Code Division Multiple Access (CDMA) and Frequency Division Multiple Access (FDMA) (both analog and digital) systems capacities are compared on the basis of identical link availabilities and physical propagation models. Parameters are optimized for a bandwidth limited, multibeam environment. For CDMA, the benefits of voice activated carriers, antenna discrimination, polarization reuse, return link power control and multipath suppression are included in the analysis. For FDMA, the advantages of bandwidth efficient modulation/coding combinations, voice activated carriers, polarization reuse, beam placement, and frequency staggering were taken into account

Lyons, R. G.

Mazur, B. A.

Yongacoglu, A.

English

NASA Technical Reports Server

N92-24176
Comparison of FDMA and CDMA for Second
Generation Land-Mobile Satellite Communications
A. Yonga§o_lu
Univ. of Ottawa
Dept. Electrical Eng.
Ottawa, Ontario
R.G. Lyons
SkyWave Electronics
300 March Road
Kanata, Ontario
B.A. Mazur
Calian Communications
300 Legget Drive
Kanata, Ontario
Abstract
CDMA and FDMA (both analog and dig-
ital) systems capacities are compared on the
basis of identical link availabilities and phys-
ical propagation models. Parameters are op-
timized for a bandwidth-limited, multi-beam
environment. For CDMA, the benefits of
voice activated carriers, antenna discrimina-
tion, polarization reuse, return link power con-
trol and multipath suppression are included in
the analysis. For FDMA, the advantages of
bandwidth efficient modulation/coding combi-
nations, voice activated carriers, polarization
reuse, beam placement, and frequency stagger-
ing have been taken into account.
1. Introduction
In the long term, the land-mobile satellite
channel will become bandwidth limited; hence
it is important from the outset to identify and
plan the introduction of techniques and tech-
nology offering maximum overall spectral ef-
ficiency. For this purpose, we have studied
the CDMA and FDMA system capacities for
a bandwidth-limited second generation land-
mobile satellite communications system.
This paper presents a capacity analysis and
comparison of CDMA and FDMA approaches
as applicable to the voice service. Both dig-
ital and amplitude companded single side-
band (ACSSB) analog modulations have been
considered with FDMA. Analysis is conser-
vative to ensure stated performance can be
achieved, notwithstanding implemention uncer-
tainties, using today's technology.
The organization of the paper is as follows.
Section 2 states an objective basis for compar-
ison and discusses the relevant system param-
eters. Section 3 presents the capacity analy-
sis for each candidate system. Section 4 com-
pares the performance of the candidate tech-
niques. Potential capacity enhancement tech-
niques are discussed in Section 5, and conclu-
sions are drawn in Section 6.
2. System Parameters
The systems are compared on the basis of
identical [1]
• C/NoW,= total received signal to noise
power ratio (L-band forward or return
link),
• O3aB = 3 dB satellite beamwidth,
• spatial coverage area (relative to O3dB).
All candidates are assumed to operate in
a circuit switched demand assignment mode,
with primarily voice traffic. Access control and
signalling overhead has been neglected as a dif-
ferentiating factor in comparing b/s/Hz of us-
able capacity versus C/NoW_. All beams are
assumed to generate and terminate the same
number of erlangs of traffic. The implications
of non-uniform traffic is discussed in [3].
The issue of voice coding rate is avoided by
expressing capacity in b/s/Hz rather than as a
number of channels/kHz. ACSSB at a specific
C/No value is treated as a 4.8 kb/s circuit to
allow comparison on the basis of b/s/Hz.
International Mobile Satellite Conference, Ottawa, 1990
639
https://ntrs.nasa.gov/search.jsp?R=19920014933 2020-03-17T12:27:50+00:00Z
The other system parameters and the as-
sumptions made are as follows.
Antenna Pattern
The assumed spacecraft antenna illumina-
tion pattern was that of a parabolic reflector
with constant illumination, and is given by
G(u) = 2(Jl(U)/U) 2 (1)
where
sin 0
u= _D--f--- (2)
In (1) and (2), ,/1( ) is the Bessel function of
the first kind and first order, D is the antenna
diameter, $ is the carrier wavelength, and 0 is
the off-axis angle.
