Abstract:
A method for determining impulsive interference applicable to an orthogonal frequency division multiple access (OFDMA) signal receiver is provided. The receiving method includes calculating a subcarrier noise of a first symbol, calculating a subcarrier noise of a second symbol, calculating a first ratio of the subcarrier noise of the first symbol to the subcarrier noise of the second symbol, determining whether the ratio is greater than a first threshold, and recognizing that the first symbol suffers from impulsive interference when the first ratio is greater than the first threshold.

Description:
This application claims the benefit of Taiwan application Serial No. 102145353, filed Dec. 10, 2013, the subject matter of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates in general to receiving signals, and more particularly, to receiving orthogonal frequency division multiple access (OFDMA) signals. 
     Description of the Related Art 
     The number of standards and specifications that transmit signals in form of orthogonal frequency division multiple access (OFDMA) is increasing, especially in the transmission of wireless signals, such as Digital Video Broadcasting-Terrestrial (DVB-T), DVB-T2, and Integrated Services Digital Broadcasting (ISDB). In the above signal transmission standards, due to the lack of time interleaving transmission specifications, these signal transmission standards are easily influenced by impulsive interference. 
       FIG. 1  shows a schematic diagram of stereotypic impulsive interference. In  FIG. 1 , the time axis is not depicted in true ratios to emphasis a part to be explained by the present invention. As shown in  FIG. 1 , there are two bursts  110  and  120 , spaced by a period of about 10 ms, along the time axis. The burst  110  includes four pulses  111  to  114 , with an interval between the pulses being about 15 us to 35 us and each of the pulses lasting for about 250 ns. Further, the pulses have different strengths. Similarly, the burst  120  includes multiple pulses that are not shown. 
       FIG. 1  shows stereotypic impulsive interference. One person skilled in the art can understand that there are other types of impulsive interference. When encountering impulsive interference, frequency bands of all or at least a part of subcarriers of common OFDMA signals suffer from interference. As such, a receiver detects an increase in the signal strength for the interfered subcarrier frequency bands. The intensity of the impulsive interference is greater than the original additive white Gaussian noise (AWGN). 
     According to the DVB-T standard, the length of a symbol under a 2K mode is 224 us, and the length of a symbol under an 8K mode is 896 us. According to the DVB-T2 standard, the symbol length may be 112 us to 3584 us. In the stereotypic impulsive interference shown in  FIG. 1 , the time interval between two bursts is about 10 ms, which is far greater than the symbol length defined by the DVB-T and DVB-T2 standards. Further, the time interval between pulses is about 15 us and 35 us, which is noticeably shorter than the symbol length defined by the DVB-T and DVB-T2 standards. Further, the duration of each pulse is only about 250 ns. In other words, it is rare that two consecutive bursts suffer from the impulsive interference, and the burst may fall between a guard period between symbols or fall in a cyclic prefix. 
       FIG. 2A  shows a schematic diagram of impulsive interference that falls in a cyclic prefix. In  FIG. 2A , three OFDMA symbols  212 ,  222  and  232  respectively correspond to respective preceding cyclic prefixes  210 ,  220  and  230 . A large part of a burst  220 A falls in the cyclic prefix  220 . Referring to  FIG. 2B  showing a schematic diagram of impulsive interference that falls in a cyclic prefix, another burst  220 B falls in the symbol  212 . 
     When encountering impulsive interference, the receiver needs to detect and recognize that the cyclic prefix of a particular symbol suffers from impulsive interference before being able to perform special processes on that symbol. Therefore, there is a need for a reliable mechanism for determining whether a symbol or a cyclic prefix of a symbol suffers from impulsive interference. 
     SUMMARY OF THE INVENTION 
     According to an embodiment of the present invention, a method for determining impulsive interference applicable to an orthogonal frequency division multiple access (OFDMA) signal receiver is provided. The receiving method includes calculating a subcarrier noise of a first symbol, calculating a subcarrier noise of a second symbol, calculating a first ratio of the subcarrier noise of the first symbol to the subcarrier noise of the second symbol, determining whether the ratio is greater than a first threshold, and recognizing that the first symbol suffers from the impulsive interference when the first ratio is greater than the first threshold. 
