'S mobile with Beg 10 MHz Lan = 31 kn.
ERP og =10W

59





Table 6. Equations of Output Signal-to-Noise Ratio, E/N? for an SS Base

Station Receiving Interference from an FM Base Station

E/N, dB
gg Foy ip! o!
(km) (km)
ED
= -P_ -c_- +
<30 <64 X, Pog Pry Cy (204Hy) log dg. + 20 log a. + G,
Ey
—= =P + +
>30 <64 N Pog Pay Co (20+Hy) log dg. + 20 log a, + G,
Ey
> —_—= = ~~ -
30 64 x, Pog Pay Cy tCq~ (204H,) log do. + (20+H,) log dy + S,
>30 >64

Pog P py Co tC qn (204H5) log agg + (20+H,) log doy + &,

Table 7. Equations of Output Signal-to-Noise Ratio, E, JN 1 for an SS Base
Station Receiving Interference from an FM mobi or an SS
Mobile Receiving Interference from an FM Base Station

agg doy EL/Nor 4B
(iam) _|_ (cm)
ES
< < —= = -
£30 <30 i Pog Pryy (204H,) log do. + (20+H,) log d,,, + S,
By
> < = = Pyan-P. tC, -Co-
30 $30 X, sg 7PpytCy Co” (204H,) log dg, + (204H,) log d+ 6,
Ey
<30 > 2. =P -
< 30 x, Pog Pry CytCo~ (204H,) log dg, + (204H,) log diy + 6,
>30 >30 | Bap -(20+H,) log d,. + (20+H,) log ad. +6
Ny ss “FM 2 ss 2 FM Pp

39



Table 4. Summary of Separation Distance, d,
Insure S/I OUT > 20 dB (SS System Bandwidth is 1 MHz)

ss

, that is Required to

Fan agg £
___Interference (km) (km) (MHz)
*
SS Mobile to FM Base 10 4-22 150
30 12-52 150
SS Base to FM Base 10 >64 150
30 >100 150
*
SS Mobile to FM Base 10 4-22 900
30 12-52 900
SS Base to FM Base 10 >80 900
| 30 >100 900
Table 5. Summary of Separation Distance, dog that is Required to
Insure S/I OUT > 20 dB (SS System Bandwidth is 10 MHz)
T
doy, deg £
Interference (im) (km) (MHz)
*
SS Mobile to FM Base 10 2-15 150
30 5-29 150
SS Base to FM Base 10 >22 150
30 >80 150
*
SS Mobile to FM Base 10 2-17 900
30 5-40 900
SS Base to FM Base 10 >35 900
30 >85 900

*
Also applicable for an SS base interfering with an FM mobile.

37



concept. The cellular concept was not analyzed in this limited study. Other
major assumptions are that the base station antenna heights are 200 m with
mobile antenna heights of 1.5 m. This, in turn, leads to the propagation
models described in (23) through (26). Another assumption is that the

increase in FM receiver output noise can range from M, = 0 for a nonfading

£

channel to Me = 29 dB in a Rayleigh fading channel.

A major unknown, which was encountered during the preparation of this
report, is the effect of FH interference on FM receivers. While it is known
that FH interference will have pulse characteristics at slow hopping rates,
it is not known what effect this has on intelligibility. Also, information
on the effects of fading on the interference portion of the received signal
is lacking. A third area where information is lacking is the behavior of SS
systems in the urban multipath channel. A particular intriguing prospect is
the fact that SS transmission can effectively combat multipath to some extent.
This study has used the values 10 dB < E/N, < 20 dB to allow for the unknowns

in system design and channel propagation conditions.

11. REFERENCES

Berry, L. A. (1978), Spectrum efficiency for multiple independent spread-
spectrum land mobile radio systems, NTIA Report 78-11, U.S. Department
of Commerce, National Telecommunications and Information Administration,
November.

