Patent Abstract:
The present invention encompasses a method and apparatus for detecting and characterizing one or more ingress events in the return band and creating a time/frequency map based on the detection of the ingress events. The time/frequency map is further characterized by marking frequency bands with ingress level above a predetermined threshold with a “1”. The time/frequency time map may be used to determine when and which return frequency band has exceeded a predetermined threshold, and to distinguish between a narrowband ingress event and a wideband ingress event. When such characterization has been made, the return path may be attenuated or disconnected at a communications gateway device located at or substantially near the subscriber location. By disconnecting the return path or attenuating the return path signal at or near the subscriber location, the ingress may be reduced and locations which are the cause of severe ingress may be effectively isolated. This allows for a high degree of reliability to be maintained on the return path, and ensures that the critical services such cable telephony may be provided with increased customer satisfaction.

Full Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation-in-part of U.S. patent application Ser. No. 09/074,851, now U.S. Pat. No. 6,321,384, having a filing date of May 8, 1998, entitled “Noise reduction in cable return paths”, which is a continuation of U.S. patent application Ser. No. 08/347,573, filed on Nov. 30, 1994, now abandoned. 
   This application also claims the benefit of U.S. Patent Application Ser. No. 60/138,933, entitled “Two-way cable system home ingress monitoring,” filed on Jun. 11, 1999. 

   BACKGROUND OF THE INVENTION 
   Cable TV systems, also known as Community Access Television (CATV) Systems, have evolved from simple broadcast systems for television services to bi-directional broadband networks which can carry voice, video and data traffic. This evolution has been accomplished in part by upgrading traditional cable networks to Hybrid Fiber Coaxial (HFC) networks which utilize fiber optic systems in conjunction with active electronics and coaxial cables to deliver broadband signals to the home. These networks also support the reception of return path signals, which are signals generated by units in or near the subscriber and which send data or voice signals from the subscriber or business location to the network through the cable system. 
   By providing advanced telecommunications services over HFC networks, network operators can enhance their service offerings to include voice, Internet access, and other new and unique multimedia services. Although some of these services may have minimal requirements for the transmission of data from the residence to the head-end and the network, many applications require the reliable transport of data and, in fact, need to have guaranteed and reliable bandwidth. 
   For example, voice communications can be carried over an HFC network based either on traditional circuit switched technology or emerging Internet Protocol (IP) standards. In either of these transport modes, an unreliable and noisy return path can cause degradation in service and even loss of an active call. 
   Because the configuration of the cable system is multipoint-to-point from the subscribers to the head-end, the return path has the undesirable characteristic of accumulating or “funneling” noise towards the head-end. The number of subscribers connected to the network is typically greater than 500, and many subscribers can have power dividers (splitters) installed in their homes to allow connection of multiple settops to the cable network. The result of the large number of subscribers and the multiple connections in the home is that there are a large number of points on the cable network where undesirable signals can enter the return path. The commonly used term for undesirable signals on the cable return path is ingress. Ingress is typically, but not limited to, AM short-wave broadcast signals and industrial and atmospheric noise, which can enter on the drop cable connecting the subscriber to the cable plant connection termed the tap, and via the coaxial wiring in the subscriber residence or business location. The coaxial wiring used in the home may be of low quality, and will allow ingress because of the low amount of shielding provided with respect to high quality coaxial cable which has a dense braided wire shield which provides high isolation of the center conductor from external electromagnetic fields. The coaxial wiring in the home is also typically unterminated, and can act as an antenna since currents generated on the outside of the shield can to some extent couple to the inside of the shield at the unterminated end and subsequently excite the center conductor. The accumulation of noise on the return path has adversely limited the use of the return path for many purposes. 
   For the foregoing reasons, there is a need for a method to detect and characterize ingress on the return path and take appropriate action based on this characterization. 
   SUMMARY OF THE INVENTION 
   The present invention encompasses a method for detecting the presence of return path ingress, and characterizing the detected ingress, and then mitigating the return path based on the characterization. This characterization may occur at a location inside or near the subscriber residence or business, or at a test point in the network, or at a head-end. For example, this location may be a communications gateway near or at the subscriber location. Alternatively, this location may be a telephone test point (TTP) in the network, or a Cable Modem Termination System (CMTS) at the head-end. 
