A method for data transmission using an envelope elimination and restoration amplifier, an envelope elimination and restoration amplifier, a transmitting device, a receiving device, and a communication network therefor The invention relates to a method for transmission of data signals according to the preamble of claim 1, an envelope elimination and restoration amplifier according to the preamble of claim 8, a transmitting device according to the preamble of claim 10, a receiving device according to the preamble of claim 13, and a communication network according to the preamble of claim 15. The coverage of a certain sex-vice area in a cellular radio network is provided by several radio base stations, which are connected to a core network to serve connections to and from mobile users within the service area. A radio base station contains a baseband unit and at least one antenna unit. In order to increase radio coverage and capacity, modern base stations use several sector antennas. In order to increase flexibility of the base stations, it is desirable to allow the antennas to be located remote from the baseband unit. This has lead to the development of active antenna systems which are also termed remote antenna heads. Typically, one remote antenna head contains one sector antenna, but there are also systems known, which have remote antenna heads with more than only one sector antenna. The base stations are preferably connected with the remote antenna heads by means of optical fibers. Conventional radio-over-fiber scenarios involve optical transmission of analogue signals between a base station and a transmitter with an amplifier in a remote antenna head. An example for the implementation of a radio-over-fiber concept in a cellular radio network using a two-fiber-ring is given in the European patent EP 1553791 Bl. However, the quality of the optical transmission suffers severely from noise, non-linearities, like e.g. chromatic dispersion, and attenuation effects. Consequently the technical implementations for radio-over-fiber concepts must involve highly sophisticated optical modulation techniques and signal conditioning. Basically, the optical transmission of analogue radio frequency signals involves high electronic efforts for signal modulation techniques and signal conditioning. Indeed, e.g. the method of intensity modulation and direct detection is straightforward, simple and allows for fairly linear optical transmission properties, but on the other hand it requires costly modulators and modulator drivers in order to meet the requirements for analogue radio frequency transmission. In case double side band modulation is used, chromatic dispersion will result in frequency and length dependent suppression of radio frequency power, which will deteriorate the transmission quality. The object of the invention is thus to propose a cost-effective and fault-tolerant method for transmission of signals from a transmitting device to at least one receiving device using an energy efficient power amplifier architecture for signal amplification with an optical connection between the transmitting device and the at least one receiving device. This object is achieved by a method according to the teaching of claim 1, an envelope elimination and restoration amplifier according to the teaching of claim 8, a transmitting device according to the teaching of claim 10, a receiving device according to the teaching of claim 13, and a communication network according to the teaching of claim 15. As mobile communication systems like e.g. Universal Mobile Telecommunications System (UMTS), Worldwide Interoperability for Microwave Access (WIMAX), or Third Generation Partnership Project Long Term Evolution (3GPP LTE) require power amplifiers with high output power at frequencies up to 2.6 GHz, preferably so-called envelope elimination and restoration amplifiers are used for signal amplification, as they feature a high degree of linearity and efficiency. In said envelope elimination and restoration amplifiers, the data signal is represented by an envelope signal component of relatively slowly varying amplitude, and a faster varying phase signal component of constant amplitude. The fast phase signal controls the input of e.g. a class E or F output stage whereas the envelope signal drives a modulator which again controls the supply voltage of said output stage. The envelope signal components or the phase signal components can be digital signals used for amplification e.g. in switched output stages. In the US patent application US 2 002/0141510 Al,' a modulator and a method of modulating is disclosed which utilizes phase or frequency modulation and amplitude modulation. A delay circuit or synchronization circuit is utilized to coordinate the performance of amplitude modulation and phase modulation. The modulation circuit can be utilized in any frequency range including high frequency and low frequency circuits. The main idea of the invention is to exploit the existence of a digital signal path inside the existing envelope elimination and restoration amplifier concept for optical transmission of data signals by dividing the envelope elimination and