Upstream Linear Distortions

Option for New DSP Family

The Upstream Linear Distortion feature for the New DSP family of field analyzers takes advantage of the pre-equalization function built in to modern DOCSIS 3.0 and DOCSIS 3.1 CMTSs. The main objective for pre-equalization is to improve upstream performance in the presence of certain RF impairments related to in-channel frequency response, micro-reflections, and group delay.

Upstream Linear Distortions

The Upstream Linear Distortion feature takes advantage of the pre-equalization function built in to modern DOCSIS 3.0 and DOCSIS 3.1 CMTSs. The main objective for pre-equalization is to improve upstream performance in the presence of certain RF impairments related to in-channel frequency response, micro-reflections, and group delay. The CMTS looks at messages coming from the cable modem and evaluates the signal quality of the messages. If the CMTS determines that the messages can be improved by pre-equalization, the CMTS sends equalizer adjustment values to the cable modem. The cable modem then applies these equalizer adjustment values, called coefficients, to its pre-equalizer.

As a result of these adjustment values, the cable modem transmits a pre-distorted signal to compensate for impairments between the cable modem and the CMTS. As this pre-distorted signal traverses the HFC network, it will experience the effects of RF impairments. By the time the pre-distorted signal from the cable modem arrives at the CMTS, it will no longer have any of the original pre-distortion, as the RF impairments will have transformed it back into a near-ideal signal that the CMTS intended to see.

These signals are illustrated with time delay equalizer taps and can be evaluated for distance to fault. Upstream linear distortion analysis takes the EQ taps from the cable modem and displays their values along with the in-channel response and group delay graphs that are calculated from these values.

Pre-Equalizer Taps

In Figure 1, the blue tap (position 8) is representative of the main modem tap with a horizontal red line that indicates the allowable correction threshold which is defined in the DOCSIS specification. If any of the green pre-main or post-main taps exceed the allowable correction threshold, it indicates that there is a likely an issue or impairment in the return path.

  • If any of the pre-main taps in positions 1–7 exceed the allowable correction threshold, it often indicates the presence of impairments causing group delay issues.
  • If any of the post-main taps in positions 9–24 exceed the allowable correction threshold, it often indicates the presence of impairments causing in channel response issues.

Technicians also have the ability to quickly and easily obtain a rough distance to the fault by using adjustable markers to select specific taps which are showing signs of impairment.

Group Delay

In Figure 2, the Group Delay of a typical upstream channel is displayed to show the difference in transmission time across the channel as measured in 1 MHz increments, which are defined in the DOCSIS specification. Technicians can use the adjustable markers to help identify the frequencies where the group delay is experiencing high levels of time delay (> 200 ns/MHz), which is indicated by the ripple measurement. Moving each marker to the edges of the channel will also provide technicians with an indication of what the peak to valley value is across the full width of the channel.

In Channel Response

In Figure 3, the In Channel Response of a typical upstream channel is displayed to show the amplitude flatness of the channel as measured in 1 MHz increments, which are defined in the DOCSIS specification. Technicians can use the adjustable markers to help identify the frequencies where the in channel response is experiencing high levels of micro-reflections (> 2 dB/MHz), which is indicated by the peak to valley measurements.

As shown in Figure 4, moving each marker to the edges of the channel will also provide technicians with an indication of what the peak to valley value is across the full width of the channel.

 

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