Abstract:
A receiver detects and recovers data from Orthogonal Frequency Division Multiplexed (OFDM) symbols, the OFDM symbols including a plurality of sub-carrier symbols formed in the frequency domain and transformed into the time domain for transmission, the time domain OFDM symbols including a guard interval. The guard interval of the OFDM symbols is one of a plurality of predetermined temporal lengths and the guard interval is formed by repeating time domain samples of the OFDM symbol from useful samples of the OFDM symbols for a period corresponding to the guard interval from the OFDM symbols. The receiver includes a correlator operable to correlate the time domain samples at a separation determined in accordance with the useful samples of the OFDM symbols and for each of a plurality of possible guard intervals combining the correlated samples for a period equal to the guard interval, for a plurality of consecutively received OFDM symbols, to form for each of the possible guard intervals a correlation output value, and a guard interval identifier arranged to receive the output from the correlator and to identify one of the guard intervals used for the received OFDM symbols from the correlation output value for each of the plurality of possible guard intervals. Accordingly in a system such as DVB-T2, where the guard interval is provided in signalling data communicated with the OFDM symbols, the guard interval can be detected without or before the signalling data is detected, thereby reducing a time for detecting the data conveyed by the OFDM symbols or the guard interval and/or improving an integrity with which the guard interval and therefore the data can be detected.

