Patent Publication Number: US-7916815-B2

Title: Method and apparatus for a multicarrier receiver circuit with guard interval size detection

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present patent application is a continuation Ser. No. 10/170,165 filed Jun. 12, 2002, now of U.S. Pat. No. 7,206,361, entitled “Method and Apparatus for a Multicarrier Receiver Circuit with Guard Interval Size Detection”, issued on Apr. 17, 2007, which claims priority under 35 U.S.C. §119 (a)-(d) to European Patent Application No. 01305149.5, entitled “Multicarrier receiver with detection of the transmission mode and length of the guard interval”, filed on Jun. 13, 2001, hereby incorporated by reference herein. 
    
    
     BACKGROUND 
     1. Field 
     This invention relates to the field of receiver circuits, more particularly to a method and apparatus for receiving signals in which a portion of a transmitted signal forms a guard interval. 
     2. Description of Related Art 
     The European DVB-T (Digital Video Broadcasting-Terrestrial) standard ETS 300 744 for digital terrestrial television (DTT) uses Coded Orthogonal Frequency Division Multiplexing (COFDM) of transmitted signals, which are therefore grouped into blocks and frames. 
     It is necessary to transmit the DTT signals over transmission paths, which are of uncertain quality. In particular, the area close to the transmission path may include objects such as tall buildings, which cause reflections. That is, a signal may be received at a receiver twice, once on a direct path from the transmitter, and then, after a short delay, as an indirect reflected path. As is well known, this can cause inter-symbol interference (ISI) in the receiver. To minimize this problem, DVB-T COFDM signals include a cyclic prefix guard interval (referred to hereinafter as “guard interval” or “guard period”) to each active symbol. Specifically, the end portion of the active symbol is repeated before the current active symbol. These cyclic signals must be removed correctly before demodulation or the demodulation performance can be seriously degraded. 
     The DVB-T standard defines two possible “modes” of operation, each of which has a specific active window size, which in turn defines the size of the Fast Fourier Transform (FFT) in the receiver, namely 2 k mode and 8 k mode. For each of these modes the standard defines four possible guard lengths, which are fractions of the respective FFT length. The combination of FFT size and guard interval length can be selected based upon the anticipated multipath delay spread (which can be a function of transmission path topology and/or network topology in the case of a single frequency network (SFN)). 
     Before the cyclic signals can be removed it is necessary to know the FFT size and the duration of the guard interval. However, the duration of the guard interval, at least, is not a constant value. The presence of the guard interval reduces the efficiency with which the transmission channel can be used, and so it is advantageous to vary the size of the guard interval to be the minimum allowed by the conditions. 
     The receiver therefore includes a mechanism for determining the size of the guard interval. The chosen mechanism should be sufficiently robust to perform adequately under noisy signal conditions and with signals that suffer from multipath and co-channel interference. 
     Therefore, a need exists for a method and apparatus for a multicarrier receiver circuit with guard interval size detection that can be easily implemented and overcomes the disadvantages of other methods and apparatuses such as the above-described known system. The present disclosure provides such a multicarrier receiver circuit method and apparatus. 
     SUMMARY 
     The present invention provides a receiver circuit that rapidly determines an FFT size and guard interval size of a received signal. In addition, the present invention provides the FFT and guard interval size determination in cases where the received signals have relatively low signal-noise ratios. 
     The present invention relates to a method and apparatus for processing a received signal comprising a plurality of symbols used to determine a size of a guard interval therein. The invention forms a correlation function from the received samples, wherein the correlation function contains peaks. The separation between peaks in the correlation function depends upon a total duration of a symbol plus a guard interval. The correlation function is passed through a plurality of resonators, wherein resonators have respective resonance frequencies corresponding to a total duration of a symbol plus an available guard interval. The inventive method and apparatus selects a guard interval corresponding to the resonator with the highest output power as the size of the guard interval of the received signal. 
     The invention also relates to a receiver circuit that determines the size of the guard interval of a received signal in the aforementioned manner, thus providing a quick and robust manner of determining the guard interval size. In one embodiment, the invention determines a mode of receiving the signals by passing the correlation function through resonators that have respective resonance frequencies, which correspond to a total duration of a symbol in an available mode plus an available guard interval. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a receiver circuit adapted for use with the present invention 
         FIG. 2  is a block diagram of an exemplary embodiment of a component of the receiver circuit of  FIG. 1 . 
