System for informing secondary users of which radio channels are usable in which geographic region

Radio frequency reuse between two or more radio frequency communications systems is facilitated by having a beacon transmitter broadcast information globally throughout an area, which information identifies to users of a second system, which radio frequency resources are already in use by a first communications system. The global broadcast of information as to what channels are in use, permits users in the second system to avoid interfering use of those frequencies.

FIELD OF THE INVENTION 
This invention relates in general to communications systems and methods. In 
particular, this invention relates to systems and methods by which radio 
frequency spectrum can be shared between different types of communications 
systems and their users. 
BACKGROUND OF THE INVENTION 
It is well known in the communications art that usable radio frequency 
spectra is becoming increasingly more scarce, particularly as the number 
of users of radio frequency based communication systems increases. When 
considering the various types of existing users of radio frequency 
spectra, which types include commercial broadcast, state, local, and 
federal governmental agency uses, cellular telephone uses, experimental, 
telephonic, and even trunked radio systems, it can be seen that available 
radio frequency spectrum is becoming increasingly more scarce. 
Even though available spectrum of radio frequencies is limited, the need 
for radio frequency spectrum continues to increase as both the number of 
users of such spectrum increases and the types of uses of such spectrum 
increases as well. In many regions of United States, for example, the 
radio frequency spectrum set aside for so-called cellular telephone users 
has already been saturated such that cellular subscribers must frequently 
wait for a usable cellular radio frequency channel to become available. As 
the number of cellular subscribers continues to increase, users will 
experience increasingly longer delays in obtaining a usable frequency 
channel. 
In addition to the well known cellular telephone service, there are 
currently in existence, plans for new two-way communications services that 
will themselves require additional radio frequency spectrum. As 
communications services increase, they will further crowd the air waves. 
Accordingly, there exists a need to be able to more efficiently use the 
radio frequency spectrum that is currently available. Sharing existing 
radio frequencies between users, and between types of users, is perhaps 
the only way to accommodate the expected demand for radio frequencies. A 
problem with sharing radio frequencies is the interference one user who 
shares the same frequency with another user, causes to the other user or 
users. Accordingly, frequency sharing schemes that limit interference to 
users that share frequencies have been developed, some of which are quite 
complicated and require a significant amount of computer overhead. (Well 
known cellular and trunked radio systems both use computers to allocate 
frequencies between potentially competing users.) 
At least one band of radio frequencies that are currently in use in the 
United States is set aside for use by so-called microwave point-to-point 
communication systems. This band of frequencies, exist generally between 
1.8 and 2.0 GHz, and, by virtue of its high frequency, is usable only over 
relatively short distances and on a virtual line of sight communications 
path. Other bands of frequencies include at least the frequencies between 
4 and 6 GHz. as well. 
This frequency band, as well as other frequency bands, is currently in use 
by governmentally licensed subscribers that in any one area, at any one 
time, typically use only a very limited portion of the entire band and as 
such, much of the radio frequency spectrum in the band assigned to these 
types of users might be reused by other, unrelated services. Since 
existing users are entitled to a relatively uninterrupted use of this 
radio frequency spectrum (by virtue of the grant of the licenses of these 
users from various governmental agencies) any new users on this band of 
radio frequency spectra will likely be permitted to share such frequencies 
only if they can do so without adversely affecting existing users. Should 
new users decide to share these radio frequencies, they must be able do so 
in a way that permits them to use such frequencies but in a way that does 
not interfere with pre-existing users. Accordingly, a method and an 
apparatus that permits sharing of radio frequency spectrum between users 
of one type of communication system, with users of a second type of radio 
communication system might permit the limited radio frequency spectrum to 
be more efficiently used. Such a method and apparatus might dramatically 
increase the number of uses and users that could be accommodated in the 
limited radio frequency spectrum. 
SUMMARY OF THE INVENTION 
There is provided a radio frequency communications system and method for 
sharing radio frequency spectrum between users of multiple types of 
services. This system and method limits interference to one type of user 
of radio frequency spectrum associated with a first type of system that 
can be caused by simultaneous use of the same radio frequency spectrum by 
another user associated with another or second system by broadcasting 
information to users in the second system that these second users need to 
avoid interfering with the first users of the radio frequency spectrum. 
