Method for seamless bit rate adaptation for multicarrier DSL
A method of mitigating transmission delay fluctuations in a communication system having a variable data rate and block-oriented subsystems, such as interleavers. The method comprises interspersing dummy data with genuine data to form a combined data stream for the subsystems. When the data rate decreases, the rate at which dummy data is supplied increases to stabilize the delay. In one embodiment, the dummy data is discarded prior to transmission and may be reintroduced as needed on the receiving end. In another embodiment, the dummy data is transmitted and only discarded after processing at the receiving end. For this later embodiment in a multi-tone system, the dummy data can be sorted onto separate sub-carriers for transmission and the power levels for those carriers can be dropped to reduce overall power consumption and crosstalk. The concept can be used to extend seamless rate adaptation to systems with forward error correction.
1. A method of mitigating a communication lag time variation in a DSL communication system, caused by a first DSL modem, comprising,
interspersing dummy data with a variable bit rate first data stream to form a combined data stream, using hardware;
supplying the first data stream to an electronic subsystem of the first DSL modem;
wherein a rate at which the dummy data is supplied increases in response to a decrease in the bit-rate for the first data stream;
whereby an increase in the communication lag time resulting from the decrease in the bit-rate is mitigated or avoided by the addition of the dummy data;
transmitting the combined data stream over a line; and
separating the first data stream from the combined data stream;
whereby the first data stream is recovered and the dummy data is discarded following transmission over the line;
wherein the electronic communication system uses frequency division duplexing and at least part of the dummy data is selectively transmitted on different frequencies from data of the first data stream; and
wherein an error rate for transmission at the frequencies at which only dummy data is transmitted is allowed to be much greater than an error rate at the frequencies at which data of the first data stream is transmitted.
2. The method of claim 1 , wherein the electronic subsystem is an interleaver.
3. The method of claim 2 , wherein the dummy data is passed through at least one additional electronic subsystem prior to transmission over the line.
4. The method of claim 3 , wherein the additional electronic subsystem is a forward error correction encoder.
5. The method of claim 1 , wherein an average power level for the frequencies on which only dummy data is transmitted is at least one order of magnitude less than an average power level for the frequencies on which the first data stream is transmitted.
6. The method of claim 1 , wherein an average power level for the frequencies on which the first data stream is transmitted is substantially reduced shortly after increasing the rate at which the dummy data is supplied.
7. A DSL communication system adapted to operate according to the method of claim 1 .
8. The DSL system of claim 7 , wherein the system provides seamless rate adaptation with forward error correction.
9. The method of claim 1 , wherein the bit rate for the first data stream is varied by discrete amounts to facilitate mitigating or eliminating the delay variation using the dummy data.