SATELLITE COMMUNICATION
INTRODUCTION
In recent year satellite communication have begun to face competition on from optical fiber system. for point to point communication between large concentrated traffic source . To retain a competitive edge ,it has been necessary to develop various new technologies .Thus phenomenal growth of few satellite technology continue the major growth now being in those area where satellite can provide unique advantage . Such applications include service provision directly to customers using small, low cost earth station s; mobile communication to ship; aircrafts, land vehicles and individuals; direct to public television/sound broadcasts and data gathering /distributing from widely distributed terminals, such as video distribution, service providers are combining the benefits of the both technology to produce the best solution to the user needs.
SATELLITE SYSTEM ARCHITECTURE
The system offer communication services to mobile user with the other mobiles or with fixed user through one of the visible satellites. User in fixed network are accessed through large fixed station called gateways which carry large amount of traffic. mobile terminals mounted on vehicles such as ships,airdraft,trucs or carried by individuals.
SPACE SEGMENT :
It consist of one or more satellies,.Telementry and control ground station , used for monitoring and controlling satellites ,a part of the space segment.
to simplyfy the mobile terminals ,complexity is shifted to the space segment and hence satellite tend to be large
SECOND GENERATION:
3-4 W geo-stationary satellite with 5-10 footprints(spot beams)
THIRD GENERATION:
5KW geostationary with 100 - 200 spot beams.
GEOSTATIONARY SATELLITE
It remains almost fixed with respect to the earth and comprise one or more static footprints.Therefore fixed stations can operate with a single antenna with minimal tracking while network topology is simple.
Third generation system sometimes called "super geo system" to distinguish them from second generation system because it provide voice & multimedia services to hand hald devices or desktop size terminal.
NETWORK CONTROL STATION(NCS)
It manages traffic flow ,broadcast network information ,Manage all call setup/de-assignment and radio resources.
On receiving a call request ,the NCS searches assign a channel from its pool, returning it at the and of the call.
HOME LOCATION REGISTER(HLR) & VISITOR LOCATION REGISTER:
It manages user mobility
NETWORK CONTROL CENTRE:
It controls the network.
SATELLITE CONTROL STATION
It manages and controls the satellite performance.
BUSINESS MANAGEMENT CENTRE
It is responsible for billing and other business functions.
HANDOVER
The architecture of a non geo stationary satellite system is more complicated due to movement of satellite beam relative to the user .Therefore a call could be routed through multiple beams and satellites .This routing is known as Hand Over.
ROUTING OF A CALL
Routing of a call is achieved by number of techniques
1.Using Satellite And Grounds Hops
2.Using Inter Satellite Links
STORE AND FORWARD SYSTEM:
Non real time system communication and discontinuous coverage is acceptable.
There are two techniques
1.Using Ground Station
2.Using Satellite links
SOME OF THE TECHNICAL CONSIDERATION:
Orbit
Tolerable Delay
Connectivity
Satellite access
Radio Link Characteristics
FUTURE ENHANCEMENT OF SETELLITE COMMUNICATION
Future telecommunication needs to comprise a mix of services requiring through puts of a few kilobits per second to hundreds to megabits per second
A flexible transport mechanism is called ASYNCHRONOUS TRANSPORT MECHANISM.
ROUTING OF A CALL
In the above figure first visible satellite transfer call to the ground fixed terrestrial network which transfers to the another satellites which is directly connected with link to the destination of the call. This technique is very useful for the non geo-stationary satellite system because they are not inter connected with the inter satellite links.
In above figure A Call may be routed through inter satellites links .In this technique first call transfer to the visible satellite this satellite transfer to the other satellites which is connected with inter satellite links to this satellite which is visible to the destination fixed or mobile devices.
STORE AND FORWARD SYATEM
This store and forward system . a discontinuous coverage is acceptable. First message is stored in the satellite buffer system then satellite moves from the sourse of the messages whenever the destination are in the visibility of the satellite then message is transfer to the destination.
In this method storing capacity provide very less storing of messages because
satellite having very less memories. and it is very costly.
