CATV RETURN PATH IMPAIRMENT DETECTION AND LOCATION SYSTEM

Abstract: A method of detecting impairments is employed in a multichannel communication system operable to transmit signals on plurality of channel frequencies. A first step of the method involves transmitting from a headend unit one or more information signals on the multichannel communication system, the one or more information signals including information identifying one or more frequencies to be measured. A second step of the method comprises transmitting a trigger signal on the multichannel communication system. In other steps of the method, first and second remote units operably connected to different locations on the multichannel communication system receive at least one of the information signals and the trigger signal. Each of the first and second remote units then perform, responsive to the trigger signal, a measurement measuring a signal level corresponding to at least a first frequency of the frequencies to be measured, the first frequency identified in the received at least one of the information signals. The first and second remote units perform the measurement at about the same time.

Description: CATV RETURN PATH IMPAIRMENT DETECTION AND LOCATION SYSTEM Field of the Invention The present invention relates generally to multichannel terrestrial communication systems and, and more particularly to methods and apparatus of impairment testing in a multichannel terrestrial communication system. Background of the Invention Multichannel terrestrial communication systems are used in a widespread manner for the transmission and distribution of signals at least partly through land-based media. Such terrestrial communication systems employ co-axial cable or other transmission media to communicate signals from one point to another. For example, one type of multichannel communication system, known as a Community Antenna Television ("CATV") system, provides radio frequency television signals, at least in part, over co-axial cable to end users, or subscribers. A CATV system typically comprises a headend facility and a distribution network. The headend facility obtains television signals associated with a plurality of CATV channels and generates a broadband CATV signal therefrom. The distribution network then delivers the CATV broadband signal to television receivers located within the residences and business establishments of subscribers. CATV networks have been increasingly employed as two-way communication networks. In particular, many CATV networks are currently capable of providing communication from the subscriber to the headend. Such communications are often referred to as reverse path communications. Technical problems, however, have inhibited wide deployment of such two-way networks. One such problem is that of interference due to ingress signals that degrade the quality of the return path communications. Ingress signals comprise noise signals that are generated by sources external to the CATV network and are radiated onto the CATV network through cable faults, terminations, and the like. Some sources of ingress include international short-wave broadcasts, citizens band and ham radio transmissions, television receivers, computers, neon signs, and electrical motors. Ingress signals are particularly troublesome in the context of return path communications because of the CATV two-way network architecture. In a CATV network, a large number of subscriber-generated reverse path communication signals are funneled toward the headend. The ingress signal power on each of the subscriber generated signals is therefore combined and amplified, resulting in a relatively high ingress signal power at the headend facility. Since consumer demand is currently high for various two-way services such as broadband internet access, interactive TV, and telephony, CATV franchises are interested in providing these services to their subscribers. However, in order to provide these services in a reliable manner, CATV franchises must eliminate or reduce ingress signals that interfere with two-way communications. Troubleshooting ingress can be a real challenge since ingress can be transient or constant, intermittent or predictably repetitive. Moreover, ingress signals may result from signal entry on one node or multiple points on one node, and may be broadband or narrow band in nature. A critical first step in resolving leakage problems is that of locating the source or sources of leakage. Several methods have been proposed and/or implemented to identify and locate leakage sources, or in other words, the location in the distribution system in which ingress occurs. U.S. Patent No. 4,520,508 to Reichert, Jr. shows a system having a central station and a plurality of subscriber terminals specially adapted to monitor signal ingress. Each subscriber terminal monitors certain frequencies and then provides signal level information to the headend controller. Once the headend controller has received signal level information from all of the subscriber terminals, the signal level information from all of the subscriber terminals are compared. By checking comparative signal levels of differently located subscriber terminals, a source of ingress may often be narrowed to a location between two of such subscriber terminals. One drawback of the system described in the above referenced Reichert Jr. patent is that each of the subscriber terminals performs its measurements independently of the other subscriber terminals. As discussed above, ingress signals are often intermittent in nature. In particular, sources of ingress often include intermittent signal sources that have continuously changing signal levels. Accordingly, a certain ingress signal may exist when one subscriber terminal is performing measurements but not exist when another subscriber terminal is performing measurements. Such inconsistent ingress signal level measurements can compromise the usefulness of the data in locating the source of ingress. A further disadvantage of the system described in the above referenced Reichert, Jr. patent arises from the method by which the monitoring equipment, . e. the subscriber terminals, are connected to the CATV system. In particular, the subscriber terminals are connected through subscriber network equipment, or in other words, the subscriber network. Subscriber networks are not closely regulated by CATV service providers and are often poorly maintained. As a result, a particular subscriber network may exhibit noise problems and ingress problems that are manifested on that subscriber network but not on the CATV service provided network. Accordingly, in the Reichert, Jr. system, it can be difficult to discern what portion of ingress signal power measurements is representative of the actual ingress power on the CATV service provider network and which portion is representative of ingress power on individual subscriber networks. Accordingly, there is a need for a impairment detection and location system having higher accuracy. To this end, a need exists for an impairment detection system that is not unduly influenced by noisy subscriber networks. Moreover, a need exists for an impairment detection and location system having an accuracy level that is not significantly affected by the intermittent nature of some leakage signal sources. Summary of the Invention The present invention addresses the above needs, as well as others, by providing an impairment detection system in which a plurality of remote units monitor one or more frequencies to be tested in a synchronized manner. By monitoring frequencies to be tested in a synchronized manner, intermittent leakage