FR2619478A1 - Retransmitter with direct amplification for extending the zone of coverage of a radio section - Google Patents

Abstract

The retransmitter according to the invention is particularly designed to afford bidirectional radio beam links between a first station, such as a telecommunication relay station, and a plurality of mobile second stations, such as vehicles. It comprises two duplexers with filters 4, 4a connecting first and second unidirectional paths S1, S2 in the retransmitter to first and second antennas AS, AM directed towards the first and second stations respectively. Each path comprises a circuit 1, 2 for channel-by-channel filtering and for amplification. The amplification of the signals is carried out at the high transmission frequencies. In the direction of transmission S2 from the second stations to the first station, for each communication channel C21 to C2N, there is provided a noise attenuator and level limiter circuit 251 to 25N for minimising the cross-modulation between the various channels.

Description

Direct amplification repeater for expansion
the coverage area of a radio station
The present invention generally relates to the field of radio-relay transmissions. More particularly, the invention relates to a re-transmitter for the coverage of small radio shadow areas, of the order of a few square kilometers.

 The repeaters or known relay station of a telecommunication network are equipment placed between first and second stations and whose function is to receive emitted signals, to amplify and rebroadcast them from one to the other station.

In order to minimize the problems of coupling between antennas while ensuring optimum use of the different frequencies carrying a frequency plan, these repeaters read a crossover of the carrier frequencies between the two directions of transmission of a bidirectional link. Thus for a link between the first and second stations, two carrier frequencies are only used. According to a first direction of transmission, from the first station to the second station, first and second frequencies are used respectively for the connection between the first station and the re-transmitter and for the connection between the transponder and the second station.

In a second direction of transmission, from the second station to the first station, the same first and second frequencies are used respectively for the connection between the second station and the transponder and for the connection between the transponder and the first station. Such a method involves the use in the retransmitter, in addition to amplifiers, transmit frequency translators or local oscillators at transmission frequencies associated with demodulation devices of the information signals, voice or digital, modulation devices, and possibly devices for filtering and regenerating the information signals. As a result, these repeaters are relatively complex and expensive and are not suitable for producing low-power, low-cost transmitters for radio coverage of small shadows.

 It is an object of the present invention to provide retransmitter equipment for radio coverage of small area areas directly amplifying and retransmitting received high frequency signals without changing or crossing carrier frequencies and without demodulating information signals.

For this purpose, a device according to the invention for retransmitting radio links between a first station and one or more second stations, said equipment comprising first and second antennas for providing bidirectional transmission links between the equipment and the first station and between the equipment and the second stations respectively, and means for coupling said first and second antennas through first and second unidirectional transmission paths in the equipment respectively associated with first and second transmission directions, is characterized in that in the first channel, it comprises means for separately filtering first signals sent by the first station to the second stations respectively, and means for simultaneously mixing and amplifying the first filtered signals before their retransmission by the second antenna to the second stations, and in the channel means, means for separately filtering second signals respectively transmitted by the second stations to the first station, means for limiting the amplitude of each of the second filtered and separated signals to a predetermined maximum value, and means for mixing and simultaneously amplifying the amplitude-limited second signals before their re-transmission by the first antenna to the first station
The means for amplifying amplify the received signals so that the emitted signals have exactly the same frequencies as the received signals, which avoids frequency beats that would not fail to produce frequency offsets between the received signals and the re-transmitted signals. in the case where these latter signals are obtained as a result of simple transpositions of frequency or carrier modulation provided by local oscillators.

 The equipment according to the invention is particularly well suited for providing radio links between a first station, such as a main station or a telecommunication relay station, and second mobile stations, such as moving vehicles.

 It is advantageously usable for the extension of radio coverage along motorways, high-speed train lines (TGV), and in road and rail tunnels.

 Other advantages and features of the invention will become more clearly apparent from reading the following description of several preferred embodiments of the invention with reference to the corresponding accompanying drawings in which - FIG. schematically shows in the form of a diagram, the environment of a transmitter according to the invention; FIG. 2 is a block diagram of a repeater according to the invention; and - FIG. 3 is a block diagram of a noise attenuator circuit and level limiter included in a channel of the transmitter.

