Abstract:
In an OFDM system having pseudo-noise (PN) sequences as guard intervals, a method for channel estimation and equalization is provided. The method comprising the steps of: providing a frequency equalization scheme; providing a time domain filter; and combining the frequency equalization with time-domain filter, thereby a time lag effect is taken into consideration for the channel estimation and equalization.

Description:
FIELD OF THE INVENTION  
       [0001]    The present invention relates generally to signal equalization, more specifically the present invention relates to equalization of TDS-OFDM signals. 
       BACKGROUND  
       [0002]    Traditional equalization of OFDM signals makes use of the cyclic property established by guard intervals (GI). The cyclic convolutional relationship between OFDM symbol and the channel impulse response calls for a one-tap frequency equalizer. In a TDS-OFDM system, pseudo-noise (PN) sequences instead of the generic GI are inserted. Typically, a straightforward method is to remove the PN sequences, to restore the cyclic property, and to apply the one-tap frequency equalizer. 
         [0003]    Therefore, for OFDM having PNs as guard intervals, an improved means for equalization can be achieved. 
       SUMMARY OF THE INVENTION 
       [0004]    In an OFDM system having PNs as guard intervals, a method for frequency equalization is provided. 
         [0005]    In an OFDM system having PNs as guard intervals, a method using a time-domain filter is provided. 
         [0006]    In an OFDM system having PNs as guard intervals, a method using frequency equalization is provided. Whereby, without adding back the tail, the improved method is the same in static situation and is better in time-varying situations as compared to existing, known methods. 
         [0007]    In an OFDM system having PNs as guard intervals, a method using time-domain filter is provided. Whereby, without adding back the tail, the improved method is the same in static situation and is better in time-varying situations as compared to existing, known methods. 
         [0008]    In an OFDM system having PNs as guard intervals, a method that combines frequency equalization together with time-domain filter is provided. Whereby, without adding back the tail, the improved method is the same in static situation and is better in time-varying situations as compared to existing, known methods. 
         [0009]    In an OFDM system having pseudo-noise (PN) sequences as guard intervals, a method for channel estimation and equalization is provided. The method comprising the steps of: providing a frequency equalization scheme; providing a time domain filter; and combining the frequency equalization with time-domain filter, thereby a time lag effect is taken into consideration for the channel estimation and equalization. A receiver is provided that comprises the above method. 
         [0010]    In an OFDM system having pseudo-noise (PN) sequences as guard intervals, a method for channel estimation and equalization is provided. The method comprising the steps of: obtaining a data frame having a first length; constructing a second length M having the length greater than the first length for further processing; using the data frame as part of M; and padding zeros after a segment within M being not occupied by the data frame. A receiver is provided that comprises the above method. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0011]    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. 
           [0012]      FIG. 1  is an example of an OFDM system having PNs as GIs in accordance with some embodiments of the invention. 
           [0013]      FIG. 2  is an example of an OFDM receiver in accordance with some embodiments of the invention. 
           [0014]      FIG. 3  is an example diagram in accordance with some embodiments of the invention. 
           [0015]      FIG. 4  is a flowchart in accordance with some embodiments of the invention. 
       
    
    
       [0016]    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  
       [0017]    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 frequency equalization (using linear de-convolution instead of cyclic de-convolution) of a TDS-OFDM signal. 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. 
         [0018]    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. 
         [0019]    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 frequency equalization (using linear de-convolution instead of cyclic de-convolution) of a TDS-OFDM signal 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 frequency equalization (using linear de-convolution instead of cyclic de-convolution) of a TDS-OFDM signal. 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. 
         [0020]    Referring to  FIGS. 1-4 , varies depictions of the present invention are shown. In  FIG. 1 , a frame structure of a TDS-OFDM system is shown. A packet of transmission or a received packet having PN sequence as guard intervals is shown. The packet is positioned sequentially within a frame among a multiplicity of packets. As can be appreciated, PNs are disposed between the OFDM symbols. It is noted that the present invention contemplates using the PN sequence disclosed in U.S. Pat. No. 7,072,289 to Yang et al which is hereby incorporated herein by reference. 
         [0021]      FIG. 2  is the typical OFDM receiver  10  is shown. The block diagram of  FIG. 2  illustrates the signals and key processing steps of the receiver  10 . It is assumed that the input signal  12  to the receiver  10  is a down-converted digital signal. The output signal  14  of receiver  10  is a MPEG-2 transport stream. More specifically, the RF (radio frequency) input signals  16  are received by an RF tuner  18  where the RF input signals are converted to low-IF (intermediate frequency) or zero-IF signals  12 . The low-IF or zero-IF signals  12  are provided to the receiver  10  as analog signals or as digital signals (through an optional analog-to-digital converter  20 ). 
