Note: Descriptions are shown in the official language in which they were submitted.
WO 98/04052 PCT/LTS97/11371
-1-
SYSTEM AND METHOD FOR EQUALIZING
THE DELAY TIME FOR TRANSMISSION PATHS
IN A DISTRIBUTED ANTENNA NETWORK
BACKGROUND
The present invention is directed to a system and method for equalizing delay
times for transmission paths which connect a plurality of remote antenna units
in a
distributed antenna network by transmission media. More particularly, the
present
invention detects the delay parameters associated with each separate
transmission path
which connects a remote antenna unit to a central unit or a base station and
then
adjusts the delay time for each transmission path so that the delay times for
all of the
remote antenna units are equalized. Because the remote antenna units are
physically
connected to the central unit or base station by separate transmission paths
of different
lengths and types of transmission media, such connections may create
intersymbol
interference that is beyond the compensation capabilities of conventional
systems.
The system and method of the present invention actively compensate for such
delay
times without any manual intervention. The present invention also synchronizes
bursts so that air frame timing between cells served by the remote antenna
units is
enhanced and the hand-off performance therebetween is improved.
As personal communication services (PCS) evolve as the next generation of
cellular telephone technology, systems and techniques for simply and
efficiently
transmitting and receiving communication signals are being investigated. One
known
system is a distributed antenna network (which is also referred to as a
multicast
network) which provides coverage over substantial areas by a plurality of
remote
antenna units. An example of a distributed antenna network is illustrated in
Figure 1
where an individual transceiver 10 is connected to a plurality of cells
20,...20" by
transmission media 30 which transports radio signals between the transceiver
unit 10
and the cells 20, . . .20~. Each of the cells 20, , . . .20n include remote
antenna units
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21,,...21". The remote antenna units 21,,...21" may be connected to the
transmission
media 30 by frequency converting circuitry 22,,...22 for certain applications.
Various infrastructures are being developed and modifications of existing
infrastructures are of great interest as alternatives for PCS because they are
fully
capable of providing high quality signals at lower costs than traditional
cellular
infrastructures. For example, CATV infrastructures have been modified for use
in
PCS. Such modifications include the CATV infrastructures using a hybrid
fiber/coax
(HFC) cable infrastructure to increase capacity and improve service quality.
Although
it is theoretically possible for any CATV infrastructure to support PCS with
the
proper modifications, the HFC cable infrastructure offers an attractive option
as an
economical alternative to wireless providers seeking to avoid the high cost of
network
construction.
Figure 2 illustrates the basic components of a CATV infrastructure used to
support PCS. In Figure 2, base station equipments 50, and 502 are connected to
a
public network such as a public switched telephone network. Remote antenna
signal
processors (RASPs) 52, and 522 connect the base station equipments 50, and 502
to a
fiber equipment 54. The fiber equipment 54 is connected to a fiber node 58 by
fiber
optic cable 56 and the fiber node 58 is connected to remote antenna driver
(RAD)
nodes 62, and 622 by two-way coaxial cable 60. The RAD nodes 62, and 622 each
include a group of RADs 64, and 642 and 66, and 662 respectively connected to
antennas 68,, 682, 70,, and 702. This CATV infrastructure converts radio
frequency
signals into CATV frequency signals usable in the existing CATV infrastructure
and
converts CATV frequency signals back into radio frequency signals for
broadcast.
More specifically, the RASPs 52, and 522 convert the radio frequency signals
from
the base station equipments 50, and 502 and then send the converted signals in
the
downlink path toward the appropriate fiber node 58 and onto the coaxial cable
60.
The RADs 64,, 642, 66, and 662 are connected to the coaxial cable 60 for
converting CATV frequency signals into assigned radio frequency signals. Radio
frequency signals may be received by the RADs 64,, 642, 66, and 662 which
convert
these signals into signals of frequencies suitable for transmission in the
uplink path of
the CATV infrastructure. Thereafter, the RASPS 52, and 522 convert the
upstream
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CATV frequency signals back into radio frequency signals for processing by the
base
station equipments 50, and 502. This CATV infrastructure also may accommodate
equipment for multiple modulation schemes, such as time division multiple
access
(TDMA), code division multiple access (CDMA) and frequency division multiple
access (FDMA).
