|Title of Invention||
METHOD FOR SYNCHRONISING OFDM SYMBOLS DURING RADIO TRANSMISSIONS
|Abstract||The invention relates to a method for synchronising OFDM symbols during radio transmissions, said method being used to facilitate strong and efficient frame and frequency synchronisation. Additional pilots are added to the OFDM symbols by the transmitter in order to form pilot pairs. A sequence is modulated on said pilot pairs and then extracted by the receiver in order to produce a measure for each OFDM symbol by comparing the extracted sequence and a stored sequence. The OFDM symbol with the largest measure is recognised as being the first OFDM symbol in a frame. The sequence is modulated using a differential modulation, QPSK or BPSK. A first rough time synchronisation is determined by correlation in the receiver using protective intervals. A non-integer frequency error can also be determined in this way in order to correct the received OFDM symbols of said non-integer frequency error. The extracted sequence and the stored sequence are compared by means of cross correlation, said cross correlation providing the measure. The integer frequency error can thus also be determined. Frame synchronisation is improved by averaging the frame synchronisation results of groups of OFDM symbols.|
ROBERT BOSCH GMBH, 70442 Stuttgart
Method for synchronization of OFDM symbols for broadcast radio transmissions
The invention is based on a method for synchronization of OFDM symbols for broadcast radio transmissions of the generic type of the independent patent claim.
It is already known that frequency and frame synchronization are carried out in digital broadcast radio transmission methods such as DAB (Digital Audio Broadcasting) and DVB-T (Digital Video Broadcasting Terrestrial), in order to find the frame start at the correct frequency in the receiver. In the case of DAB, an entire OFDM (Orthogonal Frequency Division Multiplex) symbol is in this case used for synchronization, while continuous pilot carriers are used for DVB-T.
Advantages of the invention
The method according to the invention for synchronization of OFDM symbols for broadcast radio transmissions and having the features of the independent patent claim in contrast has the advantage that the OFDM symbol, which has the pilot pairs for frame and frequency synchronization, also has user data, so that an entire OFDM symbol is not used just for synchronization. This is particularly advantageous for digital broadcast radio transmissions which are provided in the short wave, medium wave and long wave bands, and are known by the name DRM (Digital Radio Mondial) . In this case, the bandwidth must be used particularly economically.
A further advantage is that the pilot pairs can be distributed over the available channel bandwidth of an OFDM symbol, so that the influence of the frequency-selective fading on these pilot pairs is minimized. In the case of DRM, only a restricted bandwidth of 3 kHz out of a total of 10 kHz is used, for a number of reasons.
The measures and developments described in the dependent claims allow advantageous improvements to the method, as specified in the independent patent claim, for synchronization of OFDM symbols for broadcast radio transmissions.
It is particularly advantageous for the sequence to be modulated onto the pilots at the transmission end by means of differential modulation, by quadrature phase shift keying or binary phase shift keying. These types of modulation make the sequence particularly robust with respect to noise and channel disturbances.
Coarse time synchronization is initially carried out for the OFDM symbols by evaluation of the guard interval. The frequency offset, which is not an integer, can be determined from the phase of the correlations of the guard interval, in order then to correct the received OFDM symbols by this non-integer frequency offset. This OFDM symbol synchronization is in this case advantageously achieved by means of correlation, which is a very simple and known method in signal processing.
Furthermore, it is advantageous for the received sequence, which has been modulated onto the pilot pairs, to be compared with the known and stored transmission sequence by means of cross-correlation. This results in a measure, by means of which the integer frequency offset can be determined as well as
the frame synchronization, since the first OFDM symbol of a frame is identified in this way. Only the first OFDM symbol of a frame has the modulated sequence with the pilot pairs.
A further advantage is that the frame synchronization is improved by averaging the frame synchronization results over groups of OFDM Symbols, since this makes it possible to compensate for errors in the frame synchronization determination process, in an advantageous manner.
A final advantage is also that a broadcast radio transmitter and a broadcast radio receiver are provided which allow the method according to the invention to be carried out.