This system and method uses a radio frequency beacon transmitter located 
near the transmitters of pre-existing users to broadcast information on a 
radio frequency channel that users in a second or third or other system 
can monitor to identify the locations and the geographic areas and the 
frequencies that are in use by an existing system. The broadcast 
information permits users associated with a system that shares spectrum to 
know which frequencies are already in use within an area such that 
interfering re-use of such frequencies within such areas is avoided.

DESCRIPTION OF A PREFERRED EMBODIMENTS 
FIG. 1 shows an exemplary diagram of how portions of the radio frequency 
spectrum (above 800 MHz. in this figure) might be allocated for various 
communications uses. The relative frequencies of four arbitrary bands are 
shown with respect to each other. Band A is shown between 800 and 900 
MHz.; band B is shown between 850 MHz and 1990 MHz.; band C is shown 
between X.sub.1 MHz and X.sub.2 ; band D is shown as between X.sub.3 and 
X.sub.4 MHz. 
In reality, the radio frequency spectrum shown in FIG. 1 might not be 
actually allocated as it is shown in FIG. 1, and as such, FIG. 1 is 
intended to show only that radio frequency spectrum available for 
communications use is not continuous and that there are blocks, or ranges 
of spectrum available, such as the spectrum available between 1850 MHz 
1900 MHz., which today is largely used by private, point-to-point 
microwave communications users such as utility companies, state and local 
governmental agencies, etc. FIG. 1 also shows that each of these bands can 
be separated from each other by frequency ranges that might be allocated 
to other uses. It should be obvious that any of the bands shown in FIG. 1 
encompass a relatively large portion of the radio frequency spectrum and 
that most communications applications would require only a small portion 
of any band. Most frequency bands are therefore subdivided into channels, 
which channels themselves are frequency bands, to which a user is usually 
assigned the use thereof under the terms of its license from the Federal 
Communications Commission (in the United States). 
FIG. 2 shows an exemplary division of the B band shown in FIG. 1. The 
frequency bands shown in FIG. 2 in the B band are divided up into a 
plurality of radio frequency channels designated B.sub.1, B.sub.2, 
B.sub.3, etc. These radio frequency bands, B.sub.1, B.sub.2, B.sub.3, etc. 
are typically divided into transmit and receive frequency pairs where each 
of these bands B.sub.1, B.sub.2, B.sub.3, etc. in reality is a transmit 
frequency band and a receive frequency band (Transmit and receive 
designations are arbitrary. One transmitter will ordinarily transmit on 
its transmit frequency, i.e. the transmit channel, to which a receiver 
will listen to, i.e. the receive channel. Stated alternatively, each band 
shown in FIG. 2 is actually a pair of radio frequency channels.) 
In the United States, the radio frequency spectrum generally between 1850 
MHz and 1990 MHz is primarily allocated to point-to-point microwave 
communications uses. These point-to-point microwave communications users 
generally use transmit and receive pairs to provide full duplex 
communications between radio frequency stations that are communicating 
with each other. 
In any given geographic area of the United States, any one or more of the 
channels B.sub.1, B.sub.2, B.sub.3, etc. might be allocated to a 
point-to-point microwave communication system user. Such a user will in 
general have at least two stations that are communicating with each other, 
typically using one or more frequencies, such as one or more of the bands 
(a transmit and receive frequency pair) shown in FIG. 2. 
In any geographic area in the United States wherein one or more bands, such 
as the bands shown in FIG. 2, are not already allocated for use by a user, 
such an unused radio frequency band might be reused by users associated 
with another communication system. Re-using unallocated bands in a radio 
frequency spectrum can create interference between users and is preferably 
used only where users of the second system know, in advance, of the 
existence of a prior user on a frequency. 
