Method and apparatus for designing an improved transmit signal for use in a single ended line test (SELT)
View Patent ↗The present invention relates to methods and apparatuses of using a novel transmit signal that can be used to improve performance of various loop diagnostic features of SELT. In embodiments, a novel transmit signal uses a wide contiguous band of tones that includes frequencies in both upstream and downstream bands. In these and other embodiments of the invention, the SELT transmit signal is designed such that any crosstalk noise in adjacent lines appears as a burst of noise that falls within the limits of the FEC settings of the system such that it can be fully corrected without harming the performance of modems connected on the adjacent lines.
1. A method for examining a line coupled to a modem in a discrete multi-tone (DMT) communications system in which symbols communicated once every consecutive symbol period, comprising:
constructing symbols using tones in both upstream and downstream frequency bands of the DMT communications system;
forming a test signal consisting of the constructed symbols in only certain non-consecutive portions of the symbol periods and silence in the remaining symbol periods;
transmitting from the modem the test signal on the line;
receiving a reflection of the test signal from the line at the modem; and
analyzing the reflection to identify impairments in the line.
2. A method according to claim 1 , wherein the reflection comprises frequency domain S 11 data.
3. A method according to claim 1 , further comprising converting the reflection to a set of time domain data.
4. A method according to claim 3 , wherein analyzing includes identifying peaks in the set of time domain data.
5. A method according to claim 1 , wherein the modem is a CPE modem and the upstream and downstream bands are two or more adjacent bands in a xDSL system frequency band plan.
6. A method according to claim 5 , wherein the xDSL system is a vectored DSL system, and wherein transmitting is performed while one or more other lines in a vectored group including the line are operating in Showtime.
7. A method according to claim 6 , wherein the certain non-consecutive portions of symbol periods results in the constructed symbols being transmitted at a rate corresponding to an error correction capability of the vectored DSL system.
8. A method according to claim 7 , wherein the rate is a pulse of a number of symbols per DMT superframe.
9. A method according to claim 5 , wherein the constructed symbols are REVERB symbols.
10. A method according to claim 1 , wherein the tones are a contiguous set of tones spanning the upstream and downstream bands.
11. An apparatus for examining a line coupled to a modem in a DMT communications system in which symbols are communicated once every consecutive symbol period, comprising:
a SELT block that constructs symbols using tones in both upstream and downstream frequency bands of the DMT communications system, forms a test signal consisting of the constructed symbols in only certain non-consecutive portions of the symbol periods and silence in the remaining symbol periods, transmits from the modem the test signal on the line and receives a reflection of the test signal from the line at the modem; and
an analysis engine that analyzes the reflection to identify impairments in the line.
12. An apparatus according to claim 11 , wherein the reflection comprises frequency domain S 11 data.
13. An apparatus according to claim 11 , wherein the analysis engine converts the reflection to a set of time domain data to identify the impairments,
14. An apparatus according to claim 11 , wherein the modem is a CPE modem and the upstream and downstream bands are two or more adjacent bands in a xDSL system frequency band plan.
15. An apparatus according to claim 14 , wherein the xDSL system is a vectored DSL system, and wherein the SELT block transmits the test signal while one or more other lines in a vectored group including the line are operating in Showtime.
16. An apparatus according to claim 15 , wherein the certain non-consecutive portions of symbol periods results in the constructed symbols being transmitted by the SELT block at a rate corresponding to an error correction capability of the vectored DSL system.
17. An apparatus according to claim 16 , wherein the rate is a pulse of a number of symbols per DMT superframe.
18. An apparatus according to claim 17 , wherein the number is one.
19. An apparatus according to claim 14 , wherein the constructed symbols are REVERB symbols.
20. An apparatus according to claim 11 , wherein the tones are a contiguous set of tones spanning the upstream and downstream bands.