In contrast to the situation at L-band, wide bandwidths of 500 MHz or more have been allocated for mobile satellite service at frequencies between 20 and 50 GHz. These broad bandwidths are well suited for the use of spread-spectrum. Certain system considerations about the use of such high frequencies for mobile satellite service are mentioned first, and attention is then given to propagation effects on high-frequency broad-band systems. Attenuation due to rain is a constant at 20 to 50 MHz, but would not be a serious problem if outages occurring for one to three percent of the time, depending on location, are considered to be acceptable. Clear air absorption becomes a significant factor above 40 GHz, but should not exceed 2 dB at a 10 degree elevation angle and frequencies below 40 GHz. Spread-spectrum provides a form of frequency diversity that helps to minimize the effects of multipath

Flock, Warren L.

Smith, Ernest K.

English

NASA Technical Reports Server

N88-25699
PROPAGATION EFFECTS ON SPREAD-SPECTRUM MOBILE-SATELLITE SYSTEMS
DR. WARREN L. FLOCK, Department of Electrical and Computer Engineering,
University of Colorado, United States; DR. ERNEST K. SMITH, Department of
Electrical and Computer Engineering, University of Colorado, United States.
UNIVERSITY OF COLORADO
Campus Box 425
Boulder, Colorado 80309
United States
ABSTRACT
In contrast to the situation at L band, wide bandwidths of
500 MHz or more have been allocated for mobile-satellite service
at frequencies between 20 and 50 GHz. These broad bandwidths
are well suited for the use of spread spectrum. Certain system
considerations about the use of such high frequencies for mobile-
satellite service are mentioned first, and attention is then given to
propagation effects on high-frequency broad-band systems.
Attenuation due to rain is a constraint at 20 to 50 MHz but would
not be a serious problem if outages occurring for one to three
percent of the time, depending on location, are considered to be
acceptable. Procedures are available for estimating rain
attenuation for percentages of time from 0.001 to 1. Clear-air
absorption becomes a significant factor above 40 GHz but should
not exceed 2 dB at a I0 deg elevation angle and frequencies below
40 GHz. Spread spectrum provides a form of frequency diversity
that helps to minimize the effects of multipath. An experimental
program would be needed to determine the performance of spread-
spectrum systems in the presence of diffuse scatter and shadowing
by trees.
i. INTRODUCTION
The World Administrative Radio Conference for Mobile Services of October 1987
(WARC-MOB-87) allocated 3 MHz and 4 MHz bands for both downlink and uplink voice
service by land mobile-satellite systems at frequencies between 1530 and 1660.5 MHz.
The 3 MHz bands are shared with maritime mobile-satellite service, and the 4 MHz
bands are unshared except for 0.5 MHz shared with radio" astronomy. Also there are
allocations for non-speech low-bit-rate data. These allocations represent a step
forward but are small compared to the 500 MHz and wider bands allocated for mobile-
satellite service between 20 and 50 GHz. Table 1 lists allocations for mobile-satellite
servicewithin, or close to, the 20 to 50 GHz range. The desirable 19.7 to 20.2 GHz and
29.5 to 30.0 GHz ranges are available for mobile-satellite use on only a secondary basis,
it is true, and the equally appealing 20.2 to 21.2 GHz and 3010 to 31.0 bands are allocated
for governmental use in the United States. The potential of these broad-band
allocations for commercial service is nevertheless worthy of attention. Spread-
spectrum systems have a low susceptibility to interference and tend to not cause
significant interference, indeed spread spectrum is characterized by what is referred to
as a low probability of intercept (Proakis, 1983). Spread spectrum could weU be used for
mobile service in the 19.7 to 20.2 and 29.5 to 30 GHz bands without interfering with
i'_xed service that has the primary allocation for these bands.
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Table 1. Allocations for Mobile Satellite Service, 20 to 50 GHz
Frequency [GHzl Service Allocation
19.7-20.2
20.2-21.2
29.5-30.0
30.0-31.0
39.5-40.5
43.5-45.5
45.4-47.0
50.4-51.4
Downlink (Sec.)
