INTRODUCTION
To make a satellite phone call today from a location that does not offer terrestrial wireline or wireless coverage requires the use of a large, costly terminal, and entails very high per minute charges. Further, the quality of service is relatively poor because of annoying echoes, large transmission delays, overtalk associated with satellite communications using geostationary satellites. The next generation of satellite communication systems will use advances in satellite systems, wireless technologies, and miniaturization, to provide global mobile satellite services that will make calls between any two locations on earth much easier, much more affordable and much more user friendly.
Even in the year 2000, the terrestrial cellular coverage is available to less than 60% of the world’s population and only about 15% of the earth’s total surface. More than 3 billion of the world’s population have no phone service. The waiting list of landline telephone service has over 50 million names with the average wait greater than 1.5 years. Rural areas, regions, that are sparsely populated in developed countries and large parts of the developing world are destined to be underserved or to remain out of reach of terrestrial mobile services altogether. Thus, in many parts of the world, the demand for communications mobility can be met effectively only through global mobile satellite services. Handheld satellite phones are therefore forecast as the emerging mobile communications frontier with growth that could parallel recent growth in cellular mobile industry. Regardless of how you look at the numbers, there is a significant amount of people without phone service throughout the world. Mobile Satellite communication services will solve the need of worldwide travelers and provide phone services to many areas of the world that currently do not have phone service. The emerging next generating mobile systems are generally referred as GMPCS, for Global Mobile Personal Communication by Satellites.
Until now Communication Satellites have operated using Geo-Stationery Orbits (GEO), lying above 36,000 kilometers above the earth’s surface. From this Orbit the satellite appears to be stationery (fixed) above a specific location from earth, thereby ensuring continuous, uninterrupted coverage to that location. The primary role of a geostationary communications satellite is to act as a wireless repeater station in space that operates in a broadcast mode and provides a microwave link between two remote locations on earth. The key components of a communication satellite include various transponders, transceivers, and antennas that are tuned to the allocated frequency channels. Although the Geostationary Satellites have a large footprint, so that the entire surface of the earth can be covered by few such satellites, their high altitude leads to very long roundtrip signal delays and resultant degradation in service quality.
There is a trend for mobile satellite system architectures aimed at the deployment of multi-satellite constellations in Non-Geostationary Earth Orbits (NGEOs).This allows the user terminals to be small size, low cost and having low power demand.To enhance the coverage and quality of service, Low Earth Orbiting (LEO) constellations are usually selected. To supports a wide range of services and to provide superior service quality comparable to that available from terrestrial wireless and wireline networks, constellations of satellites operating in Low Earth Orbits (LEO) or Medium Earth Orbits (MEO) are considered more suitable.
A number of various global mobile satellite communications systems have already been in development stages. With the first global mobile satellite services initiated in 1998. The four such systems that are in advanced stages of planning or early implementations are Iridium, Globalstar, ICO and Teledesic.
The era of satellite-based mobile communications systems started with the first MARISAT satellite which was launched into a geostationary orbit over the
Communications.The system architectures of each system are presented along with a description of the satellite and user handset designs, the multi-access techniques employed, and an analysis of their respective cost structures.It is concluded that the technical feasibility of satellite-based mobile communications systems seems to be secure. It will be challenging however, for the vendors to actually develop and deploy these systems in a cost effective, timely, and reliable way that meets a continually evolving set of requirements driven by user expectations fueled by a rapidly changing technology base.
In order to guarantee the service quality and reliability for mobile satellite communication systems, we have to take into account outages due to obstruction of the line-of-sight path between a satellite and a mobile terminal as well as the signal fluctuation caused by interference from multipath radio waves. Thus, we need a good characterization for the satellite propagation channel. It is commonly accepted that satellite communications systems (in particular,low earth orbit LEO systems) are the de facto solution for providing the real personal communications services (PCS’s) to the users either stationary or on the move anywhere, anytime and in any format (voice, data,and multimedia).Satellite communication systems have provided international telecommunications services since the 1960’s. These systems were augmented in the 1970’s and 1980’s with regional satellite systems, national systems, and private network-based very small aperture terminals (VSAT’s). Throughout this period, systems have been based exclusively on satellites in geosynchronous orbit communicating with earth stations using high gain fixed antennas. As the systems have evolved, the original 30-m-diameter Intelsat earth stations have evolved into 1.2-m -band VSAT’s for business and home TV usage, but the basic system architecture explaining a geosynchronous spacecraft has not changed during this period. With the launch of the first Iridium spacecraft in 1997 and 1998, a significant new architecture has been introduced into the field of satellite communications. These systems are based upon the use of LEO and medium earth orbiting (MEO) systems. hese LEO and MEO systems have several advantages over geosynchronous systems. The most significant advantages are:
1) The reduction in range provides a large decrease in path loss resulting in much small receiving antennas and
2) The reduction in range provides a significant reduction in propagation delay making voice conversation more pleasing to the user and increasing the throughput of most data communication protocols. These systems can and will serve mobile and portable users
With small near omni antennas.
However, the use of the small antennas as well as the motion of the transmitter and the receiver introduces the possibility of multipath and path blockage into the link budget of these satellite systems. Moreover, the propagation channel will be time varying due to different shadowing and scattering phenomena, so traditional channel models may
not work well.This is concerned with the statistical modeling of the propagation characteristics of LEO and MEO systems. Since in these systems, satellites and mobile users are all allowed to move during communication sessions, the channel characteristics
will be different from the geostationary systems (GEO’s). Due to the movement of receivers or transmitters, the received signals may fluctuate very rapidly from time to time. This fluctuation results from the combining effects of random multipath signals and obstruction of the line-of-sight path, which induces various fading phenomena. The communication quality of service(QoS) parameters such as the word-error rate will be affected in great deal in such communication environment. For effective mobile satellite communications system design, we must quantitatively know the propagation characteristics such as signal fading due to reflection; shadowing from trees, buildings,utility poles, and terrain; Doppler effects due to movement of mobile terminals, mobile satellites, or the communication effects; and other effects such as the rainfall. Such characteristics can be studied by the statistical distribution of the received
signal envelope or received power in mobile communication systems.
MOBILE SATELLITE COMMUNICATION SYSTEM CAN BE BROADLY CLASSIFIED BY ORBIT PERIOD
1. Mobile Satellite Communication System by the Geostationary Satellite
The geostationary satellite is the artificial satellite which looks stationary from the ground. 3-4 geostationary satellites can cover almost the entire surface of the earth. Most of the artificial satellites actually used for communications or broadcasting are geostationary satellites.
