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Understanding the Anatomy of a DOCSIS OFDM Channel Based on the OCD Message

Version 3.1 of DOCSIS specifications, an international telecommunications standard for cable TV systems, introduces downstream Orthogonal Frequency Division Multiplexing (OFDM) channels, which are highly flexible in terms of frequency, bandwidth, modulation type, and active spectrum.

When a Cable Modem (CM) acquires an OFDM channel, the first step is to decode the Physical Link Channel (PLC), which is located in a 6 MHz active frequency range. The PLC has a specific structure, which allows the CM to easily decode its content without knowing all of the channel parameters.

The PLC carries critical information about the channel physical layer, such as the OFDM Channel Description (OCD) message, which is required by the CM to fully characterize the OFDM channel. The focus of this blog is on an OCD message that is extracted from the DP-1000, a DOCSIS Protocol Analyzer. The message is analyzed in order to reconstruct the channel.

OFDM Channel_Description

There are three parameters that will not be discussed: the Cyclic Prefix, the Roll-Off period, and the Time Interleaving Depth. These parameters are part of mechanisms used by the Cable Modem Termination System (CMTS) to reduce the effects of interference and maximize channel capacity. Rather, the focus here will be on the remaining parameters that define the OFDM channel in the frequency domain.

The Discrete Fourier Transform

OFDM CHannel_Discrete Fourier Transform

The Discrete Fourier Transform (DFT) Size indicates the number of subcarriers defining the OFDM channel in the frequency domain. In this case there is a channel defined over 4096 subcarriers.

The OFDM Spectrum Location

OFDM Channel_Spectrum Location_1

The OFDM spectrum location is the frequency (in Hz) of the lowest subcarrier (subcarrier 0).

OFDM spectrum location_2_no frame

The Subcarrier Assignment

OFDM Channel_Subcarrier Assignment

The Subcarrier Assignment section of the message is used to describe the nature of some subcarriers in the OFDM channel. The assignment will not change while the channel is included in the Receive Channel Set of a CM.

There are three possible subcarrier types assigned in the OCD: PLC subcarriers, excluded subcarriers, and continuous pilots.

The PLC subcarrier assignment defines which subcarriers are used to carry information on the PLC. There are 8 or 16 subcarriers allocated to the PLC, depending on the DFT size. These subcarriers are centered in a 6 MHz active frequency range.

OFDM_Subcarrier Assignment_2

OFDM CHannel_Subcarrier Assignment

Excluded subcarriers cannot be used by this downstream channel for transmission. The CMTS can use the frequency covered by these subcarriers for transmission on other channels or as guard bands between channels. Exclusion bands can be located at different locations in the OFDM channel, allowing for greater flexibility while transitioning to DOCSIS 3.1 technology.

In this case, there are two different exclusion bands between subcarriers 0-147 and 3948-4095.

OFDM_Excluded subcarriers _1

OFDM Channel_Subcarrier Assignment

The CM uses continuous pilots in the OFDM channel as reference signals to facilitate synchronization. They do not convey data but are necessary for successful decoding. Continuous pilot subcarriers have twice the amplitude of other non-pilot subcarriers. The assignment includes the continuous pilots contained in the PLC band, which are expected at specific locations.

OFDM_PLC continuous pilots

The DP-1000 DOCSIS Protocol Analyzer displays a visual representation of the OFDM channel description when locked on this kind of channel. An example for the same channel as described by the OCD message is displayed in the image below.

OFDM Channel Description

For more information about how a DOCSIS Protocol Analyzer can help in certification, design, network monitoring and debugging, contact Averna.