OFDM generation apparatus in a multi-carrier data transmission system FIELD OF INVENTION [0001] The present invention relates to an OFDM generation apparatus and method for generating OFDM transmission signals from OFDM symbols, each comprising a plurality of OFDM subcarriers, for transmission in a multi-carrier data transmission system. [0002] The present invention relates further to a transmission apparatus and method, a corresponding receiving apparatus and method, a data transmission system and a computer program for implementing the OFDM generation methods on a computer. [0003] The present invention relates particularly to the field of broadcasting, in particular of Digital Video Broadcasting (DVB), especially to devices, systems and methods in accordance with the DVB-C2 standard or the upcoming DVB-NGH standard. BACKGROUND OF THE INVENTION [0004] Broadcast systems in accordance with the DVB-C2 standard as described in the DVB-C2 specification (DVB BlueBook A138 "Digital Video Broadcasting (DVB); Frame structure channel coding and modulation for a second generation digital transmission system for cable systems (DVB-C2)") apply the concept of Absolute OFDM, in which all OFDM subcarriers are seen relative to the absolute frequency 0 MHz instead of a signal center frequency. Reason for the application of Absolute OFDM and unique pilot pattern across the medium spectrum in DVB-C2 is to avoid repeating OFDM subcarrier allocations in the frequency domain that result in an increased PAPR (Peak to Average Power Ratio). The Absolute OFDM signal is described in the final RF frequency domain. This means, however, that a baseband signal cannot be shifted to any RF carrier frequency (also called "mixing frequency" hereinafter) without the introduction of common phase rotations between OFDM symbols after the step of mixing during the OFDM generation by use of the RF carrier frequency. SUMMARY OF INVENTION [0005] It is an object of the present invention to provide an OFDM generation apparatus and method dealing with the problem of common phase rotations of the OFDM subcarriers of successive OFDM symbols, in particular by which such common phase rotations are avoided or compensated. [0006] It is a further object of the present invention to provide a corresponding transmission apparatus and method, a corresponding receiving apparatus and method, a data transmission system and a computer program. [0007] According to an aspect of the present invention there is provided an OFDM generation apparatus for generating OFDM transmission signals from OFDM symbols, each comprising a plurality of OFDM subcarriers, for transmission in a multi-carrier data transmission system, said apparatus comprising - an inverse DFT means for inverse discrete Fourier transforming an OFDM symbol into complex time-domain samples, and - a frequency mixing means for mixing said complex time-domain samples of said OFDM symbol from a baseband frequency up to a passband frequency by use of a mixing frequency to obtain said OFDM transmission signal, wherein the mixing frequency is selected such that common phase rotations of the OFDM subcarriers of said OFDM symbol with respect to adjacent OFDM symbols of said OFDM transmission signal are avoided or compensated after said mixing. [0008] According to another aspect of the present invention there is provided an OFDM generation apparatus for generating OFDM transmission signals from OFDM symbols, each comprising a plurality of OFDM subcarriers, for transmission in a multi-carrier data transmission system, said apparatus comprising - an inverse DFT means for inverse discrete Fourier transforming an OFDM symbol into complex time-domain samples, wherein the OFDM symbols are mapped onto frames of a frame structure having a channel bandwidth, said frames having a payload portion being segmented in frequency domain into data segments each covering a bandwidth portion of said channel bandwidth, and wherein data symbols are mapped onto said data segments, - a frequency mixing means for mixing said complex time-domain samples of said OFDM symbol from a baseband frequency up to a passband frequency by use of a transmitter mixing frequency to obtain said OFDM transmission signal, and - a receiver mixing frequency determination means for determining receiver mixing frequencies for mixing a received OFDM transmission signal from a passband frequency down to a baseband frequency by use of a receiver mixing frequency to obtain complex time-domain samples of a data symbol in a receiver, wherein the receiver mixing frequencies are selected such that common phase rotations of the OFDM subcarriers of a data symbol with respect to adjacent data symbols of the same data segment are avoided or compensated after mixing a received OFDM transmission signal from a passband