Multiplex communications – Communication over free space – Repeater
Reexamination Certificate
1997-11-14
2001-05-29
Vu, Huy D. (Department: 2663)
Multiplex communications
Communication over free space
Repeater
C370S437000, C375S213000
Reexamination Certificate
active
06240073
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to satellite communications and more particularly relates to a satellite based multiple access reverse communication link suitable for an Internet access network.
BACKGROUND OF THE INVENTION
Currently, communication systems around the world are growing rapidly due to the increasing need for data communication bandwidth. In particular, satellite communication systems are currently experiencing rapid growth due to growing customer demand for satellite based data communications. Most of the current demand and estimated future demand will be for Internet and other network based data communication applications. A major factor in these communication systems is the bandwidth capacity demanded by the user. Applications in widespread use today, such as video conferencing, LAN/WAN and document delivery require high speed forward and return link data capacities. Currently, however, these capabilities are not provided by present day Internet via satellite systems.
It is currently estimated that there are approximately 13 million hosts and 16 million users on the Internet. The growth rate has been approximately 10 million new users a year for the past four years. At the same time, the number of Intranets (Internet network protocols applied within an enterprise or company for sharing information) are growing at an even faster rate. Currently, accessing the Internet via satellite has gained recognition as a fast and reliable solution for fast Internet access. Current commercially available Internet via satellite solutions such as DirecPC are based on an asymmetrical approach in which the data link to the user is via satellite while the return link to the user is via telephone lines using commercially available telephony modems. The disadvantages of these asymmetrical systems is outlined below.
The asymmetric approach via satellite, in which the user receives data from the Internet via satellite, yet sends data to the Internet via telephone lines, does not take advantage of a major feature of satellite communications: wide area coverage. The asymmetric link is based on a terrestrial connection and therefore limits the ability of the fast connection to the Internet to those places in which telephone lines and Internet service providers are available and have sufficient grade of service.
The data rate of asymmetric Internet via satellite communication systems enables basically e-mail and browsing applications only. This structure is mainly targeted to consumer markets where the user is limited to sending data from their home at relatively low speeds. There are, however, many users such as small office/home office (SOHO) that desire high speed data communications in both directions yet cannot afford having dedicated lease lines for their Internet connection. In the United States alone there are approximately 3.5 million small businesses of which only 10% can justify an expensive leased line. Thus, there are a large group of users looking for an on demand economical, fast and reliable connection to the Internet with a grade of service similar to that of a leased line.
Typical applications that require high data rates in both directions include video conferencing, LAN/WAN systems, Internet applications, document delivery, audio applications such as Internet Phone, commercial web sites, net gaming, point of presence, terminal equipment, Net Meeting and collaboration software. All the above mentioned applications are currently not adequately served by the currently available asymmetric satellite communication solutions.
Spread spectrum communication systems have been used in a variety of fields for some time now. In spread spectrum communication systems, the bandwidth of the transmitted signal is much greater than the bandwidth of the information to be transmitted. The carrier signal in such systems is modulated by a function that serves to widen or spread the bandwidth of the signal for transmission. On the receive side, signal is remapped or decoded into the original information bandwidth to reproduce the desired output signal.
Spread spectrum systems can be categorized into direct sequence systems, frequency hopping systems, time hopping systems and hybrid systems which are combinations of the above three.
In frequency hopping systems a carrier frequency is shifted or hopped in discrete increments in a pattern dictated by a predetermined code or sequence, e.g., a pseudo noise sequence or code. The resulting consecutive and time sequential frequency pattern is called a hopping pattern and the duration of each hop frequency is called a chip. The transmitted information is embedded in the codes or embedded within each frequency in the carrier wave by a modulation scheme such as PSK or FSK.
In reproducing the information signal of the receiver a synchronization acquisition process is performed in which the code pattern utilized by the receiver is synchronized with the code pattern generated and used in the transmitter. Using this, de-spreading and demodulation are performed on the spread spectrum signal to extract the transmitted data. A local reference signal is used that has a frequency corresponding to the same code pattern used in the transmitter for every chip. The received signal and the local reference are mixed in order to perform a correlation or de-spreading process for converting the spread spectrum signal into a signal having a frequency bandwidth wide enough to extract the data information. More information describing the operation of spread spectrum systems can be found in “Spread Spectrum Systems,” by R. C. Dixon published by John Wiley and Sons, Inc., 1976.
Multiple user systems use multiple access techniques to allow users to share resources such as time and frequency. When the traffic from each user in the network is approximately steady it is possible to divide a single high capacity multiple access channel into a plurality of smaller orthogonal channels corresponding to individual user requirements. This can be accomplished either on a frequency basis using FDMA, on a time basis using TDMA or using CDMA. In addition, various combinations of FDMA and TDMA can also be used to minimize cost in large networks. FDMA and TDMA techniques are suitable solutions as long the traffic from each user is relatively stable. CDMA is a multiple access technique which uses spread spectrum communications. CDMA communications can be synchronous if all users are mutually synchronized in time.
TDMA communication systems are also known for providing multiple access. Theses systems partition the channel time in a fixed predetermined manner. They are efficient when the user population includes only a relatively small number of users having high duty cycles. However, many modern communication systems need to provide communication among interactive data terminals which operate in low duty cycle burst modes. Thus, TDMA is not particularly suited to this kind of communication.
In the typical modern interactive network, however, the traffic from individual terminals in the system varies as a function of time due to random traffic demands by different users at each terminal. In addition, the set of terminals active in the network can vary from moment to moment. In such systems it may be desirable to assign channel capacity to users on demand by means of a demand assigned multiple access (DAMA) architecture. In a DAMA system a separate channel called the request channel is used by individual users to request capacity as needed. The capacity can then be allocated in response to requests by a central master controller implemented by a common algorithm running in each terminal.
A DAMA system however introduces additional overhead into the multiple access channel due to the process of requesting and assigning capacity. In addition, the demand assignment process introduces a delay which can degrade the performance under the channel.
In some DAMA networks the total number of potential data terminals sharing the request channel is much larger than the numb
Barda Avi
Goldenberg Sorin
Laufer Shaul
Reichman Arie
Darby & Darby
Shiron Satellite Communications (1996) Ltd.
Vu Huy D.
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