Main Entities of Store & Forward System With Ground Station Buffer
In this system satellite transfer the messages to the ground earth station to storing the messages then terrestrial network transfer the messages to the visible satellite, which is directly connected, to the destination by the direct link.
In this system memory storing capacity is very much compare to satellite buffer system so this is very economical message transferring.
Long time message storing is done in this system easily because storage is large so for long time it stored in the storage .
TECHNICAL CONSIDERATION IN DETAIL
ORBIT:
Orbit are categorized by Altitude, Inclination, And Accentricity.The altitude determine the area covered by the satellite – higher altitude satellites cover a large region. Inclination
Influences the minimum – maximum latitude covered on the earth .The two extremes of orbital inclination are an equatorial orbit ,which has a inclination of 0, a polar orbit , which has an inclination of 90.A low earth equatorial orbit would cover a belt around the equator ,whereas polar orbit would cover a belt around the pole orthogonal to the equator ,thus covering the full earth due to the earth’s west-east motion beneath .
TOLERABLE DELAY:
Delay tolerated by a system is application dependent . An end to end delay of more than 400 ms is quit distributing in a conversation ,but delays of minutes or hours are acceptable for e-mail delivery. The tolerable time delay influences several features of a mobile satellite communication system .A non real time system can tolerate a break in the communication link by data recovery techniques ,whereas in a circuit mode service, a continuous end to end connection must be maintained .
Non real time system systems use packet switching where packets and messages are stored on buffer in intermediate nodes of a virtual links and transferred when condition are favorable.
For real time system ,
Ignoring RF link blockage due to obstruction satellite visibility remains uninterrupted as user operate static footprints. The boundaries of spot beam coverage are fuzzy, extending several tens of kilometers which leads to graceful degradation of the signal. Quality. for a slow moving vehicle. Thus slow moving vehicle do not require hand over , but fast moving vehicle hand over is required.
In non geo-stationary satellite systems,
Satellite visibility variable due to dynamic footprints. Therefore , a user is likely to communicate through different during a call, making hand over is essential for the network.
CONNECTIVITY
The network must be able to locate a called mobile and route a signal parties as efficiently as possible to maintain the call.
In ,derivatives of the GSM each mobile is registered in a database ,called the home location registered (HLR) if mobile migrates outside the home territory ,the mobile registered itself to the visitor location register (VLR).The visitor reg. Convey the mobile registration to the it’s HLR. Whenever the call is addressed to a mobile ,the mobile switching center interrogates the mobile’HLR and mobile’s VLR for the location of the mobile node.
SATELLITE ACCESS
In MSS environment, thousand of users share the satellite resources therefore satellite access efficiency is paramount .IN TDMA scheme , where pool of channels is shared by all users on a per call or packet basis. the channel pool can be managed by CENTRAL,
Or DISTRIBUTED ARCHITECTURE. In central architecture ,a pool is managed by centrally, whereas in distributed architecture ,separate pools are assigned to each participating fixed station for self – management
CDMA scheme offers an advantage of interference and multimedia mitigation ,soft handover and capacity. A number of modern systems have proffered CDMA.
In he recent year ,with proliferation of data application ,a scheme known as asynchronous transfer mode(ATM).which allows flexible combination of various types of data at the link layer , has been introduced.
RADIO LINK CHARACTERISTIC
Techniques for improving radio link reliability include use of robust modulation and forward error correcting codes, fade countermeasure embedded in the system architecture, stored and forward technique to support communication in deep fades adaptive power or code rate control.