signals may be accurately measured and located. An exemplary method according to the present invention detects impairments in a multichannel communication system, the multichannel communication system operable to transmit signals on plurality of channel frequencies. A first step of the method involves transmitting from a headend unit one or more information signals on the multichannel communication system, the one or more information signals including information identifying one or more frequencies to be measured. A second step of the method comprises transmitting a trigger signal on the multichannel communication system. In another step of the method, a first remote unit operably connected to a first location on the multichannel communication system receives at least one of the information signals and the trigger signal. The first remote unit then performs, responsive to the trigger signal, a measurement measuring a signal level corresponding to at least a first frequency of the frequencies to be measured, the first frequency identified in the received at least one of the information signals. A second remote unit operably connected to a second location on the multichannel communication system also receives at least one of the information signals and the trigger signal. The second remote unit also performs, responsive to the trigger signal, a measurement measuring a signal level corresponding to at least the first frequency of the frequencies to be measured at about the same time as the first remote unit performs the measurement corresponding to the first frequency. The ability of the remote units to perform measurements responsive to a trigger signal allows the plurality of remote units to be configured to perform the measurements contemporaneously or in other words, in a synchronized manner. The performance of measurements contemporaneously increases the accuracy of the testing method in the presence of intermittent sources of ingress. Moreover, in accordance with a preferred embodiment of the present invention, the remote units are directly coupled to the distribution network as opposed to subscriber networks. As a result, any faults or noise on individual subscriber networks will only be detected to the extent that they affect the service provider distribution network. The above described features and advantages, as well as others, will become more readily apparent to those of ordinary skill in the art by reference to the following detailed description and accompanying drawings. Brief Description of the Drawings Fig. 1 shows an exemplary embodiment of an impairment detection system according to the present invention installed in a portion of a CATV communication system-; Fig. 2 shows a block diagram of an exemplary embodiment of an ingress monitor according to the present invention; Fig. 3 shows flow diagrams of the operations performed by the various components of the impairment detection system of Fig. 1 ; and Fig. 4 shows at timing diagram of an exemplary trigger signal generated within the impairment detection system of Fig. 1 Detailed Description Fig. 1 shows an exemplary embodiment of an impairment detection system according to the present invention. The impairment detection system is shown in Fig. 1 installed in a multichannel communication system that is operable to communicate signals on plurality of channel frequencies. In the exemplary embodiment described herein, the multichannel communication system is a CATV system that includes a headend 15 and a distribution network 20. The headend 15 includes typically CATV system headend equipment for providing radio frequency ("RF") television signals to the distribution network 20 and for receiving upstream communication signals from the distribution network 20. The distribution network 20 preferably comprises the signal distribution equipment of a CATV service provider network, which does not include subscriber network equipment. The impairment detection system according to the present invention includes a headend unit 25 and a plurality of remote units including first, second, third and fourth remote units 30, 35, 40, and 45 respectively. It is noted that the four remote units 30, 35, 40 and 45 are shown by way of illustration only , and that substantially more units will typically be employed in a large CATV distribution network. The headend unit 25 is a telemetry transceiver that is operably connected transmit and receive telemetry signals to and from, respectively, the multichannel communication system. The headend unit 25 may suitably have the general architecture as the headend unit described in U.S. Patent No. 5,585,842 to Voght et al., which is incorporated herein by reference. In any event, the headend unit 25 is operable to transmit one or more information signals on the multichannel communication system, the one or more information signals including information identifying one or more frequencies to be measured. The headend unit is further operable to transmit a trigger signal to the first, second, third and fourth remote units 30, 35, 40 and 45. The headend unit 25 is coupled to the distribution network 20 and is preferably- located at the headend 15 for convenience. The first remote unit 30 is an RF transceiver that is operably connected to a first location on the distribution network 20 to receive at least one information signal from the headend unit 25. The first remote unit 30 is operable to obtain information identifying the one or more frequencies to be measured from the information signal. To facilitate the reception of the information signal, the first remote unit 30 is coupled to the distribution network 20 through an upstream directional coupler 32. The upstream direction coupler 32 provides a low impedance coupling between the "upstream" portions of the distribution network 20 and the first remote unit 30. The first remote unit 30 is preferably directly coupled to a part of the distribution network 20. as opposed to being indirectly coupled through another network that is not under test, such as a subscriber network. The first remote unit 30 is further operable to, responsive to the trigger signal received from the headend unit, perform a measurement measuring a signal level corresponding to at least a first frequency of the frequencies to be measured. To facilitate measurements of ingress, the measurement circuitry of the first remote unit 30 is preferably coupled to the distribution network 20 through a downstream directional coupler 34. The downstream directional coupler 34 provides a low impedance coupling between the downstream portions of the distribution network 20 and the measurement circuitry of the first remote unit 30. An exemplary remote unit 102 having the above described capabilities is described below in connection with Fig. 2. The second remote unit 35, like the first remote unit 30, is operably connected to the distribution network 20 to receive at least one information signal and the trigger signal from the headend unit 25. Similar to the first remote unit 30, the second remote unit 35 is directly coupled to the distribution network 20 through directional couplers. By contrast, however, the second remote unit 35 is connected to a second location on the distribution network 20 that is preferably spaced apart from the first location. The second remote unit 35, like the first remote unit 30, is operable to obtain information identifying the one or more frequencies to be measured from the information signal. The second remote unit 35 is further operable to, responsive to the trigger signal received from the headend unit, perform a measurement measuring a signal level corresponding