Patent Publication Number: US-2009225892-A1

Title: Method and apparatus for iteratively forming subspace from a dictionary for vsb channel modeling

Description:
CROSS-REFERENCE TO OTHER APPLICATIONS 
     The following applications of common assignee and filed on the same day herewith are related to the present application, and are herein incorporated by reference in their entireties: 
     U.S. aatent application No. ______ with attorney docket number LSFFT-090. 
     U.S. patent application No. ______ with attorney docket number LSFFT-092. 
    
    
     FIELD OF THE INVENTION  
     The present invention relates generally to channel estimation, more specifically the present invention relates to iteratively forming subspace from a dictionary for VSB channel modeling. 
     BACKGROUND  
     Channel modeling is one of the most important issues in a VSB communication system. It is usually done by comparing the received signal and the known transmitted signal. Typically, a known method of channel modeling is done. However, the known or initial channel modeling may not satisfy specified requirement due to estimation error caused by interference/noise and the like. 
     In a vestigial sideband (VSB) system, a channel therein can be modeled as a set of linear combinations of a group of selected vectors selected from a dictionary. It is assumed that a suitable dictionary is constructed and can be effectively used. Then, it is desirable to have a system and a method for formatting a channel subspace based on the dictionary. 
     SUMMARY OF THE INVENTION  
     In a vestigial sideband (VSB) system, a system and a method to iteratively forming subspace from a dictionary for VSB channel modeling. 
     In a VSB system, a method and receiver for channel modeling is provided. The method comprises the steps of: performing matching pursuit conducted locally with assistant information; and performing basis perturbation using the assistant information. 
     In a VSB system, a locally conducted Matching Pursuit (MP) with assistant information is conducted locally as opposed to the classical MP, which is conducted globally. Furthermore, an iteratively refined subspace is provided by exchanging some of the selected vectors with their unselected neighbors. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES  
       The accompanying figures, where like reference numerals refer to identical or functionally similar elements throughout the separate views and which together with the detailed description below are incorporated in and form part of the specification, serve to further illustrate various embodiments and to explain various principles and advantages all in accordance with the present invention. 
         FIG. 1  is an example of a relationship in accordance with some embodiments of the invention. 
         FIG. 2  is an example of a first process in accordance with some embodiments of the invention. 
         FIG. 3  is an example of a second process in accordance with some embodiments of the invention. 
         FIG. 4  is an example VSB receiver in accordance with some embodiments of the invention. 
     
    
    
     Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention. 
     DETAILED DESCRIPTION  
     Before describing in detail embodiments that are in accordance with the present invention, it should be observed that the embodiments reside primarily in combinations of method steps and apparatus components related to iteratively forming subspace from a dictionary for VSB channel modeling. Accordingly, the apparatus components and method steps have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein. 
     In this document, relational terms such as first and second, top and bottom, and the like may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. The terms “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. An element proceeded by “comprises . . . a” does not, without more constraints, preclude the existence of additional identical elements in the process, method, article, or apparatus that comprises the element. 
     It will be appreciated that embodiments of the invention described herein may be comprised of one or more conventional processors and unique stored program instructions that control the one or more processors to implement, in conjunction with certain non-processor circuits, some, most, or all of the functions of iteratively forming subspace from a dictionary for VSB channel modeling described herein. The non-processor circuits may include, but are not limited to, a radio receiver, a radio transmitter, signal drivers, clock circuits, power source circuits, and user input devices. As such, these functions may be interpreted as steps of a method to perform iteratively forming subspace from a dictionary for VSB channel modeling. Alternatively, some or all functions could be implemented by a state machine that has no stored program instructions, or in one or more application specific integrated circuits (ASICs), in which each function or some combinations of certain of the functions are implemented as custom logic. Of course, a combination of the two approaches could be used. Thus, methods and means for these functions have been described herein. Further, it is expected that one of ordinary skill, notwithstanding possibly significant effort and many design choices motivated by, for example, available time, current technology, and economic considerations, when guided by the concepts and principles disclosed herein will be readily capable of generating such software instructions and programs and ICs with minimal experimentation. 
     8-VSB (8-level vestigial sideband) is a standard radio frequency (RF) modulation format chosen by the Advanced Television Systems Committee (ATSC) for the transmission of digital television (DTV) in such countries as the United States and other adopting countries. 8-VSB is used in the transmission of video data. There is also a 16-VSB mode that has 16 amplitude levels. 8-VSB is considered effective in multi-casting in that simultaneous transmission of more than one DTV program is achieved. Further, 8-VSB is also considered effective in datacasting in that the transmission of data along with a television program is achieved. 
     In addition, VSB transmission system possesses large bandwidth, which is needed to transmit HDTV (high definition television) programming. VSB has single side band thereby having improved or better adaptability in protecting against adjacent channel interference. Further, single side band has better performance at higher bit rates. VSB uses the entire bandwidth as a single frequency having all component parts multiplexed together. The benefits therefrom include lower broadcast power and the possibility of extended station coverage. VSB further i-minimizes interference with analog NTSC signals, analog VSB modulation. Still further, VSB being a Single Frequency Network (SFN) can improve the signal strength throughout an entire service area, thereby allowing even remote and heavily walled locations to receive the desired signals. 
     In a VSB system, a transmitter transmits signals through some media such as a radio frequency channel. Due to the geographic structure between the transmitter and the receiver, signals arriving at the receiver usually undergo an inter-symbol interference because of multipath effects. In order to recover the transmitted VSB signals, time domain equalizer such as DFE is needed. To train or initialize the equalization, channel impulse response is used. On the other hand, if one uses a frequency domain equalizer, channel impulse response is also required. 
     It is noticed that performance depends heavily on the accuracy of channel modeling. Typical estimation proposals such as singular-value decomposition (SVD) has been proposed (see O. Edfors etc, IEEE trans comm, July 1998), and subspace tracking for channel modeling/refinement are known. These methods in general try to represent signal by combinations of several important vectors such as eigenvectors. Considering the linear transform of inverse Fourier transform, the response then consists of superimposition of multiple delays. As long as one can model the delay, strength, and phase; the channel is represented and Fourier Transform can be conducted to obtain the required frequency response of the channel. 
     Channel time-domain response can be modeled by a new set of basis functions. The basis functions depend on the SRRC filter frequency response and the over-sampling in time domain. In such a way, channel modeling refinement is made possible by finding the best combinations of a set of basis. It is presumed that the combined filter response of transmitting square root raised cosine (SRRC) filter, RF/IF related filter, receiving SRRC filter in a VSB system is represented as g(t). It is further presumed that the physical channel consists of N paths each with coefficient A i  and delay τ i  (i=0, . . . , N−1), with the final combined channel represented as: 
     