         FIG. 3  shows a flowchart of the method of processing a received signal. 
     
    
    
     Like reference numbers and designations in the various drawings indicate like elements. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Throughout this description, the preferred embodiment and examples shown should be considered as exemplars, rather than as limitations to the present invention. 
     The method and apparatus for a multicarrier receiver circuit with guard interval size detection determines a guard interval size from a received multicarrier signal. The multicarrier receiver circuit generates a correlation function based upon a received multicarrier signal. The correlation function is passed to a plurality of resonators, wherein the resonators have resonance frequencies corresponding to guard interval sizes. The present invention selects the guard interval size based on the resonator having the highest output. Advantageously, the method is robust in noisy environments, and also sufficiently robust to process signals having multipath and/or co-channel interference characteristics. 
       FIG. 1  is a block diagram of a receiver circuit adapted for use with the present apparatus and method. Typical digital terrestrial television signal receivers (e.g., receivers operating in accordance with the DVB-T standard with Coded Orthogonal Frequency Division Multiplexing (COFDM)) include an antenna and a tuner for receiving signals. 
     Referring now to  FIG. 1 , received signals are input to a demodulator  2 , which mixes the received RF input signal down to baseband in-phase (I) and quadrature (Q) components and produces digital samples. As shown in  FIG. 1 , the baseband I and Q components are input to a mode and guard size detection block  4 . The mode and guard size detection block  4  performs one or more of the following functions: determines whether the received signal was transmitted in “2 k” or “8 k” modes, determines the guard size (as described in more detail below), and provides corresponding mode detection and guard size outputs, and a window position output. 
     The baseband I and Q components are also input to a synchronizer circuit  6 , which removes at least one of frequency and sampling rate offsets and inputs signals to a window position block  8 . Following determination of the window position, the processed received signals are input to a guard removal block  10 , FFT block  12 , equalizer  14 , demapper  16 , and FEC block  18 . The resulting data, such as MPEG encoded signal, is output to a demultiplexing device for ultimate display. It will also be apparent that changes can be made to the receiver circuit without diminishing its end effect. For example, the order of blocks in the circuit can be changed without significant impact to its performance. 
     The mode outputs, window position outputs and guard size outputs are supplied to the window position block  8 . The mode outputs and guard size outputs are supplied to the guard removal block  10 . The mode output is input to the FFT block  12 , allowing these latter blocks to operate accurately. 
       FIG. 2  is a block diagram of an exemplary embodiment of the mode and guard size detection block of  FIG. 1 . As shown in  FIG. 2 , the mode and guard size detection block  4  includes an upper signal path and a lower signal path. The upper signal path corresponds to a 2K-carrier mode; the lower signal path corresponds to an 8K-carrier mode. The upper signal path includes blocks  42 ,  44 ,  46  and  48 . The lower signal path includes blocks  52 ,  54 ,  56  and  58 . Both the upper and lower signal paths are described in more detail hereinbelow. 
     Although other modes and guard sizes would operate with the described method and apparatus, in the DVB-T standard, there are two available carrier modes, namely “2 k” mode and “8 k” mode. In this context, “2 k” and “8 k” refer to the required effective size of the FFT block  12 . In each of these carrier modes, there are four available guard intervals, namely ¼, ⅛, 1/16 and 1/32, of the respective symbol duration. Some receivers are intended for use in transmission networks, in which only one of the available modes is used. In that case, it may not be necessary for the mode and guard size detection block  4  to provide an output that indicates the mode. 
     In one embodiment of the present invention, the I and Q samples of the received signal are supplied to an input terminal  40  of the mode and guard size detection block  4  ( FIG. 1 ). The samples are supplied to a 2 k size first-in first-out (FIFO) memory  42 . The moving average correlation of the samples over a minimum guard period is then calculated in block  44 , and the power of the correlation measured in block  46 . 
