In fixed and mobile satellite communication, the
newest MAT scheme Spatial Division Multiple Access
(SDMA), has become one of the most promising
techniques that can accommodate a continuing
increase in the number of users and traffic demands.
Thus, the technology is based on transmission
resource sharing that separates communication
channels in space. It relies on adaptive and dynamic
beam-forming technology and well-designed
algorithms for resource allocation among which
frequency assignment is considered.
The SDMA technique allows increasing the
capacity of satellite communication system,
exploiting the users spatial separation. This technique
can be integrated into hybrid combination with
conventional MAT schemes, such as Frequency
Division Multiple Access (FDMA), Time Division
Multiple Access (TDMA) or Code Division Multiple
Access (CDMA) techniques, and can be used in all the
satellite and wireless communication systems
currently operated or to be introduced in the future.
As satellite communication systems move towards an
increasing number of users and a larger throughput
for each of them, the SDMA scheme is one of the most
promising techniques that can reach three goals, such
as they move towards greater capacity, higher
flexibility (with respect to the position of the users)
and better service to the end-user.
Today SDMA is currently used by the Iridium
GMSC system in Radio Frequency (RF) L-band, a
constellation of 66 Low Earth Orbit (LEO) satellites,
thanks to time beam-switching. In addition, the
SDMA technique is also foreseen as a key enabling
technology to increase the capacity of future two-way
Analyses of Space Division Multiple Access (SDMA) Schemes
for Global Mobile Satellite Communications (GMSC)
D.S. Ilcev
Durban University of Technology, Durban, South Africa
ABSTRACT: This paper describes analyzes of the Space Division Multiple Access (SDMA) technique and their
hybrids with other Multiple Access Technique (MAT) such as Frequency Division Multiple Access (TDMA),
Time Division Multiple Access (TDMA), and Code Division Multiple Access (CDMA) for implementation in
Global Mobile Satellite Communications (GMSC). In fixed satellite communication systems, as a rule, especially
in mobile satellite systems many users are active at the same time. The problem of simultaneous
communications between many single or multipoint mobile satellite users, however, can be solved by using
different MAT schemes. Since the resources of the systems such as the transmitting power and the bandwidth
are limited, it is advisable to use the channels with a complete charge and to create a different MAT scheme to
the channel. This generates a problem of summation and separation of signals in the transmission and reception
parts, respectively. Deciding this problem consists in the development of orthogonal channels of transmission in
order to divide signals from various users unambiguously on the reception part.
the International Journal
on Marine Navigation
and Safety of Sea Transportation
Volume 14
Number 4
December 2020
DOI: 10.12716/1001.14.04.05
satellite communications systems in low-frequency
bands (typically lower than 5-6 GHz) through the
interference mitigation and high-frequency reuse.
The satellite beam-former optimizes the antenna
diagram with respect to the positions of the users in
order to maximize the gain while mitigating
interferences. The resource allocation algorithm
carefully designs a frequency plan that (a) prevents or
limits interferences between users, and (b) tailors the
allocated bandwidth to the user need in order to save
the spectrum. However, the main future terrestrial
communication standards in wirelless
communication systems, such that Worldwide
Interoperability for Microwave Access (WiMAX), 3rd
Generation Partnership Project (3GPP), Long-Term
Evolution (LTE), and New Generation Cellular
Systems, also use the SDMA scheme. The SDMA
technique basically relies on adaptive and dynamic
beam-forming associated to a clever algorithm in
charge of resource allocation.
The Geostationary Earth Orbit (GEO) and Not-
GEO mobile satellite communication systems are
currently characterized by an ever-growing number
of users, which however is coupled with limited
available resources, in particular in terms of the
usable frequency spectrum. The technologies are
therefore oriented towards developing new access
techniques, for more efficient employment of
available frequency bands, such as the SDMA
scheme that allows enhancing the capacity of a GMSC
system by exploiting spatial separation between
mobile users. The mobile users can be oceangoing
ships, land vehicles (road and rail), aircraft, and ships
containers. In an SDMA system, the Ground Erath
Station (GES) terminal does transmit the signal
throughout the coverge area via satellite, as is the case
of conventional access techniques, but rather
concentrates power in the direction of the mobile
units, known as Mobil Eart Stations (MES), the signal
is meant to reach and reduces power in the directions
where other units are present. The same principle is
applied to the reception.
In traditional GMSC systems the GES terminal,
having no information on the position of mobile
units, is forced to radiate the signal in all directions,
in order to cover the entire area of the satellite
coverage. This entails both a waste of power and the
transmission, in the directions where there are no
mobile satellite terminals to reach, of a signal which
will be seen as interfering for spot beams, which are
using the same group of RF bands. Analogously, in
reception, the antenna picks up signals coming from
all directions, including noise and interference. These
considerations have to lead the development of the
SDMA technique, which is based on deriving and
exploiting information on the spatial position of MES
terminals. In particular, the radiation pattern of the
GES terminal, both in transmission and reception, is
adapted to each different MES to obtain the highest
gain in their directions.
Figure 1. Common FDMA, TDMA and CDMA Techniques
The GEO and Non-GEO GMSC systems are a
communications node through which all types of thge
mobile users in the network must be interconnected
as flexibly as possible. At the same time, two key
resources, such as bandwidth and spacecraft power -
must be utilized efficiently. However, for some
applications, it may be necessary that a satellite is
simultaneously accessed by hundreds of mobile
satellite users, making accessing problems more
complex. Further complications are added when
factors such as a requirement for handling a mix of
Voice, Data, and Video (VDV) satellite transmissions,
traffic variations, and a necessity to incorporate
communication growth are considered. Therefore, at
this point in the fixed and mobile satellite
communication systems are implemented the
common MAT access schemes to improve
modulation and transmission problems.
A single technique cannot optimize all these
parameters and therefore a trade-off analysis using
the applicable conditions is necessary, provided that
the choice of an accessing scheme is not obvious. For
example, if the application at hand is the provision of
communication to a large number of low-cost mobile
terminals, the accessing scheme should be simple but
robust so as to permit the use of low-cost mobile
receivers. At the same time, a certain degree of
flexibility is necessary to enable sharing of the
spectrum between a large number of mobile terminals
and to accommodate the addition of mobiles to the
network. Compare this with an application where a
relatively few large MES terminals, each carrying
heavy traffic, need to be interconnected. In this case,
the accessing scheme can be complex and the main
optimization criterion would be the optimal use of
the available bandwidth and satellite power rather
than the need for the simple mobile terminal.
A number of the following MAT accessing
schemes have evolved over the years:
1 Frequency Division Multiple Access (FDMA) -
This is a MAT scheme where each concerned GES
or MES terminal is assigned its own different
working carrier RF inside the spacecraft
transponder bandwidth, which schematic diagram
is shown in Figure 1 (Left). At the introductory
phase of satellite technology, FDMA appeared to
be the best candidate because of the initially
established FDMA technology, from-the terrestrial
radio relay system, its simple network control
requirement, and the consequent low cost. The
technology became widely used in all first-
generation wireless and after that in satellite
systems. This scheme, however, is inefficient with