Title of Invention

METHOD FOR FRAME AND FREQUENCY SYNCHRONIZATION OF AN OFDM SIGNAL AND METHOD FOR TRANSMITTING AN OFDM SIGNAL

Abstract

A method is proposed for frame and frequency synchronization of an OFDM signal and of a signal for transmitting an OFDM signal, which is used to apply a pilot phase profile to pilots which are already included in the OFDM signal for channel estimation, with this pilot phase profile then being used for frame and frequency synchronization at the receiving end. This has the advantage that no additional transmission capacity need be used for synchronization. The method according to the invention is initiated by an upstream, rough time synchronization unit, which looks for the start of the guard interval in the OFDM signal. The stored pilot phase profile is compared with the received subcarrier symbols by means of cross-correlation, whose result is then assessed in order to determine the frame and frequency synchronization. (Figure 3)

Full Text

03.27.01 Vg/Pz ROBERT BOSCH GMBH, 70442 Stuttgart > Method for frame and frequency synchronization of an OFDM signal, and a method for transmitting an OFDM signal Prior Art i The invention is based on a method for frame and frequency synchronization of an OFDM signal, and of a method for transmitting an OFDM signal of the generic type of the independent patent claims. A worldwide consortium (DRM - Digital Radio Mondiale) is developing a new digital broadcast radio transmission standard for the frequency range below 30 MHz. The multicarrier method OFDM (Orthogonal Frequency Division Multiplexing) is intended to be used as the modulation method here (to be more precise, the intention is to use a coherent OFDM transmission method). The OFDM signal comprises OFDM symbols, which each in turn contain subcarrier symbols. Predetermined subcarrier symbols at the transmission end are in the form of pilots, so that channel destination is thus possible at the receiving end. The pilots are in this case distributed between the subcarriers in the time and frequency domains. Advantages of the invention The method according to the invention for frame and frequency synchronization of an OFDM signal, and the method for transmitting an OFDM signal having the features of the independent patent claims, have the advantage that the pilots which are provided in any case are now also used at the receiving end for frame and frequency synchronization, by applying a pilot phase profile, which is unique within a frame, to the pilots at the transmission end. Each OFDM symbol in a frame can then be distinguished by means of its pilot phase profile. The pilots are therefore used for an additional purpose, and there is no need to provide any additional transmission capacity for frequency and frame synchronization. Furthermore, the method according to the invention for frame and frequency synchronization is distinguished by being very robust with regard to poor propagation and reception conditions. This can be improved by using two or more (different) pilot phase profiles in one transmission frame for frame and frequency synchronization. According to the invention, it is also possible to carry out the frequency and frame synchronization even within one transmission frame. This is because the DRM (Digital Radio Mondiale) have subdivided the OFDM symbols into transmission frames. A further advantage is that use of the distributed pilots makes it possible to achieve a wide pull-in range for rough frequency estimation. The pilot phase metric makes it possible to uniquely detect a frequency offset of more than half the signal bandwidth. The expression pilot phase metric is used in the following text to denote a calculation rule by means of which the pilot phase profile is compared at the receiving end with the received subcarriers or subcarrier symbols. The expressions subcarriers and subcarrier symbols are used below as synonyms. The measures and developments described in the dependent claims allow advantageous improvements to the methods, as specified in the independent patent claims, for frame and frequency synchronization of an OFDM signal, and for transmission of an OFDM signal. A further advantage is that the received subcarrier symbols are compared with a stored pilot phase symbol only after an OFDM demodulator (DFT unit), since this makes it possible to use a large number of pilot subcarriers, whose main object is channel estimation, for synchronization purposes. For this reason, the OFDM demodulation window must be correctly positioned in advance, that is to say rough time synchronization must be carried out. In order to achieve rough time synchronization, it is advantageous to use autocorrelation to search for the guard interval in the received OFDM signal. The same method can also be used to achieve estimation of a fine frequency offset. However, for correct demodulation of the payload data, it is also necessary to determine the rough frequency offset, that