811

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.

813

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 tends to suppress the weaker

ones. For this reason, the carrier power has to be

controlled carefully;

5 The channel operations in FDMA are simple,

FDMA technique doesn’t need any base-control

station, there is no need for network timing and no

need for any equalization;

6 After the transmission of FDMA data, the effect of

the delay distortion will be so small and it can be

ignored, and data that is transferred between each

station to another during the transmission process

will not be lost; and

7 In FDMA, the reduction of the information bit rate

has a good effect on the capacity, because of the

transmission is continuous, there is almost no

need for bits that are responsible for

synchronization, and simplicity in FDMA

algorithms.

The disadvantages of the FDMA technique are

listed below:

1 The FDMA technique does not differ significantly

from analog transmission systems; improving the

capacity depends on the signal-to-interference

reduction, or a Signal-to-Noise Ratio (SNR);

2 The maximum flow rate per channel is fixed and

small and guard bands lead to a waste of capacity;

and

3 Hardware implies narrowband filters, which

cannot be realized in VLSI and therefore increases

the cost.

Here can be concluded, that with the FDMA

technique, the signals from the various users are

amplified by the satellite transponder in a given

allocated bandwidth at the same time but at different

frequencies. Depending on the multiplexing and

modulation techniques employed, several

transmission hybrid schemes can be considered and

in general may be divided into two categories, based

on the traffic demands of Earth stations on MCPC

and SCPC.

814

3 FREQUENCY DIVISION MULTIPLE ACCESS

(FDMA) NETWORK CONCEPT

As outlined earlier, the FDMA technique is the

earliest implemented in the wireless systems and still

one of the most commonly employed forms of

multiple access techniques for communications via

satellite. In the case of FDMA different Earth

stations are able to access the total available

bandwidth of satellite transponder by virtue of their

different carrier frequencies, thus avoiding

interference among multiple signals. However, the

FDMA technique is the first MAT technology

implemented on fixed and mobile satellite systems.

Its principle and operation are simple, which satellite

FDM/FDMA network architecture is shown in Figure

2. Thus, the GES Hub terminal with shared forward

FDM link is connecting 3 fixed or mobile VSAT

stations via GEO satellite transponder. However, in

return link, 3 VSAT stations are using separate FDMA

links at a specific assigned frequency band (f1 , f2 , f

N ) is assigned to connect GES Hub terminal via the

same via GEO satellite transponder.

Therefore, each VSAT station within the satellite’s

footprint transmits one or more signals at different

carrier frequencies. Each carrier is assigned a small

guard band to avoid the overlapping of adjacent

carriers. The transponder receives all carrier

frequencies within its bandwidth, does the necessary

frequency translation and amplification, and then

retransmits them back towards GES Hub terminals.

Different VSAT stations are capable of selecting the

carrier frequency containing messages of their

interest. The frequency diagram determines that each

VSAT station in communication via GEO satellite

with GES Hub terminals monopolizes its own

frequency band or frequency slot, which can be pre-

allocated or changed as needed. As stated, a guard

band is usually added between user bands to avoid

mutual interference. The size of the guard band is

related to the accuracy and stability of the carrier

frequency of the transmitting and receiving ground

station, and also to the difference of the maximum

Doppler shift between adjacent signals. Therefore, the

guard band set in the FDMA should be larger than

any carrier signal. The maximum drift value relative

to its nominal frequency for each station.

When the signal goes down, because the carrier

spectrum passes through the frequency-converting

satellite, the ground station needs to tune the receiver

to a specific downlink frequency to receive the

transmitting carrier of the corresponding uplink

ground station. And because the entire FDMA

spectrum is transmitted by each VSAT station on the

return link to the GEO satellite. Than from GEO

satellite on the downlink, that is, multiple carriers

exist at the same time for each VSAT station, the GES

Hub receiving station must be able to receive the

entire spectrum from and filter it to distinguish the

carrier actually sent to the station, and send it to other

VSAT stations. According to whether each ground

station uses multiplexing technology in the

transmission carrier, FDMA is divided into two

categories: FDMA (Multiple Channels Per Carrier-

Frequency Division Multiple Access, MCPC-FDMA)

and single channel per carrier. FDMA (Single

Channel Per Carrier-Frequency Division Multiple

Access, SCPC-FDMA).

