Note: Descriptions are shown in the official language in which they were submitted.
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HIGH CAPACITY SECTORIZED CELLULAR COMMUNICATION SYSTEM
Field of the Inventio~
The present invention relates generally to radio
frequency (RF) communication systems, and, more
particularly, to sector-transmit and sector-receive
cellular communication systems.
Description of the Prior Art
Mobile radiotelephone service has been in use for
some time and traditionally has been characterized by a
central site transmitting with high power to a limited
number of mobile or portable units in a large geographic
area. Mobile or portable transmissions, due to their
lower transmission power, were generally received in
previous systems by a network of receivers remotely
located from the central site and received transmission
was subsequently returned to the central site for
processing. In previous systems only a limited number of
radio channels were available, thus limiting the number
of radiotelephone conversations in an entire city to the
limited number of channels available.
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Modern cellular radiotelephone systems have a
comparatively large number of available radio channels
which can be effectively multiplied by reuse of the
channels in a metropolitan area by dividing the radio
coverage area into smaller coverage areas (cells) using
low power transmitters and coverage restricted receivers.
Such cellular systems are further described in U.S.
Patent Numbers 3,906,166, 4,485,486 and 4,549,311, each
assigned to the assignee of the present invention.
Some of the more spectrally efficient cellular
radiotelephone systems employ center illumination in
which each cell is subdivided into sectors. The sectors
are illuminated by directional antennas. Each sector is
provided with a number of dedicated voice channels. Such
a system is described in U.S. Pat. Nos. 4,128,740 and
4,696,027, each assigned to the assignee of the present
invention. The sectors are used to substantially
eliminate interference from adjacent co-channels.
Unfortunately, the known spectrally efficient
cellular radiotelephone systems cannot provide sufficient
channel capacity to accommodate the ever-increasing
demands of a cellular operation. Consider a relatively
large cell wherein each sector covers a large geographic
area. An aggregation of subscribers in a given sector
can readily occupy every available voice channel
available in that sector.
There has, however, been system developments to
overcome this problem. One particular system, as is
described in U.S. Pat. No. 4,144,411, subdivides each
cell into subcells which operate simultaneously on
independent and noninterferring voice channels. Although
this implementation has proven quite useful for
increasing channel capacity in each cell, it is rather
expensive to implement due to the duplicity of base site
equipment that is required.
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There is therefore a need for a cellular
radiotelephone system which overcomes these deficiencies.
Object of the Present Inventio~
It is a general object of the present invention to
provide a cellular radiotelephone communication system cell
site equipment which overcomes the aforementioned
deficiencies.
It is a more particular object of the present
invention to provide a cellular radiotelephone communication
system cell site equipment which includes multiple cavity
combiners switchably coupled to one or more transmitters
such that signals from each transmitter may be readily
coupled to at least two different antennas.
Brief Description of the Drawings
FIG. 1 is a block diagram of a conventional center
illuminated sector radiotelephone communication system.
FIG. 2 is a more detailed block diagram of cell site
equipment which may be used by the system illustrated in
FIG. 1, according to the present invention.
FIG. 3 is a diagram of an RF switch employable by the
system illustrated in FIG. 2, according to the present
invention.
FIGS. 4a, 4b and 4c comprise diagrams of equipment
employed at the sectors in the system illustrated in FIG. 2,
according to the present invention.
FIG. 5 is a flow chart useful for operating the system
illustrated in FIG. 2, according to the present invention.
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Detailed Description of the Preferred ~mLbodiment
Referring now to FIG. l, there is illustrated a
cellular radiotelephone communications system of the type
which may particularly benefit from the invention herein
described. Such a cellular communications systems is
further described in U.S. Patent Numbers 3,663,762 and
3,906,166; in an experimental cellular radiotelephone
system application with the Federal Communications
Commission by Motorola and American Radio-Telephone
Service, Inc., in February 1977; and more recently in a
system described entitled "Motorola DYNATAC Cellular
Radiotelephone System", published by Motorola, Inc.,
Schaumburg, Illinois in 1982. Such cellular systems
provide telephone coverage to both mobile and portable
radiotelephones located throughout a large geographical
area. Portable radiotelephones may be of the type
described in U.S. Patent Numbers 4,486,624; 3,962,553;
and 3,906,166 and each assigned to the assignee of the
present invention; and mobile radiotelephones may be of
the type described in Motorola instruction manual number
68P81039E25, published by Motorola Service Publications,
Schaumburg, Illinois, U.S.A. in 1979. Although the present
invention will be described with particularity for the
center illuminated sector cell system, it is obvious that
a person skilled in the art may be able to apply the
essence of the present invention to other types of
sectorized cellular configurations.