     According to another embodiment of the present invention, a receiver for OFDMA signals is provided. The receiver includes: a noise calculation module, configured to calculate a subcarrier noise of a first symbol and a subcarrier noise of a second symbol; a ratio calculation module, configured to calculate a ratio of the subcarrier noise of the first symbol to the subcarrier noise of the second symbol; a determination module, configured to determine whether the ratio is greater than a first threshold, and to recognize that the first symbol suffers from impulsive interference when the first ratio is greater than the first threshold. 
     A main spirit of the present invention is to determine whether the former symbol of two successive symbols suffers from impulsive interference according to a ratio of subcarrier noises of the former symbol and the latter symbol of the two successive symbols, so as to perform a process on the symbol that suffers from the impulsive interference. 
     The above and other aspects of the invention will become better understood with regard to the following detailed description of the preferred but non-limiting embodiments. The following description is made with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of stereotypic impulsive interference; 
         FIG. 2A  is a schematic diagram of impulsive interference falling in a cyclic prefix; 
         FIG. 2B  is a schematic diagram of impulsive interference falling in a symbol; 
         FIG. 3  is a flowchart of a process for determining impulsive interference according to an embodiment of the present invention; 
         FIG. 4  is a flowchart of a process for determining impulsive interference according to an embodiment of the present invention; and 
         FIG. 5  is a block diagram of a receiver according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Some embodiments are described in detail below. Apart from the disclosed embodiments, the present invention is also applicable in other embodiments. The scope of the present invention is not be limited by the description of the non-limiting embodiments, but is to be defined in accordance with the appended claims. To better describe and explain contents of the present invention to one person skilled in the art, different parts in the diagrams are not drawn according to relative sizes or ratios, and certain sizes and associated scales may be enlarged for better distinction. Further, irrelevant details may not be all depicted to maintain the simplicity of the diagrams for better understanding. 
       FIG. 3  shows a flowchart of a process  300  for determining impulsive interference according to an embodiment of the present invention. Referring to 3, the process  300  includes following steps. 
     In step  310 , a sum of noises of subcarriers of a k th  symbol is calculated. These subcarriers may be all subcarriers of the symbol, and may include pilot subcarriers and data subcarriers. In another embodiment, these subcarriers may be all pilot subcarriers of the symbol. In another embodiment, these subcarriers may be a part of pilot subcarriers of the symbol. As a pilot signal is a signal known to a receiver, the receiver can more readily filter out noises of the pilot subcarriers. It should be noted that the purpose of the subcarriers is not limited by the present invention. When OFDMA signal transmission is conducted by utilizing one single subcarrier, a noise signal of this single subcarrier is the sum of noise signals. 
     In step  320 , a sum of noises of subcarriers of a (k+n) th  symbol is calculated, where n is a positive integer, i.e., a positive integer greater than or equal to 1. In step  320 , the involved subcarriers are identical to those in step  310 . For example, if the sum of noises of all subcarriers is calculated in step  310 , the sum of noises of all subcarriers is also calculated in step  320 . The value n may be adjusted based on actual applications. For example, the value n may be adjusted according to the model of impulsive interference and the length of symbols. In one embodiment of the present invention, an upper limit of the value n may be determined according to a time interval between bursts and the length of symbols. Further, assuming that the time interval between bursts in an impulsive interference model is 10 symbol lengths, the upper limit of n is supposedly 9 to prevent from encountering two bursts within in same calculation period. In addition, although the (k+n) th  symbol is received after the k th  symbol, the sequences for performing step  310  and step  320  are not limited by the present invention. This is because original sample data of the symbols may be stored in advance, and calculations of step  310  and step  320  can be performed later. One person skilled in the art can understand that, when n is set to 1, the situation in  FIG. 2A  may be encountered, i.e., the burst  200 A falls in the cyclic prefix  220 . In such a situation, a change in the noises of two successive symbols may not be greater than the first threshold. Therefore, in a preferred embodiment, the value n may be selected from a positive integer greater than 1. 
     In step  330  after steps  310  and  320 , a ratio of the subcarrier noise of the k th  symbol to the subcarrier noise of the (k+n) th  symbol is calculated. That is, the calculation result of step  310  is divided by the calculation result of step  320 . 
