MASTER’S THESIS
REQUIREMENTS AND DESIGN OF A TELECOMMUNICATIONS INSTALLATION IN A VESSEL
Bartolome Solera Ripoll
Master’s Degree in Telecommunication Engineering Centre for Postgraduate Studies
Academic Year 2019-20
REQUIREMENTS AND DESIGN OF
A TELECOMMUNICATIONS INSTALLATION IN A VESSEL
Bartolome Solera Ripoll
Master’s Thesis
Centre for Postgraduate Studies University of the Balearic Islands
Academic Year 2019-20
Key words:
Telecommunication, Vessel, Satellite, VSAT.
Thesis Supervisor’s Name: Felip Riera Palou
Thesis Supervisor’s Name: Guillem Femenias Nadal
T ABLE OF C ONTENTS
Table of Contents ... i
Index of Figures ... v
Index of Tables ... vii
List of Acronyms ... ix
Summary ... xiii
1 Introduction ... 1
1.1 Background ... 1
1.2 Objective... 1
1.3 Report structure ... 2
2 Communication Fundamentals ... 3
2.1 General Communication Fundamentals ... 3
2.1.1 Digital Communication System at Link Layer ... 5
2.1.2 Shared Link Layer Methods ... 6
2.1.3 Some Quality of Service (QoS) parameters ... 6
2.2 Satcom Fundamentals ... 7
2.3 Very-Small-Aperture Terminal (VSAT) ... 7
2.3.1 Architectures of VSAT networks... 8
2.3.1.1 Basic Link Operation ... 8
2.3.2 Satellites for VSAT ... 9
2.3.2.1 Orbits ... 9
2.3.2.2 Transponders ... 9
2.3.2.3 Satellite Beams, footprints ... 10
2.3.2.4 Satellite Measurements ... 12
2.3.2.5 Frequency Bands and their propagation characteristics ... 14
2.3.2.6 Polarization ... 15
2.3.3 VSAT Networks ... 17
2.3.3.1 Adaptative Modulation and Coding (ACM) ... 18
2.3.4 VSAT Service Contract Details ... 20
2.3.5 VSAT Earth Stations ES and Earth Stations on Vessels (ESV) ... 21
2.4 VSAT Equipment for a vessel ... 21
2.4.1 Setup possibilities ... 22
2.4.2 Above Deck Equipment ... 24
2.4.2.1 Blockage and Hazard Zones ... 27
2.4.3 Below Deck Equipment ... 27
2.4.3.1 Automatic Beam Switching (ABS) ... 28
2.4.4 Communication between VSAT equipment ... 28
2.4.5 Pointing process for an Earth Station in a Vessel ... 29
2.4.5.1 ESV: targeting, searching and tracking ... 31
2.4.6 System power levels ... 32
2.4.7 Interference ... 33
2.5 Fleet Broadband (FBB): Iridium and Inmarsat ... 34
2.6 Conclusion ... 36
3 Internal Network Requirements for a Vessel ... 37
3.1 Internal LANs ... 37
3.2 Coverage: WiFi, Ethernet Sockets and TV STBs ... 38
3.3 Services ... 39
3.3.1 Streaming ... 39
3.3.1.1 Decoder / IPTV + STB ... 40
3.3.1.2 Multicast Channels ... 40
3.3.2 Voice over IP (VoIP) ... 41
3.3.2.1 General idea / Particularities of SIP ... 42
3.3.2.2 On Board Setup ... 43
3.3.3 Other kinds of Data (e-mail, web…) ... 44
3.4 Devices to use on board ... 45
3.4.1 Tunneling ... 46
3.5 Monitoring: data extraction ... 47
3.6 Internal Network Management ... 48
3.6.1 Controling the single device access at layer 2: MAC address ACLs ... 49
3.6.2 Controling the LAN access at layer 3: ACLs ... 50
3.6.3 Traffic limits on interfaces ... 50
3.6.4 Controling the Services access at layer 7: Service Managers ... 51
3.7 Conclusion ... 52
4 Internet Access Technologies ... 53
4.1 Shore ... 53
4.1.1 Devices ... 53
4.2 WiFi... 53
4.2.1 Devices and configuration ... 54
4.3 Cellular (3G/4G/5G) ... 56
4.3.1 Devices and configuration ... 56
4.4 VSAT ... 58
4.4.1 Devices and configuration ... 59
4.5 Fleet Broadband (Iridium / Inmarsat) ... 63
4.5.1 Devices and possible setup ... 63
4.6 Throughput aggregation techniques: Load Balancing and Channel Bonding .... 64
4.6.1 Devices and setup ... 66
4.7 Monitoring and control the WANs devices ... 66
4.8 Monitoring and Control of the different WANs ... 66
4.8.1 WAN availability monitoring ... 66
4.8.2 WAN priorities to configure ... 67
4.8.2.1 WAN priorities per LAN ... 68
4.9 Conclusion ... 68
5 Complete Installation ... 71
5.1 TV, voice and WiFi coverage ... 71
5.2 Internal LANs created ... 73
5.3 WANs available ... 73
5.4 Devices to install and configuration ... 74
5.4.1 Router ... 74
5.4.2 Switches on stack ... 88
5.4.3 WLC ... 91
5.4.4 APs ... 94
5.4.5 TV Boxes ... 94
5.4.6 VoIP devices... 95
5.4.7 VSAT dual antenna system ... 95
5.4.8 VSAT modem ... 97
5.4.9 Cellular modem ... 97
5.4.10 Isolated FBB system ... 97
5.5 Conclusion ... 98
6 Conclusion ... 99
References ... 101
I NDEX OF F IGURES
Figure 2.1 IP stack connections ... 4
Figure 2.2 Block diagram of a digital communication system ... 5
Figure 2.3 TDMA representation ... 6
Figure 2.4 VSAT link paths ... 8
Figure 2.5 Name convention for the different parts of the path ... 9
Figure 2.6 Example of transponders in a frequency plan for Ku-band ... 10
Figure 2.7 Footprint of Intelsat 33e (60°E), global beam ... 11
Figure 2.8 Footprint of Intelsat 33e (60°E), SubSaharan zone ... 11
Figure 2.9 Footprint of Intelsat 33e (60°E), U25 spot ... 11
Figure 2.10 VSAT link gains and loses ... 13
Figure 2.11 Electromagnetic spectrum with VSAT range ... 14
Figure 2.12 Frequency bands with their different implications ... 15
Figure 2.13 Different polarization possibilities ... 16
Figure 2.14 Two stations transmitting to the same satellite in different circular polarizations ... 16
Figure 2.15 Two linear transmitters sending the signal to the same satellite in different polarizations ... 17
Figure 2.16 HUB&spoke setup ... 18
Figure 2.17 BER depending on Es/No simulation for different modcods ... 19
Figure 2.18 ACM gain in changing conditions channel ... 20
Figure 2.19 ADE and BDE in a vessel ... 21
Figure 2.20 Single antenna system ... 22
Figure 2.21 Dual antenna system with arbitrator/mediator ... 23
Figure 2.22 Dual antenna system with Master-Slave setup ... 23
Figure 2.23 Feed assembly and antena reflector ... 24
Figure 2.24 RF path in the antenna ... 25
Figure 2.25 Azimuth axis rotation ... 26
Figure 2.26 Elevation axis rotation ... 26
Figure 2.27 Cross level axis rotation ... 26
Figure 2.28 True azimuth, heading and relative azimuth ... 29
Figure 2.29 ACU process to find the relative position of the satellite ... 30
Figure 2.30 Satellite network configured in the VSAT modem (IS33 U24) ... 30
Figure 2.31 Satellite network using the same satellite and frequency, not configured ... 30
Figure 2.32 Spiral search pattern ... 32
Figure 2.33 Clear sky search pattern ... 32
Figure 2.34 Sun passing behind the satellite ... 33
