WO1999049591A1 - Data packet satellite communication system - Google Patents
Data packet satellite communication system Download PDFInfo
- Publication number
- WO1999049591A1 WO1999049591A1 PCT/IE1999/000015 IE9900015W WO9949591A1 WO 1999049591 A1 WO1999049591 A1 WO 1999049591A1 IE 9900015 W IE9900015 W IE 9900015W WO 9949591 A1 WO9949591 A1 WO 9949591A1
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- WO
- WIPO (PCT)
- Prior art keywords
- frequency
- data
- data packet
- signal
- broadcast station
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Classifications
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B7/00—Radio transmission systems, i.e. using radiation field
- H04B7/14—Relay systems
- H04B7/15—Active relay systems
- H04B7/204—Multiple access
- H04B7/208—Frequency-division multiple access [FDMA]
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
- H04B1/69—Spread spectrum techniques
- H04B1/713—Spread spectrum techniques using frequency hopping
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
- H04B1/69—Spread spectrum techniques
- H04B1/713—Spread spectrum techniques using frequency hopping
- H04B1/7136—Arrangements for generation of hop frequencies, e.g. using a bank of frequency sources, using continuous tuning or using a transform
- H04B2001/71365—Arrangements for generation of hop frequencies, e.g. using a bank of frequency sources, using continuous tuning or using a transform using continuous tuning of a single frequency source
Definitions
- This invention relates to a satellite communication system comprising a central broadcast station in satellite communication with a plurality of user terminals each of which can make requests for broadcast data from the central broadcast station.
- the invention provides a satellite communication system comprising a central broadcast station in satellite communication with a plurality of user terminals each of which can make requests for 2 broadcast data from the central broadcast station, wherein each user terminal includes means for sending each user request as one or more data packets each of which is transmitted as a modulated carrier whose frequency is randomly selected from a set of N predetermined frequencies common to all the user terminals, each data packet including data identifying the particular user terminal from which it originates, and wherein the central broadcast station includes N channels each arranged to receive and demodulate data packets transmitted at a respective one of the N carrier frequencies .
- each user terminal transmits requests to the hub station as a series of data packets distributed in time, with successive data packets being sent at respective frequencies randomly selected from the given set of N frequencies.
- the user terminal "hops" between frequencies, and consequently this process is known as frequency hopping.
- the probability of two packets arriving on one of the N frequencies at the hub station at the same time is significantly reduced compared to the case of all packets being transmitted and received on a single frequency, and the origin of each data packet is determined from the identifying data included within the data packet rather than the frequency on which it is transmitted.
- Frequency hopping systems are known in the art. However, in conventional frequency hopping systems the changing between frequencies is performed according to a predetermined sequence which is known by the transmitter and receiver. A single receiver is normally used, which changes the frequency on which it receives according to the predetermined sequence in step with the transmitter.
- the hub station uses the present invention to know the sequence in which the user terminal transmitters hop between frequencies and thus truly random frequency hopping can be achieved.
- each data packet includes a preamble containing, in addition to the user terminal identifying data, a plurality of bits which alternate in value, i.e 101010101...etc.
- a preamble containing, in addition to the user terminal identifying data, a plurality of bits which alternate in value, i.e 101010101...etc.
- Fig. 1 is a block diagram of a two-way satellite communication system according to the embodiment of the invention.
- Fig. 2 is a block diagram of the frequency hopping transmission circuit in the user terminal of Fig. 1, and
- Fig. 3 is a block diagram of the data packet clock recovery scheme.
- Fig. 1 illustrates the overall two-way satellite communication system, which comprises a hub station 10 in satellite communication with a PC-based user satellite terminal 12. Although only one user terminal 12 is shown, it will be understood that the system includes many such terminals and the description of the terminal 12 herein will apply to all such terminals.
- a user can make a request for information from the internet, and this request is sent as one or more data packets 50 from the user terminal 12 via a satellite 14 to the hub station 10.
- the hub station 10 is connected to a TCP/IP server 16 and router 18 allowing the request to be passed onto the internet 60.
