RxMER

You Need RxMER for DOCSIS 3.1 OFDM

Why is RxMER per subcarrier valuable?

RxMER per subcarrier analysis is a valuable tool for cable operators deploying DOCSIS 3.1 OFDM channels, as it can provide insight into the performance of a subscriber’s modem.

RxMER stands for Receive Modulation Error Ratio, and it is a measure of the quality of the received signal. By analyzing RxMER per subcarrier, cable operators can determine how close a modem is to its threshold for error-free transmission. If the RxMER is too low, it can lead to data errors, which can cause issues for subscribers, particularly in the presence of changing weather conditions.

By analyzing the RxMER data, technicians can quickly determine whether a modem is close to its threshold for error-free transmission, which can help them understand whether issues reported by the subscriber are due to signal impairments or other factors. If the RxMER is close to the threshold, it may indicate that the subscriber is experiencing intermittent issues that are not visible during the technician’s visit but can occur during changing weather conditions.

Overall, RxMER per subcarrier analysis is a valuable tool for cable operators deploying DOCSIS 3.1 OFDM channels, as it can help them quickly identify and troubleshoot signal impairments that may be affecting their subscribers. By analyzing the RxMER data, technicians can take appropriate action to ensure that subscribers have a reliable and consistent connection, improving their quality of experience and overall satisfaction.

RxMER per subcarrier analysis provides valuable insights into the performance of a subscriber’s modem when deploying DOCSIS 3.1 OFDM channels. The RxMER data can be used to quickly determine whether the subscriber is experiencing issues with high-speed data, and if so, whether the issue is related to the modem’s ability to use the OFDM channel or other factors.

A very good RxMER across the band indicates that the subscriber should be able to achieve good data speeds with their DOCSIS 3.1 modem on the OFDM channel. On the other hand, a very low RxMER, particularly below 24 dB, may indicate that the subscriber is unable to use the OFDM channel at all and could be the reason for the complaint about high-speed data issues.

Moreover, RxMER data can be used to determine how far the modem is from the threshold of error-free transmission. This information can help technicians understand whether intermittent issues reported by the subscriber may be due to the modem crossing the threshold during changing weather conditions or other factors.

Finally, it’s worth noting that RxMER can have variations over frequency, such as roll-off, which can lead to unpredictable issues for the modem, particularly at higher frequencies. By having visibility into the RxMER per subcarrier data, technicians can gain valuable insights not available with spectrum data alone, allowing them to troubleshoot and resolve issues more effectively.

If you are deploying DOCSIS 3.1 OFDM channels today RxMER per subcarrier analysis will quickly tell you the following things about the performance of a subscriber’s modem:

  • If the subscriber has very good RxMER across the band, they should realize good data speed with their DOCSIS 3.1 modem on the OFDM channel.
  • If the subscriber’s RxMER is very low or below 24 dB, it would be expected that the subscriber is likely not able to use the OFDM channel at all and possibly the reason they are complaining about high-speed data issues.
  • RxMER data can be used to determine how far from threshold the modem is. For example, let us say the subscriber is complaining about intermittent issues. Seeing that RxMER is right at the threshold or maybe 1 dB from the threshold of 4096-QAM would help a technician understand that everything may be fine while they are at the subscriber’s home, but during changing weather, the RxMER may cross the threshold and cause the subscriber issues.
  • Consider the fact that RxMER will often have variation over frequency, such as roll-off. Low frequencies may be fine, but higher frequencies can be awfully bad. In many scenarios the modem may experience unpredictable issues. Having visibility into the RxMER per subcarrier data gives insight not available with spectrum data only.

The DOCSIS 3.1 PHY specification includes several PNM parameters and tests. One of the most useful downstream tests for OFDM is called receive modulation error ratio (RxMER) per subcarrier. RxMER per subcarrier provides an MER measurement for each subcarrier in the OFDM (downstream) or OFDMA (upstream) channel. This means if an OFDM channel has 7600 subcarriers, the RxMER per subcarrier data will provide 7600 MER results, one corresponding to each subcarrier. When plotted, it looks like a spectrum analyzer chart, but has different meanings. See Figure 1 for an example plot of OFDM RxMER per subcarrier data.

RxMER

In Figure 1 there are some important observations one can make. First, the average RxMER is 29.85, as shown in the top left of the chart. A red line is added at 39 dB MER, which is the minimum threshold for 4096-QAM per the DOCSIS PHY specification. Ideally, all RxMER datapoints should be above the red line to provide the highest level of data speed to the subscriber. If the CMTS was only configured to provide 4096-QAM, this modem would not be able to reliably receive any data. Fortunately, CMTSs are configured to support multiple downstream modulations, called profiles. This allows the modem to drop to a lower modulation, such as 512-QAM, to operate with an average RxMER per subcarrier of 29.85 dB. (See Table 1 for the mapping of RxMER to OFDM and OFDMA modulation.)

Finally, there are two sections of the RxMER plot where the chart goes off the screen: one between 749 MHz and 757 MHz, and another between 851 MHz and 865 MHz. These are called exclusion bands. Exclusion bands are configured in the CMTS to disable subcarriers where the cable operator knows high levels of RF ingress exists, such as LTE interference. Exclusion bands are helpful to disable subcarriers which we know can be severely impacted no matter how hard we work to clean up the HFC plant.

RxMER can be directly mapped to the modulation level which can be supported by a receiving cable modem. These mappings are defined in [PHYv3.1] (see Table 46 – CM Minimum CNR Performance in AWGN Channel) and the [CM- OSSIv3.1] (see Table 72 – CmDsOfdmRequiredQamMer Object).

Table 1 shows a reliability matrix from the PNM Best Practices Primer: HFC Networks (DOCSIS®3.1) document. As an example, for reliable operation at 1024-QAM with RxMER equal across all subcarriers, empirical evidence shows that the RxMER must be above 30 dB. If the ThresholdRxMerValue, the RxMER for the lowest 2% of subcarriers, falls below 29 dB, uncorrectable errors begin to appear, which means 1024-QAM can no longer be reliably supported by the relevant channel. The number of bit errors across the poor-quality subcarriers exceeds what the LDPC FEC can correct. Instead of looking at the RxMER of every individual subcarrier, the 2nd percentile threshold RxMER value can be monitored to confirm whether or not a specific modulation profile can be supported by the channel. [1]

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