Abstract:
A preamble identifier flag in a reserved portion of a data field synchronization segment in a digital television (DTV) data field identifies the presence of preamble training data in a forward error correction (FEC) encoded portion of the DTV data field. The data field synchronization segment is not FEC encoded, thereby allowing detection of the preamble identifier flag without FEC decoding. The detection at a receiver of the preamble identifier flag in a DTV data field allows receiver elements, such as an equalizer and a FEC decoder, to more readily obtain and utilize the preamble training data, thereby enhancing reception and/or simplifying receiver design.

Description:
This application claims the benefit, under 35 U.S.C. §365 of International Application PCT/US2009/006109, filed Nov. 13, 2009, which was published in accordance with PCT Article 21(2) on May 19, 2011 in English. 
     FIELD OF INVENTION 
     The present invention generally relates to digital television (DTV) systems and method, and more particularly to mobile DTV systems and methods. 
     BACKGROUND 
     The Advanced Television Systems Committee (ATSC) standard for Digital Television (DTV) in the United States requires an 8-VSB transmission system which includes Forward Error Correction (FEC) as a means of improving system performance. (United States Advanced Television Systems Committee, “ATSC Digital Television Standard”, (document A53.doc), Sep. 16, 1995.)  FIG. 1  shows a simplified block diagram of a typical ATSC compliant DTV transmitter and receiver, emphasizing the FEC subsystem. As shown in  FIG. 1 , on the transmitter side, the FEC encoding subsystem includes a Reed-Solomon (RS) encoder, followed by a byte interleaver, and a trellis encoder. The FEC encoding subsystem is preceded by a data randomizer and followed by an 8-VSB modulator. On the receiver side, there is a corresponding FEC decoding subsystem which includes a trellis decoder, a byte de-interleaver and a RS decoder. The FEC decoding subsystem is preceded by an 8-VSB demodulator and followed by a data de-randomizer. 
     The ATSC DTV transmission scheme is not robust enough against Doppler shift and multipath radio interference, and is designed for highly directional fixed antennas, hindering the provision of expanded services to customers using mobile and handheld (M/H) devices. In an attempt to address these issues and to create a more robust and flexible system, it has been proposed, among other things, to add a new layer of FEC coding and more powerful decoding algorithms to decrease the Threshold of Visibility (TOV). (See, e.g., International Patent Publication No. WO 2008/144004 A1.) The added layer of FEC coding may require decoding techniques such as iterative (turbo) decoding (see, e.g., C. Berrou et al., “Near Shannon Limit Error—Correcting Coding and Decoding: Turbo-Codes (1)”, Proceedings of the IEEE International Conference on Communications—ICC&#39;93, May 23-26, 1993, Geneva, Switzerland, pp. 1064-1070; and M. R. Soleymani et al., “Turbo Coding for Satellite and Wireless Communications”, Kluwer Academic Publishers, USA, 2002) and trellis decoding algorithms like the MAP decoder (see, e.g., L. R. Bahl et al., “Optimal Decoding of Linear Codes for Minimizing Symbol Error Rate”, IEEE Transactions on Information Theory, Vol. IT-20, No. 2, March 1974, pp. 284-287.) 
     SUMMARY 
     In an exemplary embodiment in accordance with the principles of the invention, preamble training data conveying a priori tracking information for iterative forward error correction (FEC) decoding at a receiver, is included in a data field of a data burst which is transmitted to the receiver. The preamble training data may be encoded by all levels of FEC coding. A preamble identifier flag placed in a reserved field of a data field synchronization segment of the data field indicates the presence of the preamble training data in the data field. The data field synchronization segment is not FEC encoded. Detection of the preamble identifier flag allows the receiver to identify the presence of the preamble training data prior to FEC decoding. As a result, FEC decoding and equalization at the receiver are improved with lower overall latency. 
     In view of the above, and as will be apparent from the detailed description, other embodiments and features are also possible and fall within the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       Some embodiments of apparatus and/or methods in accordance with embodiments of the present invention are now described, by way of example only, and with reference to the accompanying figures in which: 
         FIG. 1  is a block diagram of a digital television (DTV) system in accordance with the Advanced Television Systems Committee (ATSC) standard for DTV; 
         FIG. 2  illustrates the format of an ATSC-DTV data frame; 
         FIG. 3  illustrates the format of a Data Field Sync segment in an ATSC-DTV data frame; 
         FIG. 4  is a block diagram of an exemplary DTV-M/H (Mobile/Handheld) system in accordance with the principles of the invention; and 