- the requested information is retrieved from the internet it is transmitted back from the hub station 10 via the satellite 14 to the user terminal 12.
- the first stage in this general procedure is the transmission of the user internet request, or IP request, as a data packet, or number of data packets, from the user terminal 12.
- the user terminal 12 comprises a personal computer (PC) 20 which contains a transmit interface card (Fig. 2), and which is connected via such card to the integrated feed (not shown) of a small satellite dish 22.
- the transmit interface card allows the computer 10 to interface with the dish 22.
- the PC 20 may be connected to a network of computers 24 via a LAN 26 and can receive IP requests from one or more of the computers in this network.
- the interface between the PC 20 and the dish 22, as provided by the transmit interface card, is schematically shown in Fig. 2.
- digital IP request data from the PC 20 is first passed to a first in, first out (FIFO) buffer 28, which is essentially a variable length shift register.
- the buffer collects all the data corresponding to a given request (or, where the IP request is to be transmitted in more than one data packet, for the portion of the IP request data next to be transmitted) and then passes this on to be encrypted at 30.
- the length of the request or portion thereof, and hence the activation of output from the buffer, is determined by control logic 32.
- a preamble is generated for the IP request data or portion thereof.
- the preamble is, in this embodiment, 100 bits long, where the first 56 are bits which alternate in value, i.e. 101010101... etc. , and are used to correct for frequency drift as will be described, the next 24 bits define a word unique to that request data or portion thereof, and the final 20 bits contain header data.
- the unique word acts as a tag for each data packet and is also used for synchronisation purposes.
- the 20 bit header contains the length of the data packet.
- the IP address of the user terminal which originated the IP request is contained within the body of the data packet. This preamble is attached to the encrypted request data in a multiplexer 36.
- N the number of frequencies
- the control logic 32 determines when a new data packet is ready for transmission and thus instructs the local oscillator 38 to choose a new frequency, which may be randomly the same as the previous frequency but will in general be different.
- the random choice of a new frequency is effected by using a standard random number generator 40.
- the data packet including preamble is passed from the multiplexer 24 to a differential and convolutional encoder 42, and then passed to a mixer 44.
- the random frequency carrier signal generated by the local oscillator 38 is phase modulated by the data packet using binary phase shift keying, as is well known in the art.
- the modulated signal is then passed via a band pass filter 46 and transmission enable gate 48 to the integrated feed (not shown) of the satellite dish 22.
- timing, synchronisation and transmission enable are all determined by the control logic 32.
- the satellite dish 22 is preferably a standard 90cm diameter earth station.
- the integrated feed carries out the functions of frequency up-conversion to microwave carrier frequencies and provides transmission at relatively low powers, typically of about 250 mW.
- the data packets 50 from the user terminal 12, thus encoded, frequency hopped, and frequency up-converted, are transmitted, typically at 16 kBits per second, to the hub station 10 via the satellite 14. It will be understood that each of the N IF carrier frequencies generated by the local oscillator 38 will be up-converted to a different microwave carrier frequency by the integrated feed, so the principle of transmitting each packet at a random one of N different carrier frequencies is 7 preserved during the microwave transmission to the hub station 10 via the satellite 14.
- the hub station 10 contains N channels each containing a burst receiver 52 and a processing unit 54 (for simplicity only three such channels are shown in Fig. 1). Each receiver 52 is tuned to a respective one of the N possible microwave carrier frequencies. In the receiver 52 for which the currently received data packet is in band, the received signal is down-converted in a series of frequency steps to produce typically a 64 kHz IF signal which is applied to the processing unit 54 and sampled at 256 kHz by a 12 bit ADC (Analogue to Digital Converter) .
- the processing unit 54 consists of a single extended 6U multi-layer printed circuit board, containing two 80 Mhz TMS320C50 fixed point digital signal processors, jointly providing 80 MIPS.
- the incoming data packet is demodulated and decoded in the processing unit 54 and the relevant IP request is then passed to the TCP/IP server 16 via a satellite communication server 56.