Radio telephony systems may utilize this CATV infrastructure by operating on
available portions of the radio frequency spectrum over fiber optic and
coaxial cables
which are widely available in urban areas so that such systems may be
installed to
take advantage of this existing infrastructure. The large installed base of
fiber optic
and coaxial cables used by CATV operators may thereby be effectively exploited
at a
minimal cost by this infrastructure which distributes the signals to the
appropriate
antenna locations. However, in these distributed antenna systems, it is likely
that a
signal may be received by more than one of the remote antenna units and where
digital modulation is used, transmission problems often arise. Because the
remote
antenna units are connected by transmission media having physically separate
transmission paths back to the transceiver or base station, the time delays
due to the
variations in the lengths of the transmission paths and/or types of
transmission media
typically cause transmission problems that cannot be fully compensated by
conventional equalization techniques at the base station or the mobile
stations.
Figure 3 generally illustrates the problems associated with the transmission
time delay for a variety of transmission paths in a distributed antenna
network.
Remote antenna devices 21, , 212, 213 and 214 are generally shown to be
connected
with a transmission medium interface 32 by transmission paths 31,, 312, 313
and 314
respectively. The transceiver unit 10 may communicate with a mobile 35 over
this
network. As shown in Figure 3, each of the four transmission paths may have
different lengths which cause different delay times for the signals. Also,
each of the
four transmission paths may have different transmission media that also
affects the
amount of delay time. All known techniques used for equalizing the delay time
of
transmission paths in a distributed antenna network require manual
intervention with
specialized equipment. Known examples of equalization techniques include
physically
adding extra lengths of cable to the transmission path so that the delay times
of the
CA 02260049 1999-O1-11 w
~;ES ~,~y EP.a t"l_E.~CHW ~!1 l;- 9-98 y:~y --19 tj9 '399-1~~.5_ -~~l~p, ,y_~
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shorter transmission paths all become equal to the delay true oC tltr longest
transmission
path, drtd adding to the length of the electrical transmission path by the use
of active and
passive phase lag circuits, surface acoustic wave (SAW) delay devices, or
digital
intermediate frequency stage circuits for buffering.
These known delay time adjusting techniques for the transmission paths all
include manual processes in which the delay time for each transmission path is
measured using specialized equipment for such measurements. After manually
measuring the delays of the transmission paths, the appropriate delay can be
added to
the appropriate transmission path so that the delay times for all of the
transmission paths
1 U are equal. However, these known equalization techniques produce some
undesirable
problems. For instance, each of these known equalization techniques requires
that
transmission paths be out of service while the delay is measured and
necessitates a
further shutdown of service for updating and adjusting after a need is
detected, such as
any time after repairir~ or upgrading a transmission path.. Furthermore, these
known
15 techniques do not take into account possible changes ui the transtnissiun
media
characteristics due to environmental variations such as temperature.
Therefore, the
network cannot be actively adjusted for these changes.
U.S. Patent 5,222,162 to Yap et al. discloses a system for phased array
antenna
beam steering using time delay networks. In Yap et al., a Plurality of
cascaded optical
?0 time delay stages are selectively switched to set different delay times for
each antenna.
Steering of the phased array antenna beam is achieved by setting different
delay times
for the microwave signals that address each antenna element of the phased
array. The
overall delay of the array is accomplished by switching individual stages so
that the
delay branches for selected stages are included in the overall delay that
determines the
25 antenna bears steering. In one embodiment, Yap et al. discloses use of a
computer to
select the desired delay time.
U.S. Patent 4,417,249 to Zscheile, Jr. discloses an adaptive processor system
in
a phased array antenna system that uses a phase tracking Ioop to eliminate
undesired
received signals white leaving desired signals unaffected. An undesired
reference signal
3U is received at a first antenna and is subsequently processed in a system
path A. An out
of phase replica of the signal is received at a second antenna and processed
through a
AMENOE~ SHEET
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~~ts. ~;o\ E°A vLt.\CHE~ !1 li- 9-98 ; 1U:U6 ; -~9 ti9 -_'399~1tp- -
~~1'?, ,'~n~ 1~ ~5.= ~,-1.,
--~ a-
system path B. As disclosed by Zscheile, Jr., a phase tracking and feedback
path C
causes the signal in Path B to track and follow the phase of the undesired
signal in path
A. I"urthcr, an amplitude trackiug loop D adjusts the magnitude of an
attenuator to
increase or decrease the amplitude of the signal in path A to match the
amplitude of the
replica signal in path B. Thus, using the system of Zscheile, lr., an
undesired
reference signal in patl. A that is out of phase with a replica signal in path
B is
effectively canceled.
