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1 INTRODUCTION
Experimental TDMA systems were first developed
since 1965 (Sekimoto and Puento, 1968). In fact, these
early systems proved that time-interleaved, short-
interval communications were in fact technically
feasible. Later systems (IEEE, 1979; Kwan, 1973;
Maillet, 1972; Thompson, 1983; Watt, 1986)
established that advanced operational TDMA
concepts, such as high-accuracy, fast-acquisition
synchronization, and high-capacity data formats were
both possible and advantageous
In multiple access techniques for GMSC systems,
the uplink carriers from Ground Earth Station (GES)
terminals may be separate in time, not in frequency,
code, or space. For instance, instead of assigning a
frequency band, each ascending link is assigned a
specific time interval and a given station transmits
only during its assigned interval. This type of
operation is it is called Time Division Multiple Access
(TDMA) scheme. As can be seen, the TDMA scheme
theoretically avoids the problem of many carriers
trying to get through the satellite at the same time,
thus avoiding the intermodulation problem of the
Frequency Division Multiple Access (FDMA) scheme.
However, while the FDMA scheme involves
relatively simple frequency tuning for accessing, and
providing essentially independent channel on-off
operation, the TDMA scheme requires
communication concepts that are relatively new. To
accommodate many users, TDMA time intervals must
necessarily be short, requiring burst-type
transmissions, and the time intervals of all users must
be properly and accurately synchronized, requiring
several levels of timing control. Thus, the required
high-speed hardware for these operations is relatively
new and, in many cases, is still under development.
Analyses of Time Division Multiple Access (TDMA)
Schemes for Global Mobile Satellite Communications
(GMSC)
D.S. Ilcev
Durban University of Technology, Durban, South Africa
ABSTRACT: This paper provides analyzes of the Time Division Multiple Access (TDMA) techniques and their
hybrids with Time Division Multiplexing (TDM) for implementation in Global Mobile Satellite
Communications (GMSC). In satellite communication systems, and especially in GMSC networks, 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) 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 to the channels. 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: 10.12716/1001.14.04.06
832
2 PRINCIPLES OF TDMA SCHEME
In the TDMA scheme, each uplink from GES is
assigned a prescribed time interval in which to relay
through the satellite. During its interval, a particular
station has exclusive use of the satellite, and its
uplink transmission alone is processed by the satellite
for the downlink. Each carrier can use the same
carrier frequency and use of the entire satellite
bandwidth during its interval. Thus, since no other
carrier uses the satellite during this time interval, no
intermodulation or carrier suppression occurs, and
the satellite amplifier can be operated in saturation so
as to achieve maximal output power.
The TDMA downlinks always operate at full
saturation power of the satellite, while the entire
TDMA system must have all Earth terminals properly
synchronized in time so that each can transmit
through the satellite only during its prescribed
interval, without interfering with the intervals of
other stations. This time synchronization between
satellite and all Earth stations is called network
synchronization. A downlink Earth station terminals,
wishing to receive the transmissions from a particular
uplink, must gate into the satellite signal during the
proper time interval. This means that all Earth
stations, whether transmitting or receiving, must be
part of the synchronized network. Many users of the
TDMA satellite wish to establish a communication
link in real-time, the total transmission time must be
shared by all users. Thus, the time intervals of each
Earth station must be relatively short and repeated at
regular epochs. This type of short-burst, periodic
operation is most conducive to digital operation,
where each station transmits bursts of data bits
during its intervals.
However, TDMA digital transmissions require
that all receiving stations must obtain decoder
synchronization in each interval, in addition to the
required network synchronization for slot timing. For
phase-coherent decoding, decoder synchronization
requires establishing both a coherent phase reference
and a coherent bit timing clock before any bits can be
decoded within a slot. Also, word sync may be
needed to separate the digital words occurring
during a slot. This hierarchy of decoding
synchronization must be established at the very
beginning of each slot if the subsequent slot bits are to
be decoded. Furthermore, since each slot contains
data from a different source, synchronization must be
separately established for each slot being received. In
fact, even when receiving from the same station,
synchronization must generally be reestablished from
one periodic burst to the next. Hence, digital
communications with TDMA has an inherent
requirement for rapid synchronization in order to
perform successfully. The technology for short-burst
communications is rather new and will be closely
linked to the development of high-speed digital
processing hardware.
