It was proposed that a satellite-based land mobile radio system could effectively extend the terrestrial cellular mobile system into rural and remote areas. The market, technical and economic feasibility for such a system is studied. Some of the aspects of implementing an operational mobile-satellite system are discussed. In particular, two key factors in implementation are examined: (1) bandwidth requirements; and (2) frequency sharing. Bandwidth requirements are derived based on the satellite antenna requirements, modulation characteristics and numbers of subscribers. Design trade-offs for the satellite system and potential implementation scenarios are identified. Frequency sharing is examined from a power flux density and modulation viewpoint

Leroy, B. E.

English

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Satellite-Aided Land Mobile Communications
System Implementation Considerations
Bruce E. LeRoy
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135
ABSTRACT
s
It has been proposed that a satellite-based Land Mobile Radio System could
effectively extend the terrestrial cellular mobile system into rural and
remote areas. The market, technical and economic feasibility for su^'h a sys-
tem is currently being studied by NASA.
This paper deals with some of the aspects of implementing an operational
mobile-satellite system. In particular, two key factors in implementation are
rn
examined: (1) bandwidth requirements; and (2) frequency sharing.
N
Bandwidth requirements are derived based on the satellite antenna require-
L,	 merits, modulation characteristics and numbers of subscribers. Design trade-
offs for the satellite system and potential implementation scenarios are
identified. Frequency sharing is examined from a power flux density and modu-
lation viewpoint.
IMPLEMENTATION CONSIDERATIONS
Introduction
Despite numerous studies (References 1 to 5), mobile-satellite (MSAT)
service remains largely conceptual in nature. Numerous questions on the
implementation of the satellite system may not receive definitive answers
unless field f- ,y sting can be accomplished. Primary among these questions are
considerations of intersystem interference (between satellite systems of dif-
ferent nations or between satellite and terrestrial systems), and cost
justification of the satellite system.
This paper deals only with some technical aspects of implementing a mobile
satellite service. The purpuse is to expose some of the system design impli-
cations of technical alternatives. This paper examines communication links,
modulation, bandwidth requirements, and briefly indicates a possible start-up
scenario.
Satellite Transponder
Figure 1 shows some possible transponder design options. The eventual
operational design will be based on answers to traffic questions such as:
1. Of total calls, what percentage are mobile to mobile?
2. What percentage of calls are placed to units. (fixed or mobile) outside
the UHF beam area?
1
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2I
3. What percentage of calls are placed to units (fixed or mobile) outside
the gateway control?
4. Is double hop acceptable?
5. What tariff or other regulatory conditions prevail on "long distance"
and "local calls?"
Ultimately, considerations of potential gain and risk will determine how
an MSAT system is implemented. The size of the potential market, the market
elasticity and economics of scale will dictate a mature operational system
design. For now, this paper concentrates on the technical trade-offs and
indicates the potential design and performance alternatives.
To examine the impact ol, technical alternatives a baseline system is
constructed by assuming:
Number of subscribers
Peak hour usage
Blocking probability
CONUS area
Channel utilization
EIRP/channel
Channel spacing
Frequency reuse
180,000
= 0.03 Erlangs/subscriber
= 0.02
= 15 deg2
= 0.4
40 dBW
= 30 kHz
3 subbands reused continuously
With these assumptions, Table I shows antenna diameter, satellite RF
transmit power and required bandwidth as a function of the numbers of beams
covering CONUS.
Bandwidth Requirements
Currently the reserve bands in the 806-890 MHz band are allocated in such
a way that only 8 MHz (821-825 and 866-870) could be used by a satellite sys-
tem on an exclusive basis. Given the indications of Table I, we have suffi-
cient motivation to reduce the required bandwidth or increase the frequency
allocation.
Envelope normalized FM as discussed in Reference 6 requires a 15 kHz
channel spacing, thus halving the required bandwidth. Amplitude companded
Single Side Band (SSB) could achieve channel spacings of 2.5 - 5 kHz according
to Reference 7. Thus, the bandwidth could be reduced by a factor-of 0.08 to
0.17 of the Table I values.
Since signal to interference ratio considerations were not accounted for,
the satellite may be required to use a 4 or 7 frequency reuse plan. This
would increase the Table I bandwidth requirements by factors of 1.33 and 2,33,
respectively.
If the use of multiple satellites is considered, the required bandwidth
can be reduced directly by the number of satellites in orbit. This concept is
practical if satellite to satellite interference is manageable. Techniques to
control satellite pointing accuracy and shape overlapping beams to minimize
interference, need to be analyzed in detail. The mobile units would have to
be capable of discriminating between satellites thus implying higher gain
tracking antennas. While this antenna would add cost to the mobile, it would
^s
3also reduce the transmitted power required from the satellite, and therefore
its size and cost.,
Link Analysis and Frequency Sharing
A typical narrowband link analysis for MSAT is presented in Table II and
closely parallels that found in Reference (3). Note that the margin is 5 dB.
Reference 8 gives the equation for margin required to be above a specified
power level over 90 percent of small distances and 95 percent of large
distances as:
M = 12.5 + 0.17f - 0.176 + 1.65 (6.4 - 1.19f 	 0.056)
f is frequency in GHz
a is elevation angle in degrees
The required margin ranges between 16.5 and 11.4 dB for elevations between
200 and 40u , respectively. If we were to increase the margin shown in
Table II to 15 dB, then the EIRP per channe^ approaches 50 dBW. At this
value, the power flux density is -1.13 dbW/m /channel. This is a noise power
of about --133 dBW per channel that is perceived by terrestrial cellular
co-channel systems. Although additional isolation may be achieved by using
different polarizations and channel interleaving, the noise power to terres-
trial systems is still substantial when compared to the urban noise environ-
ment of about -144 dBW/ channel.
Ifas a design alternative ampli tude companded SSB and a channel inter-
leaving plan as shown in Figure 2 are considered, the satellite system could
conceivably coexist with the terrestrial cellular networks. 	 Reference 9 sug-
gests that in a multichannel environment a 10 dB gain in numbers of channels
could be achieved at equivalent transponder power levels. 	 However, a NASA
study shows only a 1 to 2 dB power advantage and an EIA committee found no
advantage to SSB (Reference 10).
	