to at least a first frequency of the frequencies to be measured responsive to the trigger signal. According to the present invention, the second remote unit 35 is operable to perform the measurement of the signal level corresponding to the first frequency at about the same time as the first remote unit 30 performs a measurement of the signal level corresponding to the first frequency. The third remote unit 40 and the fourth remote unit 45 have similar capabilities as the first remote unit 30 and are coupled to the distribution network 20 in a substantially similar manner. Each of the third remote unit 40 and the fourth remote unit 45, however, is coupled at a distinct location on the distribution network. As a result, the first, second, third, and fourth remote units 30, 35, 40 and 45 are each located at a distinct location in the distribution network 20. Because the first, second, third and fourth remote units 30, 35, 40 and 45 all perform measurements on the first frequency at a predetermined time after receiving the trigger signal, the remote units 30, 35, 40 and 45 perform signal level measurements at the first frequency and subsequent frequencies at about the same time. The performance of measurements by the first remote unit 30 and the second remote unit 35 at about the same time on the same frequency provides reliable comparison data for ingress detection, even in the presence of intermittent ingress signal sources. Specifically, any signal level fluctuation in the ingress signal source is captured by the measurements of both remote units, thereby reducing any negative impact on measurement accuracy. In operation, the headend unit 25 first transmits one or more information signals over the distribution network 20. The one or more information signals including information identifying one or more frequencies to be measured. In the exemplary embodiment described herein, the information signal includes a scan frequency plan identifying a plurality of frequencies to be measured. For example, the plurality of frequencies may be individually listed within the information signal, with a separate data value identifying each particular frequency to be measured. Alternatively, the information signal may simply provide an upper frequency value, a lower frequency value, and a step value. Still other methods of identifying frequencies to be measured may of course be employed. The information signal generated by the headend unit 25 may further include additional information, such as resolution bandwidth and dwell time information, which are discussed further below in connection with Fig. 3. The information signal preferably also includes information identifying which of the remote units 30, 35, 40 and 45 are participating in the test. In particular, due to the expansive nature of CATV networks, it may be impractical or unnecessary to perform a particular test that involves all of the remote units, which could number in the hundreds in some systems. As a result, only a select group of the remote units connected to the distribution network 20 would be employed for any one particular test. To this end, the headend unit 25 of the exemplary embodiment described herein generates a separate information signal for each of the select group of the remote units participating in the test. In this embodiment, the headend unit 25 inserts into each information signal an identifier for a particular remote unit, such as, for example, the first remote unit 30. Each of the remote units 30, 35, 40 and 45 that receives information signal with corresponding identification information becomes one of the select group. The select group of remote units then receive and process the one or more information signals. In particular, each of the select group of the remote units obtains the scan frequency plan from its information signal and then prepares to receive the trigger signal. Further detail regarding the processing of the information signal by the remote units is provided further below in connection with Fig. 2 and 3. Thereafter, the headend unit 25 then transmits the trigger signal on the distribution network 20 of the multichannel communication system. Each of the select group of the remote units involved in the test receives and identifies the trigger signal. Each of the select group of the remote units then performs, responsive to a single, broadcast trigger signal, a measurement measuring a signal level corresponding to the frequencies identified in the scan frequency plan. Because of each of the select group of remote units performs the signal level measurements responsive to a single, broadcast trigger signal, the measurements are performed substantially contemporaneously. In other words, each of the select group of remote units measures the signal level at a particular frequency at about the same time as the other remote units in the select group measures the signal level at that frequency. After the remote test units involved in the test complete the signal level measurements for each frequency identified in the information signal, the remote units provide signal level measurement test results to a measurement test results receiver, which is in the embodiment described herein is located within the headend unit 25. It will be appreciated that that the measurement test results receiver may alternatively be a separate device from the headend unit 25. The measurement test results receiver in headend unit 25 then processes the signal level measurement test results to obtain information about impairments in the distribution network 20. To this end, the headend unit 25 may provide the signal level measurement test results directly to a printer or display, not shown, from which a technician at the headend 15 may perform analysis on the results. Alternative, the headend unit 25 may provide additional processing to automatically identify abnormalities in the signal level measurement test results. Such abnormalities may then be communicated to an operator through a display, printer or other means, not shown. The exemplary test operation discussed above may, for example, be employed to determine the approximate location of ingress in the distribution network 20, and in particular, ingress affecting the reverse path communication signals. Specifically, for an) unused frequency in the scan frequency plan, the signal level measurements in ideal conditions are expected to be negligible. In the presence of ingress, however, the signal level measurements for one or more of the unused frequencies will exhibit some non-trivial power level. In an exemplary test operation, consider a test in which only the first, second and third remote monitors 30, 35 and 40 are employed to obtain the necessary test data. Also consider that the test is required to obtain ingress measurements for frequencies between 10 MHz to 30 MHz in 2 MHz increments. In such a case, the headend unit 25 generates a separate information signal for each of the remote units 30, 35, and 40. The first information signal would have information identifying the first remote unit 30 as well as the scan frequency plan in the format of {lower limit, upper limit, step value}, or { 10 MHz, 30 MHz, 2 MHz}. Similarly, the second information signal would include information identifying the second remote unit 35 and the same scan frequency plan. Finally, the third information signal would include information identifying the third remote unit 40 as well as the same scan frequency plan. After the headend unit 25 transmits the information signals onto the distribution network 20, the first, second and third remote units 30, 35 and 40 then receive their respective information signals and obtain the scan frequency plan { 10 MHz, 30 MHz, 2 MHz} therefrom. Each of the first, second and third remote units 30, 35 and 40 then monitors the distribution network 20 for trigger signal. The headend unit 25 subsequently transmits the trigger signal onto the distribution network 20, which is received and detected by the first, second and third remote units 30, 35 and 40. At a predetermined time after receipt of the trigger signal, each of the first, second and third remote units 30, 35 and 40, respectively, performs a first signal level measurement on the first frequency identified in the frequency channel plan, which in the exemplary embodiment described herein is 10 MHz. Each of the first, second and third remote units 30. 