A method for data transmission using a LINC amplifier, a LINC amplifier, a transmitting device, a receiving device, and a communication network therefor

0001The invention relates to a method for transmission of a data signal according to the preamble of claim 1, a LINC 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.

0002The coverage of a certain service 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.

0003In 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.

0004The 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.

0005An 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 <patcit id="pcit0001" dnum="EP1553791B1"><text>EP 1553791 B1</text></patcit>.

0006However, 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.

0007Basically, 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.

0008The 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 a power amplifier for signal amplification with an optical connection between the transmitting device and the at least one receiving device.

0009This object is achieved by a method according to the teaching of claim 1, a LINC 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.

0010As 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 LINC amplifiers (LINC = linear amplification by nonlinear components) are used for signal amplification, as they feature a high degree of linearity and efficiency.

0011In said LINC amplifiers, the input data signal is split into two constant envelope phase modulated signal components. Subsequently each signal is amplified by a power amplifier. The constant envelope signals allow for designing both power amplifiers optimally for high efficiency. After amplification both signals are recombined by means of a suited combiner, as e.g. a Chireix or Wilkinson combiner, in order to produce an amplified replica of the original amplitude and phase modulated input data signal.

0012The main idea of the invention is the realization of a superior radio-over-fiber architecture by dividing the LINC amplifier into two remote parts connected by at least one optical connection. As digital signals are more fault-tolerant against interferences than analogue signals, preferably digital optical signals are transmitted over the at least one optical connection.

0013According to the invention, a data signal is represented by two phase modulated signal components of constant amplitude in a first part of a LINC amplifier located in a transmitting device, at least one of the two phase modulated signal components of constant amplitude is converted from an electrical signal into an optical signal in at least one electro-optical converter located in the transmitting device, the at least one of the two phase modulated signal components of constant amplitude is transmitted over at least one optical connection from the transmitting device to at least one receiving device, the at least one of the two phase modulated signal components of constant amplitude is converted from an optical signal into an electrical signal in at least one opto-electrical converter located in said at least one receiving device, and the two phase modulated signal components of constant amplitude are amplified and combined in a second part of the LINC amplifier that is located in said at least one receiving device.

0014For transmission of signals from a base station via at least one remote antenna head to a user terminal, a LINC amplifier for signal amplification is used, and the signals are transmitted over said at least one optical connection from the base station to the at least one remote antenna head.

0015This concept enables new architectures which easily reduce costs and hardware effort, and moreover it allows for easy scaling e.g. in beamforming and MIMO applications.

0016In order to enable transmission of digital optical signals over the at least one optical connection, at least one of the constant envelope phase modulated signal components needs to be converted to digital electrical domain before passing them to at least one suitable electro-optical converter. This is facilitated by the constant amplitude of the two constant envelope phase modulated signal components.

0017Further developments of the invention can be gathered from the dependent claims and the following description.

0018In the following the invention will be explained further making reference to the attached drawings. <ul id="ul0001" list-style="none"><li><figref idref="f0001">Fig. 1</figref> schematically shows a cellular communication network with a base station and remote antenna heads in which the invention can be implemented.</li><li><figref idref="f0002">Fig. 2</figref> schematically shows a LINC amplifier architecture according to the state-of-the-art.</li><li><figref idref="f0003">Fig. 3</figref> schematically shows a transmitter and a receiver with a distributed LINC amplifier according to the invention.</li><li><figref idref="f0004">Fig. 4</figref> schematically shows a transmitter and a receiver with a distributed LINC amplifier applying the principles of optical multiplexing and demultiplexing according to the invention.</li><li><figref idref="f0005">Fig. 5</figref> schematically shows a transmitter and a receiver with two distributed LINC amplifiers and two antennas for demonstration of scaling according to the invention.</li><li><figref idref="f0006">Fig. 6</figref> schematically shows a transmitter and a receiver with two distributed LINC amplifiers and two antennas applying the principles of optical multiplexing and demultiplexing for demonstration of scaling according to the invention.</li></ul>

0019The principle structure of a communication network CN for signal transmission and reception in which the invention can be implemented is shown in <figref idref="f0001">fig. 1</figref>. The communication network CN comprises a base station BS, remote antenna heads RAHl-RAH4 and user terminals UE1-UE4.

0020Each 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, OF1, OF2, OF8, OF9 and OF10 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 <figref idref="f0001">fig. 1</figref>. The base station BS is in turn connected to a core network, which is not shown in <figref idref="f0001">fig. 1</figref> for the sake of simplicity.

0021For 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, LINC amplifiers according to the state-of-the-art that are located in the remote antenna heads RAH1-RAH4 can be used.

0022<figref idref="f0002">Fig. 2</figref> schematically shows a LINC amplifier according to the state-of-the-art.