Exemplary embodiments of the invention are illustrated in the drawing and will be explained in more detail in the following description. Figure 1 shows a block diagram of the method according to the invention, Figure 2 shows a block diagram of the overall transmission system using the method according to the invention, and Figure 3 shows an arrangement of the pilot pairs in the first OFDM symbol.
A fundamental requirement of any transmission system is rapid synchronization to an incoming data stream, so that rapid further processing and audio reproduction of audio signals contained in received broadcast radio signals are possible. This means that it is necessary to provide both frequency synchronization and time synchronization and, in the process, especially frame synchronization, if the data is transmitted in frames.
In this case, in particular, any frequency offset (that is to say any error in the frequency in the receiver, which is produced by a fixed-frequency oscillator) from the predetermined frequency must be compensated for. This error is caused by temperature effects in the oscillators as well as by ageing of the oscillators. The aim for frame synchronization in this case is to find the frame start.
Digital broadcast radio transmissions are frequently carried out by means of orthogonal frequency-division multiplexing. Orthogonal frequency-division multiplexing (OFDM) means that the data to be transmitted is distributed between a large number of carriers which are close to one another but are at different frequencies. In this case, the carriers are configured such that the information distributed between the carriers is not subject to mutual interference. This behavior is described by orthogonality. The use of OFDM leads, in particular, to the frequency-selective fading, which is known in radio transmissions, that is to say frequency-selected attenuation, not influencing all the information which is intended to be transmitted, but influencing only a few parts of it, so that these disturbances can be compensated for by means of error correction methods, also referred to as channel coding. Channel coding in this case means that redundancy is added to the data to be transmitted, from which reconstruction may be possible from the corrupted data which may be received in the receiver.
Two broadcast radio transmission methods which are already in use and make use of OFDM are DAB and DVB. DRM (Digital Radio Mondial) will also use OFDM for transmissions in the short, medium and long wave bands. A high level of robustness of the synchronization and optimum efficiency of the data to be transmitted are
particularly important for DRM, owing to the difficult wave propagation conditions and the low transmission frequencies.
Thus, according to the invention, a method for synchronization of OFDM symbols for broadcast radio transmissions is used, with pilots being added in a first OFDM symbol of a frame which is to be transmitted, so that this results in pilot pairs with the already existing pilots. According to the invention, a sequence which is extracted from the received signals at the receiving end is modulated onto the pilot pairs. The received sequence and a sequence which is stored in the receiver are compared, in particular by means of cross-correlation, in order to determine whether the first OFDM symbol is or is not present. This achieves frame synchronization. Furthermore, the integer frequency offset is determined from the measure of the cross-correlation.
In a first coarse time synchronization stage, which is achieved by evaluation of the guard interval of an OFDM symbol, the non-integer frequency offset is determined, in order then to correct this in the received OFDM symbols. Furthermore, this coarse time synchronization is necessary in order to place the window optimally for fast Fourier transformation. This is particularly due to the fact that the data for frame synchronization is in the form of the pilots, and the frame synchronization can thus be carried out only downstream from the OFDM demodulator, that is to say the FFT (Fast Fourier Transform). This two-stage method is therefore required for time and frequency synchronization.
The sequence modulated onto the pilot pairs is modulated by means of differential modulation, that is to say either quadrature phase shift keying (QPSK) or binary phase shift keying (BPSK). In the process, the
information is now carried by changing the phase between two successive pilot carriers. This leads to very robust transmission. In BPSK, by way of example, these are the phases 0 and 180°, and in QPSK there are four phases, which are each offset by 90° with respect to one another, that is say, for example, 45°, 13 5°, 225° and 315° in this case.
Averaging of the frame synchronization results over a number of groups of OFDM symbols, in which case the groups may be a subset of frames, results in an improvement in the frame synchronization results, since errors cancel one another out.