In a point-to-point communication system, the radio frequency signals 
between stations are frequently performed using highly directional 
antennas. As such, the geographic area in which a communications band 
(shown in FIG. 2) is unusable by other radio frequency communications 
services (by virtue of radiated signals emitted from and received by the 
point-to-point users) is relatively well defined. Users associated with a 
second or other communication system outside of the area in which a 
pre-existing user from a first, point-to-point system is in use, may be 
able to reuse the same frequency if the users from the second system know 
the geographic areas in which particular frequencies are already in use. 
Users associated with a second system could then refrain from using 
frequencies in areas where such simultaneous use would interfere with the 
prior users. 
Reuse of radio frequency spectrum by second users associated with a second 
communications system is accommodated in the preferred embodiment by means 
of at least one beacon transmitter, which is preferably located proximate 
to the location of a transmitter of the first type of communications 
system (a point-to-point microwave station in the preferred embodiment) 
wherein the beacon transmitter broadcasts information to identify to 
second radio stations associated with a second system, the approximate 
boundaries of a region in which channels used by the first system are not 
available for reuse by the second system. 
Referring to FIG. 3 there is depicted a geographic area A in which a 
plurality of point-to-point microwave stations communicating with each 
other use a plurality of bands of frequencies shown in FIG. 2. Within this 
area A there are shown a plurality of users associated with a second type 
of radio communication system that share radio frequency bands shown in 
FIG. 2 on a substantially noninterfering basis with the existing users 
that use the same bands of frequencies shown in FIG. 2. 
Referring to FIG. 3, there is shown a geographic area, A, in which a 
point-to-point microwave communications station (10) communicates with a 
second point-to-point communications station (12) using a radio frequency 
band B.sub.1 shown in FIG. 2. This first pair of radio frequency stations 
(10, 12) and their use of band B.sub.1 precludes reuse of the radio 
frequency band B.sub.1 in a geographic area defined within the broken line 
identified by reference numeral 14. The actual shape and area of the area 
identified by reference number 14 will depend in part on various factors 
including for example, the output power of the transmitters at the 
respective locations, the height of the antennas, the azimuth or direction 
of the antennas, the directional selectivity of the antennas, the surface 
features of the area between the two stations, and the sensitivity of the 
receivers at the respective stations. For purposes of this discussion, the 
area bounded by the broken line identified by reference numeral 14 is the 
area in which virtually any use of band B.sub.1 by other users could 
interfere with the use of B.sub.1 by the point-to point users at stations 
10 and 12. (The actual area in which a new user will interfere with use by 
a first user will of course vary with many factors including for example 
the output power of the new user's transmitter and the sensitivity of the 
first user's receiver.) 
Still referring to FIG. 3, a third point-to-point radio communications 
station (16) communicates with a fourth point-to-point station (18) on 
radio frequency band B.sub.3 which band, B.sub.3, is unusable by other any 
users throughout a second geographic area circumscribed by the broken line 
identified by reference numeral (20). (As set forth above, the actual area 
in which a new user will interfere with use by a first user will of course 
vary with many factors including for example the output power of the new 
user's transmitter and the sensitivity of the first user's receiver but 
for purposes of this illustration, these factors are ignored herein.) A 
fifth point-to-point communications station (17), communicates with a 
sixth point-to-point communications station (19) on radio frequency band 
B.sub.4 that is unusable by other users within the geographic area 
circumscribed by the broken line identified by reference numeral (21). 
At the station identified by reference numeral 16, a point-to-point 
transmitter/receiver combination (22) exchanges information with a 
point-to-point microwave transmitter/receiver pair (26) at the second 
station (18). It is well established in the microwave communications art 
that the antennas, output power levels of the transmitters and the 
physical separation between the stations will all be selected to insure 
that communications between the stations is not interrupted, despite any 
signal fading that may occur due to atmospheric changes etc. Despite these 
precautions, depending upon its antenna height, direction, and transmit 
power, it is possible that a user of a second type of communications 
system operating on channels B.sub.1 or B.sub.3 might emit sufficient 
energy to interfere with a receiver at either station 10, 18, 12, or 16. 