0023An input of the LINC amplifier, e.g. for input of baseband, intermediate frequency or radio frequency (RF) signals Vin(t), is connected to an input of a signal separator SIS.

0024A first output of the signal separator SIS is connected to an input of a first output stage OS1. A second output of the signal separator SIS is connected to an input of a second output stage OS2.

0025An output of the first output stage OS1 is connected to a first input of a combiner C, and an output of the second output stage OS2 is connected to a second input of the combiner C.

0026An output of the combiner C is provided to output an amplified signal Vout(t).

0027In a method for signal amplification using a LINC amplifier according to the state-of-the-art as shown in <figref idref="f0002">fig. 2</figref>, an analogue input signal Vin(t) is sent to the input of the LINC amplifier and thus to the input of the signal separator SIS. In the signal separator SIS, the input signal Vin(t) is split into two phase modulated signal components of constant amplitude. In other words, the amplitude modulation of the input signal Vin(t) is converted into phase modulations of two signals Vl(t) and V2(t) with constant envelope as in the following: <maths id="math0001"><math display="block"><mi mathvariant="italic">Vin</mi><mfenced><mi>t</mi></mfenced><mo>=</mo><mi>A</mi><mfenced><mi>t</mi></mfenced><mo>⁢</mo><mi>cos</mi><mfenced><mi mathvariant="italic">ωt</mi><mo>+</mo><mi>θ</mi><mfenced><mi>t</mi></mfenced></mfenced><mo>=</mo><mi>V</mi><mo>⁢</mo><mn>1</mn><mfenced><mi>t</mi></mfenced><mo>+</mo><mi>V</mi><mo>⁢</mo><mn>2</mn><mfenced><mi>t</mi></mfenced></math><img file="EP2247004B1_D0001.tif" /></maths><maths id="math0002"><math display="block"><mtable columnalign="left"><mtr><mtd><mi>V</mi><mo>⁢</mo><mn>1</mn><mfenced><mi>t</mi></mfenced><mo>=</mo><mfrac><mi>A</mi><mn>2</mn></mfrac><mo>⁢</mo><mfenced><mi>sin</mi><mfenced><mi mathvariant="italic">ωt</mi><mo>+</mo><mi>θ</mi><mfenced><mi>t</mi></mfenced><mo>+</mo><mi>ϕ</mi><mfenced><mi>t</mi></mfenced></mfenced></mfenced></mtd></mtr><mtr><mtd><mi>V</mi><mo>⁢</mo><mn>2</mn><mfenced><mi>t</mi></mfenced><mo>=</mo><mfrac><mi>A</mi><mn>2</mn></mfrac><mo>⁢</mo><mfenced><mi>sin</mi><mfenced><mi mathvariant="italic">ωt</mi><mo>+</mo><mi>θ</mi><mfenced><mi>t</mi></mfenced><mo>-</mo><mi>ϕ</mi><mfenced><mi>t</mi></mfenced></mfenced></mfenced></mtd></mtr></mtable></math><img file="EP2247004B1_D0002.tif" /></maths><maths id="math0003"><math display="block"><mi>ϕ</mi><mfenced><mi>t</mi></mfenced><mo>=</mo><msup><mi>sin</mi><mrow><mo>-</mo><mn>1</mn></mrow></msup><mo>⁢</mo><mfrac><mrow><mi>A</mi><mfenced><mi>t</mi></mfenced></mrow><mi>A</mi></mfrac></math><img file="EP2247004B1_D0003.tif" /></maths>

0028In the formulas above, <i>ω</i> is the frequency, <i>θ</i>(<i>t</i>) is the instantaneous phase variation, and A is the maximum value of <i>A(t) .</i>

0029The first phase modulated signal component of constant amplitude V1(t) is applied at the input of the first output stage OS1 and amplified in said first output stage OS1. At the output of the first output stage OS1, an amplified copy of the first phase modulated signal component of constant amplitude Vl(t) is provided for input at the first input of the combiner C.

0030The second phase modulated signal component of constant amplitude V2(t) is applied at the input of the second output stage OS2 and amplified in said second output stage OS2. At the output of the second output stage OS2, an amplified copy of the second phase modulated signal component of constant amplitude V2(t) is provided for input at the second input of the combiner C.

0031In the combiner C, the amplified copy of the first phase modulated signal component of constant amplitude Vl(t) is combined with the amplified copy of the second phase modulated signal component of constant amplitude V2(t) resulting in an amplified copy of the input signal Vin(t). Said amplified copy of the input signal Vin(t) is provided at the output of the combiner C as output signal Vout(t) of the LINC amplifier.

