Abstract:
A method and device for combating logarithmic quantization and Robbed Bit Signaling (RBS) impairments that..are typical to PCM telephone lines is descried. An apparatus is described which includes a front-end unit which receives samples of the digital PCM line, an impairment identifier unit which identifies samples that have a high likelihood to have, large impairments due to the PCM line, an impairment estimator unit which estimates the value of impairment caused by the digital line, a samples reconstructor unit which fixes received samples by subtracting from them the value of the estimated impairment and an output unit transfers the reconstructed samples to a receiver. The method allows improving signal quality at the output of the PCM line, and thus improving data rates and robustness of digital communication receivers, and particularly of V.34 receivers, or V.90 transceivers that are digitally linked to the PCM, line.

Description:
RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 60/078,372, filed Mar. 18, 1998, entitled Method and Device for Combating PCM Line Impairments, which is incorporated by reference. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates generally to telecommunication methods and devices and in particular to PCM communications over telephone lines and the like. 
     BACKGROUND OF THE INVENTION 
     The vast majority of Public System Telephone Network (PSTN) lines communicate information digitally using Pulse Code Modulation (PCM) as the main telecommunication protocol. PCM is the sampling of the analog signal (most typically voice signals) to produce pulse amplitude modulated signals in which the amplitude of each of the pulses is directly proportional to the original analog signal at the instant of the sample. Each of these amplitude samples is then quantized by measuring the amplitude and comparing the measured value against a scale of amplitude values in which each number in the scale represents an amplitude. The most widely used scale is from 0 to 255 in which each value in the scale is represented by an 8-bit code. Each amplitude sample is then transmitted as a binary coded signal representing the original amplitude of the signal at the sampling instant in time. These binary codes are then transmitted digitally over the telephone line and are decoded at the receiving end to reconstruct the original analog signal. Even if the sampling rate of the analog signal is more than twice the frequency of the analog signal, the reconstructed analog signal is not the exact duplicate of the original analog signal due to the quantization. In other words, if an analog amplitude falls between two values in the quantization scale, one of the values is chosen as the closest match. On the decoding end of the transmission, the exact amplitude value chosen on the amplitude scale is used for reconstructing the analog signal. The difference between the original amplitude and the reconstructed amplitude is called quantization error or quantization noise. There are two different PCM encoding and decoding protocols used in the world: μ-law (mu-law) coding (used the United States of America and other places) and A-law (used in Europe, Israel and other places). These two coding laws assign different quantization values for an analog signals. The two systems often have different quantization errors. 
     Modem communications over telephone lines have used a modulated analog signal which is modulated to encode digital data for digital communications between computers. The modulated analog signal of the modem is treated as any other analog signal by the PCM encoders and decoders. This form of modem communication was limited in speed due to the limited bandwidth allocated to voice telephone lines and due to quantization error or noise. 
     Recently, an improvement in the speed of modem communication has come in the form of PCM modems also known as 56K modems. These modems use the telephone line as a digital line. The International Telecommunications Union (ITU) recently issued a new recommendation for these modems designated Recommendation V.90. V.90 modems are designed for connections which are digital at one end and have only one digital-to-analog conversion in the path. That is, an information provider such as an Internet Service Provider (ISP) connects a digital modem directly to a PCM line at the telephone company while a user (such as in a home) connects a V.90 modem to an analog telephone line. Between the user and the telephone company, only a single analog to digital conversion is made. 
     From a V.90 modem user&#39;s prospective, downstream (from the Telephone Company Central Office to the user) speeds of up to 56,000 bits per second (bit/s) are possible, depending on telephone line conditions, with upstream (from the user to the information provider) speeds of up to 33,600 bit/s. The modems that conform to the ITU V.90 protocol use ITU V.34 protocol on the upstream. The downstream transmission is done by PCM (Pulse Coded Modulation) such that downstream quantization noise is eliminated. However, downstream data transmission is impaired by other factors such as distortions and non-linearity in the telephone line, length of the line and equipment at the telephone company. Recently, great progress has been made in the development of methods for combating noise when transmitting in the downstream direction. 