35 and 40 performs the first signal level measurement at about the same time. Preferably, each of the remote units 30, 35, and 40 performs the signal level measurement over a duration such that for at least some amount of time, each of the first, second and third remote units 30, 35 and 40 measure the signal level simultaneously. After a predetermined amount of time after performing the first signal level measurement, each of the first, second, third remote units 30, 35, and 40 performs a signal level measurement at the next frequency in the scan frequency plan, or 12 MHz. As before, the first, second and third remote units 30, 35. and 40 all perform the signal level measurement at about the same time. In a similar manner, the remote units 30, 35 and 40 perform a signal level measurement at each and every other frequency identified in the scan frequency plan in a synchronized manner. By "synchronized manner", it is meant that the remote units 30, 35 and 40 all perform a signal level measurement at each frequency at about the same time. When ingress is indicated on a particular frequency, the approximate location of the ingress may be determined through analysis of the signal level measurements of the remote units 30, 35, 40 and 45. In particular, a source of ingress may be located by determining a characteristic discontinuity in measured signal levels between adjacent remote units. For example, consider a situation in which a source of ingress is located between the first remote unit 30 and the second remote unit 35. Because, as discussed above, the remote units 30, 35 and 40 all employ directional couplers between the distribution network 20 and their signal level measurement circuitry, the signal level at the first remote unit 30 should be significantly higher than the signal level at either of the second remote unit 35 or third remote unit 40. In particular, the signal level in the first remote unit 30 is significantly higher because the first remote unit 30 is the only one of the remote units 30, 35 and 40 that is located upstream of the ingress source. As a result, the ingress signal is received through the low impedance path of the downstream directional coupler 34 between the first remote unit 30 and the distribution network 20, but is received through the high impedance path of the corresponding downstream directional couplers between the distribution network 20 and each of the second remote unit 35 and the third remote unit 40. For the same reasons, if the source of ingress were instead located between the second remote unit 35 and the third remote unit 40, then the measured signal levels at both the first remote unit 30 and the second remote unit 35 would be significantly higher than the level measured at the third remote unit 40. The system according to the present invention provides enhanced accuracy in ingress testing because each of the plurality of remote units involved in the test performs the signal level measurements at each frequency contemporaneously. In particular, ingress signals are often spurious and intermittent. As a result, an ingress measurement at a particular frequency at a particular time may be substantially different than an ingress measurement at the same frequency a short time later. Prior art systems, in which remote measurement units performed measurements independent of one another, suffer from the disadvantage that the same ingress signal may not be present at the different times that different remote units perform their signal level measurement. The system according to the present invention, however, employs a plurality of remote units that perform measurements at the same frequencies contemporaneously. As a result, the system of the present invention is substantially less susceptible to ingress measurement inaccuracies due to intermittent ingress signals. In an alternative embodiment of the present invention, the trigger signal may be included within the information signal itself. As a result, the headend system 25 need only generate a single broadcast signal to perform the test. However, such an arrangement would depend upon a priori knowledge that all the remote test units were available and ready for the test. Accordingly, such an arrangement may encounter difficulties if one or more of the remote units is employed to perform other monitoring functions. In particular, if a remote unit is busy performing another monitoring function when the combination information/trigger signal is transmitted, it may not be ready or available to perform the signal level measurements in a synchronized manner with the other remote units. By contrast, if the information signal is generated separately from the trigger signal as discussed above, the system may readily be designed to accommodate such multipurpose remote units. Specifically, the headend unit 5 may be configured to wait for an acknowledgment signal from each of the remote units before broadcasting the trigger signal. The headend unit 25 could thus postpone transmission of the trigger signal until all the remote units were available and ready. Fig. 2 shows an exemplary embodiment of a remote unit 102 according to the present invention which may be employed as any or all of the remote units 25, 30, 35 and 40 of Fig. 1. The remote unit 102 is shown coupled to a section 104 of the distribution network 20 of Fig. 1. The remote unit 102 includes a measurement receiver 110, a telemetry receiver 115, a telemetry transmitter 120, a signal strength detector 125, a demodulator 130, a modulator 135, an analog-to-digital ("A/D") converter 140, and a processor 145. The measurement receiver 110 is coupled to the section 104 of the distribution network 20 through a directional coupler 150. In order to carry out measurements on the reverse path communications in a CATV distribution network, the directional coupler 150 is configured to provide a low impedance coupling for signals communicated with further remote or "downstream" portions of the distribution network 20, and a high impedance coupling for signals communicated an "upstream" portions of the distribution network 20, i.e., portions closer to the headend 10. The measurement receiver 1 10 is a circuit operable to convert an input RF signal into an intermediate frequency ("IF") signal having a predetermined frequency, IFREQ. Preferably, the measurement receiver 1 10 is operable within a wide input frequency range of at least 5 MHz and 1000 MHz. The measurement receiver 110 also preferably has a variable resolution bandwidth that varies from approximately 30 kHz to bandwidth of an NTSC video channel or approximately 4.5 MHz. The detailed circuitry necessary to provide a measurement receiver having the above capabilities would be readily apparent to one of ordinary skill in the art. The measurement receiver 1 10 is further coupled to the microprocessor 145 through a control line 155. The control line 