Abstract:
The invention relates to a transmission or transmission/reception device having a transmission path structure which makes it possible to increase the range of variation of the transmission power without degrading the signal-to-noise ratio. The attenuation is achieved in part in an internal unit  5  and in part in an external unit  4  with the aid of a switching circuit  25  which selectably couples an antenna between an output or an input of an amplifier.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a transmission device of variable power. More particularly, the invention pertains to a transmission device having a very wide range of power variation. 
     Within the framework of wide-bandwidth wireless networks, it is known practice to use ever higher frequencies in order to allow the transmission of high bit rate applications such as video. These networks are also intended to allow data exchanges between at least one base station and a plurality of subscribers. These networks are commonly referred to as point-multipoint networks. 
     The point-multipoint type radio transmission systems are known to the person skilled in the art by the acronyms MMDS (standing for Microwave Multipoint Distribution System), LMDS (standing for Local Multipoint Distribution System) and MVDS (standing for Multipoint Video Distribution System). These systems used for the broadcasting of programmes permit a return path to the subscriber terminals which allows the subscriber to interact with the programme received. At present, the MPT-1560-RA standard provides for the use of frequencies situated between 40.5 and 42.5 GHz. 
     FIG. 1 illustrates an LMDS type system. A broadcasting station  1  furnished with a transmission/reception antenna  2  broadcasts information destined for a plurality of subscribers  3 . Each subscriber  3  has an external unit  4 , consisting of an antenna and means for transposing the signal received or the signal to be transmitted to an intermediate frequency, and an internal unit  5  which comprises means of channel selection in transmission and in reception as well as various means of coding/decoding for exchanging data with at least one user apparatus  6 , for example a television set, a telephone or a personal computer. 
     Transmissions in millimetre bands are strongly dependent on time, on atmospheric conditions, and on the distance D separating the subscriber  3  from the broadcasting station  1 . FIG. 2 represents the attenuation to a 40 GHz (giga Hertz) transmission as a function of distance. Curve  7  represents the attenuation in good weather and curve  8  represents the attenuation during rain. As may be observed by the person skilled in the art, the attenuation varies by around 50 dB as the distance varies from 50 m to 2 km. 
     Now, a base station is used for several subscribers and the subscribers must transmit with the least power so as not to saturate the reception of the base station and jam the other subscribers. It is therefore necessary to have an adjustment of the transmission power level for each subscriber. A dialogue between the broadcasting station and the device placed at the subscriber&#39;s home makes it possible to adjust the transmission power. 
     FIG. 3 represents an exemplary embodiment of the transmitter part of the device placed at the subscriber&#39;s home, according to the state of the art. The internal or first unit  5  comprises means of modulation  10  which receive a signal to be modulated. A mixer  11  is connected to the modulation means  10  so as to transpose the modulated signal into an intermediate frequency band, a voltage-controlled oscillator  12  delivering the transposition signal. The output of the mixer  11  is coupled to an amplification facility which comprises one or more amplifiers  13  and a variable-gain attenuation circuit  14 . A supervisory circuit  15  sends the various control signals to the modulation means  10  to the controlled oscillator  12  and to the attenuation circuit  14 . A coaxial cable is connected at the output of the amplification facility so as to transport the signal to the external or second unit  4 . The external unit  4  comprises a first filter  16  connected to the coaxial cable and to a mixer  17 . A local oscillator  18  delivers a transposition frequency to the mixer. A second filter  19  eliminates the image frequencies originating from the mixer  17 . An amplifier of SSPA type (standing for Solid State Power Amplifier) amplifies the signal before delivering it to an antenna  21 . 
     In the circuit of FIG. 3, the adjustment of the transmission gain is carried out in the internal unit  5  with the aid of the attenuation circuit  14 . One problem is that the adjustment of the power with the aid of the attenuator  14  alters the signal-to-noise ratio. It is not possible to have more than 35 dB of power variation if one wishes to comply with a signal-to-noise ratio of at least 25 dB. 
     SUMMARY OF THE INVENTION 
     The invention proposes a transmission path structure which makes it possible to increase the range of variation of the transmission power without degrading the signal-to-noise ratio. The attenuation is carried out in part in the internal unit and in part in the external unit by switching an amplifier. 
     In a first inventive arrangement a transmission device comprising: an internal unit which modulates data to be transmitted and transposes them into an intermediate frequency band and which comprises means of variable attenuation; an external unit which transposes at least one intermediate band signal into a transmission frequency band, the said external unit comprising at least one amplifier linked to an antenna; and a linking cable which links the external unit and the internal unit. The external unit comprises a switching circuit arrangement for coupling the amplifier output to the antenna or coupling the amplifier input to the antenna. 