- i. Altitude: about 36,000km
- ii. Orbit: the circle orbit cycle on the equator is the same as the earth's autorotation time.
- iii. Number of Satellites: four (service areas are duplicated.)
- iv. Principle Satellite System: Inmarsat Communication System, N-STAR Communication System, Omunitrucks Communication System
2. Mobile Satellite Communication System by the Quasi-Zenith Satellite
The quasi-zenith satellite is an artificial satellite of the satellite system where one satellite always stays near the zenith in
- i. Altitude: about 36,000km
- ii. Orbit: circle orbit crossing with the equator by the angle of 45 degrees
- iii. 3 as the minimum
- iv. The research and development of the satellite communication system is in progress
3. Mobile Satellite Communication System by the Non-Geostationary Satellite
This is roughly divided into three kinds of orbits: highly elliptic orbit, medium earth orbit, and low earth orbit. The medium and low earth orbits have lower satellite altitudes to shorten the radio transmission delay, enabling more speedy and smooth communication. Specifically, the highly elliptic orbit can obtain a higher elevation angle. It is currently being researched and developed.
i. Highly Elliptic Orbit (HEO)
- Altitude: about 40,000km
- Orbit: about 5-6 hours
- Number of Satellites: 2-3 as the minimum
- The system planning is in progress.
ii. Medium Earth Orbit (MEO)
- Altitude: several thousand - 20,000km (about 10,000km)
- Orbit: about 5-6 hours
- Number of Satellites: 8-10 (for the entire world)
- The system planning is in progress.
iii. Low Earth Orbit (LEO)
- Altitude: 500km - several thousand km (about 1,000km)
- Orbit: about 5-6 hours
- Number of Satellites: several dozen (for the entire world)
- Principle Satellite System: Globalstar Mobile Satellite Communication System, Orbcomm Mobile Satellite Communication System (IRIDIUM Mobile Satellite System (abolished))
Satellites in GSO
GSO satellites orbit the Earth in the equatorial plane with the same angular velocity as the Earth at a height of about 36 000 km above the equator. Geostationary satellites therefore appear stationary to an earth-bound observer and a single satellite can provide continuous service to roughly one third of the Earth's surface (but excluding
polar regions above ± 75 degrees of latitude). The maximum distance the satellite can "see" on the Earth's surface is about 42 000 km and means the propagation delay for a single hop via the satellite (once up and down) can be up to 280 ms. Geostationary satellites also move about their nominal positions causing a small but noticeable Doppler shift on both the feeder and mobile links.For personal and vehicle terminals, handover during a call between GSO satellites is unnecessary because the coverage is static and wide. However handover might be contemplated for aircraft terminals between different spot beams of the same satellite. In the latter case there is practically no difference in path length to consider. Within
Satellites in HEO
Satellites in HEO constellations orbit the Earth in planes that are inclined nominally 63,4° against the equatorial plane.This is necessary in order to keep the apogees in the most northern (southern) positions within their elliptical orbits.Typically HEO orbital periods are between 8 and 24 hours. HEO satellites are normally active only about their apogees where they appear nearly stationary to an earth observer for about eight hours, and then have to hand over to a following satellite.The satellites belonging to one particular system appear in time shift in the same celestial region. In the HEO track is sketched in profile showing at every point the true distance to the Earth's surface. In this specially depicted case, the orbital period is 12 hours and the satellites appear
alternatively at the opposite sides of the rotating globe. Therefore the illustrated HEO track reaches a maximum height at both ends above the geographical latitude of 63,4° North. At both upper ends (solid line), the satellite payloads are active. The dotted line constitutes the part where the satellite payloads are (typically) switched off. For comparison, see figure 2, where two HEO loops are indicated corresponding
to the two ends in profile in figure 1.Under the above conditions, the HEO apogee (maximum height above the Earth's surface) can be up to 42 000 km.However the maximum range to the Earth's surface is in the order of 47 000 km resulting in a maximum propagation delay of the order of 310 ms. HEO satellites reach high relative speeds during their active phase (order of magnitude:2 km/s), so that the Doppler shift (1.3 x 10-5 of radio frequency and bit rate) cannot be neglected: the radio frequency
shift is mainly due to the microwave feeder link and is of the order of 50 kHz for C-band feeder links. The satellite motion is mainly radial relative to the user community, so that common compensation of the Doppler main component is feasible.Irrespective of any user roaming, HEO systems require handover from the descending to the ascending satellite typically every eight hours. Depending on the specific system design, the distance to the two satellites at handover could be significant and a jump in path length cannot be excluded. However, a large Doppler jump will always happen.Within Europe HEO satellites can appear near the zenith. Therefore the user can work under vertical line-of-sight condition for most of the time, with blockage only being experienced in tunnels or under bridges, trees, etc. However vertical propagation is not very good within multi-storey buildings and hence paging, alerting, etc. may not be satisfactory.
Because vertical propagation can be in principle multipath-free, high data rate services are possible for outdoor operation.A number of HEO orbits have been studied extensively and given names such as "Molnya", "Tundra", and "Loopus".
Satellites in MEO
MEO satellites are in principle the same as LEO satellites. The differences are that MEO systems cause more propagation delay (80 ms to 120 ms), their Doppler shift is smaller, and handover happens less frequently and is less problematic. MEOs also need to work in a multipath environment as the number of satellites is usually smaller than for LEO but the average margins can be lower since many calls will be at a continuously high
elevation angle.The typical MEO altitude is between 10 000 km and 20 000 km, just outside the Van Allen belts with an orbital period of around 6 to 12 hours. A complete MEO constellation would probably require between 10 to 15 satellites. MEO satellites are used to provide current global radio navigation services and are optimum for such services.
Satellites in LEO
LEOs are typically circular orbits where satellites fly low above the Earth's atmosphere typically 700 to 1 500 km, bounded by outer atmospheric drag and the Van Allen radiation belts with an orbit time of about 90 minutes. For orbits near 1 500 km, inclinations near 50 degrees reduce the risk of debris collisions. Whereas polar orbits provide a whole Earth coverage including the poles themselves, inclined orbits can provide improved coverage over the populated areas located between latitudes -75 to +75 degrees. One proposed system is known to stay 700 km above the surface (see figure 1; LEO) where the coverage area at any point in time may measure up to 3 000 km in radius for about 10 degree elevation. This implies a maximum propagation delay of 20 ms and while higher altitude LEO systems would have higher propagation delays, they will never approach the values associated with GSO or HEO systems for a single satellite hop. However, on-board processing and Inter-Satellite Links (ISL) can increase delays considerably.LEO satellites move at very high speeds relative to the Earth's surface (7 km/s) and produce large Doppler frequency shifts (4,7x10-5 of radio frequency and bit rate). As the velocity is tangential to the Earth, Doppler compensation may need to be applied individually for each user.LEO systems, in common with HEO systems, also require to handover between adjacent satellites, but at a much more frequent rate of about ten minutes. Although the two LEO satellites are widely spaced, the individual path lengths can be similar and it is possible to minimise any path length jump. However, the Doppler shift jump will still always happen. As LEO satellites orbit very close to Earth, they can be considered as moving base stations. For the user the satellites appear most of the time below 30 degree elevation. Therefore LEO satellites work much of the time in a multipath environment. The additional satellite EIRP and receive sensitivity to compensate for multipath losses are achieved witha much smaller antenna on a LEO spacecraft (compared to GSO) because of the much shorter range (roughly 1/12th).