155 is shown as a common bus structure that couples the microprocessor 145 to each of the measurement receiver 110, the telemetry receiver 1 15, and the telemetry transmitter 120. The processor 145 controls the measurement receiver 1 10 by providing control signals that dictate the frequency to which the measurement receiver 1 10 is tuned as well as the resolution bandwidth of the measurement receiver 110. It will be appreciated that further control lines may be provided to control other circuit elements which are not shown to facilitate clarity of description. Such further control lines will vary depending on the particular circuit implementation chosen, and are well known in the art. It will further be appreciated that the illustration of the control line 155 as a shared common bus among several circuit elements is given by way of example only. In other embodiments, individual control lines may be used to connect the processor 145 to each of the circuit elements. The measurement receiver 1 10 is further operably coupled to provide the IF signal to the signal strength detector 125. The signal strength detector 125 may suitably be any analog or digital circuit that receives an oscillating signal and generates an analog signal having a DC value that is representative of the energy level of the received signal. In other words, the signal strength detector 125 is operable to receive the IF signal and generate an analog energy signal therefrom. Such circuits are well known in the art. For example, the signal strength detector 125 may suitably be a log amp detector, the design and implementation of which is well known. The signal strength detector 125 is further operably coupled to provide the analog energy signal to the A/D converter 140. The A/D converter 140 is a circuit or integrated circuit device that is operable to receive the analog energy signal from the signal strength detector 125 and generate a digital measurement signal therefrom. The A/D converter 140 preferably has a resolution of 12 bits or more. The A/D converter 140 is also operably coupled to provide the digital measurement signal to the processor 145. As apparent from the above description, the signal strength detector 125 and the A/D converter 140 operate together to receive the IF signal and generate the digital measurement signal therefrom. Those two elements may alternatively be replaced by a single integrated circuit that performs the same or a similar combined function. The telemetry receiver 1 15 is operably coupled through a second directional coupler 160 to receive RF telemetry signals from the distribution network 20. Because RF telemetry signals in accordance with the invention described herein are generated by the headend unit of the CATV network (see, e.g., Fig. 1), the second directional coupler 160 is configured to provide low impedance communication of signals between the telemetry receiver 115 and the "upstream" portion of the distribution network 20 that extends toward the headend. As a result, the second directional coupler 160 further provides a high impedance communication of signals between the telemetry receiver 1 15 and the "downstream" portion of the distribution network 20. The telemetry receiver 115 is operable to receive input RF telemetry signals from the distribution network 20 and generate IF telemetry signals therefrom. The telemetry receiver 1 15 need not have as wide a frequency range as the measurement receiver 110. Preferably the telemetry receiver 115 has a frequency range of approximately 50 MHz to 175 MHz. The telemetry receiver 1 15 may also have less sensitivity than the measurement receiver 110, as well as a fixed resolution bandwidth. As a result, the telemetry receiver 115 may constitute a less expensive circuit than the measurement receiver 1 10. The detailed circuitry necessary to provide a measurement receiver having the above capabilities would be readily apparent to one of ordinary skill in the art. The telemetry receiver 115 is operable to provide IF telemetry signals having an intermediate frequency ITFREQ to the demodulator 130. The demodulator 130 in the exemplary embodiment described herein is an FSK demodulator and manchester decoder. However, it will be noted that other modulation devices that carry out other modulation techniques, including QPSK and others, are well known in the art and may readily be used as the demodulator 130. It will be noted that the modulator 135 similarly comprises a manchester encoder. The functions of the manchester decoder and manchester encoder are often implemented in a single integrated circuit, such as. for example, the model 6409 integrated circuit available from Harris semiconductor. In any event, the demodulator 130 is operable to receive the IF telemetry signal and generate a digital telemetry signal therefrom. The digital telemetry signal may suitably constitute a baseband version of the received telemetry signal. The demodulator 130 is operable coupled to provide the digital telemetry signal to the processor 145. The processor 145 is generally operable to control the measurement receiver 1 10. the telemetry receiver 115, and the telemetry transmitter 120. In particular, the processor 145 is operable to receive a digital telemetry signal, which, as discussed below, may suitably be the baseband version of information signal or the baseband version of trigger signal, and control the operation of the measurement receiver 1 10, the telemetry receiver 1 15. and/or the telemetry transmitter 120 responsive to the digital telemetry signal. The processor 145 is further operable to receive the digital measurement signals from the A/D converter 140 and generate measurement test results therefrom. As will be discussed further below in connection with Fig. 3, the digital measurement signal is a plurality of digital samples representative of the measured signal strengths at one or more frequencies of a test performed in accordance with the present invention. Measurement test results comprises data representative of signal power measurements corresponding to the plurality of frequencies under test. The processor 145 is further operably connected to provide outgoing digital telemetry signals to the modulator 135. The outgoing digital telemetry signals may suitably include those that contain the measurement test results or other information indicative of some or all of the measured signal strengths. Additionally, outgoing digital telemetry signals may include handshaking messages for the headend. The modulator 135 is operable to receive the outgoing digital telemetry signal and generate an outgoing IF telemetry signal therefrom. The modulator 135 is operably connected to provide the outgoing IF telemetry signal to the telemetry transmitter 120. The telemetry transmitter 120 is an RF transmitter circuit operable to receive the outgoing IF telemetry signal and generate an outgoing RF telemetry signal therefrom. The telemetry transmitter 120 preferably has an output power level of between 0 dBmV and +50 dBmV. The telemetry transmitter 120 is also tunable, through the control line 155, to transmit the outgoing RF telemetry signal at a select carrier frequency between 5 MHz and 65 MHz. Fig. 3 shows flow diagrams 200 and 300 of the operation of the impairment detection system of Fig. 1 wherein the remote units 30, 35, 40 and 45 have the structure of the remote unit 102 of Fig. 2, and the headend unit 25 is assumed to have the general structure and operation of the headend unit 1 1 shown in Fig. 2 of U.S. Patent No. 5,585,842 and described therein. Flow diagram 200 shows the generalized operation of the headend unit 25 in accordance with the