Characterizing broadband communication networks
View Patent ↗A remote testing device, including a cable modem circuit, requests an IP address from a remote cable modem termination system (CMTS) and receives back IP connection information including adaptive equalizer coefficients, which represent compensation and/or correction for transmission path noise. Using the equalizer coefficients, the remote testing device can determine a phase response, phase angles for each frequency increment over a channel bandwidth, and group delay.
1. A testing device for determining a phase response in an upstream transmission path of a communication network, comprising:
a coupler for sending and receiving broadband RF signals to and from the communication network;
a cable modem circuit for generating an RF transmission signal to request an IP address from a remote cable modem termination system disposed at a headend of the communication network, and for receiving IP connection information from the cable modem termination system, including pre-equalization coefficient information relating to the transmission path between the testing device and the cable modem termination system,
wherein the pre-equalization coefficient information is generated at the headend by the remote cable modem termination system; and
a processing circuit connected to the cable modem circuit for receiving the pre-equalization coefficient information from the cable modem circuit, and for determining the phase response of the transmission path from the remotely obtained pre-equalization coefficient information,
wherein the pre-equalization coefficient information includes adaptive equalizer coefficients, and wherein in operation, the processing circuit performs: a discrete Fourier transform of the adaptive equalizer coefficients; and an inversion operation of a result of the discrete Fourier transform, to determine the phase response of the upstream transmission path.
2. The testing device according to claim 1 , wherein in operation, the processing circuit generates a frequency response from the result of the discrete Fourier transform.
3. The testing device according to claim 2 , wherein in operation, the processing circuit generates a list of phase angles for each frequency increment of the frequency response over a bandwidth of a channel.
4. The testing device according to claim 3 , wherein in operation, each phase angle is determined by calculating the tan −1 of an imaginary portion of a value of the frequency response divided by a real portion of the value of the frequency response.
5. The testing device according to claim 3 , wherein in operation, the processing circuit determines group delay from the list of phase angles.
6. The testing device according to claim 5 , wherein in operation, the processing circuit determines the group delay by calculating a difference in phase angle between two frequency points, and multiplying the difference by a scaling factor.
7. The testing device according to claim 5 , wherein in operation, the processing circuit determines the group delay by calculating a first derivative of a function defining the phase angles.
8. The testing device according to claim 5 , further comprising a display for displaying the phase response and/or the group delay.
9. A method for determining an upstream phase response in a transmission path of a communication network, comprising:
providing a testing device with a cable modem circuit, a processing circuit, and a coupler for sending and receiving broadband RF signals to and from the communication network;
generating an RF transmission signal in the cable modem circuit to request an IP address from a remote cable modem termination system disposed at a headend of the communication network;
receiving IP connection information from the cable modem termination system in the cable modem circuit, including pre-equalization coefficient information relating to the transmission path between the testing device and the cable modem termination system, wherein the pre-equalization coefficient information is generated at the headend by the remote cable modem termination system;
receiving the pre-equalization coefficient information in the processing circuit from the cable modem circuit, wherein the pre-equalization coefficient information includes adaptive equalizer coefficients;
performing a discrete Fourier transform of the adaptive equalizer coefficients;
performing an inversion operation of a result of the discrete Fourier transform to determine the phase response of the upstream transmission path.
10. The method according to claim 9 , further comprising generating a frequency response from the result of the discrete Fourier transform.
11. The method according to claim 10 , further comprising generating a list of phase angles for each frequency increment of the frequency response over a bandwidth of a channel.
12. The method according to claim 11 , wherein each phase angle is determined by calculating the tan −1 of an imaginary portion of a value of the frequency response divided by a real portion of the value of the frequency response.
13. The method according to claim 11 , further comprising determining group delay from the list of phase angles.
14. The method according to claim 13 , wherein the processing circuit determines the group delay by calculating a difference in phase angle between two frequency points, and multiplying the difference by a scaling factor.
15. The method according to claim 13 , wherein the processing circuit determines the group delay by calculating a first derivative of a function defining the phase angles.