第五章 模擬結果分析與比較
6.1 結論
在本論文中，我們分別提出了兩種適用於 OFDMA uplink 系統的載波頻率偏 移估計及頻率偏移補償演算法。在提出之CFO 估計演算法中，我們利用了 Self-ICI cancellation 及 Received windowing 兩種干擾壓抑技術，來減輕 MAI 對接收端做 CFO 估測時所造成的影響，使得[3]中的 data-aided 頻域估計演算法能夠適用於 OFDMA uplink 系統上，由模擬結果顯示，提出的兩種估計演算法之均方誤差
（MSE）效能，皆與[3]在單一用戶 OFDM 系統所達到的效能十分接近。
在 CFO 補償演算法部分，由於傳統時域補償方法在 OFDMA 系統上有無可 避 免 的 缺 點 所 在 ， 因 此 我 們 提 出 兩 種 在 頻 域 上 且 具 有 iterative interference cancellation 性質的 MMSE-BPIC 與 BSIC 補償演算法，兩者的效能均會隨著 iteration 次數增加而改善，模擬結果顯示，在 iteration 次數達三次以上，效能即 有收斂的現象。與[10]的 block based 頻率補償演算法相較，MMSE-BPIC 與 BSIC 演算法在效能上均有較良好的表現，主要的原因是因為MMSE-BPIC 與 BSIC 在 第一次 iteration 中能夠提供較為準確可靠的符元信號偵測結果；模擬結果也顯 示，與其他現有的補償演算法比較，MMSE-BPIC 與 BSIC 對於 CFO 的估計誤差 有較佳的容忍程度，因此效能不會受估計誤差而有太大的損失。
相較於OFDM 系統，OFDMA uplink 系統面臨最大的問題即是多用戶之間的 干擾MAI，本論文的目的就在於如何克服 MAI 的影響，找出適合 OFDMA 系統 的 CFO 估計及補償演算法，由模擬結果證實，我們所提出的估測及補償演算法 的確能夠適用在OFDMA uplink 系統環境上。
6.2 未來展望
在現行的無線通訊系統標準中，OFDM 已成為主流技術之一，以 OFDM 為 基礎的多用戶存取技術OFDMA 也已被無線都會網路標準 IEEE 802.16 所採用；
本論文已提出了適用於OFDMA uplink 系統的 CFO 估計及補償方法，但我們是 假設在時間完美同步，且通道狀態（channel state information，CSI）已知的情形 下所做的模擬，所以包括如何在OFDMA uplink 系統上做良好的時間同步（timing synchronization）、通道估測（channel estimation）等，仍是相當重要且具有挑戰 性的研究課題。
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