present invention. Flow diagram 300 shows the generalized operation of one of the remote units, such as the remote unit 102 of Fig. 2. The operations of the headend unit 25 and the remote unit 102 are interdependent as will be discussed below. Referring now to the flow diagram 200, the headend unit 25 initiates an impairment test according to the present invention by transmitting an information signal to a plurality of remote units (step 205). In particular, the headend unit transmits an information signal to each of a select group of remote units involved in the impairment test. The selection of the remote units involved in the impairment test may suitably be determined by an operator at the headend. Moreover, the select group of remote units in some cases may comprise all the remote units to which the headend unit 25 is connected. In any event, each information signal typically includes information identifying the particular remote unit to which it is directed as well as the scan frequency plan of the test. As discussed above, the scan frequency plan identifies a plurality of frequencies at which signal level measurements are to be taken. The information signal may further include information identifying the desired resolution bandwidth of the signal level measurements as well as the dwell time. The resolution bandwidth defines the bandwidth of the signal energy measurement to be performed at each frequency. The dwell time defines the amount of time to be spent measuring at each frequency. The information signal may also include information identifying the trigger signal frequency. In particular, it may be preferable to transmit the trigger signal on a frequency that differs from the normal telemetry signal frequency to avoid potential signal errors. In such a case, the trigger signal frequency is preferably communicated to the remote units through the information signal. Once all the information signals have been transmitted, the headend unit 25 polls for acknowledgment signals from the remote units (step 210). If acknowledgment signals for all of the select group of remote units have been received (step 215), then the headend unit 25 transmits the trigger signal (220). Otherwise, the headend unit 25 continues to poll for the additional acknowledgment signals (step 210). The trigger signal is preferably a characteristic telemetry signal which is manchester encoded and FSK modulated onto a trigger carrier frequency. The characteristic telemetry signal may suitably be a "start test" command sequence, and preferably further includes information identifying a predetermined time delay, TD, before the start of a test. The predetermined time delay TD is representative of the amount of time that will elapse between the transmission of the trigger signal and the commencement of the first signal energy measurement. The use of the predetermined time delay allows a time buffer to ensure that all of the remote units are able to start signal level measurements at the same time. In particular, as will be discussed further below, because of signal propagation delays in the distribution network 20, the remote units do not all receive the trigger signal at the same time. The predetermined time delay between trigger signal transmission and the commencement of the signal level measurements allows time for all the remote units to receive the trigger signal, regardless of the propagation delay. As discussed further below, each remote unit is further calibrated to compensate for its individual propagation delay so that all the remote units can perform the signal level measurements contemporaneously After transmitting the trigger signal (step 320), the headend unit 25 after a short duration transmits a results request (step 325). More specifically, the headend unit 25 transmits the results request after a suitable time has been allowed for the remote units to perform the signal level measurements on each frequency identified in the scan frequency plan. In a preferred embodiment, the headend unit 25 transmits a separate results request signal to each remote unit to simplify the coordination of communication of test results from the plurality of remote units to the headend unit 25. It is noted that it may be preferable to also perform signal level measurements at the headend contemporaneous with those made by the remote units. To this end, the headend may further include a separate monitor unit having a structure similar to that of the remote unit 102. Such a monitor unit may alternatively be integrally designed into the headend unit 25, similar to the structure shown in U.S. Patent No. 5,585,842. Once the headend unit 25 receives the signal level measurements or test results, the headend unit 25 communicates the test results to an operator through visual display or printer, not shown. Alternatively, or additionally, the headend unit 25 may download the information to a computer, and/or transmit the information to another facility. During the operation of the headend 25 in accordance with the flow diagram 200, the remote unit 102 and one or more other similarly constructed remote units operate in accordance with the flow diagram 300. It is noted that prior to the beginning of a test, the remote unit 102 ordinarily monitors a predetermined frequency for telemetry signals. To this end, the processor 145 provides a control signal to the telemetry receiver 115 that causes the telemetry receiver to tune to the predetermined monitoring frequency. Once the headend unit 25 transmits a telemetry signal, the remote unit 102 detects the telemetry signal (step 305). Thereafter, in step 310, the remote unit 102 determines whether the telemetry signal includes an information signal, and whether the information signal includes information identifying the remote unit 102 as participating in an upcoming impairment test. If the answers to both determinations in step 310 are "yes", then the remote unit proceeds to step 315 and stores the scan frequency plan within the information signal. If the answer to either determination in step 310 is "no", then the remote unit 102 ceases operations in the flow diagram 300 and returns to monitoring for telemetry signals. The remote unit 102 of Fig. 2 carries out the above-described steps 305, 310 and 315 in the following manner. The telemetry receiver 115 receives the telemetry signal and provides an IF telemetry signal to the demodulator 130. The demodulator 130 demodulates the IF telemetry signal and provides the resulting digital telemetry signal to the processor 145. The processor 145 then analyzes the digital telemetry signal to determine whether the proper data is present to identify the digital telemetry signal as an information signal. If so, and if the information signal further includes a remote unit identifier that corresponds to the remote unit 102, then the processor 145 obtains the scan frequency plan from the information signal and stores it in a memory, not shown. Referring again to Figs. 1, 2 and 3, after step 315, the remote unit 102 transmits to the headend unit 25 an acknowledgment signal indicating that it is ready to perform the test (step 320). To prepare the remote unit 102 to perform the test, the processor 145 causes the telemetry receiver 1 15 to tune to the channel frequency on which the trigger signal is expected, and causes the measurement receiver 110 to tune to the first frequency identified in the scan frequency plan. Thereafter, in step 325, the remote unit 102 waits to receive the trigger signal from the headend unit 25. When the remote unit 102 receives the telemetry signal, it begins performing signal level measurements at each of the