Figure 2.35 Sun trajectory over consecutive days ... 34
Figure 2.36 i-4 GEO constellation for Fleet Broadband (Inmarsat) ... 35
Figure 2.37 Iridium LEO constellation ... 35
Figure 3.1 Conventional TV setup on ships ... 39
Figure 3.2 Decoder / IPTV subscription with a STB to receive the stream ... 40
Figure 3.3 Multicast channels setup ... 41
Figure 3.4 Register process in SIP ... 43
Figure 3.5 Call in which a redirect and a proxy servers are involved in SIP ... 43
Figure 3.6 Diagram of the setup with the router as a SIP gateway using analog phones 44 Figure 3.7 Diagram of the setup using a digital PBX with digital phones ... 44
Figure 3.8 Traffic mirroring setup ... 47
Figure 3.9 Filtering table from a layer 7 service manager ... 51
Figure 3.10 QoS profile configuration ... 52
Figure 4.1 Diagram with 3 antennas showing the coverage ... 54
Figure 4.2 Wireless interface settings ... 55
Figure 4.3 Network LAN interface settings ... 55
Figure 4.4 Network WAN interface settings ... 55
Figure 4.5 Differences between cellular technologies ... 56
Figure 4.6 Cellular modem configuration ... 57
Figure 4.7 Cellular modem LAN configuration ... 57
Figure 4.8 SIM card settings ... 58
Figure 4.9 Intellian antenna network configuration ... 59
Figure 4.10 Intellian antenna satellite configuration ... 60
Figure 4.11 Intellian antenna modem configuration ... 60
Figure 4.12 Intellian mediator modem configuration and network configuration ... 61
Figure 4.13 Intellian mediator path control selector ... 61
Figure 4.14 IP interface configuration ... 61
Figure 4.15 Satellite interface configuration ... 62
Figure 4.16 GPS configuration ... 63
Figure 4.17 Possible networking setup ... 64
Figure 4.18 Load Balancing diagram ... 64
Figure 4.19 Channel Bonding diagram ... 65
Figure 4.20 4G bonding connection with tunneling ... 66
Figure 5.1 Image of the vessel ... 71
Figure 5.2 Conventional TV Diagram ... 72
Figure 5.3 VoIP Diagram ... 72
Figure 5.4 Diagram of the connectivity to the IT admin office through a tunnel ... 74
Figure 5.5 Physical Diagram, router connections ... 84
Figure 5.6 Physical Diagram, switches connections ... 89
Figure 5.7 Device configuration settings ... 92
Figure 5.8 VLAN settings ... 92
Figure 5.9 Cisco 9800 WLC series configuration model ... 93
Figure 5.10 AP with tags assigned. ... 94
Figure 5.11 VSAT RF Diagram ... 96
Figure 5.12 Net Diagram ... 97
I NDEX OF T ABLES
Table 2.1 OSI reference model. ... 3
Table 2.2 Frequency range used for uplink and downlink for the different bands. ... 15
Table 2.3 Differences between fixed and marine stations. ... 21
Table 5.1 Internal LANs implemented. ... 73
L IST OF A CRONYMS
Es/No Symbol Energy over the Noise Floor
ABS Automatic Beam Switching
ACL Access Control List
ACU Antenna Control Unit
ADE Above Deck Equipment
AP Access Point
APN Access Point Name
BDE Below Deck Equipment
BER Bit Error Rate
BUC Block Up Converter
C/N Carrier-to-Noise Ratio
C/No Carrier-to-Noise-density Ratio
CCITT International Telegraph and Telephone Consultative Committee (Comité Consultatif International Téléphonique et Télégraphique) CIR Committed Information Rate
CNR Carrier-to-Noise Ratio
CSMA/CD Carrier Sense Multiple Access with Collision Detection DHCP Dynamic Host Configuration Protocol
DID Direct Inward Dialing
DNS Domain Name Server
DVB Digital Video Broadcasting
DVB-S Digital Video Broadcasting by Satellite
DVB-S2 Digital Video Broadcasting by Satellite, second generation DVB-S2x Extension of Digital Video Broadcasting by Satellite, second gen.
EIRP Effective Isotropic Radiated Power
ES Earth Station
ESV Earth Station in a Vessel
ETSI European Telecommunications Standards Institute
FBB Fleet BroadBand
FEC Forward Error Coding
FSPL Free-Space Path Loss
GEO Geostationary Earth Orbit
GSO GeoSynchronus Orbit
GUI Graphical User Interface G/T Gain-to-noise-Temperature
IEEE Institute of Electrical and Electronics Engineers IETF Internet Engineering Task Force
IP Internet Protocol
IPSec Internet Protocol Security
ISAKMP Internet Security Association and Key Management Protocol ISO International Organization for Standardization
ITU International Telecommunication Union
ITU-T Telecommunications Standardization Sector of the ITU
HPA High Power Amplifier
HSRP Hot Standby Router Protocol
LAN Local Area Network
LEO Low Earth Orbit
LHCP Left-Hand Circular Polarization
LNA Low Noise Amplifier
LNB Low Noise Block
LO Local Oscillator
MAC Medium Access Control
MAC address Layer 2 address
MCPC Multiple Carrier Per Channel
MEO Medium Earth Orbit
MF-TDMA Multi Frequency Time-Division Multiple Access MGCP Media Gateway Control Protocol
MIR Maximum Information Rate
ML-PPP Multilink Point-to-Point Protocol
MP-TCP Multipath Transmission Control Protocol Mx-DMA Cross-Dimensional Multiple Access NAT Network Address Translation
OpenAMIP Open Antenna to Modem Interface Protocol
OMT Orthomode Transducer
OSI Open Systems Interconnection
PBX Petrol Bases Xo
PBR Policy-Based Routing
PIR Peak Information Rate
POS Point Of Sales (terminal)
PSK Phase Shift Key
PSTN Public Switched Telephony Network
QoS Quality of Service
RF Radio Frequency
RFC Request For Comments
RHCP Right-Hand Circular Polarization
RX Reception
S/N Signal to Noise Ratio SCPC Single Carrier Per Channel SIP Session Initiation Protocol
SNMP Simple Network Management Protocol
SNR Signal to Noise Ratio
SSH Secure Shell
TCP Transmission Control Protocol TDM Time-Division Multiplexing TDMA Time-Division Multiple Access
TX Transmission
UDP User Datagram Protocol
UPC Uplink Power Control
UPS Uninterruptible Power Supply
VoIP Voice over IP
VPN Virtual Private Network VSAT Very-Small-Aperture Terminal
WAN Wide Area Network
WiMAX Worldwide Interoperability for Microwave Access WLAN Wireless Local Area Network
WLC Wireless LAN Controller
S UMMARY
Due to the fact that the Internet connectivity is indispensable nowadays, vessels must have a network infrastructure with which this connectivity is available. The goal of this project is the study of the requirements a vessel may have in terms of services and infrastructures, as well as the possible Internet access technologies for a vessel.