As illustrated in FIG. 1, the geographical area is
subdivided into cells 102, 104 and 106 which are
illuminated with radio frequency energy from fixed site
transceivers 108, 110, 112, respectively. The fixed
site transceivers may be controlled by base site
controllers 114, 116 and 118 as illustrated. These base
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site controllers are each coupled by data and voice links
to a radiotelephone control terminal 120 which may be
similar to the terminals described in U.S. Patent Numbers
3,663,762; 3,764,915; 3,819,872; 3,906,166; and
4,268,722. These data and voice links maybe provided by
dedicated wire lines, pulse code modulated carrier lines,
microwave radio channels, or other suitable communication
links. Control terminal 120 is, in turn, coupled to the
switched telephone network via a conventional telephone
central office 122 for completing calls between mobile
and portable radiotelephones and landline telephones.
For a more detailed discussion of the system
illustrated in FIG. 1, reference may be made to U.S. Pat.
No. 4,696,027.
In one implementation of the present invention
realizing sectorized cells, the radio transceivers are
connected to the sector antennas as shown in FIG. 2.
Each sector antenna is fed by a multicoupler (for
example, RX multicoupler 202) to the primary transceiver
equipment dedicated to the particular sector (for
example, transceivers 204) and to both the adjacent
sector transceiver equipment (for example, transceivers
206 for sector 6 and transceivers 208 for sectors 2).
The transceivers 204, 206, or 208 provide communication
for voice channels which are dedicated for use at the
associated sector. In addition, each sector antenna is
coupled a signalling receiver allowing the signalling
receiver to have access to all six sector antennas. The
transmitters (e.g., transmitter 212) of the primary
transceiver equipment are coupled to the sector antennas,
via respective combiners 282, and duplexers (such as
duplexer 210). The duplexers may be similar to model
ACD-2802-AAMO manufactured by Antenna Specialists Co.,
Cleveland, Ohio, U.S.A. The combiners 282 may be
implemented using SRF4006B, available from Motorola, Inc.,
1501 W. Shure Drive, Arlington Heights, Illinois, U.S.A.
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A base site controller 220 is used to control a 6-way
switch 230 which couples a floating voice channel
transceiver 240 to each duplexer 210, via a combiner 282.
The transceiver 240 is referred to as a floating voice
channel transceiver because it provides, in accordance
with the present invention, a voice channel which can
float between sectors as channel requests are made in
each sector. The receiver portion of the transceiver 240
is interconnected (interconnection not shown) with the
receiver multicouplers 202 in the same manner as is
illustrated for the transceivers 204, 206, and 208.
This floating implementation is particularly
advantageous for rural cellular radiotelephone
applications wherein each geographical region covered by
an individual sector can be extended beyond typical
coverage areas. Consider, for example, a cellular
system according to the present invention, that is
designed to cover a large rural geographical area. Each
sectorized cell would include a plurality of high gain
sectorized antennas, each capable of transmitting at
significantly greater radiated power levels than present
implementations, e.g., at 500 Watts rather than at
current levels approximating 100 Watts. Such increases
in transmit power can cost effectively be accomplished by
employing conventional high gain sectorized antennas.
The signaling channel in the inventive system, as well
as in a conventional sectorized cell system, employs an
omni-directional antenna. Since the gain of the omni-
directional antenna is lower than the gain of a sector
antenna, a higher power amplifier is required for the
signalling channel in order to match the effective
radiated power levels for each type of channel.
