Note: Descriptions are shown in the official language in which they were submitted.
WO 94/24772 21~ 6 ~ PCT/IJ594/03007 }
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METHOD AND APPARATUS FOR SEPARATING CHANNEL,S
F~OM A RADIO F~<EQI~NCY TRANSMIl~ER
Field of the lnvention
The. field of the invention relates to
co~nunication systems and, more particularly, to
cellular communication systenns.
1 0
Background of the lnvention
Cellular systems simultaneously handling a
number of traffic channels through each base station are
1 S typically assigned a number of channels (f1-fn) in
support of communications with mobile communication
uni~s through local base stations. Each base station is, in
turn, allotted a subset of ~he channels (fl-fn). Of the
subset of channels assigned to a base site at least one
2 0 ~and often more) is designated as a control channel for
purposes of access control and channel set-up.
Communication with a cornmunication unit on a
traf~lc channel within a service coverage area of the
base site is often accomplished through an
2 5 omnidirectional antenna centrally locatecl within the
service coverage area. A number of communications
may be simultaneously supported through the antenna
with each individual communication supported by a
transmitter (located at the base site) assigned to the
3 0 traffic channel. Each transmitter includes a modulated
transmit signal source~ within the transceiver and a
radio frequency (RF) power ampli~ler). Each transmitter
thereby provi~es signal generation, modulation and
amplification.
3 5 ln contrast to traffic channels, control inforrnation
from a base site is often transceived from a number of
directional antennas that divide the service coverage
area into a number of sectors. Dividing the service
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coverage area into sectors for control channel purposes
provides a means for the base site ~o determine the
relative position of a communication unit for purposes
of handoff.
S The simultaneous transmission of a number of
traf~lc channel signals from the central antenna requires
that trans~mitter output of each active transceiver be
combined before application to, and transmission from,
the central antenna. In order to avoid interference-
- 10 producing intermodulation products, signals must be
combined after any non-linear steps within the
amplification process. In addition, the combinin~
topology must provide sufficien~ reverse isolation to
insure that signals of parallel amplification branches
will not be coupled into the output of other power
amplifiers, again producing intermodulation products.
Where each transceiver is equipped with its own
power ampli~ler (PA), combining must occur after the
PA where sigrlal levels, as well as combining losses, are
2 0 high. A cavity combiner, for combining such high level
RF signals while providing the necessary isolation, is
provided by U.S. Patent No. 4,667,172 assigned to the
assignee of the present invention.
An explanation of the operation of a cavity
2 5 combiner and of the interconnectin~ one-quarter
wavelength interconnect facilities follows. A cavity
combiner includes a mlmber of frequency selective
cavities, each resonant at a tuned frequency,
interconnected at a combiner junction by a transmission ~ -
3 0 line of a length essentially equal to one-quarter
waveiength at the tuned frequency. A number of such
frequency selective cavities and one-quarter
wavelength cables ~cavity assemblies) interconnected at
a combiner junction, provide a means for combining a
3 5 number of RF signals. I
A cavity combiner presents (at the tuned . '
frequency) an impedance equal to that of the
characteristic (matched) impedance of the system
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(typically 50 Q) to a desired signal frequency from an
amplifier through the frequency selective cavity to the
combiner junction. A si,,nal presentecl at the cavity
combiner, in the reverse direction, from the combiner
5 junction, at other than the tuned frequency~ would be
presented with a low impedance. The low impedance is
transformed by the one-quarter wavelength cable into a
hi~h impedance at the combiner junction. The high
impedance at the combiner junction prevents adjacent
10 combiner branches from mutually loading one another,
while the cavity selectivity provides an adequate level
of si~nal isolation between adjacent branches.
While the process of combining high level RF
signals works well the power loss, in terms of actual
15 power dissipation, is significant. Power loss within a
combiner is typically 3 db.
A circulator is known to transfer RF signals
received at a first port to a second port and to a load
interconnected with the second port. If an impedance
2 0 mismatch is presented at the second port, a proportional
amount of the RF signal is reflected back and becomes
an input at the second port of the circulator. Since the
circulator transfers input signals received at ~ second
port to a third port, the reflected RF sicnal is transferred
2 5 to (and dissipated) within a load often interconnected
with the ~hird port.
Radio ` frequency (RF) circulators are known in the
art of RF communications (see, for example, U~H~F.
