Abstract:
A method is directed to recovering a digital data content in a communication system, wherein the digital data content has been converted into an analog signal based on a primary clock, at a transmitter for transmitting to a receiver. The method comprises receiving the analog signal by the receiver. The analog signal is converted into a digital signal, based on a clock of the receiver. The digital signal is interpolated at the desired interpolation point, if digital signal in time has been shifted by an amount equal to or larger than a predetermined time length. The interpolated digital signal is recovered back to the digital data content, with the assist of an estimated channel impulse response. The channel impulse response is retrained every time when the interpolation point is significantly changed.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the priority benefit of U.S.A. provisional application serial No. 60/363,988, filed on Mar. 14, 2002, all disclosures are incorporated therewith. 
     
    
     
       BACKGROUND OF INVENTION  
         [0002]    1. Field of Invention  
           [0003]    The present invention relates to a communication technology in signal processing. More particularly, the present invention relates to a method for recovering a digital data content in a communication system and an apparatus to perform the method.  
           [0004]    2. Description of Related Art  
           [0005]    Nowadays, the wireless communication has been a very usual way taken by people for communication between two places without much restriction in space and distance. In addition, since the digital data technology has also been well developed, most of the wireless communications now have been in a digital manner. In digital communication, the digital data content at the transmitting end is converted into a radio frequency (RF) analog signal for transmission. After receiving the RF analog signal, the receiving end performs the conversion of the RF analog signal to the digital data content.  
           [0006]    At the transmitting end, the digital data content is first converted into digital signal according to the applied coding scheme, modulation scheme, and etc. The digital signal is then converted by a digital-to-analog converter (DAC) to an analog signal according to a local clock  102  shown in FIG. 1. The analog signal is then further processed, for example, shifting the central frequency of the analog signal to a radio frequency band, for transmission. At the receiving end, the received RF analog signal is first processed, for example, reducing the central frequency of the RF analog signal to a much lower one. The analog signal is then converted into a digital signal by an analog-to-digital converter (ADC), which samples the analog signal in time domain based on a local clock  104  shown in FIG. 1, and quantizes the sampled signal amplitude for further digital signal process (DSP).  
           [0007]    However, the clock rates between the transmitting and receiving ends are generally different, that is, the clock periods of the clock  102  at transmitting end and the clock  104  at the receiving end are generally different, and accordingly cause serious degradation of DSP performance at the receiving end. As an example shown in FIG. 1, the timing drift, which is the time distance between the ticks of the clock  102  and the clock  104 , increases linearly due to the mismatch of the clock rates. Usually, the time drift will cause an accumulative divergence of synchronization between two timing device or sources. When the timing drift is accumulated up to a certain quantity, it could cause serious errors in data recovery.  
           [0008]    Traditionally, one solution to timing drift problem may concentrate on increasing the timing resolution via increasing the clock rate of the clock  104  at the receiving end and accordingly increasing the ADC sampling rate. The receiver can then select those sampled digital signal at more precise sampling position so that the timing drift are controlled to be less than the ADC sampling period, which is defined to be the inverse of the ADC sampling rate. However, the use of a high-sampling-rate ADC increases not only the cost but also the complexity of the design. Although the use of a low-sampling-rate ADC is a proper way to reduce the cost as well as the complexity of the design, the timing resolution is decreased. Moreover, due to the decrease of the timing resolution, the timing drift problem gets more serious and the system performance is then degraded.  
           [0009]    Alternatively, an interpolator may be used to accept the output of the ADC and generate an output, called interpolated digital signal, with increased timing accuracy by interpolating the input signal at the desired time point, named interpolation points, so that the timing of the interpolated digital signal coincides with that of the clock at transmitting end. According to the Nyquist sampling theorem, which is commonly familiar to those skilled artisans in the art, the original analog signal waveform can be exactly reconstructed according to its sampled signal if the sampling rate is higher than a Nyquist rate, which is defined to be equal to twice of the signal spectrum bandwidth. Otherwise, signal distortion will occur so that the original analog signal will not be recovered exactly. Hence, theoretically, an ADC with sampling rate higher than the Nyquist rate plus an ideal interpolator is sufficient to generate digital signals having any required timing accuracy without signal distortion.  