   In one embodiment, each ingress event may be monitored over a pre-determined time interval, a corresponding frequency band may be measured, and a time/frequency map may be created indicating the time and frequency band of each ingress event. Then, the time and frequency map may be analyzed to determine when the events occurred and the frequency band where ingress events have been received. Furthermore, the time/frequency map may be used to determine the characteristics of each ingress event, e.g., whether it was a wide-band ingress event or a narrow-band ingress event. The ingress characteristics assist in determining if signals from a residence should be attenuated, or if the home should be disconnected from the return path. 
   The information from the time and frequency may be used alone or in conjunction with other network management information to determine offending residences, and determine the appropriate action which should take place, including dispatching of a craftsperson to look for broken cable shielding or to determine if there are radio frequency transmitters such as amateur radio systems which are the source of the ingress. 
   There are a plurality of ways to collect this time/frequency map information in accordance with the present invention. In one embodiment, the entire return band may be monitored to measure individual sub-bands within the return path spectrum, and to create a map representing the time and frequency dependence of the ingress events. Then, the time/frequency map may be used to distinguish narrowband ingress events from wideband ingress events. With the help of time/frequency map, harmful ingress events may be more readily detected, and mitigating actions may be taken more readily. The mitigation may be accomplished by either disconnecting the return path at the communications gateway or attenuating the return path signal. 
   In another embodiment, the information on channel usage may be obtained and used to distinguish active sub-bands from inactive sub-bands. The presence of ingress events may be detected in either the active or inactive sub-bands, and a time/frequency map may be created based on the detected ingress events. The channel usage information may be retrieved from the head-end that can include network management equipment and databases that have channel information available. Alternatively, the channel usage may be detected automatically at the communications gateway. Automatic detection can be accomplished by estimating the Power Spectrum Density (PSD) of a return path signal, correlating the PSD with a set of stored PSDs and determining the peak correlation frequency and frequency band in use. 
   The principles of the present invention are flexible and the detection of the ingress can occur at the head-end or may occur within the communications gateway. The detection of ingress may be accomplished by measuring an average return path signal power in the return frequency band and comparing the average return path signal power to a detection threshold. Based on the comparison, a determination that an ingress event is present is declared when the average power exceeds the detection threshold. 
   These and other features and objects of the invention will be more fully understood from the following detailed description of the preferred embodiments which should be read in light of the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and form a part of the specification, illustrate the embodiments of the present invention and, together with the description serve to explain the principles of the invention. 
     In the drawings: 
       FIG. 1  illustrates a two-way cable architecture for providing telecommunications services; 
       FIG. 2A  illustrates a communications gateway architecture; 
       FIG. 2B  illustrates a communications gateway architecture with external RF module; 
       FIG. 3A  illustrates a system for acquisition and processing of return path monitoring signals; 
       FIGS. 3B ,  3 C, and  3 D illustrate acquisition stages; 
       FIG. 4  illustrates full-band ingress monitoring; 
       FIG. 5  illustrates sub-band ingress monitoring; 
       FIG. 6  illustrates sub-band ingress monitoring with channel use information; 
       FIGS. 7A and 7B  illustrate active band monitoring; 
       FIG. 8  illustrates a flow chart for channel usage detection; 
       FIG. 9  illustrates an exemplary time/frequency map. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   In describing a preferred embodiment of the invention illustrated in the drawings, specific terminology will be used for the sake of clarity. However, the invention is not intended to be limited to the specific terms so selected, and it is to be understood that each specific term includes all technical equivalents which operate in a similar manner to accomplish a similar purpose. 
   With reference to the drawings, in general, and  FIGS. 1 through 9  in particular, the method of the present invention is disclosed. 
     FIG. 1  illustrates a bi-directional cable system comprising a head-end  100 , a network interface which is represented herein by a Cable Modem Termination System (CMTS)  105  and which may be connected to a channel usage information database  115 . Use of the CMTS  105  to represent the network interface is not a limitation to the present system. The present system can use any network interface for receiving upstream signals sent by a modem over the HFC network. The head-end  100  contains return path receiving equipment which can receive signals from fiber optic cables  108 . In a preferred embodiment, separate cables are used for the transmission of fiber optic signals to a node  120  and for the reception of fiber optic signals transmitted from node  120  to head-end  100 . Return path receiving equipment and methods of detecting return path signals at cable head-ends are well known to those skilled in the art. 
