Coordinated satellite-terrestrial frequency reuse

CA 02334447 2001-02-06

111223-122 PRO PATENT

COORDINATED SATELLITE-TERRESTRIAL FREQUENCY REUSE

DESCRIPTION BACKGROUND OF THE INVENTION

Field of the Invention

The present invention generally relates to frequency reuse and/or sharing among satellite-terrestrial communications systems and, more particularly, to a satellite-terrestrial communications system and method of operation thereof that provides frequency reuse and/or sharing between the respective portions of the satellite system and terrestrial underlay systems associated therewith, while substantially reducing interference therebetween.

Background Description

In present satellite-terrestrial systems, there is a need to separately allocate at least a portion of the frequency spectrum for the satellite, while allocating a separate portion for the terrestrial network. The present invention provides a system and method for efficiently reusing and/or sharing the spectrum between satellite and terrestrial base stations in a manner that facilitates efficient and optionally complete spectrum usage by both the satallite and terrestrial networks, while minimizing interference between the respective satellite and terrestrial systems.

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111223-122 PRO PATENT

SUMMARY OF THE INVENTION

It is a feature and advantage of the present invention to provide a satelliteterrestrial communications system and method of operation thereof that facilitates efficient spectrum usage and/or reuse.

It is another feature and advantage of the present invention provide a satellite-terrestrial communications system and method of operation thereof that minimizes interference between the satellite and terrestrial systems.

It is still another feature and advantage of the present invention to provide a satellite-terrestrial communications system and method of operation thereof that enables at least a portion of the frequency spectrum of, for example, a portion of the frequencies associated with a first satellite spot beam to be utilized by a terrestrial system having overlapping coverage with a second satellite spot beam.

The satellite-terrestrial communications system and method according to the present invention enhances spectrum usage by allocating at least a portion of the spectrum of, for example, at least a first satellite spot beam to an underlay terrestrial system preferably associated with or having overlapping coverage with, for example, at least a second satellite spot beam. The satellite-terrestrial communications system of the present invention also minimizes interference between each of the respective satellite and terrestrial systems that reuse or share a portion of the spectrum. The system and method according to the present invention will hereinafter be called the satellite-terrestrial frequency reuse system (STFRS). It should be understood that the STFRS can be deployed with all satellite (e.g., low-Earth orbit (LEO), mid-Earth orbit (MEO), geosynchronous orbit (GEO), etc.) and cellular or other terrestrial technologies as long as the frequency planning is maintained dynamically or substantially dynamically (e.g., time division multiple access (TDMA), code division multiple access (CDMA),

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111223-122 PRO PATENT

Global System for Mobile Communication (GSM), Time Division Duplex (TDD), Frequency Hopping (FHMA), etc.).

Within any given satellite spot beam of the STFRS, the frequencies (e.g., Radio frequency channels) used in a first spot beam are preferably not used in the underlay terrestrial system associated with the first spot beam. For example, an area of coverage by a satellite system may comprise seven spot beams, with each spot beam having nine channels. Thus, the system would have sixty three channels that can be allocated between the satellite and the respective underlay terrestrial systems. The satellite may use, for example, nine (9) of the channels, and the remaining fifty four (54) may therefore be allocated to the respective underlay terrestrial systems associated with each respective spot beam. In such a system, the nine channels associated with, for example, a first spot beam are preferably not utilized by the underlay terrestrial system associated with the first spot beam. The general concept is to efficiently allocate (e.g., based on demand) the total frequency band (e.g., sixty three channels) between the terrestrial and satellite systems within each of the spot beams and each of the respective terrestrial underlay systems associated therewith, while minimizing interference therebetween.

To accomplish this objective, the present invention provides both a terrestrial frequency reuse plan and a satellite frequency reuse plan. FIG. 2 shows one example of a satellite frequency reuse plan. As shown, there are seven regions/cells, each of which represents a spot beam designated by/1,/2, /3, /4, /5, ft and/7, respectively. Note that FIG. 2 does not show the terrestrial system underlaying each of the respective spot beams which will include a plurality of terrestrial cell therein. The reuse scheme employed by the satellite is generally dependent on the satellite technology being used. The technology can, for example, be GSM/TDMA based, spread spectrum CDMA, and the like, where there may be a single an tenna that is making multiple spot beams, multiple antennas, and the like.