     In step  340 , it is determined whether the ratio is greater than a first threshold. The process  300  proceeds to step  350  when the ratio is greater than the first threshold, or else the process  300  proceeds to step  360 . In one embodiment, the ratio calculated in step  330  may be inverted. That is, the ratio of the subcarrier noise of the (k+n) th  symbol to the subcarrier noise of the k th  symbol may be calculated. An adaptive adjustment is made in step  340 , e.g., it is determined whether the ratio is smaller than a second threshold. One person skilled in the art can understand that such modification is within the scope of the present invention. 
     In step  350 , the receiver recognizes that the k th  symbol suffers from impulsive interference. Thus, the receiver needs to perform a special process on the k th  symbol. Step  360  is then performed. 
     In step  360 , k is set to a new value. In one embodiment, the new k value may be the previous value added by one. In another embodiment, the new k value may be the previous value added by n to omit step  310  in the next round. 
       FIG. 4  shows a flowchart of a process  400  for determining impulsive interference according to an embodiment of the present invention. In the embodiment, considering impulsive interference occurring between two symbols, a scenario of the ratio being smaller than the second threshold is added to the process  300  in  FIG. 3 . Further, the process  400  further involves counters and an impulsive interference flag value (IIS_Detected). The impulsive interference flag value is for indicating that a symbol suffers from impulsive interference. A false alarm counter (IIS_False_Alarm_Cnt) is a counter used when the ratio is greater than the first threshold but smaller than the second threshold to prevent misjudgment resulted from using only the first threshold. A test counter (IIS_Test_Cnt), serving a purpose similar to that of the false alarm counter, is for preventing a predetermined number of subsequent symbols from being misjudged as suffering from impulsive interference after a symbol is recognized as suffering from impulsive interference. A threshold of the test counter is referred to as a clear threshold (IIS_Disappear_Cnt). The false alarm threshold and the clear threshold may be the same or different. Referring to  FIG. 4 , the process  400  includes following steps. 
     In step  410 , a sum of noises of subcarriers of a k th  symbol is calculated. Details of this step may be referred from step  310  in  FIG. 3 . 
     In step  415 , k is set to k+n, and the sum of noises of subcarriers of the k th  symbol is again calculated. In other words, the sum of noises of subcarriers of the (k+n) th  symbol is calculated. Details of this step may be referred from step  320  in  FIG. 3 . 
     In step  420 , a ratio between the sums of the subcarrier noise obtained in step  410  and  415  is calculated. Details of this step may be referred from step  330  in  FIG. 3 . This ratio is a ratio of the sum of the subcarrier noise of the k th  symbol to the sum of the subcarrier noise of the (k+n) th  symbol. 
     In step  425 , it is determined whether the ratio obtained in step  420  is greater than a first threshold. The process  400  proceeds to step  430  when the ratio is greater than the first threshold, or else proceeds to step  410 . Details of this step may be referred form step  340  in  FIG. 3 . In one embodiment, when the ratio is not greater than the first threshold, k may be set to a new k+n value, and step  410  is iterated. 
     In step  430 , k is set to k+n, and the sum of noises of subcarriers of the k th  symbol is again calculated. In other words, the sum of noises of subcarriers of the (k+2n) th  symbol is calculated. 
     In step  435 , a ratio between the sums of the subcarrier noise obtained in step  415  and  430  is calculated. This ratio is a ratio of the sum of the subcarrier noise of the (k+n) th  symbol to the sum of the subcarrier noise of the (k+2n) th  symbol. 
     In step  440 , it is determined whether the ratio obtained in step  435  is smaller than a second threshold. As previously stated, impulsive interference does not successively affect adjacent symbols. Therefore, when step  425  determines that the ratio of the sum of subcarrier noise of the k th  K symbol to the sum of the subcarrier noise of the (k+n) th  symbol is greater than the first threshold, and step  440  determines that the ratio of the sum of the subcarrier noise of the (k+n) th  symbol to the sum of the subcarrier noise of the (k+2n) th  symbol is smaller than the second threshold, it is almost certain that the k th  symbol suffers from impulsive interference. When it is determined that the ratio of the sum of the subcarrier noise of the (k+n) th  symbol to the sum of the subcarrier noise of the (k+2n) th  symbol is greater than the second threshold, it means that the determination basis may be incorrect, and so the process  400  proceeds to step  455 . 
     In step  445 , it is determined whether a value of the test counter is greater than an interference threshold. The process  400  proceeds to step  450  when the test counter is greater than the interference threshold. The test counter is for determining a minimum number of symbols between a previous occurrence of impulsive interference and a next occurrence of impulsive interference. For example, assuming that the two occurrences of impulsive interference are spaced by 5n numbers of symbols, the interference threshold may be set to 5. 
