Abstract:
Impulsive noise is detected for each discrete multi-tone (DMT) symbol. If impulsive noise is detected, all bytes, which belong to the associated DMT symbol are tagged by “erasure bits”. After interleaving, Reed-Solomon decoding is initially performed without erasures. If the decoding fails, it is performed again, this time with erasures. Reed-Solomon decoders report failure with relatively high certainty, and thus, if the first stage (decoding without erasures) includes failure or errors due to impulsive noise, the second stage of decoding is performed again with erasures.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is directed to a multi-tone receiver and a system for operating the same. 
     2. Description of Related Art 
     The communication channel is the set of a physical medium, device and system that connects the transmitter to the receiver. The transmitter and receiver include an encoder and decoder, respectively, for translating the information stream produced by the source into a signal suitable for channel transmission, and vice versa. Some communications channels are impaired by impulsive noise. One way to maintain the relatively high integrity of the channels is to use an error correcting code, which restores the original data when corrupted by impulsive noise. A conventional encoding method for impulsive noise in a channel and used for asymmetrical digital subscriber line (ADSL) is described in the ITU G.992.1 standard, which is incorporated herein by reference. This particular scheme uses a Reed-Solomon encoder followed by a byte-interleaver. An ADSL decoder typically comprises a byte-deinterleaver, which distributes the bytes hit by impulsive noise between multiple Reed-Solomon code words, and a conventional Reed-Solomon decoder, which corrects the errors in each of the code words. 
     Correction capabilities or results of a Reed-Solomon decoder may be improved by using erasures to indicate when the reliability of the input bytes are corrupted. Applying this knowledge to a multi-tone system operating in an environment of impulsive noise, impulsive noise may be detected by measuring the accumulative error over one or more tones of a multi-tone symbol, since multiple tones will be impaired by the impulsive noise. Bytes of a symbol suspected to be corrupted or hit by impulsive noise may then be marked as erasures. 
     The use of erasures, however, is disadvantageous when no impulsive noise is present, since false alarm impulsive noise indicators may reduce the correction capabilities of the Reed-Solomon decoder. In addition, the detection threshold level is very limited. Specifically, on the one hand, a relatively high detection sensitivity may produce false alarms and reduce the performance when little, if any, impulsive noise is present; whereas, on the other hand, a relatively low detection sensitivity may result in missed detection of impulsive noise, thereby reducing the correction capabilities when impulsive noise is present. 
     It is therefore desirable to develop a system and method for use on a multi-tone symbol that exploits the advantages of using erasures when impulsive noise is present, without impairing the correction performance if no impulsive noise is present. 
     SUMMARY OF THE INVENTION 
     Impulsive noise is estimated for each discrete multi-tone (DMT) symbol. If impulsive noise is detected, all bytes, within the associated DMT symbol are tagged by “erasure bits”. After interleaving, Reed-Solomon decoding is initially performed without erasures. If the decoding fails, it is performed again, this time with erasures. Reed-Solomon decoders report failure with relatively high certainty. Therefore, if the first stage (decoding without erasures) fails to decode into a proper codeword and impulsive noise is present, the second stage of decoding is performed again with erasures. 
     In a first embodiment, the multi-tone receiver in accordance with the present invention includes a decoder operable in one of two modes, a first mode without erasures for producing a first decoded data block and a second mode with erasures for producing a second decoded data block. The decoder generates a decoding-failure indicator when a decoding failure is detected. In addition, the receiver includes a controller which initiates the decoder to receive an input block of data; activates the decoder to operate in the first mode or the second mode based on the decoding-failure indicator reported by the decoder; and selects as an output from the decoder the first or second decoded data block output based on the decoding-failure indicator reported by the decoder. 
