Useful Features for Validating OFDMA in DOCSIS Systems

Learn about the different features available on VeEX CATV products for testing OFDMA channels

OFDMA (Orthogonal Frequency Division Multiple Access) is the upstream transmission method for DOCSIS 3.1 and 4.0.

Cable operators are in the midst of upgrading their upstream spectrum, from legacy "Low Split" (42 or 65 MHz) to "Mid Split" (85 MHz) or "High Split" (204 MHz). High Split systems provide the necessary spectrum space to support up to two 96 MHz wide OFDMA channels, which can yield >1 Gbps upload speeds.

OFDMA Upstream Channel Basics

Channel spacing can be from 6.4 MHz wide to 96 MHz wide.
It can be placed anywhere in the upstream spectrum between 5 MHz to 204 MHz.
Upstream spectrum >85 MHz is strictly reserved for OFDMA channels and no SC-QAM.
Below is an upstream spectrum trace captured using an AT2500-3G, showing four SC-QAM and two OFDMA channels within a High Split spectrum. In this example, notice the spectrum gap between the two OFDMAs, which is the FM band. This spectrum range is excluded from service to avoid FM radio interference.

Upstream spectrum trace showing showing SC-QAM, FM band, and OFDMA channels within a High Split spectrum

The standard measurements for OFDMA channels are shown below with a CX380C Cable Modem connection.

An Example of OFDMA Testing Providing Value

A large cable operator recently began deploying OFDMA channels alongside SC-QAM channels and they observed OFDMA channel levels that were typically 3 to 6 dB different than the SC-QAM levels. One consequence of this level discrepancy is a "false fail" declaration with HIP certification tests.

The reason for this difference is actually due to DOCSIS specifications:

OFDMA channels are measured using a 1.6 MHz bandwidth section of the entire channel (similar to how the PLC level is used for determining OFDM levels).
SC-QAM channels are measured based on 6.4 MHz bandwidth.
This results in different levels between these different upstream channel types.

To remedy this perceived issue, we modified the Location settings in the CX310 and now treat the Level Pass/Fail threshold separately between SC-QAM and OFDMA channels. These settings can be configured within the test set or VeSion R-Server profiles, allowing the user to set independent pass/fail thresholds.

Up Ranging Level: Applies for only SC-QAM Upstream channel levels.
OFDMA US Level: Applies for OFDMA Upstream channel levels.

CX380C’s OFDMA In-Channel Frequency Response (ICFR)

We recently introduced OFDMA In-Channel Frequency Response (ICFR) for the CX380C. This feature is a great troubleshooting tool for a technician to see and fix impairments in the transmitted OFDMA. Selecting the OFDMA UCD will display its Frequency Response trace and P/V value.

Return Path Monitoring for Upstream Channels

Below is the VeSion CX180R+ in action, with the return path probe continuously monitoring nodes for plant noise and SC-QAM & OFDMA traffic levels.

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One of the critical features of a return path monitoring system is to report signal outages with real-time alarming (e.g. ‘UCD No Signal’ alarm, as shown below).

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V-TEST: Proof is in the Pudding

Perhaps the best measurement proving that OFDMA channels are working properly is V-TEST, which is VeEX's Internet access speed test application. For High Split systems, properly configured OFDMA channels will transmit >1 Gbps upload rates.

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