Diversity techniques may offset some of these multipath effects.The total number of satellites required to give total global coverage depends on many factors including quality of service and system capacity but the total could be as high as 70. Lower numbers are possible using special orbits or by using a mixture of LEO and GSO (for example). The cost for large numbers of LEO satellites is offset to some extent by their lower complexity and easier launch requirements. However their orbital life tends to be half that of typical GSO satellites (10 - 13 years). Another factor in LEO design is the required battery capacity and solar panel size to allow operation for nearly 50% of time in eclipse.
SOME LEO SATELLITE SYSTEM
1. THE IRIDIUM SYSTEM
The Iridium System is not proposed to be a replacement for existing terrestrial cellular systems, but rather as an extension of existing wireless systems to provide mobile services to remote and sparsely populated areas that are not covered by terrestrial cellular services. It provides more capacity (large no of channels) and better quality of service (shorter transmission delays) to areas that currently receive mobile services from geostationary satellites. It can also provide emergency service in the event that terrestrial cellular services are disabled in disaster situations(earthquakes,fires,floods,etc.).
The concept of using a constellation of low earth orbit satellites to provide global telecommunication services to mobile users. Because the initial proposal called for 77 satellites in the constellation, the system was called IRIDIUM after the element, which has 77 electrons in its orbit.later studies indicated that only 66 satellite would be adequate to provide the targeted services and performance. The 66 satellites are are grouped in six orbital planes; there are 11 active satellites in each plane with uniform nominal spacing of 32.7”. the satellites have circular orbits at an altitude of 783 km, and for each plane an in-orbit satellite is provided.
Satellites in one plane are placed to travel out of phase with those in the adjacent planes. Except for the first and last planes, which are counterrotating where they are adjacent, all remaining planes are corotating, The distance between corotating planes is 31.6, and the distance between the counterrotating planes is 22. The reduced seperation between counterrotating planes is needed to compensate for the reduced coverage provided by satellites on counterrotating planes. In the Iridium system, each satellite is equipped with four two-way communication links(intersatellite links, or ISLs), one each with its neighbors in the same plane and with those in the adjacent planes.
Each Iridium satellite uses a 48-beam antenna pattern, and each beam, which has a minimum diameter of 600 km, can be individually switched. For example, only about two-thirds of the beams will be active at any given time because some the beams will be switched off when the satellites are in the vicinity of the poles, where beam patterns tend to overlap, or when the satellites are over countries or regions in which, Iridium does not have regulatory arrangements to operate. The switched of beams is referred a cell management. In a LEO-based system like Iridium, the beams are equivalent to cells associated with terrestrial mobile systems. However, in case of the Iridium system, it is the beams that move rapidly relative to the subscriber, who is considered to be stationary with respect to the satellite. Thus, switching of beams or cell management to provide continuity of an existing call is equivalent to handoff in terrestrial cellular mobile systems. This requirement for cell management is, of course, and additional complexity associated with LEO- based systems compared with MEO or GEO systems.
The Iridium system supports links of three types:up- and downlinks from the space vehicle (SV) to the gateway (GW) [or to the telemetry, tracking, and control (TT & C) center], using the ka band; up- and downlinks between the SV and the Iridium subscriber unit (ISU), using the L band; and two-way inter-satellite links between the SVs using the Ka band.
2. THE GLOBALSTAR SYSTEM
Globalstar is a global mobile satellite system based on a constellation of 48 LEO satellites. Unlike the Iridium system, Globalstar system does not use intersatellite links but rather depends on a large number of interconnected earth stations or gateways for efficient call routing and delivery over the terrestrial network. It is designed to complement the terrestrial cellular mobile networks to provide telephony and messaging services to subscribers in locations that are not covered, or inadequately covered, by conventional wireline or wireless networks.
Globalstar’s constellation of 48 LEO satellites is designed t orbit at an altitude of 1414 km above earth’s surface in eight orbital plans inclined at 52. With each plane to be occupied by six satellited with a provision for one in-orbit spare satellite in each plane. The nominal weight of each satellite is 450 kg, with a deployed span of 7 meters and working life of 7.5 years. Since Globalstar satellites do not employ intersatellite communication, they essentially provide only transponder functions, making their design and operation less complex and perhaps more reliable. Each satellite supports a 16-beam antenna pattern with an average beam diameter of 2250 km. To mitigate blocking and shadowing, Globalstar will deploy path diversity, whereby multiple satellites may be used to complete a call.
In the absence of intersatellite links, the Globalstar system makes maximum use of the international terrestrial networks (wireline and wireless). Calls from a subscriber are routed via a satellite to the nearest earth station/gateway, and from there they will be routed over the existing terrestrial network. To provide the interface between the ground segment (terrestrial networks) and space segment (Globalstar satellites) Globalstar design deploys 100 or more gateway stations distributed around the world with each station equipped with three or five antennas that can track the trajectories of the satellites. A Globalstar gateway is designed to serve an area 3000 km in diameter and will be designed to take into account the technical and administrative requirements of the coverage area. These requirements may include such factors as coverage, quality of service, and satellite visibility, as well as regulatory and contractual factors associated with national boundaries.
Globalstar uses two types of communication links: service links in the L/S band for communication between the terminals and the space vehicle, and the gateway links in the C band for communication between the earth stations and the space vehicle.