is to say the integer, multiple subcarrier separation and the frame starter. This is achieved by the method according to the invention. It is advantageous for the comparison of the pilot phase profile, which is split at the receiving end, with the subcarrier symbols to be carried out by cross-correlation, and for the result of the cross-correlation to be assessed in order to determine the frame and frequency synchronization. The assessment can be carried out, for example, by means of a main/secondary maximum ratio, or by means of a merit factor. It is also advantageous for the pilot phase profile which is required for frame and frequency synchronization to be determined by means of a pseudo-random sequence or by means of a deterministic function. This function as well as the pseudo-random sequence are then known at the transmission end and at the receiving end. It is also advantageous for the pilots to be uniformly distributed in an OFDM symbol, in order in this way to ■ achieve a high degree of robustness and to place the pilots optimally for channel estimation. A further advantage is the high degree of robustness of the frame and frequency synchronization method in response to noise interference. This robustness is achieved by using a large number of pilot subcarriers for the calculation of the pilot phase metric. Finally, it is also advantageous for a transmitter and a receiver to be provided in order to carry out the method according to the invention. Drawing Exemplary embodiments of the invention will be explained in more detail in the following description and are illustrated in the drawing, in which: Figure 1 shows a block diagram of an overall transmission system, Figure 2 shows a block diagram relating to the pilot phase metric, Figure 3 shows a flowchart of the method according to the invention for transmission of the OFDM symbol, Figure 4 shows a distribution of pilots in an OFDM symbol, Figure 5 shows a pilot phase metric for various OFDM symbols, and Figure 6 shows main/secondary maximum ratio values for a number of DRM frames. Description Owing to difficult wave propagation conditions, especially for short wavelengths, the synchronization algorithms that are used must have a high degree of robustness. Determination of and compensation for the frequency offset and finding the start of a frame are ■ necessary conditions m oraer to ensure reception ot digital broadcast radio programs. Owing to the narrow channel bandwidth and the low data rate associated with it, it is not possible to use a complete OFDM pilot symbol for synchronization purposes. Correct demodulation of the payload data also requires up-to-date channel estimation for the transmission channel. Thus, according to the invention, a pilot phase profile is applied at the transmission end, so that frame and frequency synchronization is possible at the receiving end. The use of the methods according to the invention is of particular interest for digital amplitude modulation (AM broadcast radio transmission) since the net bit rate with these applications is comparatively low. Figure 1 shows a block diagram of the overall transmission system. The data sources are an audio coder 1, additional data 2 and control data 3, which are each subjected to coding by means of respective coders 4, 5 and 6. The audio data and additional data coding in this way is then interleaved in the blocks 8 and 7. A multiplexer 9 then joins the audio data, the additional data and the control data together to form a data stream, which is subjected to frequency interleaving in the block 10 and inverse discrete Fourier transformation in the block 11, thus resulting in OFDM modulation. The block 11 is thus also referred to as an OFDM modulator. The pilots with the pilot phase profile from a memory 30 are added to the data stream in the OFDM modulator 11. The OFDM signal produced in this way is then added to an analog signal in the block 12. Transmission amplification and transmission of the broadcast radio signals by means of an antenna are carried out in the block 13. The OFDM signal then reaches a receiver via a radio channel 14, to be precise in a block 15 which has an antenna and a radio-frequency receiver. The received signals are then digitized in the analog/digital converter 16. The samples obtained in this way are now subjected to fast Fourier transformation in the block 17 (OFDM demodulation). The block 18 also carries out the synchronization according to the invention. The control information contained in the data is decoded in the block 19, while the deinterleaving of the audio data and additional data is carried out in parallel in the block 20. The program selection from the data stream is also carried out here, that is to say, for example, which broadcast radio program has been selected by the user. This selected data is then decoded in a block 21, in order to carry out audio decoding in the block 22, so that audio data which can be reproduced