Cahn, C. R. (1973), Spread spectrum applications and state-of-the-art equip-
ments, AGARD Lecture Series No. 52, NATO Advisory Group for Aerospace
Research and Development, pp. 5-1 through 5-1ll. (NTIS, Springfield,
VA 22161, Accession No. AD-766914).

Cline, R. Ww. (1973), Pseudo random modulation--an effective means of enhancing
PSK signal transmission in a diffuse multipath environment, 25th Annual

Southwest IEEE Conference and Exhibition, Houston, Texas, April, pp.
312-317.

Cooper, G. R., and R. W. Nettleton (1976), Efficient spectrum utilization
with wideband signals, Final Report TR-EE 77-12, School of Electrical
Engineering, Purdue University, W. Lafayette, Indiana.

Davies, N.C. (1973) , Spread spectrum communications, performance and syn-
chronization considerations, AGARD Lecture Series No. 58, NATO Advisory
Group for Aerospace Research and Development, pp. 4-1 through 4-24
(NTIS, Springfield, VA 22161, Accession No. AD-766914) .

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

62


a (4)

where es is the energy of the transmitted waveform, ty is the waveform time
duration, ny is the Gaussian noise power density, and bine is the bandwidth
of the matched filter. The quantity ts can also be thought of as the period
available to the decision device for viewing the received waveform. At the
end of this period, the device must decide as to the status of the received
waveform and output the corresponding bits.

The ratio of signal power to noise power at the input to the decor-

relator can be shown to be

oO RF

where bor is the rf bandwidth of the receiver. Thus, the ratio

= fh <6 (6)

is defined as processing gain Sy One can also show that, for a matched
filter,

~1l
Pine ~t. ' ”
s
which means that
s &s
=or=— . (8)
n n
fo}

As noted previously, the decision device can output more than one infor-
mation bit for each decision. If k information bits are obtained for each

decision, the system is said to be using M-ary transmission, where

k= log, M (9)



fj, Fraction of SS Mobile Locations Denied Access
©

0.01 ! 1 L ! ! | ! | ! i |
| 3 5 1 9 W 13 15
Nm» Number of Simultaneous FM Mobile interferers
Figure 24. Monte-Carlo simulation results for an SS system with & = 30 dB

and E/N, = 10 dB. The results also apply for a S = 40 dB
system with E/N, requirements of 20 dB at 150 MHz.
: 49



With frequency-hopping systems there is’ an additional possibility for
avoiding interference if the frequency controller can be programmed to avoid
those FM channels that are in use in any given locality. The fact that a
frequency is missing does not necessarily mean that no energy will be radiated
in this 25 kHz channel, but only that it will be reduced. The exact amount
of reduction that can be expected in a 25 kHz channel is dependent on the FH
signal design and is determined by tradeoffs such as the hopping pattern,
filter requirements,’ and equipment costs. .~

Figures 28(a) and (b) show the spectrum of a frequency hopper with and
without a missing frequency. These photographs were obtained by looking at
the spectrum of a programmable signal generator whose frequency-hopping
sequence could be programmed to miss a given frequency. The gap in Figure
28(b) is approximately 30 kHz wide at the 3 dB points and is only intended as
an example of what might be expected. The characteristics of the notch could
be sharper with proper design and filtering. This technique provides a
method for "reclaiming" spectrum that is not used in a particular area or
spectrum that is in use outside of the immediate service area.

Figures 29 and 30 show the separation distance dos versus dom that would
be required assuming an additional 30 4B suppression due to frequency hopping.
Comparing these figures with Figures 13 and 14, one can see that a signifi-

cant improvement has been achieved.

10. CONCLUSIONS

The conclusion of this report is that it would be impossible to overlay
a new spread-spectrum LMR system into a frequency band already occupied by
existing FM systems without causing interference. The definition of overlay
here has been intérpreted as meaning the unrestricted operation of both SS
and FM mobiles throughout the same service area. This conclusion results
from the fact that the reduction in interference obtained by spectrum spread-
ing is not sufficient to overcome the extreme range of propagation condi-
tions encountered in an LMR environment.

