Broadband Satellite Spot Beams While a conventional satellite using Ku-band will use a single downlink beam to cover a single area on the earth, a broadband satellite, using Ka-band, will use multiple Spot-beams to provide the same coverage. Each of the circles in the graphic below is equivalent to a "cell" in a cellular telephone network and allows multiple reuse of the frequency spectrum. On average, a broadband satellite will provide 35 to 45 subscriber spot-beams and about six Gateway Earth Station spot-beams; about one Gateway spot-beam for every seven or eight subscriber spot - beams.
The graphic below shows a typical spot-beam layout for the United States from a broadband satellite. Note that a conventional satellite, using Ku-band, would have only one beam for the entire United States.
The satellite uses on-board switching to provide two-way data paths between different spot beams and allow for reuse of the Beam Carrier Frequencies. The bent-pipe architecture allows the satellite processor to concentrate on building the data-paths because the satellite acts as a repeater. In the satellite, the data-path signal is received, retuned, amplified, and retransmitted but there is no demodulation, decoding or other type of signal processing.
Bent-pipe technology in the satellite switches a user uplink and a feeder downlink together to form the upstream data path between the customer and the service provider. The same technology switches the downstream user downlink and the upstream feeder link together to form the downstream data path between the customer and the service provider. The combination of the upstream and downstream data paths connected to a single Gateway form a sub-network.
These sub-networks are a part of the larger service provider network, interconnected with fiber rings, providing access to the Internet. The graphic below shows a portion of a broadband satellite network. Note that user links and Gateway links actually consist of one uplink and one downlink. A single line in the graphic displays an upstream and downstream data path.
Current Satellite Internet Access Satellite Capacity As the demand for satellite capacity has steadily increased, traditional broadcast satellite technology using Ku-band GHz frequencies becomes limiting. Because they are built to cover a very large downlink area, traditional broadcast satellites have difficulty supporting the flexible distribution of bandwidth needed for broadband Internet. MBA technology maximizes bandwidth capacity while minimizing required frequency allocation. To use the MBA technology efficiently, broadband satellites use the Ka-band frequencies, multiple pencil-like spot-beams and bent-pipe technology.
PC, router and controls satellite transmission. Traditional Ku- band antennas have four characteristics that affect the placement of the outdoor unit, while the Ka- band antenna has five. Setting these characteristics properly makes uplink and downlink communication with the broadband satellite possible. In Ka-band transmissions, combining the L-O-S characteristic settings with the beam assignment characteristic setting makes data transmission possible.
The equipment in the Gateway is very similar to the customer site equipment except on a much larger scale. Step-By-Step Azimuth: Azimuth is the horizontal angle from the True North point on the horizon to the point where the satellite intersects the horizon. Magnetic Declination defines the offset angle between True North and Magnetic North at a particular location on earth. Modifying the azimuth reading, based on True North, by the Magnetic Declination will provide the correct magnetic compass reading for pointing an ODU transceiver.
Only at the equator will the GEO Arch, the portion of the geostationary belt viewable from a position on earth, have a degree elevation. All other earth locations use a calculation of angles formed from the relationship between the ODU and satellite locations, along with a calculated line to the center of the earth.
Polarization: Polarization is the orientation of a signal wave produced by an antenna on a stationary plane defined by the direction of propagation transmission and the associated magnetic and electric fields.
WildBlue uses circular polarization. A corkscrew best visualizes the circular transmission. Although it has selected the so-called DOCSIS or Data Over Cable Service Interface Specification standard that has been adopted by the cable industry to speed the deployment of cable modems, Wildblue is not closing the door to other standards as it seeks to establish the lowest cost approach for consumers and businesses alike.
Networks such as Wildblue that adopt DOCSIS will leverage off of many of the proven capabilities and infrastructure that the cable industry has developed. While the communications systems group at Colorado Springs, CO-based ITT Industries is best known for its Milsatcom-related work in areas such as tactical broadband platforms, it plans to extend this capability into the commercial Ka-band market.
A good deal of energy is being devoted to exploring all aspects of modem and baseband switch interactions. Krebs, vice president of strategic programs at ITT Industries. By doing data rate adaption with this unit, we can accommodate a 12 dB fade in the carrier-to-noise ratio in the link in question.