- the frequency of the transmitted signal can drift considerably over the satellite link. Indeed, the nominal 64 kHz IF signal produced by the burst receivers 52 can be in error by as much as 10 kHz. In order to accurately demodulate the received signal, the actual frequency of the signal needs to be correctly determined.
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- Engineering & Computer Science (AREA)
- Computer Networks & Wireless Communication (AREA)
- Signal Processing (AREA)
- Radio Relay Systems (AREA)
- Data Exchanges In Wide-Area Networks (AREA)
Abstract
A satellite communication system comprises a central broadcast station (10) in satellite communication with a plurality of user terminals (12) each of which can make requests for broadcast data from the central broadcast station. Each user terminal includes means (Fig. 2) for sending each user request as one or more data packets (50) each of which is transmitted as a modulated carrier whose frequency is randomly selected from a set of N predetermined frequencies common to all the user terminals. Each data packet includes data identifying the particular user terminal from which it originates, and the central broadcast station includes N channels (52/54) each arranged to receive and demodulate data packets transmitted at a respective one of the N carrier frequencies.
Description
1 DATA PACKET SATELLITE COMMUNICATION SYSTEM
Technical Field
This invention relates to a satellite communication system comprising a central broadcast station in satellite communication with a plurality of user terminals each of which can make requests for broadcast data from the central broadcast station.
Background Art
There is at present a need to provide high speed data communication links in order to carry TCP/IP internet traffic between small user satellite terminals of around 90 cm diameter and a central broadcast station, herein referred to as a hub station. Such systems have been proposed in the past but suffer from network congestion and failure due to high peak traffic loading. In such systems the user requests are generally transmitted as data packets from the user terminal via satellite to the hub station. If many users transmit requests simultaneously, so-called packet collisions occur as a large number of data packets arrive at the hub station at the. same time .
It is an object of the present invention to provide an improved satellite communication system, in particular for, but not limited to, internet traffic, in which the incidence of packet collisions is reduced.
Disclosure of Invention
Accordingly, the invention provides a satellite communication system comprising a central broadcast station in satellite communication with a plurality of user terminals each of which can make requests for
2 broadcast data from the central broadcast station, wherein each user terminal includes means for sending each user request as one or more data packets each of which is transmitted as a modulated carrier whose frequency is randomly selected from a set of N predetermined frequencies common to all the user terminals, each data packet including data identifying the particular user terminal from which it originates, and wherein the central broadcast station includes N channels each arranged to receive and demodulate data packets transmitted at a respective one of the N carrier frequencies .
Thus, in operation of the system, each user terminal transmits requests to the hub station as a series of data packets distributed in time, with successive data packets being sent at respective frequencies randomly selected from the given set of N frequencies. In other words, the user terminal "hops" between frequencies, and consequently this process is known as frequency hopping.
The probability of two packets arriving on one of the N frequencies at the hub station at the same time is significantly reduced compared to the case of all packets being transmitted and received on a single frequency, and the origin of each data packet is determined from the identifying data included within the data packet rather than the frequency on which it is transmitted.
Frequency hopping systems are known in the art. However, in conventional frequency hopping systems the changing between frequencies is performed according to a predetermined sequence which is known by the transmitter and receiver. A single receiver is normally used, which
changes the frequency on which it receives according to the predetermined sequence in step with the transmitter.
Using the present invention, there is no need for the hub station to know the sequence in which the user terminal transmitters hop between frequencies and thus truly random frequency hopping can be achieved.
Preferably each data packet includes a preamble containing, in addition to the user terminal identifying data, a plurality of bits which alternate in value, i.e 101010101...etc. As is described below, these allow the correct frequency of the received data packets to be accurately evaluated despite frequency drift due to component tolerances and propagation over the satellite link.
Brief Description of Drawings
An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, wherein:
Fig. 1 is a block diagram of a two-way satellite communication system according to the embodiment of the invention,
Fig. 2 is a block diagram of the frequency hopping transmission circuit in the user terminal of Fig. 1, and
Fig. 3 is a block diagram of the data packet clock recovery scheme.