Radio link considerations also include the selection of an appropriate frequency band and interference management. The selection of hand is governed by ITU’s Radio Regulations in combination with engineering and commercial considerations. At present, most MSS systems operate in the L (~1.5GHz) and S(2 GHz) bands and a few systems operate in the Ku band. The L and S bands are well suited for mobile communication because of relatively benign propagation characteristics and mature technology. In the future, it is likely that higher frequency bands will be used due to severe overcrowding in the 1-2GHz bands coupled with a need for a larger bandwidth in support of higher throughput
A NETWORK ARCHITECTURE BASED ON OSI MODEL
A network architecture based on the OSI model is well suited for defining and standardizing a mobile system. An agreed standard allows manufacturers, network operators and service providers to develop products independently, yet allows them to be integrated into any operating system. Consider the example of the terrestrial GSM system. Its network standards interfaces are used by several operators in various parts of the world, allowing subscribers to use their phone in partner networks. Due to wide acceptability of standards, many MSS operators, such as Iridium, Global star and ICO, adopted a similar network model, simplifying integration of their system with terrestrial GSM systems. Figure shows a generic network model of an MSS system in an OSI hierarchy.
KEY
A/S =Application of service LACs = Space segment link access control
CCt =Terrestrial call control LACt =Terrestrial link access control
CCs =Space segment call control R =Radio access
MM =Mobility management T = Terrestrial access
RRM=Radio resource management RFM = Radio frequency monitoring
MOBILITY MANAGEMENT
The satellite networks must provide functions such as:
· Call handling
· Switching
· Mobility management
· Radio resource management
· Network management:
· Privacy, security management:
Figure shows depicts the main functional entities of a commercial MSS, segmented broadly by their physical association.
Space segment
It comprising of the one or more satellites, provides the desired connectivity between the fixed and mobile segment .Some of the functions shows belonging to the terrestrial part can be either wholly or partially performed by the space segment. When regenerative transponder are used some of the network management functions such as call may be transfer to space segment.
Constellation management
It involves standard telementry and telecommand (TT &C) functions for satellite health monitoring, controlling, spacecraft orbit raising, orbital adjustment in the case of a spacecraft failure, launch support during initial deployment or replacement of failed spacecraft, etc.
Network management (NM)
It’s function deals with real-time radio resource management, monitoring the radio spectrum, signal quality, network traffic flow, collection and dispatch of call data records to the business management system, interaction with the mobility management system for call set-up etc., radio resource and management, fault finding/diagnosis and fraud detection. NM functions provide call data records, user profiles and other user-related information to the business management system.
Mobility Management System:
It maintains user location in the user database and interacts either network management or gateways fro the call connection and user authentication, user profile etc,
Business Management System:
It constitutes a company’s business center and is responsible for the customer billing for the space segment usage, interfacing with gateways or the network management system to obtains call records ,updating user profile of existing user and add new profile of new user’s.
Mobile Services
It represent the various services on offer to mobile user to support of associated protocol. The current portfolio of services include data ,voice , facsimile, paging message delivery, and emergency call, supporting services are call transfer ,call forwarding, call waiting , call holding.
Mobility management of mobile systems requires a number of uniqye features, summarized as follow:
· Network connectivity, whereby calls are delivered to a mobile anywhere within the service area. The functionality is generally achieved through location registration, paging and call establishment. Location reporting by a terminal to facilitate to call establishment, Paging involves transmitting a message to verify the current location of a mobile prior to the establishment of a call. In general, a call connection comprises terrestrial and satellite components. We will see in a following section that there are wide variations in routing strategies.
· Roaming is a network function which allows users to migrate to other networks or to a mobile switching center (MSC) other than its home MSC.
· Handover involves handling live calls from one spot beam (or cell ) to another or from one satellite to another. Handover may be necessary at a mobile or a gateway.
The architecture of a mobility management system for MSS depends on a number of factors:
· Number of spot beams: The complexity of a mobility management system increases as the number of spot beams increases.
Satellite motion: Mobility management for LEO/MEO systems, where satellites themselves are moving, requires special features.
· Multiple operators: When gateways belong to different operator, the mobility management system must provide suitable provision for roaming between them.
· Extent of integration with terrestrial system: Depending on the extent of integration, architectural features to manage internetwork mobility gave to be introduced.
