Abstract:
A digital subscriber line network allows a plurality of remote modems to communicate without interfering with the communication to the central office. Each symbol of a superframe is converted to a tone vector, and the tone vectors are integrated over a plurality of superframes. The tone vectors of the data symbols are random, and tend to cancel each other out. The tone vector of the synchronization symbol remains constant among the plurality of superframes, and the sum of these tone vectors over a plurality of superframes becomes large. By identifying the largest integrated tone vectors, the network may identify the position of the synchronization symbol. The modems may then align using the position of the synchronization symbol.

Description:
TECHNICAL FIELD  
         [0001]    This invention relates to digital subscriber line devices, and more particularly to superframe alignment among multiple modems in a loop.  
         BACKGROUND  
         [0002]    As consumer demand for interactive electronic access to entertainment (e.g. video-on-demand) and information (Internet) in digital format has increased, this demand has effectively exceeded the capabilities of conventional voice-band modems. In response, various delivery approaches have been proposed, such as optical fiber links to every home, direct satellite transmission, and wideband coaxial cable. However, these approaches are often too costly, and cheaper alternatives have emerged, such as the cable modem which uses existing coaxial cable connections to homes and various high bit rate digital subscriber line (DSL) modems which use the existing twisted-pair of copper wires connecting a home to the telephone company central office (CO).  
           [0003]    Significant effort has been expended on utilizing existing telephone copper infrastructure and in-home twisted pair cable for communications beyond what is typically available in the DC to 4 kHz ‘plain-old telephone service’ (POTS) frequency band. Digital Subscriber Line (DSL) and Home Phoneline Networking (HPN) are two examples of such effort. Various DSL standards have been developed, including ANSI T1.413 ADSL specification, ITU G.992.1 full-rate ADSL recommendation and ITU G.992.2 splitterless ADSL recommendation, which enable high-speed communications between a subscriber premise and the central office.  
           [0004]    In some network configurations, two or more remote terminal (RT) DSL modems are physically connected to a single phoneline where either modem may communicate with the central office (CO) DSL modem. What is desired is to enable these remote terminal modems to communicate with each other without interfering with the connection between one of the remote terminal modems and the central office DSL modem.  
         SUMMARY  
         [0005]    A digital subscriber line network allows a plurality of remote modems to communicate without interfering with the communication to the central office. Each symbol of a superframe is converted to a tone vector, and the tone vectors are integrated over a plurality of superframes. The tone vectors of the data symbols are random, and tend to cancel each other out. The tone vector of the synchronization symbol remains constant among the plurality of superframes, and the sum of these tone vectors over a plurality of superframes becomes large. By identifying the largest integrated tone vectors, the network may identify the position of the synchronization symbol. The modems may then align using the position of the synchronization symbol. 
       
    
    
     DESCRIPTION OF DRAWINGS  
       [0006]    These and other features and advantages of the invention will become more apparent upon reading the following detailed description and upon reference to the accompanying drawings.  
         [0007]    [0007]FIG. 1 illustrates a Digital Subscriber Line network having multiple remote terminals according to the present invention.  
         [0008]    [0008]FIG. 2 is a representation of a superframe used to communicate information between DSL modems.  
         [0009]    [0009]FIG. 3A is a graphical representation of the addition of the synchronization symbol tone vectors.  
         [0010]    [0010]FIG. 3B is a graphical representation of the addition of a data symbol tone vectors.  
         [0011]    [0011]FIG. 4 is a flowchart illustrating the process used to integrate tone vectors over multiple superframes according to an embodiment of the present invention.  
         [0012]    [0012]FIG. 5 is a flowchart illustrating the process used to locate the superframe boundary using the integrated tone vectors over multiple superframes according to an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0013]    An exemplary Digital Subscriber Line (DSL) network  100  having multiple remote terminals is illustrated in FIG. 1. The exemplary DSL network  100  includes terminals  105 ,  110 , a telephone  115 , modems  120 ,  125 , telephone line connection plates  130 ,  135 ,  140 , a network interface device  145 , and central office equipment  150 . The modems  120 ,  125  may be RT ADSL modems and share the premise phoneline wiring with standard POTS equipment such as the telephone  115 . Although two modems  120 ,  125  are shown, it can be appreciated that any number of modems may share the premise phoneline wiring. The terminal  105  is electrically connected to the phoneline through the modem  120  and the telephone line connection plate  130 . The terminal  110  is electrically connected to the phoneline through the modem  125  and the telephone line connection plate  135 . The telephone  115  is electrically connected to the phoneline through the telephone line connection plate  140 . The telephone line connection plates  130 ,  135 , and  140  provide for a simple method of connecting the modems to the premise phoneline wiring. The connections used in the telephone line connection plates  130 ,  135 , and  140  are well know in the art and include, among other devices, RJ- 11  jacks. The premise phoneline wiring is terminated at the network interface device  145  which is connected to the local subscriber loop. The network interface device  145  allows connection of the premise phoneline wiring to the central office equipment  150  via the subscriber loop. The subscriber loop is terminated at the central office by the Public Switch Telephone Network and the CO DSL modem.  
