Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
2000-03-03
2004-12-28
Gesesse, Tilahun (Department: 2744)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S003010, C370S323000
Reexamination Certificate
active
06836658
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to satellite communications systems using multiple spot beams to selectively broadcast high bit rate broadband information to user terminals located within desired coverage areas and, more particularly, to a satellite communications system in which a high bit rate broadband data stream is divided into multiple data streams in a hub for delivery to user terminals located within desired coverage areas via a multiple-transponder satellite.
BACKGROUND OF THE INVENTION
Satellites are used extensively for a variety of communications applications as a result of some well-recognized benefits. The most important communications advantage that satellites enjoy is that they are in view of a large amount of the earth's surface. A geosynchronous satellite is in view of about one-third of the earth's surface, for example.
In addition, large amounts of frequency spectrum have been allocated to satellites for communications in the microwave and millimeter wave frequencies. For example, at C and Ku bands, the available spectrum for satellite communications is on the order of one GHz. That bandwidth can be made available to users located in the field of view of the satellite and can be multiplied through a variety of frequency reuse techniques.
Moreover, because satellite communications are carried out using radio frequencies through free space, mobile user terminals can be deployed. At the common satellite communications frequencies of C and Ku band, reasonably sized antennas and low cost user terminal hardware are readily available.
The advantages of satellite communications have been enjoyed by users in several types of applications. For example, cable TV networks deliver high bandwidth video programming to head-end distribution points around the world, and national retailers use VSAT networks to accept and distribute data to and from retail stores throughout the country.
The first applications of communications satellites were in point-to-point communications links between fixed pairs of earth stations
12
. As shown in
FIG. 1
, communications from one earth station to the other is via a dedicated path over the satellite
13
. As a result, two links are needed in order to provide full duplex communications. Before the advent of underseas fiber optic cable, many international telephone calls were accomplished with point-to-point satellite links.
Today, satellites are more commonly used for point-to-multipoint, or broadcast, applications as illustrated in FIG.
2
. In
FIG. 2
, the information to be broadcast to a number of receivers within the field of view of the satellite is delivered to a hub
14
which then uplinks the information to the satellite
13
. The satellite then relays the information to user terminals
15
located within a broad coverage footprint
16
on the earth. For example, television programming from a single network hub can be delivered to numerous ground-based broadcasters or cable operators by a geosynchronous satellite. Such broadcast applications take full advantage of the wide area coverage provided by geosynchronous satellites. Every television station in the United States owns and operates at least one broadcast receive terminal, and many stations own uplink terminals to deliver news feeds via satellite. Television programming is most commonly distributed using the C band of frequencies.
Today, roughly one-third of all satellite transponders are dedicated to the distribution of cable television programming. A typical cable system head-end in the United States will continuously receive 30 to 50 satellite-delivered video channels coming from several different satellites. To control access by cable subscribers to the individual channels, the programming is typically encoded at the uplink using standards developed by the Motion Picture Experts Group (MPEG) or according to one of the digital video broadcasting (DVB) standards. The receiving site at the cable system head-end then requires a decoder for each channel being recovered.
Direct-to-home video distribution systems such as the Hughes DirecTV system typically employ high-power Ku band satellites using the BSS channel plan. Under that plan, each orbit position has 32 channels assigned to it. A satellite might transmit 16 channels at one time, thus requiring that two satellites be operated in the same slot. The full complement of 32 channels of about 27 MHz each can deliver between 150 and 250 compressed digital video channels. In the late 1990s, the DirecTV system and other direct-to-home systems were augmented with data broadcasting to provide Internet access. In such systems, the data is typically transmitted in packets to allow the information to be addressed to individual receivers.
Various schemes for broadcasting digital data are known. In one analog technique, a low data rate data stream is inserted into the vertical blanking interval of the video transmission. The data is then removed from the video signal by a special decoder unit connected to the display terminal. In another known approach for digital data broadcasting, a medium data rate data stream is modulated onto a baseband subcarrier, and the receiving user terminal recovers the data with a subcarrier receiver and decoder. These systems suffer from bandwidth limitations and from the need for special receivers.
The satellite communications industry has long used the term “transponder” to refer to a defined RF channel of communications within a satellite communications system. A satellite transponder is essentially a microwave relay channel, taking into account the need to translate the frequency of the transponder from an uplink frequency to a downlink frequency. A transponderized satellite payload design breaks up the full downlink frequency band (for example, a 500 MHz band at C band or a 1 GHz band at Ku band) to allow more effective power amplification of the downlink signal by the satellite downlink transmitter. If the transmitter was instead required to accommodate the full bandwidth of the downlink, that requirement would greatly limit the RF output power level available from the transmitter. By dividing the fulldownlink bandwidth into several transponders, or channels, individual amplifiers dedicated to each transponder segment of the downlink (for example, 36 MHz or 54 MHz segments) can be employed and the power level of the full-bandwidth downlink signal can therefore be significantly higher.
In a typical transponderized satellite communications system, the frequency plan of the transponders is coordinated between and among the user terminals, the hub, and the satellite payload. As a result, the information to be relayed over a conventional communication satellite is limited by the bandwidth of a single transponder (which may be 27 MHz, 36 MHz, 54 MHz, for example). As a result, the amount of data that can be delivered to a single receiving terminal through a conventional communications satellite is inherently limited by transponder bandwidth.
The first geosynchronous communications satellites had uplink and downlink coverage footprints that were coincident with the earth field of view or with major continents in view of the satellite. Antennas for creating full earth coverage patterns are fairly simple and use fairly simple feed horn structures. More recently, switchable spot beam antennas have been developed for satellite communications applications to enable the reuse of the uplink and downlink frequencies across a geographic area. For example, if the United States is divided into multiple spot beam coverage areas, as illustrated in
FIG. 3
, the full frequency range can be reused in each spot beam coverage area to direct different information to different spot coverage areas. One conventional system for broadcasting information from a communications satellite to user terminals located within a plurality of spot beams is illustrated in FIG.
4
.
Spot beams are also employed to deliver more RF power over a smaller spot coverage area in order to reduce the size of the receive anten
Hafner William R.
Sharon Thomas E.
Summers Macy W.
EMS Technologies Inc.
Gesesse Tilahun
King & Spalding LLP
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