frequency down to a baseband frequency by use of said receiver mixing frequency. [0009] According to still another aspect of the present invention there is provided an OFDM generation apparatus for generating OFDM transmission signals from OFDM symbols, each comprising a plurality of OFDM subcarriers, for transmission in a multi-carrier data transmission system, said apparatus comprising - a multiplication unit for multiplying the OFDM symbols with a multiplication factor for compensating common phase rotations of the OFDM subcarriers of said OFDM symbol, which could be introduced by mixing said complex time-domain samples of said OFDM symbol from a baseband frequency up to a passband frequency by use of a mixing frequency, - an inverse DFT means for inverse discrete Fourier transforming an OFDM symbol into complex time-domain samples, and - a frequency mixing means for mixing said complex time-domain samples of said OFDM symbol from a baseband frequency up to a passband frequency by use of said mixing frequency to obtain said OFDM transmission signal. [0010] According to still another aspect of the present invention there is provided an OFDM decoding apparatus for decoding OFDM transmission signals into OFDM symbols, each comprising a plurality of OFDM subcarriers, received in a multi-carrier data transmission system, said apparatus comprising - a frequency mixing means for mixing said OFDM transmission signal from a passband frequency down to a baseband frequency by use of a mixing frequency to obtain complex time-domain samples of an OFDM symbol, and - a DFT means for discrete Fourier transforming said complex time-domain samples into a data symbol. wherein the mixing frequency is selected such that common phase rotations of the OFDM subcarriers of said OFDM symbol are avoided or compensated. [0011] According to still another aspect of the present invention there is provided an OFDM decoding apparatus for decoding OFDM transmission signals into data symbols, each comprising a plurality of OFDM subcarriers, received in a multi-carrier data transmission system, said apparatus comprising - a frequency mixing means for mixing said OFDM transmission signal from a passband frequency down to a baseband frequency by use of a mixing frequency to obtain complex time-domain samples of a data symbol, wherein the OFDM symbols are mapped onto frames of a frame structure having a channel bandwidth, said frames having a payload portion being segmented into data segments each covering a bandwidth portion of said channel bandwidth, and wherein said OFDM symbols are subdivided into data symbols mapped onto said data segments, and - a DFT means for discrete Fourier transforming said complex time-domain samples into a data symbol, wherein the mixing frequency is selected such that common phase rotations of the OFDM subcarriers of said data symbol are avoided or compensated. [0012] According to further aspects of the present invention there are provided corresponding OFDM generation methods and OFDM decoding methods, a transmission apparatus and method, a receiving apparatus and method, a data transmission system as well as a computer program comprising program means for causing a computer to carry out the steps of said OFDM generation methods or said OFDM decoding methods as defined above, when said computer program is carried out on a computer. [0013] Preferred embodiments of the invention are defined in the dependent claims. It shall be understood that the claimed devices, methods, system and computer program have similar and/or identical preferred embodiments as defined in the dependent claims defining preferred embodiment of the OFDM generation apparatus. [0014] The present invention is based on the common inventive idea that undesired common phase rotations of the OFDM subcarriers of an OFDM symbol or a data symbol (in case of using a segmented OFDM as, for instance, according to the DVB-C2 standard) are avoided or compensated by taking appropriate measures related to the carrier frequency by which the complex time-domain samples are mixed. This is of particular importance for systems according to the DVB-C2 standard that apply Absolute OFDM, since the generated OFDM signal is described in the passband and does not contain any phase rotations between adjacent OFDM symbol. It should be understood that generating an OFDM signal in the passband is very complex and costly. Therefore it is beneficial to generate the signal in the equivalent baseband and to mix it with a suitable mixing frequency into the passband. However, normally this mixing process results in the described phase rotations between OFDM symbols. [0015] To overcome this problem, according to the present invention the carrier frequency is selected such that such common phase rotations are