Downlink {Primary)
Uplink (Sec.)
Uplink {Primary)
Downllnk (Primary}
Uplink (Primary)
Uplink (Primary)
Uplink (Sec.)
Non-government in U.S.
Government in U.S.
Non-govermnent in U.S.
Government in U.S.
Gov. & Non-gov. in U.S.
Government in U.S.
Gov. & Non-gov. in U.S.
Gov. & Non-gov. in U.S.
2. SYSTEM CONSIDERATIONS
This paper is devoted to the role of propagation in a possible high-frequency (20
GHz or higher) mobile-satellite system. Certain other aspects of such a system are
discussed briefly in this section. We do not present definitive treatments of these other
topics but believe that they should be mentioned to provide a suitable background.
Antennas are crucial elements of mobfle-sateUite systems. It is desirable to
employ phased-array antennas on land vehicles used for land mobile-sateUite systems,
in order to keep the antennas pointed at the satellite as the vehicles turn and tilt. But
phased-array antennas tend to be costly--at L band and probably equally or more so
above 20 GHz. In considering possible systems operating above 20 GHz, one approach
could be to use a first-generaUon L Band system as a reference and assume that the
antenna gains above 20 GI-Iz should be the same as at L Band. Analysis shows, however,
that keeping antenna galn constant with frequency puts the higher frequencies at a
power disadvantage. That is, the ratio of received to transmitted power, PR/PT,
decreases with frequency when constant gain, GRGT, is maintained (Fig. 1). Another
possible approach, maintaining antenna areas, ARAT, constant with frequency, gives
a power advantage to the higher frequencies (Fig. 2). But a satellite antenna as large
above 20 GI-Iz as the antennas planned for L band would present major practical
problems and tend to be very costly. The satellite antenna and power problems could be
solved by keeping the satellite antenna gain constant and the land-vehicle antenna
area constant with frequency. In this case there is no advantage from the power
viewpoint of either high or low frequencies. But the cost of maintaining the vehicle
phased-array antenna area constant with frequency may still present a problem, and it
may be necessary to move from constant area of the ve-hicle antenna towards constant
gain. By such a procedure, a suitable tradeoff may be found between antenna costs and
power considerations.
A favorable point is that, if constant gain for the satellite antenna is
maintained as a function of frequency, the satellite antenna will tend to be relatively
small and llghtweight above 20 GHz. It may then be feasible to increase the power
transmitted from the satellite to compensate for the power handicap otherwise
encountered at high frequencies when constant gain is maintained.
The spread-spectrum technique could be applied to a single 4 MHz bandwidth,
such as that aUocated for land mobile-satelllte use at L band. Analysis in such a case
would indicate that a certain number, A, of signals could be carried simultaneously.
Utilizing the wide bandwidths above 20 GI-Iz, the same number A could also be carried
in a 4 MHz bandwidth, or perhaps a somewhat wider bandwidth such as 5 MHz would be
used. In addition some number, B, of 4 or 5 MHz channels could be employed. Then the
total number of signals that could be carried would be A times B. Spread spectrum has
I_4
been of interest in military circles for a considerable period of time (Sass, 1983). In
recent years commercial non-military applications have been increasing (Parker,
1984). A number of papers have attempted to define the role of spread spectrum and to
determine the number of simultaneous signals that can be accommodated in spread-
spectrum channels characterized by certain parameters (Yue, 1983). The advance
program for this Mobile Satellite Conference of May 3-5, 1988 lists two papers on these
topics. Continuing attention needs to be given to these matters. One general conclusion
is that spread spectrum is most suitable for thin-route applications, and land mobile-
satellite service falls into this category.
3. PROPAGATION CONSIDERATIONS
The logic of using 30-20 GHz frequencies for satellite service, as planned for the
ACTS program for example, is well established by now and it does not seem necessary to
further Justify it. The situation is somewhat different for fixed and mobile-satellite
service, however, and propagation problems become more severe very far above 30 GHz.
