Abstract:
A shared hybrid circuit for use in provisioning a communication system. In the method and apparatus of the present invention, a single shared hybrid separates transmitted and received signals for multiple transmission pathways between the distribution equipment and the customer premises equipment. In addition, the shared hybrid circuit of the present invention provides a method and apparatus to cancel a component of a downstream broadcast signal from a composite upstream/downstream signal detected at a node, thereby providing a significant reduction in echo and for the upstream transmission from a customer premises equipment transmitter. The present invention results in a significant cost savings over conventional hybrid circuits since a single shared hybrid can be used in provisioning service for many subscribers.

Description:
[0001]    This application is related to application Ser. No. 10/137,624, entitled Digital Subscriber Head End filed on May 2, 2002, which by this reference is incorporated herein for all purposes. This application is also related to application Ser. No. 10/376,407, entitled Impedance-Matched Interface for Broadband Data Service Provisioning filed on Feb. 28, 2003, which by this reference is incorporated herein for all purposes. 
     
    
     
       BACKGROUND  
         [0002]    1. Technical Field  
           [0003]    The present invention relates generally to broadband data communication systems. More specifically, the present invention provides an improved method and apparatus for efficient provisioning of broadband data services using an improved hybrid circuit.  
           [0004]    2. Background  
           [0005]    Most of the current systems for providing broadband Internet access are complex and expensive to deploy. As a result, the deployment of many broadband services, particularly digital subscriber line (DSL) service, has fallen far short of expectations.  
           [0006]    Many broadband service systems, such as DSL, are based on the same telephone subscriber loop that is used to provide “Plain Old Telephone Service (POTS)” and generally coexists with POTS service on the same twisted pair cable, offering simultaneous analog/digital services. In current systems for provisioning DSL, a digital subscriber line access multiplexer (DSLAM) is deployed at the central office (CO) and a relatively high power signal is transmitted over an F1/main feed distribution network that provides service to various subscriber groups.  
           [0007]    A “hybrid circuit” serves as the interface between the DSL system and the two-wire copper telephone line. The hybrid circuit serves two main purposes: 1) to interface the analog front-end (AFE) to the line (via the coupling method), and 2) to separate transmit (Tx) and receive (Rx) signals between the various system components. Basically, the hybrid circuit operates as a differential amplifier in the Tx direction and a passive network in the Rx direction.  
           [0008]    One of the major problems that must be overcome in the operation of a broadband system is the cancellation of Tx “echo” signals that appear in the Rx path. Conventional solutions to this problem require a separate hybrid circuit for each transmission pathway between the broadband distribution equipment and the customer premises equipment (CPE), thereby increasing the cost of provisioning broadband service. Moreover, in systems where the downstream broadband signal is continuously broadcast, the upstream transmissions from the CPE transmitter are degraded by the broadcast transmit signal.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention overcomes the shortcomings of the prior art by providing an improved hybrid circuit for use in provisioning a communication system, such as a broadband system. The shared hybrid circuit of the present invention results in a significant cost savings over conventional hybrid circuits since a single shared hybrid can be used to separate transmitted and received signals for multiple transmission pathways between the broadband distribution equipment and the customer premises equipment. In addition, the shared hybrid circuit of the present invention provides a method and apparatus to cancel a component of a downstream broadcast signal from a composite upstream/downstream signal detected at a node, thereby providing a significant reduction in echo and an increase in the quality of the upstream transmission from a customer premises equipment transmitter.  
           [0010]    In one embodiment of the present invention, a summing circuit is operable to detect a composite signal comprising a downstream broadcast signal from broadband distribution equipment and an upstream transmission from a CPE transmitter. A duplicate of the downstream broadcast signal component is provided to the summing circuit and is subtracted from the composite signal detected at the node. The resulting signal is transmitted upstream to the broadband access manager. The method and apparatus of the present invention is operable to handle upstream signals from multiple CPE transmitters on multiple pathways using a plurality of switches controlled by a timing control to sequentially detect composite upstream/downstream composite signals at a plurality of nodes on the various transmission pathways.  
           [0011]    For purposes of illustration, some aspects of the present invention will be described in connection with a particular broadband service, such as DSL. The advantages described herein, however, can be used to reduce cost and improve performance for many other systems for providing broadband services to subscribers.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    A better understanding of the invention can be obtained when the following detailed description of various exemplary embodiments is considered in conjunction with the following drawings.  