3.1 Multiple Channels Per Carrier (MCPC)

The main elements of the MCPC are multiplexer,

modulator, and transmitter using a satellite uplink

(forward) when GES multiplexes baseband data is

received from a terrestrial network and destined for

various MES. Moreover, the multiplexed data are

modulated and transmitted to the allocated frequency

segment, when the bandwidth of the transponder is

shared among several MES terminals, each with

different traffic requirements. The transponder

bandwidth is divided into several fixed segments,

with the several time frequency divisions allocated to

these MES terminals. Namely, between each band

segment is a guard band, which reduces the

bandwidth utilization efficiency and the loss is

directly related to the number of accessing MES

terminals in the network. Depending on the number

of receiving MES terminals, a total number of carriers

will pass through the satellite transponder.

On the other hand, the signals received from

different MES terminal extract the carrier containing

traffic addressed to LES by using an appropriate RF

filter, demodulator, baseband filter and

demultiplexer. The output of the demodulator

consists of multiplexed telephone channels for a few

MES terminals together with the channels addressed

to them. A baseband filter is used to filter out the

desired baseband frequency segment and finally, a

demultiplexer retrieves individual telephone

channels and feeds them into the terrestrial network

for onward transmission. Each baseband filter of GES

receive stations in this scheme corresponds to a

specific one in the GES transmitting station.

However, any change in channel capacity requires the

return of this filter, which is difficult to implement,

while many schemes may be categorized according to

the type of baseband signal.

3.2 Single Channel Per Carrier (SCPC)

For certain applications, such as the provision of MES

terminals to remote areas or individual MES terminal,

traffic requirements are low. In reality, assigning

multiple channels to each MES is wasteful of

bandwidth because most channels remain unutilized

for a significant part of the day. For this type of

application, the SCPC type of FDMA is used. In the

SCPC system, each carrier is modulated by only one

voice or by low to medium bit rate data channel.

Some old analog systems use Companded FM but

most new systems are digital Phase-Shift Keying

(PSK) modulated.

In the SCPC scheme, each MES carrier transmits a

single carrier. The assignment of transponder

channels to each MES terminal may be fixed Pre-

Assigned Multiple Access (PAMA) or variable

Demand-Assigned Multiple Access (DAMA), the

channel slots of the transponder are assigned to

different MES terminals according to their

instantaneous needs. In the case of PAMA, a few

SCPC channels, about 5 to 10, are permanently

815

assigned to each MES terminal. In the case of DAMA,

a pool of frequency is shared by many MES terminals.

When necessary, each MES terminal requests a

channel from frequency management of the Network

Control Station (NCS), which may always attempt to

choose the best available channel or a lower quality

channel until an unoccupied channel has been found.

The allocation is then announced on a signaling

channel known as a broadcast channel. The

announcement is received by the calling and called

MES terminal, which then tunes to the allocated

channel.

The communication takes place on the allocated

satellite channel and the end of the call is announced

by a signaling message, following which the NCS

returns the channel to the common pool. In addition,

the SCPS solution requires an Automatic Frequency

Control (AFC) pilot to maintain the spectrum

centering on a channel-by-channel basis. This is

usually achieved by transmitting a pilot tone in the

centre of the transponder bandwidth. It is transmitted

by designated reference GES and all the MES

terminals use this reference to correct their

transmission frequency. A receiving station uses the

pilot tone to produce a local AFC system which is

able to control the frequency of the individual carriers

by controlling the frequency of the LO. Drift in MSC

translation frequency and frequency variations

caused by the Doppler Effect and the carriers retain

their designated frequencies relative to each other.

This feature is essential because if uncorrected, the

sum of the total frequency error can cause carrier

overlapping, as carrier bandwidths are small. Thus, a

stable receive frequency permits the GES

demodulator design to be simplified. Centrally

controlled networks, such as Inmarsat MES standards

of B, C, M, Fleet 33/55/77, FleetBroadband, and other

GMSC networks are simple to manage missions

because they provide a higher usage of channels and

can use simple demand-assignment equipment. The

SCPS scheme is cost-effective for networks consisting

in a significant number of Earth stations, each

needing to be equipped with a small number of

channels.

The SCPC modulation systems previously

contained a 64 Kb/s Pulse-Code Modulation (PCM)

voice or data channel, superimposed on a carrier by

4-PSK modulation, using a transponder bandwidth of

about 38 kHz. With a carrier spacing of about 45 kHz,

a 36 MHz transponder could therefore carry about

800 channels of traffic. The dramatic improvement in

digital compression techniques is reducing the voice

channel bit rate down to 1 Kb/s or so. The minimum

subjective quality level is the main point of discussion

these days. Even at a voice bit rate of 16 Kb/s, which

is relatively high by today’s capability, this equates to

3200 channels per transponder. This figure can be

improved by more than a factor of 2 by carrier voice

activation. During the gaps in speech, a carrier is not

transmitted, making space in the transponder for

another carrier that has been voice-activated.

