CA2328013A1 - System and method of satellite-terrestrial frequency reuse using signal blockage, dynamic assignment of frequencies and/or switching center/base station coordinated frequency reuse - Google Patents

Description

COORDINATED SATELLITE-TERRESTRIAL
FREQUENCY REUSE
RELATED APPLICATIONS
This application claims priority from U.S.
provisional application serial number 60/222,605 filed on August 2, 2000 and entitled "Satellite-Terrestrial Frequency Reuse", the details of which are hereby incorporated by reference.
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 uses, for example, satellite downlink frequencies between a first satellite and a first handset as uplink frequencies between a second handset and a base station.

2 Background Description In present satellite-terrestrial systems, there is a need to reuse frequencies therebetween in a manner that minimizes interference. The present invention provides a system and method for efficiently reusing and/or sharing the spectrum between satellite and terrestrial systems in a manner that facilitates efficient spectrum usage, while minimizing interference between the respective satellite and terrestrial systems.
SUMMARY OF THE INVENTION
It is a feature and advantage of the present invention to provide a satellite-terrestrial communications system and method of operation thereof that enhances coverage for satellite systems.
It is another feature and advantage of the present invention to provide a satellite-terrestrial communications system and method of operation thereof that increases the effective frequency spectrum available.
It is yet another feature and advantage of the present invention to provide a satellite-terrestrial communications system and method of operation thereof that eliminates or substantially reduces interference

3 between satellite fixed and/or mobile user terminals and terrestrial fixed and/or mobile user terminals.
The satellite-terrestrial communications system disclosed herein enhances both coverage and capacity of satellite communications by using the same or substantially the same radio spectrum as that of the underlay terrestrial system. Moreover, the satellite-terrestrial communications system of the present invention accomplishes this with minimal interference to either the satellite system and/or the terrestrial system and/or the fixed and/or mobile terminal users. The system and method according to the present invention will hereinafter be called the satellite-terrestrial frequency reuse system (STFRS). It should be understood and obvious that the STFRS can be deployed with all satellites (e. g., low-Earth orbit (LEO), mid-Earth orbit (MEO), geosynchronous orbit (GSO), etc.) and cellular terrestrial technologies (e. g., time division multiple access (TDMA), code division multiple access (CDMA), global system for mobile (GSM) communication, broadband voice and data network, MMDS, LMDS, etc.).
The STFRS of the present invention achieves the aforementioned advantages by combining several elements.
Specifically, the present invention employs frequency

4 inversion to increase the effective frequency reuse between the satellite system and the underlay terrestrial system, a customized base station antenna pattern that substantially reduces or eliminates interference between S one or more satellites and one or more base stations, enhanced call setup and hand-off schemes, and a transitional spectrum which is used upon detecting proximity of satellite fixed and/or mobile user teztninals and/or terrestrial fixed and/or mobile user terminals that substantially reduces or eliminates interference therebetween. Finally, it is preferred that the base stations be strategically positioned and/or antennas be oriented to optimize STFRS performance. It should be understood that each of these techniques can be practiced alone, or in any combination with each other.
When a handset (i.e., mobile user terminal) or other subscriber device such as a fixed user terminal is communicating with a terrestrial system, a satellite will typically "see" the transmission. Accordingly, the terrestrial transmission will interfere with the satellite system. The present invention provides a method, called frequency inversion, whereby the satellite "sees" the fixed and/or mobile user terminals when communicating with the satellite, but does not see the fixed and/or mobile user terminals when being used with the terrestrial system. A first frequency, F1, is used as a downlink frequency between a satellite and a first fixed and/or mobile user terminal and as an uplink

5 frequency between a second fixed and/or mobile user terminal and a base station. A second frequency, F2, is used as an uplink between the first fixed and/or mobile user terminal and the satellite and as a downlink between the base station and the second fixed and/or mobile user terminal. Thus, the satellite will not see the second fixed and/or mobile user terminal transmitting at F1 since that is the downlink frequency at which the satellite transmits to the first fixed and/or mobile user terminal.
The present invention also takes advantage of the fixed location of the base station, and utilizes an antenna pattern having a null(i.e., null spot) in the direction of the satellite such that the. satellite does not see the energy which is being transmitted by the base station. Preferably, the base stations will be strategically positioned and/or located to optimize STFRS
performance and substantially reduce or eliminate interference between the base stations and the satellites-. The vertical and the horizontal antenna

