Discrete multi-tone (DMT) modulation is used in many types of data communication systems, among them Multi-carrier Very-high-speed Digital Subscriber Line (VDSL) modems, as well as Asymmetric DSL (ADSL). In these systems, N tones are modulated by QAM two-dimensional input frequency-domain symbols. A 2N-point Inverse Fast Fourier Transform (IFFT) then produces a corresponding time-domain symbol, expressed as a real baseband time-domain output signal of 2N real samples in each symbol period. At the receiving side, 2N samples are extracted from the time-domain signal during each symbol period. A FFT is used to demodulate the signal and recover the original QAM symbols on the N tones.
The number of bits to be encoded by each tone, known as the bit loading, is determined by the receiver according to the line conditions, which are measured as a function of frequency during a training period. The receiver passes a table of these values, known as the bit-loading table, to the transmitter, which thus determines how many bits of the input data stream to allocate to each successive tone in the tone order.
Similarly, a gain table which determines the required relative transmitter power for each tone is passed to the transmitter.
DMT systems typically support an additional, low power mode which is used whenever there is little or no data to transmit. For example, the ITU ADSL2 specification (ITU-T Recommendation G.992.3), entitled Series G: Transmission Systems and Media, Digital Systems and Networks; Digital Sections and Digital Line System—Access Networks; Asymmetric Digital Subscriber Line Transceivers 2 (ADSL2) (International Telecommunication Union, 2002), defines a low-power mode, referred to as L2 mode, in section 9.5 (pages 185-189) and section 8.7 (pages 82-84), which are incorporated herein by reference. In the L2 mode, the downstream signals are transmitted using different bit-loading and gain tables from the standard tables used in the high-power mode, which is referred to as L0. The L2 bit-loading and gain tables reflect the lower data rate and lower transmit power that are used in this mode.
Transition from low-power to high-power mode must be coordinated between the transmitter and the receiver, so that the receiver uses the correct bit-loading and gain values. In the ADSL2 specification cited above, mode switching is signaled by transmitting a unique, predetermined sequence referred to as a “transition sequence.” The transition sequence uses all of the transmitted tones for the duration of two symbols. The detailed structure of this mechanism can be found in section 8.7.6 of ITU Recommendation G.992.3 mentioned above. There are additional instances in which gain tables and/or bit-loading tables must be changed in a coordinated fashion between the transmitter and receiver. One example of such a mode switching operation is Seamless Rate Adaptation (SRA), described in the ADSL2 specification cited above.
An alternative signaling scheme was proposed by Oksman, in a paper entitled “Proposal on OLR SyncFlag for VDSL2,” (Infineon Technologies, Contribution T1E1.4/2003-466, presented at Working Group T1E1.4 (DSL Access), Charlotte, N.C., May 24-27, 2004), which is incorporated herein by reference. In this proposal, a sub-group of the transmitted tones is allocated during a single symbol period to transmit a unique symbol, which signals a transition.