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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

822

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

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respect to both the satellite power capacity and

bandwidth utilization. An improvement in FDMA

was introduced by incorporating an element of

flexibility in the form of a demand assigned

FDMA in which a central pool of frequencies is

shared by the user on a call-by-call basis.

2 Time Division Multiple Access (TDMA) - This is a

second MAT scheme where all concerned MES

terminals use the same carrier frequency and

bandwidth with time-sharing, non-overlapping

intervals, which schematic diagram is shown in

Figure 1 (Middle). Following an increase in traffic

demand, leading to a scarcity of available

bandwidth, and a trend towards digital techniques

was implemented TDMA technique as a more

efficient but complex MAT scheme. Thus,

currently, the TDMA scheme is being introduced

into most fixed and mobile satellite service

networks used for interconnecting high traffic

earth stations, although FDMA was started to be

used well into the 2000s.

3 Code Division Multiple Access (CDMA) - This is a

MAT scheme where all concerned Earth stations

simultaneously share the same bandwidth and

recognize the signals by various processes, such as

code identification. Actually, they share the

resources of both frequency and time using a set of

mutually orthogonal codes, such as a

Pseudorandom Noise (PN) sequence. For some

specialized applications where secrecy is vital or

where a channel may suffer frequency selective

fading or interference in communications, the

CDMA scheme based on spread spectrum

principles was developed.

4 Space Division Multiple Access (SDMA) - This is a

new MAT scheme where all concerned MES

terminals can use the same frequency at the same

time within a separate space available for each

link, which schematic diagram for wireless and

satellite SDMA techniques is illustrated in Figure

Figure 2. Wireless and Satellite SDMA Techniques

5 Random (Packet) Division Multiple Access

(RDMA) - This is MAT scheme where a large

number of mobile satellite users share

asynchronously the same satellite transponder by

randomly transmitting short burst or packet

divisions.

Currently, these methods and their hybrid

solutions of MAT shemes are widely in use with

many advantages and disadvantages, together with

their combination of hybrid schemes or with other

types of modulations. Hence, multiple access

technique assignment strategy can be classified into

three methods as follows: (1) Preassignment or fixed

assignment; (2) Demand Assignment (DA) and (3)

Random Access (RA); the bits that make up the code

words in some predetermined fashion, such that the

effect of an error burst is minimized.

In the preassignment method channel plans are

previously determined for chairing the system

resources, regardless of traffic fluctuations. Therefore,

this scheme is suitable for communication links with

a large amount of steady traffic. However, since most

mobile users in MSC do not communicate

continuously, the preassignment method is wasteful

of the satellite resources. In Demand Assignment

Multiple Access (DAMA) satellite channels are

dynamically assigned to users according to the traffic

requirements. Due to high efficiency and system

flexibility, DAMA schemes are suited to MSC

systems. In RA a large number of mobile users use

the satellite resources in bursts, with long inactive

intervals. In effect, to increase the system throughout,

several mobile Aloha methods have been proposed.

Therefore, the MA techniques permit more than

two Earth stations 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. Thus,

this feature allows any LES located in the

corresponding coverage area to receive carriers

originating from several MES and vice versa and

carriers transmitted by one MES can be received by

any LES. This enables a transmitting Earth station to

group several signals into a single, multi-destination

carrier. Access to a transponder may be limited to

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 SPACE DIVISION MULTIPLE ACCESS (SDMA)

SCHEME

The SDMA technique has several characteristics that

make its introduction in a mobile radio system

advantageous. In particular, all modifications

required are limited to base stations and do not

involve mobile units. Moreover, the SDMA technique

can be integrated with different MAT schemes, such

as FDMA, TDMA, CDMA, and therefore it can be

used in all mobile radio systems currently operating

or about to be introduced. This thechnique that

employ smart multiple antenna elements at the base

station in wireless systems or GES in fixed and

mobile satellite systems provide much higher

capacity than single-antenna-element systems. A

fundamental question to be addressed concerns the

ultimate capacity region of an SDMA system wherein

a number of fixed or mobile users, each constrained

in power, try to communicate with the base station or

GES in a multipath fading environment.