]3. THE TELEDESIC SYSTEM
Currently the high bandwidth, high quality fiber connectivity needed to support Internet access, computer internetworking, video conferencing, and so on is restricted to major commercial and population centers. Outside these application areas, such facilities are either too expensive or simply not available. The aim of the Teledesic network is to extend the existing terrestrial, fiber-based infrastructure to provide advanced information and communication services anywhere on earth. Whereas the target application for Iridium, Globalstar, and ICO is voice, with support of low bit rate data for facsimile and messaging for mobile subscribers, the primary target application for the Teledesic system is the provide worldwide, seamless, fiber like connectivity to support multimedia, video, and high bit rate data services. In a strict sense, Teledesic does not fall in the category of global mobile satellite systems or GMPCS because its focus is not on worldwide terminal mobility, but rather on providing the so-called Internet in the sky function. The planned target for Teledesic service availability is end of year 2002. Rather than individual end users, primary customers for the Teledesic system will be service providers in countries around the world wishing to extend their network capabilities in terms of geographic scope and the range of services, and also multinational corporations needing to extend the capabilities of their enterprise networks.
The design of the Teledesic system has not been finalized. According to the original plans, the Teledesic satellite segment was to use 840 LEO satellites in 21 planes at altitudes of 700 km. The Teledesic system now intends to deploy only 288 active LEO satellites placed in 12 planes (24 satellites per plane) at altitudes around 1350 km. Each satellite in the Teledesic constellation will have connections to eight of its neighboring satellites through intersatellite links operating in the connectionless packet mode, with each satellite in this interconnected mesh network providing necessary switching functions. The Teledesic network is designed for dual-satellite visibility with at lest one insight satellite at a minimum elevation of 40. This high elevation angle ensures an unobstructed and omnidirectional view of the sky from most building tops where Teledesic terminals may be located. Besides eliminating shadowing effects from neighboring buildings and terrain, the high elevation angel greatly reduces the fading effects of rain at high frequencies.
MEO SATELLITE SYSTEM
1. THE ICO SYSTEM
The ICO is a medium earth orbit (MEO) mobile satellite system, which is designed primarily to provide services to handheld phones. ICO will use TDMA as the radio transmission technology. The system is designed to offer digital voice, data, facsimile, and short-targeted messaging services to its subscribers. ICO’s primary target customers are users from the existing terrestrial cellular systems who expect to travel to locations in which coverage is unavailable or inadequate. Other customer groups potentially served by ICO include road transport, maritime, and aeronautical users, as well as users of semifixed terminals in rural areas and develo0ping countries, where conventions terrestrial wireline or wireless mobile satellite communications capability with the public land mobile networks like GSM, D-AMPS, and PDC and their PCS variants.
ICO system is designed to use a constellation of 10 MEO satellites in intermediate circular orbit (ICO), at an altitude of 10,355 km above the earth’s surface. The nominal weight of these satellites at launch is less than 2000 kg. The satellites, with an expected life of 12 years, are arranged in two planes with five satellites (and one spare) in each plane: orbital planes inclined at 45 relative to the equator. Each satellite has antennas to provide 163 transmit and receive service link beams. The orbital configuration provides coverage of earth’s entire surface at all times and ensures significant overlap so that two or more satellites are visible to the user and the satellite access node (SAN) at any time. Further, at least one of the satellites appears at the high elevation angle, thereby minimizing the probability of blocking due to shadowing effects.
The ground segment in the ICO system, which will link the ICO satellites to the terrestrial networks, will consist of the 12 interconnected SANs located in various parts of the world. Each SAN consists of earth stations with multiple antennas for communication with the satellites, switching equipment, and databases to facilitate interconnection with public telephone, data, and mobile networks. The interconnection to the public networks is through appropriate gateways. Whereas each SAN supports VLR functions, the HLR function can reside in one (or more) of the SANs. A SAN tracks the satellites within its sight and will direct communication traffic to the satellite, which can provide reliable, uninterrupted link for a given call, in terms of angle of elevation and duration of satellite visibility. SANs also have the capability to execute handoffs from an area covered by one satellite to another satellite’s coverage. Such handoffs are expected to be very infrequent in ICO’s MEO-based system. Besides the SANs, the ICO system deploys TT & C stations connected to a satellite control center (SCC) for monitoring and controlling the satellites, as well as one or more network control centers (NCC) for overall management and control of the ICO system. The TT & C functions are associated with 6 of the 12 interconnected SANs.
A broad overview how a mobile satellite system works
A satellite system consists of a satellite segment, ground segment, and end-user segment.
Satellite Segment
The satellite segment is a network of GEO or LEO satellites arranged in orbital planes (i.e. different parts of the sky) in such a way that they have a communications link with end-user equipment, ground gateways and other satellites. The satellites transmit a continuous signal to earth which enables the satellites, end-user equipment and
gateways to be linked together. The links allow end-users to be transferred between satellites as the satellites move overhead (LEO systems). On the ground, there is a ground control facility (or facilities) which manage the performance of the satellites and the transfer of information from the satellites to the gateways.
Ground segment - gateways
The gateway connects the satellites to the local telephone network. The gateway also transmits signals to the satellites and receives transmission from the satellites. The gateway tends to have switching capabilities along with software that allows the system provider to keep track of billing information and route calls.
End-user
The end user terminals, pagers and phones communicate with the gateway equipment, satellites, satellite and cellular phones along with the cellular base station equipment. For the Iridium and Globalstar systems, the endusers will use a phone slightly larger than the average cellular phone. Both Iridium and Globalstar plan to offer dual mode handsets which will allow users to connect to the existing cellular systems or their own satellite system.Other systems from such companies as American Mobile Satellite and Intelsat, use phones which are the size of a briefcase and must be unpacked before use. The paging equipment from Iridium, the only satellite company who currently has a paging system in place, is your normal run of the mill pager. A few satellite systems (Globalstar included) plan to offer fixed satellite terminals which are a telephone booth in rural areas. The phone booth will include one or more phones and will not look much different than a phone booth you may find on the streets of
How a satellite call gets routed
There are two types of satellite systems under development and each have a different approach to routing phone calls. The proposed satellite systems will use either a bent pipe or an intersatellite linked system.
Bent pipe
In a bent pipe system (Globalstar) a call is placed by a satellite user which is then beamed up to the nearest orbiting satellite. The satellite reflects the call to the nearest ground gateway. Once at the gateway, the call is routed through the public telephone network to the intended receiver of the call. The gateway must be in the line of sight of the satellite, so the system operator must have a significant number of ground gateways to provide direct satellite links. For the most part, a bent pipe system is less complex than an intersatellite linked system because the brains of the system (switching) are on the ground and the satellites are just reflectors in the sky. Bent pipe systems are easier to operate because most of the call is transferred over the public telephone network, this also reduces the cost of the system. Many of the technical features will be located at the gateway which will allows most technical problems to be fixed on the ground.
Intersatellite links
In an intersatellite linked system (Iridium) a user’s call is beamed up to the nearest orbiting satellite. When it reaches the satellite, the call enters the onboard satellite switching system and is routed between satellites up in space. The call is then downlinked to another satellite user or the closest local gateway to the end user. Upon
entering the gateway the call is directed to the intended receiver through the public telephone network. The major benefit of the intersatellite linked system is that it minimizes the cost of the ground segment (i.e. - the call is switched in the air therefore you do not need a ground gateway in the line of sight of each satellite) and it also
minimizes the long distance and interconnect fees (much of which is bypassed in the air). The intersatellite linked system operator is able to keep a larger dollar amount of each call as compared to the bent pipe system operator.However, there is added risk and higher costs because each satellite must have on-board switching capabilities.On-board switching also adds to the complexity of the system because repairs must be made to satellites.
Handover Management in
Due to the high mobility of low earth orbit (LEO) satellites, there is a significant number of handover attempts in a LEO-based mobile satellite communication system, causing a
high handover failure rate. This paper proposes to extend the period of which a handover request is valid, and thus rendering higher probability of successful handover. Satellite
communication service can be provided by geostationary earth orbit (GEO), medium earth orbit (MEO) or low earth orbit (LEO) satellites. Because of its much shorter distance from earth, lower power requirement and thus smaller mobile terminal (MT) size, LEO satellite system is a preferable choice. In this paper, only the LEO satellite system is considered. The satellite coverage area, or its footprint, is divided into a number of areas, each of which spotted by one of the satellite's multiple spotbeams, forming a cell. Since a LEO satellite is not located at a geosynchronous orbit, it is mobile with respect to a fixed point on earth. Hence an active MT may move from one cell to another and handover occurs. The ground velocity of the MT is ignored compared to the
much higher satellite velocity. Suppose the length of a cell is 400 km and the satellite moves at a velocity of 6.6 km/sec, the time taken for a MT to cross a cell, Tcell, is about 60 seconds. Thus handover is extremely frequent in this system. And it is probable that a call is dropped due to unsuccessful handover. Handover is prone to failure when the subsequent cell has no unused channel to offer. Drop call is a phenomenon where an ongoing call has to be discontinued, which the users find hard to tolerate with, making it a major technical issue.There have been some methods proposed to minimise handover failure. It is widely accepted that handover requests are to be prioritised over new call requests, either by allocating guard channels to the handover requests [1], or by queuing up the handover requests when all the channels in one cell are occupied [1] [2]. This is because dropping an ongoing call is less desirable than blocking a new call attempt. There are also proposals of making the handover request earlier, so that the request has longer time to wait for a free channel, thus reducing the handover failure rate [3] [4].
Gerard Maral et al. have proposed that a handover request is to be made to a cell as early as the MT enters the cell located right before the target cell [3]. In [4], the time of sending out a handover request during handover process was made available regardless of the location of MS in a cell. In all of these cases, a call somehow has to be terminated when the originating MT has crossed into the target cell and yet handover is not granted by the target cell. The termination is done since no service is provided by either the original cell or the target cell. In this paper it is proposed that in a similar situation, the call be only temporarily discontinued for a specific amount of time, before it is permanently terminated if there is still no available channel. Although no service is provided by both cells to the MT, its handover request which has been queued is ‘kept in view’ by the target cell. During this idle period, there is a chance that an originally occupied channel in the target cell is released. If this is the case, this channel is allocated to the suspended call and handover is completed. As a result, the handover failure rate is reduced.
Mathematical analysis has been carried out to verify the idea and the results are encouraging. For a user, he/she only experiences a short period of call discontinuity and is notified about the temporary discontinuity through a special tone. In terms of quality of service (QoS), this is more tolerable to the users compared to a drop call.
A handover management strategy is proposed to efficiently manage the channel resource of a cell in a multibeam mobile satellite system (MSS) and improves its service quality by reducing the interbeam handover failure rate (Phf) caused by limited number of communication channels. The Extended Queueing of Handover (EQH) technique extends the channel reservation time of a handover request into the adjacent cell that the user terminal (UT) is subsequently entering (destination cell). Both mathematical analysis and simulation show that EQH reduces Phf significantly, without compromising the new call
admission rate and efficiency of channel utilisation.
The footprint of a multibeam satellite is divided into cells where each cell is illuminated by a spotbeam. Interbeam handover is frequent in mobile satellite system (MSS) due to the high velocity of the satellite (about 7 km/h for a low earth orbit satellite). When a user terminal (UT) leaves from one cell and enters the adjacent one, a handover process must be completed for the sake of call continuity, where a
communication channel must be allocated to the UT by the adjacent cell (destination cell). In a channel resource limited system, handover is subject to failure when the destination cell has no idle channel to offer. In this case the call is dropped and this is intolerable to users. In order to provide higher handover quality, system operator has to allocate a larger portion of the channel resource to the ongoing call as compared to the new call. A method in use is by applying the blocked-calls-queued policy to the calls
requesting for handover (handover calls) [1]; and on the other hand sacrificing some new calls through the blocked-call-cleared policy. The longer a handover call stays in a queue, the higher chance of it being handed over successfully. Other methods that prioritise handover call over new call are: guard channelallocation [2], and channel reservation in advance [3] [4]. The compromise of new call causes inefficiency in channel utilisation because channels that are allocated to handover call cannot be taken up by new calls even though they are idle.
Extended Queueing of Handover (EQH) Scheme
In a handover process, a UT with unfulfilled handover request will have its call terminated once it leaves the present cell i.e. when the signal strength of the present beam drops below an acceptable level. Under the proposed Extended Queueing of Handover (EQH) scheme [5], the policies of queueing and early channel reservation also apply to handover calls. In addition to them, the queueing process of an initially unfulfilled handover is allowed to be continued in the destination cell and thus lasts longer, promising a higher chance of obtaining a free communication channel. In this case, since the UT has left the present cell and has not reserved a channel from the destination cell,
its call has to be discontinued until either a free channel is available on which the call can be resumed on,or until the tolerable suspension period is over which the call has to be permanently terminated,whichever comes first. Although this suspended call is prioritised over new call in getting a channel, it does not significantly affect the blocking rate of new call because the probability that a call get suspended is very small. From the viewpoints of the two involved communicating parties in an initially unsuccessful handover call,
the discontinuity can be notified through a special tone / message. In terms of quality of service (QoS), a suspended call that eventually gets terminated is better than a disruptive and uninformed drop call. On the other hand, if the call is able to be resumed upon the availability of an idle channel, the short term discontinuity makes it worth than having the call terminated and followed by setting up a new call again, which is harder because new call is less privileged. Hence regardless of the outcome EQH scheme promises a higher QoS.
General aspects of mobile satellite systems
Differences between satellite and terrestrial systems exist in spite of common objectives for high quality services and excellent spectrum efficiency. Some differences arise because:- user costs are closely related to satellite transmit power the satellite propagation channel is highly predictable satellite paths introduce significant propagation delays and Doppler shifts frequency co-ordination has to be on a global basis frequency re-use options are more limited, hence bandwidth is a tight constraint satellite beam shaping and sizing opportunities are limited.The first two points lead naturally to the emphasis placed on the line-of-sight satellite link budget when establishing the system design. The base link budget is derived from theoretical path losses to which link margins are added to
compensate for inevitable impairments in equipment and propagation characteristics. All impairments, even if not directly calculable in terms of signal loss (e.g. group delay and rate of change of Doppler shift), are converted accurately to dB so that the compensating increase in transmit power can be established. The total margin over the theoretical ideal
path is only a few dB and precision in calculating the contributory impairments is essential. The resulting link budget then allows the availability and quality of service to be estimated over the coverage area.Large link margins have a major impact on system build cost and operating tariffs simply because of the impact of additional power requirements on spacecraft size — a 3dB excess margin would almost double user charges. For this reason, mobile satellite communication systems have lead the way in very power-efficient modulation formats and low bit rate voice codecs (2,4 kbit/s and 4,8 kbit/s) as well as adaptive power control. The drive for efficient use of satellite power is noticeably reflected in terminal equipment design with:
- very low loss antennas coupled with very low loss receive filters;
- very tight transmit/receive filter specifications;
- very low noise amplifiers;
- excellent carrier/signal acquisition in presence of Doppler, noise and interference;
- power-saving and spectrum-efficient forward error correction;
- multi-path discrimination techniques might facilitate low signal-to-noise demodulator operation
The satellite-mobile uplink and downlink are inevitably more fragile than the corresponding feeder links (land earth station-satellite). However the feeder link itself needs a very substantial link margin in order that the aggregate up/down performance may be largely determined by the mobile link. These feeder links operate in higher frequency bands where Doppler and atmospheric/meteorological disturbances can become even more significant. The following clauses of this TR focus on particular characteristics, capabilities and limitations of mobile satellite systems together with typical values for key parameters where possible. However it must be recognised that most parameters are inter-dependent and will also vary with architecture of the ground infrastructure, the satellite orbital arrangement, and the user terminal configuration.
Capabilities of
The most significant attribute of any satellite communication system is the wide area coverage that can be provided with very high guarantees of availability and consistency of service. The satellite component of UMTS can potentially provide the terrestrial service user with a global service without regard to incompatible terrestrial standards used elsewhere. Existing satellite mobile services have proved very attractive to the maritime and aeronautical sectors and they have also been of great benefit to emergency services, relief agencies, journalists, and expeditions over recent years.
Services are now extending to the land mobile market where hand-portable voice terminals are now technically feasible. The next subclauses address the key attributes of wide area coverage and types of services appropriate for satellite UMTS.
6.1 Large area coverage
A single satellite can see very large areas of the Earth: a single LEO can illuminate an area of 6 000 km diameter and a GSO can illuminate about 1/3rd of the globe. Within these areas, the spacecraft antenna can be designed to maintain a near-constant power flux density on the Earth's surface irrespective of range. However for the GSO and HEO (and possibly the LEO or MEO), the spacecraft antenna may need to be arranged as a cluster of spot beams (1 000 to 2 000 km diameter) in order to make hand-held terminals feasible and to achieve spectrum efficiency. Such spot beams require large spacecraft antennas for either GSO or HEO systems. The advantages of HEO and GSO are that it is possible to deploy a satellite system to fulfil a regional requirement rather than a global one, and frequency planning and co-ordination may be relatively straightforward. Furthermore, the ground infrastructure to support the satellites could follow traditional Land Earth Station (LES) approaches.The only satellite system that cannot provide polar coverage is GSO. With this restriction, any satellite constellation can provide assured line-of-sight global coverage unaffected by weather. Operation to shadowed or in-building terminals would require an additional link margin in the order of 20 dB or more, depending on the coverage required. Note that in cities, the terrestrial UMTS service is likely to be available and therefore in-building and city coverage may not be essential.
The line-of-sight case requires polarisation matching between the satellite and the mobile terminal. To avoid the need for polarisation tracking, mobile communications have traditionally used circular polarisation.
6.2 Flexible networks and services
A feature of most present day satellites is the use of "transparent transponders". Compared to conventional cellular base stations, the satellite transponder is little more than a frequency shifting amplifier. This does have drawbacks with regard to some aspects of system design but it also means that any one satellite is reasonably independent of modulation system or access method, or of service data rate or networking. This has led to satellites being used for a variety of applications, each with different terrestrial architectures. Provided the basic satellite parameters are satisfactory, these services can be introduced long after launch. Future satellites may not be quite so flexible as some studies propose to use on-board processing to improve capacity, spectrum efficiency and satellite payload performance. The transparency concept has however proved extremely costeffective and any on-board processing function is likely to be at least re-configurable and re-programmable. Another feature that might be introduced for MEO or LEO is the inter-satellite link to simplify terrestrial networking between satellites during handover.
The transparency concept has enabled mobile satellite systems to efficiently support a range of services beyond that of voice telephony:
- high data rate services (up to 64 kbit/s) to larger antenna (0,15 m ～1,0 m, 8 dBi ～20 dBi) mobile or fixed
terminals;
- group call and broadcasting;
- low data rate paging, alerting and two-way messaging;
- terminal location finding.
Some current satellite systems are designed so that extra services can be provided at very little additional cost. This is particularly effective when services are offered as a package to perhaps offset the requirement for line-of-sight paths for low-cost voice telephony.
7 Limitations of
7.1 Delay and Doppler
The delay and Doppler effects associated with satellite links are due entirely to the mechanical laws governing the satellite orbit. Any system design must take full account of these effects. For example, simple delay has an impact on speech quality that will require echo cancellers to be used at interfaces with the analogue network. Delay also requires allowances to be made in signalling protocols and power level control.
Changes in delay are the result of integrated Doppler shifts on the bit data rate and are significant for all orbits except GSO during a call and particularly during satellite handover. Such changes are likely to require a data buffer to maintain the delay at a constant maximum value. The data buffer can reside in either the LES or the mobile terminal between the two echo control devices and is required for both receive and transmit. Doppler shift itself complicates signal acquisition and spectrum management. The Doppler shift will not be identical for the in-bound and out-bound links due to the different feeder and mobile link microwave frequencies. Furthermore, the shift is in different directions if corrected at the mobile terminal. For LEO and MEO orbits, the shift may need to be individually corrected for each mobile; for HEO, common Doppler compensation can be incorporated in the LES or onboard the satellite.