6 patterns are also preferably adjusted. Such adjustment can typically result in a reduction of energy of approximately 30 to 50 dB within the antenna main lobe.
Thus, the -base stations with the adjusted antenna pattern will transmit less energy than is transmitted by fixed and/or mobile user terminals (or subscriber units) when they are transmitting in the satellite mode.
Interference between base stations and the satellite is thus substantially reduced or eliminated. The satellite thus does not see either the terrestrial fixed~and/or mobile user terminals because of the frequency reuse, or the base station because of the specialized antenna and engineering design in positioning the base stations and antennas.
The present invention also accounts for the mobility of the mobile user terminals. Particularly, when a mobile user terminal operating in a terrestrial mode comes into close proximity with another fixed and/or mobile user terminal working in a satellite mode, they will interfere with each other. To prevent interference between two such handsets, the present invention also detects the proximity between the respective fixed and/or mobile user terminals, and passes off one of the fixed and/or mobile user terminals to a different frequency to

7 avoid such interference. To accomplish this, the present invention utilizes a transition channel, which is a small portion of the bandwidth allocated to the satellite system that is not available for terrestrial reuse. When two fixed and/or mobile user terminals are detected in close proximity of each other, it is preferred that the satellite fixed and/or mobile user terminal is handed off to the transition channel or alternatively, to a different channel. The proximity detection and handoff scheme of the present invention operates such that the frequency or probability of a call being dropped is less than the frequency or probability of a call being dropped by either terrestrial only or satellite only operations.
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

8 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, S may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of 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.
The preferred technical scheme to enhance coverage and capacity of a satellite system consists of using the same or substantially the same radio spectrum for an underlay terrestrial system in a manner such that any possible resulting interference to the satellite system and/or the fixed and/or mobile terminal users is substantially reduced or eliminated. This scheme is called "Satellite-Terrestrial Frequency Reuse" (STFR) and is achieved, preferably by applying the four techniques described in this section. It is thus preferred that the four techniques be practiced in combination with each other to maximize or substantially maximize system performance and substantially reduce or eliminate

9 interference between satellite and terrestrial systems.
However, it should be understood that each of these techniques can also be practiced alone, or in any combination with each other. It should also be understood that the STFR scheme can be deployed with all satellite (e. g., low-Earth orbit (LEO), mid-Earth orbit {MEO), geosynchronous orbit (GSO), etc. and cellular terrestrial technologies (e. g., time division multiple access (TDMA), code division multiple access (CDMA), global system for mobile (GSM) communication,broadband voice and data networks, MMDS, LMDS, etc.) Inversion of Transmit and Receive Frequencies The frequency inversion technique, as shown in FIGS. 1 and 2, involves reversing the satellite down-link (F1) and satellite up-link (F2) frequencies to become the terrestrial up-link ("return-link") and terrestrial down link ("forward-link") frequencies, respectively. As a result, there will be two possible interference paths:
(1) between the satellite and base stations, as return-link to down-link interference on Fl, and as up-link to forward-link interference on FZ; and (2) between the satellite fixed and/ar mobile user terminals and terrestrial fixed and/or mobile user terminals, as down-link to return-link interference on F1, and as forward-link to up-link interference on F2. The system and method according to the present invention eliminates or substantially reduces both of these possible interferences, as will be described herein. It should be S understood that the system may comprise one or more base station antennas (and associated base stations) and one or more satellites, although only one of each are shown in the Figures below. It should also be understood that the system may comprise one or more satellite fixed

10 and/or mobile user terminals and one or more base station fixed and/or mobile user terminals, although only one of each are shown in the FIGS. 1 and 2.
Use of Specially Designed Terrestrial Base Station Antenna As shown in FIG. 3, interference between the satellite and base stations (i.e., return-link to down-link and up-link to forward-link interference) is substantially reduced or eliminated, preferably by using a base station antenna having a substantially reduced gain in the geostationary arc (i.e., the elevation angle above the horizon from a base station to the satellite).
Unlike mobile user terminals, which may be oriented differently from user to user, a base station does not move and therefore forms a substantially fixed angular

