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BACKGROUND OF THE INVENTION
This invention relates to trunked radio systems. In particular, this
invention relates to more efficient use of communication resources used by
trunked radio systems.
A trunked radio system is a two-way radio system that allocates a limited
number of communication resources, such as radio frequencies, or time
slots in a time division multiplexed communication system etc., on a
time-shared basis amongst many users throughout a limited geographic area.
The users time-share these communications resources on an as-needed basis
by the users requesting a central controller for the privilege of
temporarily using a resource to communicate with other subscribers to the
system. If a communications resource is available for use when a user
request one, the controller grants the request to the requesting user by
broadcasting a message on a control channel throughout the coverage area,
(hereafter control resource) which the users monitor. This message, an
Outbound Signalling Word, (OSW) instructs the trunked radio system users
of the identity of the channel assigned to the requesting user. (Other
users may also be instructed to tune to the channel to permit
communication with them to occur.) The requesting user may thereafter
communicate with either a specific other user with whom communication was
requested or other group related users, depending upon instructions
broadcast from the resource controller.
Most trunked radio systems in operation today, are licensed to provide
service throughout a geographic area on discrete frequencies, or frequence
ranges. A trunked radio system license usually grants the licensee the
right to the exclusive use of these communication resources in the
particular coverage area. Of the communication resources used by a
trunking system most trunked radio systems actually allocate pairs of
resources or channels, where each allocated resource is comprised of a
pair channels. One channel of the pair of channels comprising a
communication resource is used for subscriber units to transmit on, (also
known as a subscriber unit talk-in channel which the system controller
listens to). The other channel of the pair of channels comprising a
communication resource is known as a repeater talk out channel, (which the
subscriber units listen to). These paired subscriber talk-in channels and
repeater talk out channels are considered communication resources,
although both a talk-in channel and a talk-out channel might also be
considered as communication resources.
It is well known that trunked radio system communication resources are
frequently unused for substantial periods of time. When a communication
resource licensed for trunked radio use is not being used by the trunked
radio system the communication resource is effectively being wasted since
any other use of the resource is precluded. A more efficient use of
available frequency spectrum might permit re-use of the licensed
frequencies (or channels, time slots, or other communication resources)
within the geographic area such that subscribers in different regions of
the coverage area could use a communication resource simultaneously
without interfering with each other. Ideally, both talk-in and talk-out
channels would be available for re-use.
Since the breadth of the coverage area of the trunked radio system is
primarily dependent upon the signal level radiated by both the resource
controller and the subscriber units, the coverage area of a trunked radio
system can be controlled by controlling the power levels of transmitters
for the resource controllers and the subscriber units. FIG. 1 shown a
schematic diagram of a trunked radio system (10) that provides radio
communications to subscriber units (12) within a geographic region (14).
This geographic region, (also referred to as the coverage area) (14) is
the distance over which signals from a resource controller (16) (which is
the administrator for the trunked radio system) have a useable signal
strength, i.e. a signal level sufficiently high such that information
therein may be recovered. Subscriber units beyond the coverage area (14)
may be unable to detect signals from the resource controller (16).
Similarly, subscriber units beyond the coverage area (14) might be too far
from the resource controller (16) to provide the resource controller (16)
with useable signals of their own. Reduced power levels of subscribers
within the coverage area (14) and providing distributed resource
controllers, also at reduced power levels, might permit a coverage area
(14) as shown in FIG. 1 to be simultaneously partitioned into secondary,
small-cell trunking systems providing communications throughout much
smaller geographic areas within the larger area (14). A trunked radio
system that permits more efficient use of limited radio spectrum, allowing
the spectrum to be re-used, would be an improvement over the prior art.
SUMMARY OF THE INVENTION
There is provided herein a method of coordinating communication resource
reuse by small-cell trunking systems within a large cell trunking system.
The small-cell trunking systems re-use the large-cell communication
resources but over limited geographic areas by limiting the power of
small-cell subscribers, small-cell repeaters/transmitters and small-cell
resource controllers. Only large-cell resources which are not being used
within the small-cell coverage area are used by a small cell trunking
system. Using this method, communication resources for the large cell may
be simultaneously reused by multiple users in multiple small cells, each
small cell lying within different portions of the geographic region
covered by the large cell trunked radio system.
The invention requires the resource controller for an existing, large-cell
trunked radio system to provide to small-cell controllers, a list of
communications resources useable within the large cell. (Communication
resources to be reused within the large cell would of course include with
the talk-in and talk-out channels of a communication resource. Although
the large cell resource controller might not identify each talk-in and
talk-out channel specifically, the large cell resource controller would
provide the identity of the large cell communication resource, which the
small-cell controllers would know were comprised of pairs of channels.)
