System and method of satellite-terrestrial frequency reuse

CA 02325150 2000-11-06 SU1VIMARY 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 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 for satellite trarumissions_ 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 between satellite handsets and teaestrial handsets.

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-terresnaal communications system of the present invention accomplishes this minimal interference to either the satellite system or the terrestrial system terminal users.

The system and method according to the present invention will hereinafter be called the satellite-te~ial frequency reuse system (STFRS).

It should be understood and obvious that the STFRS can be deployed with all satellite (e.g., lowEarth orbit (LEO), mid-Earth. orbit (MEO), geosyncbronous orbit (GSO), etc.) and cellular terrestrial technologies (e.g., time division multiple access ('fDMA.), code division multiple access (CDMA), global system for mobile (GS1V1) communication, ate.).

The STFRS of the present invention achieves the aforementioned advantages by combining several elements.

Specifically, the preset invention employs frequency inversion to increase the effective frequency reuse between the satellite system and the underlay teaaestrial system, a customized base station antenna pattern that substantially reduces or eliminates interference between 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 handsets and that substantially reduces or eliminates interFerence therebetween.

Finally, it is preferaed that the base stations be strategically positioned andlor 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 or subscriber device is transmitting to 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 handset when communicating with the satellite, but does not see the handset when it is being used with the terrestrial system. A first frequency, F,, is used as a downlink frequency between a satellite and a first handset and an uplink frequency between a second handset and a base station. A second frequency, Fz, is used as an uplink between the first handset and the satellite and as a downlink between the base station and the second handset -1 CA 02325150 2000-11-06 Thus, the satellite will not see the second handset transmitting at F1 since that is the downlink frequency at which the satellite transmits to the first handset.

The present invention also takes advantage of the fixed location of the base station, and utilizes an antenna pattern having a node (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 andlor located to optimize STF'RS performance and substantially reduce or eliminate interference between any two handsets_ The vertical and the horizontal antenna patterns are also preferably adjusted.

Such adjustment can typically result in a reduction of energy of approximately 30 to 50 dB within the node.

Thus, the node portion of the antenna pattern will transmit less energy than is transmitted by handsets (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 handset because of the Trequency reuse, or the base station because of the specialized antenna_ The present invention also accounts for the mobility of the handsets.

Pa~cularly, when a handset operating in a terresixial mode comes into close proximity with a haudset 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 handsets, and passes off one of the handsets to a different frequency to 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.

~~Jhen two handsets are detected. in close proximity of each other, it is preferred that the satellite handset is handed off to the transition channel or alternatively, to a different chann.,l.

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.

-2 CA 02325150 2000-11-06 DETALED DESCIUP'T~ON OF THE INVENTION The technical challenges of providing adequate terrestrial radio signal coverage and the limited availability of radio spectntm make the viability of satellite systems relatively poor vis-a vis terrestrial systems.

Due to the characteristics of radio propagation, satehite systems, for the most part, are unable to deliver acceptable signal levels in areas obstructed by natural and/or manmade structures.

As a consequence, satellite terminal users most likely will experience inadequate or substantially inadequate coverage in major urban areas and inside buildings.

The limited availability of spectnim is caused at least in part by the lack of a frequency reuse scheme, which is typically done for terrestrial systems.

We propose a novel scheme intended to eliminate or substantially reduce both the coverage and limited availability of radio spectrum problems for satellite systems.

This scheme will allow frequency reuse for a satellite system as well as provide enhanced coverage for satellite systems by deploying an underlay system of terrestrial base stations using the same or substantially the same frequency band.

With the help of enhanced coverage and increased capacity, satellite systems will become more competitive and will provide a much-needed se~,rice to the public in urban and rural areas using the same system.

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Technical Description The prefeaed technical scheme to enhaace coverage aad capacity of a satellite system consists of using the same or substaatially the same radio spectrum for an underlay terrestrial system in a maruier such that any possible resulting interference to the satellite system or the terminal users is substantially reduced or eliminated.

This scheme is called "SatelliteTerrestrial 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 interference between handsets.

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 (GSIvI) communication, etc.) 2.1 Inversion of transmit and receive frequencies The frequency inversion technique, as shown in the following figure, involves reversing the satellite down-link (F~) and satellite up-Link (Fz) frequencies to become the terrestrial up-link ("return Iink'~ and terrestrial downlink ("forward-link's 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 Fi, and as up-link to forward-link irnerference on F2; and (2) n the satellite user terminals and terrestrial user terminals, as downlink to return-Iink interference on F~, and as forward-Iink to up-link interference on F2. T'he system and method according to the present invention eliminates or substantially reduces both of these possible interferences, as will be descn'bed. herein.

It should be understood that the system, may comprise oae or more base station antennas (and associated base stations) and one or more satellites, although only one of each are shown in the Figure below.

It should also be understood that the system may comprise one or more satellite handsets and one or more base station handsets, although only one of each are shown is the Figure below.

_4_ CA 02325150 2000-11-06 Interference on F~.....:..................

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Interference on F2 w ' d ~ ~ N a ~ .C ~r ~I sc 0 aJ Interference on F~....., .. ..... .~ . ..

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Interference on F~ 2.2 Use ofspecially designed terrestrial base station antenna 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 antenuna having a substantially reduced gain in the geustahonary arc (i.e., the elevation angle above the horizon from a base station to the satellite).

