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1 INTRODUCTION
Fundamental to any radio transmission link is the
decision to use either analog or digital technology.
Clearly, digital transmission technology is used
wherever possible, but where radio transmission is
concerned, the carrier waves are always analog in
nature. In recent days, a digital signal is in most cases
superimposed on the analog carrier. Thus, frequency-
modulated systems are still in service in many parts
of the world, especially for TV transmission, but the
focus here will be on digital techniques. In practice, a
satellite transponder can be shared primarily in three
ways, each defined by a Multiple Access Technique
(MAT): (1) Frequency Division Multiple Access
(FDMA), 2. Time Division Multiple Access (TDMA),
and (3) Code Division Multiple Access (CDMA). The
satellite band allocation, about 500 MHz at C-band,
can be divided up into numerous single voice
channels of equal bandwidth by a multiplexed signal
containing many voice band channels, or by a digital
bitstream containing a combination of voice and
variable bit rate data. These options lead to the terms
FDMA and TDMA techniques so that each
conversation is carried on a different frequency band.
The FDMA scheme is a technique built on the
Frequency Division Multiplexing (FDM) method.
This technique can be considered to be the oldest and
the simplest form of multiplexing, which is used very
commonly in many technical fields such as the
telephone and commercial radio and television
broadcasting industries. The FDMA technique can be
applied for both digital and analog systems, although
FDMA is widely used in the analog communication
systems. The FDMA technique has spectral efficiency,
because of the transmission rate which is quite close
to the maximum rate that is needed by the user. As a
result, FDMA can be considered to be suitable for
users who don't have any serious problem with traffic
in the transmission, and most of the users' work is
predictable. Therefore, in unequal amounts of traffic
Analyses of Frequency Division Multiple Access (FDMA)
Schemes for Global Mobile Satellite Communications
(GMSC)
D.S. Ilcev
Durban University of Technology, Durban, South Africa
ABSTRACT: This paper introduces analyzes of the Frequency Division Multiple Access (FDMA) applicable in
Global Mobile Satellite Communications (GMSC). In satellite systems, as a rule, especially in GMSC 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 the Multiple Access Technique (MAT)
scheme. 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 different MAT schemes 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: 10.12716/1001.14.04.04
812
generated by users, FDMA can make some
modifications to deal and handle such a problem by
allocating and regulating the bandwidth with respect
to the amount of traffic. Since the transmission in
communication systems is continuous, FDMA will
need frequencies that can deal with different carriers
with different channels. But FDMA doesn't have the
capability to work with signals on multiple frequency
channels. Therefore, it is almost impossible to allocate
many channels for each user at the same time. In
general, FDMA is suitable for many applications with
suitable data types such as voice data.
Thus, in fixed and mobile satellite communication
FDMA and Code Division Multiple Access (CDMA)
are two common multiple access technologies that are
widely used in various transmission and hybrid
schemes. The FDMA technique is the first MAT
scheme implemented on satellite communication and
navigation networks. As stated, initially this
modulation scheme was used in the analog technique,
perhaps because it contains the FDM mode which is
indeed the analog frequency division multiplexing
technique. Before the digital revolution, all satellite
systems used FDM signals which were frequency
modulated onto a carrier within the FDMA
bandwidth available. Nowadays, FDMA uses digital
transmission packaging and is serving in modern
satellite systems.
The FDMA scheme describes the way in which the
information passes through the transponder. There
can be many carriers and the bandwidth used by each
carrier is a measure of the number of voice or data
channels transmitted. At one extreme of the FDMA is
used Multiple Channels Per Carrier (MCPC) and
there is also Single Channel Per Carrier (SCPC), or a
carrier might contain many channels in an TDM bit
stream. In FDMA systems, intermodulation (IM)
products created in the satellite transponder by the
many carriers necessitate a reduction of the output
amplifier output power to ensure that it operates in
the linear region, well below its saturation value. This
“back-off” results in a reduction of transmitted power
and consequently the total number of channels that
can be transmitted.
Therefore, the basic purpose of the FDMA
technique in GMSC systems is to share the frequency
resource among Mobile Earth Stations (MES)
terminals by use of multiple frequency slots.
Technically a frequency slot is occupied by a carrier
modulated with the data rate, including Forward
Error Correction (FEC) if necessary, wanted by a
certain subscriber. A standard channel arrangement is
to use one partial RF-band for downlink transmission
from the Ground Earth Station (GES) to all MES
terminals inside of satellite coverage, and another
partial band (normally but not necessary of the same
bandwidth) for uplink transmission from the MES to
the GES terminals.