The reduction in interference obtained by spectrum spreading has been
shown to be determined by beo/Po For a Beg = 1 MHz spread-spectrum system,
this amounts to 16 to 20 dB depending on the value of bop (25 or 10 kHz,
respectively). Although this reduction is of some benefit, it is not suf-

ficient to compensate for the wide range of signal conditions that can be

55 .



Before Multiplication by After Multiplication
Local Reference Signal

| Interference | Signal

Signal Interference

a aes

(a)

Interference Signal

|

|

| Signal jos
| igna

|

|

Le |

|__bae

(b)

Figure 2. Spectra of SS systems operating in cw interference
environments. Direct sequence system is shown in
(a) while frequency hopping is shown in (b).



TABLE OF CONTENTS

LIST OF FIGURES. - -- - +--+ + +e eee

LIST OF TABLES . - ~~... 2s ee eee

ABSTRACT - 2 - ee ee ee ee ee ee ew ee ee
1. INTRODUCTION. . 2 6 6 ee ee ee ee ee ee ee
2. BACKGROUND. - - - 2 2 ee ee ee ee ee ee ee
3. SPREAD-SPECTRUM PERFORMANCE CALCULATIONS. .... .-

10.

il.

3.1 General Description. .....
3.2 Spread-Spectrum Performance in
3.3 Spread-Spectrum Performance in
3.4 Performance in Multipath . . .
FM PERFORMANCE CALCULATIONS . . . .
PROPAGATION MODELS. . ......-.
SPREAD-SPECTRUM INTERFERENCE TO FM
FM INTERFERENCE TO SPREAD-SPECTRUM.

7.1 Single FM Interference... .
7.2 Multiple FM Interference . . .

FM SQUELCH THRESHOLD. .......
ADDITIONAL FH SUPPRESSION... . .
CONCLUSIONS ............

REFERENCES. - . 2... 2... ee ee

Gaussian Noise.
Interference. .

APPENDIX. COOPER-NETTLETON SYSTEM PARAMETERS. . ... .

1.

2.

System Description. ........

References... 2... . 2.2.04.

be
Be
Be

w

HO Ow

22

25

38

38
38

52

52

55

62

65

65

69



Separation requirements for propagation at 900 MHz are shown in Figures
12 and 13. By comparing Figures 10 and 12, one can see that SS base to FM
base interference is worse at 900 MHz than at 150 MHz since greater separa-
tion distances are required. However, the mobile-to-base or base-to-mobile
curves at the two frequencies are nearly identical. Only slight differences
occur when either dog or oom is over the effective radio horizon. The reason
for this is that the increased channel losses at 900 MHz due to the urban
environment are incurred by both the victim and interferer in the case of
mobile-to-base or base-to-mobile propagation. This is not true with base-to-
base interference since the victim path is a mobile-to-base path that has
additional losses due to the urban clutter while the interference is base-to-
base which, because of the antenna heights, is assumed to be line-of-sight

with free space propagation losses.

Additional insight can be gained if the curves are plotted in a different
manner. Figure 14 shows the base-to-base and mobile-to-base curves for the
bes = 1 MHz system. Shading is added to dramatize where interference is
considered to be "highly probable," which is defined as the condition where

S/T.
/ OUT
considered to be "propagation dependent," which means that 8/T op < 20 can

< 20 GB even with M, = 0 dB. Also shown with shading, are regions

occur depending on the channel characteristics (0 < M_ < 29 4B). Unshaded

£
regions are considered to be free from interference since S/I > 20 even if

ouT
M, = 29 dB. Figure 15 shows the corresponding results for a 10 MHz spread-
spectrum system at 150 MHz, while Figures 16 and 17 give the results for
operation at 900 MHz.

A summary of separation distances, for a 1 MHz SS system is listed

Ags:
in Table 4. This summary is for desired transmission distances of dom = 10
and 30 km. Values for Igg in the tables correspond to the range expected
with channel conditions of 0 <M, £29 dB. Table 5 gives the corresponding
results for a 10 MHz SS system.

