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1 INTRODUCTION

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.

http://www.transnav.eu

the International Journal

on Marine Navigation

and Safety of Sea Transportation

Volume 14

Number 4

December 2020

DOI: 10.12716/1001.14.04.05

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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

2 TYPES OF MULTIPLE ACCESS TECHNIQUE

(MAT) SCHEMES

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