Although Wi-Fi operates at different frequencies than the ones used by the DOCSIS system, many interference cases have been observed. Current cable modems typically have an embedded Wi-Fi Access Point and a small form factor, which means that the RF circuitry of the Wi-Fi subsystem and the DOCSIS subsystem are located closely to each other, increasing the likelihood of interference.
Different than the case of LTE interference, a typical root-cause of Wi-Fi interference is due to the fact that intermediate and local oscillator frequencies on both subsystems operate at the same frequency. The interference can have an impact on the DOCSIS system, on the Wi-Fi system or on both.
Interference to the DOCSIS system
The interference case that has been observed frequently is the degradation of the downstream signal demodulation of one or multiple DOCSIS channels at the cable modem. As a consequence the modem has significantly higher values for the number of packets (codewords) that were received incorrectly. Typically one or two DOCSIS channels are affected (although this also depends on the used Wi-Fi channel width), and the frequency of the affected DOCSIS channels is often related to the frequency of the Wi-Fi channel that is in use.
In many cases the interference is continuous, which may cause the affected DOCSIS channel to become unusable, forcing the modem into partial service mode, and causing a decrease in maximum throughput for that modem. In the case that the affected channel is the primary downstream channel for that modem, the modem will go off-line, thus breaking the service for that customer.
Detecting the problem in the field
As an operator you can easily monitor the packets received in error (uncorrectable codewords) on each of the DOCSIS channels. If by disabling the Wi-Fi these counters with uncorrectable codewords no longer go up, the root-cause has been identified.
Interference to the Wi-Fi system
Cases have been observed where the Wi-Fi performance is impacted by interference of the DOCSIS system. This case is difficult to identify in the field, since the performance of Wi-Fi is highly variable over time and channel in the field. By testing the Wi-Fi performance in a fully controlled environment (such as the Excentis Wi-Fi testing house), it can be determined if the Wi-Fi performance is depending on the DOCSIS channel in use.
Influence of power levels and frequencies
It speaks for itself that the power levels have a significant influence on the likelihood that the problem will show up. If the input level of the DOCSIS channel to the cable modem is low, the chances that interference occurs are bigger than if the input level is high. Note that due the very complicated internal circuitry of mixers and amplifiers, it is not always straightforward to predict the impact of certain conditions. Proper analysis and testing is needed to assure hassle-free operation.
Sample dependency – test before deploy
Since the root cause of the problem is often due to insufficient shielding between the two subsystems, the impact is typically highly variable from sample to sample. While one sample might show no issues at all, another sample might suffer highly. To detect problems before units get deployed it is important as an operator to do sample checking so that quality of deployed samples is ensured. At Excentis we’ve invested in a highly automated test system for Wi-Fi interference. We’ve already saved many operators from expensive truck rolls by detecting problems before the units were deployed.