 With reference to FIG. 1 a radio transmitting / receiving station SP provides radio coverage for a territory within a PS boundary beyond which the radio links between the SP station and moving or mobile vehicles are no longer insured . A re-transmitter according to the invention RE is used for example to cover inside this territory a shadow zone ZO due to particular local conditions such as, for example, an eventful terrain. The re-transmitter RE is implanted in such a way that the zone ZO shadow is within the territory covered by the RE repeater and delimited by a circular periphery PR.

The retransmitter according to the invention is of the direct amplification type. According to a first direction of transmission Si, from the station SP to mobiles, a signal n high carrier frequency f1 emitted by the station SP and intended for a mobile M moving inside the periphery PS, is received by the repeater RE which amplifies and retransmits it to the same carrier frequency fl towards the mobile M. According to a second transmission direction S2, mobiles to the station SP, a high carrier frequency signal f2 emitted by the mobile M and intended for the station
SP is received by the repeater RE which amplifies and re-transmits it at the same carrier frequency f2 to the station SP.

 According to the direction of transmission If, in a more or less extended area inside the periphery PS, the mobile M can receive two signals n the carrier frequency fi, the signal emitted by the station SP can be received directly and via the transponder RE. Likewise, in the transmission direction S2, the station SP can receive two signals with a frequency f2, the signal transmitted by the mobile M being able to be received directly and via the repeater RE. If one of the received signals is significantly higher than the other, there is a capture effect. If the two signals have neighboring levels this conventionally gives rise to fading, insofar as the two signals are received after neighboring propagation times. However, in the case of a repeater, the differences of path at a neighboring level, say Also walking differences can reach a few kilometers. An excessively high path difference causes distortion of the transmitted low frequency information signals. Experience shows that the time difference between the two signals must not exceed 10t of the period of the component of the highest frequency information signal. For example, in the case of a radio transmission according to standard 11F3 where the highest transmitted frequency is equal to 2550 Hz, and given a delay time of the order of one microsecond due to the transit of one of the signals through the transponder the calculation shows that the difference between the paths of the two high frequency signals must not exceed d = 12 km.

max
In order to satisfy this constraint, the re-transmitter RE according to the invention can have different configurations and be implemented in different ways depending on the extent of the shadow zone Z0 to be covered and the distance between the re-transmitter and the SP station. . In the case where the shadow zone ZO is included in a periphery PR of radius rv da / 2 = 6 km, the re-transmitter RE can be equipped with an antenna of
max reissue, or link with AM mobile omnidirectional; the RE repeater is located in the center of the perimeter PR and the difference of the paths d does not exceed the maximum value d = 12 km.
max the radius r of the periphery PR must be greater than two / 2 - 6 km in order for the shadow zone ZO to be covered by the repeater RE, the retransmission antenna AM must be directive in order to avoid that the difference of the paths d does not exceed the maximum value d - 12 km; in this last
In this case, the retransmitter is implanted, as shown in FIG. 1 at the periphery of the shadow zone ZO so that it is surrounded by the perimeter PR.

In reference ? FIG. 2 the transmitter according to the invention comprises first and second filter and amplification circuits 1 and 2 respectively in first and second unidirectional channels of the transponder respectively associated with the first and second transmission directions S1 and S2, first and second AS directive antennas, and
AM for providing bidirectional radio links respectively with the SP station and with the mobiles M via the RE repeater, and first and second filter duplexers 4 and 4a respectively associated with the AS and AM antennas in order to couple them to the two channels of the retransmitter. The AS and AM directional antennas are, for example, YAGI or dihedral type antennas, decoupled from each other.

The AS antenna is directed to the SP station; it is for example of YAGI type and receives from the station SP at most N communication signals respectively having high carrier frequencies f11 to f1N where N is a predetermined integer. The frequencies f11 to 1N are the center frequencies of N communication channels C11 to C1N transmitted along the direction S1 and each having a frequency bandwidth #F
The signals relating to the channels C1 to C1 are separated from the signals
1 N relative to the direction S2 in the duplexer 4 and are provided by it to the filtering and amplification circuit 1 which retransmits them after processing to the mobiles M via the duplexer 4a and the antennai.

