Maximizing data rate of discrete multitone systems using time domain equalization design
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Asymmetric Digital Subscriber Line in its standardized versions G.DMT and G.Lite uses discrete multitone modulation (DMT) for data transmission. Orthogonal Frequency Division Multiplexing (OFDM) is a similar modulation standard for wireless transmission that has been adopted in IEEE 802.11a wireless local area network, Digital Video Broadcasting and HYPERLAN/2. The transmission channel induces inter-symbol (ISI) interference and other noise sources. The traditional DMT or OFDM equalizer is a cascade of a time domain equalizer (TEQ) as a single finite impulse response filter (FIR), a fast Fourier transform (FFT) multicarrier demodulator, and a frequency domain equalizer as a one-tap filter bank. The time domain equalizer shortens the transmission channel impulse response to mitigate ISI. Previous TEQ design methods optimize objective functions not directly tied to system bit rate. I present the equalizer design that maximizes the bit rate of a DMT system at the output of the FFT demodulator. I develop a subchannel Signal-to-Noise Ratio (SNR) model where the desired signal is formed as the circularly convolved data symbol and the channel impulse response at the input of the FFT and noise is the difference between the received and the desired signal. The received signal also includes the near-end crosstalk, additive white Gaussian noise, analog-to-digital converter quantization noise and the digital noise floor due to finite precision arithmetic. Using the subchannel SNR model, I arrive at the optimal time domain per-tone equalizer filter bank (TEQFB) that maximizes a measure of the ADSL system bit rate. I propose a novel receiver architecture that uses TEQFB and a Goertzel filter bank demodulator at the receiver during data transmission. I also present the design of single FIR equalizer that on average achieves more than 99% of the performance of the TEQFB for the tested standard ADSL carrier serving area loops. Simulation results show that the TEQFB and single FIR outperform the bit rate achieved by the minimum mean-squared error design methods, maximum bit rate approach, and minimum ISI design. The TEQFB also outperforms the leastsquares initialized per-tone equalizer (LS PTE) method while the single FIR closely matches LS PTE performance.