Abstract:
A method and apparatus for time-shift extraction in a wideband transmitted signal containing strong narrowband interference or noise. The time-shift extraction is based on the time domain and frequency domain relation of symbol misalignment. The invention uses the sign of the product of a recieved signal sample and a reference symbol in the frequency domain to determine the time-shift. It does not rely on the signal magnitude and is therefore less dependent on the signal gain. It also does not rely on the soft phase values, which have ambiguity for values more than three hundred sixty (360) degrees.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This non-provisional United States (U.S.) patent application claims the benefit of U.S. Provisional Application No. 60/287,532, filed by inventors Ahmad Chini et al. on Apr. 30, 2001, titled “Wideband Symbol Synchronization In The Presence Of Multiple Strong Narrowband Interference”. 

   FIELD 
   This invention relates generally to communication devices, systems, and methods. More particularly, one embodiment of the invention relates to a method, apparatus, and system for wideband symbol synchronization in the presence of multiple strong narrowband interference. 
   GENERAL BACKGROUND 
   Receiver devices or systems in a communication system may receive signals or waveforms which are distorted by interference or noise. Some wideband communication systems are supposed to work in the presence of strong narrowband interference. 
   Despite such narrowband interference, a receiving device must be able to detect a signal and determine its content. A receiving device must be able to align or synchronize the received signal in order to determine the start of a signal or message and/or to determine whether such signal contains a message. 
   However, many time domain and frequency domain synchronization algorithms fail in the presence of such interference. Some algorithms are more complex and some do not tolerate multiple interference. Some algorithms are very sensitive to the signal gain or require specific forms of symbols or patterns for synchronization. 
   Time domain correlation synchronizers may be used for synchronization but require many high-resolution multiplications for each received sample. For instance, 256×256 multiplications are required for a time domain correlation synchronizer for a synchronization symbol two hundred fifty-six (256) samples long. 
   Some time domain synchronizers use only the received signal sign to reduce the implementation complexity. However, such sign-based synchronizers often fail in the presence of strong narrowband interference. 
   Some frequency domain correlation synchronizers have to calculate the fast Fourier transform (FFT) coefficient of the signal on each coming time sample. This obviously is very complex to implement in real-time applications. 
   There is a frequency domain symbol synchronizer, which is based on only one time FFT calculation per symbol (each symbol comprising multiple time samples). However, this approach is based on calculation of FFT output phases and requires comparison with every possible phase settings to obtain the timing reference. Phase calculations and a large number of comparisons make this approach less attractive compared to the approach of the invention. 

   
     BRIEF DESCRIPTIONS OF THE DRAWINGS 
       FIG. 1  is an exemplary embodiment of a preamble signal as may be utilized for symbol synchronization in the present invention. 
       FIG. 2  is an exemplary block diagram illustrating the operation of the present invention. 
       FIG. 3  is an exemplary code illustrating the operation at various points on the block diagram in FIG.  2 . 
       FIG. 4  illustrates an exemplary embodiment of two strong narrowband interferences as seen at the FFT output point X in the block diagram shown in FIG.  2 . 
       FIG. 5  illustrates a typical shape of an exemplary signal obtained at point C on the block diagram shown in FIG.  2 . 
       FIG. 6  illustrates an exemplary Synch signal obtained using the synchronizer of FIG.  2 . 
       FIG. 7  illustrates another exemplary Synch signal were the time-shift is in the reverse direction as that of FIG.  6 . 
   