0032In the following, the application of LINC amplifiers in four embodiments of the invention is depicted. The basic idea of all four embodiments is to generate phase modulated signal components of constant amplitude in a first part of a LINC amplifier located in a transmitting device, to transmit at least one of said phase modulated signal components of constant amplitude via an optical connection to a second part of the LINC amplifier located in a receiving device, and to amplify and combine the phase modulated signal components of constant amplitude in the second part of the LINC amplifier.

0033An embodiment of a LINC amplifier LINC1 according to the invention is depicted in <figref idref="f0003">fig. 3</figref>. The LINC amplifier LINC1 is indicated as a dashed box and comprises a signal separator, a carrier synthesizer CS, four electro-optical converters EO1-EO4, two optical adders A1 and A2, two opto-electrical converters OE1 and OE2, two phase signal re-synthesizers PSRS1 and PSRS2, two switched output stages OS1 and OS2, and a combiner C.

0034In the embodiment depicted in <figref idref="f0003">fig. 3</figref>, the signal separator, the carrier synthesizer CS, the four electro-optical converters EO1-EO4, and the two optical adders A1 and A2 are comprised in a base station BS, which is indicated as a box, and the two opto-electrical converters OE1 and OE2, the two phase signal re-synthesizers PSRS1 and PSRS2, the two switched output stages OS1 and OS2, and the combiner C are comprised in a remote antenna head RAH1, which is also indicated as a box.

0035The base station BS further comprises a third opto-electrical converter OE3 and a receiver RX.

0036The remote antenna head RAH1 further comprises an antenna network AN, a low noise amplifier LNA, a down converter DC, an analogue to digital converter AD2 and a fifth electro-optical converter EO5.

0037An input for intermediate frequency signals of the LINC amplifier LINC1 is connected to an input of the signal separator SIS.

0038A first output of the signal separator SIS is connected to an input of the first electro-optical converter EO1.

0039An output of the first electro-optical converter EO1 is connected to a first input of the first optical adder A1 through an optical connection, and an output of the first optical adder A1 is connected to an input of the first opto-electrical converter OE1 through an optical connection OF1, as e.g. an optical fiber or an optical free-space connection.

0040An output of the first opto-electrical converter OE1 is connected to an input of the first phase signal re-synthesizer PSRS1, and an output of the first phase signal re-synthesizer PSRS1 is connected to an input of the first output stage OS1.

0041An output of the first output stage OS1 is connected to a first input of the combiner C.

0042A second output of the signal separator SIS is connected to an input of the second electro-optical converter E02.

0043An output of the second electro-optical converter E02 is connected to a first input of the second optical adder A2 through an optical connection, and an output of the second optical adder A2 is connected to an input of the second opto-electrical converter OE2 through an optical connection OF2, as e.g. an optical fiber or an optical free-space connection.

0044An output of the second opto-electrical converter OE2 is connected to an input of the second phase signal re-synthesizer PSRS2, and an output of the second phase signal re-synthesizer PSRS2 is connected to an input of the second output stage OS2.

0045An output of the second output stage OS2 is connected to a second input of the combiner C.

0046An output of the carrier synthesizer CS is connected both to an input of the third electro-optical converter EO3 and to an input of the fourth electro-optical converter E04.

0047An output of the third electro-optical converter EO3 is connected to a second input of the first optical adder A1 through an optical connection, and an output of the fourth electro-optical converter E04 is connected to a second input of the second optical adder A2 through an optical connection.

0048In an embodiment of the invention, the output of the carrier synthesizer CS is connected to an input of only one electro-optical converter, and the output of the electro-optical converter is connected both to the second input of the first optical adder A1 through an optical connection, and to the second input of the second optical adder A2 through an optical connection.

0049An output of the combiner C is connected to the antenna network AN.

0050In 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 paths between the electro-optical converters EO1-EO4 and the opto-electrical converters OE1 and OE2 respectively, or in the signal paths between the opto-electrical converters OE1 and OE2 and the output stages OS1 and OS2 respectively.

0051In 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.

0052An output of the down converter DC is connected to an input of the analogue-to-digital converter AD, and an output of the analogue-to-digital converter AD is connected to an input of the fifth electro-optical converter EO5.

0053An output of the fifth electro-optical converter EO5 is connected to an input of the third opto-electrical converter OE3 through an optical connection OF3 as e.g. an optical fiber or an optical free-space connection.

0054The output of the third opto-electrical converter OE3 is in turn connected to an input of the receiver RX.

0055In an embodiment of the invention, an output of the LINC amplifier LINC1 is connected to the reception path, preferably at an input of the fifth electro-optical converter EO5, which is indicated by a dotted arrow in <figref idref="f0003">fig. 3</figref>, e.g. for linearization and optimization purposes.

