The margin that is required to mitigate the near-far problem in a Code Division Multiple Access (CDMA) mobile satellite system is determined by the radio-propagation model selected, the distribution of the users in clear and shadowed environments, and implementation techniques. The use of revenue potential as a means of evaluating the relative merits of CDMA and Frequency Division Multiple Access (FDMA) systems is a convenient way to rationalize the performance of systems using high-gain and low-gain antennas. The revenue potential of CDMA is much greater than the revenue potential for FDMA for a particular satellite design considered

Renshaw, Ken

English

NASA Technical Reports Server

- -  
System Level Comparison of FDMA vs. CDMA 
(Under Conference Guideline Constraint) 
- 
KEN RENSHAW, Hughes Communications, Inc., U.S.A. 
ABSTRACT 
The margin that is required to mitigate the "near-far" problem in 
a CDMA mobile satellite system is determined by the radio-propagation 
model selected, the distribution of the users in clear and shadowed 
environments, and implementation techniques. 
The use of revenue potential as a means of evaluating the relative 
merits of CDMA and FDMA systems is a convenient way to rationalize 
the performance of systems using high-gain and low-gain antennas.The 
revenue potential of CDMA is much greater than the revenue potential 
for FDMA for a particular satellite design considered. 
THE NEAR-FAR PROBLEM 
The "near-far" problem in CDMA systems occurs where a number of terrestrial mobile unit 
terminals communicate to a single base station. 
Some of the terminals are physically "far" from the base station, some are "far" in a radio 
propagation sense because they are shadowed, and some are "near" in both a physical and propagation 
sense. 
If the system is using CDMA, the "near" terminals are contributing more self-noise than the 
"far" terminals. The system design has to somehow allow the "far" terminals to communicate in the 
presence of the larger self-noise of the "near" terminals. If the system is using FDMA, the carrier 
levels received at the base station will differ between the "near" and "far" terminals. If the base 
station can accommodate the dynamic range of the FDMA signals, the "near-far" problem does not 
limit the system performance. 
For the class of satellite system called out in the conference guidelines a CDMA satellite system 
will be self-noise limited: the downlink signal-to-noise ratio is Eb/(No + l o ) ,  where Io is much 
larger than No.The E d  Io is established at the satellite. A partial shadow does not change the Et,/ Io: 
it only changes the ratio of Io to No. On the downlink there is no "near-far'' problem. 
The "near-far" problem is important on the uplink. The satellite system capacity is limited by 
the margin allowed for the difference in signal strength of the "near" and "far" terminals. 
There is limited fix for the uplink "near-far" problem. The mobile terminal can be equipped 
with a "short-loop" automatic level control. 
The receiver in the vehicle monitors the downlink signal level and uses that information to 
adjust the uplink signal's level. If the downlink signal fades by 3 dB the uplink transmitter level is 
increased 3 dB, for example. The uplink E d  Io for the shadowed station should remain constant. 
This ALC system could also be used with an FDMA system. However, there is no performance 
penalty in an FDMA system from operating the transmitter at full power. It  is not necessary to 
control the level. 
RADIO PROPAGATION MODEL LIMITATION 
The limitation on this near-far fix technique is the practical dynamic range of the mobile 
transmitter and the correlation between downlink and uplink fading. Field tests were undertaken in 
order to understand the effectiveness of the "short-loop" automatic level control. The field tests 
were quite surprising, not because of what was learned about "short-loop" automatic level control, 
71 
https://ntrs.nasa.gov/search.jsp?R=19890018542 2020-03-20T01:51:30+00:00Z
but because of what was learned about the limitations of the radio propagation model being used. 
to that in figure 1 was being used. 
When the experiment initially was designed the propagation modell producing a curve similar 
FIGURE 1, Selection of Link 
Margin Requirement 
For 90 96 of 
Percentage of Locations 
Where Signal Was Greater 
Than Ordinate 
B 
Flgure 2, Typical Tree 
Shadow Attenuation 
Rician Ravleleh 
These data would predict that a 6 dB margin would protect against fading and shadowing 90% of the 
time. What was experienced in a very limited field test program was quite different. If there was a 
clear, line-of-sight propagation path, very little link margin was required for unimpaired digital 
voice: about 2 dB margin was sufficient. It was found that almost any tree produced an attenuation of 
12-15 dB, far more than considered economically practical for either downlink margins or mobile unit 
transmitter dynamic range. This conclusion agrees with another line of propagation modeling being 
pursued in Europe 2. Figure 2 illustrates the general form of attenuation observed as the vehicle went 
behind a tree. 