Sensitivity to illumination pattern is not sig-
nificant to the conclusions stated herein [3].
Multi-Beam Geometry
A hexagonal coverage contour with two ap-
posite sides elongated was considered, as shown
in Fig. 1. This contour is illuminated by 10
beams touching at -3 dB or 13 beams touch-
ing at -2 dB contours. The minimum antenna
discrimination values indicate that FDMA ca-
pacity is optimized using 10 beams with 3 fre-
quency bands. In Fig. 1, the notation 1s'r
refers to frequency band 1, right hand polar-
ization with frequency staggering. Similarly 2 l
denotes frequency band 2, left hand polariza-
tion, no staggering.
For CDMA, channelization by utilizing dif-
ferent frequency bands in different beams, in
general, was not found to provide any signif-
icant capacity increase. However, using 13
beams touching at -2 dB contours provides
higher capacity than 10 beams touching at -3
dB contours. Therefore, 13 beams with a single
frequency band was used in the capacity anal-
ysis.
Frequency Staggering
Frequency staggering refers to offsetting the
FDMA system frequency plans in different
beams by one-half channel. For both digital
and ACSSB FDMA, the interference reduction
achieved by frequency staggering has been in-
cluded in the analysis.
Figure 1: lO-Beam dual polarization con-
figuration with a frequency reuse factor
of 10/3.
Power Control
Applying power control to compensate for
the static losses incurred by signals travelling
to and from the mobiles is a technique which
can reduce interference in some cases. For the
FDMA schemes, the effectivenes of power con-
trol on the worst case interference condition
was found to be minimal, hence was not in-
cluded in the analysis. Considering also imple-
mentation issues, power control was assumed
to be applied to the return link of CDMA, but
not the forward link.
Dual Polarization
Dual polarization was assumed for CDMA
and digital FDMA, but not for ACSSB FDMA.
Grade of Service
The link analysis of a mobile satellite sys-
tem involves a number of parameters which are
subject to statistical variations. We have ac-
counted for long-term and gross spatial varia-
tions (e.g., received power differences between
the centre and edge of a beam) by assuming
nominal worst-case values for such parameters.
Small spatial and short-term variations were
accounted for by applying a grade of service
(GOS) approach.
A primary contributor to short-term varia-
tions is shadowing attenuation. We modelled
shadowing as a two-state variable. Based on
empirical data [2], the channel was assumed
to be in the ON-state (i.e., errors are due to
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International Mobile Satellite Conference, Ottawa, 1990
AWGN and multipath but not shadowing) for
90% of the time. Thus, for system grades of ser-
vice of 90% or less, defined at the voice decoder
input, no particular effort to counteract shad-
owing need be applied. However, GOSs above
90% can be achieved by a power margin and/or
interleaving. In this paper we consider a GOS
of 90%.
3. Capacity Analysis 1
3.1 CDMA
The capacity of a CDMA system, r/, in units
of b/s/Hz is given by
I
1
- c (3)
71- IV, NoW, V(L, + ap(N---7_))
where
M: total number of users in the system,
Rb: information bit rate per user,
W,: total available bandwidth,
V: voice activity factor,
No: thermal noise spectral density,
(Eb/N'o)_q: energy per bit to total noise den-
sity ratio required for a specific BER,
Wn: ratio of total bandwidth to the chip rate,
L,: shadowing loss factor,
a: antenna discrimination factor,
p: polarization discrimination factor.
The parameters in (3) which are common to
forward and return links are calculated as fol-
lows.
Coherent BPSK theoretically requires an
Eb/N'o of 6.8 dB to achieve a BER of l0 -3.
For this we assumed the use of a rate 1/3, con-
straint length 6 convolutional code for which at
a BER of 10 -3 the coding gain is approximately
4 dB. Assuming an implementation margin of
2 dB, (Eb/io)r_ was found as 4.8 dB.
The voice activity factor, V, was assumed to
be 0.4.