Trying to maintain link quality by strictly using link power control is problematic given the depth and rate of fades in Ka-band, according to Krebs, so there is a need for a more responsive control system than is available with centralized link power control.
David Price, vice president of business development at San Diego-based Harmonic Data Systems, discusses what was on the horizon two years ago in order to stress how difficult it is to project where we will be two years from now. Significant efficiency improvements have come into the return channel systems in particular. People just do not know how the marketplace will look in two years.
Price says that the emphasis on two-way platforms represents a quantum shift on the ground in terms of what is required to properly install these units, and that having put the fundamentals in place at the component level is just the tip of the iceberg.
The skill set for one-way services cannot be applied to two-way services. In effect, it is a whole new business model absent the higher margins that have been part of the previous VSAT economic model. There is no point in having multiple spotbeams for a multicast service.
Ku-band will continue to have a lot of good space segment. Small networks will find it hard to justify Ka-band, and thin or no request services that just push content will probably stay with Ku-band as well. The Ka-band market will mature eventually, but it will need a bit of a boost in its formative or early stages. This is no secret. They want us to provide them with low-cost units. That is the theoretical minimum.
For geostationary satellites, there is no way to eliminate latency, but the problem can be somewhat mitigated in Internet communications with TCP acceleration features that shorten the round trip time RTT per packet by splitting the feedback loop between the sender and the receiver. Such acceleration features are usually present in recent technology developments embedded in new satellite Internet services.
Latency also impacts the initiation of secure Internet connections such as SSL which require the exchange of numerous pieces of data between web server and web client. Although these pieces of data are small, the multiple round trips involved in the handshake produce long delays compared to other forms of Internet connectivity, as documented by Stephen T.
Cobb in a report published by the Rural Mobile and Broadband Alliance. For example:. Unlike geostationary satellites, low and medium Earth orbit satellites do not stay in a fixed position in the sky. Consequently, ground based antennas cannot be easily locked into communication with any one specific satellite.
Communications may involve more diffuse or completely omnidirectional ground antennas capable of communicating with one or more satellites visible in the sky at the same time, but at significantly higher transmit power than fixed geostationary dish antennas, and with much poorer signal to noise ratios for receiving the signal. Tracking a single low earth orbit satellite with a high-gain narrow beam is possible, but requires a motorized antenna mount and complex software that can predict the path of each satellite in the constellation.
As with GPS , the small orbits may cause a low Earth orbit satellite to only be in the sky for an hour or less before it goes over the horizon and out of range, so a complex relaying and passing-off needs to be done to hand over the fixed-position terrestrial signal to other satellites passing overhead. A proposed alternative to geostationary relay satellites is a special-purpose solar-powered ultralight aircraft, which would fly along a circular path above a fixed ground location, operating under autonomous computer control at a height of approximately 20, meters.
One example of this is the United States Defense Advanced Research Projects Agency Vulture project, an ultralight aircraft capable of station-keeping over a fixed area for a period of up to five years, able to provide both continuous surveillance to ground assets as well as to provide extremely low latency communications networks.
Onboard batteries would be charged during daylight hours by solar panels covering the wings, and would provide power to the plane during night. Ground-based satellite dishes would relay signals to and from the aircraft, resulting in a greatly reduced round-trip signal latency of only 0. The planes could potentially run for long periods without refueling.
Several such schemes involving various types of aircraft have been proposed in the past. Satellite communications are affected by moisture and various forms of precipitation such as rain or snow in the signal path between end users or ground stations and the satellite being utilized. This interference with the signal is known as rain fade. The effects are less pronounced on the lower frequency 'L' and 'C' bands, but can become quite severe on the higher frequency 'Ku' and 'Ka' band.
Rain margins are the extra communication link requirements needed to account for signal degradations due to moisture and precipitation, and are of acute importance on all systems operating at frequencies over 10 GHz. The amount of time during which service is lost can be reduced by increasing the size of the satellite communication dish so as to gather more of the satellite signal on the downlink and also to provide a stronger signal on the uplink.