Very-Small-Aperture Terminal (VSAT) is today’s basic Internet connection for a vessel, since, in light of the current technology available, it allows the Internet connection almost everywhere with the best possible data transmission rates.
The contents of this document include the analysis of services any internal network with different requirements for different classification of user devices may have, with details of the infrastructure devices involved. It also describes the different Internet connection possibilities, which are the most frequent bottlenecks for the network. This is the reason why shaping the traffic becomes a crucial action in order to be able to provide a good Internet experience by means of optimizing the link usage.
The fundamentals for a better understanding of the above mentioned are also explained as well as a possible installation setup in which the previous information is used to complete a real network configuration with real commercial devices.
As detailed in the conclusion, the objectives initially set have been achieved successfully.
Hopefully, this document could be used as a guide for any support engineer to be contextualized in nowadays’ possibilities and most used setups.
Finally, from the academic point of view, the overall concepts, possibilities, and fundamentals have been assimilated, widening knowledge.
1 I NTRODUCTION
1.1 Background
The Internet has become essential everywhere, more so since smart devices (smartphones, smartwatches, tablets, smartTVs…) have been commercialized. Nowadays, almost everybody has a smartphone, tablet, laptop, and/or smartwatch, and most of the people in the world use them mainly connected to the Internet: checking the email inbox, chatting with friends, reading the news, looking for any information, watching videos, interacting in social networks, or working on line. This is the main reason for the Internet connection to be required wherever people go.
A yacht, a cruise, a research vessel, whatever the kind of ship, it is not an exception, requiring Internet connectivity in all its areas while having the same Internet experience like at home is expected. Nevertheless, due to the wireless Internet technologies is often the bottleneck in the majority of the cases, the most efficient devices and technologies available must be selected in order to make the impact of the impairments the radio channel brings along softer.
Fortunately, technology improvements are developed very quickly, in terms of more powerful devices, which integrate better algorithms or more functionalities; improved protocols, some of them just making better the treatment of data in general, and some others being more specific, working better if some characteristics are given; etc. This progress makes the Internet experience closer to the one at home day after day with wireless connections.
Despite the internal network is similar to the one in a hotel or a house of similar dimensions, the Internet connections cannot be the same. As detailed on this document, a wired connection is a possibility, but it is only available while docked in a port. Thus, wireless Internet access technologies, knowing they are not as fast and reliable as wired ones, are installed in order for the Internet to reach the vessel wherever it is.
1.2 Objective
The objective of this project is the study of the telecommunication requirements a typical vessel could have. This main goal presented is divided in the internal network requirements not only in terms of services (like telephony or television), but also regarding configuration (internal subnetworks and specific restrictions they may have) and infrastructure (number of access points, Ethernet sockets positions, etc.), and the possible Internet access technologies
for a vessel. This project also aims to explain VSAT technology in some detail due to it being the essential Internet connection for any vessel.
A complete installation illustrating a realistic deployment is introduced in order to show how the requirements and Internet access technologies can actually be implemented in an installation. For this part, decisions are taken in order to select the concrete commercial devices and configure them in order to cover the hypothetical needs a client could have. The solution for this part is a realistic setup that lots of vessels implement nowadays.
All the above mentioned needs to be supported by a theoretical basis complemented with an overview of several protocol and technology standards. Nevertheless, the scope of this project is not the review of their details, but the reasoned selection of commercial devices which follow the standards to cover the needs for a final and complete installation.
1.3 Report structure
Prior to developing the report, a qualitative description of the different chapters which composes it is presented.
Communication Fundamentals
In the second chapter of the report, the theoretical fundamentals of the communications are explained, including a brief history of OSI and TCP/IP reference models, some standard organizations, and a few link layer methodologies as well as quality of service parameters which are important in later chapters. After that, a detailed view of satellite communications is explained, focusing on VSAT.
Internal Network Requirements for a Vessel
The third chapter tackles the requirements in telecommunications that a vessel may have.
It describes solutions for the requirements that an internal network may have with their particular characteristics and possible devices to use. Finally, the monitorization and control of the onboard network is also addressed.
Internet Access Technologies
Diverse Internet access technologies possibilities are fully detailed in the fourth chapter, explaining their characteristics. It also covers ways to combine them as well as monitor and control them.
Complete Installation
The fifth chapter presents a complete installation for a given vessel, taking decisions about which setup from the explanation in the previous chapters will be implemented, and describing the configuration for the different devices to be installed.
Conclusion
The last chapter is used to present the conclusions about the whole project and report while providing hints for further work.
2 C OMMUNICATION F UNDAMENTALS
2.1 General Communication Fundamentals
Around the 1970s, networking was government-sponsored or vendor-developed with proprietary standards. Beginning in 1977, the International Organization for Standardization (ISO) and the International Telegraph and Telephone Consultative Committee (CCITT, in France) conducted a similar program to develop general standards and methods of networking, which defined similar networking models. The OSI (Open Systems Interconnection) model was published in 1984 by both the ISO, as standard ISO 7498 and the ITU-T (CCITT renamed), as standard X.200.
OSI had two major components: an abstract model of networking (Basic Reference Model or seven-layer model), and a set of specific protocols. The OSI reference model, summarized in Table 2.1, was a major advancement in the network concepts since the promotion of the idea of a consistent model of protocol layers, defining interoperability between network devices and software.
Table 2.1 OSI reference model.
Layer
Protocol Data Unit (PDU)
Function
Host Layers
7 Application Data High-level APIs, resource sharing and remote file access.
6 Presentation Translation between a networking service and an application.
Encoding, compression and encryption/decryption.
5 Session
Managing communication sessions.
Continuous exchange of information in the form of multiple back-and-forth transmissions between two nodes 4 Transport Segment or
Datagram
Reliable transmission between points on a network.
Segmentation, acknowledgement and multiplexing.
Media Layers
3 Network Packet Structuring and managing a multi-node network.
Addressing, routing and traffic control.
2 Data link Frame Reliable transmission between two nodes connected by a physical layer
1 Physical Symbol Transmission and reception of raw bit streams over a physical medium
OSI was an industry effort, attempting to get industry participants to agree on common network standards to provide multi-vendor interoperability. Nevertheless, due to the availability of an Internet protocol suite, an alternative model, engineers, organizations, and nations became polarized over the issue of which standard would be the best and most robust for computer networks.