Figure 1. Time Division Multiple Access (TDMA)
Techniques and TDMA Frame Structure
Therefore, the TDMA schemes permit more than
two Mobile Earth Stations (MES) to use the same
satellite network for interchanging information.
Several transponders in the satellite payload share the
frequency bands in use and each transponder will act
independently of the others to filter out its own
allocated frequency and further process that signal
for transmission. This feature allows any GES located
in the corresponding coverage area to receive carriers
originating from several MES terminals and vice
versa that carriers transmitted by one MES can be
received by any GES terminal. This enables a
transmitting GES to group several signals into a
single, multi-destination carrier. Access to a
transponder may be limited to a single carrier or
many carriers may exist simultaneously. The
baseband information to be transmitted is impressed
on the carrier by the single process of multi-channel
modulation.
3 TIME DIVISION MULTIPLE ACCESS (TDMA)
NETWORK CONCEPT
The TDMA scheme is a digital access technique that
permits individual satellite GES transmissions to be
received by satellite in separate, non-overlapping
time slots, called bursts, which contain buffered
information. The satellite receives these bursts
sequentially, without overlapping interference, and is
then able to retransmit them to the MES terminal.
Synchronization is necessary and is achieved using a
reference station from which burst position and
timing information can be used as a reference by all
other stations. Each MES must determine the satellite
system time and range so that the transmitted signal
bursts, typically Quadrature Phase Shift Keying
(QPSK) modulated, are timed to arrive at the satellite
in the proper time slots. The offset QPSK modulation
is used by Inmarsat-B MES. So as to ensure the timing
of the bursts from multiple MES, TDMA systems use
a frame structure arrangement to support telex (Tlx)
in the mobile-to-shore direction. Therefore, a
reference burst is transmitted periodically by a
reference station to indicate the start of each frame to
control the transmission timing of all data bursts. A
second reference burst may also follow the first in
order to provide a means of redundancy. In the
proper manner, to improve the imperfect timing of
TDMA bursts, several synchronization methods of
random access, open-loop and closed-loop have been
proposed.
In Figure 1 (Left) is shown a concept of TDMA,
where each MES or user transmits a data burst with a
guard time to avoid overlaps. Since only one TDMA
burst occupies the full bandwidth of the satellite
transponder at a time, input back off, which is needed
to reduce Intermediate Frequency (IM) interference in
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FDMA, is not necessary for TDMA. At any instant in
time, the transponder receives and amplifies only a
single carrier. Thus, there can be no IM, which
permits the satellite amplifier to be operated in full
HPA saturation and the transmitter carrier power
need not be controlled. Because all MES terminals to
transmit and receive at the same frequency, tuning is
simplified. This results in a significant increase in
channel capacity. Another advantage over FDMA is
its flexibility and time-slot assignments are easier to
adjust than frequency channel assignments. The
transmission rate of TDMA bursts is about 4,800 b/s,
while the frame length is about 1.74 seconds and the
optimal guard time is approximately 40 msec, using
the open-loop burst synchronization method.
Accordingly, in the TDMA scheme, the
transmission signals from various mobile users are
amplified at different times but at the same nominal
frequency, being spread by the modulation in a given
bandwidth. Depending on the multiplexing
techniques employed, two transmission hybrid
schemes can be introduced for use in GMSC systems.
The time slot in TDMA scheme is pre-assigned or can
be changed on demand, and guard times are used
between the time slots to avoid interference. Thus, the
TDMA scheme is most practical for digital data
transmission only, because of the burst nature of the
transmissions. Downlink transmission consists of
interleaved set of packets from all the ground
stations. Two Reference Stations (RS), which could be
one of the GES or a separate ground location, are
used to establish the synchronization reference clock
and provide burst time operational data to the
network, and complex computer procedures, for
automated synchronizations between MES terminals.
The second disadvantage of TDMA scheme us that
peak power and bandwidth of individual MES
terminals need to be larger than with FDMA, owing
to high burst bit rate.