   From node  120  signals are transmitted over coaxial cable  109  through active amplifiers  125 . Signals propagating down a coaxial cable  109  are intercepted by tap  127  which routes a portion of the signal to a Communications Gateway (CG)  140  located at or near a residence  152 . A drop cable  221  is used to connect tap  127  to CG  140 . In a preferred embodiment, drop cable  221  is a coaxial cable. When used herein, the term “communications gateway” (CG) refers to a device for transmitting and receiving data, voice or video signals over an HFC network. Alternative terminology for a communications gateway includes a Broadband Terminal Interface (BTI) or Coaxial Termination Unit (CTU). The term “communications gateway” is not intended to be limiting and encompasses equipment which is located on the outside of the home, in the home in a centralized location such as attic, basement or equipment closet, or in another location in the home. Businesses can also use communications gateway type devices for the transmission and reception of data, voice or video signals. 
   As illustrated in  FIG. 1 , the residence  152  can also contain a set-top box (STB)  155  which is typically connected to a television  150  and a PC  135  which can contain a cable modem. These units are typically connected through a splitter  145  to the communications gateway  140  which is coupled to the HFC network. 
   A telephone  147  can be supported by communications gateway  140  which provides traditional voice services by transmitting and receiving telephone signals and converting them to cable compatible signals. Telephone service supported by the CG  140  may be circuit switched or Internet Protocol (IP) based telephone service. 
   As shown in  FIG. 1 , signals from the CMTS can be routed to the Internet/private network  110 . These signals can be in IP format but may also be carried as circuit switched signals. When carried as circuit switched signals, the network used is the traditional Public Switch Telecommunications Network (PSTN). 
   A network management system  160  may also be connected to the Internet/private network  110  and may also be able to access an ingress database  165 . Ingress database  165  is used to record ingress events and can be used in conjunction with network management system  160  to establish the thresholds that indicate the presence of unacceptable levels of ingress in residence  152 . The network management system  160  can also provide alarm readouts, trends or performance reports to a network operator. In one embodiment, the network management system  160  retrieves spectrum data from CG  140  and performs fault detection analysis on the retrieved spectrum data. The network management system can also store the spectrum data in a database for historical trending or other temporal analysis. 
     FIG. 2A  illustrates an architecture for CG  140 . In one embodiment, CG  140  contains an RF module  200  which is capable of receiving signals from the drop cable  221  connected to tap  127  and transmitting those signals to an in-home cable  223  which is typically connected to the splitter  145 . A bidirectional coupler  210  is used to couple signals from drop cable  221  to a cable modem (CM) port on CG board  220 . The RF module permits the transmission of downstream signals to devices in the home and at the same time allows return path signals to be transmitted from STB  155 , PC  135 , or other devices in the home which support bi-directional communications over the HFC network. Simultaneously, the CG  140  contains a communications gateway board (CGB)  220  which can support additional voice services and receive signals from the drop cable  221  through use of coupler  210 . 
   The CG  140  can support telephone services through use of a Network Interface Module (NIM)  230  which supports phone services through a series of RJ11 jacks  231 . In a preferred embodiment, the communications gateway  140  also has a data interface which provides data services through RJ45 jack  232 . 
   Referring to RF module  200  illustrated in  FIG. 2A , the module contains a diplexer  205  which is used to separate out forward pass signals which are typically in the 50 to 750 MHz range of the spectrum from return path signals, which are typically in the 5 to 42 MHz portion of the spectrum. Diplexing of signals on bidirectional HFC plants is well known to those skilled in the art. 
   In a preferred embodiment, the RF module  200  contains a forward path attenuator  209  which is used in conjunction with an amplifier  207  to provide the appropriate levels in the forward path. 
   In the return path, a passive monitoring tap  202  is used to extract a portion of the return path signal arriving on in-home cable  223 . The extracted portion is then directed towards the CGB  220  where a monitoring function is performed. An attenuator/switch  203  is used in the RF module  200  to attenuate or remove return path signals when ingress from that location is determined to be unacceptable. In a preferred embodiment, an amplifier is also present in the return path allowing control of signal levels subsequent to the attenuator/switch  203 . An output of a downstream DC  204  can be used to capture ingress that has entered the cable drop within the frequency range 5-42 MHz as shown in FIG.  2 A. Ingress detected from this output can be reported to the network management system  160  or to Communication gateway element management. DC  204  is located on the line between DC  210  and CGB  220 , rather than between amplifier  207  and diplexer  205  to isolate drop-ingress from home ingress amplified by amplifier  207 . A switch  206  is placed at the ingress monitoring input of CGB  220  to time-sequence the outputs of DC  204  and  202 . The downstream DC  204  allows detecting of wideband ingress, which typically enters the plant after the CG  140 . 