While OSI developed its networking standards, TCP/IP came into widespread use on multi- vendor networks for internetworking. This is the reason why the OSI model is used as a reference for teaching and documentation nowadays, but TCP/IP, the Internet protocol suite, is the one which fits today's networking protocols [1].
The technical standards underlying the Internet protocol suite and its constituent protocols have been delegated to the IETF. The Internet protocol suite standard, in request for comments (RFC) 1122 [2], divides the protocols into operating scopes. This is a more comprehensive reference for general networking systems [3], and also the reason why TCP/IP stack is the one used from now on in this document. Figure 2.1 shows the TCP/IP stack connections through an example. The communication between the different devices in the network can be seen, as well as the scope of each layer in this model.
Figure 2.1 IP stack connections
This model can be found with several names in the literature, as well as with different (but similar) names for each layer. In addition, some of them take into account the physical layer while some others do not. Nevertheless, the official standard information is the one in the RFC 1122 from the IETF.
Every commercial device has the supported protocols detailed in its datasheet. This way, any user can confirm the compatibility for the different units to install. Vendors use the public standards in order for their models to follow their specifications and to be able to note this information down on the devices’ datasheet.
There are some organizations, like IETF ETSI, ITU or IEEE, that standardize protocols used by present-day devices. To name a few protocols, IP, DVB, JPEG and Ethernet (802.3) respectively are examples of their standards.
2.1.1 Digital Communication System at Link Layer
As detailed in Table 2.1, the link layer function is to communicate through a physical layer in a reliable way. This is the reason for the source message to be modified somehow to be adapted to the physical channel (minimize noise and interference) in order for the receiver to be able to recover the original information after reverting the operations. There are three logical processes which perform these modifications in transmission [4]:
Source encoding (compression): the process of efficiently converting the output of either an analog or a digital source into a sequence of binary digits.
Source encoding (compression): the process of efficiently converting the output of either an analog or a digital source into a sequence of binary digits.
Digital modulation (modulation): conversion from bits to an analog signal suitable to be transmitted through the channel.
While source encoding depends on the compression method used, the channel characteristics will determine the channel encoding and digital modulation possibilities. All three processes have their reverse process in reception: source decoding, channel decoding and digital demodulation respectively. The block diagram of one of such a digital communications system is the one shown in Figure 2.2.
Figure 2.2 Block diagram of a digital communication system
2.1.2 Shared Link Layer Methods
When using the same channel resources are shared by several users, the way the resources are divided among all of them must be well defined. For this reason, different multiplexing methods [5] are available to allow different streams to use the same physical channel, and multiple access techniques [6] are used to be in charge of the assignment of the resources to each transmitter. Thanks to this multiplexing, the receiver can distinguish and demodulate each user’s information. There are many methods of both multiplexing and access, however, the ones concerning this project are the ones described in this section.
Time-Division Multiplexing (TDM) [7] [8] is the multiplexing method in which there is a temporal assignation of the channel resources to the different information sources. Actually, the total frame is divided into time slots and each source uses some of them, the ones designated to them. Sometimes, the number of slots assigned to each source is the same, but sometimes it is not, depending on each information rate.
Time-Division Multiple Access (TDMA) [9] is the access method which uses time division as well, dividing the frame into slots. Taking Figure 2.3 as an example, each frame is divided into eight (8) slots. Multiple transmitter nodes are assigned these slots which they must use to transmit their data. There is the variant Multi Frequency TDMA (MF-TDMA) [10] which uses multiple frequency channels for the multiple access assignment. which uses multiple frequency channels for the multiple access assignation.
Figure 2.3 TDMA representation
In both TDM and TDMA, the synchronization between the different nodes is essential, since this is the way to detect the slots properly.
2.1.3 Some Quality of Service (QoS) parameters
Some QoS parameters need to be detailed for a better understanding of the information in the following chapters. The ones that will be most relevant in this work are:
Latency: the time for an IP packet to reach its destination. If the time takes into account only the time for the packet to go, the concept is one-way latency. In the event the time reported is the one between sending a packet and receiving its reply back, round-trip delay is the concept used [11].
Jitter: its generic definition is the deviation from the exact periodicity of presumably periodic signals [12]. In networking, jitter is used to quantify the time difference between latency values, and this second and more specific definition will be used for this document concern.
2.2 Satcom Fundamentals
A communications satellite is an artificial device orbiting the Earth that receives signals from Earth stations, amplifies them, and sends them back to the Earth by means of transponders. This is the way it creates communication channels between transmitters and receivers at different locations on the Earth [13] [14].
The high frequency radio waves used in satellite communications need a clear line of sight between transmitter and receiver, otherwise, they are heavily attenuated, making the quality of the signal severely impaired. Using satellites, the possible obstruction due to the curve of the Earth and possible structures is avoided as well as the communication between separated locations is possible. Taking advantage of the satellites coverage, they can also be used when in remote locations without other infrastructure (at sea, for example) to have Internet, telephony, television, etc. For the same reason, they are also used for broadcast television or radio services, and for military applications [13].
As mentioned, the frequency band allocation to the different satellite services is carried out by the ITU in order to be internationally coordinated. This allocation minimizes the risk of signal interference [15].
It is worth mentioning that there are about 2,000 communications satellites in Earth's orbit now, used by both private and government organizations, and thousands of them are expected to be launched over the next decade.
Complementing the satellites, teleports are used on the satellite communications provider’s end. They are fields with plenty of differently-sized antennas that are pointed to several satellites (usually one satellite per antenna, but some antennas could point to the same satellite as well) in order to create satellite networks.
2.3 Very-Small-Aperture Terminal (VSAT)
VSAT is a type of satellite communications using remote antennas with lower diameters than 3.8m (the majority of them are between 0.75 m and 1.2 m) [16]. This satellite communications type is characterized also by the use of the geostationary orbit (GEO) and the use of C, Ku, and recently, also Ka bands (detailed in section 2.3.2.5). Using some teleports and satellites, global coverage is possible. Nevertheless, satellite bandwidth is expensive since satellite cost is high, life span is short (around 10 or 15 years), space segment is restricted, and frequency bandwidth (bands defined by the ITU) is narrow.
2.3.1 Architectures of VSAT networks
Regarding the possible architectures, Point-to-Point, Star, and Mesh setups are usual.
Point-to-Point link is used for concrete services in which the link is dedicated.
Star system (also called hub-and-spoke) is the most frequent in non-dedicated networks which need the control to be centralized (in the HUB).
Mesh is implemented for systems with a few nodes (no more than 30 generally) in which the control is shared. This architecture is installed for specific services in which many point-to-point links are required for reasons like security or privacy, minimum possible latency, failover in case any link fails, flexibility to add and remove nodes, etc.
2.3.1.1 Basic Link Operation
A VSAT link is composed of two antennas, both acting as transmitter and receiver, using a satellite in the middle, whose work could be compared to a mirror, reflecting the received signal from an Earth station back to another Earth station, which can be as far as the satellite coverage allows it. The signal which goes up to the satellite is called uplink and the signal which comes back to Earth is called downlink. They are the two Radio Frequency (RF) links in which any VSAT connection is divided into. Thus, the signal which goes from one point to another on Earth follows the path shown in Figure 2.4.
