UBC Theses and Dissertations
Performance enhancement of xDSL systems using LDPC coding with DMT modulation Zheng, Thomas S. H.
Channel coding is playing an pivotal role for improving the performance of xDSL systems in terms of higher data rate and long loop reach. The current channel coding scheme for standard DSL concatenates Reed-Solomon (R-S) codes and multi-dimensional Trellis coded modulation (TCM) codes. Although RS-TCM coding provides a significant coding gain for DSL systems, this gain still far away from the channel capacity. Recently rediscovered low-density parity-check (LDPC) codes, which were invented by Gallager in 1962, have become the best-known practically implentmentable codes for closely approaching the channel capacity. The probabilistic decoding for LDPC codes is an effective and applicable technique based on the message-passing algorithm which employs probabilistic message-passing on a normal graph generated from the parity-check matrix of LDPC codes. Our empirical results showed that for flat AWGN channels and various Q AM modulation schemes more than 2.0 dB performance improvement of DSL systems was obtained with LDPC coding instead of the RS-TCM scheme. This thesis emphasizes the study of the performance of LDPC codes applied in xDSL systems operating on frequency-selective multilevel AWGN channels. Two different frequency selective channel models were used in our simulation studies, where bit-error rate (BER) was measured (via computer simulation) vs. received signal-to-noise ratio (SNR). LDPC codes were employed together with multilevel QAM in DMT-xDSL systems. Coding gains greater than 2.8 dB were obtained at 10~6 BER when compared with RS-TCM coding.
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