The significant factor in the performance of the

MAT scheme in a satellite communications system is

interference caused by different factors and other

users. In the other words, the most usual types of

interference are co-channel and adjacent channel

interference. The co-channel interference can be

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caused by transmissions from non-adjacent cells or

spot beams using the same set of frequencies, where

there is the minimal physical separation from

neighboring cells using the same frequencies, while

the adjacent channel interference is caused by RF

leakage on the subscriber’s channel from a

neighboring cell using an adjacent frequency. This

can occur when the user’s signal is much weaker than

that of the adjacent channel user. Signal to

Interference Ratio (SIR) is an important indicator of

call quality; it is a measure of the ratio between the

mobile phone signal (the carrier signal) and an

interfering signal. A higher SIR ratio means

increasing overall system capacity.

Taking into account that within the systems of

satellite communications, every user has their own

unique spatial position, this fact may be used for the

separation of channels in space and as a consequence,

to increase the SIR ratio by using SDMA. In effect,

this method is physically making the separation of

paths available for each satellite link. Terrestrial

telecommunication networks can use separate cables

or radio links but on a single satellite, independent

transmission paths are required. Thus, this MA

control radiates energy into space and transmission

can be on the same frequency: such as TDMA or

CDMA. and on different frequencies, such as FDMA

scheme.

In using SDMA, either FDMA or TDMA are

needed to allow LES to roam in the same satellite

beam or for polarization to enter the repeater. Thus,

the frequency reuse technique of the same frequency

is effectively a form of SDMA scheme, which

depends upon achieving adequate beam-to-beam and

polarization isolation. Using this system reverse line

means that interference may be a problem and the

capacity of the battery is limited.

On the other hand, a single satellite may achieve

spatial separation by using beams with horizontal

and vertical polarization or left-hand and right-hand

circular polarization. This could allow two beams to

cover the same Earth surface area, being separated by

the polarization. Thus, the satellite could also have

multiple beams using separate antennas or using a

single antenna with multiple feeds. For multiple

satellites, spatial separation can be achieved with

orbital longitude or latitude and for intersatellite-

links, by using different planes. Except for frequency

reuse, this system provides onboard switching

techniques, which, in turn, enhance channel capacity.

Additionally, the use of narrow beams from the

satellite allows the Earth station to operate with

smaller antennas and so produce a higher power

density per unit area for given transmitter power.

Therefore, through the careful use of polarization,

beams (SDMA), or orthogonal (CDMA), the same

spectrum may be reused several times, with limited

interference among users.

The more detailed benefits of an SDMA system

include the following:

1 The number of cells required to cover a given area

can be substantially reduced.

2 Interference from other systems and from users in

other cells is significantly reduced.

Figure 3. Nonsmart and Smart Antenna Beams

3 The destructive effects of multipath signals, copies

of the desired signal that have arrived at the

antenna after bouncing from objects between the

signal source and the antenna can often be

mitigated.

4 Channel reuse patterns of the systems can be

significantly tighter because the average

interference resulting from co-channel signals in

other cells is markedly reduced.

5 Separate spatial channels can be created in each

cell on the same conventional channel. In other

words, the intra-cell reuse of conventional

channels is possible.

6 The SDMA station radiates much less total power

than a conventional ground station. One result is a

reduction in network-wide RF pollution. Another

is a reduction in power amplifier size.

7 The direction of each spatial channel is known and

can be used to accurately establish the position of

the signal source.

8 The SDMA technique is compatible with almost

any modulation method, bandwidth, or frequency

band including GMSC, Global System for Mobile

Communications (GSM), and other Cellular

networks, Digital Enhanced Cordless

Telecommunications (DECT) solutions, IS-54, IS-

95, and other transmission formats. The SDMA

solution can be implemented with a broad range

of array geometry and antenna types.