completely avoided or compensated. According to another solution, based on the same idea, the OFDM symbols are multiplied with a multiplication factor, which artificially introduces common phase rotations between OFDM symbols in the baseband signal, which, however, balance the phase rotations caused by the mixing of the complex time-domain samples of the OFDM symbol from a baseband frequency up to a passband frequency by use of a mixing frequency, so that finally no common phase rotations appear in the passband signal (i.e. the Absolute OFDM signal contains no phase rotations between OFDM symbols). Hence, according to the present invention it can be achieved, that signals can be generated that are in conformity with the related standards, if there is any standard to be observed. [0016] It shall be noted that herein the terms "carrier" and "subcarrier" are used interchangeably and shall carry the same meaning. BRIEF DESCRIPTION OF DRAWINGS [0017] These and other aspects of the present invention will be apparent from and explained in more detail below with reference to the embodiments described hereinafter. In the following drawings Fig. 1 shows a block diagram of a data transmission system according to the present invention, Fig. 2 shows a block diagram of a first embodiment of an OFDM generator according to the present invention, Fig. 3 shows a diagram illustrating zero padding. Fig. 4 shows a diagram illustrating the generation of guard intervals. Fig. 5 shows a diagram illustrating the digital signal and its aliases. Fig. 6 illustrates the segmented frame structure as used according to DVB- C2, Fig. 7 shows a block diagram of a second embodiment of an OFDM generator according to the present invention, Fig. 8 shows a block diagram of a third embodiment of an OFDM generator according to the present invention, Fig. 9 shows a block diagram of a first embodiment of an OFDM decoder according to the present invention, and Fig. 10 shows a block diagram of a second embodiment of an OFDM decoder according to the present invention. DESCRIPTION OF PREFERRED EMBODIMENTS [0018] The DVB-C2 specification introduces the new concept of Absolute OFDM, in which all OFDM subcarriers are seen relative to the absolute frequency 0 MHz instead of a signal centre frequency. In particular, the L1 part 2 signalling blocks begin at the absolute frequency of 0 MHz and are partitioned in steps of 7.61MHz. In contrast to other DVB standards it is not possible to shift a C2 baseband signal to any RF mixing frequency rather than being defined in a unique way for the whole cable spectrum. Especially the pilot sequences of the OFDM signal are different for all different frequencies. The reason for that behavior is to avoid unwanted repetitions in the frequency domain which may cause unwanted high peak values of the OFDM signal in time domain. Furthermore the unambiguous pilot sequences allow for easy and reliable synchronization and offset compensation. Although the L1 part 2 block partitioning and the related pilot sequences are defined for the whole cable spectrum, L1 blocks are only transmitted in those frequencies where data slices are present. [0019] The formulas given in section 10.1 of DVB-C2 specification defining the passband signal emitted from the OFDM generator are of the precise theoretical mathematical description, but are impractical for real implementations. Real implementations for OFDM signal generation are normally based on the inverse Fast Fourier Transform and the equivalent lowpass representation of signals. However, the generation of a standard compliant DVB-C2 signal using the equivalent lowpass representation requires additional considerations. Otherwise, unwanted phase jumps may be generated between adjacent OFDM symbols that could disturb the synchronisation procedure within the receiver. Practical implementations based on the inverse Fast Fourier Transform and the equivalent lowpass representation are therefore proposed according to the present invention. [0020] Due to the application of Absolute OFDM the direct signal generation within the passband is complex or even impractical. Therefore, OFDM generation using the equivalent lowpass representation is proposed. The signal is generated at low frequencies and shifted to the final frequency afterwards. [0021] According to the DVB-C2 specification the emitted passband signal is described by the following expression: Formula where Formula and k denotes the carrier number; l denotes the OFDM Symbol number starting from 0 for the first Preamble Symbol of the frame; m denotes the C2 Frame number; K total is the number of transmitted carriers, i.e. K total= K max – K min + 1; LF total number of OFDM Symbols per frame (including the preamble); Ts is the total symbol duration for all symbols, and Ts = Tu + ∆; Tu