         FIGS. 5A and 5B  are block diagrams of exemplary embodiments of a receiver in accordance with the principles of the invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Other than the inventive concept, the elements shown in the figures are well known and will not be described in detail. For example, other than the inventive concept, familiarity with television broadcasting, receivers and video encoding is assumed and is not described in detail herein. For example, other than the inventive concept, familiarity with current and proposed recommendations for TV standards such as NTSC (National Television Systems Committee), PAL (Phase Alternation Lines), SECAM (SEquential Couleur Avec Memoire) and ATSC (Advanced Television Systems Committee) (ATSC), Chinese Digital Television System (GB) 20600-2006 and DVB-H is assumed. Likewise, other than the inventive concept, other transmission concepts such as eight-level vestigial sideband (8-VSB), Quadrature Amplitude Modulation (QAM), and receiver components such as a radio-frequency (RF) front-end (such as a low noise block, tuners, down converters, etc.), demodulators, interleavers, Reed-Solomon encoders/decoders, trellis encoders/decoders, FEC encoders/decoders, randomizers and derandomizers, equalizers, MAP decoders, Turbo decoders, correlators, leak integrators and squarers is assumed. Further, other than the inventive concept, familiarity with protocols such as Internet Protocol (IP), Real-time Transport Protocol (RTP), RTP Control Protocol (RTCP), User Datagram Protocol (UDP), is assumed and not described herein. Similarly, other than the inventive concept, familiarity with formatting and encoding methods such as Moving Picture Expert Group (MPEG)-2 Systems Standard (ISO/IEC 13818-1), H.264 Advanced Video Coding (AVC) and Scalable Video Coding (SVC) is assumed and not described herein. It should also be noted that the inventive concept may be implemented using various combinations of hardware and software which constitute a DTV receiver and processor. Finally, like-numbers on the figures represent similar elements. 
       FIG. 2  shows the format of an ATSC-DTV data frame as transmitted. Each data frame consists of two data fields, each containing 313 segments. The first segment of each data field is a unique synchronization segment (Data Field Sync) shown in greater detail in  FIG. 3  and further discussed below. Each of the remaining 312 segments of each data field, referred to as data segments, carries the equivalent data of one 188-byte MPEG-compatible transport packet and its associated FEC overhead. Note that while the 312 data segments of each data field contain FEC encoded data, the Data Field Sync segment is not FEC encoded and thus need not be FEC decoded at the receiver. 
     Each segment consists of 832 8-VSB symbols. The first four symbols of each segment, including the Data Field Sync segments, form a binary pattern and provide segment synchronization. As shown in  FIG. 3 , which shows a Data Field Sync segment, the first four 8-VSB symbols of each segment have values of +5, −5, −5, and +5. This four-symbol segment sync signal also represents the sync byte of each 188-byte MPEG-compatible transport packet conveyed by each of the 312 data segments in each data field. The remaining 828 symbols of each data segment carry data equivalent to the remaining 187 bytes of a transport packet and its associated FEC overhead. 
     As shown in  FIG. 2 , each segment takes 77.3 μs to transmit, thereby taking 48.4 ms to transmit one ATSC-DTV data frame. 
       FIG. 3  shows a Data Field Sync segment in greater detail. As shown in  FIG. 3 , each Data Field Sync segment starts with a four-symbol segment sync followed by several pseudo random (PN) sequences, a VSB mode field and a reserved field of 104 symbols. (Note that the last 12 symbols of the reserved field, labeled PRECODE, are used in trellis coded terrestrial 8-VSB to replicate the last 12 symbols of the previous segment.) As described in greater detail below, an exemplary embodiment of the invention makes advantageous use of all or part of the reserved field of the Data Field Sync segment to provide improved reception and a simplified receiver design. 
       FIG. 4  shows a simplified block diagram of an exemplary transmitter and receiver for a mobile/handheld (M/H) DTV system  400  which includes two layers of FEC, as exemplified by FEC encoders  410  and  430  in the transmitter. FEC encoder  410  may implement multiple block and/or convolutional codes and comprise multiple block and/or convolutional interleavers. FEC encoder  430  corresponds to the FEC encoding subsystem in the transmitter of the ATSC DTV system of  FIG. 1 . Data randomizer  420 , FEC encoder  430  and 8-VSB modulator  440  may be implemented as a conventional legacy ATSC DTV transmitter, such as shown in  FIG. 1 . 
     At the receiver, 8-VSB demodulator  450 , which can be implemented conventionally, demodulates the received signal which is then equalized by equalizer  455 . The equalized signal is provided to iterative FEC decoder  460  which performs turbo decoding of the various FEC encoders within  410  and  430 , including MAP decoding of the ATSC trellis encoding implemented by FEC encoder  430  and the additional FEC encoding implemented by FEC encoder  410 . Iterative FEC decoder  460  will perform a number of iterations (N) deemed necessary to achieve a desired performance. The decoded output of FEC decoder  460  is provided to data de-randomizer  470 , which can be implemented conventionally. Equalizer  455 , at the output of demodulator  450 , receives feed-back from FEC decoder  460 . 
     In an exemplary mobile DTV system, such as shown in  FIG. 4 , preamble training data segments, also called a priori tracking (APT) packets, may be transmitted in addition to the synchronization data present in the ATSC-DTV data frame described above. For example, the training data may be transmitted in the DATA+FEC segments shown in  FIG. 2 . Placing the preamble training data in DATA+FEC segments, however, would subject the preamble training data to all levels of FEC coding and interleaving introduced by FEC encoder  430 , as well as being randomized by data randomizer  420 . In addition, preamble training data in the DATA+FEC segments may also be subjected to all levels of FEC coding and interleaving introduced by FEC encoder  410 . 
     An example of a data burst, containing preamble training data, that can be used in an exemplary mobile DTV system is given in TABLE 1. The data burst of TABLE 1 will be referred to herein as a DTV-M/H (Mobile/Handheld) data burst. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 DTV-M/H Data burst 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Data Field F0 
                 Data Field Sync segment 
               