U.S_ Patent 4,849,990 to Ikegatni et al. discloses a digital communication
system that includes two branches of transmitters and receivers, wherein one
hranch
ilicludes a delay circuit fur ensuring an optimum delay time between the two
branches.
The optimum delay time is selected such that the best bit error rate in the
system is
obtained. This optimum delay time is set by a delay circuit which can consist
of
cascaded delay circuitry.
The documents discussed above, Yap et al., Zscheile, lr., and lkegami et al.,
do
not disclose any mechanism for determining the delay time for each separate
transmission path connected to each antenna in the array and arc therefore
incapable of
equalising the delay tune in varying transmission paths.
One object of the present invention therefore is to provide a system and
method
fns equalising the delay associated with the tra~osmission media which
connects a
plurality of remote antenna units to a central unit or a base station in a
distributed
antenna network.
Another object of tfte present invention is to provide a system and method for
detecting the delay of each separate transmission path for the remote antenna
units and
then adjusting the delay factors for the remote antenna unit in response to
this detection
so that all of the associated delay time is equalized.
A still further object of the present invention is to provide a system and
method
fur equalising delay time with a loopback mechanism is the distributed antenna
network
which measures the amount of round trip time delay by sending a pulse pattern
or the
like from a transceiver to the remote antenna units on the [CONTINUED ON PAGE
5)
AMENDED SWEET
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WO 98/04052 PCT/ITS97/11371
-5-
downlink path. Thereafter, the time until the pulse pattern is received back
from the
loopback mechanism on the uplink path may be counted and then the delay
parameters
for the network can be updated without any physical intervention by an
operator when
all of the transmission paths have been detected. Furthermore, bursts are
synchronized so that the air frame timing between cells served by the remote
antenna
units is enhanced and the hand-off performance therebetween is improved.
According to one aspect of the present invention, the foregoing and other
objects are attained in a communication system and method for equalizing delay
time
in a distributed antenna network. The system comprises a plurality of remote
antenna
units, a central unit or a base station connected to the remote antenna units
by
transmission media, where each connection between the base station and one of
the
remote antenna units forms a separate transmission path having an associated
delay
time, a delay detector for determining the associated delay times of the
separate
transmission paths, and a delay compensator for adjusting the associated delay
times
in response to the delay detector so that all of the associated delay times
are
substantially equalized. The system and method allow the delay parameters for
the
entire network to be set upon installation and then to be periodically updated
without
physical intervention by an operator. The detection and compensation allow
equalization of delay time differences that could not otherwise be compensated
in the
base stations or mobile stations of conventional systems and methods.
According to another embodiment of the present invention, the foregoing and
other objects may be attained in a communication system and method that
further
comprise sending a predetermined pulse pattern to a loopback mechanism for
each of
the remote antenna units and counting the time taken to transmit the
predetermined
pulse pattern on an uplink path of the loopback mechanism for each of the
remote
antenna units. By measuring the round-trip delay time for each of the remote
antenna
units and then sending that at delay time back to the base station or the
central unit,
the delay compensator is able to adjust the delay parameters and equalize the
delay
times of all of the remote antenna units. As a result, transmission paths
having
different lengths or types of transmission media may be equalized and the
signals will
be properly processed with minimal intersymbol interference.
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In another embodiment of the present invention, the delay detector may be
positioned in the transmission path between the central unit or base station
and the
remote antenna units. In practice, the delay detector should be positioned at
either the
central unit/base station or the remote antenna unit with each position having
associated advantages.
According to an aspect of the present invention there is provided a
communication system for automatically equalizing time delay in a distributed
antenna
network, comprising a plurality of remote antenna units, a central unit
connected to the
remote antenna units by transmission lines, each connection between the
central unit and
one of the remote antenna units forming a separate transmission path having an
associated delay time, a plurality of delay detectors for determining the
associated delay
time of the separate transmission paths for each of the remote antenna units,
a delay compensator for adjusting the associated delay time for each of the
remote
antenna units in response to the delay detectors so that all of the associated
delay times
are substantially equalized, a pulse generator for sending a predetermined
pulse pattern to
a loopback mechanism for each of the remote antenna units, via the
transmission lines
and a timer for counting the time taken to transmit the predetermined pulse
pattern on an
uplink path of the loopback mechanism for each of the remote antenna units.