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PATENT

In any given spot beam, the channels allocated to the satellite within that spot beam are used for satellite transmissions, whereas the terrestrial transmissions within that spot beam may use all channels except those allocated to the satellite. For example, with regard to FIG. 3a, a superscript T represents a terrestrial system, and the frequencies without a superscript T represent satellite systems. The terrestrial frequency sets (designated by (/3,/4,/5,/1,/0/, etc.) associated with the/7 spot beam use, in various combinations,/,/2,/3,/4,/5, and /6. However, the channels used by spot beam/7 are preferably not used by the underlay terrestrial system associated with the/7 satellite spot beam. In this manner, the different channels are preferably allocated among the various spot beams and associated underlay terrestrial systems such that any interfere between them is minimized. To accomplish this, the STFRS according to the present invention utilizes the inter-spot beam isolation (e.g., the isolation between the various spot beams). Thus, the terrestrial system associated with a particular spot beam preferably uses the channels that are not utilized by the spot beam since the spot beam provides an isolation that can be utilized in reducing interference. In other words, the present invention takes the co-channel, co-beam and “transfers” it to co-channel, adjacent beam interference.

Referring now to FIG. 3a, it is preferred that the distance between the terrestrial frequencies and the satellite frequencies be maximized. It is further preferred that the interference between adjacent satellites/spot beams and adjacent cells be minimized. However, even when these objectives are accomplished, the transmissions by the terrestrial network(s) will generally, to a certain extent and depending on the local attenuation, be “heard” by the associated satellite. As shown in FIG. 3a, frequency reuse planning must be carefully done along adjacent spot beam boundaries to ensure that interferences are minimized.

Consider FIG. 3a from a geographic perspective. As shown, New York city falls within spot beam/i, Philadelphia falls within spot beam/7, and Washington, D.C. falls within spot beam/4. In general, the channels associated

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111223-122 PRO PATENT with one particular spot beam can be reassigned for satellite and/or terrestrial reuse in conjunction with any other spot beam. The channels within spot beam/4, for example, can also be used as a terrestrial frequency in, for example, spot beam /7. It is preferred that the/4 channels are used in five of the six cell sites of/7. As shown in FIG. 3a, it is also preferred, however, that the channels of spot beam/4 not be used in the cell site of spot beam t'7 that is contiguous with the/4 spot beam.

Any energy that is being generated by the/4 channels within the/7 spot beam is attenuated by the antenna pattern of the/7 spot beam satellite, so that the /4 terrestrial frequencies used within spot beam/7 are not interfering with the f₇spot beam satellite frequencies. Nor, in a preferred embodiment, will the f4 channels within the f₇ spot beam interfere with the/4 spot beam since, as previously discussed, it is preferred that the channels of spot beam/4 not be used in the cell site of spot beam f₇ that is contiguous with the/4 spot beam. At some point the/4 terrestrial frequencies within the/7 spot beam will interfere with the/4 spot beam satellite transmissions, but any potential interference can be minimized by managing the frequency reuse and the size of these networks.

The present invention thus provides a system and method for coordinating a reuse plan between a satellite network and a terrestrial network using the same frequencies on an interference managed basis. In general, if one spot beam (e.g., /7) gets too congested, it can borrow frequency spectrum from other spot beams (e.g.,/1,/2,/3,/4,/5 and/or/0) that have available capacity. The present invention thus provides different ways of using the same frequencies between the satellite, which employs frequency reuse, and using that fact to allow one or more satellite channel sets to be selected for within a terrestrial network on a non-interfering basis with the satellite.

FIG. 3a also illustrates how to maximize the frequency distance of the terrestrial system. As shown, Î7 represents 9 frequency channels. Therefore, in the embodiment shown in FIG. 3a, each spot beam cell will have 45 terrestrial frequencies available. For example, consider spot beam cell A within/7. Since

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111223-122 PRO

PATENT each of/2,/3, /4, fs and/5 has 9 frequencies, 45 terrestrial frequencies are thus available in cell A within the/7 spot beam cell. Similarly, 45 frequencies are also available within spot beam cells B, C, D, E and F. It should be understood that this is a single embodiment, and that the present invention generally works regardless of how many spot beams there are or how many channels there are per satellite spot beam.