4 SATELLITE COMMUNICATION

ARCHITECTURE WITH SMART ANTENNA

ARRAYS

Another perspective of the realization of SDMA

systems is the application of smart antenna arrays

with different levels of intelligence consisting of the

satellite antenna array and digital processor. Since the

frequency of transmission for satellite

communications is high enough, mostly 6 or 14 GHz,

that the dimensions of an array placed in orbit are

commensurable with the dimensions of the parabolic

antenna, is a necessary condition to put such systems

into orbit. There are two constants in the satellite

communication community: demand for higher data

rates and demand for greater user capacity. In fact,

both depend on a unique factor known as spectrum

efficiency, the ratio of information bits transmitted

per amount of band spectrum space used, usually

expressed in bits/Hertz b/Hz). Improving that

efficiency generally involves tradeoffs between

quality of service, power, and coverage.

In view of explosive growth in the number of MES

subscribers, satellite operators and service providers

are becoming increasingly concerned with the limited

capacities of their existing networks. This concern has

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led to the deployment of smart antenna systems

throughout important GMSC markets. In fact, the

smart antenna systems have typically employed

multibeam satellite technologies, which have been

shown, through extensive reserach, analysis,

simulation, trials, and experimentation, to provide

substantial performance improvements in FDMA,

TDMA, and CDMA networks. Multibeam

architectures for FDMA and TDMA systems provide

the straight-forward ability of the smart antenna to be

implemented as a non-invasive add-on or applicable

to an existing cell site, without major modifications or

special interfaces.

In satellite communications and especially in

GMSC networks are used nonsmart antenna systems,

such as traditional omni-directional antennas. In

Figure 3 (Left) shown are shown these type of

antennas which act as transducers, that is, they

convert electromagnetic energy into electrical energy,

and are not an effective way to combat inter-cell and

intra-cell interferences. One cost-effective solution to

this interference challenge is to split up the wireless

cell into multiple sectors using sectorized antennas.

As Figure 3 (Middle) illustrates, sectorized nonsmart

antennas transmit and receive in a limited portion of

the cell, typically one-third of the circular area,

thereby reducing the overall interference in the

system.

Transmission efficiency can increase still further

by using either spatial diversity or by focusing a

narrow beam antenna on a single mobile user. The

second approach is known as beam-forming, and it

requires an array of antennas that together perform

“smart” transmission and reception of signals, via the

implementation of advanced signal processing

algorithms. Although beam-forming is being

seriously considered only lately for commercial

cellular systems, the concept of using multiple

antennas and innovative signal processing to serve

cells more intelligently has existed for many years. In

fact, smart antennas date back to the 1930s, although

most significant developments occurred during

World War II. Varying degrees of relatively costly

smart-antenna systems have already been applied in

defense systems for years. Thus, the cost has

prevented their use in commercial systems until fairly

recently, however. With smart antenna technology,

each user's signal is transmitted and received by the

base station or GES terminals only in the direction of

that particular user. This drastically reduces the

overall interference in the system. A smart-antenna

system, as shown in Figure 3 (Right), includes an

array of satellite antennas that together direct

different transmission and reception beams toward

each MES user in the GMSC network.

Compared with traditional omni-directional and

sectorized antenna configurations, the smart-antenna

infrastructure can provide the following advantages:

(a) Greater coverage area for each satellite beam; b)

Better rejection of co-channel interference; (c)

Reduced multipath interference via increased

directionality; (d) Reduced delay spread as fewer

scatterers are allowed into the beam; (e) Increased

frequency reuse with fewer MES terminal; (f) Higher

range in mobile and remote environments; (g)

Improved building penetration; (h) Location

information for emergency situations; (i) Increased

data rates and overall system capacity; and (j)

Reduction in dropped calls.

Figure 4. Full Dimension MU MIMO Transmision to MES

Terminals

5 MASIVE MIMO AND MU-MIMO FOR

MULTIBEAM MOBILE SATELLITES NETWORKS

The RF and satellite communication systems are

implementing new microwave systems such as RF

front-ends, tunable RF filters, antennas and antenna

arrays. Recently, satellite systems have begun to

incorporate new technologies such as high-speed

signal processing platforms ad Multiple Input-

Multiple Output (MIMO) techniques widely known

and applied in terrestrial communication systems.