Figure 1 shows the method according to the invention for synchronization of OFDM symbols for broadcast radio transmissions in the form of a block diagram. A data stream 2 enters an OFDM modulator 1, in order to be distributed, as described above, between various carriers, which are close to one another and are at different frequencies. Pilots are added to the OFDM data in block 3, with pilot pairs being produced in the first OFDM symbol of a frame. The OFDM symbols are sent with the pilots in block 4, in order then to be transmitted via the radio channel in block 5. The OFDM symbols are received in block 6, in order then to carry out coarse time synchronization by means of correlation of the guard interval with the symbol end in block 7. This evaluation process is carried out by identification of the guard interval of an OFDM symbol. The guard interval of an OFDM symbol has the object of compensating for multipath propagation since, in radio transmission, signals can travel from the transmitter to the receiver over different routes, which are of different length. This is due to the fact that the radio signals take different routes from the transmitter to the receiver by being reflected on buildings, mountains and vegetation. The guard interval
should be chosen to be sufficiently long that
successive symbols are not superimposed as a result of
multipath propagation. The guard interval accordingly
offers a type of buffer, in order to avoid
superimposition of user information.
The presence of a frequency shift between the transmitter and receiver can result in a frequency offset, which may be split into two components. Firstly a component which is an integer multiple of a carrier separation and secondly a component which forms the non-integer remainder. The integer component leads only to all the carriers being shifted at the output of the OFDM demodulator. This therefore does not interfere with the orthogonality of an OFDM transmission system. However, the non-integer component does interfere with the orthogonality of an OFDM system. This non-integer component must therefore be estimated in order to correct the OFDM symbols for this non-integer component. This may be done by evaluation of the phase of the autocorrelations of the guard interval. The following equation describes the mathematical relationship between the phase
where dopt = start position of the OFDM signal NpD = DFT length
NG = length of the guard interval in sample values Tu = Useable symbol duration
DFT means discrete Fourier transformation, while FFT is a DFT using an efficient algorithm.
The non-integer component df is then compensated for for all the sample values of the received signal. This is then done in block 8. The OFDM demodulation is then carried out in block 9. The demodulated OFDM symbols are now produced at the output of the OFDM demodulation. The sequence is now extracted from the pilot, in block 10. However, at this time, as there is no knowledge of which OFDM symbol is the first OFDM symbol, the receiver tries to extract the sequence from various OFDM symbols, in order then to compare the extracted sequence with a stored sequence, . which corresponds precisely to the transmitted sequence. Different trial positions in an OFDM symbol are also used for pilot extraction in block 10, since the first, coarse time synchronization using the guard interval does not necessarily accurately lead to synchronization with the OFDM symbol start. Other synchronization algorithms are also possible, as well as synchronization using the guard interval.
In this case, in block 10, the receiver carries out differential demodulation, in order to extract the sequence. The use of cross-correlation for the comparison of the extracted sequence and of the stored sequence results in a correlation quality measure A, which is a maximum when the demodulated sequence is identical to the stored sequence. That OFDM symbol for which the maximum of the correlation quality measure A was calculated is the first OFDM symbol of a frame. Frame synchronization is thus then achieved. The trial position for which this maximum was calculated indicates the integer frequency offset.
Mathematically, the frame and frequency synchronization can be described as follows. Let s be a sequence which is still to be defined with good correlation characteristics:
last equation (1) corresponds to cyclic autocorrelation of the sequence s, provided that all the pilot pairs are located in the evaluation window of the frames [sic] and frequency synchronization.
Various cross-correlation quality measures can be defined in order to assess the correlation results, such as the merit factor (MF), which indicates the ratio of the energy in the main value of the cyclic cross-correlation to the total energy contained in the secondary values:
This correlation quality measure can be determined for each correlation sequence determined using equation (1). It can be seen from equation (2) that the metric value becomes a maximum when the sequence r is most similar to the sequence s. In this case, the energy which is contained in the main value of the cyclic cross-correlation is a maximum, and the energy contained in the secondary values is a minimum.
A further correlation quality measure, in addition to the merit factor, is the ratio of the cross-correlation main maximum to the secondary maximum which has the largest magnitude. However, investigations have shown that the merit factor is the suitable correlation quality measure for the noise disturbances to be expected.