           [0010]    The usage of a simplified interpolator, which equipped with a small number of the interpolation filter coefficients, to replace the ideal interpolator, which would need a large number of the interpolation filter coefficients, can further decrease the complexity of the interpolator. An example of the simplified interpolator is a simplified linear interpolator that employs only two coefficients to compute the signal at a specified time point between those of two consecutive input signals. However, the simplified interpolator will cause signal distortion due to the small number of interpolation filter coefficients. And the system performance degrades.  
         SUMMARY OF INVENTION  
         [0011]    The invention provides a method for recovering a digital data content in a communication system and an apparatus to perform the method. At least, the timing drift problem can be effectively reduced. The points of this invention include the features as follows: (I) The timing drift problem can be dealt with a low-sampling-rate ADC and a simplified interpolator, instead a high-sampling-rate ADC and an ideal interpolator. (II) The cooperation between the channel estimator and the interpolator, whenever the coefficients of the interpolator are changed, the channel estimator should retrain itself.  
           [0012]    As embodied and broadly described herein, the invention provides a method for recovering a digital data content in a communication system. The method includes first receiving an analog signal and converting the analog signal in to a sampled digital signal stream, which is sampled based on a local clock. B1 An initialization process is performed to set up an initial condition. At the initial phase, the channel estimator starts to estimate the channel impulse response, which is a set of coefficients with raw quantities for representing a shape of the channel impulse response, and the interpolator initializes the interpolation filter coefficients. B1 The timing tracking unit calculates and outputs the interpolation point that is the time point that the interpolator should interpolated at. Based on the interpolation point, no matter it is changed or not, instructed by the timing tracking unit, the interpolator interpolates the sampled digital signal from ADC and outputs an interpolated digital signal. The interpolated digital signal is recovered into actual digital data content by the data detector with the assistant information of the coefficients of the channel impulse response. Once the interpolation point is changed, that is, the interpolation point is different from the previous determined one, the timing tracking unit generates a retraining signal to the channel estimator so that a retraining process in the channel estimator is performed to update the coefficients of the channel impulse response. Thereafter, the data detector will use the updated coefficients of the channel impulse response. In the forgoing procedure, once the interpolation point is changed, the estimate of the channel impulse response is then accordingly updated.  
           [0013]    The invention also provides an apparatus for recovering digital data content in a communication system. The apparatus is used in a receiver, in which the digital data content has been converted into an analog signal at a transmitter and transmitted over the channel. The apparatus has an analog-to-digital converter (ADC), used to receive the analog signal and convert into sampled digital signal according to a local clock at the receiver. An interpolation unit with input from the ADC is used to interpolate the sampled digital signal at desired interpolation point constructed by the timing tracking unit and outputs the interpolated digital signal. A channel estimator receives the interpolated digital signal from the interpolation unit and a retraining signal from the timing tracking unit and produce a set of coefficients as the estimate of the channel impulse response. The channel estimator starts to estimate the channel impulse response at the initial phase or whenever a retraining signal is received. A data detector receives the interpolated digital signal from the interpolation unit and an estimate of the channel impulse response from the channel estimator, so as to recover the interpolated digital signal back to the current actual digital data content. A timing tracking unit receives the detected digital data content from the data detector and an estimate of the channel impulse response from the channel estimator, to determine the interpolation point. The determined interpolation point is then sent back to the interpolator. Once the interpolation point is changed, a retraining signal is then generated and sent to the channel estimator for proceeding its retraining process to produce an updated coefficients of channel impulse response for the use of the other models in the system.  
           [0014]    It is to be understood that both the foregoing general descriptions and the following detailed descriptions are exemplary, and are intended to provide further explanation of the invention as claimed. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0015]    The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the descriptions, serve to explain the principles of the invention. In the drawings,  
         [0016]    [0016]FIG. 1 is a drawing, exemplarily illustrating the difference of clock rates at the transmitting end and at the receiving end; and  
         [0017]    [0017]FIG. 2 is a block diagram, schematically illustrating an apparatus for processing the received analog signal, so as to recover the digital data content in wireless communication, according to a preferred embodiment of the invention;  
         [0018]    [0018]FIG. 3 is a processing diagram, schematically illustrating a method for processing the received analog signal, so as to recover the digital data content in wireless communication, according to a preferred embodiment of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0019]    [0019]FIG. 2 is a block diagram, schematically illustrating an apparatus for processing the received analog signal, so as to recover the digital data content in wireless communications, according to a preferred embodiment of the invention. In FIG. 2, the present invention provides an apparatus used in the receiver to sample the analog signal at the desired time point by interpolation method to cope with the timing drift problem. The apparatus includes, for example, a RF receiver  214 , an ADC  200 , such as a low-sampling-rate ADC  200 , an interpolator  202 , such as a simplified interpolator  202 , a channel estimator  204 , a timing tracking unit  208 , a data detector  206 , and a message sink  212 .  