     In a second inventive arrangement a transmission/reception device comprising: an internal unit which, on the one hand, modulates data to be transmitted and transposes them into an intermediate frequency band and which comprises means of variable attenuation, and, on the other hand, transposes and demodulates data received; an external unit which transposes at least one intermediate band signal into a transmission frequency band and at least one signal received in a reception band into the intermediate band, the external unit comprising at least one amplifier linked to an antenna for the transmission of data; a linking cable which links the external unit and the internal unit. The external unit comprises a switching circuit for selectably coupling or bypassing the amplifier to the antenna. 
     According to a particular embodiment, the internal unit comprises means for delivering a switching signal to the external unit and the external unit comprises means for receiving the switching signal and controlling the switching circuit. This embodiment permits the control of switching in the external unit by the internal unit. 
     Preferably, the means of variable attenuation make it possible to have a greater range of variation of attenuation than the amplification change achieved by the amplifier bypass switch. Thus, it is possible to switch the external unit between two signal bursts without impeding transmission. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood, and other features and advantages will become apparent on reading the description which follows, the description making reference to the appended drawings among which: 
     FIG. 1 represents a point-multipoint type transmission system, 
     FIG. 2 represents curves of signal attenuation versus distance, 
     FIG. 3 represents the circuits serving for the transmission of data of a subscriber terminal of the system of FIG. 1, according to the state of the art, 
     FIGS. 4 and 5 represent the circuits serving for transmission of data of a subscriber terminal of the system of FIG. 1, according to two embodiments of the invention, and 
     FIG. 6 represents an example of integrating the transmission circuits and reception circuits of a subscriber terminal of the system of FIG. 1, according to the invention. 
    
    
     DETAILED DESCRIPTION 
     FIGS. 1 to  3  described above will not be detailed subsequently. To improve the understanding of the person skilled in the art, the same references are used for similar elements catering for the same function. 
     FIG. 4 represents an exemplary embodiment of the transmitter part of the device placed at the subscribers home, according to the invention. The internal unit  5  comprises modulation means  10  which receives a modulating signal for transmission from a device (not shown). A mixer  11  is connected to the modulation means  10  so as to transpose the modulated signal into an intermediate frequency band, a voltage-controlled oscillator  12  delivering the transposition signal. The frequency delivered by the oscillator  12  is determined by a supervisory circuit  15  as a function of the channel used in the intermediate frequency band. The intermediate frequency band lies for example between 0.4 and 0.7 GHz and is for example divided into channels of 2 MHz width. The output of the mixer  11  is coupled to an amplification facility which comprises one or more amplifiers  13  and a variable-gain attenuation circuit  14 . The supervisory circuit  15  sends the various control signals to the modulation means  10  to the controlled oscillator  12  and to the attenuation circuit  14 . The supervisory circuit is for example a microcontroller type circuit. A coaxial cable is connected at the output of the amplification facility so as to transport the signal to the external unit  4 . 
     The external unit  4  comprises a first filter  16  connected to the coaxial cable and to a mixer  17 . A local oscillator  18  delivers a transposition frequency to the mixer, for example of 41.8 GHz, so as to obtain a transmission band lying between 42.2 and 42.5 GHz. A second filter  19  eliminates the image frequencies originating from the mixer  17 . An SSPA (standing for Solid State Power Amplifier) type amplifier  20  amplifies the signal before delivering it to an antenna  21 . A switching circuit  25  is connected between the amplifier  20  and the antenna  21  in such a way that as a function of a command the switching circuit effects a connection between the antenna  21  and the output of the amplifier  20  or between the antenna  21  and the input of the amplifier  20 . 
     The switching circuit  25  makes it possible to utilize an exemplary extra gain of 20 dB from amplifier  20  or to bypass the amplifier and provide a lower signal level to antenna  21 . Amplifying or bypassing the signal before transmitting it via the antenna  21  does not affect the signal-to-noise ratio. In this first example, switching circuit  25  is for example a manual switch having a position determined as a function of the distance between the broadcasting station  1  and the location of the subscriber device. 
     The variable-gain attenuation circuit  14  makes it possible to have a range of variation of for example 35 dB in the transmission power and the amplifier  20  makes it possible to shift this range of variation by 20 dB. The system has an overall range of variation of 55 dB formed by two sub-ranges of 35 dB which overlap over 15 dB. Referring to FIG. 2, it may be appreciated that the system operates very well in a fixed position with a manual switch which is correctly positioned during installation. 
     However, if during installation the switch is poorly positioned, or a severe climatic variation occurs, proper operation may be impaired. Likewise, if the environment between the subscriber and the broadcasting station changes appreciably, and if the subscriber is placed at a critical distance, it is possible to have to change the positioning of the switch of the external unit which is not necessarily accessible. Furthermore, a device becomes unusable when employed from a non-stationary location, for example in an outside-broadcasting van. 