     In one embodiment, the correlation function is calculated in block  44  by multiplying input symbols with symbols contemporaneously obtained from the delay block  42  with the delay applied thereto, thereby obtaining a measure of the correlation between them. The results are then summed, and a running average is calculated over a number of samples “g”, which is equal to the smallest allowed guard interval size, that is, 1/32 of the FFT size. Thus, for example, g=64 samples in 2 k mode. 
     The blocks  44  and  46  therefore form a correlation function, and the separation between peaks in the correlation function depends on the total duration of the symbol plus the guard period. The resulting measurements are passed to a block  48  for decimation (i.e., removal of some proportion of the samples). This has the effect of easing implementation as it reduces the effective sampling rate. 
     As shown in the embodiment of  FIG. 2 , the input samples are also supplied to an 8 k size first-in first-out (FIFO) memory  52 . The moving average correlation of the samples over a minimum guard period is then calculated in block  54 , and the power of the correlation measured in block  56 . 
     In one embodiment, the correlation function is calculated in block  54  by multiplying the input symbols with symbols contemporaneously obtained from the delay block  52  with the delay applied thereto, thereby obtaining a measure of the correlation between them. The results are then summed, and a running average is calculated over a number of samples “g”, which is equal to the smallest allowed guard interval size, that is, 1/32 of the FFT size. Thus, g=256 samples in 8 k mode. 
     The blocks  54  and  56  therefore form a correlation function, and the separation between peaks in the correlation function depends on the total duration of the symbol plus the guard period. The resulting measurements are passed to a block  58  for decimation (i.e., removal of a proportion of the samples), as described above. 
     The samples remaining after decimation in blocks  48  and  58  are then passed through resonators  61 - 68  as shown in  FIG. 2 . Thus, samples from block  48  are passed through resonators  61 - 64 . Similarly, the samples from block  58  are passed through resonators  65 - 68 . 
     An exemplary set of resonators is implemented using a set of filters, wherein individual filters are centered at a respective resonance frequency. In this embodiment, the resonators have resonance frequencies based on the COFDM symbol frequency of a particular combination of the mode and the guard interval. 
     Thus, in 2 k mode, the active symbol duration is 224 μs, while the guard interval can be ¼, ⅛, 1/16 or 1/32 of the respective symbol duration, namely 56 μs, 28 μs, 14 μs or 7 μs. Therefore, the total duration of the symbol plus the guard period can be 280 μs, 252 μs, 238 μs or 231 μs. In 8 k mode, the active symbol duration is 896 μs, while the guard interval can again be ¼, ⅛, 1/16 or 1/32 of the respective symbol duration, namely 224 μs, 112 μs, 56 μs or 28 μs. The total duration of the symbol plus the guard period can therefore be 1120 μs, 1008 μs, 952 μs or 924 μs. 
     More specifically, resonance frequencies correspond to the sample rate divided by the number of samples in the FFT and in the guard interval together, after taking into account the sample rate decimation, for one of the possible combinations of the mode and the guard size. In one embodiment, the resonance frequency corresponds to an integer multiple of the sample rate divided by the number of samples in the FFT and in the guard interval together. 
     In one embodiment of the invention, the decimator  58  in the 8 k signal path has four times the decimation of the decimator  48  in the 2 k signal path. Specifically, the decimator  48  decimates by a factor of 8, and the decimator  58  decimates by a factor of 32. The result of this embodiment is that the two decimators ( 48 ,  58 ) each produce outputs that contain an equal number of samples per symbol. 
     As a consequence, the resonators  61  and  65  can be identical, as can the resonators  62  and  66 ,  63  and  67 , and  64  and  68 . Those skilled in the signal processing art shall recognize that non-identical resonators can be used with the present invention without departing from the scope or spirit of the present invention. It is also possible that a single bank of resonators could be provided, shared between the 2 k signal path and the 8 k signal path. 
     In one embodiment, the resonators have a resonance frequency at the third harmonic of the rate derived in the above-described manner. In this embodiment the filtering ability (or quality factor, Q) can increase for higher harmonics. The signal power decreases at higher harmonics. 
     The peak power generated by a resonator is determined over a maximal symbol length (i.e., the symbol length if it had a maximum guard). A counter (not shown) is provided at the output of each of the resonators  61 - 68 , and each counter increments when its peak power is largest. The peak powers produced by each resonator are then compared. 