Thus, in each of the seven satellite spot beams shown in FIG. 3a, the possible terrestrial frequencies that can be used are all satellite spot beam frequencies except the satellite spot beam associated with the spot beam under consideration and those used by the spot beam closest to the particular cell of the spot beam under consideration. The system and method of the present invention therefore maximizes the frequency distance between the terrestrial and the satellite frequencies.

The frequencies being assigned are thus preferably location dependent upon where the spot beam hits. Thus, if spot beam/i has nine frequencies and only three of the nine frequencies are needed for satellite transmission, the remaining six frequencies can dynamically be reassigned to either a terrestrial system or to increase capacity in, for example, an adjacent or non-adjacent satellite spot beam. The present invention thus provides a novel system and method of using satellite channels within a given spot beam and allocating at least a portion of those channel(s) to one or more ground based terrestrial networks within one or more respective spot beams.

It should be understood that repeat patterns other than a seven cell repeat pattern can be used. For example, a fourteen cell repeat pattern could provide additional separation between the terrestrial networks and the satellite networks. The allocation of frequencies between the terrestrial network and the satellite network should be managed efficiently. For example, a large reuse repeat pattern on the satellite and a small repeat pattern terrestrial network may give rise to inefficient use of spectrum on the satellite (unless there is sufficient excess

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111223-122 PRO PATENT spectrum), which could render the satellite capacity limited rather than power limited.

In this regard, it will be realized that one technique that may be practiced with the present invention is increasing the reuse cluster size. This will generally minimize the interference between the satellite and terrestrial systems. For example, in a reuse pattern traditionally associated with GSM systems, a pattern of four cells with three sectors each is used. Thus the same frequency is reused in every fifth cell.. If instead, twenty-four channels, for example, are assigned across the cells, one site in eight has the same frequency, as opposed to one site in four having the same frequency as with the four cell three sector pattern. Thus, the number of instances where the same frequency exists has been halved, and the energy density of an individual channel has also been reduced by half. In this example, the interference between the satellite and terrestrial systems would be reduced by approximately 3dB vis-à-vis the traditional four cell three sector reuse pattern.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

Before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting.

As such, those skilled in the art will appreciate that the conception, upon which this disclosure is based, may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of

CA 02334447 2001-02-06

111223-122 PRO PATENT the present invention. It is important, therefore, that the invention be regarded as including equivalent constructions to those described herein insofar as they do not depart from the spirit and scope of the present invention.

To better appreciate the novelty and advantages provided by the present invention, a brief discussion of satellite terrestrial frequency reuse management is presented. It should be understood, however, that the techniques presented are general, and are independent of any specific values of parameters used to illustrate the concepts.

The capacity of a satellite network utilizing spot beams is directly proportional to the number of times a cluster of spot beams is replicated. The cluster size, N, chosen for the satellite system is seven (7). Other sizes may optionally be used. It is assumed that each spot beam has a frequency set containing nine 200 kHz channels ( f_: = {7, ₍, <7,₂, <7,₃,..., q_iS} for i = 1...7). Other sets of frequencies may optionally be used. It is also assumed that there is a spot beam to adjacent spot beam average isolation of 8 dB. It should also be understood that the satellite terrestrial frequency reuse can operate in normal mode or reversed mode, and that the techniques presented here are applicable to both normal mode operation and reverse mode operation. As such, further reference to specific mode of operation will not be made. In the normal mode, shown in FIG. la, the terrestrial forward (Fl) and reverse (F2) bands are the same as the satellite network. However, in the reverse mode operation, shown in FIG. lb, the satellite forward band is the same as the terrestrial reverse band, and the satellite reverse band is the same as the terrestrial forward band.

Co Channel / Adjacent Beam Technique

Within each spot beam, the use of satellite frequency set by the terrestrial network results in the worst case interference, called co-channel/co-beam interference. To utilize the isolation rendered by the availability of the spot beams, satellite terrestrial frequency reuse should preferably be implemented on

CA 02334447 2001-02-06

111223-122 PRO PATENT adjacent spot beams. The resulting co-channel/adjacent beam interference will generally be approximately reduced by the spot beam to adjacent spot beam isolation factor. It should be noted, however, that in a cluster of, for example, seven spot beams, as shown in FIGs. 2 and 3, each spot beam has six adjacent spot beams that can contribute to the interference received. The advantage of cochannel/adjacent beam technique over co-channel/co-beam technique lay with the fact that not all spot beams have equal service demand. Consequently, the distribution of interference between adjacent spot beams can reduce the average interference in a high service demand beam.