The large-cell controller would preferably broadcast the list of
communication resources useable with the large cell with its outbound
signalling words on the control resource for the large cell. Small cell
trunking system resource controllers, established for each small cell,
would monitor the control resource for the large cell for the list of
communication resources from the large cell resource controller. The small
cell resource controllers would then be able to identify the
communications resources of the large cell from the large-cell broadcast.
When a first small cell resource controller identifies the communications
resources of the large cell, it monitors each of the communication
resources, including possibly both the talk-in and talk-out channels, for
useable communications signals. Useable communication signals on a
communication resource might originate from a subscriber unit for the
large cell that is using the resource, from the controller or a repeater
for the large cell, from other small cell trunked radio systems that have
begun using a large cell resource and that is within signal range of the
small cell controller as well as any other signal source using the
communication resource. (Useable communications signals for radio trunked
radio system, also referred to as useable signals, are radio signals of an
amplitude sufficiently great that a receiver will be able to demodulate
the information in the signal. In systems using different types of
modulation or spectrum allocation, useable signals will have other
characteristics. In most trunking systems, an FM signal of sufficient
strength to be demodulated would be a useable signal.)
Useable communication signals on a communication resource, (either a
talk-in channel, a talk-out channel, or both) indicate that the resource
is in use. Communication resources of the large cell (those resources that
are allocable by the large cell) upon which useable signals are detected
(from any source, including another small cell on either a talk-in
channel, a talk-out channel, or both) are designated by the small cell
resource controller as in use and not reusable within the corresponding
small cell trunked system by the small cell trunked system resource
controller. Large cell communication resources (either a talk-in channel,
a talk-out channel, or both) that have no useable communication signals
detectable by the small cell controller are identified by that controller
as re-useable by the small cell resource controller. (Talk-in channels,
talk-out channels or both, are referred to hereafter as communication
resources, unless noted otherwise.)
If any large-cell communication resources are re-useable, the small cell
resource controller designates one re-useable communication resource, (a
communication resource on the list broadcast from the large cell resource
controller and not having any useable communication signals detectable by
the small-cell controller) as the control resource for the small cell. The
small cell controller also designates some predetermined number of
re-useable communication resources as communication resources for the
small cell trunking system. The small cell controller then broadcasts its
own control resource signals on the re-useable communication resource it
claimed for use within its small cell. The signals that the small cell
resource controller broadcasts on its control resource include OSW's
normally required in trunking systems but also include a list of the
identity of the re-useable communication resources that the small cell
controller has allocated for use within the small cell. (The small-cell
controller's allocation of unused communication resource within its small
cell does not preclude the large cell from simultaneously assigning a
communication resource used by a small cell to a large cell subscriber.)
The small cell resource controller repetitively tests at least the
communication resources it allocates for its own small cell use for usage
by either the large cell or another small cell that interferes with the
small cells use. In the event the large cell controller assigns a
communication resource to a large cell subscriber that a small cell has
previously allocated to itself for its small cell use, the small cell
controller might have to suspend its use of the communication resource if
the large cell subscribers use of the resource (large cell subscriber
transmission on the resource) interferes with the small-cells use. For
example, signals from a large cell subscriber on the resource used within
the small cell might overpower signals within the small cell originating
from small cell users.)
Any second small cell resource controller will also monitor the control
resource from the large cell and identify communication resources of the
large cell. This second small cell controller will scan the communication
resources on the list broadcast from the large cell controller for useable
signals. (Like the first small-cell controller, the second small cell
controller will test the communication resources for any useable signals,
including signals from large-cell subscribers and from other small-cell
controllers and other small-cell subscribers.) If the second small cell
resource controller detects signals broadcast from the first small cell
resource controller on the communication resource allocated by the first
small-cell controller as its control resource, the second small-cell
resource controller will monitor the small-cell control resource signal to
learn what communication resources of the large cell trunked system the
first small-cell trunked system has claimed for use within its small cell.
The second small-cell controller will thereafter mark any communication
resources claimed by other small cell resource controllers as unavailable
for reuse within its small cell and pick a set of remaining communication
resources for its own control resource and communication resources. The
second small-cell controller will also mark communication resources having
useable signals from subscribers (both large cell and small cell
subscribers) as unavailable for reuse. The second small cell controller
then broadcasts its own control resource signals, which include its list
of communication resource claimed by it.