is the active symbol duration; A is the duration of the guard interval; C m,l,k is the complex modulation value for carrier k of the OFDM Symbol number l in C2 Frame number m; Tf is the duration of a frame, TF = LF TS; K min Carrier index of first (lowest frequency) active carrier; and K max Carrier index of last (highest frequency) active carrier. [0022] In order to generate this signal within using the equivalent lowpass representation, a carrier to shift the frequencies is added, which is compensated within the equation of Ψ; Formula with Formula [0023] Equation (2) cannot be directly transformed into the equation known from section 9.5 of the DVB-T2 specification (ETSI EN 302 755 VI.1.1 (2009-09) "Digital Video Broadcasting (DVB): Frame structure channel coding and modulation for a second generation digital terrestrial television broadcasting system (DVB-T2)") defining the signal emitted by the OFDM generator as used in a transmitter according to the DVB-T2 standard. The reason is the second exponential term. While the equations defined in section 9.5 of the DVB-T2 specification are independent from the actual mixing frequency fc, this initially will lead to phase jumps between OFDM symbols of the DVB-C2 signal. However, this effect can be avoided by means of a well-chosen mixing frequency fc Therefore, the mixing frequency shall be defined as: Formula where 1/Tu is the OFDM subcarrier spacing, and kc is the OFDM subcarrier index at the mixing frequency. Furthermore, k shall be substituted by k = k' + kc. This leads to: Formula Which can be reformulated Formula [0024] Equation (4) looks similar to the signal definition of the DVB-T2 signal as described in section 9.5 of the DVB-T2 specification. However, both equations differ in the last exponential term. This term is independent of the time t and causes a constant phase rotation for all OFDM subcarriers of a given OFDM symbol. Generally, it is possible to choose kc freely (and thus fc) and to compensate this phase rotation. However, this term can be avoided by choosing Kc properly. For this purpose, equation (4) can be written as: Formula (5), where (∆ /Tu) is the relative Guard Interval duration (e.g. 1/64 or 1/128 for DVB-C2). Additional simplification of (5) leads to: Formula [0025] Hence, this leads to a common phase rotation of Formula for all OFDM subcarriers between two consecutive OFDM symbols, which depends on the choice of the relative Guard Interval duration (∆ /Tu) (e.g. 1/64 or 1/128 for DVB-C2) and the OFDM subcarrier kc at the mixing frequency. [0026] If kc (∆ /Tu) is integer, the phase shift can be removed from the equation as it becomes multiples of 2n. Hence, if kc is multiple of 128 for Guard Interval 1/128, or multiple of 64 for Guard Interval 1/64, equation (6) can be written as: Formula which is similar to the equation for the generation of a DVB-T2 signal. However, it has to be noted that the mixing frequency fc is consequently not the centre frequency of the signal in most cases. [0027] As described above, a common phase rotation may be artificially introduced to the system, depending on the mixing frequency. This common phase rotation is compensated according to an embodiment of the present invention in order to obtain an output signal as defined in the DVB-C2 specification. Alternatively, according to another embodiment this common phase rotation can be avoided by carefully choosing the mixing frequency fc Therefore, the OFDM subcarrier kc at the mixing frequency fc shall be chosen as: Formula where (A/Tu) is the relative Guard Interval duration (i.e. 1/64 or 1/128 in DVB-C2). Practically, equation (9) obtains the carrier kc that is closest to the central OFDM subcarrier (K max + K min) I 2, and additionally, generates multiples of 2π in the above equation (7). Here, the operation \_x\ denotes the floor operation (largest integer not greater than x). More generally, the mixing frequency f cis selected as with the OFDM subcarrier kc at the mixing frequency f c being selected to be close or as close as possible to the central subcarrier among the subcarriers of said OFDM symbol. Here, "close" shall be understood such that not necessarily the mixing frequency fc must be located as close as possible to the central subcarrier, but can also be located farther away. For instance, one of the next possible mixing frequencies (seen from the frequency of the central subcarrier) that fulfills the above mentioned condition that kc (∆ /Tu) is integer can be selected as well. [0028] Consequently, the obtained mixing frequency fc is: Formula where l/Tu is the OFDM subcarrier spacing. Here, the resulting mixing frequency fc is not the centre frequency of the OFDM signal in most cases. [0029] In a more general embodiment the mixing frequency fc is selected as Formula with the OFDM