The STFR plan for the adjacent beam interference case is shown in FIG. 2. In this configuration, each spot beam is assigned a set of frequencies that will be used exclusively by the satellite network (/,.) and likewise the terrestrial network in each spot beam uses a set of frequency exclusive to the terrestrial network )^r. The satellite frequencies used in the center spot beam is f₇, and the terrestrial frequencies in this spot beam can include all other frequency sets {/i Ά Ά Ά }= (Λ T Note that in this configuration the entire spectral allocation is shared or substantially shared between the satellite network and the terrestrial network in each of the seven spot beams.

Terrestrial Cluster Size Technique

Cross network interference occurs when an RF channel is utilized both in the terrestrial network and in the satellite network, either in the co-beam configuration or in the adjacent-beam configuration. The severity of such interference depends on the power received by the competing network. In particular, the terrestrial networks reuse an RF channel or channels many times in an area covered by a given satellite spot beam or beams. Each occurrence of this RF channel gives rise to increased co-channel interference for the satellite network.

In the case of the co-beam configuration, the co-channel interference can

CA 02334447 2001-02-06

111223-122 PRO PATENT be approximated by Ml, where M is the number of times an RF channel is reused and I is the interference power of one source. For the adjacent-beam configuration the co-channel interference from one adjacent beam can be approximated by aMI, where a accounts for the fraction of power leaked from the adjacent beam. Thus, in both co-beam and adjacent beam configuration, the co-channel interference is directly proportional to the number of times a particular frequency is reused terrestrially.

Increasing Cluster Size

By increasing the cluster size for the terrestrial network the reuse of a particular frequency is reduced. To illustrate the point, consider a terrestrial network in the center spot beam in FIG. 4a. As shown, the terrestrial network has 12 available RF channels for reuse with a cluster size of four and three sectors per cell site. In each tenestrial cluster, the skyward energy from one sector will interfere with all satellite co-channels in the adjacent spot beams (in the same spot beam for co-beam configuration). FIG. 4b shows a terrestrial network with 16 cell sites (48 sectors), each RF channel is repeated four times in this network. In this same 16 site network, if twenty-four RF channels are used, for example, in a cluster of 8, then the number of co-channel sectors is reduced from 4 to 2. In general, this type of tradeoff between bandwidth and interference can be employed to reduce co-channel interference.

Terrestrial Frequency Plan Technique

In addition to increasing terrestrial cluster size within a satellite spot beam to decrease co-channel interference, careful frequency planning can help to reduce interference through maximizing satellite-terrestrial frequency reuse distance. To demonstrate this concept, again consider a terrestrial network in the center spot beam in FIG. 2. Suppose that the terrestrial network has 45 available RF channels for reuse with a cluster size of 15. As shown in FIG. 2, any satellite frequency sets

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111223-122 PRO PATENT that do not include f₇ can be used in the terrestrial network to provide adjacent beam isolation.

A random selection from the pool of 54 frequencies available for terrestrial use may result in areas where the distance between the terrestrial frequencies and satellite frequencies used in the adjacent spot beams is minimum. However, selective assignment of terrestrial frequencies to the immediate area adjacent to each spot beam in accordance with the present invention can result in increased satellite-terrestrial frequency reuse distance. FIG. 3a shows a terrestrial frequency allocation that result in maximum terrestrial-satellite frequency distance. As shown, the terrestrial network underlay in the center spot beam has been partitioned into six separate sections. In each section, the terrestrial frequency sets have been selected in order to maximize the frequency reuse distance from the satellite frequency sets in neighboring spot beams. For example, in section A of the center spot beam, the spot beam with the largest distance from this section has the assigned frequency set f_A. The spot beams with next largest distance have the assigned satellite frequency sets f₃ and f_s, and finally the last set of the spot beams have the assigned frequency sets f₂ and f₆.

