Please use this identifier to cite or link to this item: http://hdl.handle.net/11455/7834
標題: 利用展頻碼序列的選擇來降低OFDM-CDMA系統的峰均值比
The Selection of Spreading Sequences to Reduce the Peak-to-Average Power Ratio in OFDM-CDMA System
作者: 李欣鴻
關鍵字: OFDM-CDMA;正交分頻-分碼多工系統;Peak-to-Average Power Ratio;峰值對平均值比
出版社: 電機工程學系
摘要: 
在無線通訊中,凡是訊號經過展頻後再傳送的系統,都一定會經歷頻率選擇性衰減。為了降低它對系統性能的影響,於是發展出多載波分碼多工,其主要利用正交分頻多工調變技術來切割通道,使得每個子通道皆呈現平坦衰減,同時簡化了系統接收器的複雜度。正交分頻分碼多工是一種結合分碼多工與正交分頻多工之傳輸技術。
OFDM-CDMA結合OFDM與CDMA兩種調變方式,因此具有對多路徑的頻率選擇通道有較佳的抵抗力高速資料傳輸、抗多路徑延遲擴展、頻譜利用率高的優點,此外也能提供高容量的系統服務,它已被視為第四代無線行動通訊系統設計上的一項重要技術。
由於OFDM-CDMA使用OFDM的結構,因此OFDM-CDMA訊號有很高的傳輸功率峰值對平均值比,所以易受無線通訊系統傳輸端的功率放大器非線性扭曲失真的影響,而嚴重降低訊號傳輸的品質。所以需要有大的動態範圍的線性放大器,才能保證接收到所有PAPR值較大的的訊號,而不會有訊號因超過其接收端放大器動態範圍而被截的現象。若能找到一些規則,使得選用的展頻碼組合,PAPR值都能限制在可接受的範圍,這樣就不需消秏過多的功率。
長度 的展頻碼及固定的使用者人數 之下,會有 種的展頻碼組合,這些不同的展頻碼組合,會有不同的PAPR值。本論文的研究方向有二,其一是找到一些規則,使得在任意 個使用者時,都能在 種展頻碼組合中,找到PAPR較低的組合。其二是,在原來固定的 個使用者下,當第 個使用者加入時,如何分配第 個展頻碼,使得這 個展頻碼的組合,其PAPR值仍然是較小的。

In wireless communication systems, the transimitted signals after spreading are always suffered by frequency-selective fading. To reduce this effect, the Orthogonal frequency Division Multiplexing - Code Division Multiple Access (OFDM-CDMA) systems are proposed, where the whole channel is divided into many flat fading sub-channel. The OFDM-CDMA systems is a combination of the CDMA and OFDM and thus have better resistance against frequency-selective fading against, Other advantage of OFDM-CMA is the transmission rate and high efficiency use in spectrum. Furthermore, OFDM-CDMA systems provide high capacity for the system service, and is considered to be an important technique in future B3G wireless communication system.
A major drawback of OFDM-CDMA is that it has a high Peak-to-Average power Ratio( PAPR). The larger peak power signal requires a liner amplifier with a larger dynamic range, which is very expensive and sometimes almost impossible to implement. It is desirable to reduce the peaks in OFDM-CDMA signals. Since the larger peaks degrade the efficiency of a linear amplifier . The purpose of the is to select the combination of spreading sequences with low PAPR, that there PAPR all limiting in the accept range.
There are total combination of spreading sequences for a down-link transmitter using a length spreading code. Each spreading sequence has different PAPR value and the search of a spreading sequence with low PAPR is a challenge problem. This thesis has two research. One aspect hope to find some rules, which can make system choose lower PAPR spreading sequences combination in the total spreading sequences under arbitrarily users. Another aspect is fixed original users than choose th spreading sequences when th user accede to the system, and these spreading sequences combination, which PAPR still lower.
URI: http://hdl.handle.net/11455/7834
Appears in Collections:電機工程學系所

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