   DETAILED DESCRIPTION 
   In the following detailed description of the invention, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it is contemplated that the invention may be Practiced without these specific details. In other instances well known methods, procedures, components, and circuits have not been described in detail so as not to unnecessarily obscure aspects of the invention. 
   To address the problem of signal alignment or synchronization in the presence of interference, the invention provides a frequency domain symbol synchronization algorithm which works in the presence of multiple strong narrowband interference signals. Wideband Code Division Multiple Access (CDMA) and Orthogonal Frequency Division Multiplexing (OFDM) are among the communication systems, which could be benefited from this algorithm. 
   In one aspect of the invention, frequency analysis of the received signal confines narrowband interference signals to a small portion of the signal bandwidth. The signal is then processed to find the symbol time-shift, indicating the amount of signal misalignment, which appears as a modulated signal in the frequency domain. The modulating frequency is extracted and used to estimate the symbol time-shift. 
   For purposes of synchronization or signal alignment, a number of wideband synchronization symbols are initially appended to a signal. The synchronization symbol(s) may be of any length sufficient to allow a receiving device to determine if the received signal or waveform contains legitimate content or data or determine where the content begins within the signal. 
   According to one implementation, a synchronization symbol of length N is employed (where N is the number of samples). For example, a randomly generated symbol of length two hundred fifty-six (256) samples may be used. The synchronization symbol may be repeated multiple times to allow for better time alignment. 
     FIG. 1  is an exemplary embodiment of a preamble signal as may be utilized for symbol synchronization in the present invention. As illustrated in  FIG. 1  the same wideband synchronization symbol is repeated four times (indicated as A, B, C, and D) to form a preamble signal. 
   The preamble signal, along with a frame of data, is transmitted across the channel where it is corrupted by additive noise and narrowband interference signals. 
     FIG. 2  is a block diagram illustrating the processing and synchronization of a transmitted signal or waveform at a receiving device or system.  FIG. 3  illustrates pseudo code for implementing one embodiment of the invention according to the block diagram of FIG.  2 .  FIGS. 4-7  provide illustrative signals as they may appear at various points of the block diagram in FIG.  2 . 
   Referring to  FIG. 2 , a received signal (Input Data Stream) is processed by an analog to digital converter  102  (ADC), the ADC output  104  is framed and windowed (truncated)  106  to a length equal to the length of the synchronization symbols. For example, a two hundred and fifty-six (256) Hanning window may be used in an implementation where a synchronization symbol two hundred and fifty-six (256) samples long is employed. 
   Windowing  106  is performed to reduce the interference spread in the frequency domain. The windowed data  108  is analyzed using a FFT processor  110  of a proper length to convert the signal from the time domain into the frequency domain. 
   An example of the output of the FFT X  110  is illustrated in FIG.  4 . The presence of two strong narrowband interference signals  402  and  404  is seen in this graph. This illustrative graph, as well as those of  FIGS. 5-7 , assume a signal to noise ratio of about ten (10) decibels (dB), and two interference signals  402  and  404  of about twenty-five (25) dB and twenty (20) dB stronger than the signal, respectively. 
   The output X  112  of the FFT  110  is then correlated (multiplied in the frequency domain) with the reference synchronization symbol  138 . The reference synchronization symbol  138  is the frequency domain representation of the transmitted synchronization symbol and is known beforehand by the receiving system. 
   The result of the correlation (product)  114  is a signal with real  140  and imaginary  142  components containing time-shift information for the input data stream. 
   The sign of each signal component  116  and  118  is then obtained to provide corresponding signals A  144  and B  146  respectively. Relying on the sign of the outputs  116  and  118  of the frequency domain correlator  114  makes the invented approach less complex and more robust to signal gain or magnitude variations. Determining the sign of the outputs  116  and  118  also removes processing ambiguities associated with signal phases greater than three hundred sixty (360) degrees. 
   The resulting sign signals A  144  and B  146  are then convolved  120 . A convolved signal C  132  of typical shape is shown in FIG.  4 . Convolving the signals A  144  and B  146 , which carry common signal information, helps to reduce the noise in the resulting signal C  132 . 
   The frequency and sign of signal C  132  provide the time-shift information to align the input data signal (Input Data Stream). An exemplary embodiment of a signal (at point C  132  in  FIG. 2 ) is illustrated in FIG.  5 . To extract the time-shift information from the signal C  132 , another FFT processing  122  is performed. The real  150  and imaginary  152  components of the resulting signal are then added (sum)  124  to provide a synchronization (Synch) signal  126 . The Synch signal  126  indicates how much alignment (symbol time-shift) is necessary to synchronize the input data stream. 
   The Synch signal  126  may then be processed by a peak detection module to provide the time-shift parameters (peak  134  and index  136 ). 
     FIG. 6  illustrates an example Synch  126  signal. The index  136  and the sign of the peak  134  (positive or negative) of this signal is used by a controller  130  to determine the amount and the direction of the required time-shift  148 . 
   For the example illustrated in  FIG. 6 , a time-shift  148  of twenty (20) samples is required to align the receiver and the transmitter. The number of samples is indicated by the index  136  corresponding to the location of the peak  600 . The peak sign indicates the direction of the time-shift. 
     FIG. 7  shows the case where a time-shift  148  in the reverse direction  700  is required. The same structure of  FIG. 2  may be used to initially detect the signal. 
   According to one aspect of the invention, the presence of a preamble synchronization symbol may be first asserted by comparing the peak signal  134  with a threshold magnitude level, before proceeding with symbol synchronization as described. 
   The algorithm of the invention works with a wide range of synchronization symbols including many randomly generated ones. 
   According to another aspect of the invention, the symbol synchronizer of  FIG. 2  may be repeatedly invoked with various initial time-shifts (provided that preamble is long enough) to more accurately synchronize the input signal or data stream. In one implementation the resulting time-shift signal  148  may be integrated for more accurate symbol synchronization. 
   Various windowing functions may be employed including, but not limited to, Hanning, Hamming, Blackman, Blackman-Harris, Kaiser-Bessel, and rectangular windowing without deviating from the invention. 
   According to an alternative implementation, a single signal, either A  144  or B  146 , may be employed in obtaining the synchronization signal. In such embodiment, the convolution  120  is skipped. 
   As a person of ordinary skill in the art will recognize, a narrowband is merely narrow relative to the overall width of the communication channel employed. Thus, the width of narrowband interference need not be narrow in absolute terms but just in relative terms. 
   While certain exemplary embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative of and not restrictive on the broad invention, and that this invention not be limited to the specific constructions and arrangements shown and described, since various other modifications may occur to those ordinarily skilled in the art. Additionally, it is possible to implement the invention or some of its features in hardware, programmable devices, firmware, integrated circuits, software or a combination thereof where the software is provided in a machine-readable or processor-readable storage medium such as a magnetic, optical, or semiconductor storage medium.