EP0568143A1

EP0568143A1 - Improved transmitter network with a single transmitter frequency

Info

Publication number: EP0568143A1
Application number: EP93201154A
Authority: EP
Inventors: Paulus G.M. c/o INT. OCTROOIBUREAU B.V. De Bot
Original assignee: Koninklijke Philips Electronics NV; Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 1992-04-28
Filing date: 1993-04-21
Publication date: 1993-11-03

Abstract

In a transmitter network comprising a plurality of transmitters having the same transmitter frequency there are so-termed echo regions in which both transmitters may be received about equally strongly. In these regions the receivers are to comprise means for suppressing the received signal from one of the transmitters that has the features of an undesired echo signal. For reducing the complexity of the receivers, which complexity strongly depends on the echo time of the echo signal, the signal transmitted by one of the transmitters is delayed in such a way according to the invention that the average echo time in the echo region is minimized.

Description

The invention relates to a transmitter network comprising at least two transmitters 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 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 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 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 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 when conventional transmitter networks are used. In addition, in transmitter networks employing no more than a single transmitter frequency, there will be no additional disturbance even under special propagation conditions, because such disturbing signals are already taken into account in the receivers.
A problem for the prior-art transmitter network is that the difference in 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 10 km from the second transmitter, the difference between the distance from the first transmitter to the receiver and the distance from the second transmitter to the receiver is 30 km. If it is assumed that the transmitters transmit their information simultaneously, a 100 µs 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 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 the second transmitter. By delaying in said example the signal transmitted by the second transmitter by 100 µs, 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 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 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 strength 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, there are still non-negligible delay differences in part of the relatively large region in which significant echo signals occur.
By installing an auxiliary transmitter having a smaller range than the main transmitters in the regions in which significant echo signals occur, there is achieved that the size of the regions with still significant echo signals is strongly reduced as a result of which the delay differences may also be further reduced.
A further 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 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 boundary of the coverage area 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 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, 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.
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 of 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-dimensional 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 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 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 strength 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 W.C. Jakes, Wiley, 1974. The distance between the first transmitter and the second transmitter is 80 km in the example 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 (γ).
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 km/s the delay difference is found to have a value of -4/300,000=-13.3,µs. This means that the signal coming from the first transmitter A reaches the receiver 13.3 µs earlier than the signal coming from the second transmitter B. For point Q the wavelength difference is equal to 60-20=40km, as a result of which the signal from transmitter A arrives at the receiver 133 µs later than the signal from transmitter B. The large 133 µs 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 µs), the signal from transmitter A arrives at point P 72.45 µs earlier than the signal from transmitter B. The signal from transmitter A arrives at point Q 72.45 µs 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 µs.
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 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 E₁ is shown if no more than two main transmitters are included in the transmitter network. The echo region E₁ then has a size of about 44 km. This results in a delay difference which may lie between -147 µs and + 147 µs 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 E₁. 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 and E₃ is installed 28 km remote from the nearest main transmitter and that this centre is installed 32 km 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=13.3µs, 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 the echo region. The delay difference in the echo region may now lie between -53 µs and +53 µs. This delay difference is considerably smaller than the 147 µs delay difference that may occur in the echo region E₁.
Fig. 3 gives a two-dimensional representation of a transmitter network comprising equal (main) transmitters 1 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 mutually equal main transmitters 1 to 7 and a larger number of auxiliary transmitters s among the main transmitters 1 to 7. The main transmitters 1 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 Fig. 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 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 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 B₁, B₂ if a plurality of auxiliary transmitters B₁, B₂ are positioned 30 km apart around the 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 B₁ to B₄ having a smaller aerial height are present increasing the overall coverage area. In addition, further auxiliary transmitters D₃, D₅ and D₆ and D₁, D₂ and D₄ respectively, are 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 B₁ to B₃, the further auxiliary transmitters having an aerial height again smaller than that of the auxiliary transmitters B₁ to B₄. Finally, auxiliary transmitters E having an even smaller aerial 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 output of the receiver 12 is connected to an input of a demodulator 14. An output of the demodulator 14 carrying output signal a_r is connected to an input of the transmitter 15. The output 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 a_d 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 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 aerial 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 a_r is available at the output of the demodulator. This may be a single 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 aforementioned journal article by G. Plenge.
The digital symbols a_r 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 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 a_d are modulated on a carrier having the desired frequency by the modulator 17, and this modulated signal is amplified by the power 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 sampled at a high 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 installed near to a main transmitter, the signal is to be delayed in the main transmitter because the transmission link from main transmitter to auxiliary transmitter already realises a delay larger than the required delay.
In the delay means as shown in Fig. 8 the symbols a_r to be delayed are applied to an input port of a dual port RAM 20, whereas the delayed symbols a_d 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 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 a_r 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 a_d are read from the dual port RAM 20 when the clock 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 a_r will be written and these symbols will be read out at an instant which is D.T_clk 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 very rapidly by the selection of the value D.

Claims

Transmitter network comprising at least two transmitters having a like transmitter frequency which transmit a like signal, characterized in that one of the transmitters comprises delay means for delaying the signal transmitted by this transmitter, so that the signals transmitted by the transmitters arrive substantially simultaneously in an area of overlap of the coverage of the two transmitters.
Transmitter network as claimed in Claim 1, characterized in that at least one of the transmitters is an auxiliary transmitter installed among a plurality of main transmitters.
Transmitter network as claimed in Claim 1, 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 main transmitter.
Transmitter network as claimed in Claim 3, characterized in that the auxiliary transmitters are installed around the main transmitter.
Transmitter network as claimed in Claim 3 or 4, characterized in that the transmitter network comprises further auxiliary transmitters installed on the boundary of the coverage area of another auxiliary transmitter, the aerial height of the further auxiliary transmitters becoming ever smaller as the boundary of the coverage area of the transmitter network is approached more.
Transmitter to be used in a transmitter network comprising at least two transmitters having a like transmitter frequency, characterized in that the transmitter comprises delay means for delaying the signal transmitted by the transmitter.