Modern consumer-grade dish antennas tend to be fairly small, which reduces the rain margin or increases the required satellite downlink power and cost. However, it is often more economical to build a more expensive satellite and smaller, less expensive consumer antennas than to increase the consumer antenna size to reduce the satellite cost.
Large commercial dishes of 3. Satellites typically use photovoltaic solar power, so there is no expense for the energy itself, but a more powerful satellite will require larger, more powerful solar panels and electronics, often including a larger transmitting antenna. The larger satellite components not only increase materials costs but also increase the weight of the satellite, and in general, the cost to launch a satellite into an orbit is directly proportional to its weight.
In addition, since satellite launch vehicles [i. Modern download DVB-S2 carriers, with RCS feedback, are intended to allow the modulation method to be dynamically altered, in response to rain problems at a receive site. This allows the bit rates to be increased substantially during normal clear sky conditions, thus reducing overall costs per bit.
Typically a completely clear line of sight between the dish and the satellite is required for the system to work. In addition to the signal being susceptible to absorption and scattering by moisture, the signal is similarly impacted by the presence of trees and other vegetation in the path of the signal.
As the radio frequency decreases, to below MHz, penetration through vegetation increases, but most satellite communications operate above 2 GHz making them sensitive to even minor obstructions such as tree foliage.
A dish installation in the winter must factor in plant foliage growth that will appear in the spring and summer. The radio signal width between any two antennas is not perfectly straight and uniform, as if it were a beam of light. Instead as the signal propagates away from the transmitting antenna, it widens towards the centerpoint between the two antennas and then narrows again as it approaches the receiving antenna.
This is known as the Fresnel zone named for physicist Augustin-Jean Fresnel , and limits the usefulness of satellite dish antennas in locations where there is extremely limited open sky for signal reception. The signal path through space must be clear not only for direct line of sight, but also for the expanding Fresnel zone, which may be several meters [ citation needed ] larger in diameter than the ground-based satellite dish.
Two-way satellite Internet service involves both sending and receiving data from a remote very-small-aperture terminal VSAT via satellite to a hub telecommunications port teleport , which then relays data via the terrestrial Internet. The satellite dish at each location must be precisely pointed to avoid interference with other satellites. The two way satellite market can be divided into those systems that support professional applications, such as banking, retail etc.
The key difference between these systems can be seen in their ability to support advanced quality of service controls. While systems for professionals such as those from iDirect will allow the operator to define and meet strict service level agreements - those used for consumer access provide a 'best effort' service level.
Many customers in the Middle East and Africa are also encouraged to do self installs. At each VSAT site the uplink frequency, bit rate and power must be accurately set, under control of the service provider hub. Such systems are frequently marketed as "satellite broadband" and can cost two to three times as much per month as land-based systems such as ADSL.
The modems required for this service are often proprietary, but some are compatible with several different providers.
Satellite internet customers range from individual home users with one PC to large remote business sites with several hundred PCs. Home users tend to use shared satellite capacity to reduce the cost, while still allowing high peak bit rates when congestion is absent.
There are usually restrictive time-based bandwidth allowances so that each user gets their fair share, according to their payment. When a user exceeds their allowance, the company may slow down their access, deprioritise their traffic or charge for the excess bandwidth used. For consumer satellite internet, the allowance can typically range from MB per day to 25 GB per month.
The uplink direction for shared user customers is normally time division multiple access TDMA , which involves transmitting occasional short packet bursts in between other users similar to how a cellular phone shares a cell tower. Each remote location may also be equipped with a telephone modem; the connections for this are as with a conventional dial-up ISP. Two-way satellite systems may sometimes use the modem channel in both directions for data where latency is more important than bandwidth, reserving the satellite channel for download data where bandwidth is more important than latency, such as for file transfers.
In , the European Commission sponsored the UNIC project which aims at developing an end-to-end scientific test bed for the distribution of new broadband interactive TV-centric services delivered over low-cost two-way satellite to actual end-users in the home. Normal VSAT dishes 1. A voice call is sent by means of packets via the satellite and internet.
Using coding and compression techniques the bit rate needed per call is only Some also have an integrated Bluetooth transceiver and double as a satellite phone.
0コメント