by means of a loudspeaker and an audio amplifier is then produced at the output of the audio decoder 22. Pilots are added to the data to be transmitted, in the OFDM modulator 11. These pilots are used for channel estimation for the transmission channel 14. A phase profile is now also applied to these pilots and is referred to in the following text as a pilot phase profile. The pilot phase profile is then used for frame and frequency synchronization in the block 18 at the receiving end. Figure 4 shows a distribution of the pilot symbols in the frequency and time domains, with the pilots being identified by 0. When using coherent OFDM systems, such as those that are intended to be used for DRM, channel estimation by means of pilot subcarrier symbols is necessary, since equalization and correct demodulation must be carried out. A uniform distribution of the pilot subcarrier s in the time and frequency domains results in good channel estimation. The data subcarriers are represented by a dot in Figure 4. In general, for reliable channel estimation, there is no . need to transmit a pilot symbol on each subcarrier, since the transmission channel 14 varies only at a finite rate. Channel estimation for the subcarriers located between two pilots is thus achieved by means of interpolation. The phases of the pilot symbols are irrelevant to the quality of channel estimation. All that is necessary is to ensure that the crest factor of a multitone signal that is produced by pilot symbols is low. The crest factor of a multitone signal can be kept low by using the following simple phase law (equation 1) . The k-th pilot subcarrier in the 1-th OFDM symbol can thus be written in the form: where p(l,k) :Index of a pilot subcarrier in the 1-th OFDM symbol in a frame No : Integer It should be noted that the phase of the pilot subcarriers depends only on the subcarrier index p(l,k) in equation 1. If an additional phase rotation The phase (PRNDd/k) is in this case a pseudo-random additional phase rotation. The value of this additional phase rotation is dependent on subcarrier index k and on the OFDM symbol number 1. The additional phase rotations can be stored in a phase matrix. where NFRAME • number of OFDM symbols within a frame NCARRIERS : number of OFDM subcarriers The individual elements (pRwod/k) may in this case ideally originate from one pseudo-random noise sequence. This results in the greatest possible variation between the pilot phases of different OFDM symbols. It is also feasible to use a simpler phase law, as described in equation 3. The readings from the symbols in equation 4 are: x : Frequency subsampling factor y : Time subsampling factor TG : Guard interval Tu : Usable symbol duration Ts : OFDM symbol duration; Ts = TG+Tu ki : Index of first pilot subcarrier in the 1-th OFDM symbol ■ Pd/k) : Index of a pilot subcarrier in the 1-th OFDM symbol of a frame; p(l, k) =kx+ixy Po : Constant i : Index arg {Z(l,ki)} : Phase of the first pilot subcarrier in the 1-th OFDM symbol (= Start phase for deterministic calculation of the other pilot subcarrier phases) The phase values arg{Z(l,ki)} are chosen as elements in a pseudo-random noise sequence. The important factor is that the addition of an additional phase rotation results in a unique pilot phase profile within the transmission frame. The exact calculation rule for determining the pilot phase profile is of secondary importance to the proposed synchronization algorithm. If the aim is to use the algorithm described in the following text to carry out frame synchronization, then cpRNDd/k) must be a real function of 1 and k. If, on the other hand (PRM, (1, k) = f (1) or (PRND(1, k) = f (1) + f (§) is chosen, then the algorithm described in the following text allows the frequency offset to be determined only roughly- For frame synchronization from the distributed pilot arrangement, the pilot phases from different OFDM symbols must differ sufficiently, or - expressed mathematically - q>RND(l,k) = f(l,k) must therefore be a real function of the subcarrier index k and OFDM symbol number 1. A further important factor is that (PRND (1, k) = The following text describes how a unique pilot phase profile can. be used not only for frame synchronization but also for determining the rough frequency offset for a coherent OFDM system. This method avoids additional redundancy for frame synchronization. Before the proposed synchronization algorithm can be used, rough time synchronization must be carried out in order to position the DFT (demodulation) window. Rough time synchronization can be carried out by calculating the correlation between parts of the guard interval with the corresponding section at the end of the usable OFDM symbol. It is known that the same method can likewise be used to estimate the fine frequency offset (± 0,5 1/Tu) . However, detection of the rough frequency offset (an integer multiple of the subcarrier separation 1/Tu) and the start of the frame are