This calculator is designed for estimating the total transmit power of a DOCSIS 3.1/4.0 cable modem and its Dynamic Range Window (DRW) headroom based on various channel configurations.
This calculator helps estimate Cable Modem (CM) upstream transmit power levels based on DOCSIS 3.1 principles, particularly focusing on Total Composite Power (TCP) and Dynamic Range Window (DRW).
The calculations and concepts are largely based on the work presented in the following paper:
- Hranac, R. (2023). *DOCSIS 3.1 Upstream Transmit Power Control Revisited*. NCTA Technical Papers. Download Paper
Additional context can be found in CableLabs DOCSIS specifications (e.g., PHY, MAC) and SCTE standards.
Using the Calculator:
- Channel Inputs:
- Enable Channel: Check this box to include the channel in calculations and visualization. Unchecking disables the channel and its inputs.
- Channel Center Frequency (MHz): The center frequency of the SC-QAM or OFDMA channel.
- Channel RF BW (MHz): The total RF bandwidth of the channel (e.g., 6.4 MHz for SC-QAM, 24-96 MHz for OFDMA).
- Bandwidth in which power is measured (MHz): The reference bandwidth used by the CMTS/measurement device to report power (typically the channel width for SC-QAM, often 1.6 MHz for OFDMA as per DOCSIS spec).
- CM Transmit Power (dBmV): The power level reported by the CM for that channel, measured in the specified measurement bandwidth.
- CMTS Reported CM Power (dBmV): (Optional) The actual power level received at the CMTS for this channel. Used for calculating RX Difference.
- CMTS Set CM Receive Power (dBmV): (Optional) The target receive power level configured on the CMTS for this channel. Used for calculating RX Difference.
- Channel Visualization:
- Displays enabled channels graphically. SC-QAM channels are blue, OFDMA channels are green.
- The Y-axis represents the channel power normalized to a 1.6 MHz bandwidth (P1.6r_n).
- The shaded area indicates the **Dynamic Range Window (DRW)**, the difference between the highest and lowest P1.6r_n values among active channels.
- The area is shaded light green if DRW ≤ 12 dB (within spec).
- The area is shaded light red if DRW > 12 dB (violates spec).
- The dashed green lines show the minimum and maximum P1.6r_n levels defining the DRW.
- The info box shows:
- CM Dynamic Range Window (DRW): The calculated DRW value. Turns red with a ⚠️ if > 12 dB.
- CM Total Tx Pwr: The calculated Total Composite Power (TCP) for all enabled channels. Turns red with a ⚠️ if > 65 dBmV (typical Pmax).
- Basic/Advanced View:
- Basic View (Default): Shows Channel Inputs and Channel Visualization.
- Advanced View: Shows all sections, including the detailed report tables (Per Channel, TCS, Worst Channel, Effective DRW) and Advanced Instructions.
Note: Calculations assume standard DOCSIS power calculation methods (e.g., power measured relative to 1.6 MHz bandwidth). Ensure input values accurately reflect the measurement conditions.
Advanced Panel Instructions
These panels provide detailed calculations based on the channel inputs. Understanding these values helps in assessing modem performance and potential issues.
Per Channel Report
- Linear Transmit Power: The transmit power of the channel expressed in linear units relative to 1 mW (calculated as
10^(Channel TX Power / 10)). Useful for summing powers before converting back to dBmV for TCP.
- Channel Number of Equivalent 6 MHz Channels (Neq_n): Number of equivalent 1.6 MHz channels within this channel's RF bandwidth (
ceil(Channel RF BW / 1.6)). Represents the channel's bandwidth in standardized DOCSIS power calculation units. (Note: Label updated for clarity).
- 6 MHz Channel TX Power (dBmv): The calculated total transmit power for the entire channel bandwidth. This is derived from the reported power and adjusted based on the ratio of the channel's RF bandwidth to the bandwidth in which power was measured (
Reported Power - 10*log10(Meas BW / Channel RF BW)). This value represents the actual power output for the full channel width. (Note: Label updated for clarity).