SSB requires a higher S/N ratio than FM
because no FM improvement exists and a pilot tone i_s required.
If a SSB S/N = 13 dB (Reference 10) is assumed and all other factors from
Table II rem;in the same, then the required EIRP/ channel = 36.2 dBW (band-
width = 5 kHz) which is approximately the same power level as that required
for FM.	 However, the power flux density in ^he band is about 5 1 higher than
that for FM.	 The PFD for FM is -168.8 dBW/m !Hz and -163.8 dBW/m /Hz for
SSB.	 This will make it very difficult to integrate the amplitude companded
SSB system with the terrestrial cellular system.
	
However, proper transmit/
receive filtering should reduce the perceived interference to a level well
below ambient noise.
Finally, consider direct frequency sharing.
	
Current mobile units have 10
dB noise figure receivers and cell site receivers have a 6 dB noise figure.
This corresponds to received noise powers of -149.6 dBW and -154.4 dBW,
respectively in a 30 kHz channel.
	
These powers are achieved at a PFD of about
-.143.5 dBW/m2 /channel assuming a 5 dBi cellular unit antenna gain and 0 dBi
cell site antenna gain for angles above 20 0 e'^vations.	 Assuming a 3 dB
polarization isolation, a permissible PFD is about--140.5 dBW/m /channel,
which, in tur n, corresponds t0 an EIRP of 22.5 dBW/channel. This is consider -
ably less than the Table II value, ,vhich,
	
