TECHNICAL FIELD 
       [0001]    The present invention relates to impulse noise correction. 
       BACKGROUND 
       [0002]    OFDM or COFDM is a multicarrier modulation technology where the available transmission channel bandwidth is subdivided into a number of discrete channels or carriers that are overlapping and orthogonal to each other. Data are transmitted in the form of symbols that have a predetermined duration and encompass some number of carrier frequencies. The data transmitted over these OFDM symbol carriers may be encoded and modulated in amplitude and/or phase, using conventional schemes. 
         [0003]    In a mobile environment, a received signal undergoes signal degradation where the transmission channel is subject to a variety of fading conditions of the received signal such as fast and slow fading. Fast fading refers to changes in signal strength due to direct and reflected signals (multipath) interfering with each other, and slow fading refers to changes in signal strength due to distance and terrain effects. In particular, fast fading signal strength changes are due to relative motion and local scattering objects such as buildings, foliage, and change rapidly over short distances. Slow fading is the change in the local mean signal strength as larger distances are covered. In a highly random environment, fast fading will have a Gaussian distribution while slow fading will tend toward a log normal distribution. 
         [0004]    When dealing with fast fading conditions that are encountered in many communication scenarios in a mobile environment, a variation in the order of half a wavelength of the signal carrier is involved. In other words, 50 cm for a FR signal at 600 MHz. This results, in fact, from the superposition of constructive and destructive multipaths between a transmitter and a receiver. Thus, existing receivers use Automatic Gain Control (AGC) to counteract the substantial degradations in performance under fast fading conditions. AGC systems adapt the gain of the signal at the input of the receiver that is considered stable and a simple impulse noise detector can detect the impulse noise. In other words, AGC systems attempt to keep the receiver outputs constant in amplitude over most of the range and to set receiver gain to be inversely proportional to the input level. 
         [0005]    A well-known concern in the art of OFDM data transmission systems is that of impulse noise, which can produce bursts of error on transmission channels. Impulse noise or burst interference occurs at unexpected times, lasts for a short period of time (e.g., several microseconds), and corrupts all tones or bands. 
         [0006]    To correct the effect of impulse noise, prior systems use a system that detects signals samples with high level with respect to a constant signal level. Therefore, it requires that the AGC loop compensates exactly for all types of fading, including fast fading. 
         [0007]    In particular, when the speed of the mobile receiver increases or varies, AGC systems cannot alone effectively compensate for fast fading channel conditions. In fact, without an appropriate system in place to correct noise bound signals subject to fast fading conditions, the channel may suffer substantial degradation in performance due to errors in channel state estimations and impulse noise. 
         [0008]    Therefore, it is desirable to develop a new method to correct impulse noise components and improve the quality of the received signals under fading conditions. 
       SUMMARY 
       [0009]    Accordingly, it is an object of the invention to provide an improved method and system for impulse noise correction. 
         [0010]    With the following and other objects in view, the invention features detecting an impulse noise component of a data transmission signal in a mobile environment. The method, as described above, comprises the steps of: 
         [0011]    receiving over a communication channel a demodulated signal having an input signal level subject to a fading condition where the input signal level varies without the presence of the impulse noise component; 
         [0012]    estimating a variation of the input signal level independently of the impulse noise component under the fading condition to obtain a robust signal level estimate of the signal; and 
         [0013]    detecting the impulse noise component based on the robust signal level estimate and the input signal level. 
         [0014]    The method also provides for reducing the impulse noise component by cancelling a signal component of the received signal whose impulse noise component has been detected, as recited in claim  2 . 
         [0015]    In the above, the method deals more efficiently with fast fading conditions and also estimates the input signal level over a time interval (I) having a length adapted to provide accurate estimation of the variation of the signal level and a constant level of the signal. Therefore, the impulse noise correction significantly improves the quality of received signals. 
         [0016]    Furthermore, the method features as defined in claim  5  improve the detection of the impulse noise component. 
         [0017]    In addition, the invention concerns a communication system to detect an impulse noise component for a data transmission signal according to the above method, and other features of the communication system are recited in the dependent claims. 