4 HYBRID FDMA NETWORK ARCHITECTURE

Further enhancement can be obtained when FDMA

technique grouping is considered in combinations

with TDMA, CDMA, and SDMA schemes in order to

improve switching, transmission, and frequency

bands conditions of baseband signals, and improve

control of the satellite up and downlinks. There are

several hybrid schemes of multiplexed FDMA in

combination with FDMA/MCPC, FDMA/SCPC,

FDMA/TDMA, SCPC/FM/FDMA, SCPC/PSK/FDMA,

TDM/FDMA, TDMA/FDMA, TDM/SCPC, and

TDM/SCPC.techniques.

Figure 4. Iridium FDMA Scheme and TDMA Frame

Structure

4.1 Hybrid FDMA/MCPC Network Architecture

The MCPC technique, as its name implies, is another

FDMA technique in which each carrier contains

several channels. Again, star networks with thin

routes find MCPC to be a good alternative in some

situations. Voice, data, or fax channels are time-

multiplexed into one or several preassigned signals

and then sent via a modem for transmission. Using

speech coding to allow 16 Kb/s for each voice call,

four calls can be multiplexed into a 64 Kb/s signal for

one carrier. Data channels must be preassigned

because speech encoders cannot be used with data

traffic.

Usually, data is sent at 1.2, 4.9, 9.6, 56, or 64 Kb/s,

and several different-rate users can be multiplexed

for one carrier. Carrier preassignment is more

suitable for star or point-to-point applications where

a few VSAT stations use up to only six channels. A

VSAT network would evolve as traffic increases,

often beginning with a star network using MCPC to

an SCPC/PAMA and eventually to an SCPC/DAMA.

Further upgrades to a thin-route mesh network could

follow. A TDMAstar configuration would be a major

upgrade that would be cost-effective only with more

than 25 remote stations each allocated at least 15

voice circuits.

4.2 Hybrid FDMA/SCPC Network Architecture

This access method does not require any multiplexing

that is used for point-to-point, point-to-multipoint,

and mesh networks. It is the VSAT equivalent of the

conventional leased line, delivering up to about 2

Mb/s of bandwidth to individual VSAT terminals.

Satellite channels are either preassigned (PAMA) or

demand assigned (DAMA) mode. The SCPC/PAMA

scheme dedicates channels to specific VSAT stations

regardless of the network call activity.

The SCPC/DAMA hybrid systems are simple and

cost-effective for small networks with less than four

or five sites and several channels per site. The DAMA

is a more efficient way of using the limited frequency

816

resource. In SCPC/DAMA systems, users from

different earth stations share a common pool of

channels. For each call a request is sent and if a

channel is available, it is assigned on demand. The

DAMA system is more complex and the VSAT station

equipment is more expensive, but the recurring space

segment costs are lower. This is a type of concentrator

mechanism, and traffic requirements need to be

carefully studied; otherwise blocking can reduce the

system effectiveness.

The DAMA system is suitable for many remotes

when only a few channels are required for each

remote VSAT station. If the traffic is too light, the

additional cost of the DAMA control equipment

negates the reduction in satellite charges. The GES

Hub station controls the DAMA system by a common

Aloha signaling channel. Moreover, the Aloha

multiplexing system allows random contention (first

come, first served) until the traffic becomes relatively

heavy, at which time it changes to a reservation

mode. The SCPC VSAT satellite networks are well

suited to thin-route, rural telephony, and can even be

the primary communication method for some

developing countries. The SCPC system can

accommodate voice or data traffic, whereas TDMA is

best suited to data. Because SCPC is in direct

competition with leased lines, it is not surprising that

costs are similar, whereas TDMA services are

comparatively cheaper.

4.3 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.

4.4 Hybrid SCPC/FM/FDMA Network Architecture

The baseband signals from the satellite network or

users each modulate a carrier directly, in either

analog or digital form according to the nature of the

SCPC signal in question. Therefore, each carrier

accesses the satellite on its particular frequency at the

same time as other carriers on the different

frequencies from the same or other station terminals.

Information routing is thus, performed according to

the principle of one carrier per link.

