DMT performance improvement based on clustering modulation, applications to ADSL

Dynamic bit-loading for ADSL-DMT is driven by the signal-to-noise ratio (SNR) achieved per tone. 14.3 dB of minimum SNR is required to load a tone with the smallest square 4QAM constellation, for a fixed 10^-7 bit error rate (BER). In the presence of physical layer impairments such as loop attenuation, bridge taps or crosstalks, many tones cannot be used because their individual SNR values are too small. On the other hand the associated total unloaded bandwidth exhibits a significant capacity. To benefit from parts of this unloaded bandwidth capacity, trellis coding with one bit or fractional bit loading has been proposed, unfortunately requiring some trellis decoding modifications. This paper introduces a new method called clustering modulation (CM) that does not require any trellis change and which clusters unloaded tones together to increase the SNR after maximum likelihood cluster payload demodulation in the frequency domain. Clustering modulation achieves up to 424 kbits/s of rate improvement on a 2.8 km (9 kfeet) 26AWG loop and up to 200 m of reach increase against SDSL noise B crosstalks.