According to another aspect of the present invention there is provided a
method
for automatically equalizing time delay in a distributed antenna network,
comprising the
steps of (a) connecting a plurality of remote antenna units to a central unit
by
transmission lines, each connection between the central unit and one of the
remote
antenna units forming a separate transmission path having an associated time
delay, (b)
determining the associated delay time of the separate transmission paths for
each of the
remote antenna units, (c) adjusting the associated delay time for each of the
remote
antenna units in response to the step (b) so that all of the associated delay
times are
substantially equalized, (d) sending a predetermined pulse pattern to a
loopback
mechanism for each of the remote antenna units via the transmission lines, and
(e)
counting, through the use of a timer, the time taken to transmit the
predetermined pulse
pattern on an uplink path of the loopback mechanism for each of the remote
antenna
units.
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BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will become more fully understood by reading this
description in conjunction with the accompanying drawings, which are given by
way
of illustration only, and thus are not limitative of the present invention,
wherein:
Figure 1 illustrates a conventional distributed antenna network;
Figure 2 is a block diagram of a known CATV infrastructure which supports
FCS;
Figure 3 illustrates various transmission paths in a conventional distributed
antenna network;
Figure 4 is a block diagram showing a system for equalizing delay in a
distributed antenna network according to an embodiment of the present
invention;
Figure 5 is a block diagram of a loopback mechanism for one of the remote
antenna units according to an embodiment of the present invention;
Figure 6(a) and (b) are timing diagrams illustrating a delay measurement by
the loopback mechanism;
Figure 7 is a block diagram showing a system infrastructure in which a delay
equalization system according to the present invention is implemented; and
Figures 8(a), (b) and (c) illustrate delay detectors that may be used in
embodiments of the present invention.
DETAILED DESCRIPTION
This invention is directed to a system and method which actively equalize the
delay times associated with transmission paths between a central unit or base
station
and remote antenna units in a distributed antenna network. In distributed
antenna
networks where digital modulation is commonly used, communication signals are
WO 98/04052 PCT/US97/11371
often received by more than one of the remote antenna units and intersymbol
interference is likely to occur without the proper equalizing compensation. As
CATV
infrastructures become more widely used for cellular-like communication
networks,
the transmission paths for the communication signals are more likely to widely
vary
S with respect to the lengths of the transmission paths and the types of media
used for
the transmission paths. When the lengths and types of the transmission media
become
significantly different for each connection to the remote antenna unit, the
differences
between the delay times for each of these transmission paths may increase to
the point
where intersymbol interference is created that is beyond the compensation
capabilities
of the equalizers at the base or mobile stations. Therefore, the system and
method of
the present invention equalize the delay times in response to each delay time
detection
for the transmission paths to all of the remote antennas in a distributed
antenna
network. These delay time detections may be performed upon installation and
periodically thereafter at scheduled intervals or as problems in the system
operation
arise. The equalization system and method of the present invention also
provide
greater ease and flexibility in adjusting the delay parameters so that they
may be
updated without physical intervention by an operator.
Referring to Figure 4, the main components for actively equalizing the delay
times between remote antenna units and the base station in a distributed
antenna
network are illustrated according to one embodiment of the present invention.
A
plurality of remote antenna units 100, ,1002, . . .100n are shown connected to
transmission paths 102, ,1022, . . .102. The transmission paths 102, , . .
.102 are
connected to a central unit 120 which may be incorporated into a base station
or
connected to the base station. The central unit 120 may include a plurality of
segmented units, each corresponding to a respective one of the remote antenna
units
100, , . . .100", or the central unit 120 may be a single unit that receives
the inputs from
each of the remote antenna units 100,,...100, depending upon the
implementation
constraints. The central unit 120 includes a processing unit (computer,
microprocessor, CPU, PC or the like) for receiving delay time information for
the
remote antenna units lOb,,...100n, processing this information, and generating
control
signals for equalizing the delay time for the remote antenna units. For
instance, a
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_g_
microprocessor connected to a modulator/demodulator may be used to generate
pulse
signals which are sent to the remote antenna units. The microprocessor may
then
measure the time delay based on the returned pulse signals.
