Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
2~771
PHN 14.054 1 22.03.1993
Improved transmitter network with a single transmitter frequency.
rhe invention relates to a transmitter network comprising at least two
transrnitters having a like transmitter frequency which transmit a like signal.
The invention likewise relates to a transmitter to be used in such a
network.
Such a transmitter network is known from the journal article entitled
"DAB - A new sound broadcasting system, Status of the development - Routes to its
introduction" by G. Plenge in EBU Review no. 246, April 1991, Chapter 5.2.2, pp. 87-
112.
When a conventional transmitter network is designed, for example, for
lO broadcasting purposes, one is generally confronted with the problem that not enough
channels are available for the signals to be transmitted. In that case one resorts to
reusing frequencies whilst under normal propagation conditions it is possible to receive
in a certain area only one of the transmitters transmitting at a specific frequency, so that
no mutual disturbance need be expected under normal propagation conditions. In such a
15 conventional transmitter network, however, disturbances may nevertheless occur under
special propagation conditions, such as, for example, tropospheric ducting.
In the transmitter network known from above journal article, a signal is
transmitted with a like transmitter frequency via a plurality of transmitters, whereas a
receiver can receive signals from different transmitters. As a result, a disturbance signal
20 is developed having a characteristic corresponding to an echo signal. This (undesired)
echo signal is suppressed in the receiver by means of an echo canceller or by using a
what is commonly referred to as guard band in the time domain when the signal to be
transmitted is actually transmitted. Consequently, it is possible that this received signal
is discarded in the receiver for a specific period of time during which the received
25 signal is disturbed by the echo signals.
A great advantage of transmitter networks, in which no more than a single
transmitter frequency is used, is that much fewer channels need to be available than
wllen conventional transmitter networks are used. In addition, in transmitter networks
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PHN 14.054 2 22.03.1993
employing no more than a single transmitter frequency, there will be no additional
disturbance even lmder special propagation conditions, because such disturbing signals
are a]ready taken into account in the receivers.
A problem for the prior-art transmitter network is that the difference in
S time between the arrival at the receiver of signals coming from different transmitters
may be relatively large. For example, in a situation where a first and a second
transmitter are 50 km apart and a receiver is positioned between the first and the second
transmitter at a distance of lO km from the second transmitter, the difference between
the distance from the first transmitter to the receiver and the distance from the second
lO transmitter to the receiver is 30 km. If it is assumed that the transmitters transmit their
information simultaneously, a 100 ~s delay difference is found based on a light velocity
of 300,000 km/s.
As a result of these relatively large delay differences, the measures to be
taken in the receivers for cancelling the effect of the echo signals are rather complex.
It is an object of the invention to provide a transmitter network as defined
in the opening paragraph, in which the complexity of the associated receivers may be
reduced.
For this purpose, the transmitter network is characterized, in that one of
the transmitters comprises delay means for delaying the signal transmitted by this
20 transmitter, so that the signals transmitted by the transmitters arrive substantially
simultaneously in an area of overlap of the coverage of the two transmitters.
By delaying the signal to be transmitted by one of the two transmitters, it
becomes possible to reduce considerably the difference in time of arrival of the signals
transmitted by a first transmitter and a second transmitter within the coverage area of
25 the second transmitter. By delaying in said example the signal ~ransmitted by the second
transmitter by 100 ~s, the difference of delay is eliminated. For other positions of the
receiver the delay difference will naturally not be completely eliminated but certainly
strongly reduced.
The required complexity of the receiver may be further reduced by means
30 of a further embodiment of the invention, characterized in that at least one of the
transmitters is an auxiliary transmitter installed amongst a plurality of main transmitters.
If the transmitter network solely consists of main transmitters having
substantially equal coverage areas, there will be relatively large regions among these
7 7 ~
PHN 1 4 . 054 3 22 . 03 .1 993
transmitters in which the echo signals received from other transmitters are significant
and thus are to be suppressed. Significant echo signals occur if the received signal
s~rength of a plurality of transmitters are of a like order. As it is possible only in a
small area to provide a substantially complete compensation of the delay difference,
5 there are still non-negligible delay differences in part of the relatively large region in
wl1icll signiflcant echo signals occur.
By installing an auxiliary transmitter having a smaller range than ~he main
transmitters in the regions in which significant echo signals occur, there is achieved that
tlle size of the regions with still significant echo signals is strongly reduced as a result
10 of which the delay differences may also be further reduced.