Figure 2.4 VSAT link paths
On the satellite, there is some analog processing of the signal, mainly translation to a lower frequency (to be seen on section 2.3.2.5) and amplification.
Finally, it is worth mentioning that the two ways of signal, from the teleport to the remote, called downstream, outbound or outroute, and from the remote back to the teleport, called upstream, inbound or inroute, compose the complete VSAT link (Figure 2.5).
Figure 2.5 Name convention for the different parts of the path 2.3.2 Satellites for VSAT
The characteristics of the satellites used for VSAT are explained in the following subsections.
2.3.2.1 Orbits
Artificial satellites can be found describing several orbit patterns: geostationary orbit (GEO), at around 36.000 km above the Earth's surface over the equator, with the same translation period as the Earth’s rotation; geosynchronous orbit (GSO), also around 36.000 km above with the same period as GEO, but not always over the equator, but going some degrees North and South (the deviation in degrees from the equator is called inclination of the satellite);
medium orbit (MEO), between 12.000 km and GEO/GSO distance; and low orbit (LEO), between 200 km and 2.000 km above the surface, among others [17].
The ones used for VSAT communications follow GEO or, at least, GSO with almost no inclination (the lower the inclination, the better). However, there are communication satellites in MEO, as GPS satellite constellation, for example, at around 31.000 km above the sea level, or LEO, Iridium satellite constellation, for example, at around 781 km above.
GEO/GSO satellites have the benefit to be found (almost) at the same point in the sky, with no relative movement to the Earth. However, their distance from the Earth makes the latency to be higher than in MEO and LEO. In addition, they are not able to cover the pole areas due to being under the equator. MEO and LEO are populated by constellation satellites (satellites that operate in coordinated fashion) instead of single ones as in GEO/GEO cases, since the closer to Earth the satellite is, the smaller the satellite visible area, the more satellites are needed in order to cover all of the Earth. Furthermore, the lowest the orbit, the shortest the latency experienced, which can be a good improvement for some applications.
2.3.2.2 Transponders
The RF part of the satellites is divided into transponders. Transponders are the devices which work with a fraction of frequencies.
Transponders, and almost all the RF devices in the path, work with filters. The narrower the pass-band of the filter is, the easier it is to get a filter close to an ideal one, with a flat pass- band, no ripples on the edges and a sharp transition band, this is the goal in this matter. In addition, this way, the parameters of the physical systems which depend on the frequency can be adjusted independently on each transponder, so achieving that all the frequencies work at the same output. As an example, in Figure 2.6 the different transponders for a Ku-band frequency plan is shown.
Figure 2.6 Example of transponders in a frequency plan for Ku-band
Another reason to use that many transponders is the possibility to easily isolate the different polarizations (the electric field direction with respect to the wave trajectory determines the polarization pattern, deeper explained in section 2.3.2.6), allowing the frequency range to be used twice instead of once without adding much complexity to the system. As Figure 2.6 shows, each center frequency in one polarization is exactly in the middle between two center frequencies in the other, thus, the power on the other polarization at that frequency is (theoretically) zero. In order to make it possible, there is a frequency range not used for each polarization at the beginning of the full frequency range or at the end. Therefore, there is a tradeoff in this case: the narrower the transponder bandwidth, the narrower the unused frequency bandwidth and the better and easier the filter implementation, but also the lower the data transmission rate able to be achieved on each transponder. The limitation in data transmission rate becomes critical for the VSAT networks. Every carrier must use only one transponder to be correctly processed, so this parameter limits the maximum carrier to be used, and thus, the maximum data transmission rate for a single network.
2.3.2.3 Satellite Beams, footprints
A satellite beam is the principal lobe of a satellite antenna. Beams are represented by means of radiation diagrams. Footprints show the intersection between the lobes’ radiation and the Earth, so, the radiation from the principal lobe reaches the Earth's surface. Satellites can only cover the portion of the Earth for which they have line-of-sight. Nevertheless, they are not always covering all the zones with only one single beam, but using several different beams each covering different parts. Note that the narrower the beam is, the higher will be the radiation gain due to the use of the same power focused in a smaller zone. However, the signal is neither constant over the whole beam coverage zone, nor sharply falls down to zero on the
edge. The decrease of the signal level is graphically represented in the footprint by means of contours for the same signal-level locations on Earth. Figure 2.7, Figure 2.8 and Figure 2.9 are examples of footprints from different beams (Global, SubSaharan and U25 ones respectively) coming from the same satellite (Intelsat 33e) [18].
Figure 2.7 Footprint of Intelsat 33e (60°E), global beam
Figure 2.8 Footprint of Intelsat 33e (60°E), SubSaharan zone
In addition, in Figure 2.9, the three grey lines describing ellipses are labelled with 0°, 5°, and 10°. This is the elevation angle an antenna placed in any geographical point of the grey line will need to set in order to point to the theoretical position of the satellite. This angle is important since the lower the elevation the antenna must set is, the higher the probability for the signal LoS to be eventually blocked, as detailed in 2.3.2.5. The grey lines are depicted in all footprints for reference without being labelled, as shown in Figure 2.7 and Figure 2.8, since they always mean the same elevation angles.
2.3.2.4 Satellite Measurements
All beams have their representative footprint with the rings describing the same power level zones, as detailed in 2.3.2.3. Every ring limits the area which has at least the designed power level. Rings inside will be more powerful and rings outside will be weaker. Each ring has a particular and absolute power value when dealing with the downlink footprint of the satellite, called Effective Isotropic Radiated Power (EIRP), and another one for the uplink one, called Gain-to-noise-Temperature (G/T). They depend on the satellite systems, and are used to be able to theoretically calculate the signal a receiver antenna is expected to get, based on statistical approximations. Their definitions are stated below, and Figure 2.10 depicts them in an image.
EIRP: it is the power which an equivalent isotropic antenna would need to radiate to reach the same value on the zone [19]. For a better understanding, it is actually the power (absolute value) the antenna radiates towards a designed direction. It is usually provided in dBW.
Satellite providers can give this value since this is the power the satellite will transmit in case it receives the expected one. Therefore, satellite providers let clients know the power the ES must transmit for the EIRP values to be accurate.
G/T: receiver figure of merit. It is the ratio of the receiving antenna gain to the receiver system noise temperature. The noise temperature is not a physical temperature, but an expression of the noise power at the antenna, considering all noise contributions in the receiver system [20]. As an easier description, it is the increase of the signal to noise ratio on the receiver system. It is expressed in dB/K.
For a gain-to-noise ratio, the bandwidth must be considered following the expressions (1) and (2).
𝐺 𝑁
𝐺 𝑘𝑇𝐵
𝐺 𝑇
1 𝑘𝐵
𝐺 𝑇
1
𝑘𝐵 (1)
𝐺 𝑁
𝐺
𝑇 10 log 𝑘𝐵 (2)
Uplink and downlink footprints have different shapes since two different systems are used in the satellite for the reception and transmission of the signal. They are usually similar, but not always that this is achieved. Thus, for any VSAT link budget analysis, two different footprints will be needed to take into account: the uplink one with the G/T values and the downlink one with the EIRP values.