Subscriber units that operate within these small cells are low power
subscriber units. Similarly, the resource controllers for these small cell
systems also operate with low power. Lowering the power from the small
cell subscribers and resource controllers limits the geographic coverage
area and prevents the small cell signals from interfering with the large
cell resource controller.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a schematic diagram of some of the elements of a prior art
trunked radio system.
FIG. 2 shows a diagram of the radio frequency spectra of the communications
resources used by the mobile and base units.
FIG. 3 shows a train of outbound signalling words from a resource
controller.
FIG. 4 shows a typical format of an outbound signalling word from a
resource controller.
FIG. 5 shows a diagram of an outbound signalling word with an appended list
of reusable resources from a resource controller of the large cell trunk
system.
FIG. 6 shows a typical reusable communication resource information packet
format.
FIG. 7 shows a diagram of the reusable communications resources and their
subsequent usage by a small cell trunk system.
FIG. 8 shows a large scale trunk system and the relative geographic
coverage are of at least three small cell trunk systems.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1, a trunked radio system (10) is shown that provides
trunked radio communications to subscriber units substantially throughout
a coverage area (14). A subscriber unit (12a) requesting communication
with another subscriber unit (12b) will ordinarily send an inbound
signalling word (ISW) to the resource controller (16) that informs the
resource controller (16) of the first subscriber unit's (12a) request for
communication with a related subscriber unit (12b). The resource
controller (16) will assign an available communication resource to the
subscriber units (12a) and (12b) using an outbound signalling word (34)
(OSW) broadcast from the resource controller on the trunking system's
control resource. The control resource is usually a pair of radio
frequencies selected from a frequency range or spectrum on which control
signals are broadcast from the resource controller to the subscriber
units, i.e. the OSW's, and on which control signals are broadcast to the
resource controller from the subscriber units, i.e. the ISW's. OSW's and
ISW's will be sent and received on different frequencies, distinct from
the transmit and receive channels which are communication resources. See
FIG. 2.
Even after the resource controller (16) assigns a communication resource to
the subscriber units (12a and 12b) unassigned communications resources may
be available for reuse within the geographic coverage area (14). In the
preferred embodiment of this invention, the resource controller considers
all licensed communication resources as available for re-use and
broadcasts a list of communication resources available within the trunked
radio system with its OSW's. In a typical trunked radio system, the
resource controller (16) broadcasts outbound signalling words as shown in
FIG. 3. These outbound signalling words designated OWS1, OSWS2, . . .
OSW.sub.N, have a predefined format such as the format shown in FIG. 4.
OSW's will typically include three bit fields. The first bit field could
be considered the talk group ID. The second bit field could be a user ID,
and the third bit field might be a channel ID. The talk group ID in an
outbound signalling word, identifies users in the same or related talk
groups that should respond to the OWS. In this invention, the OSW's are
periodically appended with a list of the communication resources available
within the large cell trunking system (10). As shown in FIG. 5, OSW's from
the resource controller are periodically appended with the data words that
identify the communication resources useable within the trunking system
(10). The list of available reusable communication resources may be
appended on OSW's on an as-needed basis. Alternative embodiments would of
course include, for example, appending communication resource IDs to every
OSW.
FIG. 6 shows a prototype OSW with an appended list of communication
resources reuseable within the large cell. Predefined bit fields following
the OSW contain data that identifies the communications resources. (The
actual data in the reuseable resource I.D. word could represent many
quantities, including for example actual frequencies. It might include
encoded characters or symbols representing certain frequencies, time
slots, channels etc. that when decoded can identify communications
resources available for re-use.)
Referring to FIG. 8, there is shown a large cell trunked radio system (10)
that includes a large cell trunked radio system controller (16), and
subscriber units (12band 12a) which communicate with each other through
the large cell trunk system. The large cell trunk system (10) includes
three small cell trunked radio systems (100a, 100b, and 100c). One small
cell trunked radio system (100a) includes at least two small cell
subscribe units (1 and 2).
The small cell trunked radio systems being within the geographic coverage
area (14) of the large cell trunk system will find themselves at times
within range of radio signals originating from a large cell subscriber
unit (12a) as shown. To prevent interference to small-cell subscribers
from large cell subscribers, the small cells may use only communication
resources of the large cell, not carrying signals within range of the
small cell controller. If a large cell subscriber moves into or near the
coverage area of the small cell, and if the large cell subscriber uses a
communication resource that the small cell is using, the small cell
controller will have to de-allocate its claim on the communication
resource that the large cell subscriber is using. Due to the nature of
typical communications on a trunking system, i.e. short message duration,
the small cell controller might delay releasing its claim on a
communication resource and instead wait for the end of a short duration.