A filrther embodiment of the invention is characterized, in that one of the
transmitters is a main transmitter and one of the transmitters an auxiliary transmitter,
the auxiliary transmitter having a lower aerial height than the main transmitter, and in
that the auxiliary transmitter is installed on the boundary of the coverage area of the
15 main transmitter. By adding an auxiliary transmitter with a lower aerial height to the
main transmitter, it is possible to create a sharply defined coverage area of the
transmitter network, which is meant to imply that with a certain size of the coverage
area the disturbance caused outside this coverage area is reduced in comparison with the
use of only a single main transmitter. If the auxiliary transmitters are installed on the
20 boundary of the coverage are,. of the main transmitter, the size of the coverage area of
the overall transmitter network is determined by the coverage area of the auxiliary
transmitters. The field strength received from an auxiliary transmitter with a smaller
aerial height than that of the main transmitter diminishes more rapidly as a function of
the distance from the receiver to this auxiliary transmitter than does the field strength
25 received from a main transmitter as a function of the distance from the receiver to the
main transmitter. This is caused by the fact that with the auxiliary transmitter having a
smaller aerial height the area in which direct-sight transmission occurs, while the field
strength diminishes by the squared distance from the transmitter to the receiver, is
smaller than with the main transmitter, so that the area beyond the direct-sight distance,
30 in which the field strength is reduced by the fourth power of the distance, commences
earlier. Due to this faster reduction of the received field strength, the coverage area of
the overall transmitter network will thus be more. sharply defined than the coverage area
of a main transmitter alone.
2 ~ 7 ~ 1
PHN 14.054 4 22.03.1993
The invention will be further explained with reference to the drawing
Figures, in which like elements are denoted by like reference characters, in which:
Fig. 1 shows the field strength of a first and o.f a second transmitter as a
function of the position of a receiver between these transmitters;
Fig. 2 shows the field strength of two main transmitters and an auxiliary
transmitter as a function of the position of a receiver between the two transmitters;
Fig. 3 gives a two-din-ensional representation of the size of the echo
regions in a prior-art transmitter network;
Fig. 4 gives a two-dimensional representation of the size of the echo
10 regions with a transmitter network according to the invention;
Fig. 5 shows the variation of the received signal as a function of the
position of the receiver when only one main transmitter is used and when a main
transmitter and a plurality of auxiliary transmitters are used according to the invention;
Fig. 6 shows the coverage area of a transmitter network in which
IS auxiliary transmitters are used having an ever smaller aerial height as the boundary of
the coverage area is approached more;
Fig. 7 shows a system comprising a transmitter to be used in a transmitter
network according to the invention; and
Fig. 8 shows a delay element to be used in the system as shown in Fig. 6.
Fig. 1 shows the field streng~h of a first transmitter A and a second
transmitter B as a function of the position of a receiver on an imaginary line between
these transmitters. The variation of the field strength as a function of the distance is
determined on the basis of formulas for the received field strength as a function of the
distance of a transmitter as stated in the title "Microwave Mobile Communications" by
25 W.C. Jakes, Wiley, 1974. The distance between the first transmitter and the second
transmitter is 80 km in the exarmple of Fig. 1.
-Fig. 1 also shows the echo region E in which significant echoes occur.
The echo region is defined here as the area in which the field strength of the received
signal of both transmitters differs less than 10 dB (y).
For the points P and Q the delay difference may be determined if it is
assumed that the two transmitters simultaneously transmit the signal to be transmitted.
For point P the wavelength difference between the respective transmitters A and B and
the receiver is equal to 38-42=-4km. Reckoning with a velocity of light of 300,000
7 1
P~N 14.0~4 5 22.03.1993
km/s the delay difference is found to have a value of -4/300,000=-13.3~s. This means
that the signal coming from the first transmitter A reaches the receiver 13.3 ~LS earlier
than the signal coming from the second transmitter B. For point Q the wavelength~lff~rence is equal to 60-20=40km, as a res~llt of which the signal from transmitter A
5 arrives at the receiver 133 ~LS later than the signal from transmitter B. The large 133 ~lS
delay requires a relatively large complexity of the receiver.