Figure 1. Frequency Division Multiple Access (TDMA)
Techniques and FDMA Frame Structure
2 PRINCIPLES OF FDMA SCHEME
Both schemes, FDMA and TDMA are widely used for
digital transmission, and these subjects are covered in
wireless and satellite communication systems. Thus,
the most common and first employed MAT scheme
for satellite communication systems is the FDMA
concept shown in Figure 1 (Left), where transmitting
signals occupy non-overlapping frequency bands
with a special guard band between signals to avoid
interchannel interference. The bandwidth of a
repeater channel is therefore divided into many sub-
bands each assigned to the carrier transmitted by an
GES terminal. The MES terminals transmit
continuously and the channel transmits several
carriers simultaneously at a series of different
frequency bands. Because of interchannel
interference, it is necessary to provide guard intervals
between each band occupied by a carrier to allow for
the imperfections of oscillator and filter devices. The
downlink receiver (Rx) selects the required carrier in
accordance with the appropriate frequency. When the
satellite transponder is operating close to its
saturation, nonlinear amplification produces
intermodulation (IM) products, which may cause
interference in the signals of other users. In order to
reduce IM, it is necessary to operate the satellite
transponder by reducing the total input power
according to input back off and that the IF amplifier
provides adequate filtering.
Broadly speaking, the FDMA sample shown in
Figure 1 (Left) simply means splitting up an available
frequency band into a specific number of channels,
and the bandwidth of each channel depends on the
type of information signals to be transmitted by users.
After that, every user will be allocated with a special
channel with a channel bandwidth of (30 kHz). These
channels have a feature that the signals will be
controlled by guard bands, which have a beneficial
effect on decreasing the transmission impairments by
avoiding any interference between channels. One pair
of channels is used for fullduplex operation.
Information to be transmitted is superimposed on a
carrier at the channel center frequency. The
information can be a composite of several information
signals, which are multiplexed prior to being
superimposed on the carrier, or a single information
signal can be placed on the carrier. This would be
called a single channel per carrier (SCPC) system,
which has been widely used in satellite technology.
Years ago, the analog information was superimposed
on the carriers using Frequency Modulation (FM).
More recently, the analog signals have been
converted to digital pulse streams and the PSK and
QAM techniques employed.
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However, the FDMA wireless or satellite network
offers a much less adaptive structure than TDMA
regarding ease of reconfiguration for changing traffic
demands. In Figure 1 (Right) is depicted the signal
structure of the TDMA network, consisting of many
traffic stations or users slots. In the FDMA method,
guard bands are used between the adjacent signal
spectra to minimize crosstalk between the channels. A
specific frequency band is given to one person, and it
will be received by identifying each of the frequency
on the receiving end. It is often used in the first
generation of analog mobile phone. The total time
period that includes all traffic station bursts and
network information is called the FDMA frame.
Namely, the FDMA mobile devices are using
available bandwidth into a given number of
orthogonal channels of smaller bandwidths. A
channel is used by users continuously over the
duration of the message, and so the FDMA scheme is
limited to narrowband applications due to its limited
transmission rate. In such a way, if the same channel
is reused at another physically separate location, an
increase in transmit power will negatively affect the
carrier-to-interference ratio at that location.
Therefore, in FDMA, each user is permanently
allocated a certain frequency band, out of the total
bandwidth of the transponder. To reduce the adjacent
channel interference, it is necessary to have guard
bands between the sub-bands. Frequency drifts of the
satellite’s and mobile earth station’s frequency
converters have also to be taken into consideration.
The FDMA scheme is the traditional technique due to
its simple implementation and FDMA allocates a
single satellite channel to one user at once. In fact, if
the transmission path deteriorates, the controller
switches the system to another channel. Although
technically simple to implement, FDMA is wasteful of
bandwidth because the voice channel is assigned to a
single conversation, whether or not somebody is
speaking. Moreover, it cannot handle alternate forms
of data, only voice transmissions. This system’s
advantages are that it is a simple technique using
equipment proven over decades to be reliable and it
will remain very commonly in use because of its
simplicity and flexibility.
The FDMA technique has many advantages that
can be summarized as the following:
1 A FDMA method is the relatively inflexible system
and if there are changes in the required capacity,
then the frequency plan has to change and thus,
involve many GES terminals;
2 Multiple carriers cause IM in both the MES High
Power amplifier (HPA) and in the transponder
HPA. Reducing IM requires back off of the HPA
power, so it cannot be exploited at full capacity;
3 As the number of carriers increases, the IM
products between carriers also increase and more
HPA backoff is needed to optimize the system.
The throughput decreases relatively rapidly with
the number of transmission carriers, therefore for
25 carriers it is about 40% less than with 1 carrier;
Figure 2. Satellite FDM/FDMA Network Architecture
4 The FM system can suffer from what is known as a
capture effect, where if two received signals are
very close in frequency but of different strengths,
the stronger one te