     A second embodiment makes it possible to remedy all these drawbacks by controlling the switching circuit with the aid of the supervisory circuit of the internal unit  5 . The second embodiment is represented in FIG.  5 . In this FIG. 5, the internal unit  5  comprises a switched oscillator  30  which generates a switching signal of predefined frequency, for example 1 MHz on the coaxial cable. The oscillator being switched or otherwise controlled as a function of a signal originating from the supervisory circuit  15 . The presence or the absence of the sinusoidal switching signal indicates to the external unit whether the amplifier  20  is to be in circuit or is to be bypassed. The supervisory circuit  15  determines as a function of the requirement whether or not amplifier  20  is in circuit or is bypassed. The presence or the absence of the sinusoidal signal indicates to the external unit whether the amplifier  20  is or is not to be short-circuited. The supervisory circuit  15  determines as a function of the requirement whether or not it is necessary to short-circuit the amplifier  20 . 
     The external unit  4  is modified so as to be able to receive the sinusoidal signal and transform it into a command for the switching circuit  25 . The switching circuit  25  is, for example, a PIN-diode switch which receives a control signal. 
     The external unit  4  comprises a third filter  26 , of band-pass type, centred on the frequency of 1 MHz and connected to the coaxial cable. This filter makes it possible to isolate the sinusoidal switching signal which serves as command. A rectifying circuit  27 , for example a diode, is connected at the output of the third filter  26 . A fourth filter  28 , of low-pass type, is connected to the output of the rectifying circuit  27   50  as to recover the DC component of the rectified signal. A shaping circuit  29  is connected to the output of the fourth filter  28  and is, for example, a comparator which compares the rectified DC voltage which exits the fourth filter with a predefined threshold. The output signal from the shaping circuit  29  is the control signal for the switching circuit  25 . 
     With such a system, the supervisory circuit  15  of the internal unit has a range of variation of 55 dB. However, in order to preclude any transmission problem, the switching of the amplifier  20  is carried out, preferably, between two bursts of data. 
     Other control possibilities can be envisaged. It is, for example, possible to use a dedicated conductor for control but this has the effect of being more expensive in terms of electrical wire. 
     Other possibilities of implementation can be envisaged. By way of example, FIG. 6 represents an embodiment in which one and the same antenna is used for transmission and reception. 
     In addition to the elements described above, the internal unit  5  comprises separation filters  100 A, B, C, which are band-pass filters intended for separating the signals sent to the external unit  4  and the signals received from the external unit  4 . The separation filters cater for the frequency multiplexing on the coaxial cable for the internal unit. A first of the separation filters  100 A is connected between the amplifier  13  and the coaxial cable so as to allow through the band lying between 0.4 and 0.7 GHz. A second separation filter  100 B is connected between the switched oscillator  30  and the coaxial cable so as to allow through a narrow band centred on 1 MHz. A third separation filter  100 C is connected to the coaxial cable so as to receive and allow through an intermediate frequency band for reception lying between 0.95 and 1.95 GHz. 
     An amplifier  101  is connected to the third separation filter  100 C. A mixer  102  carries out a transposition of the amplified signal into a baseband, a voltage-controlled oscillator  103  delivering the transposition signal. The frequency delivered by the oscillator  12  is determined by a supervisory circuit  15  as a function of the channel used in the intermediate frequency band for reception. The intermediate frequency band for reception is for example divided into channels of 33 MHz width. A filter  104  connected to the output of the mixer  102  eliminates the image frequencies. A demodulation circuit  105  transforms the signal leaving the filter  104  into a data stream. 
     The external unit  4  comprises the elements described above for the transmission facility and for the control of the switching circuit  25 . An extra filter  110  is added between the antenna  21  and the switching circuit  25  so as to reduce the coupling between transmission and reception. An extra low-noise amplifier  111  can be added into the transmission facility if it is necessary to have an amplification when the amplifier  20  is short-circuited. The amplifier  111  may be placed at various points in the transmission facility. 
     The external unit  4  also includes a reception facility. The reception facility comprises a first filter  112  connected to the antenna  21 . The first filter  112  is of band-pass type with strong rejection so as to allow through for example the band lying between 40.5 and 41.5 GHz. A low-noise amplifier  113  amplifies the filtered signal. A local oscillator  114 -delivers a signal at a transposition frequency, for example equal to 40.2 GHz. A mixer  115  performs the transposition of the amplified signal with the aid of the signal delivered by the local oscillator  114  so that the useful signal is situated in the intermediate frequency band lying for example between 0.95 and 1.95 GHz. A second filter  116  is placed between the output of the mixer and the coaxial cable so as to remove the image frequencies. 
     Numerous other possibilities of implementation are possible. In particular, the frequencies indicated are done so purely by way of indication. The person skilled in the art may also use other architectures for the transmission and reception facilities, in particular architectures with common oscillator between transmission and reception.