         [0014]    ADSL modems use Discrete MultiTone (DMT) modulation for communication. In the time domain, DMT is the equivalent of adding pure tones of differing phase and amplitude over a specific time duration referred to as a symbol. During normal data exchange, the phase and amplitude of each tone changes from one symbol to the next. These changes cause a discontinuity in time at the symbol boundary.  
         [0015]    For multiple remote modems to communicate without interfering with the connection to the central office equipment  150 , the symbols of each modem should be aligned. By aligning the symbols, the impulse response caused by the discontinuity between the symbols may occur at approximately the same time.  
         [0016]    An ADSL modem may communicate using a superframe  200  as illustrated in FIG. 2. A typical superframe  200  has 68 data symbols  205 - 215  followed by a DMT synchronization symbol  220 . The synchronization symbol  220  is used to establish boundaries between superframes  200  and the synchronization symbol  220  carries no user bit level data. Each synchronization symbol  220  in successive superframes  200  uses the same tone phase and amplitude each time they are transmitted. The data symbols  205 - 215  include data that is scrambled prior to being quadrature amplitude modulation (QAM) encoded. Because the data is scrambled, each data symbol  205 - 215  sends a randomized phase and amplitude for each tone in their set. By locating the superframe  200  boundaries, the symbols of each modem not involved in the communications with the central office may be aligned, thus allowing for communication between modems.  
         [0017]    Each of the individual tones in the data symbols  205 - 215  and the synchronization symbol  220  may be represented as a vector. The tones in the synchronization symbol  220  are the same in each superframe  200 , and may be combined using vector addition. FIG. 3A is a graphical representation  300  of the addition of the synchronization symbol  220  tone vectors. In a first superframe  200 , the tones of the synchronization symbol  200  may be represented by a vector  305 . The tones of the synchronization symbol  200  in subsequent superframes may be represented by vector  310 ,  315  and  320 . Because the tones in the synchronization symbol  220  are the same in each superframe  200 , the vectors  305 - 320  are also the same and add upon one another. Thus, after the tones of the synchronization symbol  200  in a plurality of superframes are added, a single summed synchronization tone vector  322  is created.  
         [0018]    [0018]FIG. 3B is a graphical representation of the addition of a data symbol  305  tone vectors across multiple superframes  200 . As stated above, the data symbol  205  includes data that is scrambled prior to being QAM encoded. Because the data is scrambled, the data symbol  205  in a first superframe  200  may send a tone having a first phase and amplitude vector  330 . In another superframes  200 , the data symbol  205  may send a tone having a second phase and amplitude vector  335 . Because the data is scrambled, it is likely that the second vector  335  differs from the first  330 . Subsequent superframes  200  may include the data symbol  200  sending a tone having vectors  340 ,  345 , and  350 . The tone vectors  330 - 350  across multiple superframe may be combined to produce a summed symbol vector  355 . Because the vectors  330 - 350  differ, the summed symbol vector  355  is likely to be much smaller than the summed synchronization tone vector  322 . Thus, if all the symbol tone vectors are combined over a plurality of superframes  200 , the symbol having the largest summed vector is likely to be the synchronization symbol  220 .  
         [0019]    [0019]FIG. 4 is a flowchart illustrating the process  400  used to integrate tone vectors over multiple superframes  200 . The process  400  begins at a START block  400 . Proceeding to block  410 , the process receives a new symbol as part of a first superframe  200 . Each symbol has a symbol count to uniquely identify which symbol of the current superframe  200  is being sampled.  