However. the TDMA network offers a much more
adaptive structure than FDMA regarding ease of
reconfiguration for changing traffic demands. In
Figure 1 (Right) is shown the signal structure of the
TDMA network, consisting of N traffic stations or
users slots. The total time period that includes all
traffic station bursts and network information is
called the TDMA frame. The frame repeats in time
sequence and represents one complete transmission
in the network. The frame times range from 1 to 20
ms and each station burst (slot) contains a preamble
and traffic data (data bits). The preamble contains
synchronization and station identification data. The
reference burst, from the RS, is usually at the start of
each frame and provides the network synchronization
and operational information. Guard bands are
included to prevent overlap and to account for
different transmission times for each of the stations,
based on their range to the satellite. Station bursts do
not need to be identical in duration and can be longer
for heavier traffic stations or during higher use
periods. The specific allocation of burst times for each
of the stations within the frame is called the burst
time plan. The burst time plan is dynamic and can be
changed as often as each frame to adapt to changing
traffic patterns.
Figure 2. Hybrid TDM/TDMA Network Architecture
In general, preamble time should be long enough
to establish reliable synchronization but should be
short compared to the data transmission time. The
ratio of preamble time to total slot time is sometimes
called the preamble efficiency ( p), or overhead. We
often measure these times in numbers of bits or
symbols and write this efficiency as follows:
1
O
T
P
FF
b
b
bb
= = −
(1)
or, in terms of the TDMA frame elements the
efficiency will have the following relation:
( )
1
R T G
P R R T T F
T
n n b
n b n b t
r
+
= − + +
(2)
where values bT = number of bits available for traffic,
bF = total number of bits in frame, bO = number of
overhead bits, nR = number of reference stations, bR =
number of bits in reference burst, nT = number of
traffic bursts, bP = number of bits in traffic burst
preamble, bG = number of bits in guard band, rT =
total TDMA bit rate, in b/s, and tF = TDMA frame
time, in seconds (s).
The channel capacity for a TDMA network is most
often evaluated in terms of an equivalent voice-
channel capacity (nC). This allows evaluation of
capacity for any type of data source bitstream: voice,
data, video, or any combination of the three. The
equivalent voice channel capacity is defined with the
following relation:
I
C
C
r
n
r
=
(3)
where values rI = available information bit rate, and
rC = equivalent voice channel bit rate.
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4 HYBRID TDM/TDMA NETWORK
ARCHITECTURE
The Time Division Multiplexing (TDM) scheme is a
method of transmitting and receiving independent
signals over a common signal path by means of
synchronized switches at each end of the
transmission line so that each signal appears on the
line only a fraction of time in an alternating pattern. It
is a communication process that transmits 2 or more
digital signals or analog signals over a common
channel. The TDM program is used for long-distance
communication links and bears heavy data traffic
loads from end-users, so it can be further extended
into the TDMA scheme, where several stations
connected to the same physical medium, for example
sharing the same frequncy channel, can communicate.
Satellite links normally relay many signals from
many MES but to avoid interfering with each other it
is necessary for some kind of separation or division.
This separation is known as multiplexing and its
common forms are Frequency Division Multiplexing
(FDM) and TDM. The TDM is easier to implement
with digital modulation and to form hybrid solutions
applicable to all types of baseband signals.
The first generation of the Inmarsat analog
standard-A MES uses the TDM/TDMA arrangement
for telex transmission, which scenario is shown in
Figure 2. Each MES has at least one TDM carrier and
each of the carriers has 20 telex channels of 50 bauds
and a signaling channel. Moreover, there is also a
common TDM carrier continuously transmitted on
the selected idle listening frequency by the Network
Coordination Station (NCS) for out-of-band signaling.
The MES remains tuned to the common TDM carrier
to receive signaling messages when the mobile is idle
or engaged in a telephone call. When an MES is
involved in a telex forward call it is tuned to the
TDM/TDMA frequency pair associated with the
corresponding GES to send messages in shore-to-
mobile direction. Telex transmissions in the return
mobile-to-shore direction form a TDMA assembly at
the satellite transponder. Each frame of the return
TDMA telex carrier has 22 time slots, while each of
these slots is paired with a slot on the TDM carrier.
The allocation of a pair of time slots to complete the
link is received by the MES on receipt of a request for
a telex call. Otherwise, the Inmarsat-A uses for
forward signaling a telex mode, while all other Moile
Satellite System (MSS) Inmarsat standards for
forwarding signaling and assignment channels use
the TDM Binary Phase Shift Keying (BPSK) scheme.