To actively equalize the delay times in the distributed antenna network, delay
detectors 1101,1102. . .110 and delay compensators 1151,1152. . .115 are
respectively
positioned along each of the transmission paths 1021, 1022, ...102". The
positioning
of the delay detectors 110" and the delay compensators 115 are
interchangeable. The
time delay error may be first determined by the delay detectors 115" and then
adjusted
by the delay compensators 115, or alternatively, the time delay may be
adjusted by
the delay compensators 115n until no time delay error is determined to be
present by
the delay detectors 110". In practice, the physical placement of the
components for
performing the delay detection and compensation functions should be
incorporated in
the central unit 120 or the remote antenna units 1001...100 so that the
installation
complexities are reduced. When the delay detectors 1101,...110 and the delay
compensators 1151,...115 are incorporated into their respective antenna units
1001,...100n, the delay control for the network is maximized. However, this
configuration requires each of the remote antenna units 115 to be individually
calibrated and additional hardware must be included in each unit which
increases
costs.
If the delay detectors 1101, . . .110 and the delay compensators 1151, . .
.115 are
incorporated into the central unit 120, the complexity and costs associated
with
installing hardware in each of the remote antenna units are eliminated, while
adequate
delay control is still provided. It will be appreciated that the delay
detector can be
located at the remote antenna unit and the delay compensator can be located at
the
central unit, or the delay compensator can be located at the remote antenna
unit and
the delay detector can be located at the central unit.
Figure 5 illustrates a block diagram for a remote antenna unit 100n according
to one embodiment of the present invention. An antenna 130 is connected to a
duplexer 132n which is connected to radio circuitry 134" and 136" that forms a
normal
path A. A loopback mechanism 145 selectively forms a loop path B for measuring
the round-trip delay time.
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The loop path B includes radio circuitry 138 and 140 which are connected to
the transmission paths 102" by a medium interface 142". The radio circuitry
134",
136, 138", and 140" includes standard radio components such as channel
filtering,
amplifying and frequency converting circuitry. The loopback mechanism 145
includes switches 152 and 153" that are jointly controlled by a central unit
for all of
the remote antenna units of a network. The switches 152 and 153 are closed
when
the loop path B is desired to be used and opened when the loop path A is
desired to
be used. In one example of the present invention, a common controller for the
radio
in the remote antenna unit may be modified to provide the logic for
controlling the
switches 152n and 153. However, it will be appreciated that the switches 152"
and
153 may be controlled by other techniques. For instance, a decoder may be used
to
detect bit patterns and then to generate signals for opening or closing the
switches
152 and 153 responsive to the detected bit patterns.
To measure the round-trip delay time, a pulse or other identifiable pattern
may
be sent from the central unit 120 and the delay detector 110 on the downlink
path
through the loop path B. The time elapsed between sending the pulse and
receiving
the pulse back on the uplink path is determined by the delay detector 110. In
addition to direct timing, other methods are possible to measure the round-
trip delay
time. For example, a continuous pattern of pulses separated by a delay may be
sent
from the central unit 120 to each of the remote antenna units. Thereafter, a
synchronizer algorithm may be applied to the returning stream and the delays
adjusted
by the delay compensators until synchronization for all of the remote antenna
units is
achieved. As a result, the system has the capability to set the delay
parameters for
the entire network at installation and to update the delay parameters by a
system
controller without physical intervention by an operator.
Figures 6(a) and (b) are timing diagrams representing a pulse used to
determine the delay time. Figure 6(a) illustrates the initial pulse sent on
the downlink
path and Figure 6(b) illustrates the delay time, typically on the order of x
microseconds, of the round-trip delay time for the loop path B. By actively
adjusting
the delay parameters responsive to this detection, variations in the length
and types of
media used in the transmission paths may be readily compensated. This is
important
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when the differences in the delay times are significant and cause intersymbol
interference that cannot be compensated by base or mobile stations of
conventional
systems. For instance, if a CATV propagation delay between adjacent remote
antenna
units is more than 16 microseconds, intersymbol interference occurs that
cannot be
compensated by the equalization capabilities of conventional systems.