subcarrier kc at the mixing frequency f c being selected to be as close as possible to the central subcarrier among the subcarriers of said OFDM symbol, wherein Tv is the useful OFDM symbol duration. In other words, the mixing frequency f is selected such that the OFDM subcarrier kc at the mixing frequency f is selected that is nearest to Formula [0030] In the following it is assumed that the mixing frequency f is chosen as described above in equations (9) and (10). Hence, the transmitted signal can be described as: Formula with Formula and Formula where kc denotes the OFDM subcarrier at the mixing frequency fc; k' denotes the carrier number relative to the OFDM subcarrier at the mixing frequency fc,i.e..k'=k- K; φkc denotes the phase jump between two consecutive OFDM symbols as calculated according to equation (7); and where the other parameters have the above mentioned meaning. [0031] Practically, this generation is equivalent to the generation of a DVB-T2 signal as shown above. The only difference is the additional phase correction term φm that linearly increases every OFDM symbol and compensates the unwanted phase rotations in the generated output signal. The data c1k that is used for calculating the inverse FFT is the inner bracket of equation (11), i.e. Formula [0032] An embodiment of a possible implementation of a transmitter will now be described. First, in Fig. 1 a multi-carrier data transmission system, here a broadcast system, according to the present invention is shown, in particular according to the DVB-C2 standard. The multi-carrier broadcast system comprises a transmitter 1 for transmitting data and one or more receivers 2 for receiving data from said transmitter 1. [0033] The transmitter 1 is provided for processing input data, e.g. one or more MPEG-2 Transport Streams and/or one or more Generic Streams, to obtain OFDM transmission signals, which are fed into a cable network 3, to which said receivers 2 are connected. For this purpose the transmitter comprises particularly an OFDM generator 10 for generating said OFDM transmission signals from OFDM symbols obtained as input data or generated from the input data of the transmitter 1 (for which purpose the transmitter 1 may additionally comprise further elements, e.g. as described in the DVB-C2 standard). Further, the transmitter 1 comprises a transmitter unit 11 for feeding the obtained OFDM transmission signals into the cable network 3. [0034] The receivers 2 each comprise a receiver unit 20 for receiving said OFDM transmission signals from the cable network 3 and an OFDM decoder 21 for decoding OFDM transmission signals into OFDM symbols, which are then outputted for further processing or which are directly further processed in the receiver 2 (for which purpose the receiver 2 may additionally comprise further elements, e.g. as described in the DVB-C2 standard). [0035] Fig. 2 depicts a schematic block diagram of an embodiment of an OFDM generator 10a for the generation of the OFDM signal s(t), which will be described in detail in the following. Briefly summarized, the input signal to the OFDM generator is first zero padded for preparation of the inverse Fast Fourier Transform (IFFT). Then, the Guard Interval is added, the signal is converted from digital to analog, and finally, shifted to the wanted passband frequency. [0036] The zero padding in a zero padding unit 12 is preferably provided to pre-condition the signal for the transformation of the frequency domain signal into the time domain using the Inverse Fast Fourier Transform. Firstly, the signal is stuffed in order to fit the IFFT size N. Secondly, a realignment of the subcarrier positions is done to be able to use the IFFT. [0037] In order to use the Inverse Fast Fourier Transform, e.g. based on the Radix 2 algorithm, it has to hold N = 2p, p = 1,2, 3, 4, Generally, instead of using a Fast Fourier Transform it is also possible to use a Discrete Fourier Transform (DFT). Furthermore, the value N shall be significantly higher than the actual number of used OFDM subcarriers in order to avoid alias effects, i.e. Formula where x shall preferably be at least 512 for practical implementations according to DVB-C2, but could also be lower, e.g. 64 for WLAN applications. [0038] Fig. 3 depicts the principle of the zero padding. In principle, it realises a cyclic shift operation on the actually used OFDM subcarriers and inserts zeros to the remaining positions. Mathematically this operation can be described as: Formula where X(n)m, (or X„ in short) is the N element input signal of the subsequent IFFT unit 13. [0039] The output signal X„ of the zero padding unit 12 has been generated within the frequency domain. The task of the IFFT unit 13 is the calculation of the corresponding time signal. This is achieved by means of Formula for 0