In C-RAN, baseband signal processing is offloaded from individual base stations—called remote units (RUs)—to a central unit (CU). This yields many benefits, such as simplified network maintenance, smaller formfactor RUs, efficient use of processing resources through statistical multiplexing at the CU, reduced costs for equipment rooms at base station sites, and spectral efficiency gains from joint processing such as coordinated multi-point (CoMP). On the other hand, it necessitates the deployment of a very demanding fronthaul (FH) network transporting the raw in-phase/quadrature-phase (I/Q) samples from the RUs to the CUs for processing. Commonly, this FH network is implemented based on
the Common Public Radio Interface (CPRI) standard [2], which requires data rates of up to 24 Gbps per cell, a round-trip FH latency below 200 μs, low jitter, tight synchronization, and high reliability. These requirements can only be realized with high-capacity fiber or point-topoint wireless links, making the deployment of the FH network very costly, reducing the gains expected from centralization.
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These new RATs, which could include technologies such as massive MIMO [8], millimeter wave (mmWave) communication [9], and non-orthogonal waveforms [10], will have a significant impact on the transport network. At the same time, new services, such as the Internet of things (IoT) [11], the Tactile Internet [12], or vehicular communication [13], will add new requirements such as ultra-low latency and extremely high availability. Hence, it is of fundamental importance to design the 5G transport network and the corresponding NGFI in view of the requirements these technologies induce.
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4.2 Enabling transport technologies Dedicated fiber connectivity, currently favored for CPRIbased FH networks, does not offer resource sharing between different fibers or wavelengths. Packet-switched networks based on Ethernet are the most promising alternative as reflected by recent initiatives such as the IEEE 1914 Working Group [6]. Currently, synchronization is still challenging in packet-based networks and jitter introduced by switches and queues could become critical for FH traffic. In this regard, technologies such as Synchronous Ethernet [25] and the Precision Time Protocol (IEEE 1588) [26], which are currently investigated by, e.g., the IEEE 802.1 Time Sensitive Networking Task Group [27], are good candidates to efficiently support FH over Ethernet. Supporting variable data rates is also a particular challenge at the physical level, due to the inflexible, dedicated links currently utilized. A packet-based flexible optical transport will comprise both passive and active solutions. Passive optical network (PON) solutions will be based on wavelength division multiplexing (WDM)PONs, while active solutions will adopt more flexible and dynamic WDM technologies such as the timeshared optical network [28], which enable very granular sub-wavelength bandwidth allocation that is a key to efficiently utilize optical bandwidth in the aggregation/ metro segment of converged FH/BH networks. Wireless transport technologies are also considered as candidate technologies for future transport networks due to their lower cost and higher flexibility compared to fiber.