   In one embodiment, the attenuator is realized through the use of an electrically controlled variable resistor. In an alternate embodiment, active electronics configured for attenuation, such as an operational amplifier configured for less than unity gain and programmable by an external control signal, can be utilized. Electrically controlled attenuating elements are well known to those skilled in the art. Alternatively, a switch can be utilized, in which case the control signal forces open the return path and eliminates all signals in the return band that have been generated from devices in the home and have been passed up in-home cable  223  and through diplexer  205 . In one embodiment, a notch filter can be used in place of the switch to eliminate the frequencies that have a signal level above the threshold. 
   In one embodiment, adjusting the upstream attenuator/switch  203  performs upstream power-level equalization. For example, the attenuator/switch  203  may be initially set to a value that allows signals transmitted from devices in the residence to be received at the head-end. The following algorithm is periodically used to adjust the attenuator/switch  203 . This procedure can be used both when ingress control is being performed as well as when no ingress control processes are active. 
   The algorithm can be simply expressed as: 
                                               Tx_Difference =   (Max_Home_Tx_Level −               Upstream_CG_Path_Loss −               Home_Path_Loss −               CM_Tx_Level +               CM_Path_Loss +               Max_Channel_Band_Delta −               Bypass_Calc_Error_Margin);                        
where (Max_Home_Tx_Level−Upstream_CG_Path_Loss-Home_Path_Loss) is the maximum home device signal level received at the drop interface,(CM_Tx_Level−CM_Path_Loss) is the CG Cable Modem (CM) transmit level at the drop interface, Max_Channel_Band_Delta is the difference in CG CM channel bandwidth and the maximum CM channel bandwidth of 3.2 MHz and Bypass_Calc_Error_Margin is an offset value. Tx_Difference is the upstream attenuation value and is such that the in-home device transmitted signal power density, after losses in the home and the communication gateway  140 , is received at the drop interface at a level relatively equivalent to the CG  140  CM transmit level at the drop interface. Such power level equalization limits ingress coming from the home and entering the cable plant, thus reducing any high ingress levels.
 
     FIG. 2B  illustrates an embodiment in which the RF module  200  is implemented without amplifiers. In this embodiment, a bi-directional coupler  210  is present for coupling CM signals to and from drop cable  221 . A high pass filter  240  is used to bypass downstream signals from attenuator switch  203  which is also present in RF module  200 . 
   In one embodiment, an upstream directional coupler (DC) represented herein as passive tap  202  and a downstream directional coupler  204  are present in the RF module  200  of CG  140  to direct the ingress to CG board  220  for ingress monitoring purposes. In this embodiment, the passive tap  202  is used to capture ingress from the home contained in the 5-42 MHz band while the downstream DC  204  captures ingress coming through the bi-directional coupler  210  within the same frequency range. In this embodiment, as in  FIG. 1A , a switch  206  is placed at the ingress monitoring input of CG board  220  to time-sequence the outputs of DC  204  and passive tap  202 . The downstream DC  204  allows detecting wideband ingress which typically enters the plant after the communication gateway  140 . 
     FIG. 3A  illustrates one embodiment of CG board  220 . In the embodiment illustrated in  FIG. 3A , an acquisition stage  310  receives signals from an RF module/tap which, referring to  FIGS. 2A and 2B , is generated by passive tap  202 . The acquisition stage  310  is connected to a Digital Signal Processing (DSP) unit  320  which in addition to providing voice and data services can be used to process return path information to determine if ingress is present and at an unacceptable level. DSP  320  is coupled to a Random Access Memory (RAM)  340  which is further coupled to a microprocessor  330 . In one embodiment, a power estimator  300  is utilized in conjunction with acquisition stage  310  to determine the presence of ingress. Microprocessor  330  utilizes information from either power estimator  300  or DSP  320  to determine that ingress is unacceptable and to generate an ingress control signal which is used to attenuate the return path signals or open the switch. 
   In yet another embodiment, the return path signals are monitored at head-end  100  to determine if the ingress levels are unacceptable. If so, the CMTS  105  or other head-end equipment can issue a control signal to CG  140  for disconnection or attenuation of the return path. The head-end  100  may selectively disconnect residences in an effort to determine which home is the source of ingress, or may work in conjunction with CG  140  to determine if the return path signal from that particular residence should be disconnected or attenuated. 