     The invention is also directed to a method for operating the multi-tone receiver described above. Initially, an input data block is decoded without erasures to produce a first decoded data block. A determination in then made whether a decoding-failure indicator is generated by the decoder. If a decoding-failure indicator is generated, the input data block is decoded with erasures to produce a second decoded data block. The decoded data block output from the decoder is selected between the first and second decoded data blocks based on a decoding-failure indicator. Alternatively, the input data block may be first decoded with erasures and then, decoded without erasures based on the decoding-failure indicator. 
     In a second embodiment, instead of using one decoder operable in two modes, the receiver may be designed with two parallel decoders, processing the same input bytes, simultaneously. The multi-tone receiver includes a first decoder for decoding without erasures a sample block to produce a first decoded data block, a second decoder for decoding with erasures the sample block to produce a second decoded data block, means for generating a decoding-failure indicator when a decoding failure is detected by one of said first and second decoders, and means for selecting between the first and second decoded data block based on the presence of a decoding-failure indicator. 
     The invention also relates to the method for operating the multi-tone receiver configured in accordance with the second embodiment of the invention. Initially, an input data block is decoded without erasures using a first decoder to produce a first decoded data block, and is decoded with erasures using the second decoder to produce a second decoded data block. A decoding-failure indicator is generated when a decoding failure is detected by the first or second decoders and a selection is made between the first and second decoded data block based on the presence of a decoding-failure indicator. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other features of the present invention will be more readily apparent from the following detailed description and drawings of illustrative embodiments of the invention wherein like reference numbers refer to similar elements throughout the several views and in which: 
     FIG. 1 is a multi-tone receiver in accordance with the present invention; and 
     FIG. 2 a  is a flow chart of an embodiment of a method for operating the controller in the mutli-tone ADSL DMT receiver of FIG. 1 in accordance with the present invention; 
     FIG. 2 b  is a flow chart of another embodiment of a method for operating the controller in the mutli-tone ADSL DMT receiver of FIG. 1 in accordance with the present invention; and 
     FIG. 3 is another embodiment of a multi-tone receiver in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is an asymmetric digital subscriber line discrete multi-tone (ADSL DMT) receiver  100  in accordance with the present invention. Front end receiver  105  includes analog filters, an analog to digital converter (ADC), and digital filtering (not shown), as is well known in the field. For each period of a discrete multi-tone (DMT) symbol, a block of samples is grouped into a vector a i  using, for example, a windowing technique known in the field of DMT modulation, and then transformed by serial to parallel converter  110  and fed to a Fast Fourier Transform (FFT)  115 , wherein the vector is converted to a vector A i . The FFT  115  also preferably includes a frequency domain equalizer (FEQ)(not shown) to equalize the channel&#39;s phase and amplitude distortion for each tone. 
     A constellation decoder  120  is used to generate erasure bits. In particular, constellation decoder  120  maps each element in the vector A i  to a constellation point CONST i , where i represents a tone of the multi-tone symbol. In addition, the constellation decoder  120  also calculates a distance vector D i  representing the Euclidean distance between the vector A i  and its associated constellation point CONST i  as 
     
       
           D   i   2 =( A   i −CONST i ) 2   
       
     
     Each constellation point is mapped by the constellation decoder  120  to 1-15 bits, and a bit stream is produced by concatenating bits from all tones. This bit stream is then converted by the constellation decoder  120  to a data byte stream B by grouping eight consecutive bits into a byte, using operations and techniques well known to one of ordinary skill in the technology of ADSL modulation. A distance vector Di, representing a detector-error, of two or more tones of the multi-tone symbol are summed to obtain a metric (MET) related to the noise in a single DMT symbol and represented as 
     
       
         MET=Σ D   i   2   
       
     
     where, 
     i represents one tone of the multi-tone symbol. 