unknown, but are essential for correct demodulation of the payload data. These can be determined using the following method. The starting point for determining the rough frequency offset and the frame start is the calculation of the cross correlation between the received subcarrier symbols R(1,k) and the pilot phase sequence W(l,p(l,k)). The calculation rule based on equation 5 is referred to in the following text as the pilot phase metric. One precondition for the use of the pilot phase metric is that the start of the OFDM demodulation window is located in that area of the guard interval which is free of intersymbol interference (ISI-free). The meanings of the symbols in Equation 5 are: 1 : OFDM symbol number within a frame P(l,k) : Index of a pilot subcarrier in the 1-th OFDM symbol of a frame i : Trial position for determining the rough frequency offset (the index i runs in the frequency domain) s : Trial position for determining the frame start symbol (the index s runs in the time domain) ABS : Absolute value R(l,k) : k-th subcarrier symbol in the 1-th OFDM symbol Equation 5 produces a maximum value when the pilot phase sequence W(l,p(l,k)) matches the received subcarrier sequence (R(s,p(l,k)+i). In all other cases, the pilot phase metric assumes a low value when using a pseudo-random noise profile, owing to the pseudo-random noise character of the phase sequence. This situation is illustrated in Figure 5. Equation 5 must be calculated for a number of trial positions i in order to determine the rough frequency offset. If, in contrast, a deterministic pilot phase profile according to Equation 3 or Equation 4 is used, then the pilot phase metric is periodic with the pilot separation. In this case, Equation 5 allows only the frame start to be determined. The pull-in range for determining the rough frequency offset is restricted by the interval between the pilot subcarriers xy. If even exact time synchronization is known, then Equation 6 can be used as an alternative for finding the rough frequency offset and the frame start. In comparison with Equation 5, the cross-correlation between the pilot phase sequence W(l,p(l,k)) and the received subcarrier symbols is calculated in this case: Equation 6 allows unique determination of the rough frequency offset, using either a pseudo-random noise . profile or a deterministic phase profile, according to Equation 3 or Equation 4. In order to achieve frame synchronization it is possible on the one hand to correlate the received subcarrier symbols with all the possible pilot phase sequences in a frame, or on the other hand to correlate a pilot phase sequence with all the received subcarrier symbols. The estimation result can be improved by looking for not just one specific pilot phase profile W(l,p(l,k)) but also two or more at the same time; since, according to Equation 3/ the pilot phase profile for each OFDM symbol of a frame is unique. Mathematically, this means averaging of the metric results A(I,p(l,k),s,i) from Equation 5: where nb: The number of OFDM symbols with which the average process is carried out (1.. NFRAME). Various correlation quality measures may be defined in order to assess the matrix elements A(s,is), such as the HNV, which indicates the ratio of the main maximum A(s, is at the location 1 of the pilot phase metric to the secondary maximum with the largest magnitude. The HNV must be calculated for all possible positions of the frame start (that is to say a total of NFRAME times) . Figure 6 shows the HNV values for 4 DRM frames. The frames start symbol must in each case be identified uniquely. Maximum detection of HNV produces: The indi ces Smax and imax in Equation 9 for the maximum HNV indicate the position of the frame start symbol, and the rough frequency offset, respectively. Analogously to the HNV the merit factor (MF) can also be used as a correlation quality measure. The merit factor describes the ratio of the energy of the main value of the pilot phase metric A (s, is) to the total energy contained in the secondary values. The evaluation algorithm for frame and frequency synchronization is then: In this case as well, the indices s^ax and imax of the maximum MF indicate the frame start symbol and the rough frequency offset, respectively. The maximum pull-in range of the pilot phase metric is determined by the number of pilot subcarrier symbols in the evaluation range. The use of pilot arrangements as shown in Figure 4 allows the pull-in range to be more than half the DFT length. Figure 2 now shows a block diagram of the method according to the invention being used in the receiver. The samples of the received signal r, which has been attained by the analog/digital converter 16, are supplied to a time synchronization unit 27 and to an OFDM demodulator (= DFT unit) 28. The time synchronization unit 27 carries out the rough time synchronization on the basis of the guard interval contained in the received signal. To be more precise, an autocorrelation