restoration amplifier into two remote parts connected by at least one optical connection, as digital signals are more fault-tolerant against interferences than analogue signals. The invention enables transmission of digital instead of analogue signals by drawing use from a digital signal path which is already existing inside envelope elimination and restoration amplifiers. As a digital signal which is already available within the envelope elimination and restoration amplifiers is used, there is no need for additional costly high speed analogue-to-digital and digital-to-analogue conversion. Thus, compared with conventional radio-over-fiber scenarios, at zero additional costs all the concepts for optical transmission of digital signals can be applied to radio-over-fiber applications. An envelope elimination and restoration amplifier for signal amplification is used for transmission of signals from a base station via at least one remote antenna head to a user terminal, and the signals are transmitted over at least one optical connection from the base station to the at least one remote antenna head. The invention allows for realization of superior radio-over-fiber architecture by dividing the existing envelope elimination and restoration amplifier described above into two parts separated by digital optical links and placing them in a central base station and close to a remote antenna unit respectively. This concept enables new architectures which easily reduce costs and hardware effort, e.g. in beamforming and multiple input multiple output (MIMO) applications. According to the invention, data signals are represented by envelope signal components and phase signal components in a first part of the envelope elimination and restoration amplifier located in a transmitting device, at least one of the group of envelope signal components and phase signal components are converted from electrical signals into optical signals in at least one electro-optical converter located in the transmitting device, the at least one of the group of envelope signal components and phase signal components are transmitted over at least one optical connection from the transmitting device to the at least one receiving device, the at least one of the group of envelope signal components and phase signal components are converted from optical signals into electrical signals in at least one opto-electrical converter located in said at least one receiving device, and said data signals are amplified by an output stage in a second part of the envelope elimination and restoration amplifier that is located in said at least one re--ceiving device. Further developments of the invention can be gathered from the dependent claims and the following description. In the following the invention will be explained further making reference to the attached drawings. Fig. 1 schematically shows a cellular communication network with a base station and remote antenna heads in which the invention can be implemented. Fig. 2 schematically shows an envelope elimination and restoration amplifier architecture according to the state-of-the-art . Fig. 3 schematically shows a switched mode power amplifier architecture relying on a voltage switched circuit topology according to the state-of-the-art. Fig. 4 schematically shows a delta sigma modulator according to the state-of-the-art. Fig. 5 schematically shows a switched mode power amplifier with a delta sigma modulator, a switched output stage and a filter according to the state-of-the-art. Fig. 6 schematically shows a switched mode power amplifier that is divided into two parts located in a base station and in a remote antenna head respectively according to the invention. Fig. 7 schematically shows a transmitter and a receiver comprising a distributed envelope elimination and restoration amplifier according to the invention. Fig. 8 schematically shows a transmitter and a receiver comprising a distributed envelope elimination and restoration amplifier interconnected by a single optical path applying the principles of optical multiplexing and demultiplexing according to the invention. The principle structure of a communication network CN for signal transmission and reception in which the invention can be implemented is shown in fig. 1. The communication network CN comprises a base station BS, remote antenna heads RAH1-RAH4 and user terminals UE1-UE4. Each of said remote antenna heads RAH1-RAH4 is connected to the base station BS by means of an optical connection, as e.g. an optical fiber or an optical free-space connection, 0F1, 0F2, 0F11, 0F12 and 0F13 respectively. Each of said user terminals UE1-UE4 is connected to one or multiple of said remote antenna heads RAH1-RAH4, which is symbolized by double arrows in fig. 1. The base station BS is in turn connected to a core network, which is not shown in fig. 1 for the sake of simplicity. For amplification of signals that shall be transmitted from the base station BS via a remote antenna head RAH1-RAH4 to a user terminal UE1-UE4, preferably envelope elimination and restoration amplifiers are used that are preferably located in the remote antenna heads RAH1-RAH4 according to the state-of-the-art . Fig. 2 schematically shows an envelope elimination and restoration amplifier according to the state-of-the-art. An input for radio frequency (RF) signals of the envelope elimination and restoration amplifier is connected both to an input of an amplitude detector DET and to an input of a limiter LIM. An output of the amplitude detector DET is connected to a first input of an amplitude amplifier AAM. A second input of the amplitude amplifier AAM is provided to connect a supply-voltage VS. An output of the limiter LIM is connected to a first input of a phase amplifier PAM, and an output of the amplitude amplifier AAM is connected to a second input, the supply voltage input, of the phase amplifier PAM. An output of the phase amplifier PAM is provided to output an amplified phase and amplitude modulated signal. In a method for signal amplification using an envelope elimination and restoration amplifier according to the state-of-the-art as shown in fig. 2, an analogue radio frequency input signal is sent to the input of the envelope elimination and restoration amplifier. An envelope signal component of the input signal is sensed using e.g. a coupler and the amplitude detector DET, and the envelope signal component is provided to the amplitude amplifier AAM. The main amplitude and phase modulated radio frequency input signal is sent to the input of the limiter LIM, in which a complex input signal A(t)ei(ωt+ Φ(t)), with A(t) being the time-dependent amplitude, a being the carrier frequency and Φ (t) being the time-dependent phase, is clipped resulting in a phase signal component of constant amplitude ei(ωt+ Φ(t)). The envelope signal component is sensed by the amplitude detector DET sensing the signal before the limiter LIM. By the amplitude detector DET, the varying phase signal is eliminated resulting in an envelope signal component of slowly varying amplitude A (t). The envelope signal component of slowly varying amplitude is sent to the first input of the amplitude amplifier AAM. The supply voltage VS, which is connected to the second input of the amplitude amplifier AAM, is modulated with the envelope signal component of slowly varying amplitude resulting in an amplified envelope signal component. The phase signal component of constant amplitude is sent to the first input, i.e. the radio frequency input, of the phase amplifier PAM, and the amplified envelope signal component is connected as supply voltage to the second input, i.e. the supply voltage input, of the phase amplifier PAM, which results in an amplified copy of the analogue radio frequency input signal at the output of the phase amplifier PAM. According to the invention, the existence of a digital signal path inside an envelope elimination and restoration amplifier is used for optical transmission of signals. Such a digital signal path can be implemented in an envelope elimination and restoration amplifier as shown in fig. 2 e.g. by a delta sigma modulator and a subsequent switched output stage in the path of the envelope signal component of slowly varying amplitude. To perform this implementation of a digital signal path, the output of the amplitude detector DET is connected to the input of a delta sigma modulator, and the output of the delta sigma modulator is connected to the input of the amplitude amplifier AAM, which is a switched output stage, as e.g. a class D amplifier in this case. In the following, the basic principle of a switched mode amplifier using a delta sigma modulator is described, and subsequently the application of a switched mode amplifier with a delta sigma modulator in two embodiments of the invention is depicted. As an example for a switched mode power amplifier, a voltage switched power amplifier system according to the state-of-the-art is shown in fig. 3. Such a voltage switched power amplifier system comprises a delta sigma modulator DSM with inputs for reception of an analogue radio frequency input signal, or an envelope signal component according to the invention, and for reception of a clocking signal CS. An output of the delta sigma modulator DSM is connected to an input of a driver DR. Preferably, the delta sigma modulator DSM is connected to a noise-shaping filter NF or comprises a noise shaping filter. A first output of the driver DR is connected to the gate G of a first transistor Tl, and a second output of the driver DR is connected to the gate G of a second transistor T2. The source S of the first transistor Tl is connected to ground, and the source of the second transistor T2 is connected to the drain of the first transistor Tl. The drain of the first transistor Tl and the source of the second transistor T2 are connected to an output for radio frequency signals, or amplified analogue envelope signal components according to the invention, via a reconstruction filter RFILT that comprises an inductor L and a capacitor C in series. There are variants of the L-C filter topology which however are of no importance for the invention as disclosed. The drain of the second transistor T2 is connected to the supply of a constant voltage source. In a method for signal amplification using a voltage switched power amplifier