         [0022]    In the receiver  10 , the IF signals are converted to base-band signals  22 . TDS-OFDM demodulation is then performed according to the parameters of the LDPC (low-density parity-check) based TDS-OFDM modulation scheme. The output of the channel estimation  24  and correlation block  26  is sent to a time de-interleaver  28  and then to the forward error correction (FEC) block. The output signal  14  of the receiver  10  is a parallel or serial MPEG-2 transport stream including valid data, synchronization and clock signals. The configuration parameters of the receiver  10  can be detected or automatically programmed, or manually set. The main configurable parameters for the receiver  10  include: (1) Sub carrier modulation type including: QPSK, 16 QAM, 64 QAM; (2) FEC rate including: 0.4, 0.6 and 0.8; (3) Guard interval having: 420 or 945 symbols; (4) Time de-interleaver mode including three modes respectively having: 0, 240 or 720 symbols; (5) Control frames detection; and (6) Channel bandwidth including: 6, 7, or 8 MHz. 
         [0023]    The functional blocks of the receiver  10  are described as follows. 
         [0024]    Automatic gain control (AGC) block  30  compares the input digitized signal strength with a reference. The difference is filtered and the filter value  32  is used to control the gain of the amplifier  18 . The analog signal provided by the tuner  12  is sampled by an ADC  20 . The resulting signal is centered at a lower IF. For example, sampling a 36 MHz IF signal at 30.4 MHz results in the signal centered at 5.6 MHz. The IF to Baseband block  22  converts the lower IF signal to a complex signal in the baseband. The ADC  20  uses a fixed sampling rate. Conversion from this fixed sampling rate to the OFDM sample rate is achieved using the interpolator in block  22 . The timing recovery block  32  computes the timing error and filters the error to drive a Numerically Controlled Oscillator (not shown) that controls the sample timing correction applied in the interpolator of the sample rate converter. 
         [0025]    There can be frequency offsets in the input signal  12 . The automatic frequency control block  34  calculates the offsets and adjusts the IF to baseband reference IF frequency. To improve capture range and tracking performance, frequency control is done in two stages: a coarse stage and a fine stage. Since the transmitted signal is square root raised cosine filtered, the received signal will be applied with the same function. It is known that signals in a TDS-OFDM system include a PN sequence preceding the IDFT symbol. By correlating the locally generated PN with the incoming signal, it is easy to find the correlation peak (so the frame start can be determined) and other synchronization information such as frequency offset and timing error. Channel time domain response is based on the signal correlation previously obtained. Frequency response is taking the FFT of the time domain response. 
         [0026]    In TDS-OFDM, a PN sequence replaces the traditional cyclic prefix. It is thus necessary to remove the PN sequence and restore the channel spreaded OFDM symbol. Block  36  reconstructs the conventional OFDM symbol that can be one-tap equalized. The FFT block  38  performs a fixed point FFT such as a 3780 point FFT. Channel equalization  40  is carried out from the FFT  38  transformed data based on the frequency response of the channel. De-rotated data and the channel state information are sent to FEC for further processing. 
         [0027]    In the TDS-OFDM receiver  10 , the time-deinterleaver  28  is used to increase the resilience to spurious noise. The time-deinterleaver  28  is a convolutional de-interleaver which needs a memory with size B*(B-1)*M/2, where B is the number of the branch, and M is the depth. For the TDS-OFDM receiver  10  of the present embodiment, there are three modes of time-deinterleavering. For mode  1  B=52, M=48, mode  2  B=52, M=240, and for mode  3 , B=52, M=720. 
         [0028]    The LDPC decoder  42  is a soft-decision iterative decoder for decoding, for example, a Quasi-Cyclic Low Density Parity Check (QC-LDPC) code provided by a transmitter (not shown). LDPC decoder  42  is configured to decode at 3 different rates (i.e. rate 0.4, rate 0.6 and rate 0.8) of QC-LDPC codes by sharing the same piece of hardware. The iteration process is either stopped when it reaches the specified maximum iteration number (full iteration), or when the detected error is free during error detecting and correcting process (partial iteration). 