Techniques for Lumped Constant Circulators, by J. ..
3 0 Helszajn and F. M. Aitkent, Electronic En~ineering, Nov.,
1973) and used for purposes of signal steering,
switching, isolatin~, etc. The most cornrnon use of
circulators is to protect RF transmitter from damage due
to open-circuits between the transmitter and an
3 5 a~sociated load.
In other communication systems, transceivers are
not equipped with individual PAs~ instead. a common~
multitone linear PA (LPA) is used for amplification after
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the RF signals have been combined at relatively low
power levels at the output of the transceiver. The use
of such common LPA for tra~fic channels in systems
using a common antenna has resulted iII considerable
5 simplification of system topology, improvements in
system efficiency, and reduction in system size. In
other systems, operating under a sector format7 the use
of LPAs is not as attrac$ive because of the difficulty in
separa~inG RF signals following amplification in the LPA.
10 Because of ~he importance of power efficiency in
cornmunication systems a need exists for a method of
isolatinc RF channels for sector transmission following
amplification in a comrnon LPA.
Summary of ~he lnvention
A method and apparatus is provided for
separating spectrally diverse radio frequency signals.
2 O The apparatus includes means, within a signal transfer
module, for transferring an input signal received at a
first port of the signal transfer module to a second port
of the signal transfer module. At least one frequency
selective assembly is included, in~erconnected with the
2 5 second port of the signal transfer module, presenting a
characteristic impedance to a desired signal of the input
signal and reflecting other signals of the input signal to
the second port of the frequency transfer module. A
means, within the signal transfer module for
3 0 transferring a reflected signal received at the second
port of the signal transfer module to a third port of the
signal transfer module is also included. The method of
practicing the invention is also provided
Brief Description of the Drawings
:` WO941~4777 ~13fi ~ CT/US~4103007
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FIG. 1 is a block diagram of a communication ~-
system base site in accordance with an embodiment of
the invention.
Detailed Description of the Preferred Embodiment
The solution to the problem of separatin~
previously combined RF~ signals ~following amplification
in a common LPA~ lies conceptually in the use of a
signal transfer module (e.g., an RF circulator) and a
frequency selective cavity assembly interconnected
with a second port of the signal transfer module. It has
been deterr~ined that the use of a signal transfer
module in conjunction with a frequency selective cavity
assembly produces the unexpec~ed result of separating
a first RF si~,nal, of a ~requency to which the cavity is
tuned, for application ~o a first antenna while routing
other signals to a second antenna. Such a device has
been determined to be useful in cellular system base
2 0 sites (e.g., where traf~lc channels are transmitted frorn a
common, omni antenna and control channels are
transmitted from sector antennas).
The output of an LPA (containing a control channel
to be separated from a number of traffic channels) is
applied to a first port of the circulator. An RF input
presented to a first port of a circulator, as is known in
the art, will be transferred to the second port of the
circulator. In a sys~em having a characteristic
impedànce (e.~., 50 S2), the frequency selec~ive cavity
3 0 assembly, interconnected with the second port of the '.
circul`ator, presents a~ 50Q path for the control channel,
to which it is tuned, to a sector antenna and a high
impedance path at the cavity junction for the traffic
channels (outside a pass band of the cavity). The high
3 5 irllpedance presented to the traffic channel signals at
the cavity junction causes the traffic channel signals to
be reflected back into the second port of the circulator.
The circulator, receiving an input RF signal at its second
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WO 941~4772 PCT/US94/03007 ^~
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port, transfers the reflected RF signal to a third port of
the circulator. An omni antenna interconnected with
the third port of the circulator presents a load to the
circulator and a path for transmission of the traffic
5 signals.
The frequency selective cavity assembly includes
a frequency selective cavity and a transmission line
interconnecting the frequency selective cavity to the
second port of the circulator. The transmission line is
l 0 essentially one-quarter wavelength long at the
frequency ba~d of the desired control channel. Each
frequency selective cavity is tuned to the frequency of
one selected control channel.
FIG. 1 is a bloc~ diagram of a cellular base site lO
15 in accordance with an embodiment of the invention.
The base site has a single omni antenna 30 and a
number of sector antennas 31-36. The omni antenna is
used generally for transmitting traffic channels within a
service coverage area (not shown) of the base site 10.