Figure 2.10 VSAT link gains and loses
The power evaluation of a VSAT link, considering the detailed values, experiences the following changes: first of all, the transmitter antenna sends its EIRP. Then, the signal has losses due to the space traversed and some other additional losses, which are explained later in this chapter, to finally reach the receiver antenna on the satellite. Finally, the receiver antenna of the satellite applies its G/T, which increases again the signal power. The same process is repeated then from the satellite back to the receiver station.
Every VSAT system, when it is connected to a network and continuously adapting the settings to remain with the best parameters, has some indicators which tell how good the signal is in both reception and transmission. The most relevant ones in this work are the following:
Carrier-to-noise ratio (also known as CNR or C/N) [21] or signal-to-noise ratio (SNR or S/N) [22], in dB, is the comparison between the power the useful signal or carrier has to the one of the noise.
Carrier-to-noise-density ratio (symbolized by C/No), in dB/Hz, is a normalized measure to be able to compare different transmitters or carriers. This value represents the C/N normalized per Hertz, so that the comparison between different transmitters or carrier requirements can be done easier. Equation (3) shows its relation with the CNR, where:
𝐶 𝑁
𝐶
𝑁 10 log 𝐵 (3)
where B is the bandwidth of the carrier in Hz.
Transmission (Tx) power, in dBm (logarithmic representation for the power, referred to 1 mW), is the power actually transmitted by the VSAT system. Further details are explained in section 2.4.6, while focused on the VSAT equipment.
All of them are important for the transmission path, but only CNR is for the reception path.
Their importance is further explained in sections 2.3.3.1 and 2.4.6.
2.3.2.5 Frequency Bands and their propagation characteristics
The frequencies used for VSAT are located in the electromagnetic spectrum on the SHF band (1GHz – 40GHz), shown in Figure 2.11.
Figure 2.11 Electromagnetic spectrum with VSAT range
There are some propagation effects which affect the VSAT signal in transit. The most important ones are those listed below:
Free-space path loss (FSPL): the attenuation any wave experiences when traveling in the air [23]. As seen in the equation (4), its magnitude depends on the frequency and the distance between transmitter and receiver.
𝐹𝑆𝑃𝐿 4𝜋𝑑𝑓
𝑐 20 log 4𝜋𝑑𝑓
𝑐 𝑑𝐵 (4)
Propagation impairments (blockage): the antennas must have an LoS to the satellite clean of obstacles (no buildings, no mountains, no trees…). This is important to take into account on vessels as the mast or other antennas may block the signal.
Atmospheric behavior (mainly rain and snow fading): rain and snow on the path between the antenna and the satellite may cause signal fading depending on the heaviness. It will also affect the satellite bands differently. Some elements in the atmosphere absorb energy from the signal, and the atmosphere itself (the fact that the signal passes through) attenuates the signal. Also passing through the ionosphere, the signal is affected by disturbances.
In order to minimize the effects, VSAT systems usually use uplink power control (UPC), so the fade can be compensated. The same system is used when moving from one zone to another with different power coverage from the satellite.
The whole SHF band is split into 4 subbands used in VSAT: L-band, C-band, Ku-band, and Ka-band. The different frequencies used for each of them are shown on Table 2.2.
Table 2.2 Frequency range used for uplink and downlink for the different bands.
Frequency Band Downlink Frequency Range (GHz) Uplink Frequency Range (GHz)
L-band 0.95 - 2.15 0.95 - 2.15
C-band 3.70 - 4.20 5.925 - 6.425
Ku-band 10.70 - 12.75 13.75 - 14.50
Ka-band 19.70 - 20.20 26.50 - 40.00
The difference in frequencies has some implications. The throughput that can be achieved and the bandwidth available each band has, both increase with increasing frequency. For a given throughput and/or bandwidth, the higher the frequency, the smaller the antenna diameter. In regards to the susceptibility to fade due to the rain, losses are heavier with increasing frequency. They are all shown graphically in Figure 2.12.
Figure 2.12 Frequency bands with their different implications 2.3.2.6 Polarization
Polarization is one of the characteristics of electromagnetic waves, which is determined by the way the waves travel. Its definition is how the electric field is oriented with respect to the direction of the wave. It is used in order to be able to reuse the frequencies twice instead of once, as shown in the section 2.3.2.2. There are three possible polarization patterns: linear, circular, and elliptical. Their patterns can be seen in Figure 2.13 [24]. However, for VSAT, only linear and circular polarizations are used.
Figure 2.13 Different polarization possibilities
In linear polarization, the signal travels using two orthogonal axes. For VSAT, these axes are horizontal and vertical. In circular polarization, the signal can travel rotating clockwise and counterclockwise, which are known with Right-Hand Circular Polarization (RHCP) and Left- Hand Circular Polarization (LHCP) respectively, when examined from the point of view of the receiver. The two options for each polarization type creates the possibility for the satellites to reuse the frequencies, theoretically duplicating their capacity. Recalling Figure 2.6 from section 2.3.2.2 Transponders, both uplink and downlink, hads half of the transponders for horizontal polarization and the other half for vertical one (sorted in odd and even transponder numbers), which are sharing the same frequency band (Ku-band on this example). A graphical example is depicted in Figure 2.14 and Figure 2.15, in which two circular transmitters and two linear transmitters respectively are sending the signal to the same satellite using different polarizations of the same type. The satellite is able to isolate them and recover both individual signals to be processed independently.
Figure 2.14 Two stations transmitting to the same satellite in different circular polarizations
Figure 2.15 Two linear transmitters sending the signal to the same satellite in different polarizations
Finally, depending on the polarization used for the downlink and uplink from the same station (so depending on the VSAT network in the end), there are two setup possibilities: co- pol and cross-pol. While co-pol setup means that downlink and uplink, from the station point of view, use the same polarization (vertical reception and vertical transmission too, or RHCP for both, for example), cross-pol setup means they use the complementary polarization (vertical reception, but horizontal transmission, for example). Antennas are not always ready to work with both setup possibilities, since they do not have the equipment, so this is something to bear in mind.
2.3.3 VSAT Networks
VSAT networks are created by two different paths, as detailed in section 2.3.1.1. Each of these paths must use a VSAT technology to work.
There are mainly two types of VSAT technologies: Single Carrier Per Channel (SCPC, used for point-to-point links already explained in section 2.3.1) and Multiple Carriers Per Channel (MCPC, used in hub-and-spoke architectures also described in 2.3.1) [25]. While SCPC relies on a single information stream per carrier, MCPC uses multiple information streams in each carrier, being multiplexed at the HUB.
The hub-and-spoke architecture, the most used for VSAT networks worldwide, is based on a HUB transmitting a forward carrier for the downstream, multiplexing the traffic which must reach different remotes. It also plans the return carriers for the upstream (one or more), each of them also multiplexing the information from different remotes, thus, eventually allowing more than one spoke modem to connect, following HUB instructions. Figure 2.16 is an example of this setup. This method is categorized as MCPC for both inbound and outbound paths.
