EP0170847B1 - Method for the phase-locked transmission of a low-frequency modulation signal and circuit arrangement for performing this method - Google Patents

Abstract

1. Method for the phase-locked transmission of a voice-frequency modulation signal from a central signal source via feeder connections to a plurality of spatially distributed base stations of a common-frequency radio system, in which method phase rotations of the modulation signal in the feeder connection are compensated, characterized in that - an auxiliary carrier (AC) located outside the voice-frequency band (VB) of the modulation signal is generated ; - the auxiliary carrier (AC) is mixed with the modulation signal ; - the lover sideband (LSB) resulting from the mixing, together with a residual auxiliary carrier (RAC), is transmitted to the base stations (B, B1, ..., B3) ; and - the original modulation signal is recovered at the base stations (B, B1, ..., B3) from the lower sideband (LSB) by mixing with the aid of the residual auxiliary carrier (RAC) transmitted.

Description

The invention relates to a method for the phase-locked transmission of a low-frequency modulation signal according to the preamble of claim 1 and a circuit arrangement for carrying out the method. A method of the type mentioned is e.g. B. from Brown Boveri Mitt 5/6, 1983, pp. 186-188 and from DE-A-3 044 438.

Common-frequency radio is frequently used for frequency-economical radio coverage of large areas, in which a plurality of interconnected base stations are operated simultaneously on the same frequency channel. The base stations are distributed in such a way that the areas they cover adjoin one another and cover the entire radio coverage area.

In order to keep the reception interference in the unavoidable overlap areas of several base stations as low as possible, certain conditions with regard to the synchronization of the base stations must be observed.

In the case of a fully synchronous system, which is associated with a high level of technical complexity, the carrier frequencies of the different base stations are coupled to one another and absolutely identical.

In a quasi-synchronous system, a separate, highly stable oscillator for the carrier frequency is provided in each base station, the maximum deviation between the carrier frequencies of different base stations not being allowed to be more than 5-20 Hz.

In addition to the reception interference caused by carrier frequency differences in the overlap areas of a quasi-synchronous single-frequency system, there are also the modulation-dependent interference: The modulation signals are transmitted from a central signal source to the individual base stations via feeder connections, where they modulate the respective carrier. Since the base stations are at different distances from the central signal source and different technical systems (LF line, radio link, etc.) may be used for the feeder connections, there are in particular runtime and phase differences between the modulation signals transmitted on different feeder connections, which are balanced in a suitable manner must be in order to limit disturbing interference phenomena in the overlap areas.

When using LF lines or single-channel radio links as feeder connections, it is now known to compensate for the phase shift by means of suitable phase correction elements and delay elements inserted into the connection, each of these elements having to be matched to the special transmission properties of the respective feeder connections (Brown Boveri Mitt . 5/6, 1983, pp. 186-188). For each single-wave radio system, this requires a special structure in the individual feeder connections and therefore leads to increased expenditure.

In addition, any carrier frequency transmission systems cannot be used as feeder connections, because the transposition of the different channels at the different locations produces signals with different phases and frequencies, which cannot be matched to one another in the known way by inserting correction elements.

The present invention is therefore based on the object of specifying a method for the phase-locked transmission of modulation signals from the central signal source to the base stations of a single-wave radio system, in which the compensation of phase errors occurring is achieved regardless of the type of feeder connection, and which therefore also Permission to use any carrier frequency transmission systems as feeder connections.

The object is achieved in a method of the type mentioned by the features from the characterizing part of claim 1.

The essence of the invention is to mix the modulation signal with a subcarrier before transmission, to transmit at least one of the side bands resulting from the mixing together with the subcarrier to the base stations and there to mix again with the transmitted subcarrier or with a phase-locked coupling to it to demodulate locally generated subcarriers. Since the sideband signal and the subcarrier always experience the same phase shifts on the feeder connection, the phase shift of the modulation signal is automatically compensated for during demodulation by means of the transmitted subcarrier.

The side band and subcarrier preferably lie in the same channel, which is designed as a telephony channel.

The circuit arrangement for carrying out the method has a modulator with a local oscillator, preferably a quartz oscillator, and a first mixer behind each signal source and a corresponding demodulator with a second mixer and means for recovering the auxiliary carrier in front of each base station.