Figure 7 illustrates an example of a CATV system infrastructure that may
advantageously utilize Applicants' system and method for equalizing delay. The
infrastructure includes fiber nodes 200,,...200 which are connected to a CATV
head-
end 300 by respective optical fibers 250,,...250. The CATV head-end 300 may
support a number of fiber nodes 200,,...200n, each of which may generally
support
approximately 500 to 1500 homes or subscribers through a plurality of remote
antenna
units 202, , . . .202n. The remote antenna units 202, , . . . 202n are
respectively connected
by the optical fibers 250,, . . .250 to the CATV head-end 300. Also, a
plurality of the
remote antenna units 202,,...202n are connected to an amplifying and
optical/electrical
converting circuit 206 by coaxial cabling 204".
The CATV head-end 300 includes an amplifying and electrical/optical
converting circuit 302 which is used for interfacing the CATV head-end 300 to
the
optical fibers 250,,...250. The amplifying and electrical/optical converting
circuit
302 is connected to combining and splitting circuitry 304 which is connected
to a
video source 306 and cable access processing (CAP) units 308,,...308n which at
least
correspond in number to the fiber nodes 200,,...200n in the system. The CAP
units
308, , . . . 308" are connected to radio base stations (RBS) 314, , . . . 314"
and a hub 310.
The hub 310 is further connected to a remote antenna management system (RAMS)
3I2. The CAP units 308,,...308 are also connected to an operation support
system/switching center (OSS/SC) 350. The OSS/SC 350 includes a mobile
switching
center 352, a base station controller 354, an operation support system 356 and
its own
RAMS 358. The RAMS 358 of the OSS/SC 350 may communicate with the RAMS
312 of the CATV head-end 300 so that a plurality of CATV head-ends may be
controlled at a central location. The base station controller is also
connected to a
macrocell 360 which includes a (RBS) 362. The CAP units 308,,...308" provide
the
frequency conversion and power level adjustments for placing telephony carrier
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signals on the CATV infrastructure as well as controlling and monitoring the
status of
the remote antenna units 202,,...202.
The remote antenna units 2021,...202" are located in desired areas and convert
the CATV base communication signals back to their proper air frequencies and
power
levels. Each of the remote antenna units 2021,...202" is associated with a
specific
transceiver radio unit (TRU) in the RBS 3141,...314 so that the CATV network
is
totally transparent to the radio operation. The operating frequencies of the
CAP units
3081,...308 are set to exactly match the operating frequencies of the remote
antenna
transmitters. Up to six TRUs in a specific one of the RBS 3141,...314 feed a
single
one of the CAP units 308,,...308n which in turn serves several fiber nodes
2001,...200" in the CATV network. A combiner within the CAP units 3081,...308n
provide six transmit input ports from the RBS 314, , . . .314n in order to
support a
maximum of six TRUs. The CAP units 3081,...308 convert the telephony carrier
signals to CATV frequency signals and feed them to the CATV head-end 300 over
a
bi-directional coaxial cable.
In the CATV head-end 300, the telephony carrier signals are combined with
the video signals and both signals are transmitted over the fiber optic cables
250, , . . .250n to fiber nodes 200, , . . .200n. At the fiber nodes 200, , .
. .200, the signals
are converted back to electrical signals and distributed over a tree and
branch coaxial
network. Each individual one of the remote antenna units 201, , . . .202" taps
off the
coaxial cabling 204, filters the carrier signals, converts the frequency and
transmits
the carrier signals over the air interface. The video signals continue
unaltered to each
CATV subscriber. For the uplink, the remote antenna units 202,,...202n receive
two
diversity signals from the air, downconvert the frequency, and transmit the
different
frequency diversity carrier signals over the CATV infrastructure. In the CATV
infrastructure at the fiber nodes 2001,...200, the electrical signals are
converted to
optical signals and transmitted back to the CATV head-end 300. These signals
are
then converted back to electrical signals and routed to the CAP units 308, , .
. . 308 over
the bi-directional cable. The CAP units 308,,...308 convert the uplink carrier
signals
back up to frequencies for input to the TRUs. The CAP units 308,,...308
receive
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digital control information from the RAMS 312 and forward remote antenna
control
information to the desired one of the remote antenna units 202,,...202.
The fiber node delay compensation can be accomplished by inserting variable
delay elements in the signal paths between the CAP units 308,,...308 and the
remote
antenna units 202,,...202". The variable delay element circuitry may be
practically
placed in either the remote antenna units 202,,...202 or the CAP units
3081,...308".