Figure 2.16 HUB&spoke setup
Multiplexing techniques are required to be implemented when MCPC is used. The most used is TDM, already explained in the section 2.1.2. In addition, since several spoke modems are connected to the same HUB, an access technique is also required for the inbound path, from the remote modem to the HUB. The one implemented in the majority of the cases is TDMA, or its variant for some different frequency carriers MF-TDMA [10], also detailed in section 2.1.2. A newest possibility nowadays is Cross-Dimensional Multiple Access (Mx- DMATM), created by Newtec [26], which uses the satellite resources more efficiently, adapting the network to the spoke remotes needs each second [27].
After having described the possibilities, a VSAT network must decide one of them for each path, depending on the needs. While SCPC implementations use the protocol Digital Video Broadcasting (DVB, nowadays its second version DVB-S2 [28] or the second extended version DVB-S2x [29]) for the traffic in both ways, hub-and-spoke architectures use TDM with DVB technologies (also nowadays S2 or S2x versions) for a single forward carrier. For the return carriers in this setup, MF-TDMA has been used recently, however, as already commented, Mx-DMATM is becoming important as a return technology nowadays.
2.3.3.1 Adaptative Modulation and Coding (ACM)
Coding and modulation are usually addressed jointly as only one feature named modcod, as they both constrain the minimum transmission power needed for the signal to be decoded with a maximum probability of error at the other end. Nevertheless, they are two different features used for different purposes, as detailed in section 2.1.1.
There are several modulation types, each one focused on different channel natures. They can be classified mainly in analog or digital depending on the information source data or in amplitude, frequency or phase modulation if taking into account which of these parameters of the modulated signal actually carry the information. Nowadays, there are mixes trying to get improved modulations adding complexity to the system. The one used in satellite connections
are digital phase modulation (Phase Shift Key, PSK), since the great attenuation experienced on the link and the frequency change impossibility make the amplitude and frequency options not adequate.
Coding is noted as a fraction where the information bits over total bits rate is expressed.
As it has been said, the modulation and coding combination used have implications on the link performance. Figure 2.17 depicts the Bit Error Rate (BER) decrease when the Es/No (equivalent to CNR, explained in section 2.3.2.4) increases for different modcods.
Figure 2.17 BER depending on Es/No simulation for different modcods
Since channel conditions are unpredictable when dealing with a radio link, a static modcod assignation would be difficult to set as well as inefficient: the modcod would need to be the highest one to work in the worst conditions (so they will need to be determined), therefore, most of the capacity of the link when its conditions are better would not be used. However, if choosing a more efficient modcod, when the conditions are the worst, the link performance will go down.
ACM dynamically adjusts the modcod to have the highest efficiency on the channel in terms of spectral capacity while aiming at satisfying a prescribed BER constraint. It must be configured bearing in mind that, if the modcod thresholds are overambitious, the decoder may be unable to get the proper information, having too many errors, so the link becomes slower due to retransmissions. Its task is done by monitoring the channel characteristics frequently and adjusting the modcod parameters accordingly in order to squeeze the channel capacity whatever the conditions are [30]. Figure 2.18 shows a graphical example of the mentioned ACM gain over a fixed modcod like the one mentioned above, assuming unstable conditions.
Figure 2.18 ACM gain in changing conditions channel 2.3.4 VSAT Service Contract Details
Commercially, VSAT services divide the contracted data transmission rate into two different parts: dedicated data transmission rate and shared data transmission rate. For this to be further explained, we will start introducing two concepts:
Committed Information Rate (CIR): the guaranteed data transmission rate the VSAT service provider commits to provide to the client.
Maximum Information Rate (MIR) or Peak Information Rate (PIR): the maximum or peak of data transmission rate which the client will be allowed to reach if it is not used by other clients.
o MIR/PIR value must be equal or higher than CIR value.
As long as CIR is not always requested by the client, the non-used data transmission rate for some clients can be used by other clients which request more data transmission rate than their CIR. Then, the CIR is the dedicated data transmission rate for a client, and the difference between the CIR and the MIR/PIR is the shared data transmission rate for this client, since it is not reserved for this client, but shared in the VSAT network.
In addition to all of this, just mention that in shared services (MIR/PIR higher than CIR), MCPC is used, and for the dedicated services (MIR/PIR equal to CIR), SCPC is also a possibility as well as MCPC.
2.3.5 VSAT Earth Stations ES and Earth Stations on Vessels (ESV)
The VSAT stations are composed of several equipment, which is described in the following subsections. Nevertheless, the station is placed in a vessel as per this project concern, so vessel motion as pitch, roll, yaw or change of heading cannot be omitted, this is why stabilization sensors and controllers are required. Dual band antennas (C and Ku) and dual LNB (co-pol and cross-pol) setups are also used in order to have more flexibility in regards to the satellite network to connect to. Table 2.3 is a brief summary of the parts from an antenna, detailing the differences between fixed stations and marine stations.
Table 2.3 Differences between fixed and marine stations.
Fixed VSAT Station Marine VSAT Station
Radome Usually not Yes
Satellite tracking Usually not Yes
Movement compensation (pitch, yaw, roll) No Yes
Multiple antennas to avoid blockage No Yes
Antenna Control Unit Usually not Yes
Interaction with vessel navigational systems No Yes
Reflector Yes Yes
BUC/HPA Yes Yes
LNB/LNA Yes Yes
Modem Yes Yes
In the following subsections, the VSAT equipment used in a vessel is detailed.
2.4 VSAT Equipment for a vessel
The devices required for a VSAT installation are usually divided into the outdoor devices or Above Deck Equipment (ADE) and the indoor ones or Below Deck Equipment (BDE), where the ADE is the antenna or antennas with all its/their equipment (Reflector, BUC, LNBs…) and the BDE are the Antenna Control Units (ACUs), the switching unit, if any, and the modem, as Figure 2.19 shows.
Figure 2.19 ADE and BDE in a vessel
The function of each of the devices is going to be described in the following subsections.
2.4.1 Setup possibilities
An antenna system can have different configurations. The simplest one is when a single antenna system is used, so only one antenna with its ACU working directly with the modem, as shown in Figure 2.20.
Figure 2.20 Single antenna system
The disadvantage remains in the fact that this unique antenna must have 360° visibility in azimuth for 0° to 90° in elevation clear line of sight, otherwise, a blindspot to the satellite through which the connection is reached could be experienced anytime, generating an outage.