HANDOVER
It is the process of changing beams, satellite or gateway during a call. It may be necessary for a number of reasons the communicating satellite may move below the specified elevation; a better satellite path may be available when diversity is used; the user may move out of a spot beam coverage; the network resource manager may decide to divert capacity to a less congested beam; satellites may move out of visibility of feeder stations; when intersatellite links are used, handover may occur between satellites. Due to a static link geometry, handover management is simpler for a geostationary satellite system than for a non-geostationary satellite system. To minimize or eliminate handover between spot beams, non-geostationary satellites may deploy a quasi-stationary spot beam system, wherein the spot beams of each satellite remain fixed to predefined points on the surface of the Earth are vacated by the old beam. This type of Earth until the satellite moves below visibility, at which time a new satellite replaces the area vacated by the old beam. This type of Earth-fixed arrangement simplifies frequency planning but requires rapid, accurate and synchronized steering of spot beams on satellites. Figure shows various types of handover.
CALL HANDLING
In the GSM system, each mobile registered to the network is assigned a home area, User service profile, location, billing and other data are maintained in the home location register (HLR) in the home area of each mobile. When a mobile migrates outside its home mobile switching center (MSC), it registers itself with the visited MSC. The mobile identification is entered in the visitor location register (VLR) of the visited MSC and the information is communicated to the HLR, which updates the mobile’s location details. Whenever the mobile is called, the gateway where the call is placed interrogates the mobile’a HLR for location and other information for authentication and establishing appropriate routing.
A call originated by a terrestrial party is received by the MSC: the MSC interrogates the HLR service validation , authentication, location of the mobile and mobile status- busy or not Assuming that mobile is authenticated and available , the interrogation provides the location of the mobile ,and if the mobile ahs roamed to another area , address of the VLR is the location of the gateway .The radio resources manager sends a call announcement on the broadcast channel at the appropriate location of the network .If there is no response by the mobile within a specific time terrestrial party gets an unobtainable message by return signaling message. When the mobile responds to the call announcement , the radio resource manager assign resources to the gateway and the mobile. Following allocation , the mobile receives a notification such as ring ;when the devices off-hook ,a handshake protocol ensure radio connectivity. The call then proceeds ,after exchange of application-specific protocol. Any other exchanges of signaling for function such as power control ,handover ,end of call are conducted “in-band” i.e. within the assigned channel
When a call originated at a mobile a request is sent on a shared pre-assigned channel ;the mobile identity and service request are authenticated by the network by interrogation of the mobiles HLR, which maintains it’s services profile. After the authentication , the radio resource manager proceed to allocate the appropriate resource to a gateway.
APPLICATION OF MOBILITY MANAGEMENT SYSTEM
The third generation INMARSAT system uses 4-5 spot beams per satellite. Therefore it becomes essential to locate mobile for placing a call. Land and maritime systems use similar techniques ,whereas the aeronautical system which has a distributed system .In the land and marine system a bulletin board is transmitted in the global beam ,from which mobile reads frequencies of each broad cast channel are hard coded into mobiles and are fixed ,whereas changes to the spot beams identifying frequencies are possible through update to the bulletin board. For fixed to mobile call ,the mobile is paged by the NCS on the global broadcast to which all idle mobiles are tuned .On receiving a call announcement ,the mobile responds with its spot beams identity, which it derives from the spot beam identifier .The NDS assign the channel in the mobile spot beam. The size of the spot beam is governed by the level of spot beam identifier .When mobile call it request a channel in its present spot beams .The NCS then proceed to established a call in the selected beams. If all the channels for the spot are busy , a channel is borrowed from the global beam ,provided the services can be supported in the global beam. There is no beam—beam or satellite – satellite handover.
ATM OVER SATELLITE
Future telecommunication needs are expected to comprise a mix of services requiring throughputs of a few kilobits per second to hundreds of megabits per second.
Traditional transport mechanisms – circuit and packet modes cannot provide the flexibility required for supporting users with variable bandwidth/throughput requirements. A flexible support mechanism , known as the synchronous transport mechanism was developed for the purpose, with aim of implementing the Broadband-Integrated switched data network(B-ISDN).,which is anticipated to offer service ranging from voice to multimedia .
The recent popularity of Internet and intranets has created a tremendous interest in the underlying packet routing protocol , known as transmission control protocol /Internet protocol .Hence is great industry in the evolution of ATM vis a vis IP , it seems ,is indispensable for some application .
IP ia layer 3 protocol where ATM is based on layer 2 cell switching protocol .ATM multi services transport platform, whereas IP is developed for data and is currently not suitable for voice.
ATM is connection oriented protocol where as IP is connection less protocol.
AS far as satellite concerned ,both technologies may be supported over satellite channels but neither has been optimized for satellite delivery. Nevertheless ,ATM seems set to be an influence on next generation transport ,evidenced by adoption of the technology by a large number of services providers through out the world. .IP/ATM is a network supporting technology .It is anticipated that IP,ATM or a combination of the two will form the backbone of next generation broadband services with assurance of the quality of services and bandwidth on demand giving efficient resource utilization.
The terrestrial ATM is assumed to be transported on currently used terrestrial transport technologies ,such as Synchronous Optical Network/Synchronous Digital Hierarchy(SONET/STH) ,Plesiochronous Digital Hierarchy(PDH) and Physical Layer Convergence Protocol(PLCP). . The ATM network interface with the satellite network through the ATM satellite Internetworking Unit(ASIU.). The ASIU manages and controls key systems resources ,such as band with network access ,timing, call monitoring, traffic control
The terrestrial frames containing ATM cells are received at the front end, ATM cells are extracted ,suitable error correcting scheme are applied ,and cells are classified according to traffic class ,prioritized and placed in buffer .further error protection by a way of interleaving is applied for improved CLR performance and the signal transmitted using the chosen multiple access scheme.
Band With on demand satellite services require satellite resources to be assigned to users on a request basis.
Figure shows an architecture of a multi service satellite network which can support five different traffic resources –ATM ,Frame Relay, ISDN, SS7 & and IP
FUTURE SCOPE
To offer Giga bytes of information rate , several Giga-Hertz of spectrum will be necessary ,which can be available above 53 GHz ,and this where very regulatory decisions are necessary, coupled with large investment to develop the necessary hard ware ; large segments of continuous spectrum could provide optical fiber like throughputs. Wideband CDMA could permits large numbers of simultaneous accesses .Coupling this with advantage of onboard processing satellites, advanced spot beam technology and improved coding technique, it is estimated that up to a hundred-fold frequency reuses possible ,giving several hundred GHz of spectrum in the 35-50 GHz band.
An FSS-MSS integrated architecture is conceivable in the foreseeable future ,allowing a user to roam between a home ‘Giga-byte’ dish and personal communicator while away on the beach holiday
CONCLUSION
A vision of telecommunication in this century is the global information Infrastructure(GII).-a high capacity back bone network .The network will comprise a plethora of wired and wireless systems, which would to gather meets service needs economically and with the desired flexibility and mobility. Within such a mashed broadband network comprising optical fibre and terrestrial wireless communication system ,satellite systems are considered as one of the weakest link in the terms of bandwidth ,throughput ,quality of service , and latency.
Consider the current limitation of the current5 MSS system:
· Excessive transmission delay for GEO and MEO system when compared to the optical communication.
· Severe shortage of spectrum.
· Inability to provide consistently high signal quality.
· Susceptible to rain fades at mm waves ,scintillation and shadowing.
APPENDIX-A: GLOSSARY
· MSS=Mobile Satellite System
· HLR = Home Location Register
· VLR = Visitor Location Register
· SCC=Satellite Control Center
· NCC=Network Control Center
· BMC=Business Management Center
· NSC=Network Control Station
· MSC=Mobile Switching Center
· ATM= Asynchronous Transfer Monde
· ASIU=ATM Satellite Internetworking Unit
· SDH=Synchronous Digital Hierarchy
· SONET=Synchronous Optical Network
· PDH=Plesiochronous Digital Hierarchy
· PLCP=Physical Layer Convergence Protocol
TO DOWN LOAD REPORT AND PPT
