Multiplex communications – Communication techniques for information carried in plural... – Combining or distributing information via time channels
Reexamination Certificate
1998-06-26
2001-10-02
Olms, Douglas (Department: 2661)
Multiplex communications
Communication techniques for information carried in plural...
Combining or distributing information via time channels
C375S372000
Reexamination Certificate
active
06298073
ABSTRACT:
The present invention relates to satellite information transmission systems and, more particularly, to a novel method for equalizing the transmission delays from plural satellites in a non-earth-synchronous transmission system having multiple satellite repeaters.
BACKGROUND OF THE INVENTION
It is now well known to provide information transmission via a satellite station repeating incoming data, transmitted from a first station located at a first location at, or near, the surface of the Earth, for reception by another station situated at a second location on, or near, the surface. One such system using a plurality of satellite repeaters has some of the satellites in non-earth-synchronous orbits near to the planet's surface, and is classified as a Low Earth Orbit system with a large number of satellites, sometimes called a “Big LEO”; this system is designed to provide voice, data, fax, and supplementary services to subscribers worldwide. In the system, each user has a User Terminal, or UT, which can be a hand-held, vehicular-movable or fixed-mounted RF terminal transceiver, capable of connection to other User Terminals or to conventional terrestrial-based telephone networks world-wide through system Ground Control Stations (GCSs). Services are provided globally through three satellite orbital planes, two of which are called the “B” sub-constellation, and one called the “C” sub-constellation. The orbits are optimized to provide regional coverage proportional to the distribution of population on the surface of the earth. The B orbits are sun-synchronous, inclined, elliptical orbits, each with 5 satellites, whereas the C orbit is equatorial, circular and has 6 satellites. The distance from any satellite to a particular UT is constantly changing, in a manner different from the change in the distance between any other satellite and that (or another) UT; thus, the propagation time delays between any pair of system stations and locations will be constantly variable and at least one signal will have to be delayed by a variable amount in order for both signals of any pair to be simultaneously presented, as required for proper system operation.
Thus, in this and other non-synchronous satellite communications systems, the propagation delay between a user terminal and a gateway ground station can vary by dozens of milliseconds as a relaying satellite moves relative to both Earth-surface stations. If this time variation is not properly compensated for as it occurs, there will be at least the following three problems: the apparent received data bit rates will vary from the actual transmitted bit rate by several parts per million (perhaps as much as 20 PPM); diversity combination of the signals received from two, or more, different satellites will become more difficult due to the different signal times of arrival; and handoff of the signal from one satellite to another may result in either lost data or a gap between consecutive data portions, with a gap duration substantially equal to the propagation delay difference between the paths through the different satellites.
It is therefore highly desirable to provide a method for elastically (i.e. variably time delay) buffering at least one of a plurality of incident information transmissions to facilitate the required degree of simultaneous application of the plural transmissions in the system.
BRIEF SUMMARY OF THE INVENTION
In accordance with the invention, a method for fixing the propagation delay between the user terminal and another station in a non-synchronous LEO satellite communications system, to adjust plural signals to have substantially similar times-of-arrival, uses an elastic buffer to insert a variable delay in the received signal processing, so as to cause a fixed time for each data/vocoder bit, from the time that bit enters the transmit modulation process until the time that same data/vocoder bit is output to a user.
In a presently preferred embodiment, the variable delay is set to an amount equal to the difference between the maximum possible delay to any user served by a gateway, and the current value of the actual delay to the specific user. This delay will be advantageously inserted between the soft bit decisions produced by a despreading operation and an error-correction-decoding operation in the receiver, for optimal handling of diversity combination problems.
Accordingly, it is an object of the present invention to provide a method for synchronizing plural received signals via an elastic buffer in a LEO satellite information communication system.
This and other objects of the present invention will become apparent upon reading the detailed description of the presently preferred embodiment, when considered in conjunction with the associated drawings, in which like reference numerals are utilized for like elements.
REFERENCES:
patent: 3825899 (1974-07-01), Haeberle et al.
patent: 4001690 (1977-01-01), Mack et al.
patent: 4119796 (1978-10-01), Jones
patent: 4606019 (1986-08-01), Takai
patent: 4748622 (1988-05-01), Muratani et al.
patent: 5077761 (1991-12-01), Tokunaga
Olms Douglas
Vanderpuye Ken
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