in turn may be 10 dB low depending
^ on the margin used.
w.
r1	 4
PI
An obviokis consideration is to increase mobile antenna gain. Assuming
Table II values, a gain of about 15.5 dB is required for the mobile. This
would allow reduction of the transmit power to 22.5 dBW.
From the discussions above, it is apparent that there is also a powerful
motivation for considering higher gain directive antennas for MSAT mobile
units. While such an antenna would increase the cost of the mobile unit,
there are a number of potential benefits to be gained such as;
1. Reduce satellite transmit power, size and cost
2. Permit multiple satellite service
3. Permit discrimination to/from cellular systems
4. Permit discrimination between systems (U.S. system, Canadian system,
etc.)
While cost considerations have not been studied in detail, placing more
burden on the ground unit would reduce space segment capitalization and spread
the remaining ground segment capitalization to more nearly match the market
growth.
IMPLEMENTATION SCENARIO
As a possible implementation scenario assume:
1, Time zone coverage (20 x 30 beams) gain = 36.5 dB on axis
C. FM channels interleaved with cellular channels providing ,4 dB	 -'
additional isolation
3. Approximately 15 dB mobile unit antenna gain (circularly polarized)
4. Other Table II values apply
Such a system would have about 600 channels with no frequency reuse and
accommodate about 21,000 users at a loading of 35 users per channel. A second
satellite could be launched when needed and placed about 30 0 apart from the
first. The satellite UHF antenna would be about 12 m by 8 m, well within the
reach of current technology. Tiie required EIRP is 26.0 dBW/channe'I for a net
transmit power of 0.2 wattslchannel (at beam edge) or about 120 watts RF for
the satellite. This is well within current technology limits. The satellite
appears to be well within current and projected launch capability.
A variation on this system is an 8 beam system with 1.5 0 beams and a 16
m antenna which could reuse the frequency twice. A system of three such
satellites could serve 126,000 users.
CONCLUDING REMARKS
From the foregoing discussions, there appears to be a powerful_ technical
argument to pursue mobile antenna technology. In addition to the technical
considerations mentioned above, the use of higher gain mobile antennas would
tend to mitigate the passive intermodulation problems associated with using a
single transmit/receive antenna when there is a large signal imbalance.
However, economic considerations need to be studied in detail and may negate
the technical advantages.
Other mobile unit technologies for investigation appear to be a low noise
front end and changes in signalling processes,
An obvious satellite technology in need of examination is the large UHF
antenna.	 There are, however, considerations such as mobile antenna gain and
the use of interleaved narrow bandwidth channels which could permit multiple
satellite use and reduce the required satellite antenna technology to that
which is readily feasible today.
The requirements for on-board channel switching must be thoroughly
documented and Justified.	 If, for example, we were restricted to very large
satellites such as those described in Table 	 and also required full
connectivity (Figure 1c), as many as 50 x 10 6 cross-points would be required.
In conclusion, an implementation scenario has been presented which does
not stretch the technology greatly, yet offers reasonably extensive services.
REFERENCES
1.	 H. L. Loats, C. F. Sauer, and P. A. Castruccio, 	 "Land Mobile Market
Integration ;Study," T-1957 (6638-0200)-014, Battelle Labs, Columbus, OH,
Nov. 1980.
2.	 "Mobile Multiple Access Study," NASA CR-156650. Aug. 1977.	 f
3. R. E. Anderson and R. T. Milton; "Satellite-Aided Mobile Radio Concepts
Study," G.E. Document No. 79SOS4240, General Electric, Schenectady, NY,
June 1979.
4.	 R. L. Kelly, R. K. Khatri, J.	 D. Kiesling,	 and J. A. Weiss,	 "Communi-
cations System Technology Assessment Study," NASA CR-135224, Oct. 1977.
5.
	 "Technical Considerations for a Land-Mobile Satellite Service Operating in
Band 9," CCIR Report 770, Recommendations and Reports of the CCIR, 1978,
Kyoto, Japan, 1978.
6.	 A. M. Goldman,	 "Systems Studies for a Land Mobile Satellite Service
(LMSS)," JPL Document 715-86, Jet Propulsion Laboratory, Pasadena, CA,
Sept. 1980.
7.	 M. A.	 Herro, F.	 Nocedal, P. M. Wala, and B. B.	 Lusignan,	 "Amplitude
Companded Sideband for Land Mobile Radio," Inte l com 1 80, Proc. of the
International Telecommunications and Computer Exposition, pp. 130-135,
1980.
8.	 "Satellite Sound Broadcasting with Portable Receivers and Receivers
Installed in Automobiles," CCIR Report AD/10-11 (Mod F),	 ITU, Geneva,
Switzerland, Oct. 1981.
9.	 J. h. Mehring,	 "Amplitude Companded Sideband for Satellite Applications,"
Int,::lcom 1 80, Proc. of the International Telecommunications and Computer
rpos tion pp. 30-35, 1980.
10.	 "Comments on the Federal Communications Commission UHF Task force Report
Entitled 'Spectrum Efficient Technology for Voice Communications'," IEE
Communications Magazine, 	 vol. 17, no. 2, pp. 26-27, Mar. 1979.
s
1596
3390
ORIGINAL PAGE IS
OF PAR QUALITY
t
TABLE I
NUMBER OF CHANNELS ANTENNA PANDWIDTH TRANSMIT
BEAMS PER BEAM DIAMETER TRANSMIT POWER
200 36 78 3.2 210
150 47 68 4.1 272
100 65 55 5.8 383
80 77 49 6.9 457
60 100 43 9.0 578
40 145 35 13.1 843
ORIMNAt
or Poop
TABLE I I QUA
TYPICAL MSAT LINK
NARROWBAND FM
VALUE.
38.2 dBW
0
183 dB
5 dB
228.6 dB/°k
2.8 dB (3 dB CIRCULAR/LINEAR LOSS)
0
27.6 dB (5750K)
44.0 dB (25 kHz)
10.0 dB
25.5 dB
18.0 dB
9.3 dB
ITEM
EIRP/CHANNEL
AluPLEMENTATION LOSS
PATH LOSS
MARGIN
BOLTZMAN'S CONSTANT
MOBILE ANTENNA GAIN
MOBILE LINE LOSS
MOBILE TEMPERATURE
CHANNEL BANDWIDTH
C /N DOWNLINK
C/N UPLINK
/'kITCnCCnENCE N	 N	 u nTEnA
%j II. ^nrcn^ ►^t^1; A IY D IIS 1 RI'IVU
C/N TOTAL LINK
r{
r
}
z
DEMURMUX UDEMUX
BACKHAUL
N
a
DEMUX MUX
DEMUR
BACKHAUL
N
N-
(b)
UHF
x
N
UHF FILTER	 MUX	 DACKHAUL
DEMUR
N	 FILTER
(a)
UHF
(c) (a) Simple bent pipe,
(b) Hard w1re cross-strap.
(c) Switched fully connected.
Figure 1, - Transponder concepts,
-20
RELATIVE .30SIGNALLEVEL, .AO
dB
U
-10
 PAQe IS
-50	 SSB
-50
-70 C1	 5	 10	 15	 20	 25	 30	 35FREQUENCY, kHz	 C2
A Figure 2. - Channel interleaving.
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Satellite-aided land mobile communications system implementation considerations

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