         [0020]    By using, for example, the low-sampling-rate ADC  200  and the simplified interpolator  202 , the implementation complexity is reduced. This is one of the essential points of the invention. Because of the coordination operation between the channel estimator  204  and the simplified interpolator  202 , the signal distortion caused by the simplified interpolator  202  can be made up and the system performance is increased. This is also one of the essential points of the invention. For example, the performance gain can be increased up to 2 dB as compared with the case of no coordination operation between the simplified interpolator  202  and the channel estimator  204  according to simulation.  
         [0021]    In details about the coupling architecture, the RF receiver  214  receives the RF analog signal at radio frequency band from a far end transmitter (not shown), reduces the central frequency of the RF analog signal from radio frequency band to base band. The ADC  200  accepts an base-band analog signal form RF receiver  214 , performs the sampling and quantization of the analog signal, and outputs the sampled digital signal to the interpolator  202 . The sampled digital signal is not yet the actual digital data content but is the digital signal in discrete-time domain to describe the received analog signal in continuous-time domain.  
         [0022]    At the transmitter, the digital data is converted to a continuous waveform based on a local clock. Clock rates between transmitting and receiving ends are usually different and the clock rate mismatch causes serious performance degradation when recovering the data. This is called timing drift problem. As shown in FIG. 1, the clock rates at the transmitting end and the receiving end are in general not precisely matched. If the sampling points of the ADC  200  do drift away from those of the clock at the transmitter, the digital signal outputted from the ADC  200  will not be able to correctly represent the received analog signal. When the difference is accumulated up to a certain quantity, it could cause errors in data recovery due to timing drift.  
         [0023]    In order to avoid the foregoing errors, an interpolator  202  is used to interpolate the sampled digital signal from the ADC  200  and output an interpolated digital signal whose signal rate consists with the clock rate at the transmitter, as shown in FIG. 1. The interpolated digital signal is a function of the sampled digital signal from the ADC  200 . A timing tracking unit  208  is used to track the clock rate at the transmitter and then instructs which interpolation point the interpolator should interpolate at. There are various designs for the timing tracking unit  208 . The process of the timing tracking may base on the digital data content from the data detector  206 , as an example shown in FIG. 2, or the interpolated digital signal from the interpolator  202 , or even an assistant pilot signal containing timing information from the transmitter. The function of the timing tracking unit  208  associating with other units will be described later in more detail.  
         [0024]    The interpolated digital signal from the interpolator  202  is then sent to the data detector  206 . The data detector  206  converts the interpolated digital signal from the interpolator into the actual digital data content, in which the data detector  206  could use the information of the estimated coefficients of the channel impulse response from the channel estimator  204 . The detecteddigital data content can feed into the message sink  212 . In general, the data detector is used to detect the data and can have many possibilities for the functional block. Similarly, the timing tracking unit has many possibilities for the functional block. The blocks shown in FIG. 2 is just one example.  
         [0025]    Usually, when the data transmission action starts, it has an initial stage. During the initial stage, the interpolation filter coefficients in the interpolator  202  are initialized. Then, the transmitter sends some specific test signals, for example, signals specified by the relative standard, that are assumed known to the receiver so that the channel estimator  204  can estimate the channel impulse response, which is a set of coefficients used to describe the shape of the channel impulse response, based on the test signals. The theories for estimating the channel impulse response are complicated but have been well developed in the art, and the detail is not described here. Therefore, during the initial stage, the output from the ADC  200  is passed through the interpolator  202  and sent to the channel estimator  204  to estimate an initial channel impulse response.  
         [0026]    The channel estimator  204  and the timing tracking unit  208  are two essential units to be cooperated together, so as to compensate for the signal distortion due to the change of the interpolation filter coefficients in the interpolator  202 .  