• Fig. 1 das Blockschaltbild eines bekannten Gleichwellen-Funksystems mit verschiedenartigen Zubringerverbindungen,
• Fig. 2 das Blockschaltbild einer einzelnen Zubringerverbindung mit Trägerfrequenz-Übertragungssystem und Phasenkorrektur nach der Erfindung,
• Fig. 3 das Blockschaltbild eines Ausführungsbeispiels des Modulators aus Fig. 2,
• Fig. 4 das entsprechende Blockschaltbild für den Demodulator aus Fig. 2,
• Fig. 5a-c die in einem Modulator nach Fig. 3 auftretenden Frequenzen und Frequenzbänder,
• Fig. 6a-c die entsprechenden Frequenzen und Frequenzbänder im Demodulator nach Fig. 4.

The invention will now be explained in more detail below on the basis of exemplary embodiments in connection with the drawing. Show it:

• 1 shows the block diagram of a known single-wave radio system with different feeder connections,
• 2 shows the block diagram of a single feeder connection with carrier frequency transmission system and phase correction according to the invention,
• 3 shows the block diagram of an exemplary embodiment of the modulator from FIG. 2,
• 4 shows the corresponding block diagram for the demodulator from FIG. 2,
• 5a-c the frequencies and frequency bands occurring in a modulator according to FIG. 3,
• 6a-c the corresponding frequencies and frequency bands in the demodulator of FIG. 4th

1 shows an exemplary single-wave radio system according to the prior art. The radio coverage of a large area takes place via a plurality of base stations B1, ..., B3, which simultaneously transmit the carrier signal modulated with a low-frequency modulation signal via corresponding antennas A1, ..., A3.

The modulation signal comes from a low-frequency signal source 1, e.g. B. a microphone, and is distributed via feeder connections to the individual base stations B1, ..., B3. The feeder connections can be realized by LF lines 11, 12 of different lengths, but also by a single-channel radio link 13 with a radio relay 2 and a radio receiver 3.

In order to compensate for the phase differences occurring on the different feeder lines, phase correction elements 4 are provided in the lines, which compensate for phase differences by means of runtime differences and differences in the phase response. Each of these phase correction elements is matched to the transit time and the phase response of the respective transmission path and fulfills this function in a frequency channel with a relatively narrow bandwidth (e.g. telephony channel with A f = 3.1 kHz).

According to the invention, the modulation signal is now changed before it is transmitted via the feeder connection in such a way that the phase change can be corrected by the feeder connection at the base station without knowing the properties of the connection itself. This is particularly advantageous for applications in which any carrier frequency systems with FDM (Frequency Division Multiplex) are used as feeder connections.

The block diagram of a single such FDM feeder connection with phase correction according to the invention is shown in Fig. 2. Since an FDM system only makes sense if several channels have to be transmitted at the same time, a plurality of low-frequency signal sources 1 is assumed in FIG. Each of the signal sources is assigned its own channel. The modulation signals coming from the signal sources 1 are first modified within their channel by a subsequent modulator in the sense of the method according to the invention. The details of this modification will be discussed in more detail later in connection with the design of the modulator 5.

After the modulation signals in the modulators 5 have been modified, the individual channels are transposed to higher frequencies by a carrier frequency multiplexer 6 in a manner known per se and are transmitted in frequency to one another via a carrier frequency transmitter 7 and a carrier frequency path 14 to a carrier frequency receiver 8. A subsequent carrier frequency demultiplexer 9 returns the channels to their starting position in terms of frequency and forwards them separately to a number of demodulators 10 corresponding to the number of channels, in which the respective phase shift is corrected and the original modulation signal is recovered.

The recovered modulation signals are then passed on to a base station B, where they modulate the carrier signal emitted via an antenna A.

A preferred exemplary embodiment of the modulator 5 from FIG. 2 is shown with its block diagram in FIG. 3. A detailed representation of the circuitry structure of the individual blocks has been dispensed with here as well as with the corresponding circuit diagram of the demodulator 10, because the execution is known to every person skilled in carrier frequency technology.

Within the modulator 5, the modulation signal from the signal source 1 is first applied to the input of a first input amplifier 16 via a first isolating transformer 15, which preferably has a transmission ratio of 1: 1 and an impedance of 600 ohms, and there to a for further signal processing favorable level strengthened.

The first input amplifier 16 is followed by a band limit filter 17 which limits the frequency band of the modulation signal and preferably has an upper limit frequency of approximately 3 kHz. The pass characteristic of the band limitation filter is entered as a dashed line in FIG. 5 a, which shows the low frequency band NB of the modulation signal, which results from the limitation and lies between 0.3 and 3 kHz.