system according to the state-of-the-art as shown in fig. 3, analogue radio frequency input signals, or envelope signal components according to the invention, are sent to the delta sigma modulator DSM. Furthermore, clocking signals with a multiple of the radio frequency carrier frequency are sent to the delta sigma modulator DSM. In the delta sigma modulator DSM, the analogue radio frequency input signals, or envelope signal components, are converted into digital 1-bit or higher resolution signals. The sampling rate is determined by the received clocking signals. The digital 1-bit signals are provided at the output of the delta sigma modulator DSM. Preferably, the noise shaping filter NF is used to minimize quantization error by means of shifting quantization noise into frequency ranges that are less or not relevant for signal processing. Said digital 1-bit signals are sent to the driver DR that generates first driver signals based on the digital 1-bit signals and second driver signals based on the inverted digital 1-bit signals. The first driver signals are sent to the gate of the second transistor T2, and the second driver signals are sent to the gate of the first transistor Tl. The output signals of the driver DR are thus in antiphase which means if the first transistor Tl is on, the second transistor T2 is off and vice versa. The described amplifier architecture with two transistors Tl, T2 is just an example, and in alternative architectures, only one, as e.g. in Class J amplifiers, or more than two transistors are used, which has however no influence on the invention. In the latter case, such alternative architectures are e.g. multibit architectures, using two transistors more per each bit more. The capacitor C and the inductor L together build a reconstruction filter RFILT used to generate smooth analogue output signals that are provided at the output for radio frequency signals, or amplified analogue envelope signal components. Fig. 4 is illustrating a delta sigma modulator DSM according to the state-of-the-art. The delta sigma modulator DSM comprises a filter Fl, a summer SUM, a noise shaping filter F2, an analog-to-digital converter AD and a digital-to-analog converter DA. The filter Fl has an input for receiving analogue input signals.. The output of the filter Fl is connected to a first input of the summer SUM. The output of the summer SUM is connected to the input of the noise shaping filter F2, and the output of the noise shaping filter F2 is connected to the input of the analog-to-digital converter AD. The output of the analog-to-digital converter AD is on the one hand connected to the input of the digital-to-analog converter DA, and can on the other hand be connected to an external device, as e.g. the driver D of the switched output stage in fig. 3, for transmitting digital output signals. The output of the digital-to-analog converter DA is connected to a second inverting input of the summer SUM. In principle, an analogue radio frequency input signal, or envelope signal component, is encoded in the delta sigma modulator DSM into a two level digital output sequence that is appropriate for driving a switched output stage of a switched mode amplifier. Fig. 5 is illustrating in the upper row schematically a switched mode power amplifier according to the state of the art that comprises a delta sigma modulator DSM as depicted in fig. 4, a switched output stage SOS, and a filter F. The switched output stage SOS comprises e.g. the driver DR and the two transistors Tl and T2 as depicted in fig. 3. However, as the described amplifier architecture with one driver DR and two transistors Tl, T2 in fig. 3 is just an example, in alternative architectures, only one transistor or more than two transistors with more than one driver are used, i.e. the switched output stage SOS can comprise an arbitrary number of drivers and transistors, which has however no influence on the invention. The delta sigma modulator DSM has an input for receiving input signals. The output of the delta sigma modulator DSM is connected to the input of the switched output stage SOS. The output of the switched output stage SOS is connected to the input of the filter F, and the filter F comprises an output for transmitting output signals. In the middle row, 4 diagrams are showing the signal voltage in volts versus time in nanoseconds from left to right for signals at the input of the delta sigma modulator DSM, at the output of the delta sigma modulator DSM, at the output of the switched output stage SOS, and at the output of the filter F. In the lower row, 4 diagrams are showing the signal power density spectrum in decibels versus frequency in megahertz from left to right for signals at the input of the delta sigma modulator DSM, at the output of the delta sigma modulator DSM, at the output of the switched output stage SOS, and at the output of the filter F. As can be seen from the first two diagrams in the middle row, the digital signal modulator converts analogue signals into digital signals, and according to the invention, said digital signals will be transmitted over an optical fiber to the switched output stage SOS that is remotely located