Abstract:
A blanking scheme for mitigating impulsive noise in wireless networks is based on the signal-to-noise ratio (SNR) of symbols. To fully gain the benefits of the SNR-based blanking scheme, two methods are developed, namely a multi-level thresholding scheme in the time-, spatial- and frequency-domains, and a weighted-input error-correction decoding. The symbols are conditioned as a function of the estimated SNR in time-, frequency-, or spatial-domains or combinations therefore, and the conditioning is applied to an amplitude, phase, or energy level, or combinations thereof.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates in general to wireless communication systems, and in particular to methods and systems for mitigation of impulsive interference to achieve reliable wireless communication in Orthogonal Frequency Division Multiplexing based wireless systems. 
       BACKGROUND OF THE INVENTION 
       [0002]    Wireless networks, and particular wireless networks in industrial environments are susceptible to impulsive noise generated by electric equipment. This impulsive equipment noise is commonly characterized by a short duration and high power spike when compared to a desired signal. Thus, the impulsive noise incurs a sudden decrease in instantaneous SNR, which may lead to data packet loss and subsequently poor network performance. Despite the fact that data packet loss can be alleviated by retransmissions, such retransmissions induce delays, which is rather undesirable for wireless industrial networks with stringent delay constraints. 
         [0003]    Furthermore, the high-power impulsive noise is particularly detrimental to orthogonal frequency division modulation (OFDM)-based wireless industrial networks. An OFDM block comprises multiple symbols and the whole block has to be jointly demodulated in the receiver to recover the transmitted symbols. As a result, even if short-duration impulsive noise is added to a few symbols, the high-power impulsive noise will be propagated over the entire block after joint demodulation, thereby the entire block, rather than only a few symbols, has to be retransmitted. 
         [0004]      FIG. 1  shows the schematic diagram of a conventional OFDM system including a transmitter  110  and a receiver  120 . In the transmitter, a signal z(m) is encoded  111 , interleaved  112 , mapped  113 , inverse discrete Fourier transformed  114 , parallel-to-serial converted  115 , analog-to-digital converted  116 , and transmitted on a channel  117  subject to noise n(t). 
         [0005]    In the receiver, the received signal r(t) is analog-to-digital converted  121 , and blanked  122 . Then, the signal is serial-to-parallel converted  123 , discrete Fourier transformed  124 , de-mapped  125 , de-interleaved  126 , and decoded  127  to recover {circumflex over (z)}(m). 
         [0006]    In the prior art, a noise blanker protects a signal processing circuit from unwanted noise spikes by interrupting the signal path when the noise exceeds a predetermined threshold or reference level, see U.S. Pat. No. 4,479,251 “Noise blanker.” 
         [0007]    To cope with the impulsive noise, the blanking  122  is applied. The conventional blanking is characterized by single threshold based on only the amplitude of the received signal y(n) as shown in  FIG. 2 . In addition, the conventional blanking can only decrease the amplitude to zero. 
         [0008]    As shown in  FIG. 2 , the amplitude of each sample of data symbol y(n) is first compared with a pre-defined threshold T 0 . If the amplitude is larger than the threshold, then the prior art method blanks the corresponding n th  sample of the data symbol y(n) by setting the signal to zero and no other value. Otherwise, the blanker will simply output the sample without change. 
       SUMMARY OF THE INVENTION 
       [0009]    The embodiments of the invention provide a method for mitigating impulsive noise in wireless networks. The method noise mitigation is based on the signal-to-noise ratio (SNR). In contrast to the conventional blanking scheme, wherein the signal amplitude is used for blanking decision, the blanking scheme according to the invention makes a blanking decision by using the estimated SNR of each received symbol. 
         [0010]    The invention is motivated by the fact that the signal SNR is a more accurate quality metric than the signal amplitude. To fully gain the benefits of the SNR-based blanking scheme, two embodiments are developed, namely a multi-level thresholding scheme, and a weighted-input error-correction decoding. 