         [0027]    After the initial stage, the initial estimate of the channel impulse response and the initial interpolation filter coefficients are set up. Then, the data detector  206  recovers the digital data content based on the interpolated digital signal from the interpolator  202  and the estimated channel impulse response. The digital data content can be feed back to the timing tracking unit  208  to calculate the interpolation point. The timing tracking unit  208  may use the estimate of the channel impulse response from the channel estimator  204  to improve the timing tacking performance. The interpolator  202  then interpolates the sampled digital signal form the ADC  200  at the interpolation point instructed by the timing tracking unit  208 .  
         [0028]    The timing tracking unit  208  also generates a retraining signal whenever the interpolation point is changed. The channel estimator  204  usually needs not to retrain the coefficients until the retaining signal is received, in which the retaining process means the quantities of the set of coefficients to describe the channel impulse response are updated. When the interpolation point is changed, the interpolator  202  will update its interpolation filter coefficients accordingly, and the overall channel impulse response seen by the channel estimator  204  is also changed. In the invention, the channel estimator  204  is designed to accordingly perform the retraining process to update the channel impulse response once the interpolation point is changed that is indicated by the retaining signal.  
         [0029]    During stage for the channel estimator  204  to update its coefficients when the interpolation point is changed, the process of timing tracking unit  208  may be temporarily frozen. After the new updated set of coefficients of the channel impulse response is obtained, the process of the timing tracking unit is awaken. The updated set of coefficients of the channel impulse response is properly feedback to the necessary units in the system so as to increase the system performance. For example, as shown in FIG. 2, the detection error can be reduced in data detector  206  based on the re-estimated coefficients of the channel impulse response instead of the old ones.  
         [0030]    With the same design principle, the invention also proposes a method to process the received analog signal to retrieve the original digital data content. FIG. 3 shows an example of working flow of the system. In FIG. 2 and FIG. 3, during the initial stage, the analog signal from the RF receiver  214  is sampled and quantized by the ADC  200 . In the step  300 , an initialization for the interpolator  202  is performed of which the interpolation filter coefficients are initialized. Then in the step  302 , the channel estimator  204  starts the initial training process to estimate the channel impulse response and produce an initial estimate of the coefficients of the channel impulse response.  
         [0031]    In general, thereafter in the rest steps, for every sample of the analog signal, that is, the sampled digital signal, the timing tracking unit  208  is activated to perform the timing tracking process in the step  304 , and the interpolator  202  performs the interpolation in step  305 . In the step  306 , the data detector  206  outputs the detected data as the digital data content. In the step  308 , while the interpolation point instructed by the timing tracking unit  208  is unchanged, then the process goes back to the step  304 . Then the timing tracking process  304 , the interpolation process  305 , and the data detecting process  306  are continuously performed according to the incoming sampled digital signal from ADC  200 .  
         [0032]    In the step  308 , when the interpolation point instructed by the timing tracking unit  208  is changed, the interpolator  202  will adapt its interpolation filter coefficients according to the new interpolation point, as shown in the step  310 . Also the timing tracking process may be temporarily frozen, as shown in the step  312  simultaneously, so as to increase the stability of the upcoming channel estimator retraining process. After the filter coefficients of the interpolator are adapted, the retraining process of the channel estimator  204  is performed subsequently, as shown in the step  314 , to estimate the new coefficients of the channel impulse response. Meanwhile, the data detection in step  316  at the data detector  206  keeps outputting decoded data so that the data communication is still ongoing. While the retaining process  314  of the channel estimator  204  accomplishes, the timing tracking process  304  will then be activated, as shown in step  318 . Both the timing tracking process  304  and data detecting process  306  are now performed based on the updated coefficients of the channel impulse response provided by the channel estimator  204 . The steps  310 - 316  will repeat again, once the interpolation point is changed. The steps  310 - 316  are essential in the invention. It allows the estimated channel impulse response to be dynamically updated. This can significantly reduce the detection error of the data detector  206 .  
         [0033]    In conclusions, the present invention particularly introduce the method and the apparatus to convert an analog signal into the digital data content with the features that the coefficient of the channel impulse response is dynamically adapted according to the timing drift situation. Since the coefficient of the channel impulse response is dynamically updated the data detection error is greatly reduced.  
         [0034]    It will be apparent to those skilled artisans in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention covers modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.