The low-frequency band NB is fed to the one input of a first mixer 18 which mixes the modulation signal with an auxiliary carrier HT of preferably 3.3 kHz, shown on the frequency axis in FIG. 5b. The subcarrier HT is derived from the oscillator frequency of a local oscillator, preferably a quartz oscillator 20, which, for. B. 3,3792 MHz and is divided by a subsequent first frequency divider 21 with a division ratio of 210: 1 to 3.3 kHz. Other oscillator frequencies accordingly require different sub-ratios.

When the modulation signal and subcarrier HT are mixed, a lower sideband USB and an upper sideband OSB are formed symmetrically to the frequency of the subcarrier HT. Both sidebands are shown in Fig. 5c. The lower sideband USB corresponds to the low-frequency band NB in the upside-down position and, taken by itself, contains the same information. To save bandwidth, the first mixer 18 is therefore preferably followed by a first sideband filter 19 with a pass characteristic shown in dashed lines in FIG. 5c, which suppresses the subcarrier HT and the upper sideband OSB with an upper limit frequency of 3 kHz.

The remaining lower sideband USB arrives at an input of an adder amplifier 23. A subcarrier residue HTR, which is reduced in amplitude, is fed to another input of the adder amplifier 23, which is transmitted via an harmonic filter 22 with an upper cutoff frequency of e.g. B. 4 kHz, is taken at the output of the first frequency divider 21.

A modified modulation signal then appears at the output of the adder amplifier 23, which is composed of the lower sideband USB and the adjacent subcarrier residue HTR. Both are shown in solid lines in FIG. 5c. The modified modulation signal can now over any feeder connection with the appropriate bandwidth such. B. the FDM system of FIG. 2 can be transmitted to the base stations, where it is previously corrected and converted back in the demodulator 10.

A preferred embodiment of the demodulator 10 which matches the modulator of FIG. 3 is shown with its block diagram in FIG. 4. The modified modulation signal of FIG. 6a arriving via the feeder connection is first amplified in the modulator 10 in a second input amplifier 24 and fed to a second mixer 25. At the same time, the auxiliary carrier residue HTR is branched off from the amplified signal by means of a high-pass filter 29, which has a lower cut-off frequency of 3.2 kHz for the exemplary frequencies from FIG. 6a, and is further used in the demodulator 10 for the local generation of a phase-locked-coupled auxiliary carrier.

The branched subcarrier remainder HTR is amplified in a carrier signal amplifier 30 and passed on for synchronization of a PLL (phase locked loop) circuit 31 to its synchronization input. The PLL circuit preferably generates a frequency of 6.6 kHz, which is divided down in a subsequent second frequency divider 32 with a division ratio of 2: 1 to 3.3 kHz and fed to the feedback input of the PLL circuit 31 via a feedback loop.

In this way, a locally generated undisturbed subcarrier of 3.3 kHz is available at the output of the second frequency divider 32, which is phase-locked to the subcarrier residue HTR, which is also transmitted in the modified modulation signal.

The local subcarrier is mixed in the second mixer 25 with the amplified modified modulation signal, as a result of which a lower sideband USB and an upper sideband OSB are produced again according to FIG. 6b. The lower sideband USB corresponds to the original low-frequency band NB from FIG. 5a and is separated by a second sideband filter 26 (upper cut-off frequency: 3 kHz), whose pass characteristic is entered in FIG. 6b (FIG. 6c), amplified in an output amplifier 27 and passed on to the subsequent base station via a second isolating transformer 28 (transmission ratio: 1: 1; impedance: 600 ohms).

Since two signal quantities are used for the mixture in the second mixer 25 of the demodulator 10 with the transmitted lower sideband USB and the locally generated subcarrier coupled to the transmitted subcarrier residue HTR, both of which are subject to the same phase change due to the feeder connection, compensate each other the mix these phase changes automatically.

Due to the preferred limitation of the low frequency band to frequencies between 0.3 and 3 kHz and the selection of an auxiliary carrier HT with 3.3 kHz, the modified modulation signal (lower sideband USB in reverse position + auxiliary carrier rest HTR) can be transmitted via any internationally standardized telephony channel that has one Has a minimum bandwidth of 0.3 to 3.4 kHz.