in a remote antenna head. The functionality of the switched mode power amplifier depicted in fig. 5 is described above using radio frequency signals at the input. In the following, the invention will be described for input signals with lower frequencies of e.g. an intermediate or baseband frequency. However, the used frequency at the input has no major influence on the invention. In case of applying the described class S principle to the optically controlled envelope elimination and restoration amplifier concept according to the invention, the delta sigma modulator DSM and the filter F for reconstruction are preferably of low pass type. A switched mode power amplifier PA that can be used in an envelope elimination and restoration amplifier according to the invention is depicted in fig. 6. The switched mode power amplifier PA is indicated as a dashed box and comprises a delta sigma modulator DSM, an electro-optical converter EO, an opto-electrical converter OE, a switched output stage SOS and a filter F. The switched output stage in turn comprises at least one driver and at least one transistor as described above under fig. 5. The delta sigma modulator DSM has an input for receiving analogue input signals. An output of the delta sigma modulator DSM is connected to an input of the electro-optical converter EO. An output of the electro-optical converter EO is connected to an input of the opto-electrical converter OE through an optical connection 0F1, as e.g. an optical fiber or an optical free-space connection. An output of the opto-electrical converter OE is connected to an input of the switched output stage SOS, and an output of the switched output stage SOS is connected to an input of the filter F. In an embodiment of the invention, a further device for signal conditioning, as e.g. a filter, an equalizer or a preamplifier, is comprised in the signal path between the electro-optical converter EO and the opto-electrical converter OE, or in the signal path between the opto-electrical converter OE and the switched output stage SOS. An output of the filter F in turn -is connected to an antenna network via a further output stage, which is not depicted in fig. 6 for the sake of simplicity. In the embodiment depicted in fig. 6, the delta sigma modulator DSM and the electro-optical converter EO are comprised in a base station BS, which is indicated as a box, and the opto-electrical converter OE, the switched output stage SOS and the filter F are comprised in a remote antenna head RAH1, which is also indicated as a box. For implementation of the class S principle into the optically controlled envelope elimination and restoration amplifier, in the delta sigma modulator DSM, analogue signals, as e.g. analogue envelope signal components, that the delta sigma modulator DSM receives at its input are converted into digital signals. Said digital signals are sent to the electro-optical converter EO for converting the digital electrical signals into digital optical signals. Preferably, said electro-optical converter EO comprises a laser diode which is either directly modulated or externally modulated e.g. by means of an elec-troabsorption or lithiumniobate modulator. From the output of the electro-optical converter EO, the digital optical signals, as e.g. PWM signals (PWM = Pulse Width Modulation), are sent over the optical connection 0F1, as e.g. an optical fiber or an optical free-space connection, to an input of the opto-electrical converter OE. In the opto-electrical converter OE, the digital optical signals are back-converted into digital electrical signals. Preferably, said opto-electrical converter OE comprises a so-called PIN-diode or a so-called avalanche-photo-diode. The digital electrical signals are sent from an output of the opto-electrical converter OE to an input of the switched output stage SOS. In the switched output stage SOS, the digital electrical signals drive at least one transistor via at least one driver, which leads to amplified digital electrical signals at the output of the switched output stage SOS. Said amplified digital electrical signals are sent to the input of the filter F, and by means of said filter F, the amplified analogue envelope signal components are reconstructed and sent via the output of the filter F to an input of a further output stage as e.g. the second input of the phase amplifier PAM as shown in fig. 2. An embodiment of an envelope elimination and restoration amplifier EER1 according to the invention which applies the principle of optical heterodyning is depicted in fig. 7. The envelope elimination and restoration amplifier EER1 is indicated as a dashed box and comprises an amplitude detector DET, a delta sigma modulator DSM, a limiter LIM, a first analogue-to-digital converter AD1, a carrier synthesizer CS, three electro-optical converters E01, E02, and E03, an optical adder A, two opto-electrical converters 0E1 and 0E2, a phase signal re-synthesizer PSRS, a switched output stage SOS, an output stage OS and a filter F. The switched output stage SOS in turn comprises at least one driver and at least one transistor as described above under fig. 5. In the embodiment depicted in fig. 