Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1996-10-21
2003-07-01
Le, Thanh Cong (Department: 2684)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S452200
Reexamination Certificate
active
06587687
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to satellite communications systems and, in particular, to RF transmission and power control techniques for use in a satellite communications system.
BACKGROUND OF THE INVENTION
Satellite telephone systems for fixed and mobile communications are emerging as a new global business. These systems utilize many individual circuits routed through one satellite or a constellation of many satellites to effect communications. The value of the satellite telephone system is that it provides ubiquitous coverage of large areas of the earth without the construction of many small terrestrial cells. Since the allocation of frequencies for satellite services, a number of proposals have been advanced for the deployment of satellite communications systems. In general, these proposals have involved either a Time Division Multiple Access (TDMA) technique or a Code Division Multiple Access (CDMA) technique.
The communications link availability for these services are a critical factor. In high frequency bands above 3 GHz, and especially above 10 GHz, it is important to avoid a large amount of margin in the signal strength in order to avoid oversizing the satellite design. Further it is important for some systems, such as CDMA systems, to maintain the signal at a fixed level as it arrives at the satellite. An important consideration then is the method selected for compensating for rain attenuation in the frequency bands above 3 GHz, and for other types of signal path impairments as well.
Referring to
FIG. 1A
, there is shown an arrangement for communication to several satellites
2
for a typical Teleport or Mobile Satellite Service (MSS) site
1
. The site
1
, also referred to as a gateway, is a typical multi-satellite teleport having antennas
1
a
in contact with several satellites
2
, each of which is connected to one or more gateways or served entities. The multiple antennas la connected to a single gateway
1
may provide multiple satellite coverage, such as is proposed for Low Earth Orbit (LEO) Mobile Satellite Service (MSS) or Fixed Satellite Service (FSS).
FIG. 1B
illustrates the same site
1
with the addition of a spatial diversity site
1
b
separated from the primary site by a distance D. In this approach the spatial diversity of antenna sites provides alternative signaling paths to a single one of the satellites
2
. In the bands above 10 GHz, in order to maintain high levels of availability, it is conventional practice to place the redundant or diversity site
1
b
some 35 km to 100 km away from the primary site
1
. By thus separating the sites the communications can be switched to the diversity site
1
b
when atmospheric attenuation, due, for example, to a presence of a rain cell near the primary site
1
, exceeds a certain value at the primary site.
Alternatively, a second approach eliminates the diversity site
1
b
by placing the primary site
1
in a region, such as a desert, where rain is infrequent.
Neither of these approaches is satisfactory, and both incur large incremental costs. In the first approach there must be a duplication of hardware, real estate, and possibly personnel if the sites are manned. Also, some mechanism must be installed for linking the primary site
1
to the diversity site
1
b
(e.g., underground cables, microwave towers, etc.). The second approach requires the construction, provisioning, and maintenance of the site in a location which is either inconvenient or is not economically attractive (e.g., the site is too far from the PSTN connection, requiring long distance backhauls).
It can be realized that the communications capability is enhanced by selecting the communications path or paths with the lowest attenuation, or by combining lowest attenuation paths, while avoiding paths which are heavily attenuated. This approach maximizes communications signal strengths and reduces the amount of signal strength margin required. That is, instead of transmitting at a power level that is required to compensate for the heavily attenuated paths, and thus consuming a considerable amount of satellite power, a better approach is to avoid the heavily attenuated path or paths in favor of the less attenuated path or paths. In order to accomplish this technique it is necessary to make decisions based on observed amounts of path attenuation.
In previous systems known to the inventor such decisions were made based on received signal strength, at the gateway, of a signal transmitted from some source, generally located on the satellite or passed through the satellite. However, a disadvantage of this approach is that the attenuation information is only instantaneously known, thereby making impossible an ability to perform short term or long term planning of link allocation and power budgets.
Reference in regard to various satellite power control techniques can be had to the following U.S. Pat. No. 4,991,199, Saam, “Uplink Power Control Mechanism For Maintaining Constant Output Power From Satellite Transponder”; U.S. Pat. No. 4,752,967, Bustamante et al., “Power Control System For Satellite Communications”; U.S. Pat. No. 5,339,330, Mallinckrodt, “Integrated Cellular Communications System”; U.S. Pat. No. 4,752,925, Thompson et al., “Two-Hop Collocated Satellite Communications. System”; U.S. Pat. No. 5,126,748, Ames et al., “Dual Satellite Navigation System And Method”; U.S. Pat. No. 5,109,390, Gilhousen et al., “Diversity Receiver In A CDMA Cellular Telephone System”; and U.S. Pat. No. 5,138,631, Taylor, “Satellite Communication Network”.
Reference can also be had to the improved power management technique disclosed in commonly assigned and allowed U.S. patent application Ser. No. 08/467,209, filing date: Jun. 6, 1995, entitled “Closed Loop Power Control For Low Earth Obrbit Satellite Communications System”, by Robert A. Wiedeman and Michael J. Sites.
Reference may also be had, by example, to “Satellite Communications System Engineering”, 2nd Edition, W. Pritchard et al., Prentice Hall, 1993, pages 273-294, for a discussion of various noise temperatures, propagation factors, and the use of a rain attenuation model in RF link design.
OBJECTS AND ADVANTAGES OF THE INVENTION
It is a first object of this invention to provide an improved satellite communication system and method for accurately determining the presence and amount of attenuation due to rain and other weather-related events.
It is a second object of this invention to provide an improved satellite communication system and method that employs a direct measurement of severe path attenuation potential, without requiring a signal source either transmitted from a satellite or passed through the satellite.
An advantage provided by the teaching of this invention is an ability to operate a satellite communications system so as to conserve system power.
A further advantage provided by the teaching of this invention is an ability to operate a satellite communications system so as to assign and allocate resources in accordance with a current model of RF signal path impairments between gateways, satellites, and user terminals, on a local or global scale.
SUMMARY OF THE INVENTION
The foregoing and other problems are overcome and the objects and advantages are realized by methods and apparatus in accordance with embodiments of this invention.
This invention pertains particularly to satellite communications systems using GSO or NGSO satellites. This invention employs data derived from signals of various types to derive a model of atmospheric-related attenuation-inducing events so as to plan system resource allocation to minimize an amount of power required to close communication links between user terminals and the satellites. Disclosed are methods for modelling gateway to satellite links, as well as methods for modeling the user terminal to satellite links.
This invention avoids the necessity to employ diversity antenna sites, but does not preclude the use of such sites, by modeling atmospheric disturbances in real time by measurement, by predicting the “best” path to use,
Cong Le Thanh
Corsaro Nick
Globalstar L.P.
Ohlandt, Greeley, Ruggerio & Perle, L.L.P.
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