     Problems of noise on the upstream pose a different set of problems. Quantization noise is now present due to the analog to digital conversion on the upstream. Also, non-linearity in the telephone line, length of the line, analog noise and equipment at the telephone company introduce noise into the upstream. In addition, the telephone companies themselves intro noise through Robbed Bit Signaling (RBS). RBS is a technique used by the telephone companies on T1 lines that use digital transmission. The technique uses the least significant bit of the PCM code word to control telephone signaling functions such a dial tone, ring, busy, answer, etc. This signaling method negatively effects V.90 by reducing the data rate on the v.34 connections on the upstream. 
     Thus, most upstream noise can be classified as one of two factors, generally described as PCM noise. The first factor is logarithmic quantization noise such as either μ-law or A-law quantization in which the quantization interval grows with the magnitude of the signal. The second factor is Robbed Bit Signaling (RBS) in which the least-significant bit of some of the samples may be overridden by control information that is transferred over the telephone network. 
     There are several methods proposed to combat the V.90 upstream or V.34 impairments. For example, the V.34 standard includes an option for using a non-uniform constellation that has a higher spacing around its high magnitude symbols, which are likely to suffer from PCM impairments more than the low magnitude symbols. Another class of approaches advocates error correction codes that are robust to a PCM noise, such as the 64 states trellis code proposed in the V.34 standard. However, practical experience shows that the effectiveness of these methods is limited. Thus, there is a need in the telecommunications art for combating PCM noise and improving data rate on the upstream of V.90 connections. 
     SUMMARY OF THE INVENTION 
     The present invention solves the above-mentioned problems in the art and other problems which will be understood by those skilled in the art upon reading and understanding the present specification. The present invention provides a method and device for combating logarithmic quantization and Robbed Bit Signaling (RBS) impairments that are typical to Pulse Code Modulation (PCM) telephone lines. 
     An apparatus is described which includes a front-end unit which receives samples of the digital PCM line, an impairment identifier unit which identifies samples that have a high likelihood to have large impairments due to the PCM line, an impairment estimator unit which estimates the value of impairment caused by the digital line, a samples reconstructor unit which fixes received samples by subtracting from them the value of the estimated impairment and an output unit which transfers the reconstructed samples to a receiver. 
     The present invention is particularly useful as a front end to a digital communications receiver that receives samples of the digital PCM lines. For example, but not by limitation, the present invention is useful as a front end to either V.34 receiver or V.90 receiver which are digitally linked to a T1 line. The present invention is further applicable to any receiver that receives PCM data from a digital communication line. 
     The present invention includes a method for combating PCM line impairments. The present invention allows improving signal quality at the output of the PCM line, and thus improving data rates and robustness of digital communication receivers, and particularly of V.34 receivers, that are digitally linked to the PCM line. This method can be implemented in a manner that requires small computational resources and can be added on top of all the methods described above for combating PCM impairments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings, where like numerals refer to like components throughout the several views, 
     FIG. 1 depicts the structure of digital modem using the disclosed invention; 
     FIG. 2 depicts the structure of digital V.34 modem connected to a T1 digital line using the disclosed invention; 
     FIG. 3 depicts an impairment estimation algorithm used within the present invention; 
     FIG. 4 depicts an impairment identification algorithm used within the present invention; 
     FIG. 5 depicts the structure of a digital modem using the disclosed invention which compensates for RBS in both the incoming signal and the outgoing signal; and 
     FIG. 6 describes a correction algorithm of the impairment estimator of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific preferred embodiments in which the inventions may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that structural, logical and electrical changes may be made without departing from the scope of the present inventions. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present inventions is defined only by the appended claims. 