11 relationship with respect to the satellite. Within North America, the geostationary arc typically varies from approximately 30° to 70°, depending, for example, on the latitude and longitude and elevation of the base station (see Figure 2 below) . To fully take advantage of this fact, it is preferred that the base station antenna pattern have a null, and therefore significantly reduced gain, in the geostationary arc portion of its vertical pattern. As an analogy, one could consider the satellite to be in a "blind spot" with respect to the base station.
The additional signal attenuation achieved from this technique substantially reduces or eliminates interference between the satellite and terrestrial base stations. This technique will facilitate terrestrial coverage and at the same time substantially reduce or eliminate interference to the satellite system.
Selective Positioning of Terrestrial Base Stations To further enhance the performance of the system, a technique for optimally or substantially optimally locating and orienting base stations will preferably be used, to advantageously utilize the horizontal gain pattern of the antenna. The benefits of using this technique, for example, are that frequency reuse will be

12 maximized or substantially maximized, thereby enhancing the overall capacity of the system, and still further reducing or eliminating interference.
In addition to the increased isolation provided by the vertical antenna pattern, additional isolation can be obtained from the horizontal antenna pattern. For example, preferably by configuring base stations such that the azimuth to the satellite is off-bore or between sectors, several additional dB of isolation can typically be achieved. By keeping this configuration standard for, say, a cluster of base stations, frequency reuse for the terrestrial system can generally be increased.
Terminal User Proximity Detection Interference between satellite fixed and/or mobile user terminals and terrestrial fixed and/or mobile user terminals is typically a problem when the units are in relatively close proximity to one another. It is preferred that such interference be substantially reduced or eliminated by, for example, first detecting close proximity before the assignment of a radio channel (i.e., during call initialization), and secondly by providing a hand-off to a non-interfering channel if close proximity occurs after the assignment of a radio channel. The

13 proposed scheme allows for real-time or near real-time operation of this technique.
The technique provides optimum or substantially optimum radio resource allocation so that the coexistence of single-mode (e. g., satellite mode) fixed and/or mobile user terminals and dual-mode fixed and/or mobile user terminals can be accomplished. In order for this to work, it is preferred that a relatively small group of channels, called "transition channels", be reserved for single-mode user terminals. The single-mode handsets preferably use transition channels while inside base station coverage. It is also preferred that dual-mode fixed and/or mobile user terminals also use the transition channels under certain circumstances, as will be described in detail herein. This transition channel concept is illustrated in FIG. 4.
Call Initialization As shown in FIG. 5, when a user places a call, the fixed and/or mobile user terminal will request a traffic channel from the network. It is preferred that the network instruct the fixed and/or mobile user terminal to make a series of measurements. If the fixed and/or mobile user terminal is single-mode, it will preferably scan satellite channels for signal strength and interference.

14 If interference levels are acceptable, and if a satellite channel is available, then the fixed and/or mobile user terminal will preferably be assigned that channel. If a satellite channel is not available, the fixed and/or mobile user terminal will preferably retry a fixed number of times starting from the measurements, before the call is determined to be unsuccessful. If interference levels are unacceptable, the fixed and/or mobile user terminal will preferably request a transition channel. If a transition channel is available, then the fixed and/or mobile user will preferably be assigned that channel. If a transition channel is not available, the fixed and/or mobile user terminal will preferably retry a fixed number of times starting from the measurements, before the call is determined to be unsuccessful.
If the fixed and/or mobile user terminal is dual-mode, it will preferably scan both satellite and base station channels for signal strength and interference. If interference levels are unacceptable, the fixed and/or mobile user terminal will preferably request a transition channel. If a transition channel is available, then the fixed and/or mobile user terminal will preferably be assigned that channel. If a transition channel is not available, the fixed and/or mobile user terminal will preferably retry a predetermined number of times starting from the measurements, before the call is determined to be unsuccessful. If interference is acceptable, the fixed and/or mobile user terminal will preferably request the 5 system (i.e., satellite or base station) with the dominant signal. If the fixed and/or mobile user terminal requests a satellite channel and one is available, then the fixed and/or mobile user terminal will preferably be assigned that channel. If a satellite channel is not 10 available, the fixed and/or mobile user terminal will preferably retry a fixed number of times starting from the measurements, before the call is determined to be unsuccessful. If the fixed and/or mobile user terminal requests a base station channel and one is available,