The bandwidth to chip rate ratio, Wn, which
depends on the filtering used, was assumed to
be 1.125.
1Due to space limitations, the interested reader is
referred to [3-5] for the derivation of the following
equations.
Assuming that half of the users will transmit
and receive left-hand circular polarization and
the other half will use right-hand circular polar-
ization, the polarization discrimination factor
is
1
p = _(1 + Xpo,) (4)
where xpol is the cross polarization rejection
factor. For a Xpol of 0.1, the polarization dis-
crimination factor is p = 0.55.
The differences between the forward and re-
turn link capacity equations mainly lie in the
antenna discrimination factors and shadowing
loss factors.
In the forward direction, a mobile terminal
will receive all the co-channel interference from
the other users in its particular beam, plus
some additional co-channel interference from all
the other beams in the system at a reduced
power level. For the beam geometry and the
antenna radiation pattern assumed above, the
forward link antenna discrimination factor was
calculated as al = 0.255 [3].
In the return direction, a particular beam
of the satellite collects co-channel interference
from all the users, weighted by the antenna gain
in the direction of the co-channel users. The
return link antenna discrimination factor cor-
responding to this situation was calculated as
aT = 0.169. Note that these af and aT values
are for worst case user locations.
In CDMA, because it has a better antenna
discrimination factor and shadowing loss factor,
the capacity of the return link was found to be
higher than the forward link capacity.
3.2 Digital FDMA
The capacity of a digital FDMA system is
given by:
iRb_ C 1 1 (1 ShY o)(5)
71- W, NoW, Eb/N" V -_o
where N o is the total noise power density falling
within the bandwidth of the receiver and is
given by
N_ = No + YoEb. (6)
In (6), Yo lumps together the co-polar inter-
ference, cross-polar interference, and adjacent
channel interference (ACI), i.e.,
International Mobile Satellite Conference. Ottawa, 1990
641
Yo= + + L-Ac )/Eb (7)
where
/o-co: co-polar interference spectral density,
/o-x: cross-polar interference spectral density,
Io-ACI: adjacent channel interference spectral
density.
In FDMA, where the number of interferers is
small, treating interference as Gaussian noise
and adding it to the thermal noise on a power
basis is a conservative approach. However,
since the target bit error rate, 10 -3 , is not very
low, this approach is acceptable.
For the assumed beam geometry and antenna
pattern, and using dual polarization, the for-
ward and return link (Eb/Io-co) values were
calculated as 18.1 dB and 17.1 dB respectively.
Similarly the forward and return link (Eb/Io-x)
values were calculated as 21.5 dB and 20.5 dB
respectively. Intra-system interference densi-
ties are related to Eb, because increasing the to-
tal transmitted energy proportionally increases
the interference. There is little difference be-
tween forward and return link capacities at the
same C/NoW, value.
The upper limit of 7/ is determined by the
modulation technique, FEC code rate and fre-
quency reuse factor. For example, a rate r
coded PSK signal with q phases has a maxi-
mum bandwidth efficiency of
r/= fir l°g2 q1 + b (8)
where /3 is the frequency reuse factor, and b
accounts for the excess bandwidth required for
practical filters and the guard-bands between
FDMA channels. We used b = 0.2 as a typical
implementable value.
The ACI term is a function of the frequency
spacing between carriers, transmit symbol rate
(i.e., modulation and coding dependent), trans-
mit and receive filter characteristics, and fre-
quency uncertainty. (Eb/Io-ACi) values were
calculated by assuming that the transmit and
receive filters have a raised-cosine characteris-
tic with a roll-off of 0.2. Note that since ACI
is mainly due to differential frequency offset,
it was considered only for the return link. A
worst case differential frequency uncertainty of
+400 Hz was assumed for the return link.
The candidate modulation techniques con-
sidered for the digital FDMA were:
• coherent 16-state Trellis-Coded-8PSK
(TC-8PSK),
• coherent 4PSK with rate 3/4 convolutional
coding (constraint length of 6).