         [0011]    It should be emphasized that this blanking scheme is applicable to any wireless networks, regardless of their modulation schemes, e.g., single-carrier or multi-carrier. 
         [0012]    It should also be noted that the weighted-input error correction decoding and the blanking method can be used independently. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a block diagram of a prior art OFDM system having a receiver equipped with a prior art blanker; 
           [0014]      FIG. 2  is a schematic of a prior art amplitude-based single threshold for the blanker of  FIG. 1 ; 
           [0015]      FIGS. 3A-3B  are block diagrams of a receiver with a blanker according to embodiments of the invention; 
           [0016]      FIG. 4  is a schematic of an operation of the blanker with a multi-level threshold according to embodiments of the invention; 
           [0017]      FIG. 5  is a schematic of the input-output relation of a conditioner of  FIG. 4  with a multi-level threshold function according to embodiments of the invention. 
           [0018]      FIG. 6  is a schematic of the input-output relation of the conditioner of  FIG. 4  with two-level threshold function according to embodiments of the invention; 
           [0019]      FIG. 7  is a graph of weighting coefficients as a function of SNR used by the blanker according to embodiments of the invention; 
           [0020]      FIG. 8  is a schematic of a time-domain blanking process according to embodiments of the invention; 
           [0021]      FIG. 9  is a schematic of a frequency-domain SNR-based weighted output process according to embodiments of the invention; 
           [0022]      FIG. 10  is a schematic of a blanking process with multiple receive antennas according to embodiments of the invention; 
           [0023]      FIG. 11  is a schematic of a process including time-domain blanking, frequency-domain weighting and multiple antennas blanking/combining according to embodiments of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0024]      FIGS. 3A-3B  shows a receiver  330  according to embodiments of our invention. A received signal r(t) is analog-to-digital converted  331 , and blanked  310 . Then, the signal is serial-to-parallel converted, discrete Fourier transformed  333 , de-mapped  334 , de-interleaved  335 , and decoded  337  to recover a signal estimate {circumflex over (z)}(m). 
         [0025]    More specifically, the receiver first samples the received signal r(t) in the discrete time-domain before feeding the digitized data to the SNR-based blanker  310 . 
         [0026]    The detailed functional structure and operation of the SNR-based blanker  310  is shown in  FIG. 4 . For each received symbol  401  fed into the blanker  310 , our blanker  310  first estimates  410  the SNR. 
         [0027]    Next, the estimated SNR is compared  420 ,  421  and  422  against a first-level threshold T 1 , a second-level threshold T 2  and up to the N-th threshold TN, wherein T 1 &lt;T 2 &lt; . . . &lt;T N  as shown for a threshold function in  FIG. 5 , i.e., an increasing order. 
         [0028]    It should be emphasized that the input-output relation shown in  FIG. 5  can be both piece-wise linear or piece-wise non-linear. That is, our blanker uses an SNR-based blanker with multiple levels. The threshold function can be discrete or continuous. 
         [0029]    If the estimated SNR is less than T 1 , then according to the invention the sample value for the current symbol is conditioned  430  by setting it to zero. Otherwise, our method proceeds to compare  421  the estimated SNR with the second threshold T 2 . If the estimated SNR is less than the threshold T 2 , our method conditions the current symbol. As described below, the conditioning can change the amplitude, phase or energy of the received data symbol. Otherwise, our method proceeds to compare the estimated SNR with the threshold of the next level until either the estimated SNR is smaller than a threshold, or the N-th level threshold is reached. If the estimated SNR is larger than T N , then the blanker outputs  431  the current sample without change. 
         [0030]      FIG. 6  shows a particular example of  FIG. 5  with a two levels threshold function, wherein the second level is larger than the first level. 
         [0031]    In addition to the multi-threshold blanking, the SNR-based blanker can optionally also generates  450  a weighting coefficient w  511  for each data symbol. The weighting coefficients are designed to quantify a reliability of received data symbols. 
         [0032]    For example, the log likelihood of i-th bit S i  can be approximated as 
         [0000]    
       