7, the amplitude detector DET, the delta sigma modulator DSM, the limiter LIM, the first analogue-to-digital converter AD1, the carrier synthesizer CS, the three electro-optical converters E01, E02, and E03, and the optical adder A are comprised in a base station BS, which is indicated as a box, and the two opto-electrical converters 0E1 and 0E2, the phase signal re-synthesizer PSRS, the switched output stage SOS, the output stage OS and the filter F are comprised in a remote antenna head RAH1, which is also indicated as a box. The base station BS further comprises a feedback path with a third opto-electrical converter 0E3 and a receiver RX. The remote antenna head RAH1 further comprises an antenna network AN, a low noise amplifier LNA, a down converter DC, a second analogue to digital converter AD2 and a fourth electro-optical converter E04. An input for e.g. baseband frequency or intermediate frequency (IF) signals of the envelope elimination and restoration amplifier EER1 is connected both to an input of the amplitude detector DET and to an input of the limiter LIM. An output of the amplitude detector DET is connected to an input of the delta sigma modulator DSM, and an output of the delta sigma modulator DSM is connected to an input of the first electro-optical converter E01. An output of the first electro-optical converter E01 is connected to an input of the first opto-electrical converter 0E1 through an optical connection OF1, as e.g. an optical fiber or an optical free-space connection, and an output of the first opto-electrical converter 0E1 is connected to a first input of the switched output stage SOS. A second input of the switched output stage SOS is provided to connect a supply voltage VS, and an output of the switched output stage SOS is connected to an input of the filter F. An output of the limiter LIM is connected to an input of the first analogue-to-digital converter AD1, and an output of the first analogue-to-digital converter AD1 is connected to an input of the second electro-optical converter E02. An output of the second electro-optical converter E02 is connected to a first input of the optical adder A through an optical connection 0F3. An output of the carrier synthesizer CS is connected to an input of the third electro-optical converter E03, and an output of the third electro-optical converter E03 is connected to a second input of the optical adder A through an optical connection 0F4 . The output of the optical adder A is connected to an input of the second opto-electrical converter 0E2 through an optical connection 0F2, as e.g. an optical fiber or an optical free-space connection. An output of the second opto-electrical converter 0E2 is connected to an input of the phase signal re-synthesizer PSRS, and an output of the phase signal re-synthesizer PSRS is connected to a first input of the output stage OS. An output of the filter F is connected to a second input of the output stage OS, and an output of the output stage OS is in turn connected to the antenna network AN. In an embodiment of the invention, a further device for signal conditioning, as e.g. an equalizer or a pre-amplifier, is comprised in the signal paths between the electro-optical converters E01, E02 and E03 and the opto-electrical converters 0E1 and 0E2 respectively, or in the signal paths between the opto-electrical converters 0E1 and OE2 and the switched output stage SOS and the output stage OS respectively. In a reception path, an output of the antenna network AN is connected to an input of the low noise amplifier LNA, and an output of the low noise amplifier LNA is connected to an input of the down converter DC. An output of the down converter DC is connected to an input of the second analogue-to-digital converter AD2, and an output of the second analogue-to-digital converter AD2 is connected to an input of the fourth electro-optical converter E04. An output of the fourth electro-optical converter E04 is connected to an input of the third opto-electrical converter 0E3 through an optical connection OF5 as e.g. an optical fiber or an optical free-space connection. The output of the third opto-electrical converter 0E3 is in turn connected to an input of the receiver RX. In an embodiment of the invention, an output of the envelope elimination and restoration amplifier EER1 is connected to the reception path, preferably at an input of the fourth electro-optical converter E04, which is indicated by a dotted arrow in fig. 7. Preferably, said electro-optical converters E01-E04 each comprise a laser diode which is either directly modulated or externally modulated e.g. by means of a electroabsorption or lithiumniobate modulator. Preferably, said opto-electrical converters 0E1-0E3 each comprise a so-called PIN-diode or a so-called avalanche-photo-diode . In the embodiment depicted in fig. 7, an analogue data signal, e.g. on a baseband frequency fbb, preferably in the frequency range 0-200 MHz is sent to the input of the envelope elimination and restoration amplifier EER1. A main part of the data signal is sent to the input of the limiter LIM, in which the data signal A(t)exp(i(ωbbt+ Φ(t))) , with A(t) being the time-dependent amplitude,