「Low-density parity-check code」を編集中
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この方法では、プロセッサのオーバーヘッドがほとんどなく、ルックアップテーブル用のメモリのコストが僅かで、4.0＆nbsp; PICチップでもLDPCデコードが可能な非常に高い反復を使用できます。 | この方法では、プロセッサのオーバーヘッドがほとんどなく、ルックアップテーブル用のメモリのコストが僅かで、4.0＆nbsp; PICチップでもLDPCデコードが可能な非常に高い反復を使用できます。 | ||
| − | == | + | == Code construction == |
| − | + | For large block sizes, LDPC codes are commonly constructed by first studying the behaviour of decoders. As the block size tends to infinity, LDPC decoders can be shown to have a noise threshold below which decoding is reliably achieved, and above which decoding is not achieved, colloquially referred to as the [[cliff effect]]. This threshold can be optimised by finding the best proportion of arcs from check nodes and arcs from variable nodes. An approximate graphical approach to visualising this threshold is an [[EXIT chart]]. | |
| − | + | The construction of a specific LDPC code after this optimization falls into two main types of techniques: | |
| − | * | + | *Pseudorandom approaches |
| − | * | + | *Combinatorial approaches |
| − | + | Construction by a pseudo-random approach builds on theoretical results that, for large block size, a random construction gives good decoding performance. Various constraints are often applied to help ensure that the desired properties expected at the theoretical limit of infinite block size occur at a finite block size. | |
| − | + | Combinatorial approaches can be used to optimize the properties of small block-size LDPC codes or to create codes with simple encoders. | |
| − | + | Some LDPC codes are based on [[Reed–Solomon code]]s, such as the RS-LDPC code used in the [[10 Gigabit Ethernet]] standard. | |
| − | + | Compared to randomly generated LDPC codes, structured LDPC codes—such as the LDPC code used in the [[DVB-S2]] standard—can have simpler and therefore lower-cost hardware—in particular, codes constructed such that the H matrix is a [[circulant matrix]]. | |
| − | + | Yet another way of constructing LDPC codes is to use [[finite geometry|finite geometries]]. This method was proposed by Y. Kou ''et al.'' in 2001. | |
== 関連項目== | == 関連項目== | ||
