Abstract:
A method and apparatus for adaptively removing interference from a signal. In one embodiment, the invention comprises an infinite impulse response (IIR) notch filter configured to receive a wideband signal and provide a filtered output signal, and a controller coupled to the notch filter to adaptively control the null frequency of the notch filter thereby removing narrowband interference from the received wideband signal. The controller may employ a gradient-based algorithm to detect the highest power frequency band in the output signal and modify the null frequency of the notch filter to minimize the power of the output signal.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This non-provisional United States (U.S.) patent application claims the benefit of U.S. Provisional Application No. 60/290,378, filed by inventor Mehdi T. Kilani on May 11, 2001, titled “Narrowband Interference Canceller For Wideband Communication Systems”. 

   FIELD 
   This invention relates generally to communication methods, systems, and devices. More particularly, the invention relates to a filter for adaptively canceling interference in communication systems. 
   BACKGROUND 
   Receiving devices or systems in a communication system may receive signals or waveforms which are distorted by interference or noise. Some of the signal distortion may be caused by narrowband interference. Narrowband interference may be interfering noise or signals which are centered about particular frequencies. 
   Despite such interference, a receiving device must be able to detect a signal and determine its content. For example, the performance of spread spectrum communication systems such as Code Division Multiple Access (CDMA) and Orthogonal Frequency Division Multiplexing (OFDM) signals are severely degraded in the presence of strong sinusoidal interference. 
   Receiving devices use various methods, schemes, and/or systems to remove such interference. One such method employs filters, such as a notch filter, to filter out interference from a received signal. 
   Notch filters remove signal energy contained in a narrow band of frequencies. A fixed frequency notch filter removes a particular narrow band of frequencies. The narrow band of frequencies removed is determined by the value of the circuit elements employed in the filter. 
   However, in many applications, the particular frequency of the interference is unknown and, consequently, a fixed frequency notch filter is ineffective. Therefore, a filter which adapts itself to filter out interference of unknown or varying frequency is sought. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a z-domain block diagram of a direct form II notch filter structure. 
       FIG. 2  is a z-domain block diagram illustrating one structure of an adaptive notch filter as may be employed in one embodiment of the invention. 
       FIG. 3  is a system-level diagram illustrating the structure of an adaptive algorithm as may be employed in one embodiment of the invention. 
   

   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 will be obvious to one skilled in the art that the invention may be practiced without these specific details. In other instances well known methods, algorithms, procedures, components, and circuits have not been described in detail so as not to unnecessarily obscure aspects of the invention. 
   One aspect of the invention provides an adaptive notch filter to filter out interference in a signal. An adaptive structure is provided for eliminating a narrowband interference signal from the received signal prior to detection, thus improving the performance of the communication system significantly. The adaptive notch filter employs its output signal to generate a feedback error signal which serves to center the null frequency (filtered frequency band) of the notch filter on the narrowband interfering frequency. 
   Another aspect of the invention provides a novel notch filter structure with parameters that serve as absorbing scaling factors to reduce the dynamic range of the signals at the various nodes of the notch filter and provide stability to the filter. 
   An adaptive notch filter has an electrical control input to vary the null or center frequency (the narrow band of frequencies to be removed) of the notch filter. 
   According to one implementation, the invention utilizes a constrained Infinite Impulse Response (IIR) notch filter together with an adaptation algorithm in a novel structure. The adaptation algorithm may be a recursive algorithm for updating the time varying coefficient(s) of the notch filter. 
     FIG. 1  illustrates the z-domain block diagram of a direct form II notch filter structure  102 . The filter  102  receives signal X[n] and provides a filtered signal Y[n]. 
     FIG. 2  illustrates the z-domain block diagram of one specific embodiment of the adaptive notch filter of the invention. The filter  202  receives as inputs signals x[n] and h[n] and provides a filtered output signal y[n]. Signal x[n] includes an information or data signal s[n] and an interference signal I[n]. The null or center frequency of the filter  202  is turned by the adaptation signal or input parameter h[n] which itself is updated by an adaptive mechanism described below. 
   The filter transfer function is: 
         H   ⁡     (   z   )       =     a   ⁢       1   +       k   1     ⁢     k   2     ⁢     h   ⁡     [   n   ]       ⁢     k   5     ⁢     z     -   1         +     z     -   2           1   -       ak   1     ⁢     k   2     ⁢     k   3     ⁢     h   ⁡     [   n   ]       ⁢     k   5     ⁢     z     -   1         -       ak   4     ⁢     z     -   2                   
 
   In order to provide the required stability without degrading the frequency response behavior of the notch filter  202 , the coefficient “a” is introduced in the structure and is set to a value very close to unity. When implementing the system in integer arithmetic with finite word length, special care must be taken to reduce the dynamic range of the signals at various nodes of the filter. To do so, the parameters k 1 , k 2 , k 3 , k 4 , and k 5  have been introduced in the system structure. These parameters serve as absorbing scaling factors to reduce the dynamic range of the signals at the various nodes. In one implementation, these parameters have values in the range of: 
         -   1     ≤         k   1     2     ⁢     k   2     ⁢     k   5     ⁢     h   ⁡     [   n   ]         ≤   1       
       0   ⁢     &lt;&lt;   a&lt;1         
         0   &lt;     k   3       ,       k   4     &lt;   1         
 