As previously discussed, if the variable delay element circuitry is installed
at the
remote antenna units 2021, ... 202, the flexibility for delay control at each
remote
antenna unit is maximized but individual hardware and calibration is necessary
at each
of the remote antenna units which will lead to increased costs and complexity
during
installation and upgrade. Therefore, positioning the variable delay element
circuitry
at the CAP units 3081,...308 is desirable to reduce costs because the
supporting
circuitry is already present. Even though the time delay for each remote
antenna unit
is not individually controlled when the delay element circuitry is positioned
at the
CAP units 3081,...308, each remote antenna unit within a fiber node 200" will
have
the same amount of delay control. This provides adequate control because the
fiber
optic cables 250 between the CATV head-end 300 and the fiber nodes 200n are
usually the largest contributing component to the amount of delay time. In
other
words, the lengths of the fiber optic cables 250" are significantly greater
than the
lengths of the coaxial cabling 204" within the fiber nodes 200. Therefore,
compensating for the delay associated with the fiber optic cables 250n will
provide
adequate delay control in most instances.
Figures 8(a), (b) and (c) illustrate possible variable delay elements that may
be
used for the delay compensator in embodiments of the present invention. A
constant
amplitude phase shifter circuit is illustrated in Figure 8(a). This circuit
includes an op
amp 402, resistors R1, R2 and R3 404, 406 and 408 and a capacitor C410. The
phase lag introduced by this circuit is determined by the resistors Rl, R2 and
R3 404,
406, and 408 and the capacitor C410. The phase lag is about 170 degrees at an
operating frequency of IOc~RC. At the IF operating frequency, a phase shift of
170
degrees corresponds to about 1 microsecond of delay.
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In Figure 8(b), a chain of constant phase shifter circuits is illustrated. A
plurality of quad phase shifters 420,,...420" are connected to a multiplexer
422. Each
of the quad phase shifters 420,,...420 may include four quad op amps per
package.
The multiplexer 422 is controlled by digital logic for selecting one of
several delay
taps to provide delay adjustment. By using four quad op amps, up to 16
microseconds of delay can be provided in 4 microsecond steps. Alternatively,
an
analog ASIC using standard cells instead of discrete op amps may be used and
several
complete delay chains can be implemented in a single IC.
In still another aspect, units which utilize an ADC and DAC to create a
digital
IF section may obtain a delay circuit by inserting high speed FIFOs in the
digital IF
section of a device with the FIFOs being selected to operate at the desired
sampling
rate. The delay of the FIFOs are adjusted by controlling the depth of the
FIFOs.
The FIFOs may be selected with programmable depth registers that activate a
signal
when the FIFOs are at a certain depth. Digital logic controls the read and
write
clocks so that the programmable depth is maintained. A FIFO with a depth of 1K
provides up to a 51 microseconds of delay time at the desired sampling rate.
Alternatively, a digital implementation using a high speed dual port memory
device
instead of FIFOs may be used to store samples. Figure 8(c) illustrates a dual
port
memory 430 connected to control logic 432. Read and write addresses are input
to
the dual port memory 430 and an address offset is input to the control logic.
Thereby, read and write address signals maintain an adjustable offset from
each other.
By changing the offset, delay throughout the circuit may be varied.
In distributed antenna networks, the use of multicasting provides more
efficient
use of radio resources to cover a small areas by remote antenna units.
However,
when digital modulation is used in such networks and a signal is received by
more
than one of the remote antenna units, intersymbol interference may occur if
the
transmission paths for the remote antenna units are significantly different.
The
present invention is directed to compensating for the delays in the
transmission paths
due to differences in their path lengths or types of transmission media so
that the
delay times for all of the transmission paths are substantially equalized. The
present
invention also enhances air frame timing between cells served by the remote
antenna
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-14-
units which improves hand-off capabilities therebetween: Another advantage of
the
system and method of the present invention is to perform this equalization
actively
without using any specialized equipment and without requiring the transmission
link to
be out of service during the upgrades or repairs. The equalization of the
distributed
antenna network also provides compensation due to environmental variations,
such as
temperature, to be performed which cannot be readily adjusted in the known
systems.
The invention being thus described, it will be obvious that the same may be
varied in many ways. Such variations are not to be regarded as a departure
from the
scope of the invention, which is determined by the following claims. All such
modifications that would be obvious to one skilled in the art are intended to
be
included with the scope of the following claims.
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