結合載波間干擾分集技術於正交分頻多工系統之編碼技術研究

Abstract

正交分頻多工是一個引人關注的傳輸技術，此技術可有效地克服多重路徑通道的時間衰褪效應藉由在每個符元前加入循環字首。然而，使用循環字首的需求造成了傳輸上多餘的負擔，並造成通訊系統吞吐量的下降。直接將循環字首縮短可以改善頻譜效益卻會引入符元間干擾和載波間干擾這些不理想的效應，使得整體系統效能受到限制。為了提升縮短循環字首之正交分頻多工系統的效能，吾人因而進行結合載波間干擾分集技術於正交分頻多工系統之編碼技術研究。若將子載波視為虛擬的天線，縮短循環字首之正交分頻多工系統在載波間干擾下的通道可等效地視同一個虛擬的多重輸入多重輸出通道。此外，輔以空頻編碼的觀念，吾人亦發現使用不足的循環字首所引起的載波間干擾其實可當作一種頻率分集增益。本論文中，吾人基於縮短循環字首之正交分頻多工系統所造成的等效多重輸入多重輸出通道下研發虛擬的空頻編碼技術，同時以理論分析來探討此虛擬空頻編碼可達之分集增益上界，並提出相應之設計準則以供發展具良好效能之虛擬空頻編碼。從模擬結果中顯示，相較於傳統的編碼正交分頻多工系統而言，吾人之編碼技術不僅能夠達到較高之分集增益進而提升錯誤率，同時擁有更高之資料傳輸速率。

Orthogonal frequency division multiplexing (OFDM) is an attractive transmission technique, which effectively deals with the delay spread of the multi-path channel by appending a cyclic prefix (CP) in every OFDM symbol. However, the necessary insertion of CP produces a transmission overhead and reduces the throughput of the communication systems. A direct reduction of CP can improve the spectral efficiency but also introduces the undesired inter-symbol interference (ISI) and inter-carrier interference (ICI), hence limiting the overall system performance. To enhance the performance of the CP-shortened OFDM system, the CP-reduced coding techniques with ICI diversity combining for OFDM systems are studied here. Actually the ICI corrupted channel of the CP-reduced OFDM system can be viewed as a virtual MIMO channel when we regard the subcarriers as the virtual antennas. And the ICI induced by insufficient CP can be viewed as a sort of frequency diversity by the concepts of space-frequency coding (SFC). In this thesis, we investigate the virtual space-frequency coding techniques based on the effective MIMO channel of the CP-reduced OFDM system and propose the corresponding design criteria to develop good virtual space-frequency codes. The maximum achievable diversity order of the virtual space-frequency code is determined by the theoretical analysis. Simulation results are also provided to show that the proposed coding schemes not only achieve higher diversity order to enhance the error performance but also have higher data rate compared with the conventional coded OFDM systems.