However, there are also scenarios in satellite

communication, where these modern technologies are

applicable. Since they dramatically enhance

communication link reliability and capacity

compared to conventional schemes, it is an actual

cutting-edge topic. In particular, the GMSC networks

will focus on Multi-User (MU) MIMO techniques also

called precoding, which can be implemented in

GMSC networks via GEO and Non-GEO satellite

constellations, such as Medium Earth Orbit (MEO)

and Low Earth Orbit (LEO) satellites. In Figure 4 is

depicted the Full Dimension (FD) Multi User (MU)

MIMO transmission from GES terminal via

GEO/MEO/LEO satellites to many Very Small

Aperture Terminals (VSAT) onboard ships, land

vehicles (road and rail) and aircraft.

The new designed High Throughput Satellites

(HTS) networks are employing multi-beam antennas

and full frequency reuse for broadband fixed and

mobile satellite services. Such architectures offer, for

example, a cost-effective solution to optimize data

delivery and extend the coverage areas in future

satellite and cellular networks. In order to realize

such requirements, it is necessary to develop and

deploy the MIMO technology in both the satellite

feeder links and the multiuser service downlinks.

Spatial multiplexing of different data streams is

performed in a common feeder beam. In the user

links, MIMO with multiple beams is exploited to

simultaneously serve different users in the same

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frequency channel. Under particular design

constraints, effective spatial separation of the

multiple user signals is possible. To mitigate the inter

stream interference in the MIMO feeder link as well

as the multiuser downlink, precoding of the transmit

signals is applied in satellite and wireless

communication systems.

Therefore, the MU-MIMO was created to support

environments where multiple mobile users are trying

to access a GEO and Non-GEO satellite network at

the same time. Initially, this technology was

developed for wireless networks supported by

routers and endpoint devices and recently was

proposed for satellite networks as well. In fact, MU-

MIMO is the next evolution from single-user MIMO

(SU-MIMO), which is generally referred to as MIMO.

Otherwise, MIMO technology was created to help

increase the number of antennas on a wireless router

that is used for both receiving and transmitting,

improving capacity for wireless and satellite

connections.

There are two fundamental advantages of MU-

MIMO over the more traditional single user (SU-

MIMO) or point-to-point MIMO: (1) It can favourably

work in the Line-of-Sight (LOS) propagation

environment; and (2) MU-MIMO requires only single

antenna terminals. Whereas, the next generation of

Ku-band GEO fixed and mobile satellite

communication systems geostationary SATCOM

systems are aggressive in terms of throughput and

capacity, the HTS and High Capacity Satellites

(HiCapS) are the two main categories of satellite

systems that have emerged. An HTS system typically

consists of several fixed spot beams covering multiple

small footprints on the ground. The main aim of an

HTS system is to increase the overall throughput of a

satellite by frequency reuse across the spot beams,

using at least four or more colours. The frequency

reuse factor of a multi-spot beam antenna, as

compared to a standard large contour beam antenna,

is the number of spots divided by the number of

colours. Each colour in the spot zone denotes a

certain frequency or polarization, so spot beams with

different colours differ in frequency or polarization.

Reducing the number of dedicated colours in an HTS

system will increase the bandwidth in each spot and

can boost the overall throughput, at the cost of

increased interference for users at the edge of the

beams. The application of MU-MIMO, precoding and

Multi User Detection (MUD) techniques have been

been analysed to mitigate the interference and these

studies show an improvement in the system

performance.

It is common for a satellite communications

channel between the ground and a satellite to have a

strong LOS path because the LOS path is essential in

achieving a healthy link budget. However, in a

MIMO scenario, the LOS nature of the satellite

channel and the large range distance in the channel

path can increase the spatial correlation between the

channel paths. Geometrical optimisation is required

to achieve extra spatial degrees of freedom. To

achieve spatial orthogonality in the LOS fixed and

satellite communication channel, antenna separation

on the order of several kilometres, depending on the

wavelength, is required either in space or on the

ground.