       
         
           
             
               
                 
                   
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     Channel modeling is to find A i  and τ i  together with g(t). Note that due to the property of the 8-vsb signal, channel defined here shall be up-shifted a frequency to correspond to the 8-vsb signals. 
     Since g(t) is known to the designer if only two main SRRC filters are considered (e.g. roll-off is 0.11 in a VSB system) or if measured on initial system set-up, g(t) is sampled at symbol rate (10.76 MSPS) with over-sampling rate (e.g. 1/64 or 1/128 symbol for better resolution/match) to give the initial basis, e.g. g k  (k=0, . . . , 63) for one of the 64 phases. It is appreciated that other sampling rates are considered by the present invention. The sampling rate may be 2 n  with n being a finite positive integer. Alternatively any positive integer within the range would be sufficient. 
     It is important to have a high over-sampling basis in order to model the channel more accurately. Further, the over-sampling actions are performed in the time domain. For example, the covariance of a g k  (k=0, . . . , 63) consists of the following entries: 
       g i  (n−δ)g j (n)   (Equ. 2) 
     Where δ means delays :−D+1, . . . , 0, . . . , D−1 respectively. D is the non-zero width of g k . For a fixed i, j, the above shows covariance with changing delays. As can be seen, the correlation function g i  (n−δ)g j (n) aids in the formation of different elements or works of the dictionary in our invention. In other words, g i  (n−δ)g j (n) or equation 2 represent a set of correlation functions. 
     As shown in  FIG. 1 , the final sampled channel h(n)=h(t) is then modeled as the N shifted (due to delay) version of these initial basis. The equation as shown in  FIG. 1  that shows this model. G is a M×N matrix having M rows and N columns. A is a vector with N elements. It is noted that G is a sparse basis matrix. Finally, the dictionary is g k  with all possible k (or 0, 1, . . . , k−1) and shifting shown below (only g 0 , g 1 , and g k−1  are shown): 
     For g 0 (.):
         g 0 (.) 0 0 . . . 0   0 g 0 (.) 0 0 . . . 0   0 0 g 0 (.) 0 0 . . . 0   0 0 0 ∘ ∘ ∘ ∘  g 0 (.)
 
For g 1 (.):
   g 1 (.) 0 0 . . . 0   0 g 1  (.) 0 0 . . . 0   0 0 g 1 (.) 0 0 . . . 0   0 0 0 ∘ ∘ ∘ ∘  g 1 (.)
 
For g k−1  (.):
   g k−1 (.) 0 0 . . . 0   0 g k−1 (.) 0 0 . . . 0   0 0 g k−1 (.) 0 0 . . . 0   0 0 0 ∘ ∘ ∘ ∘  g k−1  (.)       