Description of Preferred Embodiments
Fig. 1 illustrates the overall two-way satellite communication system, which comprises a hub station 10 in satellite communication with a PC-based user satellite terminal 12. Although only one user terminal 12 is shown, it will be understood that the system includes many such terminals and the description of the terminal 12 herein will apply to all such terminals.
In operation a user can make a request for information from the internet, and this request is sent as one or more data packets 50 from the user terminal 12 via a satellite 14 to the hub station 10. The hub station 10 is connected to a TCP/IP server 16 and router 18 allowing the request to be passed onto the internet 60. When the requested information is retrieved from the internet it is transmitted back from the hub station 10 via the satellite 14 to the user terminal 12.
The first stage in this general procedure is the transmission of the user internet request, or IP request, as a data packet, or number of data packets, from the user terminal 12. To this end the user terminal 12 comprises a personal computer (PC) 20 which contains a transmit interface card (Fig. 2), and which is connected via such card to the integrated feed (not shown) of a small satellite dish 22. The transmit interface card allows the computer 10 to interface with the dish 22. The PC 20 may be connected to a network of computers 24 via a LAN 26 and can receive IP requests from one or more of the computers in this network.
The interface between the PC 20 and the dish 22, as provided by the transmit interface card, is schematically shown in Fig. 2. In the transmit interface card digital
IP request data from the PC 20 is first passed to a first in, first out (FIFO) buffer 28, which is essentially a variable length shift register. The buffer collects all the data corresponding to a given request (or, where the IP request is to be transmitted in more than one data packet, for the portion of the IP request data next to be transmitted) and then passes this on to be encrypted at 30. The length of the request or portion thereof, and hence the activation of output from the buffer, is determined by control logic 32.
At 34 a preamble is generated for the IP request data or portion thereof. The preamble is, in this embodiment, 100 bits long, where the first 56 are bits which alternate in value, i.e. 101010101... etc. , and are used to correct for frequency drift as will be described, the next 24 bits define a word unique to that request data or portion thereof, and the final 20 bits contain header data. The unique word acts as a tag for each data packet and is also used for synchronisation purposes. The 20 bit header contains the length of the data packet. The IP address of the user terminal which originated the IP request is contained within the body of the data packet. This preamble is attached to the encrypted request data in a multiplexer 36.
The transmit interface card also contains a local oscillator 38 which is enabled to generate IF carrier signals on any one of N different frequencies (in the present embodiment N=8) . According to the frequency hopping scheme, each successive data packet is transmitted at a carrier frequency which is chosen at random from the set of- N possible frequencies. The control logic 32 determines when a new data packet is
ready for transmission and thus instructs the local oscillator 38 to choose a new frequency, which may be randomly the same as the previous frequency but will in general be different. The random choice of a new frequency is effected by using a standard random number generator 40.
The data packet including preamble is passed from the multiplexer 24 to a differential and convolutional encoder 42, and then passed to a mixer 44. At the mixer 44 the random frequency carrier signal generated by the local oscillator 38 is phase modulated by the data packet using binary phase shift keying, as is well known in the art. The modulated signal is then passed via a band pass filter 46 and transmission enable gate 48 to the integrated feed (not shown) of the satellite dish 22. In the interface circuit, timing, synchronisation and transmission enable are all determined by the control logic 32.
The satellite dish 22 is preferably a standard 90cm diameter earth station. The integrated feed carries out the functions of frequency up-conversion to microwave carrier frequencies and provides transmission at relatively low powers, typically of about 250 mW. The data packets 50 from the user terminal 12, thus encoded, frequency hopped, and frequency up-converted, are transmitted, typically at 16 kBits per second, to the hub station 10 via the satellite 14. It will be understood that each of the N IF carrier frequencies generated by the local oscillator 38 will be up-converted to a different microwave carrier frequency by the integrated feed, so the principle of transmitting each packet at a random one of N different carrier frequencies is
7 preserved during the microwave transmission to the hub station 10 via the satellite 14.