   Referring to  FIG. 3A , a transceiver  350  is utilized for communications over the HFC network. In a preferred embodiment, transceiver  350  is based on the Data Over Cable System Interface Specification DOCSIS specification and can be realized using a number of commercially available chips including those produced by the Broadcom Corporation. Transceivers for communications over bi-directional HFC networks are well known to those skilled in the art. 
   The DSP  320 , in addition to performing voice processing for telephony services and other related tasks, can also host a digital implementation of the power estimator module. The DSP  320  can run various algorithms using assembly language, C code or other programming languages supported by the DSP. 
   The microprocessor  330  can direct the switch to isolate the line when the power exceeds the detection threshold either for a single event or for a predetermined number of ingress events. The microprocessor is connected to Random Access Memory (RAM)  340  which may contain data relative to the operation of the CG  140  and the in-home devices. The data may include information regarding the number of STB  155  and PCs  135  with cable modems in the home, the channels which these devices use, and the expected power levels. This information can be obtained from a network management system  160  and can be downloaded to the communications gateway via downstream channels in the cable system. When this information is present, the threshold can be calculated based on the expected transmissions from the in-home devices, rather than the worse case values discussed above. This allows a lower threshold to be established which still permits transmissions from the in-home devices to pass through the CG  140 , but attenuates or disconnects the return path when ingress, as determined by the threshold set in conjunction with information from the network management system  160  and ingress database  165 , is present. 
     FIG. 3B  illustrates one embodiment of acquisition stage  310 . In the embodiment shown, the return path signal from passive tap  202  is processed by a band-pass input filter  360  and subsequently by an analog to digital conversion stage (A/D)  370  which digitizes the signal for subsequent processing by DSP  320 . In this embodiment, it is necessary to sample the return path signal at a rate sufficient to prevent undersampling. In the instance of a 5 to 42 MHz return band spectrum, sampling at 84 MHz in conjunction with band path limiting of the input signal is sufficient. 
   Referring to  FIG. 3C , a down conversion technique is illustrated in which signals from passive tap  202  are received by a band-pass input filter  360  and subsequently down converted using a mixer  380  in conjunction with a local oscillator  385 . The down converted signal is passed through an intermediate frequency filter  375  and to an analog to digital (A/D) converter  370 . An advantage of this embodiment is that different sections of the return path spectrum can be digitized. For example, if IF filter  375  corresponds to a particular sub-band, that sub-band can be digitized and processed for subsequent determination of ingress. By varying the frequency of local oscillator  385  it is possible to select which sub-band will be examined. The advantage of this heterodyning technique is that different sub-bands can be selected for a return path monitoring based on local oscillator  385 , and with the width of those sub-bands being determined by IF filter  375 . In one embodiment, IF filter  375  is programmable such that the sub-bands can be varied in width. This can be useful when sub-bands vary in width from narrow bands on the order of kilohertz wide to wide return bands which may be 2 MHz or more wide. 
     FIG. 3D  illustrates the use of a variable band-pass filter  390  in conjunction with A/D converter  370 . In this embodiment, the variable band-pass filter is controlled both in frequency and width to allow direct digitization of the band of interest. 
     FIG. 4A  illustrates the monitoring of power over a contiguous section of the return band which contains several active channels. In one embodiment power estimator  300  estimates the power over a certain bandwidth Δf, determined by microprocessor  330 . In one embodiment, the power is measured over the 37 MHz band. Power estimator  300  can be implemented using a nonlinear (e.g. squaring) function and a low-pass filter to average the power over a period of time T. In an analog embodiment, a diode can be used for the nonlinear function and an RC circuit can serve as an integrator. Digital implementations are also possible and are based on using an FFT to obtain the PSD of the in-home signal. 
   In one embodiment, the averaging time is set to a value such that T·Δf&gt;&gt;1. The type of ingress to be detected depends on the choice of T, which cannot be longer than the duration of the signal to be monitored. For impulsive noise, the pulse duration is between 0.1 to 1 μsec for approximately 95% of impulse events, and between 1 to 10 μsec for the remaining impulse events. Narrowband ingress typically extends to several milliseconds and occupies a bandwidth on the order of kHz. 
