自從無線蜂巢式行動電話的發明, 無線通訊系統受到了極為廣泛的注意, 並已成為極富挑戰性的一項研究學門。為了實現高品質的多媒體影音通訊服務, 並克服地區性通訊系統間的隔閡, 第三代的無線通訊系統 (IMT-2000) , 在一群全世界最頂尖的科學家的研究討論之後, 即將與大家見面。
直序展頻多工通訊系統由於大量地增加系統效能, 已經成為第三代行動通訊系統中的主流技術。為了解決多工通訊系統中訊號之間的互相干擾, 有別於傳統的接收器, 多工的接收器引起了注意。然而, 這些直序展頻系統中的多工接收器都需要知道各個接收到訊號的時間、相位、與訊號振幅。因此, 身為提供這些必要性訊息的機制----直序展頻多工存取同步器, 就份外重要了。

在本論文中, 我們除了提出最佳品質的多工同步器之外, 更進一步地提出了兩大類有效估測訊號參數的同步器。第一類的同步器是基於線性組合充份統計量 (sufficient statistics), 並具有以下列舉的特性:

*適合於一般性的環境與應用

□*同步器的複雜度隨著使用者數量的增加是線性成長

□*具有抵抗遠近效應 (near-far effect) 的能力

□*非最佳化的品質

另一方面,利用維持頻域上的正交性, 第二大類的同步器是藉由輔助信號 (pilot signals) 的幫助來成功地估測參數。這類的同步器, 不同於第一類的同步器, 具有以下的特性。

□*需要藉助輔助信號 (pilot signals) 的幫助

□*同步器的複雜度與單工同步器相當

□*具有抵抗遠近效應 (near-far effect) 的能力

□*最佳化的品質

這兩大類的同步器根據其特性各別對不同的環境有其優勢, 但都適用於實際的直序展頻通訊系統中。

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Since the first introduction of wireless cellular systems,
wireless communication has attracted a lot of interests and

become one of the most challenging research topics.

In order to achieve the global

wireless communications systems providing

high-quality personal multimedia services, the third generation

cellular systems, known as

International Mobile Telecommunications - 2000

(IMT-2000), are extensively investigated.

DS-CDMA communication, as the primary technology in

IMT-2000, receives lots of researchers' interests due to its

increasing capacity. To mitigate

the performance degradation due to multiple access interference (MAI),

multiuser detection was proposed. However, all the multiuser

detectors assume the knowledge of received timings, carrier phases

and perhaps amplitudes of all users.

Multiuser synchronizers, which provide these necessary information,

are therefore of paramount importance.

Motivated by the introduction of optimal multiuser

synchronization, two categories of multiuser synchronizers are

proposed which are mathematically and numerically confirmed to be

effective for an arbitrary number of users in the near-far environment

with feasible complexity. The first category of multiuser synchronizers

linearly combine the sufficient statistics

as linear multiuser detectors do and

have the following characteristics:

* Suitable for general-purposes applications.

* Complexity grows linearly to the number of users.

* Near-far resistant for an arbitrary number of users.

* Suboptimal performance.

On the other hand, by maintaining frequency-domain orthogonality,

another category of multiuser synchronizers aided by

pilot signals are introduced.

These multiuser synchronizers, however, own

the properties:

* Employing pilot signals and limited capacity.

* Complexity similar to the one in point-to-point communications.

* Near-far resistant.

* Optimal performance.

Both categories of estimators have their special advantages according to their

characteristics and are applicable to practical DS/CDMA

communications systems.

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