     In a VSB system, initial channel modeling may not satisfy specified requirement due to interference and/or noise. Modeling with basis representation can be combined with locally conducted matching pursuit having assistant information to form a channel subspace. 
     Suppose the combined filter response of the transmitting filter, the analog filter in transmission, and the receiving filter (e.g. SRRC in VSB system) is represented as g (t). In an exemplified embodiment filter SRRC has a roll-off 0.11 in VSB context. The filter in transmission ideally possesses a flat state in an interested frequency band. g(t) is time-limited with the most of the filter energy contained within a predetermined time segment in interest. g(t) can be sampled at symbol rate with over-sampling within the time segment in interest to give the basis for the channel modeling dictionary, e.g. g k  (k=0, . . . , 63) for one of the exemplified 64 phases. It is noted that the sampling rate may vary; e.g. 1/64, 1/128 or other necessary fraction of symbol depending on specified modeling accuracy. Now suppose that the physical channel consists of N paths each with coefficient A i  and delay m i  (i=0, . . . , N−1). The sampled channel response is Equ. 1. Given initial estimation of path delay m i , one can search within a window associated with the segment of interest the best projection onto the given basis dictionary. That is, projecting h(m) onto each and every g k  (m-m i ), k=0, . . . , K with fixed m i  and find the largest corresponding projection. Here, locally matching pursuit (in which searching is only done on shifting m i ) is conducted since initial position is given as compared to classical MP, which is conducted globally (i.e. on both k and m i ). Global matching pursuit (MP) is not contemplated by the present invention. After no more significant paths is left, the first round of channel subspace is constructed by all these selected g k  (m-m i ) The local MP procedure is repeated by exchanging some of the selected vectors with their unselected neighbors until some predetermined criteria achieved. Finally, all the elements of the last round of selected element g k (m-m i ) is used to construct the channel subspace. These elements, together with the corresponding projections can be used to reconstruct the channel response. 
     In  FIG. 2 , a process  200  incorporating the matching pursuit channel refinement algorithm based on initial known delay m i  is shown, whereas g k  (n-m i ) shows a shifted basis version of g k . The algorithm includes the step of find the maximum projection (Step  202 ). If the projection less than a first set value, then the same is discarded or disregarded (Step  204 ). Remove the known maximum projection corresponding component to update the new residual (Step  206 ). Give a new initial delay m (Step  208 ). If the absolute value of the residual is less than a second predetermined set value, then the same is discarded or disregarded (Step  210 ). Otherwise, the process  200  reverts back to step  202 . 
     In  FIG. 3 , a perturbation process  300  is shown. Start using MP elements or words (Step  302 ). Perform OMP (see Y. C. Pati, R. Rezaiifar, and P. S. Krishnaprasad, “Orthogonal matching pursuits: Recursive function approximation with applications to wavelet decomposition,” in Proc. 27th Asilomar Conf. Signals, Systems, Computers, 1993 Step  304 ). Is residual less than a third set value? (Step  306 ). If true, stop process  300  (Step  308 ). Otherwise, perform basis perturbation (Step  302 ). 
       FIG. 4  is a block diagram of a conventional digital television receiver  100 , which can process a VSB signal, is shown. The digital television receiver  100  includes a tuner  110 , a demodulator  120 , an equalizer  130 , and a TCM (Trellis-coded Modulation) decoder  140 . TCM coding may use an error correction technique, which may improve system robustness against thermal noise. TCM decoding may have more robust performance ability and/or a simpler decoding algorithm. The output signal OUT of the TCM decoder  140  may be processed by a signal processor and output as multimedia signals (e.g., display signals and/or audio signals). 
     In the foregoing specification, specific embodiments of the present invention have been described. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the present invention as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of present invention. The benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential features or elements of any or all the claims. The invention is defined solely by the appended claims including any amendments made during the pendency of this application and all equivalents of those claims as issued. 
     Terms and phrases used in this document, and variations thereof, unless otherwise expressly stated, should be construed as open ended as opposed to limiting. As examples of the foregoing: the term “including” should be read as mean “including, without limitation” or the like; the term “example” is used to provide exemplary instances of the item in discussion, not an exhaustive or limiting list thereof; and adjectives such as “conventional,” “traditional,” “normal,” “standard,” and terms of similar meaning should not be construed as limiting the item described to a given time period or to an item available as of a given time, but instead should be read to encompass conventional, traditional, normal, or standard technologies that may be available now or at any time in the future. Likewise, a group of items linked with the conjunction “and” should not be read as requiring that each and every one of those items be present in the grouping, but rather should be read as “and/or” unless expressly stated otherwise. Similarly, a group of items linked with the conjunction “or” should not be read as requiring mutual exclusivity among that group, but rather should also be read as “and/or” unless expressly stated otherwise.