我們提出並以實驗呈現基於多頻帶直接檢測光學正交分頻多工(DDO-OFDM)系統，傳送可調性調變(AM)偏振態多工(PDM)光纖載中頻訊號(IFOF)，達到免偏振態追蹤的下傳系統及偏振態無感的OOK上傳系統。由於不需追加偏振態追蹤的機制，因此每一個無線接取單元(RAU)的設備簡單化，整個無線接取單元也更具有經濟效應及低功率損耗。另外，結合了集中式無線電接取網路(C-RAN)的架構，建立了一個靈活度高和複雜度集中之行動網路前段，這將有希望成為下一代行動通訊網路的參考指標。
在下傳系統當中，只需在每一個接收端設置一個簡單光學濾波器，就可以成功地解調相對應的偏振態多工及多頻帶訊號。我們以實驗實際呈現頻寬為3.5GHz且載有大約50GHz資訊量的可調性調變多頻帶OFDM訊號，並傳送於25km單模光纖中。由於使用偏振態多工的技術頻譜效應倍增，以可調性調變技術使傳送資訊量再增加8Gb/s。另外，此系統還提供4dB power margin，能更進一步支持無線通訊的服務[1]。對於上傳系統直接再利用兩正交偏振態載波，再不追加濾波器或是偏振態控制器的狀況下，能成功地使用對偏振態靈敏的MZM光學調變器達到偏振態無感的效果。由結果顯示，與傳送對齊晶體光軸的單偏振態訊號相比，大約有1.8dB功率損耗。整體而言，偏振態追蹤的技術不必使用，因此建立了真正的無偏振態追蹤的網路前段系統。

We propose and experimentally demonstrate a polarization-tracking-free polarization division multiplexing (PDM) intermediate frequency over fiber (IFoF) based on multiband direct detection optical orthogonal frequency division multiplexing (DDO-OFDM) with adaptive modulation (AM) technique downstream, as well as polarization insensitive on-off-keying upstream. The polarization tracking mechanism is eliminated, hence the complicated cell sites are simplified and letting more cost-effective and power-efficient for every radio access unit (RAU). Moreover, under the C-RAN architecture the mobile fronthual is thus establish with high spectra flexibility and complexity centralized which will be a promising selection for next generation mobile communication.
In the downstream system, only a simple optical filter at each receiver can passively demultiplex the corresponding signal band in both PDM scheme and multiband scheme. An adaptively modulated multiband OFDM signals with 3.5-GHz bandwidth, 50-Gb/s transmission 25-km single mode fiber is experimentally demonstrated. A spectral efficiency is doubled with applied PDM technique and data capacity is 8-Gb/s increased compare with employing AM or not. Furthermore, it provides 4 dB power margins, where may support wireless transmissions[1]. In the upstream generation, the polarization insensitive is implemented for using MZM by directly reuse the two orthogonal carriers without additional filtering or polarization controlling. The results show a 1.8dB power penalty with respect to those with traditional best polarization aligned mechanism, which can be negligible. The polarization tracking mechanisms are out of consideration and establish a true polarization tracking free mobile fronthaul system.

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