Telecommunications – Carrier wave repeater or relay system – Monitoring
Patent
1995-01-04
1998-09-29
To, Doris H.
Telecommunications
Carrier wave repeater or relay system
Monitoring
455 121, 455 134, 455 63, 455427, 455505, 455 671, H04B 160
Patent
active
058157964
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates to a method of determining parameters for the design of ground equipment for satellite link systems utilising duplex links.
DESCRIPTION OF THE PRIOR ART
Typical satellite link systems for duplex links include at least two ground stations and an orbiting satellite. The ground stations transmit and receive signals to and from one another via the satellite. Typical satellite link designs for duplex links implement a duplex link as two separate simplex links. When signals are transmitted in the higher bands, for example, the Ku or Ka band (11 to 14 GHz) the signals can be affected by rain and therefore this must be taken into account when designing satellite link systems. A location in which this factor must be considered in designing satellite link systems is in tropical regions where heavy rain can be expected in some parts of the year. The invention also has application in semi and non tropical regions as well.
In order to ensure that the system will be available for use in such environments systems are normally designed to operate or to be available a specific proportion of time, say 99.5% of the time. Availability 99.5% of the time is acceptable to commercial users utilising the satellite link system for communication purposes such as telephone communication and other types of communication. Thus, the need to ensure that the system is available 99.5% of the time means that the ability to transmit when rain is present at one or both of the ground stations must be taken into account when designing the ground equipment and in particular transmitter power requirements and antenna size. Minimization of transmitter power requirements and also antenna size is most desirable and represents significant cost saving in both production of the ground site and also use of the equipment at the ground site during operation of the system.
Conventional systems employ a facility known as uplink power control to increase availability by reducing the effect of rain fading on uplink signals transmitted from the ground station to the satellite. Uplink power control basically alters the output power of the transmitter at the ground station during rainy conditions to maintain the intensity of the signal received by the satellite at a constant level. Thus, in clear conditions power output of the transmitter is reduced but in rainy conditions the uplink power control increases transmitter power so that the uplink signals penetrate the precipitation and a signal arrives at the satellite which is generally equal to that which is provided in clear conditions.
For many duplex links designed in this way the uplink power control at a particular site maintains uplink availability long after the downlink (that is the signal transmitted from the satellite to the ground equipment) into the same site has failed. This effect is particularly true for connections which are downlink power limited, even to the extent that the uplink power control serves no purpose.
The basic problem is that calculating a duplex link as two simplex links in opposite directions ignores a fundamental principle of probability and results in the designs being optimistic in their availability claim. For example, assume a "duplex" circuit between two earth stations, X and Y, has four component links:
If any one of the component links becomes unavailable the entire duplex circuit is unavailable.
The conventional method of designing such an end to end "duplex" link or circuit is to decide the required availability, eg 99.5%. Given this target Link A could be engineered to give 99.8% availability and Link B to give 99.7%. Conventional calculations then give an end to end link availability of 99.5%. This is correct for the simplex path between X and Y. To complete the conventional "duplex" link the procedure is repeated between Y and X. For example Link C could be engineered for 99.9% availability and Link D for 99.6%. Again conventional calculations give the end to end availability of 99.5%.
Unfortunately the convention
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patent: 4752967 (1988-06-01), Bustamante et al.
patent: 4776035 (1988-10-01), Duggan
patent: 4819227 (1989-04-01), Rosen
patent: 4831619 (1989-05-01), Rosen
patent: 4888596 (1989-12-01), Conanan
patent: 4896369 (1990-01-01), Adams, Jr. et al.
patent: 4905235 (1990-02-01), Saburi
patent: 5060292 (1991-10-01), Ayukawa et al.
Moupfouma, F., "Rain Induced Attenuation Prediction Model for Terrestrial and Satellite-Earth Microwave Links," Annales des Telecomminucations, vol. 42, No. 9-10, Sep.-Oct. 1987, pp. 539-550.
Armstrong Robert Athol
Phillips John Joseph
Spatial Communications Pty. Ltd.
To Doris H.
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