Conceptually, this paper discusses strategies of digital satellite communication networks for a very large number of low density traffic stations. These stations can be either aeronautical, land mobile, or maritime. The techniques can be applied to international, domestic, regional, and special purpose satellite networks. The applications can be commercial, scientific, military, emergency, navigational or educational. The key strategy is the use of a non-orthogonal access method, which tolerates overlapping signals. With n being either time or frequency partitions, and with a single overlapping signal allowed, a low cost mobile satellite system can be designed with n squared (n squared + n + 1) number of terminals

Wu, W. W.

English

NASA Technical Reports Server

N88-25737
AN ACCESS ALTERNATIVE
FOR
MOBILE SATELLITE NETWORKS
W.W. WU
INTE LSAT
ABSTRACT
Conceptually, this paper discusses strategies of
digital satellite communication networks for a very
large number of low density traffic stations. These
stations can be either aeronautical, land mobile, or
maritime. The techniques can be applied to inter-
national, domestic, regional, as well as special purpose
satellite networks. The applications can be commercial,
scientific, military, emergency, navigational or educa-
tional. The key strategy is the use of a non-orthogonal
access method, which tolerates overlapping signals.
With n being either time or frequency partitions, and
with a single overlapping signal allowed, a low cost
mobile satellite system can be designed with n2(n2+n+l)
number of terminals.
i. INTRODUCTION
As new demands and requirements of satellite
communications change, the network strategy and the
technological utilization need to change accordingly.
Mobile satellite communication, either aeronautical,
land or maritime, has created both technical and
nontechnical challenges. Among the technical challenges
likely to be encountered would be a network consisting
of a very large number of small earth stations, where
there is signal degradation due to deflection, reflection,
shadowing, doppler shift, and other interference effects,
as a consequence of receivers in motion and operation in
various channel environments.
As an example of the number of stations in a mobile
satellite network there are about 25,000 vessels above
1600 gross tonnage registered in the three oceanic
regions, and there are around 330,000 general aviation
395
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aircrafts worldwide. By the year 2000 there are likely
to be more than 12,000 commercial passenger-carrying
airliners in operation [i]. For land mobile satellite
communication, the number of users can be easily
extended to six figures. On the other hand, because of
the need for mobility, the size of user stations are
getting smaller, which results in weaker signal
reception and is increasingly prone to interference
difficulties.
At present, all satellite networks, employing
either mobile or fixed earth stations, operate on the
principle of a controlling, or a number of controlling,
stations. The network architecture is determined
primary by the satellite access method used by the earth
stations in the network. Since multiple access tech-
nique affects a satellite network more than any other
network element, in this paper we therefore discuss
multiple access strategies particularly from a mobile
satellite network viewpoint. In Section 2, some
background information on multiple access techniques is
briefly described. In Section 3, the fundamental
characteristics of random multiple access is outlined.
In Section 4, the error probability expression is
derived. The application of random multiple access to
mobile satellite communication is discussed in the
concluding remarks in Section 5.
2. BASIC ACCESS SCHEMES
Based on the resources of frequency, time, and
space (or location), some of the basic access techniques
for satellite communication are shown in Figure i. From
this it can he observed that most multiple access
techniques can be derived from the division of combina-
tions of the basic resources. Basic orthogonal access
methods utilizing separate time and frequency division
have been designed, implemented, and become operational
on satellite networks. However, due to either frequency
instability or time jittering, finer frequency or time
division allocation alone becomes s practical difficulty
for a network with a very large number of stations_ and
other access means must be sought.
As shown in Figure i, when TDMA is combined with
SDMA we have satellite-switched TDMA. When FDMA is
combined with TDMA we have a time-frequency (TF) access
scheme, which includes frequency hopping (FH) and time