0056Preferably, said electro-optical converters EO1-EO5 each comprise a laser diode which is either directly modulated or externally modulated e.g. by means of a electroabsorption or lithiumniobate modulator.

0057Preferably, said opto-electrical converters OE1-OE3 each comprise a so-called PIN-diode or a so-called avalanche-photodiode.

0058In the embodiment depicted in <figref idref="f0003">fig. 3</figref>, an analogue data signal on an intermediate frequency f_if preferably in the frequency range 10-100 MHz is sent to the input of the LINC amplifier LINC1 and thus to the input of the signal separator SIS. In the signal separator SIS, the analogue data signal is split into two phase modulated signal components of constant amplitude.

0059The two phase modulated signal components of constant amplitude are converted from analogue to digital signals in the signal separator SIS.

0060In another embodiment of the invention, the conversion from analogue to digital signals is not performed in the signal separator SIS, but in an analogue-to-digital converter which is located in the signal path between the signal separator SIS and the electro-optical converter EO1 and E02 respectively.

0061The first phase modulated signal component of constant amplitude is sent to the to the first electro-optical converter EO1 for converting digital electrical signals into digital optical signals using an optical source with f_optical being the frequency of the light of the optical source, so that the first digital optical phase modulated signal component has a frequency f_os= f_optical± f_if·

0062From the output of the first electro-optical converter EO1, the first digital optical phase modulated signal component is sent over the optical connection to the first input of the first adder A1.

0063The second phase modulated signal component of constant amplitude is sent to the to the second electro-optical converter E02 for converting digital electrical signals into digital optical signals using an optical source with f_optical being the frequency of the light of the optical source, so that the second digital optical phase modulated signal component has a frequency f_os= f_optical± f_if·

0064From the output of the second electro-optical converter E02, the second digital optical phase modulated signal component is sent over the optical connection to the first input of the second adder A2.

0065In the carrier synthesizer CS, a radio frequency carrier signal on a radio frequency f_rf preferably in the frequency range of several hundreds of MHz to several GHz is generated. The radio frequency carrier signal is sent to the third and fourth electro-optical converter EO3 and E04 for converting the electrical radio frequency carrier signal into an optical radio frequency carrier signal using an optical source with f_optical being the frequency of the light of the optical source, so that the optical radio frequency carrier signal has a frequency f_orf= f_optical± f_if·

0066From the output of the third and fourth electro-optical converter EO3 and E04, the optical radio frequency carrier signal is sent over optical connections to the second input of the first adder A1 and the second adder A2 respectively.

0067In the first adder A1, the first digital optical phase modulated signal component is added with the optical radio frequency carrier signal, i.e. the first digital optical phase modulated signal component with the frequency f_os= f_ptical± f_if is summed up with the optical radio frequency carrier signal with the frequency f_orf= f_optcal± f_rf, which results in first combined optical data signals.

0068From the output of the first adder A1, the first combined optical data signals are sent over the optical connection OF1 to the input of the first opto-electrical converter OE1.

0069In the first opto-electrical converter OE1, the principle of optical heterodyning is applied, and the first digital optical phase modulated signal component is upconverted into a first digital electrical phase modulated signal component. The intensity of the detected first digital electrical phase modulated signal component varies with a frequency f_des=f_rf±f_if, as all higher frequency components of the intensity in the range of f_optical are time averaged in the first opto-electrical converter OE1. The first digital electrical phase modulated signal component is sent from the output of the first opto-electrical converter OE1 to an input of the first phase signal re-synthesizer PSRS1 in which the analogue first phase modulated signal component of constant amplitude is recovered.

0070The first phase modulated signal component of constant amplitude is sent from the output of the first phase signal re-synthesizer PSRS1 to the input of the first output stage OS1, which leads to an amplified copy of the first phase modulated signal component of constant amplitude at the output of the first output stage OS1.

0071In an embodiment of the invention, the analogue first phase modulated signal component of constant amplitude is recovered by means of implicit passive signal reconstruction filtering in the first output stage OS1, and no first phase signal re-synthesizer PSRS1 is needed.

0072The amplified copy of the first phase modulated signal component of constant amplitude is sent to the first input of the combiner C.

0073In the second adder A2, the second digital optical phase modulated signal component is added with the optical radio frequency carrier signal, i.e. the second digital optical phase modulated signal component with the frequency f_os= f_ptical± f_if is summed up with the optical radio frequency carrier signal with the frequency f_orf= f_optical± f_rf, which results in second combined optical data signals.

0074From the output of the second adder A2, the second combined optical data signals are sent over the optical connection OF2 to the input of the second opto-electrical converter OE2.

