We introduce a new method for constructing randomized low-density parity-check (LDPC) codes of moderate length, denoted Welding. The progressive edge growth (PEG) algorithm, a powerful method for constructing short and medium length codes, designs LDPC codes from optimum-degree distributions. The Welding algorithm uses these constructed codes as basis codes to create a longer code by switching edges in the code's Tanner graph randomly so that the capability of PEG is combined with the advantages of random graph codes. The Welding algorithm yields about 0.2 dB performance improvement for the AWGN channel over the other long codeword design mechanisms Lifting and PEG.
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