The Inmarsat-A MES standard used SCPS,

utilizing analog transmission with FM for telephone

channels. Thus, in calculating the channel capacity of

the SCPC/FM system it is necessary to ensure that the

noise level does not exceed specified defined values.

Therefore, the International Radio Consultative

Committee (CCIR) Recommendations for an analog

channel state that the noise power at a point of zero,

the relative level should not exceed 10,000 WOP with

a 50 dB test tone, namely the noise ratio. In this way,

it is assumed that the minimum required carrier-to-

noise ratio per channel is at least 10 dB.

4.5 Hybrid SCPC/PSK/FDMA Network Architecture

In this hybrid scheme, each voice or data channel is

modulated onto its own RF carrier. The only

multiplexing occurs in the transponder bandwidth,

where frequency division produces individual

channels within the bandwidth. Various types of this

multiplex scheme are used in channels of the

Inmarsat standard-B system. In this case, the satellite

transponder carrier frequencies may be PAMA or

DAMA. For PAMA carriers the RF is assigned to a

channel unit and the PSK modem requires a fixed-

frequency Local Oscillator (LO) input. For DAMA,

the channels may be connected according to the

availability of particular carrier frequencies within

the transponder RF bandwidth. For this arrangement,

the SCPC channel frequency requirement is produced

by a frequency synthesizer.

817

The forward satellite link assigned by the TDM

scheme in shore-to-ship direction uses the

SCPC/DA/FDMA solution for Inmarsat standard-B

voice/data transmission. This hybrid standard in the

return link for channel request employs the Aloha O-

QPSK multiplexing scheme and for-low speed

data/telex uses the TDMA scheme in ship-to-shore

direction. The Inmarsat-Aero standard in forward

ground-to-aircraft direction uses packet mode TDM

scheme for network broadcasting, signaling dat,a and

the circuit mode of SCPS/DA/FDMA scheme with

distribution channel management for service

communication links. Thus, the request for channel

assignment, signaling and data in the return aircraft-

to-ground direction the Slotted Aloha Binary Phase

Shift Keying (BPSK) (1/2 – FES) of 600 b/s is

employed and consequently, the TDMA scheme is

reserved for data messages.

4.6 Hybrid TDM/FDMA Network Architecture

This arrangement allows the use of TDM groups to be

assembled at the satellite in FDMA, while the PSK is

used as a modulation process at the Earth station.

Systems such as this are compatible with FDM/FDMA

carriers sharing the same transponders and the

terminal requirements are simple and easily

incorporated.

The Inmarsat standard-B system for telex low-

speed data uses this scheme in the shore-to-ship

direction only and in the ship-to-shore direction uses

TDMA/FDMA. The CES TDM and SES TDMA carrier

frequencies are pre-allocated by Inmarsat. Each CES

is allocated at least one forward CES TDM carrier

frequency and a return SES TDMA frequency. So,

additional allocations can be made depending on the

traffic requirements.

The channel unit associated with the CES TDM

channel for transmission consists of a multiplexer,

different encoder, frame transmission synchronizer,

and modulator. So at the SES, the receive path of the

channel has the corresponding functions to the

transmitted end. The CES TDM channels use BPSK

with differential coding, which is used for phase

ambiguity resolution at the receiving end.

4.7 Hybrid TDMA/FDMA Network Architecture

As previously stated, however, the TDMA signals

could occupy the complete transponder bandwidth.

In fact, a better variation of this is where the TDMA

signals are transmitted as a sub-band of transponder

bandwidth, the remainder of which being available

for example for SCPC/FDMA signals. Thus, the use of

a narrowband TDMA arrangement is well suited for a

system requiring only a few channels and has all

advantages of satellite digital transmission but can

suffer from intermodulation with the adjacent FDMA

satellite channels.

Accordingly, the practical example of this multiple

schemes is the Tlx (Telex) service of the Inmarsat

Standard-B system in ship-to-shore direction, which,

depending on the transmission traffic, offers a flexible

allocation of capacity for satellite communication and

signaling slots.

Figure 3. Hybrid TDM/SCPC Network Architecture

4.8 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.

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

818

rationales for using the TDM/SCPC hybrid networks

are weak at best.

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).

Table 2. List of TDM/SCPC Characteristics

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.

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.

4.9 Hybrid SDMA/FDMA Network Architecture

This modulation arrangement uses filters and fixed

links within the satellite transceiver to route an

incoming uplink frequency to a particular downlink

transmission antenna. A basic arrangement of fixed

links may be set up using a switch that is selected

only occasionally. Thus, an alternative solution

allows the filter to be switched using a switch matrix,

which is controlled by a command link. Because of

the term SS (Switching Satellite), this scheme would

be classified as SDMA/SS/FDMA. The satellite

switches are changed only rarely, only when it is

desired to reconfigure the satellite, to take account of

possible traffic changes. The main disadvantage of

this solution is the need for filters, which increase the

mass of the payload.

5 CONCLUSION

In this paper are provided the following conclusions:

FDMA technique is widely used in the analog

telecommunication systems. The working principle of

the FDMA as we said is dividing the signaling

dimensions along the frequency axes to create many

separate channels and allocate these channels to

users. The guard bands have an important effect on

decreasing the transmission impairments. Despite the

advantages, FDMA has many disadvantages such as

constant data rate and capacity and is used in many

applications such as analog mobiles and satellites.

Then, allocating a single frequency channel for short

time and then moving to another channel to give it its

own interval.

The performances and capacities of GMSC for

CDMA, FDMA, and TDMA/FDMA have been

analyzed many years ago for an L/C-band network

with global coverage. For the particular MSS under

discussion and for the particular antenna

configurations, both CDMA and FDMA offer similar

performance, FDMA yielding slightly higher channel

capacities at the design point and CDMA being

slightly better at higher Equivalent, Isotropically

Radiated Power (EIRP) levels.

The communication satellites for MSC provide

multiple-beam antennas and employ frequency reuse

of the allocated L-band frequency spectrum. It

appears that despite the fact that FDMA and

FDMA/TDMA are orthogonal systems, they

nevertheless suffer from bandwidth limitations and

sensitivity to interbeam interference in L-band.

However, at the chosen design point for aggregate

EIRP, a number of beams, and allocated bandwidth,

FDMA provides still the highest system channel

capacity.

REFERENCES

[1] Popoola S.I., (2016), "Mobile & Personal

Communications", Department of Electrical and

Information Engineering, Covenant University, Ota,

Nigeria, 34.

[2] Richharia M., (2020), "Satellite Communication

Systems", Springer, Boston, 484.

[3] Ilcev, (2017), “Global Mobile Satellite Communications

for Maritime, Land and Aeronautical Applications -

Volume2”, Springer, Boston, US, 686.

[4] Ivanov A., at all, (2019) Physical Layer Representation

in LEO Satellite with a Hybrid Multi-Beamforming,

819

Skolkovo Institute of Science and Technology, Moscow,

[5] Ilcev D. S. (2011), “Global Mobile CNS”, Manual, CNS

Systems, Durban, South Africa, 285.

[6] Maini A.K. & Agrawal V., (2007), “Satellite Technology

- Principles and Applications”, John Wiley, Chichester,

696.

[7] Garvey G., (2013), "Multiple Access Techniques",

Atlanta RF, LLC, Roswell, GA, 53.

[8] Xu X., at all, (2018), "Combination of Asymmetric

PCMA and Common Multiple Access Technology",

Army Engineering University of PLA, Nanjing, China,

[9] Pratt S.R. at all, (1999), "Operational and Performance

Overview of the Iridium Low Earth Orbit Satellite

System", IEEE Communications Surveys, New York, 9.

[10] Al Hares M.K, at al, (2008), "DMA & FDMA

Techniques", University of Kent, Canterbury, Kent, UK,

26.

[11] Robert G. Winch R.G., (1993), "Telecommunication

Transmission Systems", McGraw-Hill, New York, 633.

[12] Popoola S.I., at all, (2016), "Multiple Access

Techniques: Design Issues in FDMA/TDMA",

Manchester Metropolitan University, Manchester, UK,

34.

[13] Gigard R, (2020), "Satellite Network Design", Intelsat,

McLean, VA, 53.

[14] Gagliardi R.M., (1991), “Satellite Communications”,

Van Nostrand Reinhold, New York, 592.

[15] Ohmory S., Wakana. H & Kawase S. (1998), “Mobile

Satellite Communications”, Artech House, Boston, 466.

[16] Telecomunicazioni, (2008), "TDMA, FDMA, and

CDMA" University of Rome La Sapienza, Rome, Italy,

42.

[17] Kim H.W., at all, (2012), Applicability of Orthogonal

Frequency Division Multiple Access in Satellite

Communication", ETRI, Daejon, South Korea, 6.

[18] Sheriff R.E. & Hu Y.F., (2001), “Mobile satellite

communication networks”, John Wiley, Chichester, 381.