Abstract:
A method and system for reducing the power to average ratio (PAR) at the transmitter after the up sampling and interpolation filter. In the time domain, Side Tone Packets Injection (STPI) system and method detects the optimal peaks that will result in having a low PAR at a sampling rate of T/K and combines an optimal binary and a PAR-lowering sequence and an upsampled, interpolated original discrete time signal at the scale of one extended symbol for multi-carrier modulation and at the scale M of symbols for single-carrier modulation. STPI does not require peaks search and processing at the scale of many time samples, does not require iterative Fast Fourier Transform (FFT) and Inverse Fast Fourier Transform (IFFT) processing thus, allowing the receiver not to perform extra demodulation.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Priority is claimed based on U.S. Provisional Application No. 60/327,178 entitled “STPI Side Tone Packets Injection for PAR reduction” filed Oct. 4, 2001. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to side tone packets injection (STPI) for reduction of peak-to-average ratio (PAR) for single-carrier and multi-carrier modulation schemes in Digital Subscriber Lines (xDSL). 
     BACKGROUND OF THE INVENTION 
     High-bandwidth technologies which are prevalent nowadays use the existing copper wire infrastructure intended for plain old telephones (POTS) communication. One such technology is Digital Subscriber Line (DSL) which comes in multiple variations such as ADSL, HDSL, IDSL, SDSL, RADSL and VDSL collectively known as xDSL. Asymmetric digital subscriber line (ADSL) allows users a higher data rate downstream (i.e. to the customer) than upstream (i.e. the service provider). 
     These high-bandwidth systems use single-carrier modulation as well as multi-carrier modulation schemes, such as Carrier-less Amplitude and Phase modulation (CAP) and Discrete Multi-tone (DMT) for wired media and Orthogonal Frequency Division Multiplexing (OFDM) for wireless communication. One advantage of such schemes is that they are suited for high-bandwidth application of 2 Mbps or higher upstream (subscriber to provider) and up to 8 Mbps downstream (provider to subscriber). Quadrature Amplitude Modulation (QAM) utilizes a sine and cosine wave with the same frequency component to convey the information. The x and y components of the point to which the bits are mapped specify the amplitude of the sine and cosine waves and, because of the orthogonality between the waves, the data can be sent over the channel simultaneously. The amplitude (including sign and magnitude) of each wave conveys the information (bits) being sent. QAM modulation has been used in voice-band modem specifications, including the V.34. 
     CAP is similar to QAM. For transmission in each direction CAP systems use two carriers of identical frequency above the 4 KHz voice band, one shifted 90 degrees relative to the other. CAP also uses a constellation to encode bits at the transmitter and decode bits at the receiver. The x and y results from the encoding process are then used to excite a digital filter. 
     DMT modulation, sometimes called orthogonal frequency division multiplexing (OFDM), builds on some of the ideas of QAM but, unlike QAM, it uses more than one constellation encoder where each encoder receives a set of bits that are encoded and outputs the amplitude of the sine and cosine waves. However, a different sine and cosine frequency is used for each constellation encoder. The outputs from these different encoders are summed together and sent over a single channel for each direction of transmission. DMT systems divide the spectrum above the 4-KHz voice frequency band into 256 narrow channels called bins (sometimes referred to as tones, DMT tones or sub-channels). These bins are 4.3125 KHz wide. The waveforms in each bin are completely separable from one another. One key to separability is that the sine and cosine frequencies used in each bin should be multiples of a common frequency known as the fundamental frequency and in addition the symbol period τ, must be the inverse of the common frequency. Another of way of producing a DMT symbol, other than mapping the output of the constellation encoder into a sine and cosine amplitude, is that the output can be mapped into a complex number in a vector. The value from the cosine axis, or X will represent the real part and the value from the sine axis, or Y will represent the imaginary part. If the outputs of all the constellation encoders are ordered in the vector, then each vector point represents one of the DMT bins and, if N bins existed in the DMT system, the complex vector would have N entries. A suffix containing the complex conjugates of the vector&#39;s original entries can be added to the vector such that the new vector has complex conjugate symmetry. An inverse Fourier transform (IFFT) on those N frequency bins is then used to convert the data from frequency domain to a time domain signal. 
     From a circuit standpoint, and in relation to discrete multi-tone modulation, the prior art shown in  FIG. 1 , is a transmitter side of a DMT transceiver. The transmitter accepts serial data which is then converted from serial to parallel form and to provide two hundred fifty-six signals n 0 –n 255  via a serial to parallel converter  10  (SP). The sequences are then passed on to a symbol-mapper  20  where each bit is assigned or mapped into one of N-complex (QAM) multi-level sub-symbols. The two hundred fifty-six symbols are complex-valued and are fed into an IFFT  30  which provides five hundred and twelve output real signals by taking the complex conjugates of the two hundred fifty six samples. The parallel outputs of the IFFT are applied to a cyclic prefix  40  which helps to make a channel circular so that equalization can occur more easily in the frequency domain. The output of the cyclic prefix block is then applied to a parallel to serial converter  50  to provide a serial output signal which is upsampled and interpolated by a up-sampler  60 . The output is processed by a interpolator  70  which converts the discrete time signal into a continuous time signal. 