         [0013]    [0013]FIG. 1 is a system diagram illustrating an embodiment of a prior art distribution area.  
         [0014]    [0014]FIG. 2 is a system diagram illustrating a prior art embodiment of a DSL system for downstream and upstream broadcasting for a plurality of users.  
         [0015]    [0015]FIG. 3 is a system diagram illustrating an embodiment of a broadband distribution system showing the broadband service distribution equipment connected to a cross-connect box in a subscriber distribution area.  
         [0016]    [0016]FIG. 4 is a system diagram illustrating direct tap interconnections within a cross-connect box for connecting broadband distribution equipment to provide broadband services to subscribers in the distribution area.  
         [0017]    [0017]FIG. 5 is a system block diagram of a DSL distribution system utilizing an embodiment of a shared hybrid circuit in accordance with the present invention.  
         [0018]    [0018]FIG. 6 is a general illustration of the impedances for the F1 and F2 distribution cables relative to the serving area interface cross-connection box in the subscriber distribution area.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]    [0019]FIG. 1 is a system diagram illustrating an embodiment of a prior art distribution area  100  for providing broadband service to a plurality of subscribers. A central office  102  provides an F1/main feed distribution that may be employed to service different subscriber groups. In the illustration of FIG. 1, the F1/main feed provides connectivity to a number of serving area interface (SAI) cross-connect boxes  110 ,  112 , . . . , and  114 . Each of the cross-connect boxes  110 ,  112 , . . . , and  114  provide servicing via F2/distribution cables to subscriber groups/neighborhoods  116 ,  118 , . . . , and  120 , respectively. One or more of the cross-connect boxes  110 ,  114 , . . . , and  116  may employ a next generation digital loop carrier (NG-DLC)  108 .  
         [0020]    [0020]FIG. 2 is an illustration of a prior art architecture for providing DSL service from a cross-connect box to a plurality of end Users A, B, . . . N. Downstream transmission for Users A, B, . . . N is illustrated by arrows T A , T B  and T N , respectively. Upstream transmission for Users A, B, . . . N is illustrated by arrows “A,” “B” and “N,” respectively. Referring to the illustration for User A, the twisted pair  202   a  at the user site is connected to a hybrid connector  204   a  that provides interconnection of shielded copper twisted pair wires with other transmission media within the DSL distribution network. Data transmitted downstream is received by the digital-to-analog converter  206   a  and is then passed through the transmitter filter  208   a  and through the line driver  210   a  to the hybrid connector  204   a  and finally to the User A twisted pair  202   a . Data transmitted upstream passes from the User A twisted pair  202   a  through the hybrid circuit  204   a  to the receiver function  212   a . The data is then transmitted to the receiver filter  214   a  and the analog-to-digital converter  216   a  to the DSL network. In the prior art architecture illustrated in FIG. 2, it is necessary to provide duplicate system components for each of the end users. The various architectures of the present invention, discussed in greater detail below, allow a significant reduction in the number of components needed to provide DSL service to a plurality of users.  
         [0021]    [0021]FIG. 3 is a system diagram illustrating an embodiment of a distribution area  300  that is configured in accordance with the present invention. A central office  302  provides an F1/main feed cable to distribution points within the distribution area  300 . The distribution points typically include cross-connect boxes, shown as cross-connect box  310 , cross-connect box  312 , and cross-connect box  314 . The cross-connect boxes connect the F1 main feed cables to F2 distribution cables that provide service to a large number of subscribers, shown as subscriber(s)  316 , subscriber(s)  318 , . . . , and subscriber(s)  320 .  
         [0022]    In the embodiment shown in FIG. 3, broadband distribution equipment is connected to each of the cross-connect boxes  310 - 314 . For example, broadband distribution equipment  311  is attached to the cross-connect box  310 . The broadband distribution equipment  311  is operable to provide broadband service to the subscriber(s)  316 . As will be described in greater detail below, the interconnections of the broadband distribution equipment within each of the cross-connect boxes can be performed by “tapping off” each active F1/F2 pair within the cross-connect loop. In some embodiments, F2/distribution cable pairs are communicatively coupled to each subscriber even though only a fraction of the connections are actually used at the time the broadband distribution equipment is installed. Since the subscriber pairs are already connected, subsequent users can be provided with broadband service remotely, without the need for disrupting existing service.  