Figure 5. Satellite SDMA Multibeam Smart Anetnna and

Fixed VSAT Multibeam Satellite Network

6 TYPES OF SMART ANTENNA SYSTEMS FOR

MOBILE SATELLITES NETWORKS

The terms smart antenna heard today embrace

various aspects of a smart antenna system technology

include intelligent antennas, phased array, SDMA

system, spatial processing, digital beamforming,

adaptive antenna systems, and others. Smart antenna

systems are customarily categorized, however, as

either switched beam or adaptive array systems. The

following are distinctions between the two major

categories of smart antennas regarding the choices in

transmit strategy:

1 Switched Beam, which signifies a finite number of

fixed, predefined patterns or combining strategies

(sectors); and

2 Adaptive Array, which signifies an infinite

number of patterns (scenario-based) that are

adjusted in real time.

The SDMA scheme mostly responds to the

demands of MEO and LEO satellite constellations,

when the signals of users achieve the satellite antenna

under different angles (±22o for the MEO). In this

instance, the ground level may be split into the

number of zones of service coverage determined by

switched multiple-beam pattern lobes in different

satellite directions, or by adaptive antenna

separations with multi-beam smart antenna system,

which is shown in Figure 5 (Left). There are two

different beam-forming approaches in SDMA for

satellite communications: (1) The multiple spot beam

antennas are the fundamental way of applying SDMA

in large satellite systems including MSS and (2)

Adaptive array antennas dynamically adapt to the

number of users.

The smart antenna works in the way that each

antenna element "sees" each propagation path

differently, enabling the collection of elements to

distinguish individual paths to within a certain

resolution. As a consequence, smart antenna

transmitters can encode independent streams of data

onto different paths or linear combinations of paths,

thereby increasing the data rate, or they can encode

data redundantly onto paths that fade independently

to protect the receiver from catastrophic signal fades,

thereby providing diversity gain. A smart antenna

receiver can decode the data from a smart antenna

transmitter this is the highest-performing

827

configuration or it can simply provide array gain or

diversity gain to the desired signals transmitted from

conventional transmitters and suppress the

interference. No manual placement of antennas is

required, so the smart antenna electronically adapts

to the environment.

In satellite communications is deployed

multibeam coverage configuration as a beamforming

technique by which an array of antennas can be

steered to transmit radio signals in a specific

direction, which scenario of fixed VSAT multibeam

SDMA satellite network is shown in Figure 5 (Right),

and in Figure 6 is shown mobile VSAT multi-beam

satellite network. Rather than simply broadcasting

energy and signals in all directions, the antenna

arrays that use beamforming, determine the direction

of interest and transmit or receive a stronger beam of

signals in that specific direction directly to the fixed

and mobile VSAT stations. This technique is widely

used in radars and sonar, biomedical, and

particularly in Wi-Fi, wireless and new generations

cellular communications, where very high data rates

are required and the only way to support this would

be to maximize transmit and receive efficiency by

using beamforming. In this modern technique, each

antenna element is fed separately with the signal to

be transmitted. Thus, the phase and amplitude of

each signal is then added constructively and

destructively in such a way that they concentrate the

energy into a narrow beam or lobe.

Figure 6. Mobile VSAT Multibeam Satellite Network

Figure 7. Switched Beamforming Lobes Radiation Pattern

and Switched Beam Antenna

6.1 Switched Beamforming Pattern and Switch Beam

Antenna Array

The switched multi-beam antennas are designed to

track each user of a given cell for cellular and spot for

satellite systems with an individual beam pattern as

the target subscriber moves within the cell or spot

coverages. Therefore, it is possible to use array

antennas and to create a group of overlapping beams

that together result in omnidirectional coverage. This

is the simplest technique comprising only a basic

switching function between separate directive

antennas or predefined beams of an array. In fact,

switched beam antenna system has a fixed number of

beams, which one or more beams can be selected

from the array for transmission or reception. The

main motivation of the switched beam antenna is to

increase the antenna gain. For example, a four-

switched beam antenna system that used in 120°

sector antenna, the resultant increased gain (G) can be

calculated using the formula as follows:

G = 10 log (M) (1)

where M is the number of beams per sector. Thus for

a sector containing 4 beams (M=4), the gain increase is

6dB over the original sector antenna.

