Abstract:
A method of fast fading channel Fast Fourier Transform (FFT) trigger point tracking in an integrated services digital broadcasting (ISDB) receiver includes inputting a signal in a fading channel including N Orthogonal Frequency Division Multiplexing (OFDM) symbols, determining an average correlation result of a current time-domain sample of the signal and a previous time-domain sample taken previously of the signal, accumulating the average correlation result for at least one of the OFDM symbols, determining a peak of the average correlation result to obtain a peak position, and identifying the peak position as a trigger point of the input signal. The peak position may be compared with a first trigger point to generate a trigger point error signal. The first trigger point may be set at the middle of a guard of an OFDM symbol to generate the trigger point error signal.

Description:
BACKGROUND 
     1. Technical Field 
     The embodiments herein generally relate to wireless communication systems, and, more particularly, to a fast fading channel Fast Fourier Transform (FFT) trigger point tracking in Integrated Services Digital Broadcasting (ISDB) receivers. 
     2. Description of the Related Art 
     Orthogonal Frequency Division Multiplexing (OFDM) is a digital data modulating technique, which uses a large number of closely spaced orthogonal sub-carriers. The digital data is modulated to an amplitude and a phase of each of the orthogonal sub-carriers within a transmission band. In a digital broadcasting receiver design of ISDB receivers such as Integrated Services Digital Broadcasting-terrestrial (ISDB-T) and Integrated Services Digital Broadcasting-terrestrial digital sound broadcasting (ISDB-TSB) for OFDM systems, a major challenge lies in FFT trigger point tracking. 
     Terrestrial digital broadcasting using an OFDM method is susceptible to inter-symbol interference of multiple sub-carrier waves; the signal obtained is a composite wave resulting from the combination of the multiple sub-carrier waves received by an OFDM receiver. This causes fading (e.g., distortion in a carrier-modulated signal) of the transmitted OFDM symbols. The inter-symbol interference may be avoided by a FFT computation. In an OFDM receiver, the FFT computation such as trigger point tracking is performed by an FFT computing circuit in the receiver, by which the received OFDM signal is demodulated. 
     Trigger point is a point at which the sampling of an input signal starts. The traditional method for FFT trigger point tracking is based on a channel impulse response of the received OFDM signal. The channel impulse response refers to an output signal (an infinitely high peak) obtained for an input signal in a communication channel. The channel impulse response is usually obtained by performing an inverse FFT (IFFT) of the time-domain interpolated channel estimates or performing an IFFT on the scatter pilots. 
     For fast fading channel as in OFDM systems, performing an IFFT of the time-domain interpolated channel estimates or on the scatter pilots typically leads to degradation, noisy channel estimates, a short channel impulse response (e.g., due to limited scatter pilot spacing), aliasing (e.g., distortion of a frequency in a signal), and an incorrect trigger point adjustment. 
     For example, two trigger points which are Tu/12 (e.g., Tu is the useful OFDM symbol time in ISDB-T and ISDB-TSB) apart have the same channel impulse response and generally cannot be distinguished. This poses a problem while interpolating the channel in the frequency domain. Hence, the traditional method of performing an IFFT based on the channel impulse response in the frequency domain is generally not successful in FFT trigger point tracking. 
     SUMMARY 
     In view of the foregoing, an embodiment herein provides a method of fast fading channel FFT trigger point tracking in an ISDB receiver, and a program storage device readable by computer, tangibly embodying a program of instructions executable by the computer to perform the method of fast fading channel FFT trigger point tracking in an ISDB receiver. The method includes inputting a signal in a fading channel including N OFDM symbols, determining an average correlation result of a current time-domain sample of the signal and a previous time-domain sample taken previously of the signal, accumulating the average correlation result for one or more of the OFDM symbols, determining a peak of the average correlation result to obtain a peak position, and identifying the peak position as a trigger point of the input signal. 
     The peak position may be compared with a first trigger point to generate a trigger point error signal. The first trigger point may be set at the middle of a guard of an OFDM symbol to generate the trigger point error signal. The trigger point error signal may be scaled by a factor of 1/K, the K is programmable. A second trigger point may be obtained by adding the first trigger point with the scaled trigger point error signal, and selecting the second trigger point as a correct trigger point of the input signal. The correct trigger point may be adjusted for every N OFDM symbols. The average correlation result may include a moving average correlation result. 