Power control (increasing transmitted powers as needed) and forward error correction
are measures planned to mitigate signal degradation due to rain for ACTS. Power
control could also be used in mobile-satellite service, and a spread-spectrum system
may be able to employ forward error correction or measures having much the same
effect. It seems only realistic to expect that land mobile-satellite service, however, will
not be able to achieve the low outage percentage (0.01) commonly expected for fixed
service. For land mobile-satellite service it may be quite reasonable to accept outage
percentages of 1 to 3, depending on location. If such outage percentages are accepted,
rain would not present a serious problem for land mobile-satellite service even without
employing sophisticated techniques to combat attenuation due to rain.
Procedures are available for estimating attenuation due to rain exceeded for
percentages ofttme from 0.001 to 1. The CCIR procedure is described in CCIR Report 564
(CCIR, 1986a). Even though interest may lie in a percentage other than 0.01,
attenuation A for a percentage of 0.01 is calculated first by using
A=% Lrp dB (1)
where (Xp is an empiricany derived attenuation constant in dB/krn, L is path length
through rain in kin, and rp is a path reduction factor that takes account of the fact that
intense rain tends to be localized and can not be expected to occur with the same high
intensity along the entire length of the path. The path length L can be determined
from H = L/sln 0 for values of 0 above 10 deg where 0 is elevation angle and H is the
height of the 0 deg C isotherm, taken in the CCIR 1986 model to be given in km by
H=4.0-0.075 {¢-36 °} _>36 °
with H equal to 4.0 krn for latitude _ < 36 °. The path reduction factor rp is given
by
where D
other than 0.01 is computed from the value AO.Ol for 0.01 percent by use of
Ap = AO.Ol (0.12) p-{0.546 + 0.043 log p} dB
(2)
rp = 1/(1 + 0.045 D) (3)
is the horizontal projection of H. Attenuation exceeded for a percentage p
(4)
135
It developsthat the attenuationexceededfor a percentageof time of one is 0.12 of that
exceeded for a percentage of 0.01. The constant Up is determined by using the relation
= aRb dB/km (5)
where information for determining a and b are given in CCIR Report 721 (CCIR,
1986b). Values of rain rate R in mm/h exceeded for a percentage oftime of 0.01 canbe
estimated from worldwide plots provided in CCIR, 1986c and reproduced by Flock
(1988).
An alternative procedure that was presented in earlier versions of the CCIR
model involved using tables giving rain rates R exceeded for various percentages of time
and calculating attenuation directly using these rain rates. The values of rain rate
obtained by this procedure for a percentage of 0.01 and for a percentage of I are given in
Table 2. The table demonstrates how very much smaller the rain rates exceeded for a
percentage of time of 1 are than the rain rates exceeded for a percentage of time of 0.01.
It should be kept in mind that attenuation is accompanied by an increase in
noise and that the degradation in signal-to-noise ratio due to rain, clouds, or
atmospheric gases is due to both attenuation and the accompanying increase in noise.
Clouds become a significant factor for frequencies of about 20 GHz and higher.
For example, for the rather extreme case of clouds 4 km in thickness containing 1 g/m 3
of water, an attenuation of 3.9 dB and an increase in noise temperature of about 160 K
are encountered at 30 GHz (Slobin, 1982). Clouds occur for a larger percentage of time
than intense rain and tend to be relatively more important when emphasis is given to
effects occurring for one percent of the time or more instead of 0.01 percent.
Table 2. Rs/n Rates (ram/h) Exceeded for 0.01 and 1 Percent o(the Time for
Rei_ms A to P o_CCIR Model
Rate. 0.01 Rate. 1 _ Rate. 0.01 Rate. 1
A 8 <0.5 H 32 2
B 12 1 J 35 8
C 15 2 K 42 2
D 19 3 L 60 2
E 22 1 M 63 4
F 28 2 N 95 5
G 30 3 P 145 12
Clear air absorption becomes a factor above about 40 GHz as shown in Fig. 3
(Smith, 1982). Attenuation due to the clear air should not exceed 2 dB, however, for an
elevation angle of 10 deg and frequencies below 40 GHz.