Beam-switching algorithms and RF signal-

processing software are incorporated into smart

antenna designs. For each call, software algorithms

determine the beams that maintain the highest quality

signal and the system continuously updates beam

selection, ensuring that customers get optimal quality

for the duration of their call. One might design

overlapping beam patterns pointing in slightly

different directions, similar to the ones shown in

Figure 5. (Left). Every so often, the system scans the

outputs of each beam and selects the beam with the

largest output power. The blue spots reuse the

frequencies currently assigned to the mobile

terminals, so they are potential sources of

interference. In fact, the use of a narrow beam reduces

the number of interfering sources seen at the base

station. Namely, as the mobile moves, the smart

antenna system continuously monitors the signal

quality to determine when a particular beam should

be selected.

Therefore, a switched spot beam antenna provides

many narrow predefined beams and activates one or

more beam in instant way, which scenario is shown in

Figure 7 (Left). In terms of radiation patterns, the

switched beam is an extension of the current micro

cellular or cellular sectorization method of splitting a

typical cell. The switched beam approach further

subdivides macro-sectors into several microsectors as

a means of improving range and capacity. Each

micro-sector contains a predetermined fixed beam

pattern with the greatest sensitivity located in the

center of the beam and less sensitivity elsewhere. The

design of such systems involves high-gain, narrow

azimuthal beamwidth antenna elements. Smart

antenna systems communicate directionally by

forming specific antenna beam patterns. When a

smart antenna directs its main lobe with enhanced

gain in the direction of the user, it naturally forms

side lobes and nulls or areas of medium and minimal

gain respectively in directions away from the main

lobe. Different switched beam and adaptive smart

antenna systems control the lobes and the nulls with

varying degrees of accuracy and flexibility.

By referring to Figure 7 (Right), a switched beam

antenna system can be realized by breaking the whole

system down into four major building blocks for ease

828

of analysis. And shows how the system can be broken

down into a beamforming network to form

independent beams, an RF switch for switching

between input ports, a power detector to monitor

signal strength and a control logic running an

algorithm that controls the whole system. The basic

operation of the switched beam can be explained as

follows. The input of the RF switch is connected to

the beamforming network and its outputs connected

to the power detector. At instant, only one switch will

be turned on while others will be turned off. The

power detector will measure the signal strengths for

that incoming beam, which then will be connected to

control logic. The main function of this control logic

is to samples all the power and then make a

comparison between them. After that, logic control

will feedback to the RF switch and select the beam

which received the highest signal to noise ratio,

Signal-to-Noise Ratio (SNR) by sending a certain

amount of voltage to thw RF switch. Thus, since the

power detector operated in analog form while the

microcontroller in digital form, an analog-to-digital

converter may be needed for the interfacing part.

However, this is just a general idea about how a

switched beam antenna can be implemented as a

system but the complete implementation of this

switched beam system of way is not as simple as

what was mentioned above.

Switched-beam antennas are normally used only

for the reception of signals since there can be

ambiguity in the system’s perception of the location

of the received signal. In fact, these antennas give the

best performance, usually in terms of received power

but they also suppress interference arriving from

directions away from the active antenna beam’s

centre, because of the higher directivity, compared to

a conventional antenna, some gain is achieved. In

high-interference areas, switched-beam antennas are

further limited since their pattern is fixed and they

lack the ability to adaptively reject interference. Such

an antenna will be easier to implement in existing cell

structures than the more sophisticated adaptive

arrays but it gives only limited improvement.

Figure 8. Adaptive Beamforming Lobes Radiation Pattern

and Adaptive Beam Antenna

6.2 Adaptive Beamforming Pattern and Adaptive Beam

Antenna Array

Adaptive Array systems provide more intelligent

operation where it has the ability to adapt in real-time

radiation patterns to the RF environment. It arranges

beamforming lobes radiation pattern according to the

target and interferes with user locations, which is

shown in Figure 8 (Left). The adaptive system

chooses a more accurate placement, thus providing

greater signal enhancement. Similarly, the interfering

signals arrive at places of lower intensity outside the

main lobe, but again the adaptive system places these

signals at the lowest possible gain points. The

adaptive array concept ideally ensures that the main

signal receives maximum enhancement while the

interfering signals receive maximum suppression.

The adaptive antenna array systems select one

beam pattern for each user out of a number of preset

fixed beam patterns, depending on the location of the

subscribers. At all events, these systems continually

monitor their coverage areas, attempting to adapt to

their changing radio environment, which consists of

(often mobile) users and interferers. Thus, in the

simplest scenario, that of a single user and no

interferers, the system adapts to the user’s motion by

providing an effective antenna system pattern that

follows the mobile user, always providing maximum

gain in the user’s direction. The principle of SDMA

with adaptive antenna system application is quite

different from the beam-forming approaches

described in Figure 5 (Right) and in Figure 6 Figure 2.

(B).

The adaptive antenna systems approach

communication between MES and GES terminals in

different ways by adding the dimension of space. By

adjusting to the RF environment as it changes (or the

spatial origin of signals), adaptive antenna

technology can dynamically alter the signal patterns

to optimize the performance of the satellite system.

Adaptive array systems provide more degrees of

freedom since they have the ability to adapt in real-

time the radiation pattern to the RF signal

environment; in other words, they can direct the main

beam toward the pilot signal or Signal of Interest

(SOI) while suppressing the antenna pattern in the

direction of the interferers or Signals Not of Interest

(SNOI). To put it simply, adaptive array systems can

customize an appropriate radiation pattern for each

individual user. Fig. below illustrates the general idea

of an adaptive antenna system. Adaptive array

systems can locate and track signals (users and

interferers) and dynamically adjust the antenna

pattern to enhance reception while minimizing

interference using signal processing algorithms. A

functional block diagram of the digital signal

processing part of an adaptive array antenna system

is shown in Figure 8 (Righ). Thus, one of the most

famous adaptive processors in array signal processing

is the Minimum Variance Distortionless Response

(MVDR). The MVDR algorithm has been adopted by

many researchers for Angle of Arrival (AoA)

estimation, which is a process that determines the

direction of arrival of a received signal by processing

the signal impinging on an antenna array.

With reference to Figure 8 (Righ), it can be

observed that the RF signal received by each of the N

antennas comprising the array is at first brought

down to baseband and then converted into digital

form. The N complex signals obtained are then sent as

inputs to the Direction of Arrival (DSP), which

multiplies the signal of each antenna by a suitable

factor (wi), and finally adds the various terms. The

output signal is therefore given by:

829

( ) ( )

y t w x t



(2)

An appropriate choice of the weights vector values

w = [w1, w2, ...wN] allows giving the radiation

pattern the desired characteristics. In particular,

vector w is determined using an adaptive strategy.

Coefficients are therefore updated periodically, based

on the observation of data received. To assure correct

operation of the system, it is necessary that the

adaption rate could compensate for the

environmental variations, due to the mobility of the

sources and accentuated by the presence of multiple

paths. The use of an adaptive antenna array system at

the base station thus allows introducing the SDMA

technique, whose main advantage is the capability to

increase system capacity, i.e. the number of users it

can handle. Thus, this increase can be obtained in two

different ways, and therefore the following

applications are possible: (1) Reduction in co-channel

interference between the different cells using the

same group of radio channels is obtained, as above

seen, by minimizing the gain in the direction of

interfering mobile units. This technique, indicated

with the acronym Spatial Filtering for Interference

Reduction (SFIR) allows reducing frequency re-use

distance and cluster size. In this way, each cell can be

assigned a higher number of channels.and (2) Spatial

orthogonality between signals associated with

different users is obtained by transmitting them in

different frequency bands of FDMA, in different time

slots of TDMA, or using different code sequences of

CDMA scheme.

The events processed in SDMA adaptive array

antenna systems are as follows:

1 A “Snapshot”, or sample, is taken of the

transmission signals coming from all of the

antenna elements, converted into digital form and

stored in memory.

2 The SDMA digital processor analyzes the sample

to estimate the radio environment at this point,

identifying users and interfering their locations.

3 The processor calculates the combining strategy

for the antenna signals that optimally recover the

user’s signals. With this strategy, each user’s

signal is received with as much gain as possible,

and with the other users/interferers signals

rejected as much as possible.