     Another embodiment provides an apparatus for performing fast fading channel FFT trigger point tracking in an ISDB receiver, wherein the apparatus includes a memory unit having a set of computer programmable instructions, a display unit operatively connected to the memory unit, a processor that executes the computer programmable instructions and processes a signal in a fading channel including N OFDM symbols, a pair of moving average filters that determine an average correlation result of a current time-domain sample of the signal and a previous time-domain sample taken previously of the signal, an accumulator that accumulates the average correlation result for one or more of the OFDM symbols, and a peak detector that determines a peak of the average correlation result to obtain a peak position, and identifies the peak position as a trigger point of the input signal. 
     The peak detector may compare the peak position with a first trigger point to generate a trigger point error signal. The first trigger point may be set at the middle of a guard of an OFDM symbol to generate the trigger point error signal. A scaling block may scale the trigger point error signal by a factor of 1/K, the K is programmable. In addition, the scaling block may obtain a second trigger point by adding the first trigger point with the scaled trigger point error signal, and a trigger point peak detection block selects the second trigger point as a correct trigger point of the input signal. The trigger point peak detection block may adjust the correct trigger point for every N OFDM symbols. 
     These and other aspects of the embodiments herein will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. It should be understood, however, that the following descriptions, while indicating preferred embodiments and numerous specific details thereof, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the embodiments herein without departing from the spirit thereof, and the embodiments herein include all such modifications. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The embodiments herein will be better understood from the following detailed description with reference to the drawings, in which: 
         FIG. 1  is a block diagram illustrating trigger point peak detection for fast fading channel FFT in an ISDB-T and ISDB-TSB receiver according to an embodiment herein; 
         FIG. 2  is a block diagram illustrating the feedback for trigger point tracking for fast fading channel FFT in a ISDB-T and ISDB-TSB receiver according to an embodiment herein; 
         FIG. 3  is a flow diagram illustrating a method for fast fading channel FFT trigger point tracking according to an embodiment herein; 
         FIG. 4  is a schematic diagram illustrating a mobile TV receiver according to an embodiment herein; and 
         FIG. 5  is a schematic diagram illustrating a computer architecture according to an embodiment herein. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The embodiments herein and the various features and advantageous details thereof are explained more fully with reference to the non-limiting embodiments that are illustrated in the accompanying drawings and detailed in the following description. Descriptions of well-known components and processing techniques are omitted so as to not unnecessarily obscure the embodiments herein. The examples used herein are intended merely to facilitate an understanding of ways in which the embodiments herein may be practiced and to further enable those of skill in the art to practice the embodiments herein. Accordingly, the examples should not be construed as limiting the scope of the embodiments herein. 
     Referring now to the drawings, and more particularly to  FIGS. 1 through 5 , where similar reference characters denote corresponding features consistently throughout the figures, there are shown preferred embodiments. The embodiments herein provide a time-domain based approach to estimate a fast fading channel FFT trigger point tracking in an ISDB-T and ISDB-TSB receivers  100  to obtain a correct trigger point. Average correlation results of the time domain sample are obtained using moving average filters MA Tg  108 ,  110  of  FIG. 1  and are averaged and accumulated. The peak of the average correlation results is found and compared with a target trigger point to generate a trigger point error signal, which is scaled by a factor of 1/K. A new trigger point is obtained by adding the previously trigger point with the scaled trigger point error signal. 
       FIG. 1  is a block diagram illustrating a trigger point peak detection for a fast fading channel FFT in an ISDB-T and ISDB-TSB receiver  100  having OFDM symbols  102 , a delay Tu block  104 , a conjugate (conj.) block  106 , a moving average (MA) filter Tg  108 , a moving average (MA) filter Tg  110 , an accumulator  112  for N symbols, and a peak detection block  114 , which yields a trigger point  116  according to an embodiment herein. The OFDM symbols  102  pass through the delay Tu block  104  and conj. block  106 . The delay Tu block  104  is delayed by a time Tu. The conjugate (conj.) block  106  flips the sign of quadrature part of the OFDM symbol samples. 
     In one embodiment, a current time-domain sample of ISDB-T and ISDB-TSB correlates with the time-domain sample Tu taken previously (e.g., the delay Tu block  104 , the conjugate (conj.) block  106  and the multiplier  105  perform the correlation). The correlation results are passed through the moving average filter MA Tg  108  and the moving average MA Tg  110 . The MA filter Tg  108  and the MA filter Tg  110  average the correlation results. The MA filter Tg  108  and the MA filter Tg  110  have a moving average size of Tg, where Tg is a guard interval time. The accumulator  112  for N symbols then accumulates the averaged results. 
     The peak detection block  114  obtains the accumulated results and detects the peak position by searching for a peak from the accumulated results. The peak is obtained from the accumulator  112  for N symbols. The peak detection block  114  thereafter identifies the trigger point  116  as corresponding to the peak position. In one embodiment, the moving average (MA) results are then accumulated for N symbols over a symbol length (Tu+Tg) in the accumulator  112  for N symbols. The obtained peak position is compared with a predetermined target trigger point. For example, a target trigger point or a desired trigger point may be set at the middle of the guard of an OFDM symbol to generate a trigger point error signal. 
       FIG. 2  is a block diagram of a feedback control system  200  for trigger point tracking for the fast fading channel FFT in the ISDB-T and ISDB-TSB receiver  100  (of  FIG. 1 ) having a target trigger point  202 , a scaling block  204 , a triggering block  206 , a received OFDM symbols block  208 , and a trigger point peak detection block  210  according to an embodiment herein. 
     The obtained trigger point (e.g., the trigger point  116  from the peak detection block  114  of  FIG. 1 ) is compared with the target trigger point  202  to obtain a new trigger point. The target trigger point  202  is set at the middle of the guard of an OFDM symbol to generate a trigger point error signal. The scaling block  204  scales the obtained trigger point error signal by a factor of 1/K, where K is a programmable parameter. 
     A new trigger point is obtained by summing a previous trigger point and the scaled trigger point error signal. The new trigger point is obtained by using the formula: Trig(n)=Trig(n−N)+Adjustment in the triggering block  206 . The received OFDM symbols block  208  then receives the new trigger point. The trigger point peak detection block  210  detects the peak of the new trigger point. The system  200  updates the trigger point for every N symbols. The peak of the new trigger point  210  is compared with the target trigger point  202 . The above steps are repeated and the trigger point is adjusted for every N symbols. 
       FIG. 3 , with reference to  FIGS. 1 and 2 , is a flow diagram illustrating a method for the fast fading channel FFT trigger point tracking according to an embodiment herein. In step  302 , a current time domain sample is correlated with a time domain sample Tu of a previous time. In step  304 , the correlation results are passed through the moving average (MA) filter Tg  108 , and the MA filter Tg  110 . In step  306 , the moving average results are accumulated for N symbols over a symbol length (Tu+Tg) (e.g., using the accumulator  112  for N symbols). In step  308 , the peak of the accumulation results is searched. 
     In step  310 , a peak position is determined (e.g., using the peak detection block  114 ) after accumulating for N symbols. In step  312 , the peak position is compared with a target trigger point (e.g., the target trigger point  202  of  FIG. 2 ). In step  314 , a trigger point error signal is generated. In step  316 , the trigger point error signal is scaled by a factor of 1/K (e.g., using the scaling block  204 ). In step  318 , a new trigger point is obtained. In another embodiment, the new trigger point is obtained by summation of the previous trigger point and the scaled trigger point error signal. The steps from  302  may be repeated for N number of symbols. 
       FIG. 4  illustrates an exploded view of a mobile TV receiver  400  having a memory  402  with a computer set of instructions, a bus  404 , a display  406 , a speaker  408 , and a processor  410  capable of processing the set of instructions to perform any one or more of the methodologies herein. The processor  410  may also enable frequency samples to be consumed in the form of one or more displays  406  or audio for output via speaker and/or earphones  408 . The processor  410  carries out the methods described herein and in accordance with the embodiments herein. The received frequency domain sample may also be stored in the memory  402  for future processing or consumption. The memory  402  may also store specific information about the frequency domain sample available in the future or stored from the past. When the sample is selected, the processor  410  may pass information. The information may be passed among functions within mobile TV receiver  400  using the bus  404 . 
     The techniques provided by the embodiments herein may be implemented on an integrated circuit chip (not shown). The chip design is created in a graphical computer programming language, and stored in a computer storage medium (such as a disk, tape, physical hard drive, or virtual hard drive such as in a storage access network). If the designer does not fabricate chips or the photolithographic masks used to fabricate chips, the designer transmits the resulting design by physical means (e.g., by providing a copy of the storage medium storing the design) or electronically (e.g., through the Internet) to such entities, directly or indirectly. The stored design is then converted into the appropriate format (e.g., GDSII) for the fabrication of photolithographic masks, which typically include multiple copies of the chip design in question that are to be formed on a wafer. The photolithographic masks are utilized to define areas of the wafer (and/or the layers thereon) to be etched or otherwise processed. 
     The resulting integrated circuit chips can be distributed by the fabricator in raw wafer form (that is, as a single wafer that has multiple unpackaged chips), as a bare die, or in a packaged form. In the latter case the chip is mounted in a single chip package (such as a plastic carrier, with leads that are affixed to a motherboard or other higher level carrier) or in a multichip package (such as a ceramic carrier that has either or both surface interconnections or buried interconnections). In any case the chip is then integrated with other chips, discrete circuit elements, and/or other signal processing devices as part of either (a) an intermediate product, such as a motherboard, or (b) an end product. The end product can be any product that includes integrated circuit chips, ranging from toys and other low-end applications to advanced computer products having a display, a keyboard or other input device, and a central processor. 
     The embodiments herein can take the form of an entirely hardware embodiment, an entirely software embodiment or an embodiment including both hardware and software elements. The embodiments that are implemented in software include but are not limited to, firmware, resident software, microcode, etc. 
     Furthermore, the embodiments herein can take the form of a computer program product accessible from a computer-usable or computer-readable medium providing program code for use by or in connection with a computer or any instruction execution system. For the purposes of this description, a computer-usable or computer readable medium can be any apparatus that can comprise, store, communicate, propagate, or transport the program for use by or in connection with the instruction execution system, apparatus, or device. 
     The medium can be an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system (or apparatus or device) or a propagation medium. Examples of a computer-readable medium include a semiconductor or solid state memory, magnetic tape, a removable computer diskette, a random access memory (RAM), a read-only memory (ROM), a rigid magnetic disk and an optical disk. Current examples of optical disks include compact disk-read only memory (CD-ROM), compact disk-read/write (CD-R/W) and DVD. 
     A data processing system suitable for storing and/or executing program code will include at least one processor coupled directly or indirectly to memory elements through a system bus. The memory elements can include local memory employed during actual execution of the program code, bulk storage, and cache memories which provide temporary storage of at least some program code in order to reduce the number of times code must be retrieved from bulk storage during execution. 
     Input/output (I/O) devices (including but not limited to keyboards, displays, pointing devices, etc.) can be coupled to the system either directly or through intervening I/O controllers. Network adapters may also be coupled to the system to enable the data processing system to become coupled to other data processing systems or remote printers or storage devices through intervening private or public networks. Modems, cable modem and Ethernet cards are just a few of the currently available types of network adapters. 
     A representative hardware environment for practicing the embodiments herein is depicted in  FIG. 5 . This schematic drawing illustrates a hardware configuration of an information handling/computer system in accordance with the embodiments herein. The system comprises at least one processor or central processing unit (CPU)  10 . The CPUs  10  are interconnected via system bus  12  to various devices such as a random access memory (RAM)  14 , read-only memory (ROM)  16 , and an input/output (I/O) adapter  18 . The I/O adapter  18  can connect to peripheral devices, such as disk units  11  and tape drives  13 , or other program storage devices that are readable by the system. The system can read the inventive instructions on the program storage devices and follow these instructions to execute the methodology of the embodiments herein. The system further includes a user interface adapter  19  that connects a keyboard  15 , mouse  17 , speaker  24 , microphone  22 , and/or other user interface devices such as a touch screen device (not shown) to the bus  12  to gather user input. Additionally, a communication adapter  20  connects the bus  12  to a data processing network  25 , and a display adapter  21  connects the bus  12  to a display device  23  which may be embodied as an output device such as a monitor, printer, or transmitter, for example. 
     The embodiments herein provide a time-domain based approach for fast fading channel FFT trigger point tracking in an ISDB-T and ISDB-TSB receiver  100  overcomes the problem of the FFT calculated by the time-domain interpolated channel estimates based on scatter pilots which are subject to degradation, noisy channel estimates, aliasing and lead to a poor channel impulse response. Also, the embodiments herein are advantageous over the estimation of channel impulse based on the IFFT of the scatter pilots technique, which due to the limitation in scatter pilot spacing can only see a very short channel impulse response. 
     Generally, the time-domain based approach to estimate the fast fading channel in an ISDB-T and ISDB-TSB receiver  100  provided by the embodiments herein is useful in obtaining the correct trigger point. The average correlation results of the time domain sample obtained using moving average filters Tg  108 ,  110  are averaged and accumulated. The peak of the results is found (which is the trigger point  116 ) and compared with a target trigger point  202  to generate a trigger point error signal, which is scaled by a factor of 1/K. The new trigger point is obtained by adding the old trigger point with the scaled trigger point error signal. 
     The foregoing description of the specific embodiments will so fully reveal the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the appended claims.