         [0023]    By using the configuration illustrated in FIG. 3, broadband service capabilities may be offered to the subscriber(s)  316 ,  318 , . . . , and  320  without a radical overhaul of the system&#39;s communication hardware or significant man-hours to enable those services. Moreover, the broadband service can be provided with far less power than is currently required using broadband distribution equipment that is connected to the distribution network at the central office.  
         [0024]    Broadband signal transmission to the broadband distribution equipment  311 ,  313 , . . . ,  315  at the cross-connect boxes  310 ,  312 , . . . , and  314  can be provided via broadband data transmission equipment  306  that can be implemented in a number of different configurations. For example, the broadband data can be transmitted to the broadband distribution equipment  311 ,  313 , . . . ,  315  using dedicated cables in the F1 main feed to transport Ti or other broadband service, as illustrated by the pathway  307 . In this embodiment, a predetermined number of cable pairs in the F1 bundle are dedicated for broadband data transmission. In addition, some of the F1 cable pairs can be dedicated to provide power to the broadband distribution equipment and/or can be used by the hybrid circuit, as described in greater detail below. The broadband data bandwidth carried over the F1 is aggregated and distributed to subscribers by the broadband distribution equipment  311 ,  313 , . . . ,  315 . Alternatively, the broadband data can be transmitted to the broadband distribution equipment  311 ,  313 , . . . ,  315  using a separate transmission pathway illustrated by reference numeral  308 . The separate transmission pathway can be implemented using a number of techniques known in the art, including fiber optic media or point-to-point radio transmission.  
         [0025]    [0025]FIG. 4 is a system diagram illustrating an embodiment of interconnections between the F1 and F2 cables and the broadband distribution equipment  400 . In one embodiment, the F1 cables can be connected directly to the broadband distribution equipment  400  as illustrated by the connection of terminals  410  and  412 . The F1 terminals  410  and  412  are also connected to F2 terminals  411  and  413  that correspond to subscribers. Alternatively, the various F1 cables can be connected to the F2 cables, which are further connected to the broadband distribution equipment  400 . For example, the F1 cable terminals  414  and  416  are shown connected to F2 cable terminals  418  and  420 , respectively, which are further connected to the broadband distribution equipment  400 . In each of the embodiments discussed above, the broadband distribution equipment  400  is “tapped” to the respective F1/F2 connections resulting in a parallel impedance relationship that will be discussed in greater detail below.  
         [0026]    As was discussed above, each of the F1 cables can be connected to respective F2 terminals and to broadband distribution equipment  400  even though the customer premises equipment corresponding to a particular F2 terminal may not be activated at the time the connection is initially established. Various users can subsequently be provided with DSL service by remotely activating the broadband service without the need to have a technician physically return to the cross-connect box, thereby reducing the cost of provisioning DSL service.  
         [0027]    [0027]FIG. 5 is an illustration of an architecture for delivering DSL service using multipoint downstream broadcast and frequency and/or time-division multiplexing for upstream transmission. Data transmitted downstream is received by the digital to analog converter  506  and is then passed through the transmitter filter  508 . The downstream transmitted data is carried on transmission line  507  and distributed to each of the users A, B . . . N, designated by CPE&#39;s  502   a ,  502   b , . . .  502   n , along the transmission path illustrated by the arrows labeled “T.” For example, downstream data for user A, illustrated by customer premises equipment (CPE)  502   a , travels through line driver  510   a  and is coupled through line coupling  504   a  and SAI  509   a  and is then distributed on a transmission line  511   a  comprising F2 cable bundles. Upstream data transmitted from CPE  502   a  is generated in CPE transmitter, designated in FIG. 5 by CPE 1 TX which is transmitted through transmission line  511   a  and SAI  509   a  to the line coupling  504   a.    
         [0028]    A shared hybrid circuit  500  receives a composite signal at Node A comprising the combination of a downstream broadcast signal&#39;s echo from driver  510   a  and an upstream transmission from CPE 1 Tx. The downstream component of the composite signal detected by the shared hybrid at node A comprises gain GI which is defined by the transmit voltage, Vtx, at the output of driver  510   a , the source impedance of driver  510   a , illustrated by Z Tx1 , and by the combined impedance of all of the components in the transmission pathway between Node A and the customer premises transmitter CPE 1 Tx. The upstream component of the composite signal detected at Node A is characterized by gain GR 1  which is defined by the upstream signal voltage, VCPE 1 , the combined impedance of all of the components between Node A and the CPE transmitter CPE 1 Tx, and by the source impedance, Z Tx1 , of the driver  510   a . The composite signal at node A is transmitted to the shared hybrid circuit  500  through switch  522   a  which is controlled by a receiver select control  524 . The broadband distribution system illustrated in FIG. 5 can operate in a time-division multiplexing mode by controlling the operation of the switches  522   a ,  522   b , . . . ,  522   n  to coordinate the arrival at the summing circuit  526  of the upstream transmissions from the CPE transmitters CPE 1 TX, CPE 2  TX, . . . , CPE N  TX.  
