White Paper by Brady Volpe

The return path in any DOCSIS network is most frequently the weakest link of the DOCSIS network. It is the source of most RF impairments and also the location of RF ingress, which disrupts DOCSIS communication of any type. Why is this? Unlike the downstream, all signals from every subscriber’s home are funneled back to the headend or hub site to a single point. That single point is the CMTS.

Unfortunately for the CMTS the noise and DOCSIS signals are in competition with each other for the demodulation on the input of the CMTS. If the RF ingress noise is too high, then DOCSIS signals from cable modems will not be demodulated and subscribers will be unhappy with their DOCSIS service.

The best way of detecting noise in the upstream is by using a spectrum analyzer in the headend or hub site. A spectrum analyzer is simply a device that scans across a band of frequency defined by the user, say 5 MHz to 42 MHz, and then displays the RF energy that exists in that band. For example the following image using the Nimble Spectrum Analyzer shows a simple representation of a spectrum analyzer plotting the return path with one upstream DOCSIS channel from 5 MHz to 42 MHz.


Figure 1: Spectrum Analyzer 5-42 MHz, One DOCSIS Channel at 36 MHz

The left hand vertical axis of the spectrum analyzer indicates the amplitude of the trace on the display. Notice this axis in units of dBmV (decibel millivolts), which ranges from ­60 dBmV to +10 dBmV. The horizontal x-axis on the bottom of the analyzer starts at 5 MHz on the left hand side and stops at 42 MHz on the right hand side. The trace itself is the yellow line and represents RF energy as received at the input to the spectrum analyzer.


Types of Spectrum Analyzers

Spectrum analyzers come in many different varieties from different vendors. At the high end you will find vector signal analyzers that are capable of capturing very short transient events on the return. Vector signal analyzers are also able to demodulate bursty cable modem signals into their demodulated baseband signal, such as QPSK, 16­QAM or 64­QAM. They are great devices for detailed troubleshooting however they also cost roughly US $80k and are very difficult to operate.

Lower cost spectrum analyzers are available in the $15k ­ $25k range that offers basic spectrum analyzer functionality. In addition these analyzers provide the ability to demodulate downstream signals into their 64­QAM or 256­QAM signals. A limitation of these analyzers is that they are not able to see impairments under upstream DOCSIS signals, like a vector signal analyzer can. This is because most lower cost spectrum analyzers do not have sufficient speed to capture the microsecond transmission speeds of cable modems.

Other common headend spectrum analyzers are the costly headend return path monitoring systems. These are widely deployed for monitoring each upstream port destined for the CMTS. Generally return path monitoring systems are capable of measuring signals up to 85 MHz. However there are some fundamental limitations with any return path monitoring systems. First, in order to install the return path monitoring system one must put into place splitters in the return path to split some signal from the CMTS to the return path monitoring system. This adds loss between the cable modem and the CMTS as well as another point of failure in the headend or hub site. It also places the return path monitoring system on a leg that is not always identical to what the CMTS is seeing. Second, return path monitoring systems, like lower cost spectrum analyzers, are not capable of seeing noise under the cable modem. Finally, return path monitoring systems are expensive and complicated to install, configure and use.


Which Spectrum Analyzer is for You?

The best headend / hub site spectrum analyzer is the one that you already have. The one in your CMTS. Why? Because it’s already installed. It requires no additional cabling. Further the RF spectrum that is displayed by the spectrum in the CMTS is exactly what you should be looking at.

Most CMTSs today have the ability to act as a return path spectrum analyzer. This is because most CMTSs are built around a Broadcom chipset for return path demodulation. This can be used to make a very powerful and useful tool for cable operators. In addition to giving visibility to the return path spectrum, the CMTS knows when cable modems are transmitting and even more important the CMTS knows when cable modems are not transmitting. This gives the ability to see any noise that is located under the cable modem QAM channels as shown below in figure 2.