7.2 Low link margins
Emphasis has already been made on the importance of keeping impairment margins low. An illustration can be based on the calculation of Carrier-to-Noise (C/N) ratios for uplink, downlink, and the total link. Assuming the downlink has C/Nd = 10 dB and is near the performance threshold, the feeder will need a 13 dB margin (C/Nu = 23 dB) to maintain
the degradation to less than 0,2 dB (i.e. C/Nt = 9,8 dB). Operation at levels just above threshold are only feasible for satellite because of the stable propagation path and because most impairments (including the large noise contribution) can be considered to be random. These low margins, compared to the terrestrial environment, result in longer signal acquisition times.
All impairments must be carefully analysed and include: imperfect in-band filtering, group delays, out-of-channel emissions (which demand very tight power amplifier linearity requirements, carrier to interference ratios, etc.) Multi-path also requires especial attention. In the terrestrial environment, multipath propagation normally results in inter-symbol interference that can be compensated with equalisers. The effects of multipath fading itself are often negligible within the main service areas because the detected signal level is sufficiently above threshold. In the satellite path, multipath delays are often short enough to be ignored (except for aircraft and ships) due to the comparatively high elevation angle of the radio path. However multi-path fading, in which a multipath signal partially cancels the main signal, can reduce the final signal below the modem operating threshold. Hand-offs between successive LEO, MEO, or HEO satellites will be more complex because of the small operating margins which makes it difficult to promptly detect signal disappearance. Satellite signal qualities often cannot be assessed from signal level (which is swamped by thermal noise) but are often estimated from the activity within the forward error correction algorithm. This requires time averaging and cannot be an instantaneous measurement. Satellite diversity reception
might alleviate some of these issues. Mobile terminals with high gain (directive) antennas have further problems with signal acquisition as the antenna may need to be mechanically or electronically steered towards the satellite before the signal rises above the detection
threshold.
7.3 Spectrum and orbit matters
Limited spectrum availability will constrain the potential capacity of the satellite component and hence will orientate personal satellite services towards low bit rate voice and data. Spectrum issues are very complex but can be broadly classified into three areas:
- feeder link planning;
- mobile frequency co-ordination;
- mobile frequency re-use and spectrum efficiency.
Global agreements exist for planning GSO systems via the ITU RS (formerly IFRB) for designated frequency bands. Feeder links are normally in one of the established Fixed Satellite Service (FSS) bands and are straightforward except for the large bandwidths required to support peak traffic on each satellite. Mobile frequency co-ordination is not simple however, particularly as their antenna patterns are near omni-directional and any mobile system is likely to require exclusive access to a frequency band. The next problem, that of re-using the frequencies as frequently as possible, is very similar in concept to terrestrial cellular planing except that isolation is provided by satellite antenna beam shaping rather instead of geographical spacing. Feeder links for non-GSO satellites are more complex, particularly because of the lack of established procedures for the
many possible orbits. Furthermore, there is no orbital registration akin to that in the GSO orbit where orbital positions are assigned to particular operators and countries. LEO and MEO may require several widely spaced feeder LESs per satellite sector or inter-satellite links to prevent the feeder link interfering with the geostationary orbit. In either case,
there will be additional delay and Doppler jumps. For HEO orbits where the satellites appear to operate at the same part of the celestial sphere, feeder link planning may not be difficult as GSO-type procedures could be applied. The magnitude of the orbit and spectrum planning problems is partly illustrated by figure 2 which shows an azimuth -
elevation diagram for a fixed land earth station site at a latitude of approximately 50° North (It is not computed from simulated systems but shows only the principle. Therefore slight differences to simulated orbit constellations may exist)
The dotted line, extending from East to West in the shape of an arc, represents the geostationary orbit with two fixed GSO satellites designated 1 and 2. The three LEO tracks belong to one system of approximately polar orbits. The LEO satellites designated 1 and 2 travel North-South. LEO satellite 1 is about to hand over to LEO satellite 2. The LEO satellite 3 travels South-North, but this satellite No. 3 appears here at this track, due to the Earth's revolution, only at a time shift of half a day with respect to the satellites travelling North-South. The slight drift of the three LEO satellites towards West is caused by the Earth's rotation, and hence the rotation of the earth station site, towards East.
At the north-north-western horizon and in the East of the zenith there are two loop-shaped tracks of the HEO satellites designated 1, 2 and 3. The dotted lines extending from the loop near the zenith show the branches of the track where the communication payloads are inactive, as is here the case for the HEO satellite 2. From the diagram one can conclude that a fixed earth station (according to CCIR Recommendation 465 [1]) at this site can communicate with GSO satellite 1 and HEO satellite 1, even when the LEO system is in operation. On the contrary, the links with GSO satellite 2 and HEO satellite 3 could not co-exist with LEO satellite 3, since it passes both the other satellite positions.
Assuming, the LEO satellites' orbit period were not adjusted with the Earth's rotation, then the LEO satellite tracks would scan across the sky like the lines on a television screen, and co-existence with neither GSO nor HEO satellites on the same frequencies would be possible.
7.4 Scope for technical developments
7.4.1 Signal to Noise (S/N) levels
Satellite systems operate very close to theoretical signal to noise demodulation thresholds. There is virtually no scope for reduction in receive thermal noise levels at the satellite or at the mobile terminal as noise levels are dominated by the Earth's background thermal noise (290 K). The noise performance of modern amplifiers is almost
insignificant against this background. The only scope to improve signal to noise margins (for example to provide shadow or in-building operation) is to improve satellite antenna gain.
7.4.2 Hand-held terminal antennas
Present operational mobile satellite systems provide voice services with medium gain steered antennas in the gain range 8 dBi to 15 dBi. Low data rate services can use unsteered lower gain antennas with gains between 0 dBi and 4 dBi.The challenge for UMTS is to provide voice telephony to hand-held terminals using unsteered low gain antennas. The hand-held target imposes practical limitations on the form of antenna and it is unlikely that antennas will have usable gains greater than 0 dBi. However this does not prohibit the use of higher gain antennas for particular applications or circumstances.
7.4.3 Satellites
Satellite technology and commercial launcher capabilities have matured over the past ten years allowing systems planners to design complex systems with confidence. However, reliability is paramount for commercial satellite services and therefore only well established technology, proven in space, is normally considered for major projects.