In general, the terrestrial network within each spot beam must also be sectioned in the same way that has been done for the center spot beam as shown in FIG. 3 a.

Satellite spot beams at the edge of the service area do not have the lull complement of six neighbors. As such, the terrestrial network within the areas covered by this type of spot beams will have slightly different configuration. FIG. 3b shows the terrestrial network frequency plan for such a spot beam. The spot beam with assigned satellite frequency of f₇ has only three adjacent spot beam The spot beams with frequency assignment f₂, f₃, and f_A are missing from the f₂ cluster. As a result, the f₂, f₃, and f_A frequency sets can be assigned to all terrestrial underlay partitions in f₂ spot beam. The remaining terrestrial frequency assignments for this spot beam follow the procedure described above with the exception of section B. In section B, there are two

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111223-122 PRO PATENT choices for one frequency set assignment, f_s and f_t, either of these frequency sets can be assigned to this section. This is because both/j and/5 are equidistant from section B.

Frequency Borrowing Technique

In highly populated areas where terrestrial coverage can present great spectral efficiency over the satellite coverage, the terrestrial cell site density will be high. Accordingly, the interference generated in these cell sites will also be high. In such circumstances, it is advantageous to trade part of the satellite frequency spectrum of the spot beam (and even part of adjacent spot beams) to the terrestrial network. Such a trade off results in lower co-channel interference levels. As an example of a frequency borrowing technique, consider the example discussed in the previous section where each of the terrestrial networks have been configured with 45 RF channels and the satellite network in the corresponding spot beam has been configured with nine (9) RF channels. To reduce the interference by increasing the cluster size, three (3) RF channels from the satellite network can be reassigned to the terrestrial network resulting in 48 RF channels for reuse terrestrially and six (6) RF channels for the corresponding satellite spot beam.

Use Of And Prediction Of Terrain Clutter To Protect The Satellite From Terrestrial Energy

Terrestrial units are always going to be working in some sort of clutter. For example, handsets, say, six feet above the ground generally must work through buildings, trees, etc. The base stations themselves can be installed in such a manner as to provide as much isolation from the satellite as possible by, for example, putting antennas on sides of buildings (e.g., away from direct line of

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111223-122 PRO PATENT sight from the satellite).

This present invention provides a system and method that ensures that potential attenuators are used to provide maximum separation between a terrestrial base station and a satellite. The present invention considers the morphological and/or topological characteristics to increase the separation between the satellite and the terrestrial system . This enables, for example, the capacity and density of terrestrial networks in urban areas to be increased without giving rise to objectionable interference on the satellite or satellites.

The present invention utilizes shadow analysis (e.g., modeling tools) that, as shown in FIG. 5, have information about not only the terrain in an area but the buildings, trees, etc. As shown in FIG. 6, a three dimensional picture of the morphology in the area is thus provided. This information is used to do a shadow analysis between the satellite and any potential base station in the area so that base stations can be optimally located in areas that do not, for example, have a line of sight to the satellite. Base stations can thus be optimally located such that they preferably have no view or minimal view of the sky, and thus, no view or a minimal view of the satellite. Energy is thus optimally directed to covering ground users within an area covered by the base station.

In accordance with the present invention, a standard system is provided that enables standard measurements to be taken pertaining to how much satellite energy and/or how much satellite visibility there is in any particular location. This data can in turn be used in deciding upon base station location(s). Generally, base stations should be located where there is more shadowing, and where there is minimal signal strength directed towards the satellite and therefore interference. The system and method quantifies the shadowing, where the more shadowing, the better possible location for a terrestrial base station.

For example, going from the south side of a building to the north side of a building (or vice-versa) can add a significant attenuation towards the satellite because these may be additional signal blockage. The present invention thus

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111223-122 PRO PATENT enhances spectral efficiency and interference protection, particularly within a combined satellite-terrestrial network.

The many features and advantages of the invention are apparent from the detailed specification, and thus, it is intended by the appended claims to cover all such features and advantages of the invention which fall within the true spirit and scope of the invention. Further, since numerous modifications and variations will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, and accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention. While the foregoing invention has been described in detail by way of illustration and example of preferred embodiments, numerous modifications, substitutions, and alterations are possible without departing from the scope of the invention.

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