hopping (TH). When a spreading effect is applied to
396
time we have time spreading (T-SPD); when it is applied
to frequency we have frequency spreading (F-SPD) or a
spread spectrum (SS) arrangement. From a mechanization
standpoint, FH can produce F-SPD, as TH can produce
T-SPD.
As indicated in Figure I, we classify ALOHA
multiple access systems as time division systems. ALOHA
or ALOHA-derived systems are different from conventional
TDMA systems in that ALOHA is not periodic in time and
most message slots are not fixed in length. In ALOHA
systems, although one can transmit messages randomly,
the system cannot guarantee their reception randomly
because of possible message collisions from other
accessing stations. For this reason, we do not consider
ALOHA-type access schemes to be truly random. The
discussion of other access schemes such as satellite
polling is omitted here and they can be found in [2].
In the next section, we discuss a different type of
random multiple access, unrelated to ALOHA, for mobile
satellite applications.
3. RANDOM MULTIPLE ACCESS
Starting with the use of the TF matrix for station
access, depending on the division of time and frequency,
with different detection and implementation process,
variations of TF systems may be designed. Basically, a
TF system makes use of the elements or cells of the time
and frequency two-dimensional matrix resulting in the
use of both time and frequency divisions. A specific
type of access method, which utilizes the TF matrix and
with deterministic signatures of prescribed character-
istics, has been referred to as random multiple access
(RMA). The definition, configuration, and the tradeoffs
of such an access scheme are described in [2], and the
signature generation algorithm for the purpose of random
multiple accessing is illustrated in [3].
Depending on the practical constraints as to whe-
ther time or frequency can be more finely divided, n can
be either the number of time or the number of frequency
divisions. If n is the number of time divisions as
shown in Figure 2, then it has been worked out by Wu
[I], that:
397
The number of frequency divisions f = n 2
The number of symbols in the TF matrix = n 3
The number of stations in the network = n2(n2+n+l).
In the derivation and proof of the above results,
the assumptions were that n is a power of a prime (i.e.,
n = pm), and single element overlap in the TF matrix is
tolerated. In the case that time can be more finely
divided as shown in Figure 3, then n becomes the number
of frequency divisions, and n 2 the number of time
divisions. The number of entries or symbols in the TF
matrix and the number of earth stations in the network
remain the same under the same condition of single
element overlapping. The number of divisions n needed
with respect to the number of stations in a network is
shown in Table I, with n up to 31 for near one million
stations.
4. PERFORMANCE WITH ONBOARD QUEUE
In RMA, a collection of s signatures (addresses, or
sequences) is assigned to each of the stations in the
network. Each signature assembles E number of time-
frequency elements out of a possible total number T x F,
where T and F are the number of time and frequency
division elements in a given frame. The receiver must
determine which one of the s signatures was transmitted,
based on the received E elements. Let us assume that
the collection of E elements is randomly chosen, and the
probability of the event for a particular E element to
occur is P(E) = E/TF. The probability for this event
not to occur is P(_.) = 1 - P(E). For k users in the
network not to have any one of the particular E
elements, the probability is
E
= [1 - P(E)]k = 1 (1)
But for false detection to occur, all E elements must
match, and this can occur with the probability
Ek(E) = (1 - Pk(E)) E
/ [ lk}= 1- 1 ETF (2)
398
which can happen for any one of the E element collec-
tions. Since there are s number of collections
(signatures), we then have for each user
F(k, E, s) = s F k (E). (3)
Now assume that a mobile satellite with an onboard
queue, and a network in which all users are statist-
ically independent, each user i with Poisson arrival
rate X i and departure rate gi. If the users can only be
increased or decreased one at a time (a realistic
assumption), then the network can be modelled as a
Markovian birth-death process. In this case, the
probability of a station accessing in At is XnAt, and
that of a station dropping out is _nAt. The steady
state probability of n accessing stations remaining in
operation at time t is
n _ki_ i
P" = P° iM=, -tx-i
(4)
For a maximum number of K stations we have
K K n __i- 1
n=O = ----
[ _i_,_ki_i . ]=Po 1+ =1
n=l =
(5)
or
Po = _ " hi- I1+ I]