frequencies identified in the information signal. To this end, a telemetry signal that may include the trigger signal is received at the telemetry receiver 1 15 and provided to the processor 145 through the demodulator 130. The processor 145 receives the digital telemetry signal and determines whether the digital telemetry signal includes the "start test" command sequence of the trigger signal. If so, then, after a predetermined time, the processor 145 takes a signal level measurement at the first frequency in the scan frequency plan. In particular, as noted above, the measurement receiver 110 is already tuned to the first frequency. It is noted that even before the trigger signal is received, the signal strength detector 125 and the A/D converter 140 may already be generating digital measurement signals. The processor 145, however, only begins receiving, processing and storing the digital measurement signals at the predetermined time, PT. after receiving the trigger signal. The predetermined time PT is preferably derived from the predetermined time delay, TD, which is received in the trigger signal, and the propagation delay value, PD, for the remote unit 102. The propagation delay value PD is representative of the propagation delay of signal transmission between the headend unit 25 and the remote unit 102. Each remote unit has its own propagation delay value which is determined by number of factors, including distance from the headend unit 25. The propagation delay value for each remote unit involved in a test is determined prior to the test. Fig. 4, discussed further below, shows a flow diagram of a headend unit performing an operation for determining the propagation delay value for each remote unit in a test system. Accordingly, the remote unit 102 commences the first signal level measurement, after a predetermined time, PT, after receiving the trigger signal, where PT = TD - PD. In other words, if the predetermined time delay transmitted in the trigger signal to all the remote units is 100 ms, and the propagation delay for the remote unit 102 is 20 ms, then the remote unit 102 commences the actual signal level measurements 80 ms after receiving the trigger signal. It is noted that if the value PD for each remote unit is properly determined, the plurality of remote units will perform the first signal level measurement at substantially the same time. The processor 145 at commencement of the signal level measurement accumulates a plurality of digital measurement signals over a predetermined time window and then stops receiving digital measurement signals. The predetermined time window is based on the dwell time information received in the trigger signal. The plurality of digital measurement signals accumulated over the time window are used to develop a signal level measurement for the first frequency. Upon completing the signal level measurement for the first frequency, the processor 145 causes the measurement receiver 1 10 to tune to the second frequency in the scan frequency plan. After a predetermined time from the end of the time window for performing the first signal level measurement, the processor 145 again begins receiving, storing and processing digital measurement signals received from the A/D converter 140. Such digital measurement signals are again accumulated over a predetermined window and are used to formulate a second signal level measurement, which is associated with the second frequency in the scan frequency plan. The processor 145 repeats the above processing until a signal level measurement has been performed for each frequency identified in scan frequency plan. The signal level measurements for all of the frequencies together constitute the measurement test results. After completion of step 330, the remote unit 102 awaits a telemetry signal from the headend unit 25 requesting the measurement test results. Upon receiving the request results telemetry signal (step 335), the remote unit 102 in step 340 generates and transmits an outgoing telemetry signal that includes the measurement test results to the headend unit 25. To this end, the processor 145 provides the measurement test results as a baseband digital signal to the modulator 135. The processor 145 also tunes the telemetry transmitter 120 to an appropriate telemetry communication frequency. The modulator 135 then generates an outgoing IF telemetry signal containing the measurement test results and provides the outgoing IF telemetry signal to the telemetry transmitter 120. The telemetry transmitter 120 provides to the coupler 160 an outgoing RF telemetry signal having a carrier frequency equal to the telemetry communication frequency. The outgoing RF telemetry signal that includes the measurement test results then propagates to the headend unit 25 over the distribution network 20. Fig. 4 shows a flow diagram of an exemplary propagation delay calibration operation performed by the headend unit 25 of Fig. 1 in conjunction with a plurality of remote units. The purpose of the propagation delay calibration operation is to provide each remote unit with its own propagation delay value, PD, which is then used by the remote unit to facilitate synchronization of measurements with other remote units involved in a test. The operations of Fig. 4 are typically performed before commencing the operations shown in Fig. 3. However, the operations of Fig. 4 need not be associated with a particular test. Once the value PD is stored in each of the remote units, those remote units may participate in a plurality of subsequent tests. In step 402, the headend unit 25 addresses an uncalibrated remote unit and requests that the uncalibrated remote unit prepare for a calibration test signal. By "uncalibrated", it is meant that the remote unit has not been calibrated in the present calibration operation. The headend unit 25 then, in step 404, waits for the remote unit to return an acknowledgement signal signifying that is prepared for the calibration test signal. To prepare for the test signal, the remote unit performs the requisite functions to enable itself to transmit a calibration return signal immediately upon reception of the calibration test signal. Those of ordinary skill in the art may readily configure a remote unit such as the remote unit 102 to prepare itself adequately for the calibration test. Once the acknowledgement signal is received, the headend unit 25 in step 406 transmits a calibration test signal and starts a counter. The remote unit receives the test signal and immediately transmits a calibration return signal to the headend unit 25. The headend unit 25, in step 408 receives the test signal, stops the counter, and records the counter value, CV. The headend unit 25 then, in step 410, derives the propagation delay value, PD, for the remote unit and transmits the value to the remote unit for future use. The headend unit derives PD from the counter value CV. The value CVis representative of approximately twice the relevant propagation delay, inclusive of signal processing time at the remote unit. As a result. J /> CV represents a fair approximation of the propagation delay value PD for the remote unit. The headend unit 25 then, in step 412, determines if there are any other uncalibrated remote units. If so, then the headend unit returns to step 402. If not, then the headend unit ends the propagation delay calibration operation. It will be note that the above described embodiments are merely illustrative, and that those of ordinary skill in the art may readily devise their own implementations and modifications that incorporate the principles of the present invention and fall within the spirit and scope thereof.