     FIG. 4A  illustrates monitoring of the return path over a monitoring band  404 . In many cable systems monitoring band  404  will be 5 to 42 MHz. As shown in  FIG. 3A  an ingress signal  400  is present, along with a STB signal  410 , and a CM signal  420 . An ingress detection threshold  402  is established, based on the well known general characteristics of ingress, the specific characteristics of ingress for that plant, or a combination of the two. The ingress detection threshold  402  may change over time depending on the characteristics of the plant and the services in place. As an example, if more services are utilized the ingress detection threshold  402  may be lowered as compared to an initial value, since ingress may be more critical with additional services than when the services provided were minimal or at a low penetration rate. 
   In one embodiment, ingress detection threshold  402  is established in head-end  100  and transmitted to communication gateway  140 . In an alternative embodiment, communication gateway  140  establishes ingress detection threshold  402  locally. As previously discussed, ingress detection threshold  402  can vary over time. Ingress database  165  can be utilized to determine an appropriate ingress detection threshold based on historical ingress data. In addition, historical ingress data can be combined with information regarding channel usage, which is available in channel usage information database  115 , to determine the appropriate ingress detection threshold. 
   As shown in  FIG. 4B , the ingress power density over the monitored band  440  can be calculated. When the ingress power density over the monitored band  440  does not exceed ingress detection threshold  402 , no action is necessary. When the ingress power density over the monitored band  440  exceeds the ingress detection threshold  402 , the return path signal can be attenuated, or the return path can be opened through use of a switch to eliminate the ingress. 
   As an example of full band monitoring, a CM can transmit up to 58 dBmV within a bandwidth of 3 MHz and an STB can transmit up to 60 dBmV within 150 kHz. If the transmit power for these devices is lowered from the maximum transmit power by 10 dB, the worst case threshold can be calculated as the estimated PSD from these data sources over the entire return path. 
     FIGS. 5A and 5B  illustrate sub-band monitoring, in which ingress is monitored within monitoring bands  424 . These monitoring bands can correspond to DOCSIS channels, ranging from 200 kHz to 3.2 MHz, or can be set to a bandwidth which is sufficiently wide to permit rapid acquisition of the power spectral density but narrow enough to resolve a portion of the return band. 
   As illustrated in  FIG. 5B , the ingress power within monitoring bands  500  is measured, and when the ingress power density is greater than the ingress detection threshold, an unacceptable ingress event  510  is declared. Unacceptable ingress event  510  indicates that an unacceptable level of ingress is present in the monitoring bands  500 . 
     FIGS. 6A and 6B  illustrate the detection of ingress with further discrimination between used and unused bands. In this embodiment used bands  610  are distinguished from unused bands  600 , with unused bands  600  not containing any return path signals. The advantage of this embodiment is that an ingress detection threshold within used bands  610  can be established which is different than the ingress detection threshold within unused bands  600 . As illustrated in  FIG. 6B , unacceptable ingress events in used bands  620  are distinguished from unacceptable ingress events in unused bands  630 . The different ingress detection thresholds are established based on the expected power levels in the used bands and the allowable ingress levels as determined with respect to the return path signal levels. 
     FIG. 7A  illustrates the monitoring of active channels  710  in a return frequency band  700 . In this embodiment, the communications gateway only monitors channels which have been identified as having return path traffic. As shown in  FIG. 7A , channels may be identified as active, and the ingress monitoring techniques previously discussed used to determine if there are unacceptable levels of ingress on these channels. 
   Information regarding which channels are in use may be obtained from a number of sources, including the CMTS  105  at head-end  100 , or by the CMTS  105  in conjunction with the channel usage information database  115 . The CMTS  105  incorporates channel usage information as part of its operation in allocation of return path bandwidth using the DOCSIS protocol. Since CMTS  105  transmits downstream channel usage information to the CG  140  as part of the DOCSIS protocol, the CG can determine which return path channels are in use and monitor those channels correspondingly. Information regarding the use of return path channels by other in-home devices including STB  155  can also be transmitted to CG  140 , either through DOCSIS or as part of another communication protocol. 
     FIG. 7B  illustrates the monitoring of an active CG channel  720  in the return frequency band  700 . In this embodiment only the return channel being actively used by the CG  140  is monitored. Because this channel is likely to contain critical return path payloads carrying voice and data, it is important to maintain the integrity of the active CG channel  720 . Information from STB  155  which is being carried on another return channel may not be as critical as the information being carried in the active CG channel  720 , and by monitoring the active CG channel  720  only ingress which is present in the active CG channel  720  will result in disconnection or attenuation of the return path. This embodiment offers the advantage that ingress signals at frequencies outside of the active CG channel  720  will not result in disruption of communications in the active CG channel  720 . 