The satellite antenna is a critical system element. In order to allow operation with low-performance hand-held PES's, the satellite antenna must provide a high gain. This can only be achieved by using advanced array-type antenna technology, including electronic beam forming and beam steering. The resulting spot (cell) diameters on the Earth's surface are typically in the range 1 000 km to 3 000 km.
7.4.4 Digital modulation techniques
The potential capacity of any satellite system is limited essentially by the availability of frequency spectrum and onboard satellite DC power. Hence, for most cost effective operation, it is of paramount importance that power and spectrally efficient transmission schemes are employed. Current research is continuing to make worthwhile progress in
this area.
7.4.5 Voice coding
Lower bit rate voice codecs have been widely used in mobile satellite systems compared to terrestrial systems to reduce power and spectrum requirements. Continuing codec development, coupled with advances in semiconductor integration, is likely to yield improved speech quality and some reduction in overall power/spectrum demands. Target performances for UMTS speech codecs have been set for both terrestrial and satellite components, taking into account the progress that is expected to be made by the time UMTS is introduced.
Growth drivers of mobile satellite communication services
Deregulation
Governments throughout the world are opening up their telecommunications systems whether it be through spectrum allocations, privatization, competition or access. Governments have realized that there is a strong correlation between telecommunications services and economic growth. Therefore they have started to knock down the walls that existed within their telecommunications markets and are encouraging investment in the latest technologies so that their countries do not fall behind in communications.
Technology
Technological developments have improved the power and versatility of satellites, today they have greater capacity and lower costs. For instance, the smaller size of many of today’s satellites lowers the cost of launching satellites. At the same time recent digital technologies (TDMA - Time Division Multiple Access, CDMA - Code Division
Multiple Access) are being applied to satellite systems which a increases capacity and lowers the cost of launching a system.
Globalization
People no longer are isolated from the world. People are affected by trade like never before; Nike and Gillette are no longer just
to be able to make a phone call and receive one -- they want one telephone number that can be used anytime, anywhere in the world. Thus, we feel the development of the global economy is a key driver of the mobile communications business.
Economic growth
Economic growth throughout the world has increased living standards which also drives demand for communications services. As individuals increase their economic stature, one of the first things they desire is a phone. This is a positive for satellite service providers. As developing economies continue to grow and enter the global economy, the demand for satellite services will increase because people will be able to afford it, and the need for mobile services will increase.
Demand for phone service
More than 3 billion of the world’s people do not have phone service. The waiting list for landline telephone service has over 50 million names with the average wait greater than 1.5 years. On average, there are slightly fewer than 12 phones lines per every 100 people in the world. This is far lower than what exists in developed countries such as
not even attempt to get service -- this could understate the actual number of people
waiting for phone services. Atthe same time, Iridium (through research by Booz Allen and Gallup) has determined that the demand from worldwide travelers for mobile satellite services will be 42 million people by the year 2002. Regardless of how youlook at the numbers, there is a significant amount of people without phone services throughout the world. Also, phone services are not developed in many countries, so travelers are unable to access a reliable phone. Satellite communications services will solve the needs of worldwide travelers and provide phone services to many areas of the world that currently do not have phone service.
Mobile communications trends
Cellular demand continues to explode throughout the world with some estimates of 500 million subscribers by the year 2002. Cellular phone bills in Third World countries are higher than the average bill in the
countries are willing to pay for phone services. An ubiquitous phone service offered by satellite companies will benefit from these trends in cellular communications.
Who will use satellite communications systems?
Global roamer
The first type of satellite user will be the global roamer. The global roamer consists mainly of business travelers who want to have the ability to make and receive calls anywhere in the world. Iridium has conducted extensive analysis of this market and concluded that this market will consist of 42 million people by the year 2002.
Cellular extension
The second type of user will be individuals who wish to extend their cellular coverage to areas where no service currently exists. Both Globalstar and Iridium plan to offer dual mode phones which will work with GSM/TDMA/CDMA cellular systems and satellite communications systems. An example of a dual mode user would be would be an individual who lives in
person would normally have cellular service from Ameritech but that coverage does not include upstate
individual got out of the range of their Ameritech systems, they could hit a switch on their Iridium phone and make or receive calls outside of their Ameritech coverage zone routed through the Iridium satellite system. This would allow for ubiquitous service for cellular users even when they are out of range of their current cellular system.
Landline extension
The third type of satellite user will an individual who wants landline extension. In this instance a satellite company would install a fixed telephone booth in a rural area (e.g. in the outskirts of
Global
Satellite communication systems are designed to provide voice, data, fax, paging, video conferencing and internet services to users worldwide. Through satellite based systems, users will be able to make a phone call from an African safari or while sailing around the world. No matter where users are, they will be able to communicate with clients, customers, associates, friends, and family anywhere in the world. In addition, satellite communications will allow countries to provide phone services without large investments in landline or wireless systems. Satellite communications will be one of the fastest growing areas within the communications industry.
GEO satellite networks offer great potential for multimedia applications with their ability to broadcast and multicast large amounts of data over a very large area thus achieving global connectivity. Internet via satellites, in particular, GEO satellites, have the following merits:
High bandwidth.
A Ka-band (20-30Ghz) satellite can deliver throughput of gigabits per second rates.
Inexpensive.
A satellite communications system is relatively inexpensive because there are no cable-laying costs, and one satellite covers a very large area.
Untethered communication.
Users can enjoy untethered mobile communication anywhere within the footprints of the satellite.
Simple network topology.
Compared with the mesh interconnection model of the terrestrial Internet, GEO satellite networks have much simpler delivery paths. The simpler topology often results in more manageable network performance.
Broadcast/multicast.
Satellite networks are naturally attractive for broadcast/multicast applications (such as MBONE). In contrast, multicast in a mesh interconnection network requires complicated multicast routing. Performance can vary for each multicast group member and is dependent on the route from the source.
Video Conferencing:
GEO satellites can provide better quality in video conferencing due to the available bandwidth and simpler network topology.
Satellite networks promise a new era of global connectivity, where geographical isolation will no more be the barrier among continent. In this work, we have shown that indeed many popular Internet applications perform to user expectation over satellite networks, such as video teleconferencing, bulk data transfer, background electronic mail, and non-real time information dissemination. Some other applications, especially highly interactive applications such as web browsing.
Even though the systems will eventually be built, conflicts over billing and control still remain. Operators of phone services in
Thus the field mobile satellite communication have better prospectus in the future. As conditions on mars are seems to be favourable for human life.
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