Abstract
Discrete multitone (DMT) uses an inverse discrete Fourier transform (IDFT) to modulate data on the carriers. The high sidelobes of the IDFT filter bank used can lead to spurious emissions (egress) in unauthorized frequency bands. Applying a window function within the DMT symbol can alleviate this. However, window functions either require additional redundancy or will introduce distortions that are generally not easy to compensate for. In this paper, a special class of window functions is constructed that corresponds to a precoding at the transmitter. These do not require any additional redundancy and need only a modest amount of additional processing at the receiver. The results can be used to increase the spectral containment of DMT-based wired communications such as ADSL and VDSL (i.e., asymmetric, resp., very-high-bitrate digital subscriber loop).
Highlights
Discrete Fourier transform (DFT-) based modulation techniques [1] have become increasingly popular for high-speed communications systems
To illustrate the egress reduction, the spectra are compared for a very-high-bitrate DSL (VDSL) scenario based on the power spectral density mask Pcab.P.M1 from [9]
The most important features are that the frequencies between 3000 kHz and 5200 kHz and above 7050 kHz are reserved for upstream communications, and that the power is lowered by 20 dB in the amateur radio bands, from 1810 kHz to 2000 kHz, and from 7000 kHz to 7100 kHz
Summary
Discrete Fourier transform (DFT-) based modulation techniques [1] have become increasingly popular for high-speed communications systems. While DMT seems attractive because of its flexibility towards spectrum control, the high sidelobe levels associated with the DFT filter bank form a serious impediment, resulting in an energy transfer between in-band and outof-band signals This contributes to the crosstalk, for example, between different pairs in a binder, especially for next-generation DSL systems using dynamic spectrum management (DSM), where the transmit band is variable [4]. Controlling egress is usually done in the frequency domain by combining neighbouring IDFT-inputs (such as in [10]) or, equivalently, by abandoning the DFT altogether and reverting to other filter banks, such as, for example, in [11] Another approach would be to apply an appropriate time-domain window (see [12] for an overview) at the transmitter.
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