2 0 The sector antenna 31-36 are each used for transmitting
control information within a portion (e.g., a 60 degree
sector of the service coverage area.
Traffic channel information originating from
within a public switch telephone network (PSTN) or
2 5 another base site 10 is routed to appropriate traffic
channel transceivers 13-14 by the controller 12.
Control information originating within the controller 12
is also routed to control transceivers 15-20. The low-
level output signals of the transceivers 13-20 are
3 0 combined within the combiner 21 through resistive
combining techniques for i subsequent amplification
within the LPA 22. Within the LPA 22 the combi~ed
signals are amplified to a level sufficient for
transmission from the omni 30 or sectored 31-36 `
3 5 antennas
Following amplification within the LPA 22 the
combined signal is applied to port 1 of the circulator 23.
The combined signal is transferred to port 2 of the
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circulator 23. At the output of port 2 the signal passes
through a splitting junction 43 and a number of
transmission lines 37-42 to frequency selective cavi~ies
24-29 .
Each frequency selective cavity 24-29 is tuned to
the frequency of a control channel to be transmitted
through ~the associated sector antenna 31-36. The
interconnec~ed cables 37-42 have a length substantially
equal to one-quarter the wavelength at the desired
control channel frequency.
The use of the one-quarter wavelength cable (of
the typical characteristic impedance of typically 50Q)
and frequency selective cavity presents a low loss path
to the desired control channel signal for transmission
from the associated sector antenna. The characteristic
impedance presented by the frequency selective cavity
assembly also prevents a reflection of the desired
control channel signal back to the junction 43 or to the
second port of the circulator 23.
2 0 Traffic channel frequencies reachin~ the junction
43, on the other hand, are presented with a high
impedance. Upon reaching the junction 43, the reflected
traf~lc channel signals (undesired signals) are reflected
back to the second port of the circulator 23. Upon being
2 5 reflected into the the circulator (as an input), the
undesired signals are trans~erred to the third port of th
circulator 23. The ornni antenna 30 interconnected to
the third port of the circulator 23 presents a load for the
traf~lc channel signals and a path for transmission of the
3 0 - si~nals.
The use of the circulator 23 and frequency
selective cavity assemblies improves communication
systems by reducin~ the cost and improvin~ the
reliability and efficiency of such systems. The use of a ~-
3 5 cornmon LPA reduces the cost of transmitters within
such a system by reducing the parts required to build
such as system and by utilizino the LPA to a higher
capacity for a better efficiency. The use of a common
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WO 94/24772 PCT/US94/03007
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LPA allows for a reduction in the size of such a system
by concentrating power amplification to a single area.
The use of a common LPA further allows the dyn~m~c
changin(J of system topologies (e.g, the reassignment of
S transceiver from one service coverage area to another).
The many features and advanta,~es of this
invention are apparent from the detailed specification
and thus it is intended by the appended claims tO cover
all such features and advanta,~es of the system which
fall within the true spirit and scope of the invention.
Further, since numerous modifications and changes will
readily occur to those skilled in the art (e.g., use within
commercial transmission systems)~ it is not desired to
limit the invention to the exact construction and
operation illustrated and describe~d~ and accordin~ly all
suitable modi~lcations and equivalents (e.g., orn~i-sector,
sector-omni, ornni-omni, and sector-sector channel
separators) may be resorted to, falling within the scope
of the inventian.
?. O An omni-sector separator may be used, for
instance, where a separated control channel is
transmitted on an omni antenna and reflected traffic
channels are transmitted from directional antennas. An
omni-sector channel separator may also ~lnd usefulness
2 5 in -cases where geographic obstructions cause poor
illumination of a service area from the central omni
antenna, which can be remedied by a single channel
sector antenna placed in close proximity and powered
from the same LPA. An omni-sector channel separator
3 0 may further find usefulness in cases where geo~raphic
cond;tions cause interfere;nce to other service areas on
selected channels radiated from the central omni
antenna by separating those channels and radiatin~
them from sector antennas into specific service areas. A
35 sector-sector channel separator may be useful where
directional antenna for traffic and control do not
coincide.
WO9S12477~ 2136~ PCTluS94/03007 h~
It is, of course~ to be understood that the present
invention is, by no means, limited to the specific
showin~ in the drawing, but also comprises any
modification within the scope of the appended claims.
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