         [0029]    The TDS-OFDM modulation/demodulation system is a multi-rated system based on multiple modulation schemes (e.g. QPSK, 16 QAM, 64 QAM), and multiple coding rates (0.4, 0,6, and 0.8), where QPSK stands for Quad Phase Shift Keying and QAM stands for Quadrature Amplitude Modulation. The output of BCH decoder  46  is bit by bit. According to different modulation schemes and coding rates, the rate conversion block  44  combines the bit output of BCH decoder  46  to bytes, and adjusts the speed of byte output clock to make the receiver  10 &#39;s MPEG packets outputs evenly distributed during the whole demodulation/decoding process. 
         [0030]    The BCH decoder  46  is designed to decode codes such as BCH ( 762 ,  752 ) code, which is the shortened binary BCH code of BCH ( 1023 ,  1013 ). The generator polynomial is x̂10+x̂3+1. 
         [0031]    Since the data in the transmitter has been randomized using a pseudo-random (PN) sequence before BCH encoder (not shown), the error corrected data by the LDPC/BCH decoder  46  must be de-randomized. The PN sequence is generated by the polynomial 1+x 14 +x 15 , with initial condition of 100101010000000. The de-scrambler/de-randomizer  48  will be reset to the initial condition for every signal frame. Otherwise, de-scrambler/de-randomizer  48  will be free running until reset again. The least significant 8-bit will be XORed with the input byte stream. 
         [0032]    The data flow through the various blocks of the modulator is as follows. The received RF information  16  is processed by a digital terrestrial tuner  18  which picks the frequency bandwidth of choice to be demodulated and then downconverts the signal  16  to a baseband or low-intermediate frequency. This downconverted information  12  is then converted to the Digital domain through an analog-to-digital data converter  20 . 
         [0033]    The baseband signal after processing by a sample rate converter  50  is converted to symbols. The PN information found in the guard interval is extracted and correlated with a local PN generator to find the time domain impulse response. The FFT of the time domain impulse response gives the estimated channel response. The correlation  26  is also used for the timing recovery  32  and the frequency estimation and correction of the received signal. The OFDM symbol information in the received data is extracted and passed through a 3780 FFT  38  to obtain the symbol information back in the frequency domain. Using the estimated channel estimation previously obtained, the OFDM symbol is equalized and passed to the FEC decoder. 
         [0034]    At the FEC decoder, the time-deinterleaver block  28  performs a deconvolution of the transmitted symbol sequence and passes the 3744 symbols to the inner LDPC decoder  42 . The LDPC decoder  42  and BCH decoders  46  which run in a serial manner take in exactly 3744 symbols, remove the 36 TPS symbols and process the remaining 3744 symbols and recover the transmitted transport stream information. The rate conversion  44  adjusts the output data rate and the de-randomizer  48  reconstructs the transmitted stream information. An external memory  52  coupled to the receiver  10  provides memory thereto on a predetermined or as needed basis. 
         [0035]      FIG. 3  shows the frames with the PNs removed. But the removed frames still retain some unwanted effects due to the nature of wireless transmission in that some part or tail of data information falls outside the N 1  or length A-B. The tail is defined as delta. The tails preferably need be restored or added-back. In other words, each of the received signal y has a tail delta. For details of the restoring process, see  FIG. 4 . 
         [0036]    In  FIG. 4 , an equalization flowchart  400  is shown. First, the PNs interposed between the OFDM frames are removed (Step  402 ). The data frame are then acquired (Step  404 ). There is a choice in the acquiring of data frames in that the starting point is A and the ending points can be any point between C to D. Therefore, the acquired data frame length ranges between length A-C and length A-D. Select a length M that is longer than the acquired data length and accommodating same; pad zeros thereafter for a Fourier transform such as fast Fourier transformed for both the received information y and the characteristics h over M (Step  406 ). M is a value greater than the sum of N 1  and N 2  (M&gt;N 1 +N 2 ). In an exemplified application, N 1 +N 2 =4200, and M=8192. The transformed FFT M (y) is then divided by FFT M (h) (Step  408 ). Note that the non-data sections of M are filled or padded by zeros. The quotient or a derivative of Step  408  undergoes a frequency domain filter (with bandwidth defined in time domain) to be rid of anything beyond N 1  because the transmitted data (not shown) is time limit to A-B (Step  410 ) after equalization. The TD is defined as follows: 
         [0037]    1, at A-B; 
         [0038]    δ, otherwise, where δ→0. 
         [0039]    The filter co-efficient is obtained by applying FFT to the time domain. 
         [0040]    Perform interpolation from M to N 1  (Step  412 ). Interpolation may be performed by a sinc function from M to N 1  so as to obtain the equalized symbol in FFT size N 1  as defined in the transmitter. 
         [0041]    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. 
         [0042]    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.