However, if the signal transmitted by transmitter B is delayed by the
average of the delay difference between the points P and Q (59.15 ~s), the signa! from
transmitter A arrives at point P 72.45 ~s earlier than the signal from transmitter B. The
10 signal from transmitter A arrives at point Q 72.45 ,us later than the signal from
transmitter B. It appears that when this invention is implemented, in the echo region
one only needs to take an echo signal into account having a delay of 72.45 ~s.
Fig. 2 shows the field strength of a main transmitter A, a main transmitter
C and an auxiliary transmitter B as a function of the position of the receiver on an
15 imaginary line between the two transmitters. The distance between the main transmitters
A and C is equal to 100 km and the auxiliary transmitter B is 50 km remote from the
two main transmitters. In Fig. 2 the echo region El is shown if no more than two main
transmitters are included in the transmitter network. The echo region El then has a size
of about 44 km. This results in a delay difference which may lie between -147 ~s and
20 + 147 ~s in the echo region.
If an auxiliary transmitter B is inserted between the two main transmitters,
there are two echo regions E2 and E3 having a size which is considerably smaller than
that of the echo region El. The size of the echo regions in the situation as shown in
Fig. 2 is equal to 16 km. From Fig. 2 it appears that the centre of the echo regions E2
25 and E3 is installed 28 km remote from the nearest main transmitter and that this centre
is installed 32 krn remote from the auxiliary transmitter. If, according to the inventive
idea, the signal transmitted by the auxiliary transmitter is delayed by a period of time of
(32-28)/300,000= 13.3~s, the signals from auxiliary transmitter B and the main
transmitter belonging to the echo region will arrive at the same instant at the centre of
30 the echo region. Tlle delay difference in the echo region may now lie between -53 ~LS
and ~53 ~s. This delay difference is considerably smaller than the 147 ~s delay
difference that may occur in the echo region El.
Fig. 3 gives a two-dimensional representation of a transmitter network
2 ~ ~ ~ 1 7 ~
PHN 1 4 . 054 6 22 . 03 .1 993
comprising equal (main) transmitters I to 7, installed equidistantly. The echo regions
are shaded and cover a considerable area.
Fig. 4 gives a two-dimensional representation of a transmitter network
comprising mlltually e4~1al main transmitters I ~o 7 and a larger n~lmber of auxiliary
5 transmitters s among the main transmitters I to 7. The main transmitters I to 7 exactly
simultaneously transmit the signal to be transmitted, whereas all the auxiliary
transmitters s transmit the signal to be transmitted, delayed by a .same period of time. In
~ig. 4 the echo regions are also shaded. A comparison of Fig. 4 to Fig. 3 shows that
the size of the echo regions and hence the size of the possible delay difference in the
10 transmitter network as shown in Fig. 4 is considerably smaller than the size of the echo
regions in Fig. 3.
The dashed line a in Fig. 5 shows the field strength of the received signal
as a function of the position of a receiver whilst assuming that no more than a single
main transmitter is used. There is further assumed that the coverage area is to have the
15 size as denoted by the letter D and that the relative field strength within the coverage
area is to be at least -90 dB. This -90 dB value may be determined, for example, by
disturbance caused by transmitters from a neighbouring area. The solid lines show the
received signal coming from the main transmitter A and the auxiliary transmitters Bl,
B2 if a plurality of auxiliary transmitters Bl, B2 are positioned 30 km apart around the
20 main transmitter A. Fig. 5 distinctly shows that the size of the coverage area may be
maintained with a considerably lower transmitter power of the main transmitter A. This
lower power of the main transmitter leads to a smaller field strength of the received
signal outside the coverage area, as a result of which the disturbance caused outside the
coverage area is reduced proportionally.
In the transmitter network as shown in Fig. 6 there is a main transmitter
A supplying a signal to a large part of the coverage area. On the boundary of the
coverage area of the main transmitter A four auxiliary transmitters Bl to B4 having a
smaller aerial height are present increasing the overall coverage area. In addition,
furtller auxiliary transmitters D3, Ds and D6 and Dl, D2 and D4 respectively, are
30 present on part of the boundary of the coverage area of the main transmitter A and on
the boundary of the coverage area of the auxiliary transmitters Bl to B3, the further
auxiliary transmitters having an aerial height again smaller than that of the auxiliary
transmitters Bl to B4. Finally, auxiliary transmitters E having an even smaller aerial
2 ~ r~
PHN 14.054 7 22.03.1993
height are present for completely covering the desired coverage area.