Due to the fact that the clear line of sight over the sky (360° in azimuth and 0° to 90° in elevation) is almost impossible to achieve, single antenna systems almost always have the risk to get into blockage at some point (a mast, another antenna, a radar, etc.). Nevertheless, depending on the shadow zones the antenna has, the probability that the satellite is actually blocked is low (in case of a fine mast, a short antenna, etc.). If the risk is too high from the customer’s point of view (a big mast, sails, etc.), a dual system can be installed, so the complete system’s blockage zones are reduced notably. Dual systems, apart from having two antennas instead of one, need a switching unit which decides which of the antennas is being used every time. This setup is shown in Figure 2.21. The antennas themselves work independently (any antenna model works), trying to reach the satellite signal as if a single antenna system is installed. However, the switching unit, also called arbitrator or mediator, is the one which decides which of the reception signals passes to the modem and which of the antennas will receive the transmission signal to send to the satellite, making the standby antenna be ready to work, but discarding the signal it receives and making it transmit nothing.
Figure 2.21 Dual antenna system with arbitrator/mediator
There are some antenna models which are created to work without an arbitrator/mediator as an isolated device when a dual system is created with them. As shown in Figure 2.22, they follow a Master-Slave setup, in which one of the antennas, the master one, in addition to the antenna task, takes also the arbitrator/mediator job, deciding which antenna must be active every time. The slave antenna only does the antenna work, receiving the orders from its master one.
Figure 2.22 Dual antenna system with Master-Slave setup
All kinds of antennas are able to work in a single antenna system and dual antenna system.
Depending on the antenna model, one or the other dual antenna setup will be implemented.
Nevertheless, bear in mind that in dual systems, regardless of the implementation, both antennas must work the same way, with the same gains and losses, the same parameters sent and expected to be received, since the modem does not take care of the antennas setup.
The easiest way to achieve this is by means of using the same antenna model and software version for both antennas (actually, this is the way it is done). Anyway, if a difference in performance or parameters exists, it must be adjusted using the software possibilities (offset adjustments, for example) or adding hardware (physical attenuator devices) for the system to work smoothly.
2.4.2 Above Deck Equipment
ADE is composed by the antenna itself and its parts, which are described in this section.
The structure is composed of the two elements listed below:
Radome: protection for the antenna, which allows the RF signals to pass through.
The antenna is installed inside.
Stabilized pedestal: antenna base.
Regarding the RF, the parts are the following:
Antenna reflector: dish which reflects the received signal from the satellite to the feed and does it in the opposite way for the transmitted one. Beamwidth decreases as the size of the reflector increases.
Feed horn assembly: RF part which directs the transmitted signal to the reflector and concentrates the one received from it. Transmitted and received signals travel together using its structure. As shown in Figure 2.23, the phase center of the feed horn is installed on the focal point of the reflector for an efficient usage of the dish.
Figure 2.23 Feed assembly and antena reflector
Block Up Converter (BUC): composed of an upconverter first, which moves the frequency up from the one received from the modem (intermediate frequency in L- band) to the one which is required to be transmitted by the antenna (RF in C, Ku or Ka band) using an internal local oscillator (LO), and then, a power amplifier,
which amplifies the signal prior to be transmitted. Note that each BUC has only one fixed LO, thus, they are manufactured to work only for one satellite band each.
Low Noise Block (LNB): analog device to the BUC for reception, composed by a Low Noise Amplifier (LNA) first, which amplifies the signal received from the satellite, and then a downconverter, which moves the frequency back from the one received from the satellite (RF) to a lower one (L-band) using an internal LO. The LO embedded in the LNB downconverter can have a fixed frequency, limiting the bandwidth the received signal must be in, or a programmable one, so that it can adjust the down conversion for the result to be a L-band frequency. Multiband LNBs choose one of the four LO pre-programmed frequency availabilities by receiving a voltage (13V/18V) and a tone (0kHz/22kHz) from the antenna controller.
Waveguides: structure that guides the electromagnetic waves with almost non- existent loss at microwave frequencies.
Diplexer: combiner for two signals of the same polarization.
Orthomode transducer (OMT): waveguide component that separates or combines two orthogonally polarized microwave signal paths.
Tx reject filter: part which filters out the high transmit frequencies from the BUC to prevent it from interfering with the received signal entering the LNB.
Rotary joint: particular waveguide which permits the physical structure to rotate.
Figure 2.24 RF path in the antenna
Due to the nature of the vessels, changing their location and having rolling movements while not sailing, the azimuth, elevation and polarization angle, moves. Therefore, stabilization elements are required on the antennas to be able to get constant signals, as sensors and references to detect where the antenna is pointing to, and motors with encoders which make the antennas move in all three axes, as Figure 2.25, Figure 2.26 and Figure 2.27 show.
Figure 2.25 Azimuth axis rotation
Figure 2.26 Elevation axis rotation
Figure 2.27 Cross level axis rotation The elements are the ones listed below:
Home sensor: this is the reference in azimuth for the antenna to know when it is aligned to the bow of the vessel. An offset can be adjusted if it is not exactly installed.
Tilt sensors: typically used in elevation and cross level axis to know when the antenna is in parallel to the horizon and detect errors between the desired pointing angle and the real pointing angle.
Rate sensors: they provide movement information of the antenna. Typically one per axis.
Motors: one per axis installed. They provide the movement required to the antenna axis they are taken care of.
Encoders: can be separated or built-in, give a digital reading of the motor movement.
Finally, in order to concentrate the information or send the commands to each single device, and communicate to the ACU, there is a control unit inside the antenna radome.
2.4.2.1 Blockage and Hazard Zones
Antennas may have pointing directions towards which their transmission is disabled by software. The reasons for setting this configuration are safety or optimization in dual systems.
BUCs are powerful and it is not safe that an antenna transmits towards a zone where people may be in. This is the reason for the hazard zones. However, since antennas are usually installed in the mast and no one is expected to be around that zone (above 0° elevation on the antenna), usually no hazard zones are configured. In addition, there are directions in which the antenna is blocked by vessel structures. If dealing with a single antenna system, blockage zones are hardly ever configured, since it is better to have bad signal than none. Nevertheless, when a dual system is implemented, they are fully set in order for the antenna switch from the arbitrator/mediator or master antenna to work smoothly. In any of these cases, the zones are configured to prevent the antenna transmission while pointing to any direction inside them.
Reception is not a problem, since the power is many dBm lower (around 100dB difference).
Furthermore, reception is coming from the satellite, so there is nothing to do on the antenna for the zone not to receive this radiation.
2.4.3 Below Deck Equipment
All the VSAT equipment installed in a room, usually using a rack, is named the BDE.
The devices which compose it are the ones described below.
ACU: this is the device which interacts with the control unit inside the radome and communicates to the modem, arbitrator/mediator or master antenna depending on the setup (discussed over section 2.4.1).
Arbitrator/Mediator (only in dual systems with switching unit setup): device which selects one antenna or the other. It has a selector to use one antenna manually or let it switch automatically based on blockage zones configured or signal level received.
Satellite modem: it is the endpoint of the satellite link. Its tasks are not only to modulate and demodulate the signal, but also to encode/decode it. Then, it acts as a transducer between its Ethernet port(s) and its satellite interface.
Other equipment (LAN, VoIP, etc).
ACUs, arbitrator/mediator and satellite modem must communicate for the system to properly work. This communication can be implemented serially or, in case both modem and antenna system allow it, via Open Antenna to Modem Interface Protocol (OpenAMIP) through