Referring again to Fig. 1, the hub station 10 contains N channels each containing a burst receiver 52 and a processing unit 54 (for simplicity only three such channels are shown in Fig. 1). Each receiver 52 is tuned to a respective one of the N possible microwave carrier frequencies. In the receiver 52 for which the currently received data packet is in band, the received signal is down-converted in a series of frequency steps to produce typically a 64 kHz IF signal which is applied to the processing unit 54 and sampled at 256 kHz by a 12 bit ADC (Analogue to Digital Converter) . The processing unit 54 consists of a single extended 6U multi-layer printed circuit board, containing two 80 Mhz TMS320C50 fixed point digital signal processors, jointly providing 80 MIPS. The incoming data packet is demodulated and decoded in the processing unit 54 and the relevant IP request is then passed to the TCP/IP server 16 via a satellite communication server 56.
As -mentioned earlier, the frequency of the transmitted signal can drift considerably over the satellite link. Indeed, the nominal 64 kHz IF signal produced by the burst receivers 52 can be in error by as much as 10 kHz. In order to accurately demodulate the received signal, the actual frequency of the signal needs to be correctly determined.
This is achieved in the processing unit 54 which determines the power spectrum of the signal in the frequency domain and hence at which frequency the power is greatest. In order to do this a Fast Fourier
Claims
1. A satellite communication system comprising a central broadcast station in satellite communication with a plurality of user terminals each of which can make requests for broadcast data from the central broadcast station, wherein each user terminal includes means for sending each user request as one or more data packets each of which is transmitted as a modulated carrier whose frequency is randomly selected from a set of N predetermined frequencies common to all the user terminals, each data packet including data identifying the particular user terminal from which it originates, and wherein the central broadcast station includes N channels each arranged to receive and demodulate data packets transmitted at a respective one of the N carrier frequencies .
2. A system as claimed in claim 1, wherein each user terminal includes means for modulating each data packet on a randomly selected one of N predetermined intermediate carrier frequencies and means for up- converting each intermediate carrier frequency to a respective one of N microwave carrier frequencies prior to transmission of the data packet.
3. A system as claimed in claim 2, wherein the central broadcast station includes means for down-converting the microwave carrier frequency of each received data packet to a nominal intermediate frequency, analog-to-digital converting the down-converted data packet, estimating the actual frequency of the down-converted data packet by applying a Fast Fourier Transform to the digital data
11 packet to provide a frequency domain power spectrum, and demodulating the digital data packet using the frequency thus estimated.
4. A system as claimed in claim 3, wherein each data packet includes a preamble containing digital data which provides, in the frequency domain power spectrum, two local maxima centered about the actual frequency of the down-converted data packet.
5. A system as claimed in claim 4, wherein the digital data comprises a plurality of bits which alternate in value.
6. A system as claimed in claim 3, 4 or 5, wherein the Fast Fourier Transform is an Offset Fast Fourier Transform.
7. A system as claimed in any one of claims 3 to 6, wherein the central broadcast station includes means for recovering the bit clock of the demodulated data packet by delaying the data bits and adding them to the undelayed bits to provide a signal having a strong frequency component at the bit clock rate, applying the lastmentioned signal to a filter, and limiting the output of the filter to provide the bit clock, the central broadcast station further including means for decoding the data packet using the bit clock so recovered.
8. A system as claimed in any preceding claim, wherein the central broadcast station includes means for passing the decoded user request to the internet and for receiving data from the internet in response to such request .
12
9. A system as claimed in claim 8, wherein the central broadcast station includes means for broadcasting the data received from the internet to all user terminals.
