Note: Descriptions are shown in the official language in which they were submitted.
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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.
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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
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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
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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
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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
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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
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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
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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
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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-
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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
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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
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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
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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.
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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
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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
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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
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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
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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
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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
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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.
