Patent Application
20070281611
The above set forth and other features of the invention are made more apparent in the ensuing Detailed Description of the Invention when read in conjunction with the attached drawings, wherein:
Overall MSS and ATC capacity is optimized by switching in a filter setting corresponding to the relative MSS and ATC traffic anticipated. This may be done dynamically, on a beam-by-beam and satellite-by-satellite basis, or on a predetermined basis based on time of day and traffic projections, or using any other method of determining how many channels are needed for MSS versus ATC. Thus in filter setting L4, only 2 MSS channels are sufficient for MSS traffic. In this setting, any amount of ATC traffic can be put into the remaining 11 channels of the 13 possible without affecting the MSS traffic in that beam. However, practical limitations arise from adjacent beam interference, but even then, actual ATC capacity per beam is significantly enhanced over the case where there is no such filter setting.
For example, it is currently found that the Globalstar® satellite system as above defined can support 5.88 million ATC subscribers per 1.23 MHz channel while still maintaining excellent MSS capacity to serve rural areas. This provides a total ATC subscriber base of 23.5 million ATC subscribers in 5.5 MHz ATC spectrum over the continental United States (CONUS). Globalstar® may further increase ATC capacity by making use of the narrowest L band filters available on the Globalstar® satellite constellation to result in about 411 million subscribers for ATC services in CONUS while still serving 107,000 MSS subscribers in CONUS.
It is found that employing the system and method of the instant invention that S band use of ATC does not noticeably impact satellite capacity since forward link capacity is determined by total satellite power and there are, for example, 13 channels available on the currently employed Globalstar® system available on the forward link and up to seven channels contemplated for ATC use. Consequently, it is found that MSS capacity limitations due to the employment of ATC become noticeable with the interference limited to L band on the return link.
Accordingly, the discussion hereinafter will be directed to the return link wherein the ATC capacity available is calculated employing the steps which are discussed hereinafter.
A first determination is made to assess the interference an average ATC terminal contributes towards co-channel MSS users by evaluating the various factors that diminish the impact of an ATC terminal's interference into the satellite's L band antenna. These factors include the difference in propagation loss towards the satellite and base station, differences in antenna gain in the two directions, and polarization losses, as well as the reduced power expected on average for ATC transmissions. Under these circumstances, a loss of about 26.9 dB or a factor of 490 when considering the impact of an ATC subscriber as opposed to an MSS subscriber or, in other words, 490 ATC subscribers are equivalent to one MSS subscriber co-channel in the same beam. Each 1.23 MHz channel is found to carry 60 simultaneous MSS users per beam, which translates to about 60×490 or about 29,400 simultaneous ATC users per beam or about 176,400 simultaneous ATC users over six beams covering CONUS before it exhausts the MSS channel. When considering the standard traffic intensity of 30 milli-Erlangs for cellular use, the total number of ATC subscribers in CONUS is about 5.88 million per 1.23 MHz channel. It is contemplated that many more subscribers can be accommodated without affecting the same beam; however, as more ATC subscribers are added beyond the number that causes interference equal to the MSS capacity of that channel, the ATC interference starts affecting adjacent beams as well. For example, if each beam is attenuated on average by 6 dB, or a factor of 4, in the region of overlapping beams, then if there were 5.88 ×4 million ATC subscribers in one channel in one beam, then that would effectively be depriving the adjacent beam of MSS use of that channel.
Employing the assumption that there are 50 major trade areas (MTAs) in CONUS and that ATC is rolled out progressively across these MTAs, on average, if only 10 MTAs have ATC rolled out, then the capacity impact on each beam is about ⅕ the effect it would have if all 50 MTAs were rolled out. Of course, the specifics of the MTA locations would affect the actual result; nevertheless, this approximation is substantially correct to calculate the loss of MSS capacity as ATC is rolled out.
In the instance where Globalstar® is authorized ATC in all nine L band channels, it would be possible to do a band segmentation and allocate separate MSS and ATC channels by employing the L band surface acoustic wave (SAW) filter settings available on each Globalstar® satellite. Globalstar® satellites, as originally built and employed, included these filters by virtue of the uncertainty at the time of designing the satellites as to which channels would be allocated to Globalstar® and which, for example, to Iridium and they comprise four settings: L1 (all 13 channels), L2 (9 channels), L3 (6 channels), and L4 (2 channels) as shown conceptually in
In the case where MSS were restricted to only 2 channels available in L4 mode, then ATC subscribers in the other 7 channels could be increased to 10×5.88 million per channel before impacting adjacent beams of other satellites that overlap the area and are in L2 mode. This would result in about 411 million ATC subscribers and about 107,000 MSS subscribers in CONUS.
Any suitable method of assigning a first frequency (F1) to the ATC component may be employed in the system of the instant invention. Typical methods of assigning a first frequency include random assignment, or use of an algorithm that determines which frequency is least occupied by desired users and/or interference.
Any suitable method of dynamically setting a filter on the satellite to suppress F1 frequency may be employed in the system of the instant invention. Typical methods for setting filters include switching in a filter based on a command transmitted from the ground, or on a predetermined time schedule.
Any suitable method of allocating frequencies other than F1 to the MSS component may be employed in the system of the instant invention. Typical methods of allocating include random assignment, or use of an algorithm for selecting frequencies sequentially based on a determination of which ones have least interference.
Although the method and system of the instant invention has been illustrated by employing a LEO constellation comprising at least one satellite in LEO orbit, the method and system of the invention may also be practiced employing medium earth orbiting (MEO) satellites comprising at least one satellite in MEO orbit, geosynchronous earth orbiting (GEO) satellites comprising at least one satellite in GEO orbit, and highly elliptical earth orbiting (HEO) satellites comprising at least one satellite in HEO orbit.