     Spurious high amplitude peaks in the composite time signal occur when the signals from the different tones add constructively. Compared to the average signal power, the instantaneous power of these peaks are high, and consequently, so is the peak-to-average power ratio (PAR). These large peaks require a large dynamic range of the analog-to-digital converter (ADC) and analog front end (AFE) which result in inefficient amplifiers with excessive power dissipation and expensive transceivers. To overcome the drawbacks of the high PAR, many solutions and techniques have been proposed, one of which is, tone reservation method in which a pre-selected number of tones are set aside for PAR reduction. The information transmitted in these tones is used to subtract from the signal envelope, thus reducing the PAR, but at a cost of increased complexity at the transmitter. 
     Another method depends upon applying a combination of saturating non-linearity (clipping) at the transmitter and having the knowledge of the saturating characteristics at the receiver to iteratively decode the data vector without distortion by performing a number of tedious and costly fast FFT and IFFT computations. 
     Yet another method, known as selective mapping, involves generating a large set of data vectors all representing the same information in which the data vector with lowest PAR is selected. However, potential problems may arise with decoding the signal in the presence of noise where errors in the reverse mapping may result in the data of whole symbols being lost. Other drawbacks also exist. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method and system for reducing the PAR of single carrier and multi-carrier modulation such as DMT, (sometime called OFDM) as well as CAP. Reducing the PAR also reduces the complexity as well as the power consumption of the DAC. The present invention as described herein provides a reduction in the power to average ratio, while maximizing the computational capabilities of the processor by minimizing millions of instructions per second (MIPS). Embodiments of the invention implement a method and system of injecting side tone packets in the time domain signal by choosing a basis sequence of the available packets at the regular rate of T for multi-carrier modulation and at the baud rate for single-carrier modulation and calculating sub-sequences of the afore-mentioned basis sequence, and combining the sub-sequences with signal symbols. 
     The PAR-lowering method and system may be applied after the upsampling and interpolation filters and before the digital-to-analog converter. In contrast with the known methods of controlling the PAR in multi-modulation schemes at both the transmitter and the receiver, the present invention reduces the DMT (OFDM) and single carrier modulation peak to average ratio PAR at the transmitter and because of that, the receiver is not at involved in the PAR reduction mechanism. Other advantages also exist. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The purpose and advantages of the present invention will be apparent to those of ordinary skill in the art from the following detailed description in conjunction with the appended drawings in which like reference characters are used to indicate like elements, and in which: 
         FIG. 1  is a schematic block diagram of a prior art Discrete Multi-tone (DMT) transmitter. 
         FIG. 2A  is a frequency-domain response of a prior art DMT signal. 
         FIG. 2B  is a is a time-domain response of a prior art DMT signal. 
         FIG. 3  is a flow chart of a PAR reduction technique using Fractional Local Peak Detection and Mitigation (FLPDM) according to some embodiments of the invention. 
         FIG. 4  is an interpolation impulse response at T/2 according to some embodiments of the invention. 
         FIG. 5  shows where a negative value of T leads to a large negative value after upsampling and interpolation according to some embodiments of the invention. 
         FIG. 6  is a high-PAR boundary situation according to some embodiments of the invention. 
         FIG. 7  is an FLPDM simulation result of a DMT PAR distribution density according to some embodiments of the invention. 
         FIG. 8  is a flow chart of a PAR reduction technique using Synchronized Local Peaks Detection and Mitigation (SLPDM) according to some embodiments of the invention. 
         FIG. 9  is an SLPDM simulation result of a DMT PAR distribution density according to some embodiments of the invention. 
         FIG. 10  is a frequency-domain side tone injection according to some embodiments of the invention. 
         FIG. 11  is an Side Tone Packets Injection (STPI) simulation PAR result applied to ADSL-DMT modulation according to some embodiments of the invention. 
         FIG. 12  is an STPI simulation PAR result applied to CAP modulation according to some embodiments of the invention. 
         FIG. 13  is an STPI simulation PAR result applied to CAP modulation at T/2 according to some embodiments of the invention. 
         FIG. 14  is a PAR histogram for classical CAP according to some embodiments of the invention. 
         FIG. 15  is a PAR histogram for classical CAP+STPI according to some embodiments of the invention. 
         FIG. 16  is a flow chart of a PAR reduction technique using STPI according to some embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description is intended to convey a thorough understanding of the invention by providing a number of specific embodiments and details involving the PAR reduction techniques. Various improvements and modifications to the preferred embodiments will apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments. Therefore, the present invention is not limited to the specific embodiments shown and described, but interpreted to a larger scope consistent with the novel features disclosed herein. 
       FIG. 3  shows a PAR reduction technique where the original signal  100  is at the regular sampling rate (T) for multi-carrier and the baud rate for single-carrier modulation. Prior to implementing the necessary steps for lowering the PAR, signal  100  is stored as indicated in step  110 . The signal may then be extended by adding a suffix T+1/K as indicated in step  120 , by appending the new samples at the rate of T, such that the new samples repeat the last available samples. Such suffixes are chosen according to the last “current” symbol values which is expected to have a high PAR value of the next symbol. At step  130 , the extended signal is upsampled and interpolated by an integer factor depending on the signal bandwidth and timing recovery. 
     At step  140 , a threshold is set by calculating the standard deviation σ of the signal. This is achieved by taking the square root of the variance,
 
σ=√{square root over (σ 2 )},
         where,       

     
       
         
           
             
               
                 σ 
                 2 
               
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                   1 
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                   N 
                 
                 - 
                 
                   1 
                   ⁢ 
                   
                     
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                         i 
                         = 
                         0 
                       
                       
                         N 
                         - 
                         1 
                       
                     
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       ( 
                       
                         
                           x 
                           1 
                         
                         - 
                         μ 
                       
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             , 
           
         
       
         
         
           
             σ 2  is the variance, 
             x 1  is the signal samples, 
             μ is the average value of the signal, 
             N is the number of samples;
 
and hence the threshold peak is determined from the probability density function.
 
           
         
       
    
     Normally, any signal peaks that exceed the DAC threshold can not be accurately converted and thus, may be clipped which will result in loss of information (e.g.,  FIG. 2B ). Once the threshold is set in step  140 , a peak-detection algorithm may be used to detect for signal peaks (P) that exceed the set threshold. In the event that no signal peaks exceed the threshold, the extended original signal is interpolated and then transmitted, as indicated at step  150 . 
     One of the exemplary steps in the invention is to localize such peaks (P), into the neighborhoods where they occur. Neighborhoods may be formed at the regular sampling rate of T, as indicated in step  160 , as well as the extended rate of T/K, as indicated in step  170 . In step  180 , a sequence basis (Q) may be generated by injecting in the time domain an optimal binary (+1, −1) combination of the Q basis sequence, where Q is any number (L) of packets of 4 tones each. Such combination takes place at the scale of one extended symbol for multi-carrier modulation and at an integer M for single-carrier modulation. As indicated in step  190 , another sequence (J), which a sub-sequence of the Q basis sequence is locally generated at (T), such that J=2 Q , when Q=4, for example, J will be 2 4 =16 sub-sequences. 
     In step  200 , one of the locally generated sub-sequences is combined with (P), the localized highest peak sources that exceeded the threshold. Such a combination takes place locally at the rate of (T). Next, in step  210 , the combination is upsampled and interpolated at the fractional rate of (T/K), and then the PAR is determined from the upsampled, interpolated combination of J+P as indicated in step  220 . The PAR is stored and the process may be revisited again where another of the locally generated sub-sequences is combined with (P). The highest of the locally detected peaks and the combination upsampled, may be interpolated and the PAR determined and stored. 
     After an iterative operation, as indicated in step  240 , the lowest PAR is chosen from all the stored PAR at the local fractional rate of T/K. In step  250 , the PAR lowering sequence which produces the lowest PAR is generated at the rate for all the samples where it is combined in step  260  with the originally stored signal of step  110  and then transmitted. 
     In another embodiment, as shown in  FIG. 8 , a PAR reduction technique may be implemented where the original signal  300  is at the regular sampling rate (T) for multi-carrier and the baud rate for single-carrier modulation, and prior to implementing the necessary steps for lowering the PAR, signal  300  is stored as indicated in step  310 . A threshold is tuned  320 , similarly to the tuning indicated in step  140 . At step  330 , a peak-detection algorithm is used to detect for signal peaks that exceed the tuned threshold, such high peaks are localized to the regular rate (T) which is the classical sampling rate for multi-carrier modulation and the baud rate for single-carrier modulation. In step  350 , a sequence basis (Q) is generated by injecting in the time domain an optimal binary (+1, −1) combination of the Q basis sequence, where Q is any number (L) of packets of 4 tones each, such combination takes place at the scale of one extended symbol for multi-carrier modulation and at an integer M for single-carrier modulation; and in step  360 , another sequence (J) which a sub-sequence of the Q basis sequence is locally generated at (T), such that J=2 Q , when Q=4, for example, J will be 2 4 =16 sub-sequences. 
     In step  370 , one of the locally generated sub-sequences is combined with (P), the localized highest peak sources that exceeded the threshold, such a combination takes place locally at the rate of (T). Next, as indicated in step  380 , the PAR is determined from the combination of J+P found in step  370 . The PAR-lowering sequence is stored as indicated in step  390  and the process shown in steps  350 ,  360  and  370  are re-visited to calculate another PAR-lowering sequence that is then stored. After an iterative operation, and as indicated in step  400 , the best PAR-lowering sequence is chosen from all the stored PAR-lowering sequences at the local rate of (T). In step  410 , the best PAR-lowering sequence is then generated at the local and then the original stored signal in step  310  is added to the lowest PAR-lowering sequence at (T) and then transmitted. 
     Another embodiment of this invention may be illustrated with reference to  FIG. 10 .  FIG. 10  shows that tone packets may be added on either side of the spectral mask. This embodiment provides a method of injecting, in the time domain, an optimal binary combination (+1, −1) which leads to the minimum PAR desired, and combining of Q tones. Such a combination takes place at the scale of one symbol for multi-carrier modulation and at the scale of any number of symbols for single-carrier modulation, where such tones (or packets) are loaded into the first quadrant of a QAM-4. 
       FIG. 9 , shows a PAR reduction technique where a discrete time signal, as indicated in step  500 , is sampled at the regular sampling rate (T) for multi-carrier and the baud rate for single-carrier modulation. Prior to implementing any of the necessary steps for lowering the PAR, the signal  500  may be stored as indicated in step  510 . The signal is then extended by adding a suffix T+1/K, as indicated in step  520 . The DMT real-valued symbol k is denoted by N time samples at the regular sampling rate; by appending new samples at the rate T in which new samples would be repeating the last available samples. Such suffixes are chosen according to the last “current” symbol values which is expected to have a high PAR value of the next symbol. At step  530 , the extended original symbol is upsampled so that the actual output sample rate matches the desired rate, and interpolated by an integer factor, which depends upon the signal bandwidth and the accuracy of the timing recovery. At step  540 , a suffix is added to the upsampled, interpolated symbol of step  530 . In step  550 , a sequence basis (Q) is generated by injecting in the time domain an optimal binary (+1, −1) combination of the Q basis sequence which are the number of signal packets, such combination takes place at the scale of one extended symbol for multi-carrier modulation and at any integer M for single-carrier modulation. In step  560 , another sequence (J) which a sub-sequence of the Q basis sequence is locally generated at (T/K), such that J=2 Q , when Q=4, for example, J will be 2 4 =16 sub-sequences. 
     In step  570 , one of the generated PAR-lowering sub-sequences is combined with the upsampled, interpolated extended symbol at the rate of (T). Next, as indicated in step  580 , peaks are searched that result from the combination produced in step  570 . In step  590 , optimal peaks, which will result in a low, acceptable PAR are determined from the detected peaks and the optimal peaks are upsampled, interpolated and stored. In the event that an optimal peak is not determined in step  590 , the process of steps  550 ,  560 ,  570  and  580  are re-visited until a combination is determined that yields an optimal peak. 
     After an iterative operation, and as indicated in step  610 , the optimal peak and the original signal stored in step  510  are combined and transmitted. In reference to  FIGS. 12 ,  13 ,  14  and  15  side tone packets injection (STPI) is shown to lower the PAR in single-carrier CAP modulation at the scale of 544 symbols and that a PAR of 1.8 dB is achieved at 10 −4  error probability, which yields a 2 dB PAR improvement at 10 −7 . Also shown is a PAR comparison of CAP and DMT modulations. 
     Although a number of exemplary methods for lowering the PAR have been discussed, other variations of the techniques may be implemented without departing from the spirit or the scope of the present invention. 
     Other embodiments, uses, and advantages of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. The specification and drawings should be considered exemplary only, and the scope of the invention is accordingly intended to be limited only by the following claims and equivalents thereof.