4 An analogous calculation is done to allow

spatially selective transmission from the array.

Each user’s signal is now effectively delivered

through a separate spatial channel.

5 The system now has the ability to both transmit

and receive information on each of the spatial

channels, making them two-way channels.

As a result, the SDMA adaptive array antenna

system can create a number of two-way spatial

channels on a single conventional channel, be it

frequency, time, or code. Of course, each of these

spatial channels enjoys the full gain and interference

rejection capabilities of the antenna array. In theory,

an antenna array with (n) elements can support (n)

spatial channels per conventional channel. In practice,

the number is somewhat less because the received

multipath signals, which can be combined to direct

received signals, takes place. In addition, by using

special algorithms and space diversity techniques, the

radiation pattern can be adapted to receive multipath

signals, which can be combined. Hence, these

techniques will maximize the SIR or Signal to

Interference and Noise Ratio (SINR).

7 HYBRID SDMA NETWORK ARCHITECTURES

Further enhancement can be obtained when SDMA

technique grouping is considered in combinations

with FDMA, TDMA, and CDMA schemes in order to

improve switching, transmission and frequesny

bands conditions of baseband signals, and improve

control of the satellite up and downlinks.

7.1 SDMA/FDMA

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.

7.2 SDMA/TDMA

This solution is similar to the one previously

explained in that a switch system allows a TDMA

receiver to reconfigure the satellite. Under normal

conditions, a link between beam pairs is maintained

and operated under TDMA conditions. The

utilization of time slots may be arranged on an

organized or contention basis. Switching is achieved

by using the RF signal. Thus, onboard processing is

likely to be used in the future, allowing switching to

take place by the utilization of baseband signals. The

signal could be restored in quality and even stored to

allow transmission in a new time slot in the outgoing

TDMA frame. This scheme is providing up and

downlinks for the later Intelsat VI spacecraft, known

as SDMA/SS/TDMA.

7.3 SDMA/CDMA

This arrangement allows access to a common

frequency band and may be used to provide the MAT

technique to the satellite when each stream is

decoded on the satellite in order to obtain the

destination addresses. Thus, on-board circuitry must

be capable of determining different destination

addresses, which may arrive simultaneously, while

also denying invalid users access to the downlink.

However, on-board processors allow the CDMA

bitstream to be retimed, regenerated and stored on

830

the satellite. Because of this possibility. the downlink

CDMA configurations need not be the same as for

uplink and the Earth link may thus, be optimized.

8 CONCLUSION

The performances and capacities of MSC 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 EIRP levels. As the MSS grows and the

antenna beam size decreases, CDMA appears to be a

very efficient system, because it is not limited by L-

band bandwidth constraints.

However, CDMA is wasteful in feeder link

bandwidth, and the choice of a multiple access

system must take all parameters into consideration,

such as oscillator stability, interference rejection,

system complexity, etc. as well as system cost before

deciding on a particular multiple access systems.

The communication satellites for GMSC systems

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 inter-beam interference in L-band.

The CDMA scheme is better at absorbing Doppler

and multipath effects, and it permits higher rate

coding, but it suffers from self-jamming and from

bandwidth constraints in the feede rlink. In general,

all three multiple access systems show similar

performance. However, at the chosen design point for

aggregate EIRP values, the number of satellite beams,

and allocated bandwidth, FDMA provides still the

highest system channel capacity.

The narrowness of the frequency spectrum

allocated to MSC means that it has to be explored to

the full. Methods available for effective spectrum

utilization include efficient signal design and

subdivision of the total coverage area into narrow

illumination zones. Modern satellites for MSC have

also onboard processors, which connect an uplink

band to a downlink beam. Processors use A/D

conversion and digital filtering. The A/D converters

quantize the signal and produce quantization noise.

Recently is developed SDMA as an advanced

solution where all concerned MES terminals can

share the same frequency at the same time within a

separate space available for each link. On the other

hand, the RDMA scheme is suitable for a large

number of users in GMSC systems, where all MES

terminals share asynchronously the same transponder

by randomly transmitting short burst or packet

divisions. In addition is developed several mobile

Aloha methods, which successfully increase the

system throughout.

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