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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The disclosed invention generally relates to the field of simulcast
transmission systems, and more particularly a method and apparatus for
automatically synchronizing the transmissions in a wide area simulcast
transmission system.
2. Description of the Prior Art
A number of methods have been proposed or are in use today for
automatically synchronizing the message transmissions of transmitters
utilized in simulcast transmission systems. One such system is described
in U.S. Pat. No. 4,718,109 to Breeden et al., entitled "Automatic
Synchronization System" which is assigned to the Assignee of the present
invention. A simulcast transmitter system is described wherein a master
transmitter was centrally located within a plurality of secondary
transmitters disposed in an annular fashion around the central
transmitter. The innermost annular ring of transmitters were synchronized
to the master transmitter, while the remainder of the system transmitters
were disabled. The next adjacent annular band of transmitters were then
synchronized to the innermost annular band and the process was repeated
until every annular band in the system was synchronized. Such a
synchronizing arrangement guaranteed adjacent annular bands were properly
synchronized, however such a system did not necessarily provide for
variations in delay which were introduced do to not utilizing a common
signal source for making the delay measurements.
An alternate method of synchronizing the transmitters in a simulcast
transmission system having a large number of transmitters is shown in FIG.
1. An important factor in determining the regularity to which the
transmissions in such a simulcast transmission system was synchronized was
the time required to complete the transmitter propagation delay
measurement sequence. For a large simulcast transmission system, such as
one having forty transmitters, delay measurement times of forty seconds
and more were typical when each region was sequentially accessed for
measuring the individual transmitter propagation delays. FIG. 1 shows a
typical large multi-transmitter simulcast system 100 which has been
divided into a plurality of smaller transmission regions 102, each
transmission region 102 having a plurality of regional transmitters 104
responsive to a regional controller 106 for controlling the transmission
of messages and further for controlling the transmission of information
utilized for synchronization of the transmitter transmissions. Each
transmission region 102 included one or more regional receivers 108 (only
one of which is shown), which was coupled to the corresponding regional
controller to provide monitoring of delay measurement signals required to
enable the measurement of the inter-regional propagation delays for each
of the regional transmitters in each transmission region 102. By splitting
the simulcast transmitter system 100 into the smaller transmission regions
102, the inter-regional propagation delay measurements could be
simultaneously measured for regional transmitters in alternate
transmission regions, such as shown for regional transmitters 104 and 104"
within transmission regions 102 and 102", respectively, thereby reducing
the total time required to synchronize transmissions within the system.
Measurement of the transmitter propagation delays as shown in FIG. 1,
while speeding up the inter-regional propagation delay measurement
process, presented a new set of problems, such as that of measuring the
intra-regional propagation delays required to synchronize the transmitters
in adjacent transmission regions.
In order to measure these intra-regional propagation delays, an output 110
of one of the regional controllers 106 was redirected to a regional
transmitter 104 in an adjacent transmission region, as shown in FIG. 1, in
order to establish a signal source for the intra-regional measurements.
Once the intra-regional transmission propagation delays were measured, the
intra-regional differential propagation delays were computed and then
added to the inter-regional differential propagation delays for each
transmission region to determine the total transmission delay required for
each transmitter to synchronize the transmissions of the transmitters
within each transmission region and between transmission regions.
A number of problems arose from the method of FIG. 1 for synchronizing the
transmissions of such a simulcast transmitter transmission system. The
intra-regional transmission propagation delays required a means for
switching between two transmission sources for the same transmitter, i.e.
