METHOD AND APPARATUS TO IMPROVE THE LINEARITY OF THE TIME DOMAIN TRACE DERIVED FROM A SINGLE ENDED LINE TEST
The present invention is related to improvements to SELT testing, and more particularly to methods and apparatuses for reducing the effects of distortions of the reflected signal without sacrificing the integrity of the original signal. In embodiments, a method according to the invention consists of first modifying the transmit sequence to utilize and better “excite” the low frequency tones. In embodiments, the method includes compensating for transformer roll-off at low frequencies. In other embodiments, the method includes filling gaps in the received frequency domain S11 sequence with a predicted, or estimated, version of the S11 signal.
1 . A method of performing a single ended line test (SELT) in a xDSL modem comprising:
constructing a sequence of transmit symbols using available tones in a bandplan for the xDSL modem;
modifying the sequence of constructed transmit symbols to utilize and better excite low frequency tones in the constructed transmit symbols; and
transmitting the sequence of modified constructed transmit symbols through a hybrid of the xDSL modem while simultaneously sensing received symbols corresponding to the modified constructed transmit symbols through the hybrid.
2 . A method according to claim 1 , wherein constructing includes mapping a common QAM constellation point to the available tones, and wherein modifying includes mapping the common QAM constellation point to additional low frequency tones that are not included in the bandplan for the xDSL modem.
3 . A method according to claim 2 , wherein the bandplan is associated with a VDSL2 system and wherein the additional low frequency tones are tones 1 to 5.
4 . A method according to claim 1 , wherein modifying includes applying a compensation gain to the low frequency tones.
5 . A method according to claim 4 , wherein the compensation gain corresponds to a roll-off of a transformer in the xDSL modem.
6 . A method according to claim 5 , wherein the roll-off has been predetermined and stored in the xDSL modem.
7 . A method according to claim 1 , wherein constructing includes using both upstream and downstream tones in the bandplan.
8 . A method according to claim 7 , wherein constructing includes mapping a common QAM constellation point to the upstream and downstream tones, and wherein modifying includes mapping the common QAM constellation point to additional low frequency tones that are not included in the bandplan for the xDSL modem.
9 . A method according to claim 1 , wherein the constructed transmit symbols are REVERB symbols.
10 . An apparatus for performing a single ended line test (SELT) for a xDSL modem comprising:
a mapper that constructs a sequence of transmit symbols using available tones in a bandplan for the xDSL modem;
a SELT block that modifies the sequence of constructed transmit symbols to utilize and better excite low frequency tones in the constructed transmit symbols;
a transmit path for transmitting the sequence of modified constructed transmit symbols through a hybrid of the xDSL modem; and
a receive path for simultaneously sensing received symbols corresponding to the modified constructed transmit symbols through the hybrid.
11 . An apparatus according to claim 10 , wherein the mapper constructs the sequence of constructed transmit symbols by mapping a common QAM constellation point to the available tones, and wherein the SELT block causes the mapper to further map the common QAM constellation point to additional low frequency tones that are not included in the bandplan for the xDSL modem.
12 . An apparatus according to claim 11 , wherein the bandplan is associated with a VDSL2 system and wherein the additional low frequency tones are tones 1 to 5.
13 . An apparatus according to claim 10 , wherein the SELT block includes a low frequency gain compensation block that applies a compensation gain to the low frequency tones.
14 . An apparatus according to claim 13 , wherein the compensation gain corresponds to a roll-off of a transformer in the xDSL modem.
15 . An apparatus according to claim 14 , wherein the roll-off has been predetermined and stored in the xDSL modem.
16 . An apparatus according to claim 10 , wherein the mapper uses both upstream and downstream tones in the bandplan.
17 . An apparatus according to claim 16 , wherein the mapper constructs the sequence of constructed transmit symbols by mapping a common QAM constellation point to the upstream and downstream tones, and wherein the SELT block causes the mapper to further map the common QAM constellation point to additional low frequency tones that are not included in the bandplan for the xDSL modem.
18 . An apparatus according to claim 10 , wherein the constructed transmit symbols are REVERB symbols.