低密度校驗碼LDPC
2.6 複雜度、吞吐量和解碼時延
| 2.8 LDPC 碼在 3GPP 中的應用 |
目前，LDPC 碼已廣泛應用到諸多通信系統中，例如，全球微波互聯接入 （WiMAX）（IEEE 802.16e）、Wi-F（i IEEE 802.11n）[10]、DVB-S2[12] 等。
在 3GPP 讨論 LTE 方案的早期，LDPC 碼也是 4G 信道編碼的候選方案 [49]。 但考慮到實作複雜度，3GPP 最終選擇 Turbo 碼作為 3G 和 4G 信道編碼的方 案而沒有選擇 LDPC 碼。經過學術界和工業界的努力研究，LDPC 碼最終進入 要求嚴格的 3GPP 5G-NR 标準 [13]。見表 2-17，LDPC 碼在 3GPP 中是如何應用的 [14]。
| 2.9 未來發展 |
将來，LDPC 碼将進一步發展。作者認為，其可能的方向如下。
（1）對短碼更好的支援（優化出新的 BG3、BG4）。在物聯網（IoT）和機 器類通信（MTC）中，常常需要發送小的資料塊，而 IoT 和 MTC 又需要十分 節能，不能消耗過多的電量，這就需要性能好的基礎矩陣。
（2）多邊 LDPC 碼 [23] 具有更低的誤碼平台，這有利于對誤碼嚴格要求的業 務（URLLC）。
（3）類似于 Turbo 碼的并行級聯 Gallager 碼（PCGC，Turbo 碼的内部 編碼器是 LDPC 碼）[50]。目前，PCGC 在碼率 R = 0.3367 時，離香農限約 0.4 dB[50]。
（4）使用階梯碼（Staircase Code）[51] 形式的LDPC碼 [52]。階梯碼的好處是， 可以做到很低的誤碼平台。
| 2.10 小結 |
這一章主要描述了 LDPC 碼的産生和發展、基本原理、準循環 LDPC 碼、 QC-LDPC 譯碼結構、LDPC 碼在 5G-NR 标準中的進展、複雜度、吞吐量、 鍊路性能以及 LDPC 碼在 3GPP 中的應用、未來發展等。總的來說，LDPC 碼 的性能非常優越、複雜度較低、吞吐量高，可以進行并行解碼，解碼時延小。當然，LDPC 碼也具有構造複雜、不适合短碼等缺點。相信随着編碼研究人員 的深入研究，LDPC 将有更為廣泛的應用。
| 參考文獻 |
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