 The AM antenna is for example of dihedral type and radiates in the area where mobile M move; it receives at most N communication signals transmitted by the mobiles M; these N signals have respectively high carrier frequencies f21 to f2N different frequencies f11 to f1N, for example higher or lower than them. The frequencies f21 to 2N are the center frequencies of N communication channels C21 to G2N each having the bandwidth aF. The signals relating to the channels C21 t C2N are separated from the signals relating to the channels C11 to C1N in the duplexer 4a and are supplied by the latter to the filtering and amplification circuit 2 which retransmits them after processing to the station SP via the duplexer 4 and the antenna AS.

 The duplexer 4, 4 consists of two bandpass filters 41 and 42, 41a and 42a. The filter 41, 41a has a center frequency f10 equal to (f11 + f12 +. + F1N) / N and a bandwidth pass #B equal to @flN - f11 + AF The filter 42, 42a has a center frequency f20 equal to (f21 + f21 + f2N) / N and the same bandwidth width AB is also equal to 2N f2N - f21 + AF center frequencies f10 and f20 of filters 41, 41a, and 42, 42a are such that # f20 - f10 #>. # B; in other words, the frequencies f1 È 1N and f21 a f2N are not interleaved in the frequency spectrum.

 Channel channel filtering of the signals relating to the channels C11 to N and C21 to C2N are carried out respectively in the filtering and amplification circuits 1 and 2 in order to eliminate inter-channel noise signals that can lead to a saturation of power amplifiers 11 and 21 output in the circuits 1 and 2 respectively. Indeed, the amplifiers 11 and 21 are preferably optimized in power as a function of the number N of channels, in order to reduce their cost.

 It can be demonstrated in the case of the simultaneous amplification of N different signals of the same amplitude by the same amplifier, that if P is the effective power to be delivered by an amplifier for a signal, the nominal power of the amplifier must at least be equal to NP and its peak power at least equal to NP In the case where third-order intermodulation products are likely to occur between the N signals whose carrier frequencies are regularly distributed in the frequency spectrum the power of peak of the amplifier must be at least equal to N. (N / 2 - 1) .P. It is therefore necessary to optimize power amplifiers 11 and 21 to eliminate all the signals having frequencies that are within the bandwidths of these amplifiers and which do not correspond to the channels to be amplified.

 In the filter and amplification circuit 1, the filtering of the component signals relating to the channels C1 to CiN are carried out in the mid-frequency range by means of a battery of 12 to 12N crystal filters, after transpositions at intermediate frequencies in a frequency converter 14 and before reverse transpositions in another frequency converter 15. Also the N bandpass filters 121 to 12N have inputs connected to the output of the duplexer filter 41 through the converter 14 to filter the relative signals. to the different channels C1 to CiN The case according to the characteristics of the duplexer 41 the circuit 1 may comprise a bandpass filter 16 interconnected between the output of the filter 41 and the input of the converter 14 and having similar filtering characteristics but more The converter 14 transposes the frequency band of the signals relating to x C11 to C1N, centered on the frequency fi0, from the high frequency domain to the medium frequency domain and thus performs a conversion at intermediate frequencies included in all the passbands of the filters 121 to 12N For this purpose a local oscillator ( not shown) provides the converter 14 with a signal OL having a frequency equal to the amplitude of the frequency transposition performed by the converter 14.