Description:
FIELD OF INVENTION 
       [0001]    The present invention relates to receivers and methods for detecting and recovering data from Orthogonal Frequency Division Multiplexed (OFDM) symbols, the OFDM symbols including a plurality of sub-carrier symbols, the OFDM symbols being transmitted with a guard interval. 
       BACKGROUND OF THE INVENTION 
       [0002]    There are many examples of radio communications systems in which data is communicated using Orthogonal Frequency Division Multiplexing (OFDM). Systems which have been arranged to operate in accordance with Digital Video Broadcasting (DVB) standards for example, utilise OFDM. OFDM can be generally described as providing K narrow band sub-carriers (where K is an integer) which are modulated in parallel, each sub-carrier communicating a modulated data symbol such as Quadrature Amplitude Modulated (QAM) symbol or Quadrature Phase-shift Keying (QPSK) symbol. The modulation of the sub-carriers is formed in the frequency domain and transformed into the time domain for transmission. Since the data symbols are communicated in parallel on the sub-carriers, the same modulated symbols may be communicated on each sub-carrier for an extended period, which can be longer than a coherence time of a channel via which the data is to be communicated. The sub-carriers are modulated in parallel contemporaneously, so that in combination the modulated carriers form an OFDM symbol. The OFDM symbol therefore comprises a plurality of sub-carriers each of which has been modulated contemporaneously with different modulation symbols. 
         [0003]    In order to allow data to be recovered from the OFDM symbols in the presence of multi-path which causes the same OFDM symbols to be received via echo paths and/or simulcast in which the same OFDM symbols are received from more than one transmitter, it is conventional to include a time domain guard interval between successive OFDM symbols. The guard interval is formed by repeating samples in the time domain from a ‘useful’ part of the OFDM symbols. The useful part of the OFDM symbols correspond to the samples in the time domain which are formed when the modulated sub-carriers are transformed into the time domain from the frequency domain. As a result of the guard interval, all of the samples from the useful part of the OFDM symbols can be received by a receiver provided that the multi-path or the simulcast delay between versions of the same OFDM symbols does not exceed the guard interval. 
         [0004]    However, detecting and recovering data from the useful part of the OFDM symbols at the receiver can nevertheless present a technical problem. 
       SUMMARY OF INVENTION 
       [0005]    According to the present invention there is provided a receiver for detecting and recovering data from Orthogonal Frequency Division Multiplexed (OFDM) symbols, the OFDM symbols including a plurality of sub-carrier symbols formed in the frequency domain and transformed into the time domain for transmission, the time domain OFDM symbols including a guard interval. The guard interval of the OFDM symbols is one of a plurality of predetermined temporal lengths and the guard interval is formed by repeating time domain samples of the OFDM symbol for a period corresponding to the guard interval from the OFDM symbol. The receiver includes a demodulator arranged in operation to detect a signal representing the OFDM symbols, and to generate a sampled version of the OFDM symbols in the time domain, a guard detector, which is arranged in operation to identify from the time domain samples of the OFDM symbols, useful samples of the OFDM symbols, a Fourier transform processor arranged in operation to receive the time domain version of the OFDM symbols and to perform a Fourier transform on the useful samples of the OFDM symbols to form a frequency domain version of the OFDM symbols, from which the data symbol bearing sub-carriers can be recovered. A detector is arranged in operation to recover the data from the data bearing sub-carriers of the OFDM symbols. The guard detector includes 
         [0006]    a correlator arranged to correlate the time domain samples at a separation determined in accordance with the useful samples of the OFDM symbols (Tu) and for each of the plurality of possible guard intervals (Tg) to combine the correlated samples for a period equal to the guard interval, for a plurality of consecutively received OFDM symbols, to form a correlation output value for each guard interval, and 
         [0007]    a guard interval identifier arranged to receive the output from the correlator and to identify one of the guard intervals used for the received OFDM symbols from the output correlation value for each of the plurality of possible guard intervals. 
         [0008]    In some systems, which convey data using OFDM symbols, a plurality of different guard intervals are used in the transmitted OFDM symbols. For example in the DVB-T2 system there are at least seven possible guard intervals. In some circumstances it may be advantageous to detect the guard interval from the received OFDM symbols themselves. Performing a correlation of samples separated by a period of time corresponding to the useful part of the OFDM symbols, and combining the correlated samples by forming an average value of the correlated samples within a moving window equal to a temporal length of the guard interval, a correlation output value can be formed, which depends on the length of the guard interval. By forming such a correlation output value for all possible guard intervals, an estimate can be made of the guard interval from a highest averaged value of the correlated samples produced by a moving averaging filter for a plurality of consecutive OFDM symbols. Since the correlation output value is formed from consecutive symbols, a correct one of the possible guard intervals corresponding to the actual guard interval will reinforce the correlation output value, whereas an incorrect one of the possible guard interval values will reduce the correlation output value when the moving average is applied over consecutive symbols, because non-guard interval samples will appear noise like. As such, an efficient and robust technique can be formed for detecting the guard interval. 
         [0009]    In one example, the correlator includes a multiplier operable to multiply signal samples separated by the useful number of samples in the OFDM symbols one of which is conjugated, and a moving averaging filter for each of the possible guard intervals which forms an average of the samples at an output of the multiplier, within a sliding window of samples. The number of samples averaged by the moving average filter is equal to the corresponding guard interval, the guard interval identifier being arranged to identify the guard interval from the moving averaging filter output having a greatest value after the consecutive number of symbols has been received. In combination, the multiplier and the moving averaging filter form a classic correlation sum, each moving averaging filter thereby forming a correlation sum for a guard interval period. 
         [0010]    Useful samples of the OFDM symbol correspond to the OFDM symbol samples in the time domain (Tu) before the guard interval (Tg) is applied by repeating samples from the OFDM symbol, which has been generated. 
         [0011]    In one example the number of the plurality of consecutively received OFDM symbols is determined in accordance with a minimum time to achieve a reliable detection of the guard interval, for example this may be two or between two and three symbols. 
         [0012]    For some examples, the received OFDM symbols may include signalling data indicating the guard interval of the received OFDM symbols, and the detector is operable to recover the signalling data from the OFDM symbols and to provide the guard detector with the indication of the indicated guard interval. The guard interval identifier within the guard detector maybe arranged to estimate the guard interval from a combination of the guard interval indicated by the signalling data and the guard interval identified from a value of the output of the correlator. As such, depending on the time taken to recover the signalling data, compared with the time taken to detect the guard interval from the correlation output value from the correlator, it may take less time to detect the guard interval by decoding the signalling data. For example, the number of samples of the useful part of the OFDM symbols may vary in accordance with a transmitter operating mode between a predetermined number of discrete values. As such the guard interval identifier maybe arranged in some embodiments to determine the guard interval from the output of the correlator or the signalling data depending on whether the signalling data is available before the guard interval can be determined from the output of the correlator. In other examples, both the guard interval detected from the correlation output value and the guard interval determined from the signalling data maybe used to improve an integrity of the estimated guard interval value. 