               
                   
                   
                 260 Legacy ATSC Data Segments 
               
               
                   
                   
                 52 Preamble Data Segments 
               
               
                   
                 Data Field F1 
                 Data Field Sync segment 
               
               
                   
                   
                 52 DTV-M/H Data Segments 
               
               
                   
                   
                 26 Legacy ATSC Data Segments 
               
               
                   
                   
                 104 DTV-M/H Data Segments 
               
               
                   
                   
                 130 Legacy ATSC Data Segments 
               
               
                   
                 Data Field F2 
                 Data Field Sync segment 
               
               
                   
                   
                 312 Legacy ATSC Data Segments 
               
               
                   
                   
               
             
          
         
       
     
     As shown in TABLE 1, each DTV-M/H data burst comprises three data fields, F0, F1 and F2. Each data field F0-F2 is analogous to a legacy ATSC-DTV data field, such as those shown in the legacy ATSC-DTV frame of  FIG. 2 . As such, each exemplary DTV-M/H data burst set forth in TABLE 1 corresponds to 1.5 frames of the legacy ATSC-DTV standard. Note that each data field F0-F2 starts with a Data Field Sync segment (such as shown in  FIG. 3 ) followed by 312 FEC encoded data segments (or DATA+FEC segments). In typical operation, it is contemplated that a DTV-M/H receiving device will receive multiple legacy ATSC-DTV data fields or frames with one or more DTV-M/H data bursts in between. 
     In the exemplary DTV-M/H data burst of TABLE 1, preamble training data is contained in the first data field F0 as 52 data segments. When receiving an exemplary DTV-M/H data burst, a DTV-M/H receiver will discard the 260 Legacy ATSC data segments in Data Field F0 and process the remaining data including the 52 preamble training data segments. The preamble training data is to be utilized by a DTV-M/H receiver as training in order to enhance its performance. As described in greater detail below, signaling the presence of a preamble within a data field allows a simplified receiver design with improved performance. 
     In an exemplary embodiment of the invention, a subset or the entirety of the reserved field ( FIG. 3 ) of the Data Field Sync segment of one or more data fields F0-F1 of the DTV-M/H data burst of TABLE 1 contains an indicator to identify the presence or absence of preamble training data in the data field. The indicator is referred to herein as an identifier flag. This flag may be a particular pattern, data sequence, or a PN sequence, which preferably can be easily regenerated by a receiver field sync detector. If a PN sequence is used, it could be a portion of the PN511 or the PN63 sequences already used in the Data Field Sync segment ( FIG. 3 ) or a linear combination of these sequences. Because, as shown in TABLE 1, the preamble data signals the beginning of a burst of mobile data to the receiver, the use of such a flag in the Data Field Sync segment of Data Field F0 allows a receiver to identify the mobile data burst. This allows for improved reception and a simplified receiver design. 
     In an embodiment employing the exemplary DTV-M/H data burst structure of TABLE 1, the aforementioned identifier flag is placed in the Data Field Sync segment of data field F0. A further, complementary flag, such as a logical inverse of the preamble identifier flag in data field F0, may be placed in the Data Field Sync segments of the other two data fields, F1 and F2. One skilled in the art will understand that there are multiple possible alternatives. For example, the identifier flag could be placed in data field F1, and its logical inverse in data fields F0 and F2. In this case, the identifier flag would identify data field F1, with the preamble being in the previous data field F0. Likewise, the identifier flag could be placed in field F2 and its inverse in fields F0 and F1. In this case, the flag would identify data field F2, and the data field containing the preamble, F0, would be the subsequent field. In other embodiments, preamble training data for a data burst may be contained in one or more locations other than that shown in TABLE 1, including other data fields of the data burst. 
       FIG. 5A  is a block diagram of an exemplary embodiment of a DTV-M/H receiver  500 A in accordance with the principles of the invention. The receiver  500 A comprises demodulator  510 , equalizer  520 , iterative FEC decoder  530  and data de-randomizer  540 . 