Both rate 3/4 coded 4PSK and TC-8PSK mod-
ulations were assumed to employ a short inter-
leaver (with about 60 ms of total delay) to ran-
domize most of the errors due to multipath.
The modem implementation loss was as-
sumed to be 2 dB for 4PSK and 2.5 dB for
TC-8PSK.
After AWGN, multipath and implementation
loss are taken into account, the Eb/N: values
required to obtain a BER of 10 -3 are 7.3 dB
for rate 3/4 coded 4PSK and 9.1 dB for TC-
8PSK.
The maximum spectral efficiency of rate 3/4
coded 4PSK (including frequency reuse factor
of 10/3) is 4.17 b/s/Hz, while TC-8PSK can
achieve 5.56 b/s/Hz. Based on equation (5)
and the above given Eb/N'ovalues , it can be con-
cluded that in the power limited region rate 3/4
coded 4PSK is more efficient. However, if suffi-
cient power is available, TC-8PSK can achieve
a better spectral efficiency.
3.3 ACSSB
In ACSSB, the mean opinion score (MOS)
is used as a mechanism for quantifying sub-
jective acceptability. C/No required per chan-
nel, (C/No)ch, was obtained from experimental
data [6,7] by determining the operating point
from the channel parameters and the interfer-
'ence levels.
Considering a voice activity factor of 0.4 and
a channel spacing of 5 kHz, the total C/NoW,
can be expressed as
No---_, = 0.8 × 10-4/3( )ch (9)
where fl = 10/3 is the frequency reuse factor.
642
International Mobile Satellite Conference, Ottawa, t990
For a MOS of 2.5 and the multi-beam geom-
etry of Fig. 1, the required (C/No)ch for the
forward and return link are found as 47 dB-
Hz and 48 dB-Hz respectively. These values
include a margin of 1 dB for phase noise, am-
plifier nonlinearity and filtering.
Note that for the beam geometry assumed,
ACSSB can readily achieve its bandwidth-
limited capacity without utilizing dual polar-
ization. Hence only single polarization has been
considered.
4. Capacity Comparisons
In this section the forward and return link
capacities of CDMA and FDMA schemes are
compared. The C/NoWs values used in the
comparisons are 10 dB and 20 dB. Table 1
presents the capacities for the 10 dB C/NoW,
value which corresponds to a power limited
condition for the candidate systems. Ta-
ble 2 presents the capacities for the 20 dB
C/NoW, value, at which the FDMA candidates
have reached their bandwidth-limited capaci-
ties, and CDMA curves are approaching their
asymptotes.
Note that an excess power margin (beyond
the bandwidth-limited C/NoW,) does not in-
crease capacity but increases the GOS to above
90%.
5. Capacity Enhancement
Some further potential capacity enhance-
ment techniques which were not included in the
analysis are briefly discussed below.
5.1 CDMA
For CDMA, using 4PSK rather than BPSK
would distribute the self noise among orthogo-
nal dimensions of the signal space. Thus 4PSK
could increase the capacity by 60% and 90%
at C/NoWs values of 10 dB and 20 dB respec-
tively.
Bit or chip synchronization permits one to
choose spreading codes which are approxi-
mately orthogonal, thus reducing the self noise.
As a consequence of above mentioned enhace-
ments, the capacity of CDMA would signifi-
cantly improve. However, this may be practical
only in the forward link. Since the CDMA sys-
Table 1: Capacity at C/NoW,= 10 dB.
System
CDMA
Forward Link
Capacity
(b/s/Hz)
2.3
CDMA
Return Link 4.1
Digital FDMA
Forward Link 4.171
Digital FDMA
Return Link 3.71
ACSSB
Forward Link 2.42
ACSSB
Return Link 2.4 a
1. 4PSK with rate 3/4 coding
2. 2 dB excess margin available
3. 1 dB excess margin available
Table 2: Capacity at C/NoW,= 20 dB.