The reader is again cautioned that the results here are based on the
median values of S/I IN. In reality both S and I can be expected to have a
variability due to the motion of the FM mobile and the interfering SS mobile.
The effect of the varying I on the system performance is another unknown that

needs to be explored.

30



15. ABSTRACT (Cont'd.)

simulates the operation of a spread-spectrum system in a multiple FM
interferer environment. The conclusions are that an overlayed spread-
spectrum system also would receive significant interference.

The report also describes a frequency hopping, spread-spectrum
system that is programmed to miss those FM channels in use at a given
locality. The advantages obtained with this technique are briefly
discussed.

WU.S. GOVERNMENT PRINTING OFFICE: 1979-0-681-011/26


| a
n=32 4
Scatter Component Only (a=0) |
No Multipath (Be/ Bar =) 7”
o ~]
- Scatter Component Only (a =O)
S 192 — Relatively Heavy Multipath (Bc /Bar=L)—J
ir] = a
s [= qa
8 =
E | |
S
wn = 4
°
S i val |
~ Specular Only (a =1)
= 3 No Multipath (Be /Bpe=)
10° F— —
3 [= a
a2 }— —
2 = |
a = 4
Equal Specular and Scatter (a=0.5)
Relatively Heavy Multipath
10-4 (Be/ Bar = 76)
pree_t_tt | [oy
0 5 10 15

E,/No, 4B

Figure 7. Expected performance of an n = 32 frequency hop system for
various channel conditions.

18




[[]]_ FM Base interference to SS Base ~ Propagation Dependent
FM Base Interference to SS Base - Highly Probable

FM Base (Mobile) Interference to SS Mobile (Base) - Propagation Dependent
fi] FM Base (Mobile) Interference to SS Mobile (Base) - Highly Probable

dem, FM Separation (km)

dgg, SS Path Length (km)

Figure 20. Separation distance Gout versus deg that is required to insure

10 dB < E,/NG S 20 dB, with S = 30 dB, 900 MHz.

42


Table

2. Equations of Output Signal-to-Noise Ratio for an FM Base Station

Receiving Interference from an SS Base Station

s
Sem | ss w OOF
Gm) _ | om)
s Pss 2
$30 } <64 | Four = Pp y-P..-c, -(20+H,) log dpyt20 log 4,410 log Bop 720 tos 26*8+)~ Mp
s Pss 2
>30 | <64 | Four = Pry -Pog-C, -(204H,) log 4.4420 log 4.410 log Bop 20 tog 38° BHD Hy
s Pss 2
30 | 64 | F OUT = PLy-Pao-C)4C,~(204H,) log d,+(204H4) log 4,410 log Bop ‘10 19938" (B+2)~
s ®ss 2
230 J >64 | Four = PL Pog-CotC4-(204H,) log dat (20+H,) log d..+10 log Bop 20 tos 28*84)- Hy
Table 3.

Equations of Output Signal-to-Noise Ratio for an FM Base Station

Receiving Interference from an SS Mobile or an FM Mobile Receiving

Interference from an SS Base Station

Soy gg s
Gn) | om our
s Pcs 2
S30 | S30 | OUT = PLy-Pog-(204H, log dpyt(20+H,)log 4.410 log ae +10 log 387 (B+1)- My
s Pss 2
>30 330 y OUT = PrP gg ty Ca7 (20+H,) log Apt (2048, ) leg aggt10 log ia +10 log 387 (8+1)- My
s Pog 2
S$ our = pi - > sh 1) =m,
$30} >30 | Four = PLP. -c14C,- (2048, Jog d.y+(204H,) 10g ag. #10 log Bee + 10 tog 38° (Be te
s Pos 2
$ our = Pp -Po_- os +1) =
730] 230 | GOUT = Pear Pgg (20H) 108 Ayyt(204H,) 105 dygt10 Log pS +10 log 28*(B41)~ My

26


SS Base Interference to FM Base - Propagation Dependent
SS Base Interference to FM Base - Highly Probable
SS Base (Mobile) Interference to FM Mobile (Base) - Propagation Dependent

SS Base (Mobile) Interference to FM Mobile (Base) - Highly Probable

100

i on

dgg, SS Separation (km)
s
L

dey»FM Path Length (km)

Figure 16. Separation distance agg versus deny that is required to insure
that S/I OUT > 20 dB with a 1 MHz SS system at 900 MHz.