The filters 111 to 11N respectively have as central frequencies intermediate frequencies resulting from the transposition of the high frequencies fl1 to flN in the converter 14 and all have the bandwidth AF After this filtering channel by channel the signals relating to the channels C1 to CiN are mixed in a combiner 13 and supplied at the input of the frequency converter 15 whose output is connected to the input of the power amplifier 11. The converter 15 performs a transposition of frequencies opposite to that performed by the converter 14 and receives also the local oscillator signal OL. The amplifier 11 operates in the frequency band between the frequencies fi1 - aF / 2 and flN + AF / 2; it supplies to the antenna AM the amplified signals relating to the channels C1 to CiN through the filter 41a of the duplexer 4a
According to the second transmission direction S2, the signals relating to the channels C21 to C2N received by the antenna AN have levels which can be very different from each other, because the mobiles M are at different distances from the antenna AN. The signals corresponding to the mobiles M very close to the antenna AN are resus with a very high level and can disturb the other signals by intermodulation. For this second direction of transmission it is necessary in order to ensure a good quality of communications to limit the amplitude of a signal when it exceeds a maximum level. Similarly, it is necessary to limit the level of the "squelch". that is, the low-frequency noise generated by the signal intermodulation, when the signal-to-noise ratio of the signal becomes less than a predetermined value.

 In addition to the power amplifier 21, the filtering and amplifying circuit 2 comprises two frequency converters or translators 22 and 23 N bandpass filters 241 24N N noise attenuation circuits and level limiters 251 to 25N and a combiner 26. The where appropriate, a band-pass filter 20 similar to the filter 42a and playing the same role as the filter 16 is interconnected between the output of the filter 42a and the input of the converter 22.

The converter 22 transposes the frequency band of the signals relating to the channels C21 to C2N centered on the frequency f20, from the high frequency domain to the medium frequency domain and thus performs a conversion at intermediate frequencies within the operating frequency band of the limiting circuits 251 to 25N
For this purpose a local oscillator (not shown) or preferably, the local oscillator connects to the converters 14 and 15 in the first channel provided to the converter 22 a signal OL having a frequency equal to the amplitude of the transposition of frequencies achieved by the converter 22.

 The N filters 241 to 24N have inputs connected to the output of the converter 22 and function as well as the filters 121 to 12N to separate the signals relating to the different transposed channels.

The filters 241 to 24N provide the filtered signals relating to the channels
C21 to C2N at limiting circuits 251 to 25N respectively.

A block diagram of an attenuator and limiter circuit 25n where n is an integer between 1 and N is shown in FIG. 3. The attenuator and limiter circuit 25 comprises a noise attenuator 251
n and an automatic gain control (AGC) level limiter 252.

The noise attenuator 251 consists of a low-variable-cut-off-frequency quartz band-pass filter 2511 receiving as input the signal relating to the channel C2 delivered by the band-pass filter 24, and
nn of a signal-to-noise (S / N) measurement circuit 2512. The circuit 2512 measures the signal-to-noise ratio at the output of the filter 2511 and adjusts the low cut-off frequency of the filter 2511 to a value even more great that the signal-to-noise ratio is even lower. In other words, the attenuator 251 strongly attenuates the low components of the signal having a high noise and therefore "squelch" the noise, in the proper sense of this English verb.

The automatic gain control level limiter 252 comprises a variable gain amplifier 2521, a level measurement circuit 2522, and a threshold control amplifier 2523. The signal relating to the channel C2 is supplied to the amplifier 2521 by the filter 2511 in
n the noise attenuator 251. The circuit 2522 measures the level at the output of the amplifier 2521 and more precisely detects the envelope of the signal coming from the amplifier 2521 and delivers to a direct input (+) of the amplifier of control 2523 a positive voltage of amplitude proportional to the level measured. An inverse input (-) of the amplifier 2521 receives a voltage representative of a maximum level Nmax. When the level measured exceeds the maximum level Nmax, the amplifier 2523 supplies a gain control input of the amplifier 2521 with a positive voltage which proportionally decreases the gain of the amplifier 2521.

According to another simpler and less expensive embodiment, each of the circuits 251 to 25N comprises only a level limiter 252, the noise attenuator 251 not being necessary for certain operating conditions.
Referring again to FIG. 2 in the circuit 2 the combiner 26 mixes the signals relating to the channels C21 to C20 delivered by the limiting circuits 251 to 25NI and supplies them to the frequency converter 23. The converter 23 performs a transposition of frequencies opposite to that carried out by the converter 22 For this purpose it also receives the local oscillator signal OL.

The power amplifier 21 receives from the converter 23 the signals relating to the channels C11 to C2N amplifies them and transmits them to the antenna As, via the filter 42 of the duplexer 4 for transmission to the station SP.