         [0013]    For the example of DVB-T2, for example, the operating modes include 1K, 2K, 4K, 8K, 16K or 32K signal samples, and the guard interval identifier is operable to determine the guard interval from the signalling data, if the operating mode is one of the 16K mode or the 32K mode. 
         [0014]    Furthermore in some embodiments, the guard interval identifier may be arranged to determine a fine frequency offset of the OFDM symbols in the frequency domain by determining an argument of the correlation output value for the identified guard interval. The fine frequency offset may be used to adjust a position of the OFDM symbol in the frequency domain to improve a likelihood of correctly detecting the data conveyed by the OFDM symbols. 
         [0015]    In some embodiments the OFDM symbols are transmitted in accordance with a Digital Video Broadcasting standard, such as DVB-T, DVB-T2, DVB-H or DVB-C2. 
         [0016]    Accordingly in a system such as DVB-T2, where the guard interval is provided in signalling data communicated with the OFDM symbols, the guard interval can be detected without or before the signalling data is detected, thereby reducing a time for detecting the data and/or the guard interval and/or improving an integrity with which the guard interval and therefore the data can be detected. 
         [0017]    Various aspects and features of the present invention are defined in the appended claims. Further aspects of the present invention include a method of detecting and recovering data from Orthogonal Frequency Division Multiplexed (OFDM) symbols. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0018]    Embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, wherein like parts are provided with corresponding reference numerals, and in which: 
           [0019]      FIG. 1  is a schematic block diagram of an OFDM transmitter which may be used, for example, with the DVB-T2 standard; 
           [0020]      FIG. 2  is an example illustration of a super frame structure according to the DVB-T2 standard; 
           [0021]      FIG. 3  is a schematic block diagram of an OFDM receiver which may be used, for example, with the DVB-T or DVB-T2 standard; 
           [0022]      FIG. 4  is a schematic representation illustrating example guard bands for OFDM symbols according to the DVB T2 standard; 
           [0023]      FIG. 5  is a flow diagram illustrating a process according to the present technique for recovering signal information within the DVB T2 standard to identify a guard interval being used or to detect the guard interval without the signalling data (blind detection); 
           [0024]      FIG. 6  is a schematic block diagram illustrating a detector for identifying a guard interval of symbol according to the DVB T2 standard without recovering signalling information (blind guard detector); 
           [0025]      FIG. 7   a  is a schematic representation of a correlator for detecting a guard interval when the assumed spacing between OFDM symbols and the guard interval does not equal the actual spacing between OFDM symbols and guard interval;  FIG. 7   b  is an example of an output from the correlator for the example shown in  FIG. 7   a;    
           [0026]      FIG. 8   a  is a schematic block diagram of the operation of the correlator for detecting the guard interval when the assumed separation between the OFDM symbols is equal to the actual separation between OFDM symbols and the guard interval is correct;  FIG. 8   b  is an illustration of an output of the correlator for the example shown in  FIG. 8   a;    
           [0027]      FIG. 9  is a graphical representation of a result of an output of an accumulated correlation magnitude for four possible guard intervals; 
           [0028]      FIG. 10   a  is a graphical representation of accumulated correlation magnitudes for four possible guard intervals where the next maximum value falls below a threshold limit for detecting a correct guard interval; whereas  FIG. 10   b  provides an example where the next maximum correlation value does not fall below the threshold set to identify the correct guard interval; 
           [0029]      FIG. 11  is an example representation of a simplified architecture for detecting a guard interval according to the present technique; 
           [0030]      FIG. 12  is a further illustration of an operation of the simplified architecture shown in  FIG. 11  for a subsequent second symbol; 
           [0031]      FIG. 13  is an illustration of an optimum starting point for detecting a guard interval; 
           [0032]      FIG. 14   a  is a graphical illustration of a performance of correctly identifying a guard interval against additive white Gaussian noise for an FFT size of 8 k and a one path channel with a guard interval of 1/32;  FIG. 14   b  is a graphical representation of a plot of mean phase error and a standard deviation of phase error between a detected fine frequency offset and the actual fine frequency offset for the example of  FIG. 14   a ; and 
           [0033]      FIG. 15   a  is a graphical illustration of a performance of correctly identifying a guard interval against additive white Gaussian noise for an FFT size of 8 k and a one path channel with a guard interval of 1/16; and  FIG. 15   b  is a graphical representation of a plot of mean phase error and a standard deviation of phase error between a detected fine frequency offset and the actual fine frequency offset for the example of  FIG. 15   a.    
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0034]      FIG. 1  provides an example block diagram of an OFDM transmitter which may be used for example to transmit video images and audio signals in accordance with the DVB-T, DVB-H, DVB-T2 or DVB-C2 standard. In  FIG. 1  a program source generates data to be transmitted by the OFDM transmitter. A video coder  2 , and audio coder  4  and a data coder  6  generate video, audio and other data to be transmitted which are fed to a program multiplexer  10 . The output of the program multiplexer  10  forms a multiplexed stream with other information required to communicate the video, audio and other data. The multiplexer  10  provides a stream on a connecting channel  12 . There may be many such multiplexed streams which are fed into different branches A, B etc. For simplicity, only branch A will be described. 
         [0035]    As shown in  FIG. 1  an OFDM transmitter  20  receives the stream at a multiplexer adaptation and energy dispersal block  22 . The multiplexer adaptation and energy dispersal block  22  randomises the data and feeds the appropriate data to a forward error correction encoder  24  which performs error correction encoding of the stream. A bit interleaver  26  is provided to interleave the encoded data bits which for the example of DVB-T2 is the LDCP/BCH encoder output. The output from the bit interleaver  26  is fed to a bit into constellation mapper  28 , which maps groups of bits onto a constellation point of a modulation scheme, which is to be used for conveying the encoded data bits. The outputs from the bit into constellation mapper  28  are constellation point labels that represent real and imaginary components. The constellation point labels represent data symbols formed from two or more bits depending on the modulation scheme used. These can be referred to as data cells. These data cells are passed through a time-interleaver  30  whose effect is to interleave data cells resulting from multiple LDPC code words. 
         [0036]    The data cells are received by a frame builder  32 , with data cells produced by branch B etc in  FIG. 1 , via other channels  31 . The frame builder  32  then forms many data cells into sequences to be conveyed on OFDM symbols, where an OFDM symbol comprises a number of data cells, each data cell being mapped onto one of the sub-carriers. The number of sub-carriers will depend on the mode of operation of the system, which may include one of 1 k, 2 k, 4 k, 8 k, 16 k or 32 k, each of which provides a different number of sub-carriers according, for example to the following table: 
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                 Data Sub- 
               