controller 106 and controller 106'. This switching of sources added errors
to the measurements consisting of delays introduced by the added signal
path utilized to make the intra-regional measurements, which could easily
approach hundreds of microseconds of added delay. When multiply adjacent
transmissions regions occurred, i.e. where more than two transmission
regions overlaped, additional switching hardware was required to
interconnect each of the regions for measurement, further contributing to
errors in the propagation delay measurements, and adding substantially to
the cost of the system. The method of FIG. 1 also restricted cross check
measurements between the adjacent regions without the utilization of
additional hardware to provide such cross check measurements. The method
of FIG. 1, also became inoperative in those instances when the transmitter
used to compare intra-regional measurements became inoperative. To resolve
this problem required additional hardware in the form of redundant
switching to other transmitters within the transmission regions to be
available when the primary transmitter failed. The method of FIG. 1 also
precluded restructuring of the transmitters in the system as the system
operator deemed appropriate, such as when a better combination of
transmitters was determined to provide for more accurate propagation delay
measurements within the simulcast transmitter system.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method for
synchronizing the transmissions of a simulcast transmitter transmission
system.
It is a further object of the present invention to provide a method for
synchronizing the transmissions of the simulcast transmitter transmission
system which provides improved measurement capability.
It is a further object of the present invention to provide a method for
synchronizing the transmissions of the simulcast transmitter transmission
system which provides measurement reconfigurability.
It is a further object of the present invention to provide a method for
synchronizing the transmissions of the simulcast transmitter transmission
system which provides simple cross-check measurement capability.
These and other objects of the present invention are achieved by providing
a method for synchronizing message transmissions in a simulcast
transmitter system. The system comprises at least two transmission
regions, each transmission region having at least one regional receiver
coupled to a regional controller for receiving delay measurement signals
generated for the measurement of transmission propagation delays. Each
transmission region includes a plurality of regional transmitters having
adjustable transmission delays which are responsive to the regional
controllers for transmitting the messages and the delay measurement
signals. A master controller couples to each regional controller for
enabling the message transmissions and for initiating the transmission of
delay measurement signals for the measurement of propagation delays. The
master controller generates a first delay measurement signal for the first
region, and effects the transmission of the first delay measurement signal
from a selected one of the plurality of regional transmitters operating
within the first transmission region. The regional controller within the
first region measures the propagation delay from the selected transmitter
within the first transmission region. The master controller next generates
a second delay measurement signal and effects the transmission of the
second delay measurement signal from a selected one of the plurality of
regional transmitters operating within a second transmission region
adjacent to the first transmission region. The regional controller within
the first region measures the propagation delay from the selected
transmitter within the second transmission region. The intra-regional
differential propagation delay between the transmission of the first and
second delay measurement signals is computed. The transmission delays for
each regional transmitter operating within the first and second
transmission regions are computed based on the computed intra-regional
differential propagation delay, and the transmission delays for each
transmitter within the first and second transmission regions are adjusted
to equalize the intra-regional differential transmission delays between
each transmission region.
BRIEF DESCRIPTION OF THE DRAWINGS
The features of the invention which are believed to be novel are set forth
with particularity in the appended claims. The invention itself, together
with its further objects and advantages thereof, may be best understood by
reference to the following description when taken in conjunction with the
accompanying drawings, in the several figures of which like reference
numerals identify identical elements, in which, and wherein:
FIG. 1 is an electrical block diagram showing the implementation of the
inter-regional and intra-regional delay measurements in a prior art
simulcast transmission system.
FIG. 2 is an electrical block diagram showing the implementation of the
inter-regional and intra-regional delay measurements in a prior art
simulcast transmission system of the present invention.
FIGS. 3A and 3B are signal flow diagrams illustrating the inter-regional
delay measurement procedure and the intra-regional delay measurement
procedure, respectively, for the simulcast transmission system of the
present invention.
FIG. 3C is a diagram illustrating one embodiment of an intra-regional
differential propagation delay measurement sequence utilized in the
simulcast transmission system of the present invention.
FIG. 4 is an electrical block diagram showing a first embodiment for the
implementation of the regional controllers utilized in the simulcast
transmission system of the present invention.
FIG. 5A is a signal waveform showing the delay measurement utilized in the
simulcast transmission system of the present invention.
FIG. 5B is an electrical block diagram showing the stop detector utilized
in the first embodiment of the simulcast transmission system of the
present invention.
FIG. 6 is an electrical block diagram showing the transmitter utilized in
the first embodiment of the simulcast transmission system of the present
invention.
FIGS. 7A-7C are flow charts showing the procedures for equalizing the
propagation delays of the simulcast transmission system of the present
invention.
FIG. 8 is an electrical block diagram showing a second embodiment for the
implementation of the regional controllers utilized in the simulcast
transmission system of the present invention.
FIG. 9 is an electrical block diagram showing a second embodiment for the
implementation of the regional transmitters utilized in the simulcast
transmission system of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
FIGS. 2-9 show the preferred embodiment of the present invention, a
simulcast transmission system providing improved propagation delay
measurement capability which is required for the synchronization of
message transmissions from a plurality of transmitters operating in a
plurality of transmission regions. As shown in FIG. 2, the simulcast
transmitter system of the present invention comprises at least two
transmission regions 102. FIG. 2 in particular shows three regions,
although it will be appreciated from the description to follow, any number
of regions may be accommodated by the system. Each transmission region
includes at least one regional receiver 108 for receiving transmitted
delay measurement signals. The regional receivers, such as conventional FM
(frequency modulated) receivers, are well known in the art. Each regional
receiver 108 is coupled to a regional controller 106 through any of a
number of well known communication links 109, such as wireline links, RF
links employing link transmitters and receivers, and microwave links. It
will be appreciated, the number of regional receivers required within each
transmission region is a function of such parameters as the size and
topography of the transmission region. Transmission regions covering large
cities, as for example the New York Metropolitan area, would generally
require multiple regional receivers due to the enhanced propagation delays
encountered between the transmitters and receivers within such large
cities. When multiple receivers are required in the simulcast transmission
system, it will be appreciated a signal strength voting means (not
illustrated), which is well known in the art, is utilized to select the
regional receiver which provided the greatest signal output for
propagation delay measurements from each transmitter within the
transmission region. It will also be appreciated, cross delay
measurements, i.e. the comparison of the propagation delay measurement
between each of the regional receivers, is required to account for
differences in the signal paths between each of the plurality of receivers
and the regional controller.
Each transmission region includes a plurality of regional transmitters 104
which are responsive to the regional controllers 106 for transmitting the
messages, and for transmitting the delay measurement signals, as shown in
FIG. 5, and which will be described in detail below. Returning to FIG. 2,
each regional controller 106 may couple to a splitter 112, or
point-to-multipoint transmission device when more than one transmitter is
provided within a region. Splitters, providing such point to multipoint
transmission are well known in the art. The regional controller is to be
described in detail with FIG. 3 below.
The simulcast transmission system of the present invention also includes a
master controller 116, unlike that of the prior art systems, which couples
to each regional controller, for enabling the distribution of the message
transmissions, and for initiating the transmissions of the delay
measurement signals which are utilized for the measurement of the
inter-regional and intra-regional propagation delays, as will be explained
below. A paging terminal 118 couples to the master controller 116 to
provide the messages which are inputted into the system by the message
originators over the public switched telephone network (PSTN), which is
not shown. The operation of paging terminals such as shown in FIG. 2 are
well known in the art.
The method of measuring the propagation delays within the simulcast
transmitter system of the present invention is further best understood by
way of the signal flow diagrams of FIGS. 3A and 3B. The values of ultimate
interest are the differences in the transmission, or propagation delays
between transmitters. The differential propagation delays are calculated
from the measured propagation delays in two stages, the inter-regional
differential propagation delay measurements and calculations which are
illustrated with FIG. 3A, and the intra-regional differential propagation
delay measurements and calculations, which are illustrated with FIG. 3B.
The inter-regional and intra-regional propagation delay measurements are
initiated at predetermined times, such as once each day, although it will
be appreciated other measurement intervals, such as twice each day or
every other day, can be utilized as well depending upon the stability of
the transmission delays of the overall simulcast transmission system. The
following nomenclature is used to identify the system elements to allow
computation of the differential propagation delays from the corresponding
propagation delay measurements:
Cn, Ck--regional controller in nth and kth region
Xmn, Xmk--transmitter m in nth and kth region
Rjn, Rjk--Receiver j in nth and kth region
M--Master Controller
The propagation delays within the system are identified using the following
notation:
T(source)(destination)
where T is the propagation delay time for a signal, in this instance the
delay measurement signal, to propagate from the signal source to the
signal destination. As an example, the notation TCnX1n identifies the
transmission time, or propagation delay, encountered between the regional
controller in transmission region n and transmitter 1 in transmission
region n.
FIG. 3A is a signal flow diagram for the inter-regional propagation delay
measurements. Each measurement is initiated by the master controller which
generates a measurement control signal which is suitably encoded to select
the regional controller for the region in which the measurements are to be
made, and which identifies the measurement initiated as an inter-regional
propagation delay measurement. The regional controller so selected then
sequences through each of the transmitters in a predetermined order to
make the individual propagation delay measurements for each transmitter
within the transmission region.
The inter-regional transmission delay for each of the regional transmitters
is determined by measuring the loop back delay. The loop back delay is
defined as
TmCnCn=TCnXmn+TXmnRjn+TRjnCn
where TmCnCn is the loop back delay which is being measured for transmitter
m within transmission region n. The loop back delay is measured for each
transmitter m within each transmission region n of the system. The loop
back delay represents the time required for the regional controller Cn to
originate the delay measurement signal and then to receive the delay
measurement signal after being transmitted by transmitter m. TCnXmn
(TCnX1n and TCnX2n in FIG. 3A) is the inter-regional transmitter delay,
the delay encountered in the transmission of the delay measurement signal
from the regional controller Cn to the regional transmitter Xm in region
n. TXmnRjn (TX1nR1n and TX2nR1n in FIG. 3A) is the RF delay encountered in
the transmission of the delay measurement signals between the selected
transmitter Xm and regional receiver Rj in region n. This parameter is
calculated in a manner well known in the art, and is based on the
propagation time required for the delay measurement signal to travel the
measured distance between the selected regional transmitter Xm and the
regional receiver Rj. TRjnCn (TRICn in FIG. 3A) is the receiver delay, or
delay encountered in the transmission of the delay measurement signal
between the regional receiver Rj and the regional controller Cn in region
n.
Once the loop back delay has been measured, the transmitter delay can be
computed as follows:
TCnXmn=TmCnCn-TXmnRjn-TRjnCn
It will be appreciated from the equation presented above, only two of the
three quantities on the right hand side of the equation are known at this
time, TmCnCn which is the loop back delay measured, and TXmnRjn which is
the computed RF delay. TRjnCn remains as of yet unknown, and as a result,
the actual value for the transmission delay is unknown and cannot be
computed. As will be shown below, an actual value for TRjCn need not be
known to determine the transmission delays which are required to
synchronize the regional transmitter transmissions within the simulcast
transmission system of the present invention.
After the loop back delays for each transmitter have been measured, the
inter-regional differential propagation delays are computed by subtracting
the computed transmission delays for the `mth` transmitter within each
transmission region n from the `ref`, or reference transmitter within each
transmission region n.
##EQU1##
As can be observed in the above equation, all of the values on the right
side of the equation are now known since the unknown quantity TRjCn drops
out of the equation when a common receiver is used for the loop back delay
measurements. It will be appreciated, any transmitter within each
transmission region may be designated as the reference transmitter for the
purposes of the differential propagation delay calculations. Depending
upon the magnitude of the loop back delay and the RF delay for the
reference transmitter Xref within each transmission region n compared to
that of the other transmitters within the transmission region n, it will
also be appreciated, additional delay may have to be added to, or
subtracted from, each of the transmission paths of the transmitters within
the particular region in order to synchronize the transmission delays of
all transmitters within the particular transmission region.
The transmission delay which is inserted into, or removed from, each
transmitter transmission path is calculated as follows:
Transmission Delay(Xmn)=.DELTA.Trefn-.DELTA.Tmn
where the transmission delay for transmitter m in region n, Xmn, is
computed by subtracting the differential propagation delay of the mth
transmitter (.DELTA.Tmn) from the differential propagation delay
(.DELTA.Trefn) for the reference transmitter within region n. An alternate
method of determining the additional transmission delay would be to
determine the maximum differential propagation delay .DELTA.TMAXn of all
transmitters in region n and to then subtract the differential propagation
delay of the mth transmitter (.DELTA.Tmn) in order to determine the
additional transmission delay required.
Transmission Delay(Xmn)=.DELTA.TMAXn-.DELTA.Tmn
FIG. 3B is a signal flow diagram for the intra-regional propagation delay
measurements. The intra-regional propagation delay measurements are
initiated by the master controller which generates a measurement control
signal which is suitably encoded to select one of the regional controllers
in the transmission region in which the measurement is to be made, and
also identifies the measurement as an intra-regional propagation delay
measurement. Unlike the inter-regional propagation delay measurements, the
master controller generates both the measurement control signal and the
delay measurement signal for the intra-regional propagation delay
measurements.
In order to determine the intra-regional propagation delays, the loop back
delay for a selected transmitter Xmn within a selected transmission region
n with the master controller M generating the delay measurement signal is
measured. The loop back delay is defined as
TMCnCn=TMXmn+TXmnRjn+TRjnCn
where TMCnCn is the loop back delay which is being measured for the
selected transmitter within transmission region n. The loop back delay
represents the time required for the master controller M to originate the
delay measurement signal and for the regional controller to receive the
delay measurement signal after being transmitted by the selected
transmitter. TMXmn (TMX1n in FIG. 3B) is the intra-regional transmitter
delay, the delay encountered in the transmission of the delay measurement
signal from the master controller M to the regional transmitter Xm in
region n. TXmnRjn (TX1nR1n in FIG. 3B) is the RF delay encountered in the
transmission of the delay measurement signals between the selected
transmitter Xm and regional receiver Rj in region n, as described above.
TRjnCn (TR1nCn in FIG. 3B) is the receiver delay, or delay encountered in
the transmission of the delay measurement signal between the regional
receiver Rj and the regional controller Cn in region n.
The loop back delay for a selected transmitter Xm within an adjacent
transmission region k with the master controller M generating the delay
measurement signal is next measured. The loop back delay for this
measurement is defined as
TMCkCn=TMXmk+TXmkRjn+TRjnCn
where TMCkCn is the loop back delay which is being measured for the
selected transmitter Xm within transmission region k. The loop back delay
represents the time required for the master controller M to originate the
delay measurement signal which is routed through regional controller Ck,
and for the regional controller Cn to receive the delay measurement signal
after being transmitted by the selected transmitter Xm in region k. TMXmk
(TMX1k in FIG. 3B) is the intra-regional transmitter delay, the delay
encountered in the transmission of the delay measurement signal from the
master controller M to the regional transmitter Xm in region k. TXmkRjn
(TX1kR1n in FIG. 3B) is the RF delay encountered in the transmission of
the delay measurement signals between the selected transmitter Xm in
region k and regional receiver Rj in region n, as described above. TRjnCn
(TR1nCn in FIG. 3B) is the receiver delay, or delay encountered in the
transmission of the delay measurement signal between the regional receiver
Rj and the regional controller Cn in region n.
Loop back delay measurements as described above are required for selected
transmitter pairs within each adjacent pair of the plurality of
transmission regions as shown in FIG. 3C for a large transmission system
having a large number of transmission regions. It will be appreciated more
or less transmission regions may be required in a particular simulcast
transmission system, than that shown as an example in FIG. 3C.
Returning to FIG. 3B, once the loop back delays has been measured for each
adjacent pair of transmission regions n and k, the transmitter delays for
region n and k are computed as follows:
TMXmn=TMCnCn-TXmnRjn-TRjnCn and
TMXmk=TMCkCn-TXmkRjn-TRjnCn
It will again be appreciated, as in the inter-regional differential
propagation delay calculations, only two of the three quantities on the
right hand side of the equations above are known at this time, TMCnCn and
TMCkCn which are the loop back delays measured, and TXmnRjn and TXmkRjn
which are the computed RF delays.