15 then the fixed and/or mobile user terminal will preferably be assigned that channel. If a base station channel is not available, the fixed and/or mobile user terminal will preferably retry a fixed number of times starting from the measurements, before the call is determined to be unsuccessful. It should be obvious to those skilled in the art that many variations of FIG, 5 are available that would accomplish the call initialization objective. For example, the specific sequence of steps may be altered or re-ordered, such that

16 the overall functionality is substantially the same or similar. For example, the determination whether the user is in dual-mode may be juxtaposed after measuring satellite and base station channels.
Satellite-to-Base Station Hand-Off With regard to FIG. 6, as a mobile user terminal approaches a base station, it will preferably alert the network of its proximity. If the mobile user terminal is single-mode, then one of two things can generally happen.
If, for example, the single-mode mobile user terminal is being served by a transition channel, then hand-off is not required. If, for example, the single-mode mobile user terminal is being served by a satellite channel, then a request to hand-off to a transition channel is preferably made. If a transition channel is available, then the hand-off procedure preferably takes place. If a transition channel is not available, then the mobile user terminal preferably checks if, its current interference level is acceptable. If interference is acceptable, then the mobile user terminal preferably camps on the satellite channel, preferably for a pre-specified period of time before another request to hand-off to a transition channel is made. If interference is not acceptable, the mobile user terminal preferably

17 determines if another satellite channel is available for use. If not, then the current call is preferably dropped.
If so, the mobile user terminal is preferably re-assigned to a new satellite channel which it camps on, preferably for a pre-specified period of time before another request to hand-off to a transition channel is made.
If the mobile user terminal is dual-mode, then a request to hand-off to a base station channel is preferably made. If a base station channel is available, then the hand-off procedure preferably takes place. If a base station channel is not available, then a request to hand-off to a transition channel is preferably made. If a transition channel is not available, then the mobile user terminal preferably checks if its current interference level is acceptable. If interference is acceptable, then the mobile user terminal preferably camps on the satellite, preferably for a pre-specified period of time before another request to hand-off to a base station channel is made. If interference is not acceptable, the mobile user terminal preferably determines if another satellite channel is available for use. If not, then the current call is preferably dropped.
If so, the mobile user terminal is preferably re-assigned to a new satellite channel, which it preferably camps on

18 for a pre-specified period of time before another request to hand-off to a base station channel is made.
If the first attempt to hand-off to a transition channel was successful, then the mobile user terminal preferably camps on this channel, preferably for a pre specified period of time before comparing the signal levels of the transition channel and the base station channel. If the signal of the base station is not stronger by a pre-specified margin, then the mobile user terminal preferably camps on the transition channel, preferably until the base station channel becomes the stronger channel. If the base station signal is stronger by a pre-specified margin, then a request to hand-off to a base station channel is preferably made. If a base station channel is available, then the hand-off procedure preferably takes place. If a base station channel is not available, then the mobile user terminal preferably camps on the transition channel, preferably for a pre-specified period of time before comparing the signal levels of the transition channel and base station channel again. It should be obvious to those skilled in the art that many variations of FIG. 6 are available that would accomplish the satellite to base station hand-off objective. For example, the specific sequence of steps may be altered or

19 re-ordered, such that the overall functionality is substantially the same or similar.
Base Station-to-Satellite and Base Station-to-Base Station Hand-Off As shown in FIG. 7, as a dual-mode mobile user terminal moves away from the base station it is served by, it will eventually take appropriate measures upon sensing a stronger channel, either from the satellite, another base station, or a system or device associated therewith. If a satellite channel is stronger than a neighboring base station channel, then a request to hand-off to a satellite channel is preferably made. If a satellite channel is available, then the hand-off procedure preferably takes place. If a satellite channel is not available or if a neighboring base station channel is stronger than a satellite channel, then a request to hand-off to a neighbor base station channel is preferably made. If a base station channel is available, then the hand-off procedure preferably takes place. If a base station channel is not available, then the mobile user terminal preferably camps on its current channel, preferably for a pre-specified period of time before making measurement comparisons again. It should be obvious to those skilled in the art that many variations of FIG. 7 are available that would accomplish the base station-to-satellite and base station-to-base station hand-off objectives. For example, the specific sequence of steps may be altered or re-ordered, such that the 5 overall functionality is substantially the same or similar.
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 10 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 15 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

20 embodiments, numerous modifications, substitutions, and alterations are possible without departing from the scope of the invention.

Claims