The new generation of Inmarsat digital standard-B
(inheritor of standard-A) uses the same modulation
TDM/TDMA technique but instead of Aloha BPSK
(BCH) at a data rate of 4800 b/s for the return request
channel used by Inmarsat-A, new standard-B is using
Aloha Offset. Quadrature Phase Shift Keying (O-
QPSK), 1/2 – FEC, at a data rate of 24 Kb/s. This MAT
satellite network is also useful for the Inmarsat
standard-C MES terminal for maritime, land (road
and rail), and aeronautical applications. In this case,
the forward signaling link and sending of messages
in the ground-to-mobile direction use a fixed
assigned TDM carrier. The return signaling channel
uses hybrid, slotted Aloha BPSK (1/2 FEC) with a
provision for receiving some capacity and the return
message channels in the mobile-to-ground direction
are modulated by the TDMA system at a data rate of
600 b/s.
The TDM/TDMA technology uses a single high-
speed TDM carrier transmitted from the central GES
site or Hub, from which many Very Small Aperture
Terminal (VSAT) stations can receive information.
For this TDM forward link, the DVB-S2 of Digital
Video Broadcasting-Return Channel via Satellite
(DVB-RCS) standard is most commonly used. It is
also the most flexible for multiplexing many
concurrent streams of traffic to different sites, and the
most efficient with its support of Adaptive Coding
and Modulation (ACM). The ACM mode
dynamically adjusts the modulation and coding on
the "virtual link" to each VSAT individually, as local
conditions (e.g., weather, interference) at the VSAT
change. To transmit back to the central site efficiently,
the VSATs in a TDM/TDMA network are
synchronized, and they transmit information in "burst
mode" within a series of short, scheduled timeslots.
Timeslots may be assigned across multiple TDMA
carriers and accessed using "fast frequency hopping".
Timeslots are assigned to each VSAT exclusively (i.e.,
without contention) based on their current traffic
needs. This is called Dynamic TDMA, and it is the
most advanced form of TDM/TDMA.
The TDMA technology is fully standardized
internationally by the DVB group under the DVB-
RCS family of standards. In Table 1 are presented the
advantages and disadvantages characteristics of the
TDM/TDMA technology with the cost of remote
(VSAT). For instance, the low-cost TDMA/DVB-RCS
Indoor Unit (IDU) or VSAT stations have dropped in
price to $1,000 including Outdoor Unit (ODU) or
VSAT antenna, while the cost of SCPC modems is
$6,000. The Antenna unit and its ODU sizing are
based on either shared carrier size or dedicated
carrier size. Thus, the supplying cost advantages of
TDM/TDMA presented earlier still apply when
comparing against SCPC with bandwidth
cancellation.
Table 1. List of TDM/TDMA Characteristics
The TDMA networks allow all VSATs to
dynamically share multiple TDMA carriers as if they
were a single large pool of bandwidth. Each TDMA
carrier group may contain dozens of carriers, with up
to 32 carriers per carrier group in a satellite network.
Therefore the "return link" may contain huge
amounts of capacity, in aggregate. In the TDM/TDMA
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network, the TDMA carriers may operate at widely
different symbol rates, e.g., from 500 ks/s to 5 Ms/s
and even higher. To determine which VSAT will use
which timeslots on which carrier at any moment, the
satellite link uses Adaptive Carrier Selection (ACS).
The ACS mode is applied dynamically for each
VSAT, given its local weather conditions,
configuration, e.g., antenna and Block Upconverter
(BUC) size, and service policy, e.g., maximum rate
requirements. In fact, the ACS mode determines what
carrier & symbol rate will work best at the current
signal levels of those available in the carrier group.
In addition, in a satellite DVB-RCS2 (2nd
Generation) network, ACM per burst is supported for
each VSAT and on all TDMA carriers in the carrier
group. This further optimizes efficiency, throughput,
and reliability for each VSAT and greatly simplifies
network operations. Any VSAT can use any
MODulation and CODing (MODCOD), on any
carrier, if necessary. DVB-RCS2 SatLink TDM/TDMA
networks now surpass SCPC networks not only in
efficiency, but also in throughput and link availability
for almost any conceivable network configuration
and satellite bands, such as C, Ku, and Ka-band
domain.