     In step  450 , the impulsive interference flag is set. Further, it is indicated that the k th  symbol suffers from impulsive interference, and so it is necessary that other processing circuits or software perform a special process on the k th  symbol. After setting the impulsive interference flag, it is similarly checked whether the ratio between the sums of subcarrier noise of a pair of subsequent symbols is smaller than the second threshold, and so the process  400  proceeds to step  465 . Before entering step  465 , details of a process when a false alarm occurs in step  440  are first given below. 
     In step  455 , a value of the false alarm counter is increased. 
     In step  460 , it is determined whether the value of the false alarm counter is greater than a false alarm threshold. When the false alarm counter is greater than the false alarm threshold, the process  40  returns to step  430  to calculate the sum of noises of subcarriers of a next symbol (e.g., the (k+3n) th  symbol). Step  435  is then performed to calculate a pair of symbols (e.g., the ratio between the (k+2n) th  symbol and the (k+3n) th  symbol) and step  440  is performed for a next round of determination. The process  400  proceeds to step  499  when it is determined in step  460  that the value of the false alarm counter is greater than the false alarm threshold. 
     In step  465 , k is again set to k+n, and the sum of noises of subcarriers of the k th  symbol is again calculated. 
     In step  470 , the ratio of the sum of subcarrier noise of the symbol previously calculated to the sum of subcarrier noise of the symbol calculated in step  465  is calculated. 
     In step  475 , it is determined whether the ratio obtained in step  470  is smaller than the second threshold. When the ratio is smaller than the second threshold, it means that the ratio of a pair of symbols after setting the impulsive interference has restored to a normal value. Thus, the process  400  proceeds to step  480 , or else the process  400  proceeds to step  490 . 
     In step  480 , the value of the test counter is increased. 
     In step  485 , it is determined whether the value of the test counter is greater than a clear threshold. When the value of the test counter is greater than the clear threshold, it means that the ratio of certain pairs of symbols after setting the impulsive interference flag has restored to normal, and so the process  400  proceeds to step  499 ; or else, the process  400  returns to step  465 . 
     In step  490 , the test counter is reset to zero. 
     In step  499 , the impulsive interference flag is cleared. Regardless of whether the impulsive interference flag is previously set, the impulsive interference flag is altogether cleared in this step. 
       FIG. 5  shows a block diagram of a receiver  500  according to an embodiment of the present invention. The receiver  500 , being adapted to receive signals transmitted in form of OFDMA, includes a receiving front-end  510 , a memory module  520  and a processing module. The receiving front-end  510  may include circuits such as an antenna, an analog-to-digital converter (ADC), a sampler, a fast Fourier transform (FFT) converter, and is for analyzing individual symbols to be stored in the memory module  520 . 
     The processing module  530  includes a noise calculation module  532 , a ratio calculation module  534  and a determination module  536 . The processing module  530  may be utilized to perform the processes  300  and  400  in  FIG. 3  and  FIG. 4 . In one embodiment, the noise calculation module  532  is utilized to perform steps  310  and  320  of the process  300 , the ratio calculation module  534  is utilized to perform step  330  of the process  300 , and the determination module  536  is utilized to perform steps  340  to  360  of the process  300 . 
     In another embodiment, the noise calculation module  532  is utilized to perform steps  410 ,  420 ,  430  and  465  of the process  400 , the ratio calculation module  534  is utilized to perform steps  420 ,  435  and  470  of the process  400 , the determination module  536  is utilized to perform the remaining steps of the process  400 , and the memory module  520  is utilized to further record the counters and thresholds required in the process  400 . 
     One person skilled in the art can understand that, the processing module  530  may be implemented by software, hardware, or software collaborating with hardware. The implementation of the processing module  530  is not limited by the present invention. 
     In conclusion, a main spirit of the present invention is to determine whether the former symbol of two successive symbols suffers from impulsive interference according to a ratio of subcarrier noises of the former symbol and the latter symbol of the two successive symbols, so as to perform a process on the symbol that suffers from the impulsive interference. 
     While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited thereto. On the contrary, it is intended to cover various modifications and similar arrangements and procedures, and the scope of the appended claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and procedures.