A margin of 6 dB was of little value in improving the communications while the vehicle was behind 
a tree. 
As the result of this limited experience, a conclusion was reached which agrees with that in CClR 
IWP 8/14 : 
"In general the propagation data that is available today is not sufficient 
to characterize the land mobile case." 
It can be concluded that there are two general schools of thought. One fundamentally believes 
that margins for shadowing are effective. The other believes that margins for shadowing would have 
to be impracticably large. 
SYSTEM IMPUCATIONS OF PROPAGATION MODELS 
If the channel capacity for the class of satellite system called out in the conference guidelines is 
calculated, a CDMA satellite system will be self-noise limited. The CDMA system capacity will 
generally be determined by the link margin assigned to the vehicle-to-satellite link. 
For the same satellite the FDMA capacity will be limited by the link margin assigned to the 
satellite-to-vehicle link. 
In designing a satellite system it is not appropriate to bet the cost of a satellite system 
(hundreds of millions of dollars) on the universality of whatever propagation model one believes in. 
It is safer to provide a variety of differently priced services (with different margins) and let the 
consumer determine the "right" propagation model. 
Some users will prefer to use a low-margin service (at a lower per-minute rate) which is 
restricted to line-of-sight conditions. Users in the unforested Southwestern area of the country or 
those that operate mainly from clear interstate highways might be happy with a low-cost, 
low-link-margin service. Users in deeply forested areas might find that they can only communicate 
from locations where there is line-of-sight. They will be willing to be selective in where they 
attempt to communicate, rather than pay for a 20 dB margin. Other users will insist on either 
72 
having toll-quality speech (not broken up by fading and shadowing) or none at all and will use the 
low-margin service only in selected locations. 
Some users such as emergency vehicles and law enforcement officers will not be able to be 
selective in where or when they communicate and insist on a service with a large link margin. They 
may be tolerant of the voice quality varying from "good" to "very poor" as long as they have some 
communications. Others that live in forested areas with low elevation angles to the satellite may have 
no choice but to use the high-margin service. 
The system engineer must guess at the national market distribution and guess at what the 
average or aggregate margin will be when they design the system. Since there is no experimental or 
factual data to support a presumed aggregate margin, it is a matter of personal opinion or 
conjecture. 
The specification of aggregate margin can determine the outcome of the FDMA versus CDMA 
tradeoff. If a low-aggregate margin is specified, CDMA will have the greatest channel capacity. If a 
high margin is specified and the FDMA and CDMA systems being compared have a high percentage of 
the users in each antenna beam, FDMA will have the greatest channel capacity. If the aggregate 
margin is greater than 4-6 dB FDMA for that class of system described before in reference 4, FDMA 
will have a greater capacity. Below 4-6 dB link margin CDMA has a greater channel capacity. 
In the time since reference 4 was written the CDMA system technology has been improved. The 
relative efficiency of CDMA and FDMA is not so dependent on margin selection if channelization of the 
CDMA is used. In the last section of this paper further examples will be shown of channelized CDMA. 
Antenna Costs: Manufacturing, Parts, Labor +Manufacturer's Profit + 
Inventory/Distribution Costs +Sales Cost + Installation Cost - Omni Antenna 
Costs +Warranty/Service Cost + Structual Modifications to Vehicle +Repair of 
Damage to Vehicle = $ 2,000 
REVENUE POTENTIAL AS AN EFFECTIVENESS MEASURE 
"Revenue potential" is a more encompassing measure of system effectiveness than simple 
channel capacity. It  can include the cost of the user's vehicle antenna and eliminate the "apples 
versus oranges" comparison of systems using directional and omnidirectional antennas. Since CDMA 
systems (under the conference guidelines) are self-noise limited, there is little advantage for them 
to use directional antennas. FDMA systems are power limited and can gain in capacity by using 
directional antennas. The total investment of the users in terminal equipment plus the investment in 
the space segment are different in CDMA and FDMA systems if different vehicle antennas are used. In 
the following revenue potential calculation the variable is removed by assuming that all users pay 
the same monthly charge and then calculating the satellite operator's revenue. 