5 HYBRID TDM/SCPC NETWORK
ARCHITECTURE
The TDM/TDMA and Single Channel Per Carrier
(SCPC) architectures are the main alternative
technologies for satellite networking in the world
today. The modem and management technologies
underlying both approaches have been advancing
rapidly in recent years, causing some confusion as to
which technology is better for a given set of
networking requirements. The SCPC network refers
to using a single signal at a given frequency and
bandwidth. Most often, this is used on broadcast
satellites to indicate that radio stations are not
multiplexed as subcarriers onto a single video carrier,
but instead independently share a transponder.
Figure 3. Hybrid TDM/SCPC Network Architecture
The SCPC mode is using for a VSAT satellite
transmission system that uses a separate frequency
carries for each of its communication channels, as
opposed to frequency division multiplexing that
combines many channels on a single carrier. It can be
used for broadcast data and full-duplex audio and
video communications. In an SCPC system,
transmissions are sent to the satellite continuously on
a single satellite carrier. The satellite signal is received
at a single location, in the case of a point-to-point
system, or at many different locations in a broadcast
system, providing hubless connectivity among
multiple sites. This technology a system where each
sub-division carries only one 4-kHz voice channel
enables companies and corporate organizations to
establish their own private network to connect sites
into a single network with highly reliable
performance with very low latency.
Due to the increasing dominance of IP traffic,
many former SCPC networks have already been
converted to TDM/TDMA network architecture.
However, some SCPC networks have converted only
"half-way", whereby a DVB-S2 TDM carrier is used
on the forward link, but SCPC links are used for
return link communications. This hybrid
configuration is called TDM/SCPC scheme and its
network architecture is illustrated in Figure 3. If using
DVB-S2 it gets the full benefits of statistical
multiplexing and ACM on the forward link, but these
benefits are non-existent on the return link in this
hybrid network. Therefore, the technical and business
rationales for using the TDM/SCPC hybrid networks
are weak at best.
Nonetheless, the TDM/SCPC hybrid configuration
is commonly promoted and used in certain types of
VSAT networks, in particular in cellular backhaul
networks and in some other types of networks where
fast access to large amounts of capacity for the return
link (upstream) traffic must be guaranteed. There are
four possible reasons for the continued use of this
form of SCPC confirguration:
1 The possibility that SCPC in "continuous mode"
will provide better modem efficiency (in b/s per
Hz) than TDMA burst mode due to lower
overhead and ability to use higher-rate, more
efficient MODCOD scheme;
2 The possibility that SCPC links are better at
providing guaranteed capacity and will operate
more reliably against rain fades, interference, or
congestion;
3 The possibility that SCPC links will provide lower
latency or less total delay; and
4 The pssibility that SCPC links can be operated at a
higher speed, when necessary, for any or all sites
within the satellite transponder footprint.
These possibilities or some of them are true with
respect to the limitations of some popular
TDM/TDMA technologies. For those technologies, the
hybrid TDM/SCPC option is useful and may even be
"cost-effective" in networks with nearly constant
levels of traffic in the peak hour at each site, a
consistent peak hour time each day. In Table 2 are
presented the advantages and disadvantages
characteristics of the TDM/SCPC technology with the
cost of remote (VSAT).
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Table 2. List of TDM/SCPC Characteristics
However, in comparison to satellite TDM/TDMA
networks using the DVB-RCS2 standards, these
conditions do not hold true. In fact, the opposite is
true, because, in terms of total network efficiency, a
satellite DVB-RCS2 return link operating in TDMA
burst mode will deliver 2x more in bps/Hz than some
popular SCPC options, even before adding in the
benefits of statistical multiplexing with TDMA
configuration.
Figure 4. Iridium FDMA Scheme and TDMA Frame
Structure
6 HYBRID FDMA/TDMA NETWORK
ARCHITECTURE
The Iridium GMSC system employs a hybrid
FDMA/TDMA access scheme, which is achieved by
dividing the available 10.5 MHz bandwidth into 150
channels introduced into the FDMA components.
Each channel accommodates a TDMA frame
comprising eight-time slots, four for transmission,
and four for the sgnal reception. Each slot lasts about
11.25 msec, during which time data are transmitted in
a 50 Kb/s burst. Thus, each frame lasts 90 msec and a
satellite is able to support 840 channels. In such a
way, a mobile satellite user is allocated a channel
occupied for a short period of time, during which
transmissions occur. The Iridium satellite system
supports full-duplex voice channels at 4800 b/s (2400
b/s according to and half-duplex data channels at
2400 b/s. The IRIDIUM network utilizes multiple spot
beams on each satellite that divide the satellite
footprint into smaller cells. However, to provide two-
way satellite communications, the IRIDIUM system
uses a combination of Frequency Division Multiple
Access (FDMA) and Time Division Multiple Access
(TDMA) techniques.