     FIG. 1 is a block diagram which describes a digital modem (modulator and demodulator) interface of the present invention. This modem interface would typically be connected directly to a digital telephone line to provide an interface for an Internet Service Provider (ISP) or some other information provider. This modem interface would typically be located in the telephone company&#39;s central office or at an ISP location which is connected to a telephone company central office by a PCM line. 
     In FIG. 1, the line interface (I/F) unit  101  connects the modem to a digital PCM line, such as T1, ISDN, or E1 line, and outputs to unit  102  an 8 kHz stream of PCM characters which are logarithmically quantized (by μ-law or A-law quantization) on the upstream. The least significant bit of some of this upstream data may be corrupted by Robbed Bit Signaling (RBS). The line interface unit  101  also receives 8 kHz PCM characters from the transmitter unit  107  and sends it over the digital line for the downstream path to a user. The line interface unit may send channel information  108  to unit  102  to extract information such as the type of line (for example, but not by way of limitation, T1, E1, ISDN, etc.), the amount and timing of data samples which are corrupted by robbed bit signaling (RBS), and the values of the bits that are planted on the least significant bits of the transmitted PCM characters using RBS. 
     The Impairment Identifier unit  102  identifies the timing of data samples where robbed bit signaling (RBS) occurs both for the incoming data, and for the outgoing data whose line echo may be added to the incoming data. Unit  102  synchronizes unit  103  to the timing of the RBS in the incoming data, and synchronizes unit  106  to the timing of the RBS in the outgoing data. The timing phases of the incoming and the outgoing RBS can be identified by providing different timing phases of the RBS patterns to unit  103  and  106 , respectively, and choosing the phases which correspond to lowest cumulative error magnitude at the output of the equalizer and the echo canceler, respectively, at the receiver unit  105 . 
     The Impairment Estimator unit  103  generates an estimate of the RBS impairment. When unit  102  identifies that the PCM character is corrupted by RBS, the RBS impairment is estimated as half of the difference between the levels of the received PCM and the adjacent PCM character which differs from the received character only by its least significant bit. When unit  102  identifies that the PCM character is not corrupted by RBS, the RBS impairment is estimated by zero. In that manner, the peak error and the mean square error due to RBS are reduced significantly. The probability density function of a PCM quantization error in the presence of RBS is:                  f   e          (   e   )       =     {               1   /   4        Δ     ,       …                   Δ   /   2       &lt;        e        ≤     3        Δ   /   2                         1   /   2        Δ     ,       …                      e          ≤     Δ   /   2                   0   ,       …                      e          &gt;     3        Δ   /   2                           (   1   )                                
     where Δ is the quantizer step at the magnitude of the quantizer input. The power of the error is 7Δ 2 /12 and its peak value is 3Δ/2. The probability density function error after applying in the present invention in the mode where unit  103  estimates the RBS impairment as half of the difference between the levels of the received PCM and the adjacent PCM character which differs from the received character only by its least significant bit is:                  f   e          (   e   )       =     {               1   /   2        Δ     ,       …                      e          ≤   Δ                 0   ,       …                      e          &gt;   Δ                       (   2   )                                
     The power of the error at the input to the receiver unit  105  is Δ 2 /3 and -its peak value is Δ. Therefore, the present invention is capable of reducing the peak of the error by a factor of 3/2 (3.53 dB), and reduced the error power by a factor of 7/4 (2.42 dB). 
     In operation, impairment estimator unit  103  of FIG. 1 may receive an error indication from the receiver unit  105 , and as a result it may apply a correction algorithm shown in FIG.  6 . When an error detection is made at  601 , the correction algorithm of the impairment estimator chooses a PCM character which was corrupted by RBS at  602 . This character is drawn from a PCM segment that has a large quantization interval which was received near the time of the detection of the error by the receiver. The algorithm then estimates the RBS impairment by zero at  603 . The algorithm then initiates re-processing of the data block where the error has occurred at  604 . If the receiver detects an error in the data block again at  605 , then the algorithm estimates the RBS impairment at  607  as being the difference between the levels of the received PCM character and the adjacent PCM character which differs from the received character only by its least significant bit. The algorithm then initiates reprocessing of the data block where the error has occurred. If an error occurred again at  608 , then unit  103  will choose another PCM character from the block of characters and repeat the process by returning to  603 . Impairment estimator unit  103  may apply a more elaborate algorithm such as the one depicted in FIG.  3  and explained below. 