The small cell controller might wait ten seconds for example, thereafter
re-check the communication resource for activity. If the resource is still
used, the small cell controller could then release the resource.
To coordinate usage of communications resources by small cell trunked radio
systems (100a, 100b, and 100c), each of the small cell trunked radio
systems repetitively checks the large-cell communications resources for
communications signals and adjusts their claims for a subset of the
communications resources not having any detectable signal. One
communication resource claimed by a small cell controller will serve as
the control resource for the small cell, carrying OSW's and ISW's for the
small cell. The identity of communication resources claimed by the small
cell resource controller is announced to geographically adjacent small
cells by the small cell broadcasting, on the communication resources it
claimed for its control resource, a list of the identity of communication
resources claimed by that controller. As small cell controllers release
their claim on communication resources, the list of communication
resources claimed by them should change accordingly.
Prior to claiming a communication resource each small cell resource
controller for a small cell (200, 220, and 240) identifies the large cell
communication resources and tests each for a useable signal or prior claim
(by another small cell controller). (Useable communications signals, also
referred to as useable signals, are signals of an amplitude sufficiently
great than an FM receiver will be able to demodulate the information in
the signal.) Communication resources that have no useable signals and not
claimed by a small cell controller are considered as being unused by a
small cell controller and subject to the small cell controllers claim for
its use within the small cell. Stated alternatively, each small cell
resource controller (200, 220, and 240) tests the large-cell
communications resources for a control resource signal or other useable
signal, that would indicate prior assignment, claim, or usage of the
communication resource.
If a small cell controller (200, 220, and 240) does not detect a control
resource signal, claim, or other signal from another small cell resource
controller or other user, the small cell resource controller may claim at
least one unallocated reusable communications resource as its own control
resource and begin broadcasting its own outbound signalling words such as
those shown in FIG. 8. The outbound signalling words shown in FIG. 7
include bit fields following the small cell outbound signalling words that
inform other adjacent small cell resource controllers of the number and
identity of communications resources that the small cell resource
controller claims for its own exclusive use within its small cell coverage
area (140, 160, and 180) for example.
In the preferred embodiment, the large cell resource controller would
ordinarily broadcast a list of reusable communication resources on its own
control resource as described above. An alternate embodiment would include
transferring the list of reusable communication resources to the small
cell resource controllers (200, 220 and 240) by a wire-line, such as the
telephone network.
Since the available communication resources of the large-cell may change
over time, the small cells should periodically check the list of available
communication resources that are available to the small cells for
communication signals. The steps of the method shown in FIG. 10 should be
repeated.
Upon finding a large cell control resource, the small cell resource
controller tabulates the identity of communication resources designated as
useable by the large cell controller. Each small cell controller then
scans the communication resources for any signals that indicate that the
communication resource is in use and thereby unavailable for re-use by the
small cell.
If a signal is detected on a communication resource, the detecting small
cell controller should monitor that detected small cell control resource
to determine if the signal is from a small cell control resource and
tabulate from the small cell control resource the identity of
communication resources allocated by that small cell controller to itself.
Remaining available communication resources can be used by the small cell
controller as its own control resource and assignable or reusable
communications resources.
Implementation of a small cell trunking system within a large cell trunking
system would require much of the infrastructure found in a large cell
trunking system albeit adapted for small geographic areas. A resource
controller for the small cell would typically include a base site at which
a radio receiver detects RF signals on which the ISW's are carried. The
resource controller would also include a radio transmitter to broadcast
the OSW's. A computer coupled to the receiver and transmitter would keep
track of assignments of communication resources as well as the list of
available communication resources. The computer would likely administer
other tasks as well.
The small cell trunking system might include repeater stations, such as
those shown in FIG. 1 and well known in the art, for the reception and
distribution of signals throughout the coverage area of the small cell.
Small cell subscriber units within the small cell would of course have to
be able to detect control signals (OSW's) from the small cell control
resource on any large cell communication resource. The small cell
subscribers would also have to generate ISW's accordingly. The repeaters
are typically coupled to the resource controller so that the resource
controller knows of the use and non-use of a communication resource.
Transmitters used within the small cell (including subscriber unit
transmitters, resource controller transmitters and repeaters) will
typically have low output power levels, ten milliwatts for example.
Increasingly high output power levels will expand the coverage area of the
small cells.
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