Abstract. S. Kim et al. have been analyzed the girth of some algebraically structured quasi-cyclic (QC) low-density parity-check (LDPC) codes, i.e. Tanner (3, 5) of length 5p, where p is a prime of the form 15m + 1. In this paper, by extension this method to Tanner (3, 7) codes of length 7p, where p is a prime of the form 21m + 1, the girth values of Tanner (3, 7) codes will be derived. As an advantage, the rate of Tanner (3, 7) codes is about 0.17 more than the rate of Tanner (3, 5) codes
