中文摘要：

The girth plays an important role in the design of LDPC codes. In order to deter^nine the girth of Tanner (5 ,7 ) quasi-cyclic (QC) LDPC codes with length I p for p being a prime with the for^n 35m + 1, the cycles of lengths 4 ,6 ,8 ,a n d 10 are analyzed. Then these cycles are classified into sixteen categories,each of which can be expressed as an ordered block sequence,or a certain type. It is also shown that the existence of these cycles is equal to polynomial equations over Fp who has a 35th unit root. We check if these polynomial equations have a 35th unit root and obtain the girth values of Tanner (5 ,7 ) QC LDPC codes.