The broad bandwidths employed in spread-spectrum systems tend to minimize
the effects of multipath, a common source of signal degradation in land mobile-
satellite systems. Multipath effects can be expected when driving on broad interstate
highways, etc. A number of experimental studies, however, have tended to emphasize
the importance of shadowing by trees instead of multipath effects (Vogel and Smith,
1985; Goldhirsh and Vogel, 1986). It appears that additional experimental efforts are
needed to determine the performance of spread-spectrum systems in the presence of
diffuse scarer and shadowing by trees.
136
4. CONCLUSION
The broad bandwidths a11ocated for the mobile-sateUite service at frequencies
above 20 GHz suggest that serious consideration should be given to the exploitation of
these frequencies by the use of spread spectrum, for commercial as well as
governmental applications.
-4O
A
¢n
v
F-
n
%.
n
I I IIIIII I I t IIIIIIJ
L..
-80
-120
-150
-200 --
-240 I f,,.,,l i ,,.i.I _JJ..,I
i07
I I I_11111 I I IIIll
80dB - - 60
_0 _ -I00
£
fl.ae -140
-180
I I Illl
I08 109 I0 I0 i0 II
FREQUENCY (Hz)
Fig. I. PR/Pr as a function of
frequency and GR GT
-20 tilt.ill, li.,lll lilllllJl _
l
-220 I itltttd f I tJitltl I ttlttltl I I,Itt.
I07 108 109 I010 I011
FREQUENCY (Hz)
Fig. 2. PR/PT as a function of
frequency and AR AT
iO 3
5
RANGE OF VALUES I
2
i0-2
I 2 3 IO 2 5 IO 2 2 4
FREQUENCY (GHz)
Fig. 3. Vertical one-way attenuation through atmosphere for a water vapor content of
7.5 g/m 3 at the surface
137
REFERENCES
CCIR 1986a. Propagation data and prediction methods required for earth-space
telecommunication systems, Report 564-3. Volume V, Propagation in Non-
ionized Media, Recommendations and Reports of the CCIR. 1986 (Geneva: Int.
Telecomm. Union).
CCIR 1986b. Attenuation by hydrometers, in particular precipitation, and other
atmospheric particles, Report 721-2. Volume V, Propagation in Non-ionized
Media, Recommendations and RePorts of the CCIR. 1986 [Geneva: Int.
Telecomm. Union).
CCIR 1986c. Radiometeorological data, Report 563-3. Volume V, Propagation in Non-
ionized Media, Rq¢Qmmendations and RePorts of the CCIR. 1986 (Geneva: Int.
Telecomm. Union).
Flock, W.L. 1988. Propagation Effects on Satellite Systems at Frequencies Below 10
GHzo NASA Reference Publication 1108(02). (Washington, DC: National
Aeronautics and Space Administration ).
Goldhirsh, J. and W.J. Vogel 1987. Roadside tree attenuation measurements at UHF for
landmobfle satellite systems. IEEE Trans. Antennas ProDagat. AP-35: pp. 589-
596, May.
Parker, E. B. 1984. Micro earth stations as personal computer accessories. Proc. IEEE.
72: pp. 1526-1531, Nov.
Proakis, J.G. 1983. Digital Communications. (New York: McGraw-Hill).
Sass, P.F. 1983. Propagation measurements for UHF spread spectrum mobile
communications. IEEETrans. Veh. Technol. VT-32: pp. 168-176, May.
Slobin, S.D. 1982. Microwave noise temperature and attenuation of clouds. Radio
Science. 17: pp. 1443-1454, Nov. -Dec.
Smith, E.K. 1982. Centimeter and millimeter wave attenuation due to atmospheric
oxygen and water vapor. Radio Science. 17: pp. 1455-1464, Nov.-Dec.
Vogel, W.J. and E.K. Smith 1985. Propagation Considerations in land Mobile Satellite
Transmission, MSAT-X Report No. 105. (Pasadena: Jet Propulsion Laboratory).
Yue, O. 1983. Spread spectrum mobile radio, 1977-1982. IEEE Trans. Veh. Technol. VT-
32: pp. 98-105, Feb.
138


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