         [0018]    As recited in claim  11 , the invention also features an article (e.g., a chip) including a computer-readable storage medium bearing computer-readable program code capable of causing a processor to:
       receive over a communication channel a demodulated signal having an input signal level subject to a fading condition in a mobile environment where the input signal level varies without the presence of the impulse noise component;   estimate a variation of the input signal level independently of the impulse noise component under the fading condition obtain a robust signal level estimate of the signal; and   detect the impulse noise component based on the robust signal level estimate and the input signal level.       
 
         [0022]    Other features of the article are further recited in the dependent claims. 
         [0023]    These and other aspects of the impulse noise correction method will be apparent from the following description, drawings, and from the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0024]      FIG. 1  is a schematic diagram of a receiving unit according to the present invention; 
           [0025]      FIG. 2  is a schematic diagram of a noise detection unit of the receiving unit of  FIG. 1 ; 
           [0026]      FIG. 3  is a flow chart of a method to correct an impulse noise component; 
           [0027]      FIG. 4  is a schematic diagram of another noise detection unit of the receiving unit of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0028]    Referring to  FIG. 1 , a communication system  2  includes a transmitter  4  and a receiving unit  6 . The transmitter  4  transmits a modulated wave  8  to an antenna  10  associated with the receiving unit  6 . The modulated wave  8  is converted by the receiving antenna  10  into a Radio Frequency (RF) signal processed in the receiving unit  6 . The receiving unit  6  includes a receiver  12 , a noise detection unit  14 , a noise reduction unit  16 , and a signal processing unit  18 . 
         [0029]    The modulated wave  8  is directed to the receiving unit  6  where it is initially processed by the receiver  12 . The receiver  12  may include conventional signal processing systems such as a tuner, an amplifier, and the like. The modulated wave  8  is also A-D converted in the receiver  12 . The receiver  12  outputs a pre-processed signal  20 , defined as x(t), that is subsequently subject to further processing in the noise detection unit  14 . The noise detection unit  14  carries out the detection of the impulse noises by obtaining a meaningful impulse noise value as distinguished from the signal values. This mechanism is described in greater detail in  FIG. 2 . In the noise detection unit  14 , the impulse noises are detected from the pre-processed signal  20  x(t), which outputs in addition to the pre-processed signal  20 , a noise reduction control signal  22 . These signals are, in turn input into a noise reduction unit  16 , which reduces or eliminates the impulse noise component from the pre-processed signal  20  x(t). This is achieved by cancelling a signal component of the received signal whose impulse noise component has been detected, thus outputting a noise free signal  24 . The noise free signal  24  is then sent onto the signal processing unit  18  for higher level signal processing. 
         [0030]    Referring now to  FIG. 2 , the noise detection unit  14  receives the pre-processed signal  20  x(t) from the receiver  12 . The noise detection unit  14  includes a signal sampling unit  30 , a robust level estimate circuit  32 , and a noise detection circuit  34 . After the pre-processed signal  20  x(t) is sampled by the signal sampling unit  30 , a sampled signal  36  is input onto the robust level estimate circuit  32 . 
         [0031]    In particular, the robust level estimate circuit  32  is a circuit adapted to withstand insensitivity against deviations, i.e., conditions departing from an assumed distribution or model outside of normal specifications. Thus, the robust level estimate circuit  32  estimates a variation of the level of the sampled signal  36 , for example, in small time intervals (I) referred to as x(t). In this case, if we represent the sampled signal  36 , P(I) represents the square root of the mean of the level of the sample signal  36 , namely |x(t)| 2 . Furthermore, the length of the interval (I) is sufficiently large to have the most accurate estimation, but sufficiently small to also ensure that the level of |x(t)| 2  remains constant over the time interval (I). In the robust level estimate circuit  32 , the calculation for the estimation must be robust against the impulse noise component of the signal  20  x(t). This means that the estimate must not be significantly affected when sampled signals, x(t), are corrupted by impulse noise component. Different techniques may be applied to make the estimation robust, such as removing high values over a given threshold from the computation of the estimate or to make a simple rough estimate of the impulse noise position and to remove these points from the computation of the sampled signal  36 . 