10. A satellite communication system substantially as described herein with reference to the accompanying drawings .
Priority Applications (1)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
AU30505/99A AU3050599A (en) | 1998-03-23 | 1999-03-23 | Data packet satellite communication system |
Applications Claiming Priority (4)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
IES980213 IES980213A2 (en) | 1998-03-23 | 1998-03-23 | A satellite communication system |
IES980213 | 1998-03-23 | ||
IE980749A IE980749A1 (en) | 1998-03-23 | 1998-09-08 | A Satellite Communication System |
IE980749 | 1998-09-08 |
Publications (1)
Publication Number | Publication Date |
---|---|
WO1999049591A1 true WO1999049591A1 (en) | 1999-09-30 |
Family
ID=26320169
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
PCT/IE1999/000015 WO1999049591A1 (en) | 1998-03-23 | 1999-03-23 | Data packet satellite communication system |
Country Status (3)
Country | Link |
---|---|
AU (1) | AU3050599A (en) |
IE (1) | IE980749A1 (en) |
WO (1) | WO1999049591A1 (en) |
Cited By (7)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
WO2001080452A1 (en) * | 2000-04-14 | 2001-10-25 | Hughes Electronics Corporation | Interfaces between terminal users, satellites and hubs in a two-way satellite system |
WO2001080455A1 (en) * | 2000-04-14 | 2001-10-25 | Hughes Electronics Corporation | System and method for providing a scalable architecture in a two-way satellite system |
US6441782B2 (en) | 2000-04-14 | 2002-08-27 | Hughes Electronics Corporation | Method and system of directing an antenna in a two-way satellite system |
US6650869B2 (en) | 2000-04-14 | 2003-11-18 | Hughes Electronics Corporation | System and method for managing return channel bandwidth in a two-way satellite system |
US6965581B2 (en) | 2000-04-14 | 2005-11-15 | Hughes Electronics Corp. | Transceiver in a two-way satellite system |
US6987741B2 (en) | 2000-04-14 | 2006-01-17 | Hughes Electronics Corporation | System and method for managing bandwidth in a two-way satellite system |
US7164661B2 (en) | 2000-04-14 | 2007-01-16 | Hughes Networks Systems, Llc | System and method for providing a two-way satellite system |
Citations (2)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
EP0423715A2 (en) * | 1989-10-16 | 1991-04-24 | Hughes Aircraft Company | Synchronization for entry to network in a frequency hopping communication system |
US5539769A (en) * | 1994-03-28 | 1996-07-23 | University Of Southern California | Adaptive fuzzy frequency hopping system |
-
1998
- 1998-09-08 IE IE980749A patent/IE980749A1/en not_active IP Right Cessation
-
1999
- 1999-03-23 WO PCT/IE1999/000015 patent/WO1999049591A1/en active Application Filing
- 1999-03-23 AU AU30505/99A patent/AU3050599A/en not_active Abandoned
Patent Citations (2)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
EP0423715A2 (en) * | 1989-10-16 | 1991-04-24 | Hughes Aircraft Company | Synchronization for entry to network in a frequency hopping communication system |
US5539769A (en) * | 1994-03-28 | 1996-07-23 | University Of Southern California | Adaptive fuzzy frequency hopping system |
Cited By (8)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
WO2001080452A1 (en) * | 2000-04-14 | 2001-10-25 | Hughes Electronics Corporation | Interfaces between terminal users, satellites and hubs in a two-way satellite system |
WO2001080455A1 (en) * | 2000-04-14 | 2001-10-25 | Hughes Electronics Corporation | System and method for providing a scalable architecture in a two-way satellite system |
US6441782B2 (en) | 2000-04-14 | 2002-08-27 | Hughes Electronics Corporation | Method and system of directing an antenna in a two-way satellite system |
US6650869B2 (en) | 2000-04-14 | 2003-11-18 | Hughes Electronics Corporation | System and method for managing return channel bandwidth in a two-way satellite system |
US6965581B2 (en) | 2000-04-14 | 2005-11-15 | Hughes Electronics Corp. | Transceiver in a two-way satellite system |
US6987741B2 (en) | 2000-04-14 | 2006-01-17 | Hughes Electronics Corporation | System and method for managing bandwidth in a two-way satellite system |
US7164661B2 (en) | 2000-04-14 | 2007-01-16 | Hughes Networks Systems, Llc | System and method for providing a two-way satellite system |
US7463582B2 (en) | 2000-04-14 | 2008-12-09 | Hughes Network Systems, Llc | System and method for scaling a two-way satellite system |
Also Published As
Publication number | Publication date |
---|---|
IE980749A1 (en) | 1999-10-20 |
AU3050599A (en) | 1999-10-18 |
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