   According to one embodiment, these parameters are set to integer power of 2 −1  to simplify the notch filter implementation. 
   The value of h[n] is updated using gradient-based algorithms such as Recursive Prediction Error and Pseudolinear Regression algorithms. The signals w[n- 1 ], w[n- 2 ], and h[n] (shown in  FIG. 2 ) are scaled down by appropriate factors so that when the system is implemented in integer arithmetic, the dynamic range associated with each node always remains within the desired range. This prevents the structure from overflow non-linearity effects. As mentioned earlier, according to one implementation, all the coefficients ‘a’, k 1 , k 2 , k 3 , k 4 , and k 5  are designed in the form of integer power of two to enable a computationally simple hardware structure. 
   The null or center frequency of the notch filter is allowed to vary adaptively using gradient-based algorithms such as Recursive Prediction Error (RPE) and Pseudolinear Regression (PLR). The adaptation is based on the minimization of the error signal which is the output signal y[n] of the notch filter  202 . Hence, no reference signal is required. Once convergence takes place, the null or centre frequency of the notch filter is placed at the frequency corresponding to the unwanted narrowband interfering signal thus effectively eliminating the interference I[n]. The IIR filter structure is specially parameterized so that it remains stable within the whole input frequency range. This stability is achieved by selecting the value of coefficient ‘a’ to be less than but close to unity. In conventional structures, the parameter ‘a’ is set equal to one (1) which makes the system prone to instability at frequencies very close to half the sampling frequency. 
   An advantage of the proposed structure is that no prior information regarding the frequency location of the interfering signal is required. That is, the adaptation signal h[n], which serves to configure or set the null frequency of the notch filter, is based on the output signal y[n]. 
     FIG. 3  illustrates a block diagram of the structure  302  of an adaptive algorithm that may be employed in one embodiment of the invention. The adaptive algorithm is gradient based and its task is to minimize the power of the signal at the output of the notch filter. Once the minimization has been accomplished, the null frequency of the notch filter is found at the frequency of the narrowband signal. 
   The error surface associated with the system is proven to be unimodal produced that the desired wideband signal is corrupted with single narrowband interference. That is, the signal is corrupted by a single dominant interference narrowband. This means that the convergence of gradient-based algorithm is guaranteed and also the minimum of the error surface corresponds to the interfering frequency line with a negligible bias. 
   The error signal and its derivative with respect to h[n] are given by:
 
For RPE algorithm: 
             ∂   e       ∂   h       =     w   ⁡     [     n   -   2     ]         ⁢               
               ⁢         e   ⁡     [   n   ]       =     y   ⁡     [     n   -   1     ]         ;         
 
For PLR algorithm: 
             ∂   e       ∂   h       =     x   ⁡     [     n   -   2     ]         ;       
               ⁢         e   ⁡     [   n   ]       =     y   ⁡     [     n   -   1     ]         ;         
 
   The signals are delayed by one extra sample to reduce the number of operations per cycle. The step size factor is absorbed inside the scaling coefficients m 1 , m 2  and m 3 . In one implementation, these parameters are designed to be integer power of 2 −1  to simplify the implementation further and to reduce the dynamic range of the signals at various nodes. The required frequency resolution, interference to signal ratio improvement factor depend on the word length of the adaptive coefficient h[n]. The bandwidth is controlled by the parameters k 3  and k 5 . The proposed structure is able to place the null frequency of the notch filter within a frequency band ranging from 0.004 (normalized frequency) to half the sampling frequency. 
   It must be understood that the adaptive notch filter described herein is not limited to any particular communication protocol or scheme and may be implemented on both wired and wireless communication, transmission, and/or reception systems. 
   Moreover, the adaptive notch filter is not limited to any particular frequency or range of frequencies. The proposed structure shown in  FIG. 2  is capable of removing single narrowband interference, however multiple filters embodying the adaptive filter describe herein may be configure in series to remove more than one interference signal. 
   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. 
   Additionally, various aspects of the invention are novel over the prior art. For instance, no prior information regarding the interfering frequency is required with the adaptive notch filter described. Moreover, the parametization described makes the structure highly stable with minimized dynamic range for signals at various filter nodes. Also no reference signal is necessary for the adaptive structure described, whereas conventional interference cancellers based on the finite impulse response (FIR) filter structure require a reference signal to be available for effective cancellation. 
   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. The invention is not limited to the specific constructions and arrangements shown and described since various other modifications may occur to those of ordinary skill in the art. Additionally, it is possible to implement the present invention or some of its features in hardware, programmable devices, firmwave, integrated circuits, software or a combination thereof where the software is provided in a processor readable storage medium such as a magnetic, optical, or semiconductor storage medium.