35



OUT, 4B

S
N

STITT TTT TTT TA TTT TAT TTT TITTY
40|— 4
[- 2
74

L v4
| 4 a
y ma
L- | / |
30 |— | 4 YF
i. i 7 7
[- : A
= | 7
= ' fou
L | 7
[- . 7 4
[ V 7 q
20-— H J
FO! q
[- | 7 7 a”
[- H Yo 7 a
10 -— | 4 + 7 —l
[ : 4 + 7 4
L | fy 7 =
oa ' 4 + 7 —— Specular Channel 4
[_ | 7 Ss 7 ——— Rayleigh Channel 7
C i Lard 7 —-— Specular Channel with |
4 7 7 Threshold Extension _|
ma Ae / q
[- 4 |v 7 a

poewid Mito tii tii tid itis
0 10 20 30 40

Ss
ny DIS, 38
Figure 8. Signal-to-noise ratio out at an FM discriminator as a

function of the signal-to-noise ratio into the discrimina-
tor for nonfading and Rayleigh fading channels.

21



Examples of other techniques that exist but are not being examined in this
report are chirp modulation, time hopping, and hybrid techniques composed of
combinations of the preceeding techniques (Dixon, 1976).

Direct sequence spectrum spreading is accomplished by modulo-2 addition
of a high rate (typically 1 Mbps or greater) pseudo-random (PRN) code to a
slower (50 kHz or less) digitized voice signal. The resulting wideband binary
signal is then converted into a bi-phase, phase-shift-keyed signal for trans-

mission over the channel.
Frequency hopping, in contrast, spreads the spectrum by changing the

frequency in discrete hops according to some predetermined pseudo-random pat-
tern. The output of a frequency hopper is essentially a narrowband, bi-
phase, phase-shift-keyed signal whose center frequency is changing in discrete,
psuedo-random hops. The advantages of frequency hoppers are generally said
to be in the reduced requirements for time synchronization. With direct
sequence, the smallest element of interest is the PRN code element, often
called a chip, which for a 10 MHz system is 100 ns. Conversely, the shortest
element of a frequency hopper is the time duration between hops which can be
as great as 100 Us for a similar 10 MHz system. Unfortunately, the synchroni-
zation advantage leads to a related disadvantage since the performance of a
frequency hopper in multipath is usually poorer than that of a direct sequence
system. The performance of each of these systems in multipath will be dis-
cussed in Section 3.4.

Spread-spectrum systems have other advantages in addition to their
ability to combat interference and jamming. Some of the advantages are as
follows:

(a) Relatively low power density: Power spectral density of

the radiated signal is generally lower than for narrowband
systems. This means the potential for interference to a
narrowband system is less.

(b) Resistance to multipath: A spread-spectrum system exhibits
an inherent resistance to multipath. Sufficiently delayed
multipath signals are transformed into noise in the
decorrelation process.

(c) Security: The system has security advantages because the

low power spectral density characteristics make the signal



SS Base Interference to FM Base - Propagation Dependent

SS Base Interference to FM Base - Highly Probable

SS Base (Mobile) Interference to FM Mobile (Base) - Propagation Dependent
SS Base (Mobile) Interference to FM Mobile (Base) — Highly Probable

100 1

dgg, SS Separation (km)
s

dey, FM Path Length (km)

Figure 29. Separation distance a... versus dy that is required to insure

S/I OUT > 20 dB with a 1 MHz SS system at 150 MHz. Assumes
additional 30 dB suppression due to FH pattern.