For this analysis it is assumed that the average user leases his terminal equipment and uses 
150 minutes of airtime per month. Each satellite channel is shared by 60 users. For this example 
the user cost structure shown in figure 3 will be used. 
Figure 3, User Terminal Cost Assumetions 
Radio Costs: Manufacturing, Parts, G/A, Labor +Manufacturer's Profit + 
InventoryDistribution Costs +Sales Cost + Installation Costs + Omni Antenna Cost I +Warranty/Service Cost = $3,000 
I 73 
I 
($400) whether they have an omnidirectional antenna or a directional antenna. 
If the user has a directional antenna, they pay the satellite operator $150 per month. If they 
have an omnidirectional antenna, they pay the satellite operator $250 per month. If there are 60 
users per channel, then the satellite operator's revenue potential is either $9,000 /channel/month 
or $1 5,000 /channel/month, depending on the antenna used. 
COMPARISON OF REVENUE POTENTIALS 
The satellite that will be assumed is the fan-beam satellite that was described in a previous 
related paper 4. The 7 MHz of bandwidth (allocated by the conference committee, not the FCC or 
WARC) asshown is assumed. For an FDMA system the band is divided into three 2.33 MHz 
sub-bands. For the CDMA system the band is channelized into two 3.5 MHz sub-bands. 
Figure 4 shows how the frequencies are allocated to antenna beams for an FDMA system. For 
beams limited to CONUS the frequency-use factor is 2.33. 
Figure 4, FDMA Frequency Use 
Is 2.33 Times 
b 
Figure 5, CDMA Frequency Use 
Is 3 5  x 2 =7 Times 
In an FDMA system the polarization cannot allow an additional frequency use. Multipath 
reflections from buildings and other objects reverse the sense of the polarization. A signal 
transmitted on one sense of polarization appears as interference on the opposite polarization after 
reflection. Since FDMA receivers require 1 0-dB or more signal-to-noise ratios they cannot tolerate 
interference that is near the same level. 
In figure 5 the CDMA system frequency allocation to beams is shown. CDMA has 3.5 frequency 
uses and a polarization reuse to give a total use of 7 times. 
By alternating the frequency assignment the cochannel interferers are limited to the side of the 
antenna beam and to the sidelobe regions. 
Every other antenna beam uses the same frequency subband. The users in the adjacent beam do 
not use the same frequency subband and therefore do not contribute to the self-noise of that band. 
This is in contrast with the system of reference 4 where the cochannel interferers were equally 
distributed in all beams. 
The EdN0 requirements for CDMA and FDMA are quite different. The CDMA requirements are 
2.9 dB in the hub-to-mobile direction and 3.9 dB in the mobile-to-hub direction based on actual 
laboratory tests of rate 1/3 coded BPSK equipment. The satellite-to-mobile link requirement is 
lower than the mobile-to-satellite link. On the satellite-to-mobile link a single high power 
reference beacon is used to provide high signal-to-noise timing to all users. On the 
mobile-to-satellite link the timing must be derived from the data carrier. The FDMA Eb/No 
requirements are for rate 2/3 TCM/D8PSK are 9.5 dB from reference 5. 
The difference in FDMA and CDMA Eb/No requirements is substantial. Because the CDMA 
requirements are so low polarization reuse of frequencies is possible. The multipath reflected 
cross-polarized interference levels are much smaller: the signal-to-interference level is much 
higher than the signal-to-noise required. 
74 
The system capacity has been calcillated for 2-dB and 8-dB margin requirements and with the 
use of both directional and omnidirectional antennas. In figure 6 it can be seen that the CDMA channel 
capacity is substantially greater than the channel capacity of FDMA for both 2-dB and 8-dB 
aggregate margins. 