         [0029]    The summing circuit  526  also receives a compensation signal equal to the magnitude of the downstream broadcast signal via line driver  510   x  that is provided as a negative input to the second input port of the summing circuit. The magnitude of the compensation signal that is provided to the negative input of the summing circuit  526  is defined by a transfer function “H” as a function of Vtx, the source impedance of the driver  510   x , Z TXX  and by the compromise impedance connected to the second input of the summing circuit as discussed below. The output of the summing circuit  526  is provided to receiver driver  512  and the signal is transmitted upstream via receiver filter  514  and analog to digital converter  516 .  
         [0030]    The magnitude of the downstream Tx signal seen at node A is determined by a voltage divider relationship characterized by Vtx, Vcpe N , the source impedance line driver  510   a  (Z Tx1 ), and the combined impedance of the line components between the CPE  502   a  and Node A. To ensure that the component of the downstream transmit signal provided to the negative input of the summing circuit  526  correctly compensates for the component of the transmit signal seen at Node A, a compensating impedance is connected to the output of the line driver  510   x . This compensating impedance is defined by the combination of the impedance of the line coupling  504   x  and one or more of a plurality of compromise impedances, Z 1 , Z 2 , . . . , Zm. The value of the compromise impedance is selected such that the combination of the impedance of the line coupling  504   x  and the impedance provided by one or more of the compromise impedances Z 1 , Z 2 , . . . , Zm will be approximately equal to the combined impedance of the elements between the CPE  502   a  and the Node A as discussed above. A calibration control  528  can be used to calibrate the shared hybrid to ensure that the proper compromise impedance is selected to optimize performance for each of the transmission pathways for CPEs  502   a ,  502   b , . . . ,  502   n . The switch  530  at the negative input to the summing circuit  526  is closed during normal operation of the shared hybrid circuit  526 , but is opened when the calibration procedure is implemented.  
         [0031]    The composite signal at the output of the receiver driver  512  is determined by the equation:  
         [V CPE N *G RN +V TX *(G N −H)] 
         [0032]    The term (G N −H) will be driven toward zero if the transfer functions of the downstream broadcast signal and the gain of the signal provided to the negative input of the summing circuit  526  are approximately equal. If this result is achieved, the echo effects of the downstream broadcast will be essentially eliminated.  
         [0033]    [0033]FIG. 6 is a generalized illustration of the equivalent impedances resulting from line lengths of the F1 and F2 distribution cables connected to the SAI  409   n  and cross-connect box  310   n  in the subscriber distribution area. The SAI  409   n  has a source impedance Z s . The impedance of the portion line from the SAI  409   n  to the central office  302  is Z 1 . The impedance of the portion of the line from the SAI to the customer premises equipment of the subscriber  402   n  is Z 2 .  
         [0034]    In some operating environments, the length of the F1 cables may be relatively short, thereby creating a situation where the echo rejection is degraded due to the impedance characteristic of the F1 cable bundle, thereby resulting in undesired signal errors. In one embodiment of the present invention, this problem is solved by attaching an additional available F1 cable pair in parallel with the compromise impedance. Referring again to FIG. 5, the shared hybrid circuit is shown to comprise an SAI termination  509   x  that is connected to F1 cable pairs  530  that can be used to generate a supplemental impedance to reduce error signals. The SAI termination  509   x  is also connected to a compromise impedance Z 0  that approximates the impedance of the components between the SAI termination and the CPEtx in the various transmission pathways. For example, Z 0  can be used to approximate the combined impedances of the Tx Line  511   a , and the CPE  502   a . The SAI termination  509 × and F2 cable pairs  350  can be selectively connected to the line coupling  504   x —either alone, or in combination with compromise impedances Z 1 , Z 2 , . . . , Zm—to generate an appropriate impedance to compensate for impedance problems associated with short F1 cables as discussed above.  
         [0035]    In view of the above detailed description of the invention and associated drawings, other modifications and variations will now become apparent to those skilled in the art. It should also be apparent that such other modifications and variations may be effected without departing from the spirit and scope of the invention.