 The re-transmitter as described above may present a risk of looping from one channel to another through the duplexers 4 and 4a and by coupling antennas AS and AN. Since the amplifiers 11 and 21 and the duplexers 4 and 4a are assumed to have the same characteristics respectively, it is demonstrated that the following conditions must be satisfied in order to eliminate this risk of loopback: 2G - 2R - a - 3E <O and G + Fb + S <D, where G, R, a and E respectively denote the gain of the amplifiers 11 and 21, the resection of the duplexers 4 and 4a the attenuation provided by the filters 121 to 12N the filters 241 to 24N and the circuits 251 25N out-of-band and in the band and where Fb S and D respectively denote the noise factor of a channel, a safety margin and the decoupling between the antennas AS and AN.

 Advantageously for the radio coverage of a rail or road tunnel, the antenna AM ensuring the link with the mobiles can be a coaxial cable pierced with slots said radiating cable or a helix-type antenna providing a circular polarization of the airwave.

In the latter case an optimization of the location of the antenna and its orientation provides for tunnels having lengths of the order of a kilometer results comparable to the radiating target.

Claims (9)

 1 - Equipment for retransmitting radio links between a first station (SP!) And one or more second stations (M), said equipment comprising first and second antennas (AS, AM) for providing bidirectional transmission links between the equipment (RE ) and the first station (SP) and between the equipment (RE) and the second stations (M) respectively and means (44a) for coupling said first and second antennas through first and second unidirectional transmission paths in the first station. equipment respectively associated with first and second directions of transmission (S1 S2)  characterized in that it comprises  in the first lane  means (121 to 12N) for separately filtering first signals (C11 to C1N) transmitted by the first station (SP) respectively to the second stations (M) and means (13 11) for simultaneously mixing and amplifying the first signals filtered before being re-transmitted by the second antenna (AM) to the second stations (M),  and in the second way,  means (241 to 24N) for separately filtering second signals (C11 to C1N) transmitted respectively by the second stations (M) to the first station (SP) of the means (251 to 25N, 252) for limiting the amplitude each of the second filtered and separated signals at a predetermined maximum value and means (26, 21) for simultaneously mixing and amplifying the amplitude-limited second signals prior to being retransmitted by the first antenna (AS) to the first station (SP).  2 - Equipment according to claim 1, characterized in that it comprises in the second channel (S2), means (251 to 25N 251) for attenuating each of the second filtered and separated signals (C21 to C2N) as a function of the value of a signal / noise ratio (S / B) of said signal so that the low frequency components of said signal are all the more attenuated as the value of said ratio is low.  3 - Equipment according to claim 1 or 2 characterized in that it comprises in the first channel (S1) means (14) for transposing the transmission frequency band of the first signals (C11 to C1N) in a band lower frequencies before separation and filtering of said first signals and means (15) for transposing the low frequency band of the first filtered and mixed signals into the transmission band of said first signals before their amplification and retransmission.  4 - Equipment according to any one of claims 1 to 3 characterized in that it comprises in the second channel (S2) means (22) for transposing the transmission frequency band of the second signals (C21 to C2N) in one a lower frequency band before separation and filtering of said second signals and means (23) for transposing the low frequency band of the second filtered and amplitude limited signals and mixed in the transmission band of said second signals before their amplification and reissue.  5 - Equipment according to claims 3 and 4 characterized in that the two means for transposing (14 15) in the first channel (S1) and the two means for transposing (22 23) in the second channel (S2) are connected to a single local oscillator (OL) to perform frequency transpositions having the same amplitude.  6 - Equipment according to any one of claims 1; 5, characterized in that the second antenna (AM) is an omnidirectional antenna.  7 - Equipment according to any one of claims 1 to 5 characterized in that the second antenna (AN) is a directional antenna.  8 - Equipment according to any one of claims 1 to 7 characterized in that the first antenna (AS) is a directional antenna.  9 - Equipment according to any one of claims 1 to 8 characterized in that said second stations are mobile stations (M) such as vehicles.