57



FM Base Interference to SS Base - Propagation Dependent

FM Base Interference to SS Base - Highly Probable

FM Base (Mobile) Interference to SS Mobile (Base) - Propagation Dependent
FM Base (Mobile) Interference to SS Mobile (Base) ~ Highly Probable

100

dem, FM Separation (km)
I

Pigure 21.

dgg, SS Path Length (km)

Separation distance dom versus dog that is required to insure
10 dB < E/N, S 20 dB, with S, = 40 dB, 900 MHz.

43



With multiple interferers, the concept of separation distance dou becomes
vague. For example, consider the case where there are ten interfering trans-
mitters that are all located such that their separations satisify the minimum
requirements for dou: Although individually none of the transmitters singly
would degrade performance, collectively their total power may be sufficient
for interference. This situation is considered in this section with the aid
of a Monte-Carlo, computer simulation program.

The geometry involved in the Monte-Carlo simulations is shown in Figure
22. Basically, the simulation model assumes that SS mobiles and interfering
FM mobiles are randomly located throughout a circular service area of radius
R. Interfering base stations, however, are restricted to the perimeter of
this circle and are assumed to be randomly located around the outer perimeter.
The reason that base stations are restricted to the perimeter is that we have
already seen that base-to-base is a dominate source of interference. In fact
it is easy to see from Tables 8 and 9 that one of the requirements to avoid
base-to-base interference is that Gay > agg Thus, it is mandatory that FM
base stations be located outside of the SS service area or unacceptable
interference to the SS base will be very likely. The victim SS base station
is assumed to be at the center of the service area.

A Monte-Carlo computer program simulates this model by randomly gener-
ating interfering and desired locations and computing the resulting inter-
ference. The probability density function for the interfering mobile distance
is (Berry, 1978)

2

P (diy) eR day (36)

and the probability density function for the SS mobile distance is

ar (37)

P(dcg) = RF gg

The sequence used in the simulation program is:
(a) simulate a single desired SS mobile distance deg as
per (37),
(b) simulate Ny interfering FM mobile distances Fin as
per (36),

45



and M is the number of waveforms that must be recognized by the decision
device. For binary encoded transmission, M = 2 and k = 1. The ratio of

energy-per-bit ey to noise power density ny is also useful and is defined as

e. e
22s. (10)
n n ¢*k

° fo}

3.3 Spread-Spectrum Performance in Interference

Unfortunately, the simple tractable solutions common to Gaussian noise
disappear when one considers performance in interference. The reason for

this is due, at least in part, to the fact that the interferer's statistics

are either unknown or mathematically complex. Thus, in the following material,

we will be forced to make some estimates based on current SS systems.

One of the facts that is often overlooked is that an optimum matched
filter receiver is optimized only for additive Gaussian noise. This means
that its performance can be surprisingly poor in interference. This is
particularly true for the frequency-hopping receiver where interference can
cause significant problems if the receiver has not been properly engineered
with interference in mind.

The interference environment of concern here is one composed of multiple,
narrowband, FM, land-mobile signals whose spectra are typically about 10 kHz
wide. For all practical purposes, the FM signal will appear to be cw inter-
ference to the much wider bandwidth, spread-spectrum system. If the spread-
spectrum system has an rf bandwidth of 10 MHz and the FM channel assignments
are 25 kHz, one could expect a maximum of 10/.025 - 400 such interferers
within the receivers operating passband. Practically, of course, only a
relatively small fraction of these interferers will be active at any given
instant in time.

Figure 2 shows how the DS and FH receivers handle interference. The
response of the DS receiver is shown in Figure 2(a), where the various spec-
tra are given before and after multiplication in the receiver by the local
PRN reference signal. As can be seen, the desired signal is collapsed into
the original narrowband information signal of bandwidth Dine’ while the

. interference is spread in frequency into a noise-like signal of bandwidth