(If a new user is in the area bounded by the intersection of the areas 
identified by reference numerals 14 and 20, such a new user might 
interfere with either point-to-point station.) In order to ensure that a 
radio station associated with a second type of radio communications system 
does not interfere with the use of the band B.sub.3 (or B.sub.1 for that 
matter) by these first and second stations (16 and 18), a beacon 
transmitter (24) which is located proximate to the tower associated with 
the first of these stations (16), broadcasts pre-determined information on 
a control or information channel (shown arbitrarily as channel C in FIG. 
2) that identifies the area in which channels should not be re-used. 
In FIG. 3 there are shown at least three different subscribers (U1, U2, and 
U3) which might be associated with a second type of communication system, 
which system might be a personal communications system for example. These 
new users might also be cellular telephone users in a cellular system 
sharing the resources depicted in FIG. 2. 
In the preferred embodiment of the invention, in order to avoid interfering 
with the use of the bands used by the point-to-point stations (10, 12, 16, 
18, and 17, 19), the users associated with the second system (U1, U2, and 
U3) are informed by beacon transmitters located at each of the 
point-to-point stations (10, 12, 16, 18, 17, and 19) of the frequency and 
the geographic area over which channels (B.sub.1, B.sub.3, and B.sub.4) 
are used by the point to point stations (16 and 18). Each beacon 
transmitter broadcasts information on the geographic area over which 
frequencies used by the corresponding point-to-point station is using. In 
addition to broadcasting the particular frequencies, broadcasted 
information might include, the output signal level from the stations, the 
antenna height and direction, the geographic boundaries or coordinates of 
the areas in which new users are prohibited or discouraged (in latitude 
and longitude for example). 
In an alternate embodiment of the invention, a single beacon transmitter 
(24 for example) might broadcast the information that identifies the 
location (by latitude and longitude coordinates for example) of all the 
other stations, the frequencies used by each, the carrier or output power 
level of each, the azimuth or the direction in which they are all 
broadcasting, the broadcast signal strength of the beacon transmitter, and 
also possibly the geographic coordinates of the areas considered by each 
of the stations to be off-limits to new users. 
A user associated with a second type of communication system, depicted in 
FIG. 3 as user U2, when within range of the signal broadcast from the 
beacon transmitter (24), if it has the capability of detecting and 
demodulating and identifying the information broadcast from the beacon 
transmitter, could adjust its own transmitter and receiver to completely 
lock out its use of any band used by a point-to-point station it might 
interfere with. (B.sub.3 is the band used by the first and second stations 
16 and 18.) If the user U2 has the ability to determine its location 
(either Loran, dead reckoning or GPS for example) the user U2 might be 
able to selectively lock out from re-use only those frequencies used in or 
near the area in which the user U2 is located. 
If for example the beacon signal strength and direction is adjusted to 
cover an area in which re-use of band B.sub.3 by a second user, U2, would 
interfere with the prior use of B.sub.3 by the point-to-point stations 16 
and 18, the second user, U2 for example, that cannot detect the beacon 
signal could reasonably assume that since no beacon signal were detected, 
that any of the channels B.sub.1 -B.sub.n are available for re-use. 
Adjusting the signal strength and/or direction of the beacon signal could 
be used to expand or contract the area in which re-use of a frequency, 
B.sub.3 in this example, is prohibited. 
In FIG. 3, the first and second point-to-point stations (10 and 12) are 
shown communicating with each other using band B.sub.1. Like the third and 
fourth point-to-point stations, (16 and 18), these first and second 
stations (10 and 12) each have a point-to-point transmitter/receiver (30) 
at the first station (10) and a point-to-point transmitter/receiver at the 
second station (12) (which point-to-point transmitter/receiver pair at the 
second station (12) is not shown for clarity). 
As an alternate embodiment of the invention and as a way of enhancing the 
reuse of radio frequency spectrum, the beacon transmitter (32) coupled to 
the first point-to-point station location (10) includes the capability of 
adjusting its output power level so as to change the geographic area over 
which its signal can be detected by a subscriber, such as the subscriber 
U1 shown in FIG. 3. 
