Chen Yuanxiang, Li Juhao, Zhu Paikun, Peng Huanfa, Zhu Jinglong, Tian Yu, Wu Zhongying, Ge Dawei, Zhou Peng, Liu Ke, Xu Yongchi, Chen Jingbiao, He Yongqi, Chen Zhangyuan
Opt Express. 2016 Oct 3;24(20):22413-22422. doi: 10.1364/OE.24.022413.
In this paper, we propose a cost-effective wavelength-reused mode-division-multiplexing (MDM) system for high speed symmetrical bidirectional mobile fronthaul application. At the base band unit (BBU) pool, one of the spatial modes is used to transmit signal carrier while the others are used for downstream (DS) signal channels. At the remote radio unit (RRU) side, the signal carrier is split and reused as modulation carrier for all the upstream (US) signal channels after mode demultiplexing. Thanks to the low mode crosstalk characteristic of the mode multiplexer/demultiplexer (MUX/DEMUX) and few-mode fiber (FMF), the signal carrier and each signal channel can be effectively separated. The spectral efficiency (SE) is significantly enhanced when multiple spatial channels are used. Compared with other wavelength reused scheme in which the downstream and upstream be modulated in orthogonal dimension, the modulation format of both directions are independent in the proposed wavelength reused MDM system. Therefore, it can easily achieve symmetrical bidirectional transmission without residual re-modulation crosstalk. The proposed scheme is scalable to multi-wavelength application when wavelength MUX/DEMUX is utilized. With the proposed scheme, we demonstrate a proof of concept intensity modulated 4 × 25-Gb/s 16-QAM orthogonal frequency division multiplexing (OFDM) transmission over 10-km FMF using low modal-crosstalk two-mode FMF and MUX/DEMUX with error free operation. The downstream receiver sensitivity is -21 dBm while the upstream receiver sensitivity is -18 dBm for bidirectional transmission. Due to the Rayleigh backscattering and other spurious reflections, the upstream suffers 2 dB power penalty compared with unidirectional transmission without downstream. To mitigate bidirectional transmission impairments, we propose a simple and effective method to suppress Rayleigh backscattering by shifting the downstream subcarrier frequency. A receiver sensitivity improvement of up to 2.5 dB is achieved for upstream with different downstream power.
在本文中，我们提出了一种具有成本效益的波长复用模式分割复用（MDM）系统，用于高速对称双向移动前传应用。在基带单元（BBU）池，其中一个空间模式用于传输信号载波，而其他空间模式用于下行（DS）信号通道。在远端射频单元（RRU）侧，信号载波在模式解复用后被分割并重新用作所有上行（US）信号通道的调制载波。由于模式复用器/解复用器（MUX/DEMUX）和少模光纤（FMF）的低模式串扰特性，信号载波和每个信号通道能够有效分离。当使用多个空间通道时，频谱效率（SE）显著提高。与其他在正交维度上调制下行和上行的波长复用方案相比，在所提出的波长复用MDM系统中，两个方向的调制格式是独立的。因此，它可以轻松实现对称双向传输，而不会产生残留的再调制串扰。当使用波长MUX/DEMUX时，所提出的方案可扩展到多波长应用。通过所提出的方案，我们展示了一个概念验证，即使用低模式串扰的双模FMF和MUX/DEMUX在10公里的FMF上进行强度调制的4×25 Gb/s 16正交幅度调制（QAM）正交频分复用（OFDM）传输，且无差错运行。对于双向传输，下行接收器灵敏度为-21 dBm，而上行接收器灵敏度为-18 dBm。由于瑞利后向散射和其他杂散反射，与没有下行的单向传输相比，上行遭受2 dB的功率代价。为了减轻双向传输损伤，我们提出了一种简单有效的方法，通过移动下行子载波频率来抑制瑞利后向散射。对于不同的下行功率，上行的接收器灵敏度提高了高达2.5 dB。