A preferred implementation of a floating voice
channel system is described with reference to FIGS. 3,
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4a, 4b and 4c. FIG. 3 includes a flow diagram of a
floating voice channel targeted for one of six sectors of
a cell. FIG. 3 also includes a conventional internally
terminated RF coaxial switch 310, such as the 6-IT-2L31
available from DB products, Pasadena, California, U.S.A.
switch is used in a manner similar to the switch 230 of
FIG. 2. This preferred implementation differs from the
implementation of FIG. 2 in that the system employs a
plurality of floating voice channels rather than only one
floating voice channel. The switch 310 of FIG. 3 is a
representative switch for one voice channel (one such
switch is utilized for each floating voice channel) that
allows the base site controller to dynamically switch the
one associated voice channel to any particular sector.
Such dynamic switching is useful, for example, when a
subscriber transits from one sector to another. In this
instance, the base site controller needs only to switch a
floating voice channel assigned to the subscriber to the
next sector. More importantly, as previously discussed,
this dynamic switching provides significant improvement
to the system in that each available floating voice
channel can be switched to any sector to service an
influx of subscribers demanding service thereat. The
availability of a single floating channel provides a
channel capacity increase for every sector in a cell. At
least in part, because radiotelephone calls are
relatively brief and because communication systems can be
designed such that a request for an additional
communication channel in a particular geographic region
is unlikely, the effective channel capacity improvement
is significantly greater than what would otherwise be
achieved by adding additional fixed channels in each
sector. For example, the effective channel capacity
improvement is illustrated by comparing two cells
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providing substantially identical traffic density
characteristics (e.g., using conventional Erlang unit
measurements ~. The first cell is a conventional cell
having 6 sectors with 3 dedicated channels per sector and
having no floating channels, and the second cell is a
cell, designed in accordance with the present invention,
having 6 sectors with 1 dedicated channel per sector and
having 5 floating channels employable within the cell.
It is readily apparent that 18 channels are required for
the conventional implementation, while only 11 channels
are required for the inventive implementation.
For a statistical analysis on the practicability of
"overflow" channels in a general communication system,
reference may be made to "Practical Traffic Engineering
of Least Cost Routing Systems - Part 5, 'Peaked' Traffic:
What It Is and When you Should Worry About It", Michael
T. Hills, Business Communications Review, July-August,
1983.
One skilled in the art will recognize the need to
avoid co-channel interference between sectorized regions.
For example, it is useful to avoid such interference by
employing floating channels that have ample reuse
distance. The orientation of the dedicated channels is
preferably designed as is illustrated and described in
U.S. Pat. No. 4,128,740, assigned to the assignee of the
present invention.
One skilled in the art will further recognize that
any number of dedicated channels per sector may be
employed and that any number of floating channels per
cell may be employed without departing from the scope of
the present invention. For example, it may be desirable to
employ zero dedicated channels per sector and a plurality
of floating channels per cell.
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In Fig. 4a, a general block diagram of the floating
voice channel interconnection is illustrated for sector 1
of a representative cell. A voice channel transmitter
450 is shown coupling a sector dedicated (or fixed) voice
channel, i.e., a voice channel which does not float, to a
conventional channel combiner 455. Also intercoupled to
the combiner 455 are signal paths 460, 462, 464, and 466
for respectively carrying four floating voice channels
from an RF coaxial switch 310. The combiner 455 operates
conventionally to intercouple the voice channels (paths)
to the sector's antenna 458.
Fig. 4b illustrates a general block diagram of the
equipment for sector 2 of the representative cell, which
equipment is essentially identical to the equipment
illustrated in Fig 4a. In Fig. 4b, a voice channel
transmitter 470 is shown coupling another sector
dedicated voice channel to a similar channel combiner
475. Coupled to an antenna 478, via the combiner 475,
are signal paths 480, 482, 484 and 486 for respectively
carrying the same four floating voice channels from
another RF coaxial switch 310 dedicated for use at sector
2 of the representative cell.
In more detail than the block diagram of Figs. 3a and
3b and consistent with the equipment of Fig. 2, Fig. 4c
illustrates a block diagram of equipment for a
representative sector of such an inventive system.