- Channel Power Spectral Density in 1.6 MHz Chunks (dBmV) (P1.6r_n): The calculated power within a standardized 1.6 MHz equivalent bandwidth for this channel (
Reported Power - 10*log10(ceil(Meas BW / 1.6))). This normalization allows for direct comparison of power density between channels of different widths and is critical for calculating the Dynamic Range Window (DRW). (Note: Label updated for clarity).
- CM Channel TX Headroom (db): The difference between the highest P1.6r_n among all active channels (
P1.6r_l from Worst Channel Report) and this specific channel's P1.6r_n. It shows how much "quieter" this channel's power density is compared to the "loudest" channel. Lower headroom values mean the channel is closer to the maximum power density being transmitted. (Note: Label updated for clarity).
- Channel RX Difference from CMTS Set Point (dB): The difference between the
Actual Receive Power at CMTS input and the CMTS Set CM Receive Power input. A positive value means the signal is arriving hotter than expected, negative means lower. This requires accurate input from CMTS measurements.
- Adjusted Channel TX Headroom (dB): The
CM Channel TX Headroom adjusted by the Channel RX Difference. This attempts to factor out receive level deviations to estimate the true headroom based on transmit levels.
TCS Report
- Number of Equivalent 1.6 MHz Ch: Total count of 1.6 MHz equivalent channels across the specified group (Whole TCS, Legacy SC-QAM, OFDMA). Calculated from the sum of individual channel Neq_n values. (Note: Label updated for clarity).
- Occupied US Spectrum (MHz): Total RF bandwidth occupied by all enabled channels in the specified group (sum of Channel RF BW values).
- Total Composite Power (TCP) (dBmV): The logarithmic sum of the linear powers (calculated from P1.6r_n and Neq_n) of all enabled channels within the specified group. Represents the total power the modem transmits for that group. This is compared against Pmax.
- Pmax (dBmV): The maximum allowable Total Composite Power for the cable modem (typically 65 dBmV for DOCSIS 3.1 in North America, but can vary based on CM capabilities and configuration).
- TCS TCP Headroom (dB): The difference between Pmax and the calculated TCP (Whole TCS) (
Pmax - TCP (Whole)). Indicates how much power margin the modem has before hitting its maximum output capability. Lower values indicate the modem is transmitting closer to its limit.
- Combined TCS RX Difference from CMTS Set Point (dB): The bandwidth-weighted average of the individual
Channel RX Difference values across all active channels in the Whole TCS. Provides an overall indicator of how the entire transmit signal set is being received relative to target levels.
- Adjusted TCS TCP Headroom (dB): The
TCS TCP Headroom adjusted by the Combined TCS RX Difference. Attempts to estimate the true headroom available if all channels were received exactly at their set points.
Worst Channel Report
- P1.6hi (dBmV): The theoretical maximum P1.6r_n per channel if the total Pmax were distributed perfectly evenly across all Neq for the Whole TCS (
Pmax - 10*log10(Neq (Whole TCS))). Represents an ideal equal power distribution scenario.
- P1.6r_l (dBmV): The highest actual P1.6r_n value observed among all active channels. This represents the power density of the "loudest" 1.6 MHz equivalent channel segment.
- Min Ch Headroom (dB): The minimum headroom calculated across all active channels (
P1.6hi - P1.6r_l). This indicates the headroom of the "loudest" channel relative to the theoretical maximum equal distribution (P1.6hi). A small value suggests the loudest channel is near the theoretical maximum PSD.
- Worst Rx Channel Difference from CMTS Set Point (dB): The most negative
Channel RX Difference value found among all active channels (i.e., the channel arriving lowest relative to its set point).
- Worst Adjusted Min Ch Headroom (dB): The minimum
Adjusted Channel TX Headroom value found among all active channels. This highlights the channel with the least headroom after accounting for receive level deviations.
Effective DRW
- Effective DRW (dB): Dynamic Range Window - the difference between the highest (P1.6r_l) and lowest P1.6r_n values among all active channels (
P1.6r_l - min(P1.6r_n)). Per DOCSIS 3.1 PHY specification [CM-SP-PHYv3.1], this value should generally not exceed 12 dB to ensure proper CMTS receiver operation.