               
                   
                 Mode 
                 carriers 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 1K 
                 853 
               
               
                   
                 2K 
                 1705 
               
               
                   
                 4K 
                 3409 
               
               
                   
                 8K 
                 6913 
               
               
                   
                 16K  
                 13921 
               
               
                   
                 32K  
                 27841 
               
               
                   
                   
               
               
                   
                 Maximum Number of Sub-carriers per mode. 
               
             
          
         
       
     
         [0037]    The sequence of data cells to be carried in each OFDM symbol is then passed to the symbol interleaver  33 . The OFDM symbol is then generated by an OFDM symbol builder block  37  which introduces pilot and synchronising signals fed from a pilot and embedded signal former  36 . An OFDM modulator  38  then forms the OFDM symbol in the time domain which is fed to a guard insertion processor  40  for generating a guard interval between symbols, and then to a digital to analogue convertor  42  and finally to an RF amplifier within an RF front end  44  for eventual broadcast by the COFDM transmitter from an antenna  46 . 
       Frame Format 
       [0038]    For the DVB-T2 system, the number of sub-carriers per OFDM symbol can vary depending upon the number of pilot and other reserved carriers. An example illustration of a “super frame” according to the DVB-T2 standard is shown in  FIG. 2 . 
         [0039]    Thus, in DVB-T2, unlike in DVB-T, the number of sub-carriers for carrying data is not fixed. Broadcasters can select one of the operating modes from 1 k, 2 k, 4 k, 8 k, 16 k, 32 k each providing a range of sub-carriers for data per OFDM symbol, the maximum available for each of these modes being 1024, 2048, 4096, 8192, 16384, 32768 respectively. In DVB-T2 a physical layer frame is composed of many OFDM symbols. Typically the frame starts with a preamble or P1 symbol as shown in  FIG. 2 , which provides signalling information relating to the configuration of the DVB-T2 deployment, including an indication of the mode. The P1 symbol is followed by one or more P2 OFDM symbols  64 , which are then followed by a number of payload carrying OFDM symbols  66 . The end of the physical layer frame is marked by a frame closing symbols (FCS)  68 . For each operating mode, the number of sub-carriers may be different for each type of symbol. Furthermore, the number of sub-carriers may vary for each according to whether bandwidth extension is selected, whether tone reservation is enabled and according to which pilot sub-carrier pattern has been selected. As such a generalisation to a specific number of sub-carriers per OFDM symbol is difficult. 
       Receiver 
       [0040]      FIG. 3  provides an example illustration of a receiver which may be used with the present technique. As shown in  FIG. 3 , an OFDM signal is received by an antenna  100  and detected by a tuner  102  and converted into digital form by an analogue-to-digital converter  104 . A guard interval removal processor  106  removes the guard interval from a received OFDM symbol, before the data is recovered from the OFDM symbol using a Fast Fourier Transform (FFT) processor  108  in combination with a channel estimator and corrector  110  and an embedded-signalling decoding unit  111 . The demodulated data is recovered from a de-mapper  112  and fed to a symbol de-interleaver  114 , which operates to effect a reverse mapping of the received data symbol to re-generate an output data stream with the data de-interleaved. Similarly, the bit de-interleaver  116  reverses the bit interleaving performed by the bit interleaver  26 . The remaining parts of the OFDM receiver shown in  FIG. 3  are provided to effect error correction decoding  118  to correct errors and recover an estimate of the source data. 
         [0041]    As shown in  FIG. 3 , the embedded-signalling decoding unit provides, via a connecting channel  119 , a control signal. According to the present technique the control signal indicates the guard interval which has been detected from signalling data provided with the OFDM symbols as explained in the following paragraphs. However, advantageously the guard detector  106  also performs a blind detection process for detecting the guard interval without the signalling data, which may be used separately or in combination to determine the guard interval. 
       Blind Guard Interval Detection 
       [0042]    A simplified and yet robust algorithm has been developed to detect blindly a selected guard interval of received OFDM symbols. As will be appreciated, the present technique finds application in detecting the guard interval of OFDM symbols for any system and is not limited to DVB-T2. However motivation for the use of a blind detection arrangement for detecting the length of the guard period has been encouraged by the development of the DVB-T2 specification. This development included modifications to extend the number of guard intervals, from a set of guard intervals which were provided in the DVB-T standard (‘old’ guard intervals) and an extended set of guard intervals (‘new’ guard intervals). The guard intervals are signaled to the receiver in L1 signaling data to inform the receiver of the use of ‘old’ or ‘new’ guards as defined below. 
         [0000]    
       
         
               
             
               
               
               
               
             
           
               
                   
               
               
                 Signaling Format for the Guard Interval 
               
             
          
           
               
                   
                 Classification 
                 Value 
                 Guard Interval Fraction 
               
               
                   
                   
               
               
                   
                 ‘Old’ Guards 
                 000 
                  1/32 
               
               
                   
                   
                 001 
                  1/16 
               
               
                   
                   
                 010 
                 ⅛     
               
               
                   
                   
                 011 
                 ¼     
               
               
                   
                 ‘New’ Guards 
                 100 
                      1/128 
               
               
                   
                   
                 101 
                  19/128 
               
               
                   
                   
                 110 
                  19/256 
               
               
                   
                   
                 111 
                 reserved future use 
               
               
                   
                   
               
             
          
         
       