0075In the second opto-electrical converter OE2, the principle of optical heterodyning is applied, and the second digital optical phase modulated signal component is upconverted into a second digital electrical phase modulated signal component. The intensity of the detected second digital electrical phase modulated signal component varies with a frequency f_des=f_rf±f_if, as all higher frequency components of the intensity in the range of f_optical are time averaged in the second opto-electrical converter OE2. The second digital electrical phase modulated signal component is sent from the output of the second opto-electrical converter OE2 to an input of the second phase signal re-synthesizer PSRS2 in which the analogue second phase modulated signal component of constant amplitude is recovered.

0076The second phase modulated signal component of constant amplitude is sent from the output of the second phase signal re-synthesizer PSRS2 to the input of the second output stage OS2, which leads to an amplified copy of the second phase modulated signal component of constant amplitude at the output of the second output stage OS2.

0077In an embodiment of the invention, the analogue second phase modulated signal component of constant amplitude is recovered by means of implicit passive signal reconstruction filtering in the second output stage OS2, and no second phase signal re-synthesizer PSRS2 is needed.

0078The amplified copy of the second phase modulated signal component of constant amplitude is sent to the second input of the combiner C.

0079In the combiner C, the amplified copy of the first phase modulated signal component of constant amplitude is combined with the amplified copy of the second phase modulated signal component of constant amplitude resulting in an upconverted and amplified copy of the analogue data signal provided at the input of the LINC amplifier LINC1. Said amplified copy of the analogue data signal is provided at the output of the combiner C as output signal of the LINC amplifier LINC1.

0080The upconverted and amplified copy of the analogue data signal is sent to the antenna network AN for transmission over an air interface.

0081In the reception path, analogue electrical signals are sent from the antenna network AN to the low noise amplifier LNA for signal amplification.

0082The amplified analogue electrical signals are sent to the down converter DC in which the signals are downconverted from the radio frequency to the intermediate frequency.

0083The downconverted analogue electrical signals are sent to the analogue-to-digital converter AD for digitalization, and the digital electrical signals are sent to the fifth electro-optical converter EO5 for converting the digital electrical signals into digital optical signals.

0084The digital optical signals are sent from the fifth electro-optical converter EO5 to the third opto-electrical converter OE3 through the optical connection OF3.

0085In the third opto-electrical converter OE3, the digital optical signals are back-converted into digital electrical signals and sent to the receiver RX for further processing.

0086In an embodiment of the invention, if linearization of the LINC amplifier LINC1 should become necessary, an output signal of the LINC amplifier LINC1 is fed back via the fifth electro-optical converter EO5 and the optical connection OF3 to the base station BS, and based on said fed back output signal, the LINC amplifier LINC1 is controlled, which is indicated by a dotted arrow between the receiver RX and the signal separator SIS.

0087Advantages of the above described embodiment of the invention are, that by using optical heterodyning, the need for electrical upconversion is dropped, and the signal processing related to the phase modulated signal components of constant amplitude is performed on an intermediate frequency level at considerably reduced speed and hence reduced power dissipation and chip complexity.

0088However, the application of optical heterodyning is not mandatory for the invention. Thus, in an embodiment of the invention, the phase modulated signal components of constant amplitude are electrically upconverted before they are sent to the first and second electro-optical converters EO1 and E02 respectively, and the digital optical phase modulated signal components are sent directly over an optical connection to the first and second opto-electrical converters OE1 and OE2 respectively. As a consequence, the carrier synthesizer CS, the third and fourth electro-optical converters EO3 and E04, and the adders A1 and A2 are not necessary in this embodiment.

0089In principle, it is not necessary for the application of the invention that both phase modulated signal components of constant amplitude are transmitted as an optical signal from the base station BS to the remote antenna head RAH1, i.e. it is possible that either the first phase modulated signal component of constant amplitude or the second phase modulated signal component of constant amplitude is transmitted electrically from the base station BS to the remote antenna head RAH1. Thus, in embodiments of the invention, either only the first phase modulated signal component of constant amplitude or only the second phase modulated signal component of constant amplitude is transmitted optically from the base station BS to the remote antenna head RAH1.

0090In an embodiment of the invention, the analogue data signal which is sent to the input of the LINC amplifier LINC1 is not on an intermediate frequency, but on a baseband frequency.

0091In the embodiment depicted in <figref idref="f0004">fig. 4</figref>, a distributed LINC amplifier LINC2 applying the principles of optical multiplexing and demultiplexing according to the invention is shown. The basic structure of the distributed LINC amplifier LINC2 is similar to the structure of the distributed LINC amplifier LINC1 depicted in <figref idref="f0003">fig. 3</figref> and described above. Thus, in the following only the differences compared to the distributed LINC amplifier LINC1 depicted in <figref idref="f0003">fig. 3</figref> are described.