For various reasons, radio frequency signals may from time to time 
experience increased fading which fading effects the distance over which 
communications can be maintained. At times during which the signal between 
the stations (10 and 12) fades, the area in which a user such as the user 
U1, is proscribed from using the channel B.sub.1, which channel is used by 
the stations (10 and 12), will increase. (When the signal on the channel 
B.sub.1 between the stations is fading, the output power from the 
transmitter of the subscribe U1 is more capable of interfering with the 
use of channel B.sub.1 by the stations (10 and 12). 
During times when the signal strength of the frequency on channel B.sub.1 
decreases, the area over which reuse of channel B.sub.1 by a second 
subscriber associated with a second system (U1) should decrease. In such 
cases, the output power of the beacon transmitter (32) when increased, 
will change the distance over which its signal can be detected from the 
distance D1 as shown to the distance D2. 
Modulating the output power of the beacon transmitter (32) might be 
controlled based upon the bit error rate or other error rate detected by 
the station (10 or 12) in response to fades caused by any source or 
reason. It should be noted that the area enclosed by the broken lines 
designated by references D1 and D2 are circular corresponding to the fact 
that the antenna that might be used with the beacon transmitter (32) is an 
omnidirectional antenna where it is desirable to have a beacon signal that 
can be detected from any direction. Alternatively, this radiated beacon 
signal may be directional in order to further limit any interference 
caused to the stations (10 and 12) on channel B.sub.1. 
Still referring to FIG. 3, the fifth and sixth point-to-point stations (17 
and 19) communicate with each other using a third channel B.sub.4 as 
shown. The geographic area in which reuse of channel B.sub.4 is not 
permitted is identified by the broken line identified by reference numeral 
(21). 
The intersection of the geographic area identified by reference numeral 
(20) and the geographic area identified by reference numeral (14) in FIG. 
3 comprises an area in which neither frequency B.sub.1 nor frequency 
B.sub.3 would be reusable by subscribers associated with the second radio 
system. 
In FIG. 3, a separate beacon transmitter (13) might broadcast the 
geographic coordinates of geographic areas in which certain communications 
channels are not to be reused by subscribers of a second radio system. If 
a beacon transmitter (13) coupled to a point-to-point station (17 for 
example) broadcasts appropriate information, it can by broadcasting such 
information proscribe reuse of frequencies within an area, such as the 
area enclosed by the broken lines and identified by reference numeral 
(23). 
It should be pointed out that areas in which frequencies can be re-used can 
vary with both the horizontal distance away from a beacon transmitter and 
vertical elevation as well. Upper floors in a building for example might 
comprise areas in which one or more re-useable frequencies that are used 
in a nearby area for point-to-point communications are available under 
certain conditions to re-users in the building. In such an application, to 
avoid interfering use caused by virtue of the elevation available in a 
building, re-users at high elevations might be required to use very low 
power to offset the advantage their emitted signals would likely have by 
virtue of their height. 
In any embodiment of this invention, information from the beacon 
transmitter might be broadcast on a frequency or carrier that is used by 
the point-to-point microwave stations for example. In using the carrier 
information that an existing user is occupying, the communications 
equipment used by the second system's communications equipment may of 
course be more complex to be able to extract the relative information. 
In most instances, a beacon transmitter located approximate to broadcasting 
towers that are used by existing services will broadcast beacon 
information on a separate channel such as the channel C shown in FIG. 2. 
While the embodiment depicted in FIG. 3 contemplates locating or 
co-locating the beacon transmitters proximate to the location of the 
transmitters and antennas of the pre-existing users, an alternate 
embodiment would include using a single beacon transmitter (13) such as 
the one shown in FIG. 3 which transmitter would have sufficient output 
power to permit any subscriber, anywhere within the geographic coverage 
area (A) of the beacon transmitter (13) to receive and detect information 
broadcast from this centralized beacon transmitter (13). To be able to 
permit reuse of channels within various areas, the centralized beacon 
transmitter (13) would of course have to include the capability of 
broadcasting geographic information that would be sufficiently detailed to 
permit users within these areas to know which frequencies to use and which 
to not use.