     The receiver  500 A also comprises preamble flag detector  560 A which detects the preamble identifier flag in the data stream at the output of demodulator  510  or equalizer  520 . As discussed, the preamble identifier flag in the Data Field Sync segment of a data field provides an indication as to whether or not the data field contains the preamble training data. If the preamble flag detector  560 A detects the preamble identifier flag, it provides an indication to equalizer  520  and iterative FEC decoder  530  accordingly. Once said indication is provided, equalizer  520  and FEC decoder  530  can expect to start receiving the preamble training data a known number of segments (e.g.,  260 , per TABLE 1) following the Data Field Sync segment containing the preamble identifier flag. Upon reception of the preamble, equalizer  520  and FEC decoder  530  can use the training data in the preamble to process the received encoded data; equalizer  520  uses the preamble training data to perform equalization of the demodulated signal at the output of demodulator  510 , and FEC decoder  530  uses the preamble training data to perform iterative FEC decoding of the equalized signal at the output of equalizer  520 . Data de-randomizer  540  de-randomizes the decoded data and can be implemented conventionally. 
     By contrast, because the preamble training data is FEC encoded, a conventional receiver would need to iteratively FEC decode the received data stream by multiple iterations before it can reliably detect the preamble training data and feed back the a priori information contained therein to the equalizer and/or other iterations of the FEC decoder. By utilizing a preamble identifier flag of the present invention, which is not FEC encoded, as described above, the exemplary receiver  500 A can identify the presence of the preamble training data prior to the FEC decoder. Moreover, detection of the preamble identifier flag can be implemented more simply than detection of the preamble data itself. As such, replacement of a preamble data detector with preamble flag detector  560 A represents a net simplification. In addition, the feedback path from iterative decoder  530  to equalizer  520  can be shortened (i.e., feedback from an earlier iteration of decoder  530  can be used), resulting in lower overall latency and thus improved receiver performance. 
     In a further embodiment of the invention shown in  FIG. 5B , receiver  500 B comprises field sync detector  550  which detects the Data Field Sync segment in the data stream at the output of demodulator  510  or equalizer  520  using correlation or any other suitable method. In the embodiment of  FIG. 5B , preamble flag detector  560 B receives an indication from field sync detector  550  of the presence of a Data Field Sync segment, thereby alerting the preamble flag detector  560 B to search for the possible presence of a preamble identifier flag in the reserved field of the Data Field Sync segment. Upon detection of a Data Field Sync segment by detector  550 , preamble flag detector  560 B looks for the above-described identifier flag in the reserved field of the Data Field Sync segment. It should be noted that a field sync detector, such as  550 , will typically be included in any receiver designed to receive data bursts with Data Field Sync segments, such as shown in  FIG. 3 . Moreover, by using the indication provided by the field sync detector  550 , it may be possible to simplify the implementation of preamble flag detector  560 B relative to that of detector  560 A of the embodiment of  FIG. 5A . 
     The principles of this invention can be extended to other mobile DTV systems and data frame and preamble training structures. 
     In view of the above, the foregoing merely illustrates the principles of the invention and it will thus be appreciated that those skilled in the art will be able to devise numerous alternative arrangements which, although not explicitly described herein, embody the principles of the invention and are within its spirit and scope. For example, although illustrated in the context of separate functional elements, these functional elements may be embodied in one, or more, integrated circuits (ICs). Similarly, although shown as separate elements, some or all of the elements may be implemented in a stored-program-controlled processor, e.g., a digital signal processor or a general purpose processor, which executes associated software, e.g., corresponding to one, or more, steps, which software may be embodied in any of a variety of suitable storage media. Further, the principles of the invention are applicable to various types of wired and wireless communications systems, e.g., terrestrial broadcast, satellite, Wireless-Fidelity (Wi-Fi), cellular, etc. Indeed, the inventive concept is also applicable to stationary or mobile receivers. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention.