Abstract:
A system that detects and corrects for phase offset between a subscriber and a service provider. The phase offset detection and correction system provides for improved performance of Pulse Code Modulation encoding in the upstream direction.

Description:
FIELD OF THE INVENTION 
   This invention relates to apparatus and methods for compensating for phase offset errors between a service provider and a subscriber. Particularly, the invention concerns apparatus and methods for correcting for phase offset errors between an analog modem and a digital network. 
   BACKGROUND OF THE INVENTION 
     FIG. 4  shows the basic elements of an end-to-end transmission within the Public Switched Telephone Network (hereinafter “PSTN”). The PSTN shown includes first and second Subscribers, first and second Service Providers, and a Switched Digital Network. Analog Subscriber Loops connect the Subscribers to their respective Service Providers, and the Switched Digital Network connects the Service Providers together. The term “Subscriber” refers to any device that transmits information signals over the analog subscriber loop, such as a telephone, voice band modem, ISDN line, Discrete Multi-tone System (e.g. digital subscriber line), or a computer equipped with one of these devices. The term “Service Provider” refers to a device connected between the analog subscriber loop and the Digital Network. Typical Service Providers include Central Offices and Internet Service Providers. 
   The Analog Subscriber Loops are conventional twisted pairs that transport analog signals from the Subscriber Equipment to the associated local Service Provider. At the Service Provider, the analog signals are converted to 64 kbps DS0 digital data streams by a channel unit filter and codec, which together implement a bandlimiting filter followed by subsequent analog to digital conversion using a nonlinear encoding rule. The resulting DS0 streams are transported to their respective destination Service Provider via the Switched Digital Network. 
   At the Service Provider  1 , Subscriber&#39;s  1  loop signal is first bandlimited. The bandlimited analog signal is then sampled at a rate of 8 ksamples/second, and then converted into an 8-bit digital representation using a nonlinear mapping rule referred to as Pulse Code Modulation (“PCM”) encoding. This encoding is approximately logarithmic, and its purpose is to permit relatively large dynamic range voice signals to be represented with only 8 bits per sample. Exemplary PCM encoding schemes include A-law and mu-law encoding; See S. Haykin, “Communication Systems”, John Wiley &amp; Sons, 1983, 2 nd  edition, pages 407–438, the contents of which are incorporated herein by reference. 
   Subscribers  1  and  2  may use a conventional modem to transmit digital data over the configuration of  FIG. 4 . The conventional modem encodes the subscriber&#39;s digital data into a symbol sequence. The symbol sequence is then represented as an appropriately bandlimited analog signal which can be transmitted over the approximately 3.5 kHz bandwidth available on the end-to-end connection. An exemplary modem can include a Digital to Analog converter (i.e. D/A),) an Analog to Digital converter (i.e. A/D), and a hybrid. 
   To increase the rate of data transmission in the upstream, i.e. from the Subscriber to the Service Provider, it has been suggested that the subscriber transmit using PCM encoding. However, using PCM encoding in the upstream direction faces special problems. In particular, the phase offset between the Subscriber and the Service Provider greatly degrades the performance of PCM encoding in the upstream direction. 
   Accordingly, there is a need for an apparatus and method that accounts for the phase offset between the Subscriber and the Service Provider. 
   SUMMARY OF THE INVENTION 
   In accordance with the invention, transmission in the upstream direction is improved by correcting for a phase offset between a subscriber and a service provider. One aspect of the invention provides for a method of sending a training signal from the subscriber to the service provider; receiving the training signal at the service provider; calculating the phase offset between the subscriber and the service provider based upon the received training signal; transmitting the calculated phase offset from the service provider to the subscriber; and pre-adjusting a new signal transmitted from the subscriber to the service provider based upon the transmitted phase offset. The service provider, under this method, is synchronized to a network clock. 
   Another aspect of the invention provides for a subscriber including a digital to analog converter, an analog to digital converter, a clock recovery circuit, and a control element. The digital to analog converter converts a digital signal into an analog signal in preparation for transmission over the analog subscriber loop, while the analog to digital converter converts analog signals received from the analog subscriber loop into a received digital signal. The clock recovery circuit recovers a clock signal from the received digital signal. The subscriber also includes a control element that generates a phase adjusted clock signal by adjusting the phase of the recovered clock signal by a phase offset. The phase offset is based upon the arctangent of a training signal that was modulated by the service provider. The subscriber can correct for the phase offset by synchronizing subsequent conversions by the digital to analog converter with the phase adjusted clock signal. This synchronization to the phase adjusted clock signal causes signals transmitted by the subscriber to be in phase with the network clock when received at the service provider. 
   In a further aspect of the invention, a service provider determines a phase offset between the service provider and a subscriber operably coupled to the service provider. The service provider that determines the phase offset includes a cosine modulator, a sine modulator, and a processor. The cosine modulator modulates a training signal received from the subscriber by a cosine function to generate a signal Rx, and the sine modulator modulates the training signal received from the subscriber by a sine function to generate a signal Ry. The processor then determines the phase offset based upon the arctangent of (Rx/Ry). 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features and advantages of the invention will be apparent from the following description, as illustrated in the accompanying Figures in which like reference characters refer to the same elements throughout the different views: 
       FIG. 1  is a block diagram of a service provider, in accordance with the invention; 
       FIG. 2  is a block diagram of a subscriber, in accordance with the invention 
       FIG. 3  is a flow chart illustrating a method of correcting for phase offset, in accordance with the invention; and 
       FIG. 4  shows a block diagram of a conventional Public Switch Telephone Network. 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a block diagram of a service provider  10 , in accordance with the invention. The service provider  10  determines a phase offset between the service provider  10  and a subscriber  20  of  FIG. 2  operably coupled to the service provider. The service provider includes a modulator  16  and a processor  18 . The modulator  16  includes a cosine modulator for modulating a training signal received from the subscriber by a cosine function to generate a signal Rx. The modulator  16  also includes a sine modulator for modulating a training signal received from the subscriber by a sine function to generate a signal Ry. The processor  18  receives signals Rx and Ry as input from the modulator  18 . The processor then determines the phase offset based upon a computation of the arctangent of (Rx/Ry). 
   The modulator  16  and the processor  18  can be implemented using a data processor, such as a digital signal processor or a microcontroller executing software instructions. Alternatively, the modulator and the processor can be implemented entirely in software executing on a computer, entirely in electrical circuitry, or in a hybrid of software and electrical circuitry. 
   The service provider  10  can also include a hybrid  12 , and A/D converter  14 , and a D/A converter  20 . The A/D  14  is operably coupled between the hybrid  12  and the modulator  16 , while the D/A  20  is operably coupled between the hybrid  12  and the processor  18 . The hybrid  12  operably couples the service provider  10  to the analog subscriber loop. 
   The hybrid  12  can generally be described as a passive device used for converting a dual analog signal that is carried on one pair of conductors (i.e. the analog local loop) to separate analog signals that are carried on two pairs of conductors. Those skilled in the art are familiar with the use and operation of hybrid devices and, thus, a detailed description thereof is not necessary to enable one of skill in the art to make and practice the present invention 
   The D/A  20  converts digital signals to analog signals for transmission over the analog local loop, and the A/D  14  converts analog signal received from the analog local loop to digital signals. The A/D converter and the D/A converter can also be described as capable of implementing a CODEC (coder/decoder) function. The A/D can implement a mu-law CODEC. Those skilled in the art are familiar with the non-linear mu-law and A-law signal compression algorithms. The mu-law algorithm includes 255 discrete signal conversion values; A-law uses 256 values. The broad principles of the invention are not, however, limited to a specific quantization scheme. The A/D  14  can utilize  255  non-uniformly spaced quantization levels, which are closer together for small analog signal values and spread further apart for large signal values, to convert an analog signal received from the analog subscriber loop to one of 255 unique “symbols” or “levels”. 
   The operation of the service provider  10  can be further explained with reference to the following mathematical analysis. In particular, if we assume that the subscriber  20  transmits a training signal, S(t), with all harmonic terms as follows: 
               S   ⁢           ⁢     (   t   )       =       ∑     i   =   1     ∞     ⁢           ⁢     sin   ⁢           ⁢     (     i   *     ω   0     ⁢           ⁢   t     )                 (   1   )             
         where ω 0  is the base frequency.
 
Then the received signal, R(t), at the service provider  10  will be: 
               R   ⁢           ⁢     (   t   )       =         ∑     i   =   1     ∞     ⁢           ⁢       C   i     ⁢           ⁢     sin   ⁡     [     i   *     ω   0     ⁢           ⁢     (     t   -     t   0     -     Δ   ⁢           ⁢   t       )       ]           +     n   ⁢           ⁢     (   t   )                 (   2   )             
   where Δt is a phase offset between the receiving service provider  10  and the transmitting subscriber  20  due to the sampling offset, and   where t 0  is the time delay at the receiving service provider  10  and C i  is constant coefficient.       

   The modulator  16  can then calculate the signals Rx and Ry. In particular, modulator  16  modulates R(t) by a cosine signal to generate Rx and modulator  16  modulates R(t) by a sine signal to generate Ry. That is: 
             Rx   =       ⁢       ∫   T             ⁢     R   ⁢           ⁢     (   t   )     *   cos   ⁢           ⁢     (       ω   0     ⁢           ⁢   t     )     ⁢           ⁢     ⅆ   t                     =       ⁢         ∑     i   =   1     ∞     ⁢           ⁢       ∫   T             ⁢       C   i     ⁢           ⁢     sin   ⁡     [     i   *     ω   0     ⁢           ⁢     (     t   -     t   0     -     Δ   ⁢           ⁢   t       )       ]       ⁢           ⁢   cos   ⁢           ⁢     (       ω   0     ⁢           ⁢   t     )     ⁢           ⁢     ⅆ   t           +     N   ⁢           ⁢     (   t   )                     =       ⁢         C   1     ⁢           ⁢     sin   ⁡     [       ω   0     ⁢           ⁢     (       t   0     +     Δ   ⁢           ⁢   t       )       ]         +   N                 Ry   =       ⁢       ∫   T             ⁢     R   ⁢           ⁢     (   t   )     *   sin   ⁢           ⁢     (       ω   0     ⁢           ⁢   t     )     ⁢           ⁢     ⅆ   t                       =       ⁢         ∑     i   =   1     ∞     ⁢           ⁢       ∫   T             ⁢       C   i     ⁢           ⁢     sin   ⁡     [     i   *     ω   0     ⁢           ⁢     (     t   -     t   0     -     Δ   ⁢           ⁢   t       )       ]       ⁢           ⁢   sin   ⁢           ⁢     (       ω   0     ⁢           ⁢   t     )     ⁢           ⁢     ⅆ   t           +     N   ⁢           ⁢     (   t   )           ⁢                       =       ⁢         C   1     ⁢           ⁢     cos   ⁡     [       ω   0     ⁢           ⁢     (       t   0     +     Δ   ⁢           ⁢   t       )       ]         +   N               
         where 
       T   =       2   ⁢           ⁢   π       ω   0           
       

   The processor  18  can determine the phase offset by calculating the arctangent of (Rx/Ry). In particular, analysis of the above cited equations for Rx and Ry reveals that: 
         (       t   0     +     Δ   ⁢           ⁢   t       )     =         1     ω   0       ⁢           ⁢   arctan   ⁢           ⁢     Rx   Ry       +   n         
         or   Δt=(1/ω 0 )*arctangent (Rx/Ry)+n−t 0 ,       

   If this adjustment in sent from the service provider  10  back to the subscriber  20 , then the subscriber  20  can send a new signal that is pre-adjusted to remove any phase offset at the service provider. In particular, the received signal at the service provider  10  after pre-adjusting in accordance with the invention is: 
         R   ⁢           ⁢     (   t   )       =         ∑     i   =   1     ∞     ⁢           ⁢       C   i     ⁢           ⁢   sin   ⁢           ⁢     (     i   *     ω   0     ⁢           ⁢   t     )         +   noise         
 
     FIG. 2  is a block diagram of a subscriber  20 , in accordance with the invention. The subscriber  20  is operably coupled via an analog subscriber loop to a service provider  10  of  FIG. 1 . A phase offset can exist between the clock in the subscriber  20  and a network clock to which the service provider  10  is synchronized. The subscriber  20 , according to the invention, removes this offset by adjusting its local clock signal. 
   The subscriber  20  includes a D/A converter  22 , and A/D converter  24 , a clock recovery circuit  26  and a control element  28 . The subscriber can also include a hybrid  12  for coupling the A/D and the D/A converters to the analog subscriber loop. The D/A converter  22  converts a digital signal into an analog signal in preparation for transmission over the analog subscriber loop. The A/D converter  24  converts analog signals received from the analog subscriber loop into a received digital signal. The output of the A/D converter  24  is applied to the clock recovery circuit  26 . 
   The clock recovery circuit  26  recovers a clock signal from the received digital signal. The recovered clock signal is applied to the delay element  30 . The clock recovery circuit  26  and the delay element  30  are conventional elements, well known in the art. 
   The control element  28  generates a phase adjusted clock signal by adjusting the phase of the recovered clock signal by a phase offset. Particularly, the control element  28  directs the delay element  30  to adjust the phase of the recovered clock signal output by the clock recovery circuit  26 . The control element determines the extent of the phase adjustment based upon control signals received from the service provider  10 . The service provider  10 , as discussed above, determines the extent of phase adjustment based upon a training signal sent by the subscriber  20 . For example, the phase offset is based upon the arctangent of a training signal that is modulated by the service provider. 
   The subscriber  20  corrects for the phase offset between the subscriber  20  and the service provider  10  by synchronizing subsequent conversions by the D/A converter  22  with the phase adjusted clock signal. This synchronization to the phase adjusted clock signal causes signals transmitted by the subscriber  20  to be in phase with the network clock when received at the service provider  10 . 
     FIG. 3  is a flow chart illustrating a method of improving transmission in the upstream direction by correcting for a phase offset between a subscriber  20  and a service provider  10 . One aspect of the invention provides for a method of sending a training signal from the subscriber (step  42 ), receiving a training signal (step  44 ), calculating the phase offset (step  48 ), transmitting the calculated phase offset (step  50 ), and pre-adjusting a new signal transmitted from the subscriber (step  52 ). 
   The method begins at step  40 , and processing proceeds to step  42 . At step  42 , the subscriber  20  sends a training signal to the service provider  10 . After step  42 , the method proceeds to step  44 . 
   At step  44 , the service provider  10  receives the training signal from the subscriber  20 . 
   At step  46 , the service provider  10  modulates the received training signal by a cosine function to generate the signal Rx. The service provider  10  also modulates the received training signal by a sine function to generate the signal Ry. Processing next proceeds to step  48 . 
   At step  48 , the service provider  10  calculates the phase offset between the subscriber and the service provider based upon signals Rx and Ry generated at step  46 . In particular, the processor  18  determines the phase offset based upon a computation of the arctangent of (Rx/Ry). 
   At step  50 , the service provider  10  transmits the calculated phase offset to the subscriber  20 . 
   At step  52 , the subscriber  20  pre-adjusts a new signal transmitted from the subscriber to the service provider based upon the transmitted phase offset. 
   While the invention has been shown and described having reference to specific preferred embodiments, those skilled in the art will recognize that variation in form and detail may be made without departing from the spirit and scope of the invention. Thus, specific details of the disclosure herein are not intended to be necessary limitations on the scope of the invention other than as required by the prior art.