     
         [0043]      FIG. 4  provides an illustrative representation of a P2 symbol which may include various possible guard intervals according to the table shown above. As shown in  FIG. 4  a P2 Symbol  120  may be followed by further data symbols  124 . The P2 symbol is made up of a first useful part Tu and a guard interval Tg, which includes signal samples repeated from the useful part of the burst Tu. 
         [0044]      FIG. 4  illustrates the guard interval detection process inside the DVB-T2 receiver. Initially, a P1 symbol is detected and validated by the receiver. Decoding the P1 symbol provides signaling data which indicates the FFT size. For 16 k and 32 k FFT sizes, it should be possible to demodulate the P2 symbols to obtain the corresponding guard interval mode, before the blind guard detection process can determine the guard interval. However, the proposed blind guard interval detection (GID) is a generic scheme, which can be used to detect guard intervals for all possible FFT sizes if that proved to be advantageous. The output of the blind GID provides a ‘detected’ GF, which is then used to demodulate and decode the P2 symbols to get the guard interval information. The fine frequency offset information that can be provided by the P1 symbols detection can now be replaced. The proposed blind guard interval detection is capable of providing this information more accurately, because there is no need for up scaling for FFT sizes above 1K. 
         [0045]    In exceptional circumstances where the ‘decoded’ and ‘detected’ guard intervals would not agree, the decided guard interval would favour the ‘decoded’ value. 
         [0046]    The blind GID should perform accurately. Otherwise, the L1 decoding would suffer a breakdown and a loop needed to be repeated with a fresh set of samples. The information extracted in the current T2 frame can only be used in the next T2 frame. Therefore, we should be able to repeat the loop for a number of times and only through simulations we would be able to establish what this number might be. 
         [0047]    As illustrated in  FIG. 4  the guard interval Tg could be one of seven possible values as indicated in the above table. Thus the present technique is arranged to detect the correct guard interval before and without recovering the L1 signal information which explicitly identifies the size of the guard interval from the value of the signalling field as represented in the table above. Accordingly, because the guard interval can be detected without detecting each of the P2 symbols and performing the necessary processing and decoding operations in order to recover the signal information, the detection of the guard interval and correspondingly the recovery of data from the OFDM symbol stream can be reduced. 
         [0048]    The guard detector  106  and the embedded signalling decoder  111  operating in accordance with the present technique to detect the guard interval will be explained in the following paragraphs. Essentially,  FIG. 5  summarises a process wherein a receiver detects both the P1 symbol within the super frame of the DVB T2 system and subsequently detects and demodulates the P2 symbol in order to recover the guard interval from the L1 signal information and performs the blind GID.  FIG. 5  is briefly summarised as follows: 
         [0049]    S 1 : The process begins with an initialisation in which the P1 symbol of the OFDM super frame is first detected. 
         [0050]    S 2 : The receiver determines whether or not the P1 symbol has been detected. If yes, then processing proceeds in parallel with steps S 3  and S 4 . 
         [0051]    S 3 : The receiver then decodes the P1 symbol to identify the FFT size which is provided within that symbol. 
         [0052]    S 4 : At decision point S 4  the receiver determines whether Nmax samples have been stored, Nmax being equal to the maximum number of valid samples within the OFDM symbols. If Nmax samples have been stored the processing proceeds to step S 6 . 
         [0053]    S 6 : At decision point S 6  it is determined whether or not the FFT size of the OFDM symbol is less than 16K. If the FFT size is less than 16K then according to the present technique a time taken to acquire the guard interval using the blind technique which will be explained in the following paragraphs has been determined to be less than the time taken to acquire all of the P2 symbols and to decode and identify the guard interval from the L1 signalling held within the P2 symbol. Accordingly, if the FFT size is less than 16 k then the processing proceeds to step S 8 . 
         [0054]    S 8 : Without the FFT size determined and step S 3 , the guard interval is detected using the blind technique, which will be explained in the following paragraphs. An output of step S 8  is to provide a detected guard interval on an output  130  and on second output  132  a fine frequency off set, which, as will be explained in the following paragraphs, is required to correctly detect the received OFDM symbols. 
         [0055]    S 10 : If on the other hand the FFT size is greater than 16K, i.e. 16 k or 32 k for DVB-T2, then processing proceeds to demodulate the stored P2 symbols. 
         [0056]    As already explained the number of P2 symbols will vary depending on the mode of operation or in other words the FFT size concerned. Therefore, in order to recover the actual guard interval concerned, which can be identified by recovering the L1 signalling data, steps S 12  and S 14  are performed. These steps are also performed, when the guard interval is detected blind, so that a confirmation can be made as to the correct guard interval. If there is a discrepancy between the bind detected guard interval and that identified by the L1 signalling data then the guard interval identified by the L1 signalling data is used to determine the guard interval. 
         [0057]    S 12 : At step S 12  the L1 data is recovered from the P2 symbols and is decoded to find the guard interval. Thus and output of step S 12  is to provide the decoded GF data for all models. 
         [0058]    S 14 : The GF data identifies the decoded guard interval which is to be used to decode the received OFDM symbols. 
         [0059]    As will be explained in the following paragraphs the present technique provides an arrangement for detecting the guard interval without detecting and demodulating the P2 signals to recover the L1 data from which the guard interval can be detected. As explained above, for certain modes, for example, for modes less than the 16 k mode, the guard interval can be detected more quickly using the blind detection technique, rather than waiting for all of the P2 data and then decoding the L1 signalling data. Performance results which identify this conclusion will also be explained. 
       Blind Guard Interval Detection 
       [0060]    The P1 detection would signal the start position of a first P2 symbol. Knowing the FFT size, we would ‘roughly’ locate the end position of the first P2 symbol. Performing a conventional windowed correlation only around this point should be sufficient to flag up the end position and thus make available the guard interval information. The accuracy and reliability of the extracted information is increased by performing the same process over a number of OFDM symbols and accumulating the complex correlation results. 