     The resonator having a resonance frequency corresponding most closely to a harmonic of the COFDM symbol frequency (for the combination of the mode and the guard interval that is actually in use) produces the highest output power. Therefore, the comparison of the resonator peak powers indicates the frequency corresponding to the actual symbol duration (active symbol duration plus guard period) in the received signals. 
     Thus, in this embodiment, by examining the counter values after a number of symbols, the counter with the highest value is determined to be the one that corresponds to the mode (either 2 k or 8 k) and guard period used by the transmitted signal. As previously described, these output parameters could then be used in the further processing of the received signals. 
       FIG. 3  shows a flowchart of the method of processing a received signal. As shown in  FIG. 3 , the method  300  begins at a STEP  310  whereat the method  300  generates digital samples and delayed digital samples from a received signal. After the STEP  310 , the method  300  proceeds to a STEP  320  whereat the method  300  generates a correlation function based on the digital samples and delayed digital samples generated in the STEP  310 . In one embodiment, the correlation function is also based on a total symbol duration and a guard period. After the STEP  320 , the method  300  proceeds to a STEP  330  whereat the method  300  passes the correlation function through a plurality of resonators. In one embodiment, resonators correspond to various guard interval sizes and have resonance frequencies based on a mode and a guard interval. After the STEP  330 , the method  300  proceeds to a STEP  340  whereat the method  300  selects a guard interval size based on the resonator having the highest power output of the plurality of resonators. In one embodiment, the STEP  340  determines a peak power output over a maximal symbol length. 
     In an exemplary embodiment, the STEP  310  generates a first set of delayed digital samples and a second set of delayed digital samples, wherein the first set and the second set have different delays. In the exemplary embodiment, the STEP  320  generates correlation functions corresponding to both sets of delayed digital samples. The STEP  330  uses these correlation functions generated in the STEP  320 . 
     In one embodiment, the STEP  320  further comprises measuring a correlation function power from the correlation function. In one embodiment, the STEP  320  further comprises averaging the correlation function over a number of samples, wherein the number of samples corresponds to a smallest allowed guard interval size. In one embodiment, the STEP  320  generates a correlation function by multiplying a digital sample symbol by a delayed digital sample symbol. 
     In one embodiment, the STEP  330  further comprises decimating the correlation function prior to passing the correlation function through the plurality of resonators. In another embodiment, the STEP  330  comprises passing the correlation function through the plurality of resonators, wherein resonators have a resonance frequency of a third harmonic of a sample rate divided by a number of samples in an FFT and a guard interval. In another embodiment, the STEP  330  comprises passing the correlation function through the plurality of resonators, wherein resonators have a resonance frequency of a sample rate divided by a number of samples in an FFT and a guard interval. In another embodiment, the STEP  330  comprises passing the correlation function through the plurality of resonators, wherein resonators have a resonance frequency of an integer multiple of a sample rate divided by a number of samples in an FFT and a guard interval. In another embodiment, the STEP  330  comprises passing the correlation function through the plurality of resonators, wherein resonators have a resonance frequency of an integer multiple of a sample rate divided by a number of samples in an FFT and a guard interval, and further divided by a sample rate decimation. 
     SUMMARY 
     A novel multicarrier receiver apparatus and method with guard interval size detection have been described. The described method and apparatus for a demodulator multicarrier receiver circuit with guard interval size detection allows the FFT size and guard interval size to be detected, even in the presence of relatively noisy input signals. The method is robust in noisy environments, and also sufficiently robust to process signals having severe multipath and/or co-channel interference. Moreover, the method is of low complexity. The apparatus and method are described with particular reference to the DVB-T standard. However, the same are applicable to any receiver that is intended for use in transmission systems having multiple available combinations of guard intervals and FFT sizes. 
     A number of embodiments have been described. Nevertheless, it will be understood that various modifications may be made without departing from the scope or spirit of the invention. For example, the method and apparatus can be implemented in software, hardware, or in a software/hardware combination. Furthermore, the method and apparatus can be used in virtually any type of communication system. Its use is not intended to be limited to a European DVB-T standard-based communication system. For example, the present invention can be used in a North American television standard-based communication system. Accordingly, it is to be understood that the invention is not to be limited by the specific illustrated embodiment, but only by the scope of the appended claims.