System Capacity
(b/s/Hz)
CDMA
Forward Link 3.1
CDMA
Return Link 8.3
Digital FDMA
Forward Link 5.561'2
Digital FDMA
Return Link 5.561'3
ACSSB
Forward Link 3.24
ACSSB
Return Link 3.25
1. TC-C8PSK
_" 6.5 dB excess margin available
3. 5.2 dB excess margin available
4. 8.8 dB excess margin available
5. 7.8 dB excess margin available
International Mobile Satellite Conference, Ottawa. 1990
643
tem has a lower relative capacity in the forward
link, the associated complexity might be justi-
fiable.
5.2 FDMA
Subdividing each beam into many sectors de-
scribed by constant interference level bound-
aries and assigning location dependent fre-
quency or polarization is expected to improve
the capacity.
Capacity enhancement with ACSSB is lim-
ited to reducing spectral occupancy and chan-
nel spacing. A 20% increase (corresponding to
a reduction in channel spacing from 5 kHz to 4
kHz) is achievable in the bandlimited region; re-
sulting increased sensitivity to interference may
be offset by resorting to dual polarization.
6. Conclusions
Based on the results presented above, the fol-
lowing conclusions can be drawn.
• For both FDMA systems, the disparity be-
tween the forward and return link capaci-
ties is negligible.
• The CDMA capacity for the forward and
return links display a wide variance. In all
cases the return link provides higher ca-
pacity.
• At low C/NoWs values (i.e., up to 10 dB)
the capacity of digital and ACSSB FDMA
are comparable. At higher C/NoW_ val-
ues digital FDMA provides more capacity
then its analog counterpart, while ACSSB
FDMA has larger excess margins available
(which yields better voice quality/higher
GOS).
• In the forward link, digital FDMA provides
more capacity than CDMA. The differen-
tial between digital FDMA and CDMA de-
creases with increasing GOS.
• In the return link, CDMA provides more
capacity than digital FDMA. The differen-
tial between digital FDMA and CDMA re-
mains fairly constant as GOS is increased.
• Both FDMA system capacities are sensi-
tive to channelization, while CDMA is not.
• In [3] other beam contours have been con-
sidered. Capacity increases with increased
coverage area, while the relative perfor-
mance of the various schemes was the same
as for the smaller coverage area.
It can be concluded that all systems consid-
ered possess various possibilities for enhance-
ment, but the capacity improvements available
to CDMA are probably larger and more easily
attainable.
References
[1] A.J. Viterbi, "When Not to Spread Spec-
trum- A Sequel", IEEE Com. Magazine, Vol.
23, pp. 12 - 17, April 1985.
[2] Standard-M Technical Note TN/1/3; "Sim-
plified Land Mobile Burst Channel Model for
Standard-M', August 1989.
[3] "Study of Modulation Techniques in View
of Frequency Reuse for Land-Mobile Satellite",
Final Report to European Space Agency by
SkyWave Electronics, April 1990.
[4] T. Murphy, "Calculation of FDMA and
CDMA System Capacity for a Land Mobile
Satellite System", MobileStar sm Technical Re-
port TM-C1412, Hughes Communications Inc.,
April 1988.
[5] K.G. Johannsen, "CDMA versus FDMA
Channel Capacity in Mobile Satellite Commu-
nication", Int'l. J. of Satellite Commun., Vol.
6, pp. 29 -39, 1988.
[6] S.C. Morris, "A Subjective Evaluation of the
Voice Quality of MSAT Modulation Schemes",
BNR Report OCTR 85-0006, Dept. of Comm.
Contract No, 01SM.36001-4-4005, Dec. 1985
[7] J.T. Sydor and B.AS. Chalmers, "Co
and Adjacent Channel Performance of Am-
plitude Companded Single Sideband Modula-
tion", Comm. Research Centre, Mobile Satel-
lite Communications SS # 4/87, June 1987.
Acknowledgement
This work is based on a study performed for
the European Space Agency by SkyWave Elec-
tronics Ltd. and Calian Communications Sys-
tems. The views expressed are those of the au-
thors.
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International Mobile Satellite Conference, Ottawa, _1990


Comparison of FDMA and CDMA for second generation land-mobile satellite communications

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