         [0061]    A schematic blocked diagram illustrating the technique performed by the guard detector  106  for blind detection of the guard interval is provided in  FIG. 6 , which provides an example of the process performed in step S 8  in  FIG. 5 .  FIG. 6  is supported with pictorial illustrations in  FIGS. 7   a ,  7   b ,  8   a  and  8   b . In  FIG. 6  the received OFDM symbol signal samples (K) are fed to a first input of a multiplier  140 . A second input of the multiplier  140  receives the signal samples which have been delayed by a value of N-samples which corresponds to the time delay Tu of the temporal period correspondent to the useful part of the OFDM symbol. Thus a delay element  142  delays the signal by N-samples, which corresponds to Tu. The phase of the signal samples from the output of the delay element  142  are then conjugated by an element  144  before being multiplied by the multiplier  140  to form a correlation sample before these correlation samples are summed by a moving averaging filter to form a correlation sum. 
         [0062]    A summation process to effect the correlation is made by one of J possible moving average filters each of which corresponds to one of the possible guard intervals Tg. Thus each of the moving averaging filters  146  serves to perform a moving averaging window corresponding to one of the possible guard interval periods Tg. Furthermore, the output of the moving averaging filter is then added to the moving averaging filter results from the next consecutive OFDM symbol using a summing element  148 . An output of the summing elements  148  are fed to a guard interval identifier  150  which serves to receive the output from the summing circuits  148  to identify a peak value of one of the output correlation values from the moving averaging filters corresponding to each of the possible guard intervals Tg. The output of the guard interval identifier  150  therefore provides on one output the detected guard interval  152  and on another output the fined frequency offset  154 , is generated by an argument of the correlation value, which corresponds to the detected guard interval. 
         [0063]    The guard interval identifier  150  also receives on the channel  119 , an indication of the guard interval from the L1 signalling data, which is recovered from the embedded-signalling decoding unit  111 . This indication of the guard interval may be received before the guard interval can be determined from the correlation output values, depending on the operating mode. 
         [0064]    The detected guard interval is received from the channel  152  by a guard remover  153 , which removes the guard period samples from the received OFDM symbols to provide the useful samples of the OFDM symbols on an output channel  155  to the FFT processor  108  as shown in  FIG. 3 . 
         [0065]    Returning to the explanation of the blind GID, given that the FFT size is known, for every possible guard interval (J=4 for ‘old’ guards and J=3 for ‘new’ guards), a single windowed correlation is performed per symbol for a defined number of symbols {1 . . . N}, where N=5 in the example shown in  FIGS. 7   a  and  8   a . The ‘N’ values are weighted and summed together. This means for ‘J’ possible guard interval, there should be ‘J’ values computed. As  FIG. 9  shows, with J=4 and N=2, the peak value corresponds to the actual guard interval. 
         [0066]    The basic principle should now be clear. For the very first symbol, the windowed correlation for all guards would detect some amount of correlation but as the ‘fingers’ advance by the possible ‘Tsi’, the correlation reduces except for when the possible ‘Ts’ equals the actual ‘Ts’. In this case, the correlation should remain more or less constant for all ‘N’ symbols (as shown in  FIG. 5 )). Thus, when all ‘N’ values are summed together and weighted accordingly, the actual guard interval would correspond to a maximum ‘accumulated correlation magnitude’ value. The fine frequency offset can also be measured by computing the argument of the corresponding correlation value. 
         [0067]    Numerous simulations were run to select an optimum value for ‘N’. There are 6 possible FFT sizes when including 16K and 32K in the blind search. This results in the possible guard interval durations encompassing a rather wide range. In ‘clear’ channels ‘N’ can be as low as 2 symbols but when considering more difficult channels this number might need to be increased to 3. 
         [0068]    A pictorial representation of the effect of the correlation when the assumed guard interval is not equal to the actual guard interval is represented in  FIGS. 7   a  and  7   b .  FIG. 7   b  provides a schematic block diagram illustrating the correlation process which is performed by the circuit shown in  FIG. 6 . However, elements have been expanded in order to illustrate the effect of the output of the moving averaging filter when the guard interval Tg is not equal to the actual guard interval. As shown in  FIG. 7   a , a plurality of received OFDM symbols are fed to the correlator  142 ,  144 ,  140 , which correlates the signal samples which are separated by an assumed value Ts whereas in fact the actual value of Ts is shown  162 . The output of the correlation formed by registers  166 ,  168 , and the multiplier  140  and complex conjugator  144  are fed to a summing unit  146  which is then fed to the moving averaging filter formed by summing unit  148 . An output of the moving averaging filter  146  is formed at an output  178 .  FIG. 7   b  shows a plot of the correlation magnitude at the output  178  with respect to the number of OFDM symbols received. 
         [0069]    As can be seen from  FIGS. 7   a  and  7   b , when the assumed value of the guard is such that the correlation separation Ts is not equal to the actual separation, the correlation magnitude starts from a high point  180  but proceeds downwards to a noise level value  182  for all substance symbols. Effectively, this is formed because of the negative reinforcement that each of the correlation values for subsequent symbols will not reinforce the signal samples, a part of which will be correctly correlating with samples repeated from the useful part of the burst within the guard interval, but the remainder of which will not be correlated because these are not aligned with the samples from the guard interval. Accordingly, this negative reinforcement effectively causes the moving averaging filter  148  to increasingly produce a noise like output as represented by  FIG. 7   b.    
         [0070]      FIGS. 8   a  and  8   b  provides a corresponding example to that shown in  FIGS. 7   a  and  7   b  but for the case where the assumed value of the separation between the signal samples Ts is equal to the actual separation, in other words, the guard interval Tg assumed for the moving averaging filter is the same as the actual guard interval in the received signal samples. Parts shown in  FIG. 8   a  corresponds with the parts shown in  FIG. 7   b  and so these will not be further explained.  FIG. 8   b  corresponds to  FIG. 7   b . However, as can be seen from  FIG. 8   b  the result of the correlation value at the output of the moving average filter  146  is to start from a highpoint of  180  but to increase or at least provide a constant output from the moving averaging filter as the number of OFDM symbols are received. This is because the correlation of the sample with the guard interval and the useful part of the burst now coincide with the more OFDM symbols are received the more the correlation result is reinforced so therefore each of the OFDM symbols received as indicated by the arrows  184  the correlation result provided at the output of the moving averaging filter  178  is reinforced. 
         [0071]      FIG. 9  provides some example results for correlating for possible guard intervals with respect to correlation magnitude. Once the correlation process over two OFDM symbol completes, both the maximum and the ‘next maximum’ correlation magnitudes are computed. The correlation process would then continue for an extra OFDM symbol if the ratio of ‘max’ to ‘next max’ falls below a value which can be adjusted by a user controlled register. Simulations have shown that this remedy increases the detection reliability considerably in multipath channels and in channels with high levels of CWI or noise. As can be seen the actual correlation magnitudes for guard interval number 2 is a “clear winner” with the correlation magnitude of 0.25 compared to the next noise value of 0.06. 
         [0072]      FIGS. 10   a  and  10   b  illustrate corresponding results for the example shown in  FIG. 9  where the maximum correlation value exceeds a designed threshold of 0.15, for example. For the example, shown in  FIG. 10   a  the next maximum value is below the example threshold of 0.15 whereas in  10   b  the next correlation magnitude is 0.2 which is above the correlation threshold 0.15. 
       Simplified Architecture 
       [0073]    Further modifications of the algorithm were made to reduce hardware complexity.  FIGS. 11 and 12  provide a simplified representation illustrating the operation of the correlator shown in  FIG. 6 . As for the example shown in  FIGS. 7   a  and  8   a , a correlation is performed from a separation of signal samples corresponding to the value of Tu, that is the number of useful signal samples in the OFDM symbols. However, for each of the possible guard intervals a different summation is performed as represented by a multiple summing unit through which possible guard values are included  200 . Thus  FIG. 11  shows the output for a first symbol after the P1 symbol and  FIG. 12  provides the example of the second symbol after the P1 symbol. 
         [0074]    A single ‘windowed correlator’ is used for all guards (maximum of four guards at a time). The two ‘fingers’ of the correlator are separated by the FFT size and the output of the correlation is directed into a number of adders (maximum of four) at different times. Let us describe the process in the first symbol after P1 symbol by examining  FIG. 11 , where a ‘triguard possibility’ is considered. For this symbol, the first locations of the ‘first finger’ and the ‘second finger’ are the same, P 0  and P 4  respectively, for all possible guards. Therefore, the correlation is turned on at P 0  and the correlation values are summed together. Once the first finger reaches P 1 , the sum value is stored away, SUM 0 , and the correlation continues without resetting the sum value. At P 2  and P 3 , the same process is repeated and the other two sums generated, SUM 1  and SUM 2 . The sum value is now reset ready for the next symbol. The correlation can now be switched off until the first finger reaches the next P 0  i.e. second symbol. As shown in  FIG. 12 , the first location of the two ‘fingers’ at this symbol might now be different. In this example, the correlation values between P 0  and P 1  are summed together to form SUM 0 , values between P 1  and P 3  are summed together to form SUM 1  and values between P 2  and P 4  are summed together to form SUM 2 . 
         [0075]    Before computing the ‘correlation magnitude’, some appropriate weighting needed to be applied to the generated SUM values. This is because, the correlation windows are not the same and the computed maximum can be misleading. The weighting is a simple mechanism in which the SUM values corresponding to smaller windows are scaled up accordingly. The weighting mechanism can also be reversed. This is to scale down SUM values corresponding to larger windows. This is an implementation decision as one requires a ‘multiplication’ and the other a ‘division’ operation. 
         [0076]    As can be seen from  FIG. 12  after the first symbol, the signal samples for guard zero and guard two are no longer aligned with the signal samples correlated from a useful part of the OFDM burst. Accordingly, the output value of the correlation is not reinforced. In contrast the guard correlation value for guard one is reinforced. 
         [0077]    As illustrated in  FIGS. 11 and 12  the start value for the correlation as shown by an upwardly pointing arrow and the stop value is represented by a downwardly pointing arrow. 
         [0078]    The guard interval detection process as illustrated by the above example is dependent on the accuracy of the P1 symbol detection algorithm. The end of P1 symbol is required to be detected accurately and large ‘P1 position errors’ could lead to significant loss of performance. The present examples have been optimized to tolerate P1 position errors of up to a +/− (half the actual guard), as shown in  FIG. 13 . 
         [0079]      FIG. 13  provides and example illustration of the process starting the correlation with respect to the possible value of the guard interval. As shown in  FIG. 13  there is a possible range of starting points over a value of the guard interval Tg/2 as represented by a block  210 . Thus, after detecting the P1 symbol the next possible starting point can vary over a range of positions as illustrated by Tg/2. However, an optimum starting point  212  is shown. There are various techniques for identifying the possible staring point for performing in correlation from the P1 symbol. As those acquainted with the DVB-T2 standards will appreciate, the P1 symbol is a robustly encoded OFDM symbol with two guard intervals as a preamble and a post-amble from the useful part of the burst. Accordingly, but with correlating the pre-amble and the post-amble with a useful part of the symbols, there is a greater likelihood of detecting the FFT size and also an optimised starting point  212 . 
         [0080]    In our co-pending patent application number GB0909590.2 a scheme for detecting the P1 symbol is disclosed, which offers high reliability with a reduced position error range. The contents of our co-pending UK patent application number GB0909590.2 are incorporated herein by reference and in particular the technique for detecting the P1 symbols as disclosed therein. This is well within the tolerance of the guard interval detection design. The Guard interval detection technique can therefore used in combination with the P1 symbol detection technique to provide an improved reliability of detecting and recovering data from the OFDM symbols. 
       Results: Acquisition Time 
       [0081]    A table below provides a summary of different configurations and corresponding acquisition times for all FFT modes in an 8 MHz system. In general, a maximum acquisition time, Acq, can be computed using Equations (1) and (2), where ‘N’ represents the number of OFDM symbols and ‘T’ is the elementary period as given in Table (3). For FFT modes 1 k to 16K, the maximum guard size is ¼ whereas it is 19/128 for 32 k, hence Equations (1) and (2). 
         [0000]        Acq =(5/4)* N *FFT Size* T ; for 1K to 16K FFT  (1) 
         [0000]        Acq =(147/128)* N *FFT Size* T ; for 32K FFT  (2) 
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                   
               
               
                   
                 N = 2 
                 N = 3 
               
               
                   
                 Minimum Acquisition 
                 Maximum Acquisition 
               
               
                 FFT Size 
                 Time (msec) 
                 Time (msec) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1K 
                 0.28 
                 0.42 
               
               
                 2K 
                 0.56 
                 0.84 
               
               
                 4K 
                 1.12 
                 1.68 
               
               
                 8K 
                 2.24 
                 3.36 
               
               
                 16K  
                 4.48 
                 6.72 
               
               
                 32K  
                 8.232 
                 12.348 
               
               
                   
               
             
          
         
       
     
         [0082]    As indicated by the table above, the acquisition time for the 16K and 32K modes is significant and therefore may be acquired more quickly by detecting decoding the L1 signaling data from the P2 symbols. 
       Results: Performance Evaluation 
       [0083]      FIGS. 14   a ,  14   b ,  15   a  and  15   b  illustrate the typical performance of the guard interval detection process described above for the two possible guard interval values of 1/32 and 1/16 respectively. As shown in  FIGS. 14   a  and  15   a , a pass rate is shown at 100% with respect to the accuracy with which the guard interval is detected using the blind detection technique, as the signal to noise ratio of additive white Gaussian noise increases. As shown in  FIGS. 14   b  and  15   b  the mean phase error and the standard deviation of the fine frequency offset reduces sharply with respect to signal to noise ratio. As can bee seen, the fine frequency offset error converges to zero for high signal to noise ratios. In general, the proposed scheme shows a very high degree of robustness both against AWGN and CWI (continuous wave interference) in single and multi-path channels. The fine frequency offset estimation accuracy is also high and it might be a better alternative for the value computed at P1 detection block as there is no issue of mapping for FFT sizes above 1 k. 
         [0084]    As will be appreciated the transmitter and receiver shown in  FIGS. 1 and 3  respectively are provided as illustrations only and are not intended to be limiting. For example, it will be appreciated that the present technique can be applied to a different transmitter and receiver architecture. 
         [0085]    As mentioned above, embodiments of the present invention find application with DVB standards such as DVB-T, DVB-T2, DVB-C2 and DVB-H, which are incorporated herein by reference. Services that may be provided may include voice, messaging, internet browsing, radio, still and/or moving video images, television services, interactive services, video or near-video on demand and option. The services might operate in combination with one another. In other examples embodiments of the present invention finds application with the DVB-T2 standard as specified in accordance with ETSI standard “EN 302 755 and ETSI EN 300 755, “Digital Video Broadcasting (DVB): Frame structure, channel coding and modulation for a second generation digital terrestrial television broadcasting system (DVB-T2)”, May 2008. In other examples embodiments of the present invention find application with the cable transmission standard known as DVB-C2. For the example of DVB-C2, it will be appreciated that the OFDM symbols are not transmitted and received via a radio frequency carrier, but via cable and so an appropriate adaptation of the transmitter and receiver architecture can be made. However, it will be appreciated that the present invention is not limited to application with DVB and may be extended to other standards for transmission or reception, both fixed and mobile.