無線通訊已成為現今通訊的一大熱門議題。無線通訊系統的技術
以正交分頻多工(OFDM) 為核心的技術不斷的發展，且正交分頻多
工技術被廣泛的利用在新穎的標準無線通訊規格上，在本論文中我
們所使用的多通道通訊系統即為正交分頻多工。通道編碼也是現今
無線通訊中一大熱門議題。在嚴重雜訊與有限頻寬的無線衰減通道
中傳輸，通道編碼為必要的技術。其中渦輪碼已被廣泛使用且證實
擁有非常好的效能。渦輪碼的使用能有效的降低系統位元錯誤率。

正交分頻多工系統能有效對抗無線傳輸中多路徑效應所造成的
符元間干擾，藉由將整個可用頻寬分成多個子通道同時作訊號的傳
輸，每個子通道有不同的振幅增益，為因應每一子通道擁有不同增
益與雜訊影響，我們必須針對每一子通道決定在該子通道上分配所
需之能量，使得整體效能有所改善。本論文所使用的功率分配方式
是基於最佳化系統整體位元錯誤率在未使用錯誤更正碼情況下，接
下來我們進而將系統加入錯誤更正碼機制-渦輪碼，並期望加入錯誤
更正碼後搭配我們所提出的功率分配後其系統位元錯誤率亦可為最
佳化，本論文也將針對多種不同功率分配做比較，經由模擬實驗證
實我們所提出的功率分配能有較佳的系統效能。

本論文我們亦探討放大與前傳的中繼正交分頻多工通訊系統，中
繼系統的傳輸概念大量被運用到高速的無線通訊服務，可用來對抗
通訊時的遮蔽或路徑損失降低錯誤率。我們基於實際通訊環境考量
下在中繼系統中考量訊號源與中繼端有各自的總能量限制，我們同
樣基於最小化系統位元錯誤率在系統未加錯誤更正碼情況下來做多
通道的功率分配，並針對中繼系統的子載波不同配對方式做比較。
接下來我們來做我們所提出的功率分配搭配子載波配對方式與其他
的功率分配方式的系統效能比較，經由模擬實驗結果的呈現，系統
搭配我們所提出的功率分配方式有較佳的錯誤率系統效能。

Wireless communication is one hot topic in today's communication.
Among the techniques for wireless communication, orthogonal frequency
division multiplexing (OFDM) is the core technolology. It is under continous
development and is included in many wireless communication protocols.
In this thesis, the OFDM is effectively regarded as a multicarrier
communication system. Channel coding is another important topic in today's
wireless communication. It is an essential technique when the channel
is noisy and fading . Among the channel coding techniques, turbo codes
have been shown to exhibit excellent performance and are adopted widely.
They can reduce the bit error rate (BER) in a significant fashion.

OFDM is capable of fighting the inter-symbol interference (ISI) problem,
which is majorly due to the multipath effect in wireless communication.
In OFDM, the available spectrum is divided into many sunchannels.
Each subchannel has its own amplitude gain. According to the influence
of each subchannel and noise, we propose that differnet subchannels be
allocated with different power. Through proper power allocation to the
subchannels, we expect that the system performance can be improved. In
this thesis, the power allocation we used is based on optimal BER of the
system when there is no error correction coding in the system at first. Then
we hope the BER performance is also optimal after imposing the error correction
mechanism into the system.In our research we also compare the
performance with various existing power allocation schemes. Through the
simulation results it is confirmed that system with our power allocation produces the best performance.

Another topic we investigate in this system is the use of amplify-andforward
relay in the communication system. Relayed transmission has already
found applications in high speed wireless communication service. It
can be used to combat the shadow or the path loss to reduce the bit error
rate. Through our thesis, it is assumed that the power at the source and
the relay is separately constrained. We also allocate the power to the subchannels based on the minimization of the system bit error rate. Different
subcarrier pairings are also tried and compared. Then, our power allocation
method is combined with the best paring scheme. The combination turn out
to produce very good BER performance as compared to other combinations
of power allocation and pairing.

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