         
           
             
               
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         [0000]    where μ denotes the mean of the input when S i =v and σ l   2  is the noise energy. Conventionally, it is assumed that σ l   2 =σ 2  is a constant. However, in the presence of impulsive noise, σ l   2  is time variant. 
         [0033]    Thus, we can model σ l   2 =σ T   2 +σ I   2 , where σ T   2  is the constant Gaussian noise level and σ I   2  is the time-varying energy of the impulse noise. 
         [0034]    The likelihood function can be expressed as 
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         [0000]    where W i  is a weighting coefficient for the i-th data symbol. The value of w i  is computed based on the estimated noise level at the i-th data sample. Generally, the weight assignment function is designed such that the weight decreases as the total noise level increases. 
         [0035]      FIG. 7  shows an example of the weight assignment function given by 
         [0000]    
       
         
           
             
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         [0000]    where k is a constant much greater than one. For OFDM system, the W i  is estimated for the entire OFDM symbol in which the bit S i  belongs. 
         [0036]    As shown in  FIG. 3A , the blanker can output both the processed data samples and the weighting coefficients  511 . The processed data samples are then transformed  333  into the frequency-domain via the Discrete Fourier Transform (DFT) operation. 
         [0037]    After that, the DFT output is first de-mapped  334  to P(n), and de-interleaved  335  to Q(n). The corresponding weighting coefficients w(n)  511  obtained from the blanker is also de-interleaved  335  into {tilde over (w)}(n). Finally, the weight {tilde over (w)}(n) is then fed into the error correction decoder  337  and used to generate cost metrics for the decoder. 
         [0038]    A decoder  337  example using Q(n) and {tilde over (w)}(n) is shown in  FIG. 3B  in which the de-interleaved weighting coefficients {tilde over (w)}(n) are multiplied  326  with Q(n). The resulting samples {tilde over (Q)}(n)=Q(n)×{tilde over (w)}(n) are then fed into a Viterbi decoder  328 . 
         [0039]    It is worth noting that the SNR estimation can be implemented using different methods. In one implementation, the noise level can be estimated by counting the total number of time-domain samples with energy exceeding a pre-determined threshold. 
         [0040]    In another implementation, the blanker can estimate the noise power from the total symbol energy, when if the network employs constant-energy symbols. 
         [0041]    For OFDM networks, the blanker can also determine the noise level based on the energy within null subcarriers over which no signals are transmitted. 
         [0042]    Spatial-Domain Noise Reduction 
         [0043]    If the receiver is equipped with multiple antennas, the blanking can be applied to the received signal from each antenna as shown in  FIG. 10 . 
         [0044]    Here, the SNR  1011  is estimated  1010  for the input symbol  1001  from each antenna. The factors γ  1021  generated  1020  using the functions  1002 . The symbols are conditioned  1030 , and the combined  1009 , before being passed to the DFT  1035 . 
         [0045]    Time-Domain Noise Reduction 
         [0046]      FIG. 8  shows the essential steps for time-domain noise reduction. The invention first uses the input symbol  801  to estimate  810  the SNR  811  and subsequently generate  820  the weighting coefficient α  821  according to the multi-level threshold function  802 . After that, the symbol is conditioned  830  by α  821  before output  809  to the DFT  835 . 
         [0047]    Frequency-Domain Noise Reduction 
         [0048]      FIG. 9  shows the essential steps for frequency-domain noise reduction. The invention first converts each time-domain input symbol  901  into the frequency-domain via the DFT  835 . After that, the frequency-domain symbol is used to generate  920  the weighting coefficient w  921  based on estimated  910  SNR  911  and a multi-level threshold function  902 . Finally, the frequency-domain symbol is conditioned  930  by w before the output symbol  909  is decoded. 
         [0049]    Combined Time and Frequency-Domain Noise Reduction 
         [0050]      FIG. 11  shows a receiver wherein the time, spatial and frequency-domain noise reduction are used in combination. If there is only one receiver antenna, then we can respectively collapse either the time or frequency-domain noise reduction by setting w=1 or α=1. 
         [0051]    Conditioning 
         [0052]    It should be noted that the conditioning can be applied to any aspect of the data symbol signal, e.g., the amplitude, phase, or energy, or any combination therefore. The conditioning can either increase or decrease the phase or energy, or shift the phase. 
       EFFECT OF THE INVENTION 
       [0053]    The invention improves the network bit-error-rate (BER) after error correction and decoding in an OFDM network, when impulsive noise is present. 
         [0054]    Although the invention has been described by way of examples of preferred embodiments, it is to be understood that various other adaptations and modifications may be made within the spirit and scope of the invention. Therefore, it is the object of the appended claims to cover all such variations and modifications as come within the true spirit and scope of the invention.