Unlike a user terminal, which is mobile and may be oriented differently from user to user, a base station does not move and therefore forms a substantially fixed angular 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 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 pattezn_ 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 stafions.

This technique will facilitate terrestrial coverage and at the same time substantially reduce or eliminate interference to the satellite system.

-5 CA 02325150 2000-11-06 2. 3 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 maximized or substantially maximized, thereby enhancing the overall capacity of the system, aid further reducing or elfminating int~'erence.

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 d~ 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.

2.4 Terminal user proximity detection Interference between satellite user terminals and terrestrial user terminals is typically a problem when the units are in relatively close proximity to one another.

It is prefeaed that such interference be substantially reduced or eliminated by, far example, first detecting close proximity before the assignment of a radio channel (i.e., during Gall.

fnitiali~ation~, and secondly by providing a hand-off to a non-interfering channel if close proxaanity occurs after the Geostationary Arc CA 02325150 2000-11-06 assignment of a radio channel.

The 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 terminals (satellite mode) and dual-mode 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 terminals.

The single-mode channels preferably use transition channels while inside base station coverage.

It is also preferred that dual-mode terminals also use the transition channels under certain circumstances, as will be described in detail herein.

This transition channel concept is illustrated in the following figure.

Satellite ~ o Satellite ~ Down-link e~ ~ Up-link ~T a ' W 3 H Terrestrial ~ ~ Terrestrial ~ 3 Reverse-lmk ~ Forward-link ~c frequency (not to scale) Channel Plan Call Initialization (~t'owchart on page 10) When a user places a call, the user terminal will request a tratFtc channel from the network.

It is preferred that the network instruct the terminal to make a series of measurements.

If the terminal is single-mode, it will preferably xan satellite channels for signal strength and interference.

If interference Ievels are acceptable, and if a satellite channel is available, then the terminal will preferably be assigned that channel.

If a satellite channel is not available, the 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 terminal will preferably request a transition channel.

If a transition channel is available, then the terminal will preferably be assigned that channel.

If a transition channel is not available, the terminal will preferably retry a fixed number of times starting. from the measurements, before the call is determined to be unsuccessful.

If the 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 terminal will preferably request a transition channel, If a transition channel is available, theft the terminal will preferably be assigned that channel.

If a transition channel is not available, the 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 terminal will preferably request the system (i.e., satellite or base stafion) with the dominant signal.

If the terminal requests a satellite channel and one is available, then the terminal will preferably be assigned that CA 02325150 2000-11-06 chaanel.

If a satellite channel is not available, the will preferably retry a fixed number of times starting from the measurements, before the call is determined to be unsuccessful.

If the terminal requests a base station channel and one is available, then the terminal will preferably be assigned that channel.

If a base station channel is not available, the 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 variaxion_s of the Figure shown on page 10 are available that would accomplish the call initialization objective, For example, the specific sequence of steps may be altered or re-ordered, such that 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 (flowchart on naQe 11 As a user terminal approaches a base station, it will preferably alert the network of its proximity.

If the terminal is single-mode, then one of two things can generally happen.

If, for example, the single-mode terminal is being served by a transition channel, then hand-off is not required.

Ify for example, the single-mode terminal is being served by a satellite channel, then a request to hand off to a transition channel is prefezably made.

If a transition channel is available, then the Band-off procedure preferably takes place.

If a tran_sifion channel is not available, then the terminal preferably checks if its current interference level is acceptable.

If interference is acceptable, then the terminal preferably camps on the satellite, 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 terminal preferably determines if another satellite channel is available for use.

If not, then the current call is preferably dropped.

If so, the terminal is preferably reassigned to a new satellite channel which it camps on, preferably for a preSpecified period of time before another request to hand-off to a transition channel is made.

If the 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 terminal preferably checks if its current interference level is acceptable_ If interference is acceptable, thcn the temainal preferably camps on the satellite, preferably for a pre-specified period of time before another request to hand-off to a base station chanacl is made.

If interference is not acceptable, the terminal preferably determines if another satellite channel is available for use.

If not, then the cuzrent call is preferably dropped_ If so, the ternninal is preferably re-assigned to a new satellite channel, which it preferably camps on for a pre-specified period of time before another request to handoff to a base station channel is made.

If the first attempt to hand-off to a transition channel was successful, then, the terminal preferably camps on this channel, preferably for a pre-specified period of time before comparing the signal Ievels of the transition channel and base station.

If the base station is not stronger by a prespecified margin, then the terminal preferably camps on the transition channel, preferably until the base station channel becomes the stronger channel.

If the base station is stronger by a prespecified margin, then a request to hand-off to a base station channel is preferably made.

If a -g CA 02325150 2000-11-06 base station channel is available, then the hand-ofd procedure preferably takes place.

If a base station channel is not available, then the terminal preferably cramps on the transition channel preferably for a pre-specified period of time before comparing the signal Levels of the transition channel and bast station again.

It should be obvious to those skilled in the art that many variations of the Figure shown on page 1 I are available that would accomplish the satellite to base station hand-off objective.

For example, the specific sequence of steps may be altered or reordered, such that the overall functionality is substantially the same or similar.

Base Station-tn-Satellite and Base Station-to-Base Station Hand Ot~'! owchart on page I2) t~ls a dual-mode 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 as 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 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 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 is the art that many variations of the Figure shown on page I2 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 overall functionality is substantially the same or similar.

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