In the Embedded Signal Processing Laboratory, our work in ADSL transceiver design has focused on the design of equalizer structures to maximize achievable bit rate. We have solved the problem of maximizing the bit rate for three different equalizer structures:
  1. single-path time domain equalizer (TEQ) structure: single finite impulse response filter TEQ plus FFT plus one-tap frequency-domain equalizer (FEQ)
  2. dual-path TEQ structure: two parallel TEQs plus FFT plus one-tap FEQ
  3. per tone equalizer structure: equalization is customized for each subchannel (tone)

For the single-path TEQ, we have optimized bit rate at the output of the TEQ by developing the

The Min-ISI method is suitable for real-time implementation on fixed-point digital signal processors. The Min-ISI reaches 99% of the channel capacity as measured by the matched filter bound (MFB) with a three-tap TEQ. Since the D/A, transmit filter, receive filter, and A/D are not included in the modeling, a slightly longer TEQ (perhaps five taps) would be needed in practice. This is a significant reduction over the 20 or so TEQ taps used in commercial ADSL transceivers, which use a Minimum Mean Squared Error (MMSE) method to design the TEQ. One advantage to having such a short TEQ is that we can move the TEQ from hardware to software in echo-canceled ADSL. The min-ISI method is a generalization of the Maximum Shortening SNR (MSSNR) method in that the ISI is weighted in frequency. The min-ISI method essentially puts ISI in low SNR (i.e. noisy) subchannels.

In the dual-path TEQ, one TEQ is design so as to optimize the achievable bit rate over the entire data transmission spectrum. The other TEQ is biased to selected subchannels to increase transmission throughput. Both the MBR and Min-ISI methods can be used to design both TEQs to optimize bit rate at the TEQ outputs.

In our version of per tone equalization, we use a bank of TEQs where each TEQ is tuned to a particular subchannel. A single-tap FEQ would still be used. The dual-path TEQ can be viewed as a special case of a time-domain per-tone equalizer, which uses a bank of TEQs plus FFT.

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