         [0032]    Therefore, the robust level estimate circuit  32  produces an estimate of the variation of the pre-processed signal  20  level independently from the impulse noise component under a fast fading condition. This results in a robust signal level estimate for the signal  20  x(t), namely P(I). Thereafter, the noise detection circuit  34  detects the impulse noise component based on the robust signal level estimate P(I) and the signal  20  x(t), and outputs the noise reduction control signal  22  defined as D(t) that is sent to the noise reduction unit  16  for further processing. Moreover, as noted, the signal  20  x(t) is also output directly to the noise reduction unit  16  as shown in a line  26 , so that the impulse noise component can be cancelled and the noise free signal  24  can be processed 
         [0033]    The framework of the detection algorithm used in connection with  FIGS. 1 and 2  above includes defining a detection function D(t) as the probability of an impulse noise component in the signal  20  x(t) at a time t. The detection function D(t) may be determined by comparing the signal  20  x(t) to a threshold value such that if the signal  20  x(t) is greater or lesser than a given threshold A, for instance, then the detection function D(t) will indicate that the signal energy of the signal  20  x(t) is considered to have the presence of an impulse noise component. In other words, if |x(t)|&gt;A, then D(t)=1, and if otherwise, D(t)=0. 
         [0034]    Referring back to  FIG. 2 , if the robust signal level estimate P(I) generated by the robust level estimate circuit  32  is now taken in account to determine the noise reduction control signal  22 , then the above described algorithm is further refined and adapted. If the |x(t)|&gt;A·P(I), i.e., the adapted threshold, then the detection function D(t)=1, and if |x(t)| is otherwise, D(t)=0. This can also be written as D(t)=1 if |x(t)|/P(I)&gt;A and if otherwise, D(t)=0. As a result, |x(t)| is normalized using P(I). 
         [0035]    Referring now to  FIG. 3 , a method  40  for correcting impulse noise is illustrated. In the method  40 , a signal time interval is used to estimate the level of the signal during a particular time interval in a step  42 . As a result, a level of the signal, x(t) is generated. Next, using the generated signal, x(t) as the input, the robust signal level estimate is calculated in a step  44 . The resulting output is the robust level of the signal. This is, in turn, used to detect an impulse noise component in a step  46 . Here, the detection algorithm is used a detection function defined as a probability of the presence of impulse noise component in the signal as a function of time. Consequently, the output of the detection step  46  generates an impulse noise detection value. 
         [0036]    If the impulse noise detection valued has been detected (step  48 ), then the impulse noise component is removed in an impulse noise removing step  50 . Thereafter, the method  40  continues by inputting a next signal time interval to estimate the level of the signal (step  42 ). On the other hand, if the impulse noise detection value has not been detected (step  52 ), then the method  40  directly proceeds to the step  42 . 
         [0037]    Many additional embodiments are possible. For example, referring to  FIG. 4 , another noise detection unit  70  analogous to the noise detection unit  14  of  FIG. 2  is shown. In this noise detection unit  70 , a noise detection circuit  72  detects the impulse noise component based on the signal  74  x(t) and a threshold value  76  generated by a noise reduction unit  78 . The noise reduction unit  78  generates the noise free signal  80 , defining an impulse noise component in the signal  74  x(t), namely, D(t). In other words, the threshold value  76  is used to compare the signal  74  x(t) to the noise free signal values generated by the noise reduction unit  78  so that the detection of an impulse noise component can be done more accurately with this feedback mechanism. As a result, the noise detection unit  70  can further refine the detection of impulse noise components of signals in a mobile environment. 
         [0038]    In addition, the method and systems described above have been described using a particular detection algorithm, but other detection functions are possible.
A method of detecting an impulse noise component for a data transmission signal in a mobile environment includes receiving over a communication channel a demodulated signal having an input signal level subject to a fading condition where the input signal level varies without the presence of the impulse noise component; estimating a variation of the input signal level independently of the impulse noise component under the fading condition to obtain a robust signal level estimate of the signal; and detecting the impulse noise component based on the robust signal level estimate and the input signal level. The method also includes reducing the impulse noise component by cancelling a signal component of the received signal whose impulse noise component has been detected.