The Hybrid FDMA/TDMA Network Architecture
is established when two slots (same position in time)
of the user are allocated in two different narrow-band
radio channels. Iridium satellite system uses
frequencies in the L-band of 1616 MHz to 1626.5 MHz
for the user’s uplink and downlink with the satellites.
This gives the system 10.5 MHz of bandwidth. As
shown in Figure 4 (Left), the Iridium FDMA scheme
divides the available bandwidth into 240 channels of
41.67 kHz for a total of 10 MHz. This leaves 500 kHz
of bandwidth for guard bands, which amounts to
approximately 2 kHz of guard band between
channels.
The TDMA frame is 90 ms long and it contains
four full-duplex user satellite channels at a burst data
rate of 50 kb/s. The four full-duplex channels consist
of four uplink time slots and four downlink time
slots, as depicted in Figure 4 (Right). The eight user
time slots take up a total of 69.12 ms, which leaves
20.88 ms of the TDMA frame for framing bits and
guard time slots. A possible frame structure is to use
a framing time slot twice as long as an individual user
time slot. This would result in 864 framing bits taking
up 17.28 ms. Subtracting this value from the 20.88 ms
remaining in the TDMA frame leaves 3.6 ms for
guard time slots. This can be divided into eight 400
microsecond guard time slots between time slots in
the frame, and two 200 microsecond guard time slots
at each end of the frame. Although the exact frame
structure is not published in the open literature, this
approach is reasonable. Thus, it uses 4.6 percent of
the 90 ms frame for guard timeand utilizes 76.8
percent of the frame for actual data bits.
7 CONCLUSION
The performances and capacities of the Time Division
Multiple Access (TDMA) network architectures and
their hybrids with Time Division Multiplexing
(TDM)for GMSC applications have been analyzed an
implemented many years ago for an C, Ku and
newest Ka-band. The Multiple Access Technique
(MAT) schemes are is the use of multiplexing
techniques to provide communication service to
multiple fixed and mobile satellite users over a single
channel. It allows for many users at one time by
sharing a finite amount of spectrum. The
TDM/TDMA, Single Channel Per Carrier (SCPC), and
their hybrid solutions are the main alternative
technologies for mobile satellite network
architectures in the world today. The VSAT satellite
modems and management technologies underlying
both approaches have been advancing rapidly in
recent years, causing some confusion as to which
technology is better for a given set of networking
requirements. In fact, the main architecture for design
hybrid TDMA satellite networks is Time Division
Multiplexing (TDM) technique and its combination
with TDM/TDMA, TDM/SCPC and FDMA/TDMA
hybrid network architecture for GMSC applications.
In such a way, implementing these hybrid MAT
networks promise many improrovements in satellite
transmission systems.
The Frequency Division Multiple Access (FDMA)
technique is widely used in the general analog
telecommunication and satellite communications
systems for all mobile applications at sea, on the
ground, and in the air. The working principle of the
FDMA as usual is dividing the signaling dimensions
along the frequency axes to create many separate
channels. After that, allocates these channels to fixed
or mobile satellite users. The guard bands have an
837
important effect on decreasing the transmission
impairments. The FDMA technique has many
advantages such as good capacity, simple algorithms,
and so on. Despite the advantages, FDMA has many
disadvantages such as constant data rate and channel
capacity. Therefore, the FDMA technique is used in
many applications such as analog cellular and
satellite systems.
On the other hand, the TDMA technique is an
advanced digital multiple access technology that
allows more than one user to access radio frequency
(RF) channels in satellite and other
telecommunications systems. The principle work of
TDMA is that the signals should be divided into
milliseconds-long packets. Then, allocating a single
frequency channel for short time and then moving to
another channel to give it its own interval. Like the
FDMA technique, the TDMA scheme has guard times
that prevent any interference between channels, and
decrease the factors of transmission impairments. The
TDMA technique has many advantages such as good
capacity with high data rates. However, the TDMA
scheme has many disadvantages such as attenuation
impairment.
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