Claims: We claim: 1. An impairment detection system for use in a multichannel communication system, said multichannel communication system operable to communicate radio frequency (RF) signals on plurality of channel frequencies, the impairment detection system comprising: a) a headend unit operably connected to the multichannel communication system, said headend unit operable to transmit one or more information signals on the multichannel communication system, the one or more information signals including information identifying one or more frequencies to be measured, and transmit a trigger signal; b) a first remote unit operably connected to a first location on the multichannel communication signal to receive at least one of the information signals, said first remote unit further operable to obtain information identifying at least a first frequency of the one or more frequencies to be measured, and perform a measurement measuring a signal level corresponding to at least the first frequency responsive to the trigger signal; c) a second remote unit operably connected to a second location on the multichannel communication signal to receive at least one of the information signals, said second remote unit further operable to obtain information identifying at least the first frequency of the one or more frequencies to be measured, and perform a measurement measuring a signal level corresponding to at least the first frequency responsive to the trigger signal at about the same time as the first remote unit performs the measurement corresponding to the first frequency. 2. The impairment detection system of claim 1 , wherein the multichannel communication system includes a distribution network and a plurality of subscriber networks operably connected to the distribution network, and wherein the first and second remote units are coupled directly to the distribution network. 3. The impairment detection system of claim 1, wherein the one or more frequencies to be measured comprises a plurality of frequencies; the first remote unit is further operable to obtain information identifying the plurality of frequencies and perform a measurement measuring a signal level corresponding to each of the plurality of frequencies; and the second remote unit is further operable to perform a measurement measuring a signal level corresponding to each of the plurality of frequencies at about the same time the first remote unit performs the measurement corresponding to each of the plurality of frequencies. 4. The impairment detection system of claim 1, wherein the first remote unit is further operable to provide a measurement results signal to a measurement results receiver, the measurements results signal including information identifying the measured signal level of at least the first frequency. 5. The impairment detection system of claim 4 wherein the headend unit includes the measurement results receiver. 6. The impairment detection system of claim 1 , wherein the first remote unit is further operable to receive a first information signal of the one or more information signals and the second remote unit is further operable to receive a second information signal of the one or more information signals. 7. The impairment detection system of claim 1, wherein headend unit is further operable to generate the trigger signal as part of the one or more information signals. 8. A method for detecting impairments in a multichannel communication system, said multichannel communication system operable to transmit signals on plurality of channel frequencies, the method comprising the steps of: a) transmitting from a headend unit one or more information signals on the multichannel communication system, the one or more information signals including information identifying one or more frequencies to be measured b) transmitting a trigger signal on the multichannel communication system; c) receiving at a first remote unit operably connected to a first location on the multichannel communication system at least one of the information signals and the trigger 2? signal; d) performing at the first remote unit, responsive to the trigger signal, a measurement measuring a signal level corresponding to at least a first frequency of the frequencies to be measured, the first frequency identified in the received at least one of the information signals; e) receiving at a second remote unit operably connected to a second location on the multichannel communication system at least one of the information signals and the trigger signal; f) performing at the second remote unit, responsive to the trigger signal, a measurement measuring a signal level corresponding to at least the first frequency of the frequencies to be measured at about the same time as the first remote unit performs the measurement corresponding to the first frequency. 9. The method of claim 8, wherein the multichannel communication system includes a distribution network and a plurality of subscriber networks operably connected to the distribution network, and further comprising the steps of coupling the first and second remote units directly to the distribution network. 10. The method of claim 8, wherein the one or more frequencies to be measured comprises a plurality of frequencies, step d) further comprises performing at the first remote unit a measurement measuring a signal level corresponding to each of the plurality of frequencies, and step f) further comprises performing at the second remote unit a measurement measuring a signal level corresponding to each of the plurality of frequencies. 1 1. The method of claim 8, further comprising a step g) of transmitting from the first remote unit a measurements results signal to a measurements results receiver, the measurements results signal including information identifying the measured signal level of at least the first frequency. 12. The method of claim 11 , wherein step g) further comprises transmitting from the first remote unit the return signal to the measurements results receiver, wherein the headend unit includes the measurements results receiver. 13. The method of claim 8, wherein step c) further comprises receiving at the first remote unit a first information signal of the one or more information signals and step e) further comprises receiving at the second remote unit a second information signal of the one or more information signals. 14. The method of claim 8, wherein step b) further comprises transmitting the trigger signal as part of the one or more information signals. 15. A remote unit for use in an impairment detection system in a multichannel communication system, said multichannel communication system operable to communicate radio frequency (RF) signals on plurality of channel frequencies, the impairment detection system including a headend unit coupled to the multichannel communication system, the remote unit comprising: at least one coupling to the multichannel communication signal, said at least one coupling operable to receive an information signal from the headend, the information signal including information identifying a plurality of frequencies to be measured, receive a trigger signal, receive a measurement signal at each of the plurality of frequencies to be measured; a processor operably coupled to the at least one coupling, the processor operable to: receive the information identifying the plurality of frequencies to be measured, receive the trigger signal; obtain a signal level measurement for a first frequency of the plurality of frequencies to be measured at a predetermined time after receiving the trigger signal. 16. The remote unit of claim 15 wherein the processor is further operable to obtain the signal level measurement for the first frequency at about the same time as a plurality of remote units obtain signal level measurements for the first frequency. 17. The remote unit of claim 15 wherein the at least one coupling includes a first coupling for coupling to the multichannel communication system, the first coupling operable to receive the information signal and trigger signal; a second coupling for coupling directly to the multichannel communication system, the second coupling operable to receive the RF measurement signals. 18. The remote unit of claim 15 wherein the processor is further operable to obtain the signal level measurement for the first frequency of the plurality of frequencies to be measured at the predetermined time after receiving the trigger signal, wherein the predetermined time is based in part on a propagation delay between the headend and the remote unit. 19. The remote unit of claim 15 further comprising a circuit coupled between the at least one coupling and the processor, the circuit operable to receive RF measurement signals and generate digital measurement signals therefrom. 20. The remote unit of claim 15 further comprising a measurement receiver coupled between the at least one coupling and the processor, the measurement receiver further coupled to the processor through the control line, the measurement receiver operable to: tune to the first frequency response to a control signal received from the processor, receive RF measurement signals within a resolution bandwidth of the first frequency and provide a first frequency measurement signal therefrom, the first frequency measurement signal including a first frequency energy level. and wherein the processor is further operable to obtain the signal level measurement representative of the first frequency energy level.

Inventors: GOTWALS MICHAEL D, JONES RICHARD E JR, GOTWALS, MICHAEL, D., JONES, RICHARD, E., JR.
Applicants: WAVETEK WANDEL GOLTERMANN INC, WAVETEK WANDEL GOLTERMANN, INC.
Application year: 1999
Main classification: H04N17
IPC codes: H04N7, H04N17