n=l i=1 _.Li
(6)
The series in the denominator of (6) converges when
(_i-i/_i) < 1 for all i. Combining the results of (4),
(6), (I), and (3), we have the false detection error
probability,
399
KF(k,E,s,K) = _] Pk " F(k,E, s)
k=O
s 1- 1 E k hi_l
TF iOl _Li
-- K k
1 + '_ i-i hi-I
k=] i=] la,i
(7)
where K is the maximum number of stations designed for
the system, and k is the number of simultaneous stations
accessing the satellite channel during the time of
detection.
Equation (7) can be simplified for identical user
parameters, i.e., X i = _, _$ = _ for all i. A sample
evaluation of (7) is shown in Table 2 which assumes Xi_ 1
= X, _i = _ for all i and K = 1,000; k varies from i0 to
60. For K equal to 60, E changes from 3 to 16, and p
equal to 0.9, the false detection error probability F
(k, E, s, K) is shown in Figure 4.
5. CONCLUDING REMARKS
Without referring to a specific application of
aeronautical, land mobile, or maritime, this paper
conceptually discusses the use of a random multiple
access method with prescribed deterministic signatures
for mobile satellite networks. In the special case of
single element overlap being allowed, over a million
station network can be designed for the signature length
n = 32.
A possible RMA for mobile satellite network arrange-
ment is shown in Figure 5, where RMA generation and
detection functions may be combined into a coded phase
modem. Such a modem is extensively discussed in the
session on Modulation and Coding in this conference and
elsewhere. On the other hand, for minimization of com-
plexity or low cost reasons, digital frequency modu-
lation or frequency shift keying scheme can also be used.
A new investigation on RMA is presently under way
jointly by the University of Technology, Sydney and the
Overseas Telecommunications Commission of Australia [4,
5].
400
REFERENCES
[1] A. Ghais, G. Berzins, and D. Wright, "INMARSAT and
the Future of Mobile Satellite Services," IEEE
Journal on Selected Area in Communications, Volume
SAC-5, No. 4, May 1987.
[2] W.W. Wu, Elements of Diqitai Satellite
Communication, Volume I - System Alternatives,
Analyses and Optimization, Computer Science Press,
1984, Chapter 3.
[3] W.W. Wu, Elements of Diqital Satellite
Communication, Volume II - Channel Coding and
Integrated Service Digital Satellite Networks,
Computer Science Press, 1985, Chapter 3.
[4] M. Yates and T. Stevenson, Private Communications,
12 February 1988.
[5] G. Brann, G. Richardson, T. Percival, M. Friedgut,
and E. Bachmann, Private Communication, 12 February
1988.
Table 1
Parameters for RMA sequences up to length 31.
n
3
4
5
7
8
9
II
13
16
17
19
23
27
29
31
m
1 I
2 I
3'
2 2
5'
7 I
23
3 2
11'
13'
24
17'
19'
23'
33
29 I
31'
Number of
Symbols n 3
1
8
27
64
125
343
512
729
1331
2197
Number of
Stations
n2( n2 + n + 1)
3
28
117
336
775
2793
4672
7371
16093
30927
4096
4913
6859
12167
19683
24389
29791
69888
88723
137541
292537
551853
732511
954273
401
Table 2
Calculations of false detection error probability in random multiple access.
K(Max. Stations) = 1000
E p k F(k, E, s, K)
3 .1
4 .1
8 .1
16 .1
3 .2
4 .2
8 .2
16 .2
3 .5
4 .5
8 .5
16 .5
3 1.0
4 1.0
8 1.0
16 1.0
3 .1
4 .1
8 .1
16 .1
3 .2
4 .2
8 .2
16 .2
3 .5
4 .5
8 .5
16 .5
3 1.0
4 1.0
8 1.0
16 1.0
3 .1
4 .1
8 .1
16 .1
3 .2
4 .2
8 .2
16 .2
3 .5
4 .5
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
40
40
40
40
40
40
40
40
40
40
40
40
40
4O
40
40
60
60
6O
60
60
60
6O
60
60
60
402
.60333491E - 05
5.12182990E - 07
.65816949E -15
2.05177104E -38
.60493668E- 05
5.12694672E - 07
.65982604E -15
2.05382081E - 38
.60493878E- 05
5.12695343E-07
.65982821E -15
2.05382349E -38
.60493811E -05
5.12695129E - 07
.65982752E -15
2.05382264E - 38
4.00833630E - 06
1.28045777E -07
4.14542372E - 16
5.12942975E - 39
4.01234463E -06
1.28173822E-07
4.14956914E-16
5.13455918E-39
4.01234312E-06
!.28173774E-07
4.14956757E -16
5.13455724E- 39
4.01234312E-06
1.28173774E - 07
4.14956757E -16
5.13455724E- 39
2.67222463E -06
8.53638462E -08
2.76361563E-16
3.41961936E - 39
2.67489686E -06
8.54492100E -08
2.76637925E -16
3.42303898E -39
2.67489688E - 06
8.54492107E - 08
SPACE
TIME I I
FH CDMA TH
Figure I
MULTIPLE ACCESS METHODS
fn2 - 1
fn 2 - 2
fl
fo
to tI
Figure 2
'n- 1
TF MATRIX WITH f = n2
tn2-1
tn2- 2
t 1
to
fo fl fn- 1
Figure 3
TF MATRIX WITH t = n2
403
A
v
v
u.
10-10.
10 - 20.
10 - 30.
10-40
_, _: _ooo
4 8 12 1
Figure 4
FALSE DETECTION ERROR PROBABILITY OF
RANDOM MULTIPLE ACCESS SYSTEMS
Inp_ut Coded _._ RMA
Phase Generation
Modulation .._ _ _._coded
Satellite RMA Phase
Channel Detection Demodulation
Figure 5
MOBILE SATELLITE APPLICATIONS WITH RANDOM MULTIPLE ACCESS
404


An access alternative for mobile satellite networks

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