When choosing the correlation sequence s, it should be remembered that: in principle, sequences with perfect
autocorrelation characteristics can be found for each length of sequence, in other words, the number of pilot pairs is a parameter which can be selected. The individual OFDM symbols are subject to major disturbances at the receiver due to multipath propagation. However, since no channel estimate is yet available during the synchronization phase, the correlation sequence r at the receiver must be determined by differential demodulation from the individual subcarrier symbol pairs. In order to achieve a high level of robustness against noise disturbances, the number of phase states in the correlation sequence s should be as small as possible. Binary phase shift keying or quadrature phase shift keying, one of which has two phase states and the other of which has four phase states, is thus used. The frame synchronization 11 and the integer frequency offset 12 are then available as output values from the block 10.
Figure 2 shows a block diagram of the overall transmission system using the method according to the invention. This shows the data sources 13, audio coding 15, other data and control data 19. These data sources 13, 15 and 19 each lead to channel coding 14, 16 and 20. The channel coding 14 and 16 is in each case followed by resorting of the data in time in the blocks 17 and 18, which is known as time interleaving. The resorting of the data on the basis of time has the advantage that adjacent data, for example speech data, is not completely disturbed by one disturbing burst, since the data items are well separated from one another in time after the time interleaving. The effect of this burst can then be corrected more easily at the receiving end by means of error correction measures.
The output data from the time inter leavers 17, 18 and from the channel coding 20 is combined in a multiplexing stage in block 21. In block 22, the data
is distributed between various carriers, that is to say in this case OFDM modulation takes place. In block 23, the OFDM signals are transformed to the time domain, with the pilots being added to the OFDM symbols in block 24. In block 25, the OFDM signals are converted to analog signals, by means of a digital/analog converter 25, for transmission. The subsequent radio-frequency transmitter 26 with an antenna transmits the analog signals via the radio channel 27.
A radio-frequency receiver 2 8 receives the OFDM signals which have been transmitted via the radio channel 27. The radio-frequency receiver 28 passes the received OFDM signals to an analog/digital converter 29, which converts the received OFDM signals to digital signals. OFDM demodulation is carried out in block 30, with coarse time synchronization having previously been carried out in block 31, as described above. The demodulated signals are then sent to a block 32 for decoding the control information and/or to a block 33 for so-called deinterleaving, that is to say to reverse the resorting of the data in time, so that the data is now once again available in the correct time sequence, and is sent for program selection. Channel decoding is carried out in block 34, while decoding of the audio data or other data is carried out in block 35.
Figure 3 shows an arrangement of the pilots in the first OFDM symbol of a frame. The numbers in the horizontal direction indicate the various carriers for an OFDM symbol. The symbols 38 represent pilots which have already been provided for channel estimation. The symbol 39 represents the pilots which, according to the invention, are added to the first symbol in order to produce pilot pairs or even pilot triples. These additional pilots are in this case also used to improve the channel estimate, which is required for coherent demodulation.
The arrangement of the pilot pairs is initially random, with a large number of . pilot pairs onto which the sequence is modulated leading to better frame synchronization than a smaller number of pilot pairs. User information is located between the pilots, and is also transmitted in the first symbol, which is used for frame synchronization. Different ' merit factors A have been calculated for different known sequences with very good correlation characteristics, such as CAZAC sequences, Milewski sequences, Frank sequences, Lemple sequences, difference set sequences and quadratic residue sequences, and it has been shown that it is impossible to calculate a good merit factor for sequences with sequence lengths of 10, 14 and 18 with a small number of phase states. However, a sequence with 16 pilot pairs, a Frank sequence, has a particularly high merit factor and is thus particularly robust. Four phase states, that is to say QPSK, were used in this , case.
The performance of the synchronization method can be increased further by averaging over groups of OFDM symbols, in which case the groups of OFDM symbols may be parts of a frame. This is due to the fact that individual errors can be canceled out by an averaging process.