0092The distributed LINC amplifier LINC2 depicted in <figref idref="f0004">fig. 4</figref> comprises an optical multiplexer MUX which is located in the base station BS and an optical demultiplexer DEMUX which is located in the remote antenna head RAH1.

0093The output of the first optical adder A1 is connected to a first input of the optical multiplexer MUX through an optical connection.

0094The output of the second optical adder A2 is connected to a second input of the optical multiplexer MUX through an optical connection, and an output of the optical multiplexer MUX is connected to an input of the optical demultiplexer DEMUX through an optical connection OF4.

0095A first output of the optical demultiplexer DEMUX is connected through an optical connection to the input of the first opto-electrical converter OE1, and a second output of the optical demultiplexer DEMUX is connected through an optical connection to the input of the second opto-electrical converter OE2.

0096In the embodiment depicted in <figref idref="f0003">fig. 3</figref>, the first and second optical phase modulated signal components of constant amplitude are transmitted over separate optical connections OF1 and OF2 from the base station BS to the remote antenna head RAH1.

0097In the embodiment depicted in <figref idref="f0004">fig. 4</figref>, the first and second optical phase modulated signal components of constant amplitude, and the optical radio frequency carrier signal are multiplexed in the optical multiplexer MUX, and transmitted over the common optical connection OF4 to the optical demultiplexer DEMUX, in which the optical signals are demultiplexed. As optical multiplexing and demultiplexing is applied in this embodiment, the optical frequency that is used for upconversion in the electro-optical converters EO1 and EO3 is different from the optical frequency that is used for upconversion in the electro-optical converters E02 and E04. The first optical phase modulated signal component and the respective optical radio frequency carrier signal are sent to the first opto-electrical converter OE1, and the second optical phase modulated signal component and the respective optical radio frequency carrier signal are sent to the second opto-electrical converter OE2.

0098In the embodiment depicted in <figref idref="f0005">fig. 5</figref>, two distributed LINC amplifiers LINC3 and LINC4 each connected to an antenna network AN1 and AN2 respectively and comprised in a base station BS and in a remote antenna head RAH1 are shown for demonstration of scaling according to the invention. The basic structure and function of the distributed LINC amplifiers LINC3 and LINC4 is similar to the structure of the distributed LINC amplifier LINC1 depicted in <figref idref="f0003">fig. 3</figref> and described above. Thus, in the following only the differences compared to the distributed LINC amplifier LINC1 depicted in <figref idref="f0003">fig. 3</figref> are described.

0099In the embodiment depicted in <figref idref="f0005">fig. 5</figref>, a carrier synthesizer CS is used for generation of radio frequency carrier signals for both the upper distributed LINC amplifier LINC3 and the lower distributed LINC amplifier LINC4 instead of having a separate carrier synthesizer for each distributed LINC amplifier LINC3 and LINC4. An output of the carrier synthesizer CS is connected to four inputs of four electro-optical converters E03, E04, E06 and E07. In further embodiments for multiband application, more than one carrier synthesizer CS is used in order to provide different carrier frequencies.

0100An output of the electro-optical converter E03 is connected to an input of the first optical adder A1 of the upper distributed LINC amplifier LINC3, and an output of the electro-optical converter E04 is connected to an input of the second optical adder A2 of the upper distributed LINC amplifier LINC3.

0101An output of the electro-optical converter E06 is connected to an input of the first optical adder A3 of the lower distributed LINC amplifier LINC4, and an output of the electro-optical converter E07 is connected to an input of the second optical adder A4 of the lower distributed LINC amplifier LINC4.

0102In an embodiment of the invention, the output of the carrier synthesizer CS is connected to an input of only one electro-optical converter, and the output of the electro-optical converter is connected to inputs of the four optical adders A1-A4 through optical connections.

0103Each of the two distributed LINC amplifiers LINC3 and LINC4 has a reception path as described above in <figref idref="f0003">fig. 3</figref> with the difference that they have a common optical reception path between the remote antenna head RAH1 and the base station BS performed by optical multiplexing and demultiplexing.

0104An output of the antenna network AN1 of the upper distributed LINC amplifier LINC3 and an output of the antenna network AN2 of the of the lower distributed LINC amplifier LINC4 are each connected to an input of the respective low noise amplifier LNA1 and LNA2.

0105In an embodiment of the invention, an output of the upper LINC amplifier LINC3 and an output of the lower LINC amplifier LINC4 are connected to the respective reception path, preferably at an input of the fifth electro-optical converter EO5 and the tenth electro-optical converter EO10 respectively, which is indicated by dotted arrows in <figref idref="f0005">fig. 5</figref>. The receiver RX is connected both to the signal separator SIS1 of the upper distributed LINC amplifier LINC3 and to the signal separator SIS2 of the lower distributed LINC amplifier LINC4, which is indicated by two dotted arrows in <figref idref="f0005">fig. 5</figref>. Alternatively, the receiver RX is connected to a digital processing unit in which a linearization algorithm is normally performed.