     The signal re-constructor unit  104  reconstructs the incoming signal by adding the estimated RBS impairment to the level of the received PCM characters, and outputs the result to the receiver unit  105 . Unit  105  is a known digital receiver such as a V.34 receiver, which is capable of re-processing a data block. Unit  105  identifies and indicates decoding errors by for example and not by limitation, examining the path consistency of a Viterbi detector algorithm in the receiver. Receiver  105  may use an IF echo-canceler and may provide an indication of cumulative residual echo magnitude at the output of the echo-canceler filter. It also provides an indication of the cumulative residual noise magnitude at the incoming signal. 
     The Echo RBS Correction unit  106  receives from unit  102  an indication on the timing of the outgoing characters that are corrupted by RBS, and estimates the RBS corrupted outgoing character. One algorithmic approach is to calculate the median between the levels of the received PCM character and the adjacent PCM character which differs from the received character only by its least significant bit. In cases where the control bits that are transferred between the telephone company central offices using RBS have a periodic pattern, the unit  106  will generate a PCM character whose 7 most significant bits are equal to the 7 most significant bits of the transmitted character and its least significant bit is equal to the value of the control bit which is estimated according to the known periodical pattern, where the phase of the pattern is estimated by,unit  102 . For example, in some T1 systems, the pattern has a period of 24 bits. In this manner, the effect of robbed bit signaling on the performance of line echo can be significantly improved. 
     Transmitter  107  is a known digital transmitter. A system controller  109  sends the impairment identifier unit  102  information on the type of the digital channel such as T1, E1 or ISDN lines. This channel type is used by unit  102  to choose the algorithm for identifying the line&#39;s impairments. 
     FIG. 2 depicts the structure of digital V.34 modem connected to a T1 digital line using the disclosed invention. FIG. 2 is another embodiment of the invention described in conjunction with FIG.  1 . Unit  201  sends and receives 8 kHz streams of PCM characters over a T1 or other digital line. Unit  202  identifies PCM characters which are infected by RBS. In many cases these characters occur every sixth character. Receive RBS Estimator  203  estimates the error due to RBS using the methods described above and below in conjunction with FIG.  3 . Transmit RBS corrector unit  204  compensates for RBS error in the transmitted signal. V.34 transmitter  205  and V.34 receiver  206  are known devices. 
     FIG. 3 depicts an impairment-estimation algorithm used within the unit  103  or  203 . The estimated impairment value is selected by the impairment mode (output of unit  102 ) and it can be one of three values. First, the value can be estimated as zero  303  in the simplest case. Second, the estimated impairment value  304  can be estimated as the difference between the level of the received PCM character and the output of smoothing filter unit  301 . The smoothing filter  301  performs a weighted linear combination which estimates the signal before the PCM impairment, and gives low weights to PCM characters which were impaired by RBS and/or had high magnitude. Third, the estimated impairment value  305  is the difference between the level of the received PCM character and the level of a PCM character which is generated by unit  302 . Unit  302  performs PCM quantization of the output of unit  301 , and its output level is restricted to be the level of the PCM character which is equal to the received PCM character by its seven most significant bits. 
     The first type of output (zero)  303  output will typically be chosen in case there is no RBS in the PCM character and the character has low magnitude. The second type of output  304  will typically be chosen in case of a PCM character which either has a high magnitude or suffers from RBS. The third type of output  305  will typically be chosen in case of a PCM character which suffers from RBS. 