In the system as shown in Fig. 7 the output of a receiver aerial 10 is
connected to a receiver 12. The OUtpllt of the receiver 12 is connected to an input of a
dernodlllator 14. An OUtpllt of the demodulatol 14 carrying output signai ar is connected
5 to an input of the transmitter 15. The OUtpllt of the transmitter 15 is connected to the
delay means 16 in accordance with the inventive idea. The output of the delay means 16
carrying output signal ad is connected to an input of a modulator 17. The output of the
modulator 17 is connected to an input of a power amplifier 18, the output of the power
amplifier 18 being connected to a transmitter aerial 19.
Generally, transmitter networks having a transmitter frequency for all
transmitters are preferably used for transmitting digital signals, because the measures
necessary for suppressing echo signals are harder to realise when analog signals are
transmitted, although the application to analog signal transmission is highly conceivable.
The system as shown in Fig. 7 is arranged for processing such digital
15 signals. The signal to be transmitted is received through an aerial 10. This may be an
aerial for a radio link but this may also be an aeriat for satellite reception. The output
signal of aerial 10 is processed in receiver 12 to an intermediate frequency signal and
then demodulated in the digital demodulator 14 and detected, so that a sequence of
digital symbols ar is available at the output of the demodulator. This may be a single
20 sequence of digital symbols but this may also be a large number of symbol sequences as
occur in OFDM signals for digital broadcasting as this has been proposed in afore-
mentioned journal article by G. Plenge.
The digital symbols ar are delayed by the desired period of time by the
delay means 16. For determining the delay of the delay means 16, one not only has to
25 take the delay into consideration necessary for simultaneously receiving in the middle of
an echo region signals transmitted by two transmitters, but also the delay difference
occurring in the transmission links between the studio and the various transmitters.
The delayed symbols ad are modulated on a carrier having the desired
frequency by the modulator 17, and this modulated signal is amplified by the power
30 amplifier 18 to a signal having the desired power.
It is likewise conceivable that the system as shown in Fig. 7 is arranged
without a demodulator 14 and a modulator 17. As a result, however, the delay means
are then to be arranged in an analog form, or the signals are to be sarnpled at a high
7 ~ ~
PHN 14.054 8 22.03.1993
rate if the delay means are arranged in a digital form.
If the transmission links to the auxiliary transmitters are realised via a
beam transmitter installecl near to a main transm;tter, the signal is to be delayed in the
main transmitter because the transmission link from main transmitter to auxiliary
5 transmitter already realises a delay larger than the required delay.
In the delay means as shown in Fig. 8 the symbols ar to be delayed are
applied to an input port of a dual port RAM 20, whereas the delayed symbols ad are
available at an output port of the dual port RAM 20. A clock signal CLK is applied to a
clock input of a counter 26, to a control input of the multiplexer 22 and to a read/write
10 control input of the dual port RAM 20. An output of the counter is connected to a first
input of the multiplexer 22 and to a first input of an adder 24. The digital representation
of the desired delay time D is fed to a second input of the adder 24. The output of the
adder 24 is connected to a second input of the multiplexer 22, whereas the output of the
multiplexer is connected to the address input of the dual port RAM 20.
The symbols ar to be delayed are written in the dual port RAM 20 when
the clock signal is a logic "0". For this purpose, the clock signal activates the write
mode of the dual port RAM 20 and likewise provides that the multiplexer 22 applies the
sum of the count of the counter 26 and the value D to the address input of the dual port
RAM 20. The delayed symbols ad are read from the dual port RAM 20 when the clock20 signal is a logic " 1". For this purpose, the read mode of the dual port RAM 20 is
selected when the clock signal gets the logic " 1" value and the count is applied to the
address input of the dual port RAM via the multiplexer 22.
Since the write address is always a value D larger than the read address,
at a specific address first the symbols ar will be written and these symbols will be read
25 out at an instant which is D.Tclk later. If the adder 24 generates a carry, this carry may
be ignored as a result of which the writing in the dual port RAM 20 again starts at
address 0.
The use of delay means as shown in Fig. 7 are advantageous relative to
the use of a shift register for delay means, in that the delay can be set very simply and
30 very rapidly by the selection of the value D.
