Modulation switching for DSL signal transmission
View Patent ↗A digital subscriber line (DSL) communication system that utilizes the high frequency band of a standard telephone line does not require the use of a plain old telephone service (POTS) splitter in the resident's home, which provided isolation between the POTS frequency band (0 to 4 kHz) and the DSL frequency band. A digital subscriber line modem utilizes either constant envelope modulation or quadrature amplitude modulation for outputting DSL signals upstream to a central office. When a telephone in the resident's home is detected as being off-hook, then the constant envelope modulation is used by the DSL modem in order to lessen the intermodulation product distortion that results in audible noise heard by a user of the telephone. When the telephone is on-hook, then another type of modulation, such as QAM, is used to maximize the upstream data rate capability in the DSL frequency band, since any noise generated by the QAM is not a problem due to the non-use of the POTS frequency band.
1. A modem for coupling directly to a subscriber line, the subscriber line providing a communication path for telephone equipment, the modem comprising:
an on-hook/off-hook detector means for detecting whether the telephone equipment is in an in-use state or a not-in-use state, and to output an in-use/not-in-use signal indicative thereof; and
a modulator means for receiving the in-use/not-in-use signal and to provide, based on the in-use/not-in-use signal, one of a plurality of types of modulations to signals that are transmitted by the modem,
wherein, when the in-use/not-in-use signal indicates that the telephone equipment is in the not-in-use state, the modulator means provides an amplitude modulation to the signals that are transmitted by the modem, and
wherein, when the in-use/not-in-use signal indicates that the telephone equipment is in the in-use state, the modulator means provides a shift key modulation to the signals that are transmitted by the modem.
2. The modem of claim 1 , wherein the shift key modulation is constant envelope modulation and provides less intermodulation product distortion than the amplitude modulation; and
wherein the intermodulation product distortion is due in part to a non-linearity in the telephone equipment utilizing the subscriber line.
3. The modem of claim 2 , wherein the constant envelope modulation is one of a frequency modulation and a phase modulation.
4. The modem of claim 2 , wherein the constant envelope modulation is a minimum shift keying modulation.
5. A method of providing communications over a subscriber line connecting a central office to a remote location, the remote location including POTS equipment and a DSL modem, the method comprising:
a) detecting whether the POTS equipment is in an in-use state or a not-in-use state; and
b) based on the detection made in the step a), providing one of a first modulation technique and a second modulation technique to signals transmitted by the modem,
wherein the first modulation technique is an amplitude modulation that is used when the POTS equipment is in the not-in-use state, and
wherein the second modulation technique is a shift key modulation that is used when the POTS equipment is in the in-use state.
6. The method according to claim 5 , wherein the POTS equipment is one of a telephone and a facsimile machine.
7. A DSL modem for transmitting modulated signals on a subscriber line, the subscriber line being coupled to at least one telephone device, the DSL modem comprising:
a detector coupled to the subscriber line, the detector providing a status signal indicative of an operational status of the telephone device; and
a modulator coupled to the detector, the modulator providing the modulated signals in accordance with one of at least two different modulation techniques, one of the modulation techniques being selected in response to the status signal,
wherein the modulation technique is a key modulation when the status signal indicates that the telephone device is off-hook, and
wherein the modulation technique is an amplitude modulation when the status signal indicates that the telephone device is on-hook.