     In the situation where the operation of RBS by the telephone company results in flipping (changing) the least significant bit of a PCM character and the output of unit  302  recovers the original PCM character prior to flipping its least significant bit, then the PCM quantization error is uniformly distributed in the range −Δ/2 where Δ is defined above after equation (1). In reviewing equation (1) above, one skilled in the art will observe that in such a case described here, the peak of the quantization error is reduced by a factor of 3 (which is 9.54 db) and its power is reduced by a factor of 7 (which is 8.45 dB). 
     FIG. 4 shows an algorithm for identifying the phases of RBS which may be applied in unit  102 . The incoming signal is subtracted from an output of a smoothing filter  401 , which is designed to reduce the effect of RBS and which performs a weighted linear combination which estimates the signal before the PCM impairment, and gives low weight to current PCM character. The bandwidth and pass-bands of the filter  401  are matched to the spectrum of the received signal. Particularly, in case the incoming signal is sine wave or a combination of a few sine waves, then the smoother will be a narrow band-pass filter which is passing only the frequencies around the frequencies of the transmitted sine waves. The power of the difference signal  405  is averaged by unit  403 . The power of the signal  405  is also averaged by units  402  each of which accumulate separately the power averaged over N sub-sequences which are obtained from sequence  406  by decimation at ratio of N in N different phases, where N is the expected period of the RBS pattern. The existence and the phase of the RBS bits are identified by examining the ratio between values of N accumulators of units  402  to the accumulator of unit  403 , wherein a high ratio implies the existence of RBS at the corresponding phase. The phase and pattern of the RBS may be identified reliably during periods in which sine waves are transmitted, for example phase 2 of the training sequence according to the ITU V.34 standard, and filter  401  can have a very low bandwidth. 
     FIG. 5 depicts the structure of a digital modem using the disclosed invention which compensates for RBS in both the incoming signal and the outgoing signal, in PCM lines where the RBS pattern can be predicted. Units  501 ,  503 ,  504 ,  505 , and  509  are similar to units  101 ,  103 ,  104 ,  105  and  109  respectively, which were described above. Unit  502  is similar to unit  102 , and it is particularly capable of identifying the pattern and the phase of the robbed bits that are added over the least significant bits of the outgoing PCM characters. Unit  507  is a known digital, transmitter that outputs a 8 kHz samples signal with low level of quantization error, for example, in 16-bit linear quantization. 
     Unit  506  is an adaptive PCM quantizer which operates in the following manner. If impairment identifier unit  502  identifies that the PCM character does not suffer from RBS, adaptive PCM quantizer unit  506  quantizes its input to a quantization grid that includes all 256 PCM levels. If impairment identifier unit  502  identifies that the PCM character will suffer from RBS and that the value of the least significant bit (the robbed bit) is zero, then unit  506  quantizes its input to a quantization grid that includes the levels of the 128 PCM characters whose least significant bit is zero. If impairment identifier unit  502  identifies that the PCM character suffers from RBS and that the value of the least significant bit (the robbed bit) is one, then unit  506  quantizes its input to a quantization grid that includes the levels of the 128 PCM characters whose least significant bit is one. In this manner, the effect of robbed bit signaling on the transmitted data can be significantly reduced, and the data rate of the transmitted data can be increased without increasing the error rate of the receiver which is connected to the other side of the line. The probability density function of the error in the transmitted signal due to the PCM quantization and RBS is given by equation (1) above. The probability density function of the error in the transmitted signal due to the PCM quantization and RBS when the disclosed invention is applied and when unit  506  receives the correct RBS pattern from unit  502  is given by equation (2) above. The power of the error and the peak of the error are reduced by the present invention by factors of 7/4 (2.42 dB) and 3/2 (3.53 dB). 
     CONCLUSION 
     Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement which is calculated to achieve the same purpose may be substituted for the specific embodiments shown. This patent is intended to cover any adaptations or variations of the present invention. Therefore, it is manifestly intended that this invention be limited only by the claims and the equivalents thereof.