Figure 6, Satellite Channel Capacity 
12000 fl 
10000 
8000 
6000 
4000 
2000 
0 
CDMA 
12 dBi antenna 
4 dBi antenna 
2-dB 8 d B  2 d B  8-dB 
Margin Margin Margin Margin 
In figure 6 the system capacity does not change in proportion to the antenna gain changes. For 
the FDMA high-margin cases a substantial amount of power is used in the K-band backhaul to 
provide the margin on that link. That is why 8 dB change of antenna gain only increases the capacity 
by approximately a factor of three. The FDMA low-margin case with the high-gain antenna is 
bandwidth-limited to 3200 channels: the difference in capacity between the low-gain and high-gain 
cases is not as large as expected because of the bandwidth limit. In the CDMA low-margin cases the 
system is self-noise limited and the additional gain of the antenna does not make much difference. In 
the CDMA high-margin examples the 4-dB antenna causes the system to operate in a power limited 
mode. The 12-dB antenna causes the system to approach self-noise limitation: the capacity does not 
increase proportional to the antenna-gain difference. 
It should also be noted in figure 6 that a single CDMA satellite will allow the capacity of 
approximately three comparable FDMA satellites. 
Figure 7, Revenue Potential 
- 
.d I 12 dBi antenna 
8 4 dBi antenna 
a .s 
E *  
a 
Y 
Y 
0 
2 3  
5 .= 
$ E  
4 
- m 
E 
CDMA CDMA FDMA FDMA 
Margin Margin Margin Margin 
2-dB 8-dB 2-dB 8-dB 
If the channel capacities are multiplied by the revenue potential for each class of link, as outlined 
above, the results shown in figure 7 are developed. Using the revenue potential criteria tends to even 
75 1 
I 
out the differences between the high-gain and low-gain cases. It can be seen that CDMA has a much 
greater revenue potential than FDMA.The presentation of the same data in the form of figure 8 can bring 
some interesting additional insight. Previously it was mentioned that there is room for disagreement 
about the propagation model and the aggregate margin required. The consumers' preference for 
directional or omnidirectional antennas is also not well understood. We can also speculate on the 
consumers' willingness to pay for margin. Figure 8 provides the range of annual revenue potential as a 
function of margin and antenna preference. 
Figure 8, Range of CDMA and FDMA Revenue 
Potential Versus Link Margin 
CDMA 
FDMA 
8 dB Marein 2 dB 
8 dB 2 dB 
12 dBi antenna 
4 dBi antenna 
0 5 0 0  1 0 0 0  1 5 0 0  
Annual Revenue Potential, $ Millions 
It is clear from figure 8 that even with the uncertainties of margin requirements CDMA will offer 
hundreds of millions of dollars per year greater revenue potential. The system operator has the 
potential of making more money offering a CDMA service with 8-dB aggregate link margin than they 
would offering a FDMA service with 2-dB margin. The system operator also has the potential of making 
more money with one CDMA satellite than three comparable FDMA satellites. 
References 
1. J.S. Butterworth, "Propagation Measurements for Land Mobile Satellite systems at 1542 
MHz," Communications Research Centre Technical Note No. 723, Department of 
Communications, Canada, Aug 1984. 
2. E. Lutz, "Land Mobile Channel - Recording and Modelling," Fourth International Conference on 
SATELLITE SYSTEMS FOR MOBILE COMMUNICATIONAS AND NAVIGATION, London, England, 
17-1 9 October 1988. 
U.S. SG8C/228/1 , October 151 988. 
the Mobile Satellite Conference, May 3-5, 1988, Jet Propulsion Laboratory. 
15,1988, JPL Publication 88-12, MSAT Report 148 
3. J.Vorhies, "MSS Propagation Modeling for MSS" CClR XVII Plenary Assembly, IWP 8/14-16, 
4. I.M. Jacobs, et all "Comparison of CDMA and FDMA for the Mobilestar System," Proceedings of 
5. K. Dessousky and M.Sue, "MSAT-X A Technical Introduction and Status Report," April 
76 


System level comparison of FDMA vs. CDMA (under conference guideline constraint)

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