     FIG. 8  represents a flowchart for a channel recognition method which can be used to identify active channels in the return frequency band  700 . The advantage of this technique is that the CG  140  can automatically identify which channels are in use in the return frequency band  700  and mark them as active channels  710 . 
   Referring to  FIG. 8  the method is performed by receiving an in-house signal  800 . This can be accomplished through use of the CG  140  with return path monitoring capability as illustrated in FIG.  2 A. Once the in-home signal has been received, the power spectral density (PSD) can be determined in an estimate PSD  810  step. Determination of the PSD can be accomplished through use of DSP  320  as illustrated in FIG.  3 A. Upon obtaining the PSD of the received return path signal, a correlation calculation step  820  is performed using the PSD determined in step  810  and a stored PSD  825 . The stored PSD  825  contains a representation of an expected return path PSD. As an example, DOCSIS based devices can utilize Quadrature Phase Shift Keying (QPSK) transmission or Quadrature Amplitude Modulation (QAM). The spectra for these transmissions, including channel width, can be known a priori and stored as part of stored PSD  825 . 
   Once the correlation calculation step  820  is complete, a “determine frequency at peak correlation” step  830  is performed, followed by a “determine minimum frequency band in use” step  840 . In the “determine minimum frequency band in use” step  840 , the limits of the active channel  710  are determined. These limits are used in conjunction with a stored or calculated PSD mask to determine the parameters for monitoring of the return path frequency band  700 . 
     FIG. 9  illustrates an exemplary time/frequency map  900 . In the exemplary case of  FIG. 9 , the ingress events are monitored over a pre-determined period of time and the results of the monitoring are represented in a matrix format on the map  900 . As shown in  FIG. 9 , the time frames (N time frames) are tracked on the horizontal axis and the various frequency bands (K frequency bands) are tracked on the vertical axis. In this exemplary case each of the frequency bands and each of the time frames has a predetermined width. 
   For each time frame, the PSD of the signal is obtained and the frequency bands having a signal power above the detection threshold are given an integer value  1 . The presence of a “1” indicates the detection of a signal above the detection threshold. Over a window of M time frames a map as shown in  FIG. 9  can be obtained. Furthermore, a cumulative sum  920  over the K frequency bands at time frame n (CSF n ) may be computed. Additionally, a cumulative sum  910  over the N time frames at frequency band k(CST k ) may also be computed. The CSF n  and CST k  may then be used to distinguish a narrowband ingress event from a wideband ingress event. In particular, a narrowband ingress event is declared when the CST k  exceeds a threshold, for example, S nb  and, a wideband ingress event is declared when CSF n  exceeds a threshold, for example, S wb . The principles of the present invention are flexible and different values of threshold may be used. A user may select suitable threshold values based on the application/system in use. 
   In one embodiment, the width of the frequency band is set equal to 200 kHz. In this embodiment, the threshold S wb  is equal to 5, which corresponds to an ingress signal with a bandwidth of 1 MHz. More generally, the threshold S wb  can be determined from this equation: 
         S   wb     =     W     Δ   ⁢           ⁢   f           
 
where W is the minimum bandwidth of a wideband signal and Δf is the width of the frequency band used.
 
   In this embodiment, a narrowband ingress event declaration may be based on a threshold S nb  corresponding to an ingress duration of several milliseconds. For example, by considering a narrowband ingress event of duration 100 ms and a 10 ms time between received detection measurements, the threshold S nb  can be set to 10. 
   The time/frequency map results may be stored and time averaged at the subscriber location or near the subscriber location with the information being used to set thresholds for entire return band monitoring or sub-band monitoring, or to adjust the time or frequency windows for ingress monitoring and for the declaration of ingress events. For example, this location may be a communications gateway near or at the subscriber location. The map can also be transmitted to the head-end or other location in the network where it can be used by a network management system to determine the status of the return path network, isolate faults, and generate work orders to send craft to a residence. 
   Furthermore, this characterization may only occur at a test point in the network, or at a head-end in the network. For example, the characterization may occur at a telephone test point (TTP) in the network, or a Cable Modem Termination System (CMTS) at the head-end. 
   Although this invention has been illustrated by reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made that clearly fall within the scope of the invention. The invention is intended to be protected broadly within the spirit and scope of the appended claims.

Technology Classification (CPC): 7