Communications systems may use radio-frequency (RF) electromagnetic signals to communicate information. The RF electromagnetic signals used for communication may include free-space radio waves propagating in air, guided electromagnetic waves propagating in coaxial cable, or both. A concept of so-called “frequency reuse” allows cable operators to reuse the air RF frequencies by carrying within the cable networks signals at frequencies overlapping with the air RF frequencies. The frequency reuse is made possible by the electromagnetic shielding effect exhibited by coaxial cables.
In bidirectional cable networks, upstream, i.e. towards the headend, and downstream, i.e. away from the headend, signals occupy separate frequency bands called upstream and downstream spectral bands. In the United States, the upstream spectral band typically spans from 5 MHz to 42 MHz, while the downstream spectral band typically spans from 50 MHz to 1000 MHz. Together, the upstream and downstream spectral bands overlap a so-called Very High Frequency (VHF) band spanning between 30 MHz and 300 MHz. Downstream spectral band also partially overlaps a so-called Ultra High Frequency (UHF) band spanning between 300 MHz and 3 GHz.
Ideally, cable network signals do not interfere with the over-the-air signals. However, when the cable shielding, or shielding of cable network components is degraded or compromised for some reason, e.g. improper installation, rodent chews, etc., electromagnetic signals propagating in the cable network may leak out to the over-the-air environment, and vice versa. If the leakage field is sufficiently strong, interference between over-the-air and guided electromagnetic waves may occur, causing service quality degradation in over-the-air communications, cable communications, or both. The impact of signal leakage on over-the-air users may, depending on the ratio of the desired signal strength to the leakage strength and other factors, cause a mild or moderate degradation, or may even cause a complete signal disruption in extreme cases.
Licensed over-the-air users must be able to operate without interference degrading or disrupting their services. Paragraph 76.613 of Telecommunications Chapter I of Federal Communications Commission (FCC) states: “An MVPD that causes harmful interference shall promptly take appropriate measures to eliminate the harmful interference.” If a leakage-related interference occurs, the operator of the cable network causing the leakage must fix the problem. In cases where the interference may impact “safety of life and protection of property”, the FCC has an authority to require a cable operator to shut down offending signals until the problem has been resolved. Clearly, this represents a serious concern for cable network operators.
Although FCC Rules specify signal leakage limits over a wide range of frequencies, the emphasis is made in the Rules on the aeronautical band spanning between 108 MHz and 137 MHz, which is within the VHF band. This is perhaps why the signal leakage has been historically monitored mostly in the aeronautical band. Furthermore, the assumption has been that absence of leakage in the aeronautical band indicates the likely absence of leakage at other frequencies as well.
Recently, new data has emerged indicating that some frequencies may be radiated much more efficiently from a leakage location than others. Depending on the nature and geometry of the leak, there may be little VHF leakage and significant UHF leakage, and vice versa. Thus, UHF leakage still needs to be measured, even when the VHF leakage is measured. Measuring UHF leakage has become increasingly important in view of a growing cellular network usage, in particular in Long Term Evolution (LTE) spectral band. The LTE band spans between 698 MHz and 960 MHz, which is in the UHF frequency band. UHF leakage in the 470-698 MHz range may affect antenna-based TV reception and some non-TV services that may exist in the 470-698 MHz range.
For technical reasons, measuring leakage in the UHF band has proven much more difficult than in the VHF band. To provide adequate sensitivity, the VHF leakage has typically been monitored using a resonant antenna tuned to an injected “tag” signal having a very narrow bandwidth of 10-15 KHz. Digitally-modulated UHF leaking signals are much broader in bandwidth than 10-15 KHz, and often appear noise-like in a frequency spectrum. For instance, quadrature amplitude modulated (QAM) signals occupy 6 MHz or 8 MHz frequency band per channel, and there may be many (up to a hundred or more) leaking downstream QAM channels in the UHF band, increasing the total radiated bandwidth to tens, and perhaps hundreds of megahertz. Because of this, UHF signals are more difficult to detect than VHF signals.
One known approach to monitoring cable network signal leaks in the UHF band is to provide a narrowband UHF pilot signal between two active QAM carrier signals, preferably at a lower level than the adjacent active carriers, to avoid disturbing the latter; and to provide field test equipment tuned to detect the pilot signal. Thus, the pilot signal operates as an indicator of signals leaked from a cable network under test. The field test equipment may be outfitted with global positioning system (GPS) locators for automatic mapping of the pilot signal detected in the field, as a truck carrying the test equipment is driven in vicinity of the cable network. Detrimentally, this method requires modification of the downstream cable signal at the headend of the cable network, which may be expensive to implement. Furthermore, the leaking signal in the UHF band may be highly frequency dependent. Therefore, the strength of a narrowband pilot signal may not always be representative of the strength of an actual leaking downstream cable signal.
Another approach to monitor UHF signal leaks in a cable network includes disposing a detector proximate a QAM modulator source for sampling the source QAM signal. These samples are then transmitted to a field device, which compares off-air measured signals to the received samples by computing, in real time, a time-domain correlation function. If the source samples and field samples match, then the signal is identified as leaking out of the cable network. Although this approach does not require pilot signals, in that the QAM signal itself is used as a “pilot” signal, this approach does require a modification of the headend equipment. Furthermore, field test equipment needs to be upgraded with test devices including complex digital signal processors and correlators, to determine a real-time match of QAM time traces. This makes the testing solution based on signal correlation rather costly, as well.
Yet another approach to monitor UHF signal leaks in a cable network includes obtaining and displaying a frequency spectrum of electromagnetic waves in vicinity of a suspected leakage location, to assist a field technician in visually determining if the detected signals resemble QAM signals of a cable network. Although simpler and less expensive than the previously described methods, this method of cable signal leak detection relies on the field technician's knowledge and ability to interpret complex frequency spectra of detected radio waves, which requires extensive training, and may impact veracity and reliability of the test results obtained. Adding to the complexity of relying on visual cues, off-air QAM signals displayed on a frequency spectrum may rarely resemble QAM signals propagating in coaxial cable lines.