三階碼在同步展頻通訊系統中的應用

Abstract

由於正交展頻序列可以確保任何展頻碼之間在理想情況下皆無交互存取干擾，因此如同Walsh序列般的正交展頻碼大量地被使用在新式的同步展頻系統中。然而，我們使用的傳統二元正交碼長度受限於二的次方長，此往往在特定的系統考量下，侷限了頻譜的配置。有鑑於此，本文提出一種三階正交碼的編碼系統，在此編碼架構之下我們可以產生任意整數長度的正交碼。並且依據三階正交碼的效能評估，我們詳細分析其表現，並且討論相關的特性，同時和傳統二元正交碼作比較。我們可以發現，三階碼在直接序列展頻系統中的表現可以達到跟二元正交碼相同的程度。進一步地，我們還提出兩種改善現有三階碼干擾量的方法，並和二元碼比較。結果顯示出在不同的特定系統設計考量之下，這些方法可以對三階碼系統有相當程度的改善。

Orthogonal spreading codes such as Walsh sequences are extensively employed in modern synchronous spread spectrum systems, since there is ideally no mutual access interference between any pair of sequences. However, the conventional orthogonal codes we adopted are binary with amplitudes belong to {a, -a}, i.e., the code length can only be made of 2’s power. This will sometimes restrict the spectrum allocation under certain specific processing gains. In the thesis we introduce a new encoding scheme of ternary orthogonal codes, with chip numbers of arbitrary integer values and amplitudes belong to {a, 0, -a}. Based on the performance evaluations, the analyses and related properties are discussed with the comparison of binary coding systems, and we conclude that the ternary codes generated by our systematic construction algorithm behave as well as the binary codes in a DS spread spectrum system. Furthermore, we provide two methods of reducing the proposed ternary codes, to enhance the MAI performance. It is shown that the properly reduced form of ternary orthogonal codes can improve the system performance to a certain extent under different system considerations.