0106The usage of a common carrier synthesizer CS and a common reception path for both LINC amplifiers LINC 3 and LINC 4 reduces the costs and makes the system scalable for the usage of several antennas applying so-called beamforming or multiple-input-multiple-output.

0107In an embodiment of the invention, by combining several LINC amplifiers as described in <figref idref="f0005">fig. 5</figref> with a specific radio carrier frequency for each LINC amplifier generated in a specific carrier synthesizer for each LINC amplifier, e.g. multiband coverage or a fragmented transmitter with a dedicated frequency band for signal transmission of each LINC amplifier can be achieved.

0108In the embodiment depicted in <figref idref="f0006">fig. 6</figref>, two distributed LINC amplifiers LINC5 and LINC6 each connected to an antenna network AN1 and AN2 respectively and comprised in a base station BS and in a remote antenna head RAH1 applying the principles of optical multiplexing and demultiplexing are shown for demonstration of scaling according to the invention. The basic structure and function of the distributed LINC amplifiers LINC5 and LINC6 is similar to the structure of the distributed LINC amplifier LINC2 depicted in <figref idref="f0004">fig. 4</figref> and described above. Thus, in the following only the differences compared to the distributed LINC amplifier LINC2 depicted in <figref idref="f0004">fig. 4</figref> are described.

0109In the embodiment depicted in <figref idref="f0006">fig. 6</figref>, a carrier synthesizer CS is used for generation of radio frequency carrier signals for both the upper distributed LINC amplifier LINC5 and the lower distributed LINC amplifier LINC6 instead of having a separate carrier synthesizer for each distributed LINC amplifier LINC5 and LINC6. An output of the carrier synthesizer CS is connected to four inputs of four electro-optical converters EO3, E04, E06 and E07.

0110An output of the electro-optical converter EO3 is connected to an input of the first optical adder A1 of the upper distributed LINC amplifier LINC5, and an output of the electro-optical converter E04 is connected to an input of the second optical adder A2 of the upper distributed LINC amplifier LINC5.

0111An output of the electro-optical converter E06 is connected to an input of the first optical adder A3 of the lower distributed LINC amplifier LINC6, and an output of the electro-optical converter E07 is connected to an input of the second optical adder A4 of the lower distributed LINC amplifier LINC6.The base station BS in <figref idref="f0006">fig. 6</figref> comprises an optical multiplexer MUX which is used by both distributed LINC amplifiers LINC5 and LINC6, and the remote antenna head RAH1 comprises an optical demultiplexer DEMUX which is also used by both distributed LINC amplifiers LINC5 and LINC6.

0112The output of the first optical adder A1 of the upper distributed LINC amplifier LINC5 is connected to a first input of the optical multiplexer MUX through an optical connection, and the output of the second optical adder A2 of the upper distributed LINC amplifier LINC5 is connected to a second input of the optical multiplexer MUX through an optical connection.

0113The output of the first optical adder A3 of the lower distributed LINC amplifier LINC6 is connected to a third input of the optical multiplexer MUX through an optical connection, and the output of the second optical adder A4 of the lower distributed LINC amplifier LINC6 is connected to a fourth input of the optical multiplexer MUX through an optical connection.

0114An output of the optical multiplexer MUX is connected to an input of the optical demultiplexer DEMUX through an optical connection OF7.

0115A first output of the optical demultiplexer DEMUX is connected through an optical connection to the input of the first opto-electrical converter OE1 of the upper distributed LINC amplifier LINC5, and a second output of the optical demultiplexer DEMUX is connected through an optical connection to the input of the second opto-electrical converter OE2 of the upper distributed LINC amplifier LINC5.

0116A third output of the optical demultiplexer DEMUX is connected through an optical connection to the input of the first opto-electrical converter OE4 of the lower distributed LINC amplifier LINC6, and a second output of the optical demultiplexer DEMUX is connected through an optical connection to the input of the second opto-electrical converter OE5 of the lower distributed LINC amplifier LINC6.

0117In the embodiment depicted in <figref idref="f0003">fig. 3</figref>, the first and second optical phase modulated signal components of constant amplitude are transmitted over separate optical connections OF1 and OF2 from the base station BS to the remote antenna head RAH1.

0118In the embodiment depicted in <figref idref="f0006">fig. 6</figref>, the first and second optical phase modulated signal components of constant amplitude, and the optical radio frequency carrier signal of both LINC amplifiers LINC5 and LINC6 are multiplexed in the optical multiplexer MUX, and transmitted over the common optical connection OF7 to the optical demultiplexer DEMUX, in which the optical signals are demultiplexed.