is calculated in order to look for the start of guard interval, and hence for the start of an OFDM symbol. The data R(l,k) which is demodulated using the OFDM modulator 28 is then passed on for calculation of the pilot phase metric in a processor 29. The value A obtained from this is passed on for averaging over a predetermined number of OFDM symbols, in order to calculate a mean value of A and this is also done in the processor 29. This is then followed either by assessment of the correlation value A with a mains/secondary maximum ratio or, as described above, with a merit factor, with this assessment also being carried out in the processor 29. The indices for the maximum value, calculated in this way, of the correlation quality measure indicate the position of the frame start symbol and the rough frequency offset. In other words, the frequency offset is produced in integer multiples of the subcarrier frequency separation as the result at the output of the processor 29, and the frame start symbol is found during the detection of the maximum value. The receiver thus uses a stored pilot phase profile to search the . received subcarrier symbols, value by value. When the best possible match is achieved between the stored pilot phase profile and the received pilot phase profile, the frame start has then been found, and the rough frequency offset has been detected. Figure 3 shows a flowchart of the method according to the invention, being used in the transmitter. The pilots and the payload symbols to be transmitted are mapped onto an OFDM symbol in a first method step 23. At the same time, the unique pilot phase profile is applied to the pilot (method step 24) . The OFDM symbol produced in this way is then passed to the OFDM modulator 10 and 11 (method step 25), in order to produce an OFDM signal. A guard interval is also added in the OFDM signal. The OFDM signal is transmitted in the block 13 (method step 26). 03.27.01 Vg/Pz ROBERT BOSCH GMBH, 70442 Stuttgart Claims 1. The method for frame and frequency synchronization of an OFDM (Orthogonal Frequency Division Multiplex) signal, with OFDM symbols which each have subcarrier symbols being received with the OFDM signal, characterized in that the received subcarrier symbols are compared with at least one stored pilot phase profile, and in that the frame and frequency synchronization of the OSDM signal are carried out as a function of the comparison. 2. A method for transmitting an OFDM signal, with OFDM symbols being transmitted with the OFDM signal, to each of which OFDM symbols a guard interval has been added and which each have subcarrier symbols, with predetermined subcarrier symbols being transmitted as pilots, characterized in that, before transmission, a respective phase is in each case applied to the pilots such that this results in at least one pilot phase profile. 3. The method as claimed in Claim 1 and 2, characterized in that, before the comparison of the received subcarrier symbols with the stored pilot phase profile, rough time synchronization is carried out by searching for the guard interval in the received OFDM signal. 4. The method as claimed in Claim 1 or 3, characterized in that the comparison is carried out by cross-correlation, and the cross-correlation is then assessed for determining the frame and frequency synchroni zation. • 5. The method as claimed in Claim 1 or 4, characterized in that the comparison is carried out using the following equation: 6. The method as claimed in Claim 2, characterized in that the pilot phase profile is determined by means of an equation or by means of a pseudo-random sequence. 7. The method as claimed in Claim 6, characterized in that the equation is, 9. The method as claimed in Claim 2, characterized in that the pilots are distributed uniformly in an OFDM symbol. 10. The method as claimed in Claim 4 or 5, characterized in that a main/secondary maximum ratio is used for assessment of the cross-correlation. 11. The method as claimed in Claim 4 or 5, characterized in that a merit factor is used for assessment of the cross-correlation. 12. A transmitter for carrying out the method as claimed in Claim 2, 3, 6, 7, 8 and 9, characterized in that the transmitter has a memory (30) with a pilot phase profile, an OFDM modulator (10, 11) , an antenna (12) for transmitting the OFDM signal, and an apparatus for feeding in (11) the pilot with the pilot phase profile. 13. A receiver for carrying out the method as claimed in one of Claims 1, 3, 4, 5, 10 or 11, characterized in that the receiver has a first time synchronization unit (18, 27) for rough time synchronization, an OFDM demodulator (17, 28) and a processor (29) with a memory for carrying out the comparison between the received subcarrier symbols and the stored pilot phase profile. A method for transmitting an OFDM signal substantially as herein described with reference to the accompanying drawings.

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