Potential 'Gotchas' in an 85 MHz Return Path

Sept. 16, 2015
Changing an HFC network from a 42 MHz split to an 85 MHz split presents significant challenges in the home and local access network. These ...

Changing an HFC network from a 42 MHz split to an 85 MHz split presents significant challenges in the home and local access network. These challenges result from the dramatic change in RF level experienced in the home and network as the frequency band shifts from downstream to upstream usage. However, new strategies are emerging to allow the shift to an 85 MHz split without requiring a wholesale upgrade of in-home equipment.

Why is there an issue?

Ideally, the change of the HFC frequency split would not cause any issues. But, the tuners within set-top boxes, cable modems and cable-ready TV sets are not perfect. The presence of significant energy in their originally designed reception band even outside of the nominal tuned channel can cause distortions due to automatic gain control (AGC) circuitry.

An AGC circuit exists to protect a device's internal tuner from too much energy in the incoming signal and to boost the signal when it is too low. Many AGC circuits integrate the power received over a wide bandwidth, not just the narrow channel currently desired, and adjust the level into the tuner accordingly. When the AGC circuit detects that the incoming signal power is too high, it will attenuate the signal to protect the tuner. Harmonic distortion or similar issues caused by a powerful upstream signal above 54 MHz in the tuner may also contribute to the problem.

The upstream transmit signal from an 85 MHz cable modem is potentially quite powerful. Therefore, the transmitted signal will generate reflections within each splitter that it crosses. That reflected energy travels back through the rest of the home network and can cause a device's AGC circuit to attenuate the entire downstream signal and create video problems.

The problem of unexpected reflected energy in the downstream tuner's receive band is not limited to the home where a new 85 MHz modem is deployed. When a signal is sent from one home into a port on a tap, reflections are generated in the splitter circuitry within the tap. Therefore, another home that shares a tap's internal splitter can receive a high level of reflected energy. In testing, reflections with as little as 24 dB of loss were seen.

Diagnosing 85 MHz related issues in the field

Problems caused by AGC overload due to upstream transmissions below 85 MHz tend to be intermittent, showing up only when the new modem is transmitting. A report of occasional video breakups can have many possible causes, and 85 MHz reflections add yet another.

Further complicating the diagnosis, subscribers are likely to be watching television at the same time that others in their home or neighborhood are using high-speed data services. If a reflected signal issue exists, it may be seen in the evening, but may disappear the following morning. A technician who arrives at the home may not see any video degradation and, unless trained to also look for 85 MHz modem issues, may dismiss the problem as a temporary interruption in the plant.

Preventing 85 MHz related issues

Several alternatives exist to remediate a home where a new 85 MHz modem has been installed; the sensitive CPE devices can be protected individually or addressed by remediating the home overall.

One approach to preventing in-home issues is to use a two-port cable modem for 85 MHz deployments, where one port addresses the HFC network and the other port addresses the home network. Using this device eliminates the possibility of interference in the home by directing all 85 MHz upstream transmissions directly onto the HFC network.

Alternatively, if a traditional single-port modem configuration is used, then filters that block signals between 42 MHz and 85 MHz should be added to each set-top box or other cable receiving device or alternatively to a splitter port leg to protect multiple devices. The additional filtering helps prevent reflections in the 42-85 MHz band from reaching the other ports used for the home.

Preventing issues in the local access network

A more challenging situation is present if the operations department determines that some tap ports do not have limited port-to-port isolation and therefore may pass 85 MHz energy from one port to another. One option is to direct installers to place bandpass filters on other ports of a tap potentially affected when the new modem is installed.

Another possibility is to replace the tap faceplate or even the entire tap as a part of the 85 MHz plant upgrade. This solution can help prevent 85 MHz crosstalk and makes the most sense if the tap is to be replaced as part of a 1.2 GHz downstream upgrade.

Advance deployment of 85 MHz modems before plant upgrades

Some MSOs are considering deploying 85 MHz modems ahead of planned 85 MHz plant upgrades while continuing to use the 54-85 MHz frequency band for downstream services. Unfortunately, problems can be caused by an 85 MHz modem even when it operates below 42 MHz if it does not have a built-in switchable filter. Testing indicates that at least some digital-to-analog converters (DACs) used for modem transmitters have a noise floor that pops up during transmissions. The levels seen were fairly low, but may cause distortion to analog video signals in the 54-85 MHz band. To avoid this problem, service providers can deploy a device with a built-in switchable filter.

Conclusion

Consumer demand for upstream bandwidth is increasing, causing service providers to evaluate new methods of delivering it. The latest DOCSIS standards provide support for upstream operation above the current operating 5 to 42 MHz or 5 to 65MHz bands. Successful expansion of the upstream band requires that actions be taken to prevent problems with legacy equipment already deployed in the field.

A two-port cable modem or gateway provides the most operationally friendly option, but other options can also provide acceptable performance. Improving the port-to-port loss in splitters in the home as well as within taps supporting homes with new 85 MHz modems can prevent problems from showing up after the new modems are deployed.

Carol Ansley is Senior Director, CTO Office, for ARRIS.

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