Band-AMC Scheme with Hybrid ARQ in Downlink OFDMA Systems

正交分頻多工存取技術已經成為無線寬頻存取系統和 Evolved UTRA (UMTSerrestrial Radio Access) 下行鏈路的正式規格。其主因在於其本身較易對抗無線環境中的多重路徑干擾 (multipath interference) 以及同時可以彈性分配時間和頻率兩個維度的資源來取得多用戶分集增益 (multiuser diversity gain)。單一載波系統下的動態調變編碼技術已經逐漸成熟。其特性在於根據即時頻道狀況而挑選適當的調變編碼機制 (MCS) 來傳送資料並期許達到最大輸出 (throughput)。將動態調變編碼技術應用在正交分頻多工存取系統下，因其俱備同時追蹤頻道在時間和頻率上的變化之能力，故在資源分配上會有更高的彈性。由頻道的時變特性所造成的延遲或不正確的頻道狀況回饋而引發的效能下降，能夠以結合混合式自動重傳要求機制的方式來得到整體的系統改善。今在正交分頻多工存取系統的動態調變編碼技術的研究中，幾乎沒有研究針對動態調整調變編碼機制 (MCS) 的切換點來追蹤時變的頻道統計特性。因，在本篇論文當中，我們將提出一個在正交分頻多工存取下行鏈路系統下之頻動態調變編碼及混合式自動重傳要求機制的系統架構，該架構的動態調變編碼術是採用動態調整切換點。透過模擬的結果顯示，我們所提出的動態調整切換的方法優於滿足某給定的封包錯誤率 (frame error rate) 之固定切換點的傳統方。Orthogonal Frequency Division Multiple Access (OFDMA) technique wasdopted for the broadband wireless access (BWA) system because of its in-erent immunity to multipath interference and flexible resource allocation inoth the time and frequency domains to obtain the multiuser diversity gain.daptive modulation and coding (AMC) technique in single-carrier sys-em has been studied to do the adaptive transmission with the modulationnd coding scheme (MCS) adjusted according to the instantaneous chan-el condition for throughout maximization. Exploiting AMC technique inFDMA system can be more flexible due to the ability to track the channelariation in both the time and frequency domain. In time-varying channels,he performance degradation of adaptive transmission caused by delayed ornaccurate channel condition feedback can be mitigated with the use of hy-rid automatic repeat request (HARQ).o far, few studies focus on dynamical adjustment of the MCS switchinghresholds tracking the time-varying wireless channel in OFDMA system.n this thesis, we will propose a framework of downlink OFDMA systemmploying band-AMC scheme with HARQ. And the numerical result showshat our work outperforms the traditional method which is adopting fixed-hreshold scheme with a target frame error rate (FER).1 Introduction 1.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3 Thesis Organization . . . . . . . . . . . . . . . . . . . . . . . . 5 Adaptive Adjustment of MCS Thresholds for AMC Systemsith HARQ 6.1 AMC Concepts for Throughput Maximization . . . . . . . . . 6.2 Related Work . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.1 System Model . . . . . . . . . . . . . . . . . . . . . . . 8.2.2 Adaptive Threshold Adjustment Algorithm . . . . . . . 9.2.2.1 Throughput Analysis: . . . . . . . . . . . . . 9.2.2.2 Operations: . . . . . . . . . . . . . . . . . . . 10 Proposed Algorithm for Variable MCS in the Retransmis-ions 12.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.2 System Model . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.3 Problem Analysis . . . . . . . . . . . . . . . . . . . . . . . . . 13.4 Proposed Algorithm . . . . . . . . . . . . . . . . . . . . . . . 17.4.1 When in initial transmission: j = 1 . . . . . . . . . . . 17.4.2 When in retransmissions: j > 1 . . . . . . . . . . . . . 18.4.3 Operations: . . . . . . . . . . . . . . . . . . . . . . . . 23.5 Numerical Results . . . . . . . . . . . . . . . . . . . . . . . . . 24 Proposed Framework of OFDMA Downlink System 29.1 System Model . . . . . . . . . . . . . . . . . . . . . . . . . . . 29.2 Subchannel Allocation Scheme . . . . . . . . . . . . . . . . . . 32.3 Rate Matching Method . . . . . . . . . . . . . . . . . . . . . . 32.3.1 Proposed Method . . . . . . . . . . . . . . . . . . . . . 33.4 Adaptive Threshold Adjustment Algorithm in the Proposedramework . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34.5 Numerical Results . . . . . . . . . . . . . . . . . . . . . . . . . 36 Conclusion and Future Work 41.1 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41.2 Future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42eferences 4