0119As optical multiplexing and demultiplexing is applied in this embodiment, the optical frequencies that are used for upconversion in the electro-optical converters EO3, E04, E06 and E07, which correspond to the optical frequencies that are used for upconversion in the electro-optical converters EO1, E02, E09 and EO8 respectively are all different from each other.

0120The first optical phase modulated signal component and the respective optical radio frequency carrier signal of the upper LINC amplifier LINC5 are sent to the first opto-electrical converter OE1 of the upper LINC amplifier LINC5, and the second optical phase modulated signal component and the respective optical radio frequency carrier signal of the upper LINC amplifier LINC5 are sent to the second opto-electrical converter OE2 of the upper LINC amplifier LINC5.

0121The first optical phase modulated signal component and the respective optical radio frequency carrier signal of the lower LINC amplifier LINC6 are sent to the first opto-electrical converter OE4 of the lower LINC amplifier LINC6, and the second optical phase modulated signal component and the respective optical radio frequency carrier signal of the lower LINC amplifier LINC6 are sent to the second opto-electrical converter OE5 of the lower LINC amplifier LINC6.

0122The two distributed LINC amplifiers LINC5 and LINC6 have reception paths as described above in <figref idref="f0005">fig. 5</figref>.

0123An output of the antenna network AN1 of the upper distributed LINC amplifier LINC5 and an output of the antenna network AN2 of the of the lower distributed LINC amplifier LINC6 are each connected to an input of the respective low noise amplifier LNA1 and LNA2.

0124In an embodiment of the invention, an output of the upper LINC amplifier LINC5 and an output of the lower LINC amplifier LINC6 are connected to the respective reception path, preferably at an input of the fifth electro-optical converter EO5 and the tenth electro-optical converter EO10 respectively, which is indicated by dotted arrows in <figref idref="f0006">fig. 6</figref>. The receiver RX is connected both to the signal separator SIS1 of the upper distributed LINC amplifier LINC5 and to the signal separator SIS2 of the lower distributed LINC amplifier LINC6, which is indicated by two dotted arrows in <figref idref="f0006">fig. 6</figref>. Alternatively, the receiver RX is connected to a digital processing unit in which a linearization algorithm is normally performed.

0125The usage of a common carrier synthesizer CS, a common multiplexer MUX, a common demultiplexer DEMUX, and a common reception path for both LINC amplifiers LINC 5 and LINC 6 reduces the costs and makes the system scalable for the usage of several antennas applying so-called beamforming or multiple-input-multiple-output.

0126In an embodiment of the invention, by combining several LINC amplifiers as described in <figref idref="f0006">fig. 6</figref> with a specific radio carrier frequency for each LINC amplifier, e.g. multiband coverage or a fragmented transmitter with a dedicated frequency band for signal transmission of each LINC amplifier can be achieved.

0127In a further embodiment of the invention, signals from the base station BS are transmitted through at least two optical connections OF9, OF10 via at least two remote antenna heads RAH3, RAH4 to a user terminal UE4 applying so-called beamforming or multiple-input-multiple-output procedures, as depicted in <figref idref="f0001">fig. 1</figref> for the case of two remote antenna heads RAH3 and RAH4 that are used for signal transmission to the user terminal UE4.

0128In the embodiments described above, analogue data signals are provided at the input of the LINC amplifiers LINC1-LINC6. However, in further preferred embodiments, the input data signal provided at the input of the LINC amplifiers LINC1-LINC6 respectively is a digital signal, as e.g. a so-called I or Q value provided from a digital processing unit on a baseband frequency. In the signal separator SIS, the digital data signal is split into two digital phase modulated signal components of constant amplitude. In embodiments using digital input data signals, no conversion from analogue to digital signals in the signal separator SIS as described in the embodiments using analogue input data signals is necessary, but instead a dedicated signal processing must be performed.

0129In embodiments using digital input data signals, the signal separator SIS is preferably implemented in a field programmable gate array (FPGA), but could also be implemented in an application-specific integrated circuit (ASIC).

0130Such a LINC amplifier according to the invention can be used e.g. for a communication network applying the standard Universal Mobile Telecommunications System, Third Generation Partnership Project Long Term Evolution, or Worldwide Interoperability for Microwave Access.

0131In the embodiments above, the invention is described for the transmission of signals from a base station BS via at least one remote antenna head RAH1 to a user terminal UE1, however the invention is also applicable for transmission of signals from an arbitrary transmitting device to an arbitrary receiving device, as e.g. used in a point-to-point radio system or used for connection of devices within a rack.