After the intra-regional loop back delay measurements are made for each
adjacent pair of transmission regions, the intra-regional differential
propagation delays are computed by subtracting the computed transmission
delays for the `mth` transmitter within each transmission region k from
the `mth` transmitter within each adjacent transmission region n.
##EQU2##
As can be observed in the above equation, all of the values on the right
side of the equation are now known from the intra-regional propagation
delay measurements made for each adjacent pair of transmission regions.
The intra-regional differential propagation delay calculations resulting
from the intra-regional propagation delay measurements are shown in FIG.
3C as .DELTA.2-1, .DELTA.3-2, and so forth. While a sequential progression
of intra-regional propagation delay measurements are indicated to obtain
the intra-regional differential propagation delay results shown in FIG.
3C, it will be appreciated other combinations of transmission region
pairs, such as region 10 with region 2, region 9 with region 2, and so
forth, can be selected for the measurement and computation of
intra-regional differential propagation delays required to synchronize the
message transmissions in the system.
Since the inter-regional differential propagation delay values for each
transmission region are independently derived for each transmission
region, the inter-regional differential propagation delay values can be
simply compared to determine the maximum inter-regional differential
propagation delay for all transmission regions within the simulcast
transmission system as described above. The computations of the additional
transmission delays for each transmitter in each transmission region is
therefore straight forward. However the intra-regional differential
propagation delay calculations rely on measurements made using at least
n-1 transmission region pairs. As a result the determination of the
additional transmission delays required to synchronize the intra-regional
transmissions is considerably more complicated. One approach determines
the additional transmission delays for groups of transmission regions. One
example of this approach is to synchronize the transmissions between
regions 1, 2 and 3 of FIG. 3C using the intra-regional differential
propagation delay values .DELTA.2-1 and .DELTA.3-2. Likewise, regions 3, 4
and 5 could be synchronized using the intra-regional differential
propagation delay values .DELTA.4-3 and .DELTA.5-4. Regions 1, 2 and 3
would then be synchronized with regions 3, 4 and 5, since each group of
regions shares the measurements made in common in region 3. Regions 6-10
would be synchronized in a similar manner as for regions 1-5. One or both
of the intra-regional differential propagation delay values .DELTA.6-5 and
.DELTA.1-10 would then be used to synchronize the transmissions between
the larger transmission region groups. It will be appreciated, other
methods may be utilized to synchronize the intra-regional transmissions,
such as sequentially equalizing each transmission region with the
previously synchronized transmission regions. In this method region 2 is
synchronized to region 1, and then region 3 is synchronized to regions 1
and 2, and so forth until all transmission regions are synchronized.
Because of the simplicity of the measurements and the basic calculations
for determining the intra-regional transmission delays, the same set of
measurements can be made with any of the transmitters within each
transmission region. Consequently, several measurements can be made using
several transmitters to check the accuracy of the measurements and provide
cross checking of the measurements. In addition, since no additional
switching hardware is required as in the prior art systems, any
transmitter within each transmission region can be used as a back-up
provided the transmission can be received by a receiver in the adjacent
region pair. This is extremely advantageous should the transmitter
selected as the reference become inoperative.
FIG. 4 shows an electrical block diagram of the regional controllers 502
utilized in a first embodiment of the present invention. A second
embodiment of the present invention is shown and will be described in FIG.
8. In the embodiment of the present invention shown in FIG. 4, the master
controller 500 and regional controllers 502 are co-located in a common
area, such as being mounted in a common card rack in a central office
building. The master controller 500 shares common "backplane"
interconnections 504 with each of the regional controllers 502 for
communication of control signals 508, such as the measurement control
signals, and audio and data signals 506, such as the delay measurement
signals generated by the master controller 500 for the intra-regional
propagation delay measurements. As a result of the close proximity between
the master controller 500 and each o | | |
