Research On Discrete Multitone Modulation Technology

G.DMT and G.lite Asymmetric Digital Subscriber Line (ADSL) modems and some Very-high-speed Digital Subscriber Line (VDSL) modems rely on discrete multitone modulation (DMT). Due to a variety of advantages, such as high bandwidth efficiency and excellent anti-multipath performance, DMT has received considerable attentions and widely used in many fields. The transmission channel is the physical medium that connects the transmitter and receiver and induces inter-symbol (ISI) interference and other noise sources. One efficient way to counteract channel distortion in the transmission band is to combine a cyclic prefix with a time domain equalizer in DMT transceivers. The traditional DMT 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 aim is shortening the transmission channel impulse response to mitigate ISI. Previous TEQ design methods such as minimum mean-squared error (MMSE), maximum shortening signal-to-noise ratio (MSSNR), and maximum geometric signal-to-noise ratio (MGSNR) optimize objective functions not directly optimize system bit rate. In this dissertation, I make a systemic research on basic issue and principle and the key technologies of DMT transmission system used in ADSL modem first. On the basis of traditional TEQ design structure and algorithm, the important is to present the equalizer design that maximizes channel capacity of a DMT system, meanwhile develop a subchannel Signal-to-Noise Ratio (SNR) model. I partition an equalized multicarrier channel into its equivalent signal, noise, and ISI paths to develop a new subchannel signal-to-noise (SNR) definition. Using the new subchannel SNR definition, I derive a nonlinear function of TEQ taps that measures channel capacity. Based on the nonlinear function, I propose the optimal maximum-bit-rate (MBR) TEQ and near-optimal minimum-ISI (Min-ISI) method. To sum up, I have a systemic and comprehensive analysis and explanation on three TEQ equalizer structures design structure: (1) Single-Path, (2) Dual-Path, and (3) Per-Tone. Simulating the performance of TEQ algorithms by MATLAB computer simulation. The results compare all the time domain equalization algorithms with simulation and analyze their advantage and disadvantage. The research work and its conclusion afford a valuable reference and directive meaning for the choice of equalization algorithm and equalizer structure and optimal parameter design in the future. Based on simulations using eight different carrier-serving area ADSL channels show: (1) The proposed MCC and Min-ISI methods yield higher bit rates than the MMSE, MSSNR, and MGSNR; (2) The Min-ISI method achieves 99% of the available bit rate of the optimal MBR method; (3) The Per-Tone equalization structure with a Nw-taps FEQ optimizes the SNR for each subchannel separately. The results of simulations show that Per-Tone achieves higher bit rate than single-FIR TEQ. (4)Dual-path time-domain equalizer achieved better tradeoff of complexity vs. bit rate.