Included therein are blocks representative of those
illustrated in Fig. 2: base site controller 220, (voice
channel) transceiver 240, switch 230, transceivers 204,
duplexer 210, receiver multicoupler 202, and 5 channel
combiner 282, representative of the combiner 455 or 475
of Figs. 4a and 4b. The combiner 282 is illustrated
combining the outputs of the switch 230, the other
switches (not shown) and the outputs of transceivers
(transmitters) 204 to the duplexer 210. Other
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interconnections are consistent with the illustration of
Fig. 2
Fig. 5 illustrates a flow chart useful for operating
channel assignments in a particular cell for the system
illustrated in Fig. 2, according to the present
invention. This flow chart accommodates assignment
operation for both floating voice channels as well as
fixed (dedicated) voice channels for a cell. The flow
chart begins at block 510 where the base site controller
is depicted monitoring the signalling channel for a
subscriber radiotelephone call request. From block 510,
flow proceeds to block 515 where a test is performed by
the base site controller to determine if a subscriber
station has requested the use of the voice channel in a
sector. If not, flow proceeds to block 510.
If a subscriber station has requested a voice channel
in a particular sector, flow proceeds from block 515 to
block 520 where another test is performed to determine if
a fixed channel is available in the particular sector.
If a fixed channel is available, flow proceeds to block
525 where a channel assignment is made to the available
voice channel. From block 525, flow returns to block
510.
If a fixed channel is not available in a particular
sector, flow proceeds from block 520 to block 530 where a
test is performed to determine if a floating channel is
available in a particular sector. If a floating channel
is available, flow proceeds to block 535 where the base
site controller (BSC) controls the sector's RF coaxial
switch to connect the available floating channel to the
requesting subscriber station. From block 535, flow
proceeds to block 540 where the actual channel assignment
is made. Once the radiotelephone call terminates on the
floating channel, depicted by block 540, the base site
controller disconnects the connection of the floating
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channel to the particular sector, depicted at block 550.
From block 550, flow returns to block 510.
From block 530, if a floating channel is not
available in he particular sector, flow proceeds to
block 560 where a test is performed to determine if a
fixed channel is available in a sector which is adjacent
to the particular sector in which a subscriber station
requests a channel. This test illustrated in block 560
is typical (often referred to as sector sharing) in
conventional sectorized telephone communication systems
when there are no voice channels available at the sector
from which a channel is requested. Thus, if a fixed
channel is available in an adjacent sector, flow proceeds
to block 565 where the requesting subscriber station is
assigned a channel from the adjacent sector. From block
565, flow returns to block 510.
If a fixed channel is not available in an adjacent
sector, flow proceeds from block 560 to block 570 where
the base site controller determines if the requesting
subscriber can be assigned to a channel in an adjacent
cell, preferably to the nearest sector in the nearest
cell, with respect to the subscriber unit. This is
conventionally accomplished by determining if the
assignment of a channel from an adjacent cell would
cause undue cochannel interference in another cell. More
specifically, if the subscriber signal strength in the
target cell exceeds the system's cochannel interference
threshold, flow proceeds to block 580 where the call
request is denied ~blocked). If the subscriber signal
strength is not above the threshold, flow proceeds from
block 580 to block 575 where the call is assigned to the
adjacent cell. From block 575, flow returns to block
510.
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If the signal strength of the requesting subscriber
station is above the cochannel interference threshold,
flow proceeds from block 570 to block 580 where the base
site controller informs the subscriber station that the
call request cannot be served. From block 580, flow
returns to block 510.
The foregoing sequence illustrated in the flow chart
of Fig. 5 incorporates a preferred sequence of channel
availability checking. The base site controller first
checks if the fixed channel is available in the
particular sector, and if not, proceeds to check if a
floating channel is available. After the base site
controller has determined that neither a fixed nor a
floating channel is available in a particular sector,
only then does the base site controller check for channel
availability in adjacent sectors. This sequence is
important for channel usage efficiency. Optimally, the
fixed channel(s) in each sector is (are) utilized before
any floating channel is utilized.
One skilled in the art will recognize that various
modifications may be made to the above described system
without departing from the spirit or scope of the present
invention.
We claim:
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