     The metric MET is then compared to a threshold, representing the noise level within a symbol which is regarded as an effect of impulsive noise. The threshold value may be a fixed value, or alternatively, may be changed dynamically based on channel conditions. For instance, the threshold value may be increased if no impulsive noise is detected for a given period of time. If MET is greater than the threshold then all of the data bytes B associated with the DMT symbol are tagged by an erasure bit E, resulting in a 9 bit word W. Alternatively, the sum of the additive inverses of each detector error may be compared to a threshold and the data bytes tagged if the sum is less than the threshold. The word W is then fed to a deinterleaver  125 , similar in operation to a conventional ADSL deinterleaver, except that 9-bit words are deinterleaved instead of bytes. The 9-bit word output WD of the deinterleaver is again separated into an eight bit data byte BD and an erasure bit ED, which are fed to a conventional Reed-Solomon decoder  130  that is controlled by controller  135 . In particular, controller  135  is capable of causing the decoder  130  to receive a block of samples from the de-interleaver  125 , outputting from the decoder a decoded data block, activating the decoder to operating in a first mode without erasures, activating the decoder to operate in a second mode with erasures, and receiving a decoding-failure indicator from the decoder, as will be described in further detail below. 
     FIG. 2 a  is a flow chart of a first embodiment of the operation of the controller  135  in the multi-tone receiver  100  in FIG.  1 . Initially, in step  205 , controller  135  causes the decoder  130  to receive a block of data from the de-interleaver  125 . Then in step  210 , controller  135  activates the decoder  130  to operate in a first mode without erasures and a first decoded data block without erasures is produced. A determination is made in step  215 , whether a decoding-failure indicator was generated or reported by the decoder  130 . If a decoding-failure indicator is generated by the decoder, then in step  220  the second mode of operation of the decoder with erasures is activated by the controller  135  and a second decoded data block with erasures is reproduced from the original input data block and output from the decoder. Otherwise, if a failure is not reported by the decoder, then the first decoded data block is output from the decoder in step  225 . 
     Detection of the decoding failure is not exact, especially when the redundancy of the Reed-Solomon code is relatively small. Therefore, it is possible for a failure or error to exist without being detected. Performing the decoding without erasures (in FIG. 2 a ) insures that the performance of the decoder will not be degraded with respect to a conventional (single-iteration) decoder without erasures. This scheme is applicable when performance in a channel without impulsive noise must not be compromised. In an alternative embodiment in accordance with the present invention and shown in FIG. 2 b , a first stage of decoding may be performed with erasures, followed by a second decoding stage without erasures. Performing the decoding with erasures (in FIG. 2 b ) insures that the performance of the decoder will not be degraded with respect to a conventional (single-iteration) decoder with erasures. The scheme shown in FIG. 2 b  is suitable when performance in a channel with impulsive noise must not be compromised. 
     Furthermore, although the multi-tone receiver shown in FIG. 1 has a single decoder  130  that is switched between two modes, one with erasures and the other without erasures, in an alternative embodiment in accordance with the present invention, decoding may be performed in parallel by two conventional Reed-Solomon decoders, one with erasures and the other without erasures. This alternative embodiment shown in FIG. 3 is similar to the embodiment shown in FIG. 1, except that instead of the receiver  100  (FIG. 1) including a single decoder  130  operable in two modes and a controller  135 , receiver  300  includes a first decoder  330   a , a second decoder  330   b , a logic circuit, and a switch  345 . The first decoder  330  operates without erasures, whereas the second decoder  330   b  operates with erasures. Logic circuit  340  receives from decoders  330   a ,  330   b  a decoding-failure indicator. Based on the logic results of the decoding-failure indicators of the two decoders  330   a ,  330   b , the logic circuit will cause switch  345  to output the decoded data block produced by the first decoder  330   a  or the second decoder  330   b.    
     Thus, while there have been shown, described, and pointed out fundamental novel features of the invention as applied to a preferred embodiment thereof, it will be understood that various omissions, substitutions, and changes in the form and details of the devices illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit and scope of the invention. For example, it is expressly intended that all combinations of those elements and/or steps which perform substantially the same function, in substantially the same way, to achieve the same results are within the scope of the invention. Substitutions of elements from one described embodiment to another are also fully intended and contemplated. It is also to be understood that the drawings are not necessarily drawn to scale, but that they are merely conceptual in nature. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.