Overall, the method according to the invention provides a method to easily compensate for phase shifts of the modulation signal to be transmitted when controlling base transmitters of a single-wave radio system, regardless of the type of feeder connection.

Claims (9)

1. Method for the phase-locked transmission of a voice-frequency modulation signal from a central signal source via feeder connections to a plurality of spatially distributed base stations of a common-frequency radio system, in which method phase rotations of the modulation signal in the feeder connection are compensated, characterized in that

- an auxiliary carrier (AC) located outside the voice-frequency band (VB) of the modulation signal is generated;

- the auxiliary carrier (AC) is mixed with the modulation signal;

- the lover sideband (LSB) resulting from the mixing, together with a residual auxiliary carrier (RAC), is transmitted to the base stations (B, B1,. .., B3); and

- the original modulation signal is recovered at the base stations (B, B1, ..., B3) from the lower sideband (LSB) by mixing with the aid of the residual auxiliary carrier (RAC) transmitted.

2. Method according to Claim 1, characterized in that the voice frequency band (VB) of the modulation signal and the auxiliary carrier (AC) are located within a telephony channel, the voice frequency band (VB) preferably extending from 0.3 to 3 kHz and the auxiliary carrier (AC) preferably exhibiting a frequency of 3.3 kHz.

3. Method according to Claim 2, characterized in that at the base stations (B, B1, ..., B3), the transmitted residual auxiliary carrier (RAC) is separated from the lover sideband (LSB), and a local auxiliary carrier of the same frequency, which is coupled locked in phase to the residual auxiliary carrier (RAC), is generated and used for mixing with the lower sideband (LSB).

4. Method according to Claim 1, characterized in that the lower sidebands (LSB) of several modulation signals are multiplexed vith the associated auxiliary carriers (AC) in accordance with the carrier frequency method and are transmitted as multiplex signal to the base stations (B, B1, ..., B3), vhere they are demultiplexed before the original modulation signals are recovered.

5. Circuit arrangement for carrying out the method according to Claim 1, characterized in that in each feeder connection from the signal source (1) to a base station (B, B1, ..., B3), a modulator (5) is arranged after the signal source (1) and a demodulator (10) is arranged before the base station (B, B1, ..., B3), in that the modulator (5) contains a local oscillator for generating the auxiliary carrier (AC) and a first mixer (18) for mixing the modulation signal with the auxiliary carrier (AC), and in that the demodulator (10) exhibits a high-pass filter (29) for separating out the transmitted residual auxiliary carrier (RAC), a second mixer (25) for recovering the original modulation signal with the aid of the transmitted residual auxiliary carrier (RAC), and a second sideband filter (26), which is arranged at the output of the second mixer (25), for suppressing the auxiliary carrier (AC) and the upper sideband (USB) produced during the mixing.

6. Circuit arrangement according to Claim 5, characterized in that in the modulator (5) after the output of the first mixer (18), a first sideband filter (19) is provided for suppressing the auxiliary carrier (AC) and the upper sideband (USB) produced during the mixing, and in that the residual auxiliary carrier (RAC), which is taken from the crystal oscillator (20), is added to the remaining lower sideband (LSB) in an adding amplifier (23).

7. Circuit arrangement according to Claim 6, characterized in that the local oscillator is a crystal oscillator (20), oscillates at a frequency which is a multiple of the frequency of the auxiliary carrier (AC), and in that the crystal oscillator (20) is followed by a first frequency divider (21) for generating the auxiliary carrier (AC).

8. Circuit arrangement according to Claim 7, characterized in that in the demodulator (10) following the high-pass filter (29), a carrier signal amplifier (30) and subsequently a PLL (Phase Locked Loop) circuit (31) is arranged which generates a local auxiliary carrier of the same frequency, which is coupled locked in phase to the residual auxiliary carrier (RAC), and forwards it to the second mixer (25).

9. Circuit arrangement according to Claim 8, characterized in that

- the crystal oscillator (20) generates a frequency of 3.3792 MHz,

- the first frequency divider (21) has a dividing ratio of 210 : 1,

- the first and second sideband filter (19, 26) exhibit an upper cut-off frequency of 3 kHz,

- the high-pass filter (29) exhibits a lower cut-off frequency of 3.2 kHz, and

- before the first mixer (18), a band-limiting filter (17) having a band width of 0.3 to 3 kHz is arranged.

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