Figure 2: Spectrum Analyzer with Ingress Under Carrier

In figure 2 the max hold line is blue, the live trace is yellow and the red line is the showing the time that no modems are transmitting. This red line of no transmit time becomes important to view because it shows the noise that is present under the DOCSIS channel, centered around 23 MHz. Notice that the red line has two (2) spikes near the DOCSIS channel, one just to the left of the DOCSIS channel and one directly under the DOCSIS channel. The spike directly under the DOCSIS channel would normally be impossible to see with typical spectrum analyzers. But this is the type of ingress that would cause correctable and uncorrectable codeword errors. These have been covered in previous papers and are known to be the worst types of errors because they have a direct negative impact on subscribers, especially related to voice impairments.


Identifying Ingress

Using the CMTS as a spectrum analyzer will not only tell you if your DOCSIS upstream channel is clear, but will also aid in identifying ingress. When ingress is present in your upstream plant this means more RF energy on return path lasers, which often results in laser clipping. Laser clipping means data loss! Identifying the source of ingress noise often means identifying the frequency of the noise and then finding where the noise is leaking into the HFC plant. Once you identify the source of the leak, possibly a bad or unterminated drop at a disconnected subscriber’s house, you rectify the situation and reduce the amount of ingress noise. Figure 3 shows a couple of ingress sources using the CMTS upstream spectrum analyzer.

Identifying the source of ingress noise often means identifying the frequency of the noise and then finding where the noise is leaking into the HFC plant. Once you identify the source of the leak, possibly a bad or unterminated drop at a dis-connected subscriber’s house, you rectify the situation and reduce the amount of ingress noise. Figure 3 shows a couple of ingress sources using the CMTS upstream spectrum analyzer.


Figure 3: Upstream Spectrum Analyzer with Ingress Noise Markers

In figure 3 two vertical markers are identifying two ingress noise sources. One source at 26.9 MHz and the second at 15.5 MHz. Tofind out what may be transmitting at 26.9 MHz we can use the frequency allocation lookup table, frequently available in many headends or simply Google “what transmits at 26.9 MHz”. A Google search returns what was expected; Citizen Band radio (or CB radio) transmitting unlicensed from 26.9 MHz to 27.2 MHz This is the likely source of our Vertical Marker 1 ingress. For Vertical Marker 2 a Google search indicates that short wave radio’s 19 meter band is 15.1 MHz to 15.8 MHz, which is the likely source of the displayed ingress.

Now two sources of ingress have been quickly identified, which are over­ the ­air vs. in-house. So it is time to go out into the plant and start tracking down the location where the ingress is finding its way in. Since the CMTS upstream spectrum analyzer is all software­based vs. a heavy piece of hardware, a technician can traverse the HFC plant with nothing more than a tablet or even iPhone / Android phone. Quickly pulling pads and using the divide­ and­ conquer method will isolate the ingress location allowing the technician to resolve the noise ingress location and ultimately create a clean return path.

With some diligent work it is possible to have a return path that looks like the one in figure 4, which is a near ideal return path.


Figure 4: Upstream Spectrum Analyzer with Near Ideal Noise Floor



This paper examined upstream spectrum analysis, types of spectrum analyzers and using the CMTS as one of the best upstream spectrum analyzers possible. The CMTS can prove truly valuable as a spectrum analyzer because you already have it installed in every headend and hub site. The resources in the CMTS exist to process upstream spectral data because the spare FFT analyzer is built into the chipset. Having the CMTS be your spectrum analyzer is quite ideal because it does not require any additional cabling from return path receivers to the CMTS. You can be confident that what you see on the upstream spectrum analyzer is exactly what the CMTS is seeing. Finally, if you do not currently have a return path spectrum analyzer you likely do not know what you are missing. This paper will shed some light on how a return path spectrum analyzer can be used to see noise under the upstream DOCSIS carrier and also see other RF ingress carriers, which can then be used in the field to clean up the plant. Remember, the best spectrum analyzer is the one that you already have and use.

DOCSIS is a registered trademark of Cable Television Laboratories, Inc.

Nimble This™ Spectrum Analyzer patent pending