         [0020]    Proceeding to block  415 , the tone vectors of the current symbol are determined. The process  400  then proceeds to block  420  where the tone vectors are added to an integrated total for the current symbol. The integrated total represents the summation of each of the tone vectors for the current symbol location in each of the preceding superframes. If the current superframe  200  is the first to be sampled, the integrated total would become the value of the current tone vectors. Thus, after multiple superframes  200  are sampled, the integrated total is representative of the summed vector  355 .  
         [0021]    Proceeding to block  425 , the process check if the current symbol is the final symbol in the superframe  200 . If there are more symbols in the superframe  200 , the process  400  proceeds along the NO branch to block  430 . In block  430 , the symbol count is incremented to indicate the next symbol is to be processed. The process  400  then returns to block  410  to receive the next symbol for vector summation.  
         [0022]    Returning to block  425 , if there are no further symbols in the current superframe  200 , the process  400  proceeds along the YES branch to block  435 . In block  435 , the symbol count is reset in the event more superframes  200  need to be counted.  
         [0023]    Proceeding to block  440 , the process  400  determines if more superframes  200  are to be summed. The number of superframes to sum prior to completing the process  400  may be predetermined, adjusted based on historical data, or set in any other matter. If more superframes  200  are to be added, the process  400  proceeds along the YES branch back to block  410  to receive the first symbol of the next superframe  200 . Returning to block  440 , if no additional superframes  200  are to be summed, the process  400  proceeds along the NO branch to terminate in an END block  445 .  
         [0024]    [0024]FIG. 5 is a flowchart illustrating the process  500  used to locate the superframe boundary using the integrated tone vectors over multiple superframes  200  acquired according to the process  400  in FIG. 4. The process  500  begins at a START block  505 . Proceeding to block  510 , the process  500  resets the symbol count counter and a memory location labeled MaxPower. The memory location MaxPower stores the value of the largest summation vector sampled to date.  
         [0025]    Proceeding to block  515 , the process  500  retrieves the integrated total of the symbol based on the current value of the symbol count. The integrated total represents the value of the summed vectors for the current symbol count from all the sampled superframes  200 .  
         [0026]    Proceeding to block  517 , the process  500  converts the integrated total to a power value. The sum of the power value is equivalent to the sum of the absolute values of the summed vectors for the current symbol, and may be defined as the power figure for the specific symbol position of the superframes.  
         [0027]    Proceeding to block  520 , the process  500  compares the symbol power figure to the value of MaxPower to determine if the symbol power figure is greater than the value of MaxPower. For the first symbol, the value of MaxPower is zero, and thus the first symbol should always be greater. For each subsequent symbol, having a symbol power figure greater than the value of MaxPower indicates that the value of the summed vectors from the current symbol count is larger than each of the previous summed vectors. If the value of the symbol power figure is not greater than MaxPower, the process  500  proceeds along the NO branch to block  530 . If the value of the symbol power figure is greater than MaxPower, the process  500  proceeds along the YES branch to block  525 . In block  525 , the value of MaxPower is set to the value of the current symbol power figure. This creates a new standard to which each subsequent symbol power figure may be compared. The process  500  also sets a boundary flag to the current symbol count to indicate the symbol count that resulted in the changed value of MaxPower. Of course, the boundary flag indicated the symbol count of the largest summed vector.  
         [0028]    Proceeding to block  530 , the process  530  determines if additional symbols are available for comparison. Because the superframes  200  have a predetermined number of symbols, the process  500  may simply compare the symbol count to a predetermined number to determine if additional symbols are available. If additional symbols are available, the process  500  proceeds along the YES branch to block  535 . In block  535  the process increments the symbol count and then returns to block  515  to retrieve the integrated total for the new symbol position. The process  500  remains in this loop until the power figures for each symbol position is compared.  
         [0029]    Returning to block  530 , if no additional symbols are present, the process  500  proceeds along the NO branch to block  540 . In block  540 , the process  500  indicates that the superframe boundary is located at the position of the boundary flag. The boundary flag was updated each time the integrated total exceeded the value of MaxPower. Because the summed synchronization vector  322  should have the largest power figure, the boundary flag indicates the symbol position of the synchronization symbol  220 . This position may be communicated to each remote modem, thereby enabling communication among the modems without interfering with the connection to the central office. The process  500  then terminates in an END block  545 .  
         [0030]    Numerous variations and modifications of the invention will become readily apparent to those skilled in the art. Accordingly, the invention may be embodied in other specific forms without departing from its spirit or essential characteristics.