In the DRM GROUND mode, which is used in the medium wave band and in the long wave band, the additional pilots in order to form the pilot pairs are arranged in accordance with the following Table 1 for the corresponding carrier indices, specific frequencies and phases of the additional carriers:
Table 1: Carrier indices of the additional pilots and the phases of the additional carriers in the DRM GROUND mode
The following Table 2 shows the carrier indices and the phases of the additional carriers for the pilots to be added for the DRM SKY mode. The DRM SKY mode is intended for the short wave band.
Table 2: Carrier indices of the additional pilots and the phases of the new carriers in the DRM SKY mode
ROBERT BOSCH GMBH, 70442 Stuttgart We Claims
1. Method for synchronization of OFDM symbols for broadcast radio transmissions, with the OFDM symbols being transmitted in frames, and with the OFDM symbols having user data, pilots and guard intervals, characterized in that, at the transmission end, additional pilots are added to a first OFDM symbol of a frame, so that pilot pairs are formed, in that, at the transmission end, a sequence is modulated onto the pilot pairs, in that pilot pairs are demodulated for the received OFDM symbols in order to extract the sequence, in that, at the receiving end, the extracted sequence and a stored sequence are compared by producing a measure for each OFDM symbol, and in that the OFDM symbol having the largest measure is identified as the first OFDM symbol in a frame.
2. Method according to Claim 1, characterized in that the sequence is modulated onto the pilot by means of differential modulation.
3. Method according to Claim 2, characterized in that quadrature phase shift keying or binary phase shift keying is used as the differential modulation.
4. Method according to one of the preceding claims, characterized in that, at the receiving end, symbol synchronization and determination of a non-integer frequency offset are carried out on the basis of the guard intervals of the OFDM symbols, and in that the received OFDM symbols are corrected by the non-integer frequency offset.
5. Method according to Claim 4, characterized in that the symbol synchronization is achieved by means of correlation.
6. Method according to Claim 4 or 5, characterized in that the non-integer frequency offset is determined on the basis of the. phase of the autocorrelations of the guard interval.
7. Method according to one of the preceding claims, characterized in that the extracted sequence and the stored sequence are compared by means of cross-correlation.
8. Method according to one of the preceding claims, characterized in that an integer frequency offset is determined by means of the measure.
9. Method according to one of the preceding claims, characterized in that the frame synchronization is carried out via groups of OFDM symbols by averaging the frame synchronization results of the groups of OFDM symbols.
10. Method according to one of the preceding claims, characterized in that, in the DRM GROUND mode, the carrier indices (V7, 19, 21, 28, 29, 32, 33, 39, 40, 41, 53, 55, 56, 60, 61, 63, 71 as well as 73 are used as the carrier indices for the additional pilots, and 270°, 180°, 0°, 0°, 0°, 180°, 0°, 90°, 180°, 180°, 90°, 0°, 0°, 0°, 0°, 270°, 0° and 180° are used as the phases which are respectively associated with these carrier indices.
11. Method according to one of the preceding claims, characterized in that, in the DRM SKY mode, the carrier indices 14, 18, 20, 24, 26, 32, 36, 42, 44, 50, 54, 56, 62, 66 and 68 are used as the carrier indices for the
additional pilots, and 90 °, 180°, 0°, 0°, 180°, 90°, 0°, 180°, 270°, 0°, 0°, 270°, 0° and 180° are used as the phases which are respectively associated with these carrier indices.
12. Broadcast radio transmitter and broadcast radio receiver for carrying out the method according to one of Claims 1 to 11.
13. Method for synchronization of OFDM symbols for broadcast radio
transmissions, substantially as hereinabove described and illustrated with reference to the accompanying drawings.
|Indian Patent Application Number||IN/PCT/2002/1943/CHE|
|PG Journal Number||26/2007|
|Date of Filing||26-Nov-2002|
|Name of Patentee||ROBERT BOSCH GMBH|
|Applicant Address||Postfach 30 02 20 70442 Stuttgart|
|PCT International Classification Number||H04L27/26|
|PCT International Application Number||PCT/DE2001/001569|
|PCT International Filing date||2001-04-26|