Pacing the way toward 5G has lead researchers and industry in the direction of centralized processing known from Cloud-Radio Access Networks (C-RAN). In C-RAN research, a variety of different functional splits is presented by different names and focusing on different directions. The functional split determines how many base station functions to leave locally, close to the user, with the benefit of relaxing fronthaul network bitrate and delay requirements, and how many functions to centralize with the possibility of achieving greater processing benefits. This paper presents for the first time a comprehensive overview systematizing the different work directions for both research and industry, while providing a detailed description of each functional split option and an assessment of the advantages and disadvantages. This paper gives an overview of where the most effort has been directed in terms of functional splits, and where there is room for further studies. The standardization currently taking place is also considered and mapped into the research directions. It is investigated how the fronthaul network will be affected by the choice of functional split, both in terms of bitrates and latency, and as the different functional splits provide different advantages and disadvantages, the option of flexible functional splits is also looked into.

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A Survey of the Functional Splits Proposed for 5G Mobile Crosshaul Networks

The exponential traffic growth, demand for high speed wireless data communications, as well as incessant deployment of innovative wireless technologies, services, and applications, have put considerable pressure on the mobile network operators (MNOs). Consequently, cellular access network performance in terms of capacity, quality of service, and network coverage needs further considerations. In order to address the challenges, MNOs, as well as equipment vendors, have given significant attention to the small-cell schemes based on cloud radio access network (C-RAN). This is due to its beneficial features in terms of performance optimization, cost-effectiveness, easier infrastructure deployment, and network management. Nevertheless, the C-RAN architecture imposes stringent requirements on the fronthaul link for seamless connectivity. Digital radio over fiber-based common public radio interface (CPRI) is the fundamental means of distributing baseband samples in the C-RAN fronthaul. However, optical links which are based on CPRI have bandwidth and flexibility limitations. Therefore, these limitations might constrain or make them impractical for the next generation mobile systems which are envisaged not only to support carrier aggregation and multi-band but also envisioned to integrate technologies like millimeter-wave (mm-wave) and massive multiple-input multiple-output antennas into the base stations. In this paper, we present comprehensive tutorial on technologies, requirements, architectures, challenges, and proffer potential solutions on means of achieving an efficient C-RAN optical fronthaul for the next-generation network such as the fifth generation network and beyond. A number of viable fronthauling technologies such as mm-wave and wireless fidelity are considered and this paper mainly focuses on optical technologies such as optical fiber and free-space optical. We also present feasible means of reducing the system complexity, cost, bandwidth requirement, and latency in the fronthaul. Furthermore, means of achieving the goal of green communication networks through reduction in the power consumption by the system are considered.

Toward an Efficient C-RAN Optical Fronthaul for the Future Networks: A Tutorial on Technologies, Requirements, Challenges, and Solutions

Mobile fronthaul is an important network segment that bridges wireless baseband units and remote radio units to support cloud radio access network. We review recent progresses on the use of frequency-division multiplexing to achieve highly bandwidth-efficient mobile fronthaul with low latency. We present digital signal processing (DSP) techniques for channel aggregation and deaggregation, frequency-domain windowing, adjacent channel leak age ratio reduction, and synchronous transmission of both the I/Q waveforms of wireless signals and the control words (CWs) used for control and management purposes. In a proof-of-concept experiment, we demonstrate the transmission of 48 20-MHz LTE signals with a common public radio interface (CPRI) equivalent data rate of 59 Gb/s, achieving a low round-trip DSP latency of <2 μs and a low mean error-vector magnitude (EVM) of ∼2.5% after fiber transmission. In a follow-up experiment, we further demonstrate the transmission of 32 20-MHz LTE signals together with CPRI-compliant CWs, corresponding to a CPRI-equivalent data rate of 39.32 Gb/s, in single optical wavelength channel that requires an RF bandwidth of only ∼1.6 GHz. After transmission over 5-km standard single-mode fiber, the CWs are recovered without error, while the LTE signals are recovered with an EVM of lower than 3%. Applying this technique to future 5G wireless networks with massive multiple-input multiple-output is also discussed. This efficient mobile fronthaul technique may find promising applications in future integrated fiber/wireless access networks to provide ultrabroadband access services.

Efficient Mobile Fronthaul via DSP-Based Channel Aggregation

Fiber-Wireless (FiWi) network, as an integration of optical fiber network and wireless access network, has attracted intensive research interest and enjoyed significant growth in its applications over the last two decades. Besides bringing great opportunities for the development of FiWi networks, the new applications and techniques, like the emerging Internet of Things (IoT), raise unprecedented challenges in different aspects of FiWi integration. Based on such observation, we provide in this paper a comprehensive survey of recent progress and ongoing research works in the aspects of network architecture, QoS provisioning, scalability improving, reliability enhancing, energy saving, as well as standardization activities and case studies. Furthermore, new trends and challenging issues on future smart FiWi networks are identified and discussed in details.

New Perspectives on Future Smart FiWi Networks: Scalability, Reliability, and Energy Efficiency

In this  study, we demonstrate the intermediate-frequency-over-fiber (IFoF)  based mobile fronthaul technology implemented in a 28-GHz millimeter-wave (mmWave) based 5G prototype supporting Giga-bit mobile broadband services. In order to confirm the technical feasibility of the proposed IFoF-based mobile fronthaul, the clock signal for frequency synchronization and mobile payload data are simultaneously transmitted in the form of IF carriers. Phase noise and error vector magnitude measurements were performed on separate and simultaneous clock transmission paths, respectively. The results confirm an acceptable impact on performance, and the simultaneous clock transmission method reduces system costs and complexity, and improves the flexibility and efficiency of wavelength operation. We also successfully demonstrate the operation of real-time Giga-bit mobile services such as 4K video streaming over IFoF-based mobile fronthaul for 5G prototypes with a 28-GHz mmWave. The achieved peak data-rate per user is up to 1.5 Gb/s, which complies with the requirements of the 5G vision of IMT-2020.

Demonstration of IFoF-Based Mobile Fronthaul in 5G Prototype With 28-GHz Millimeter wave

We proposed a cost-effective next generation mobile fronthaul architecture with multi-IF carrier transmission scheme and investigated the system performances under various IF carrier operating conditions.

Cost-effective Next Generation Mobile Fronthaul Architecture with Multi-IF Carrier Transmission Scheme

A relaying method of an optical signal of a hybrid relaying apparatus in a gigabit passive optical element includes: selecting and receiving any one of a first serial electrical signal corresponding to a downlink wavelength division multiplexing-passive optical line terminal (WDM-PON OLT) optical signal and a second serial electrical signal corresponding to a downlink gigabit passive optical network optical line terminal (GPON OLT) optical signal; modulating the downlink serial electrical signal to a downlink GPON transmission convergence (GTC) frame; extracting control information for uplink transmission from the downlink GTC frame; converting the downlink GTC frame into the downlink serial electrical signal; and converting the converted serial electrical signal into an optical signal and transmitting the converted optical signal in a GPON OLT optical transceiver.

Apparatus and method for relaying in gigabit passive optical network

Disclosed is a cloud base station in an orthogonal frequency division multiplexing (OFDM)-based fixed-mobile converged access network and an operation method thereof. The cloud base station may include a physical (PHY) layer unit to perform a quadrature amplitude modulation (QAM) of a parallel signal received from a media access control (MAC) layer per subcarrier, and an optical orthogonal frequency division multiplexing (OFDM) transceiver to transform the QAM modulated subcarriers into a time domain to generate an OFDM sample per subcarrier, add a cyclic prefix (CP) and control information to for operating an enhanced radio unit (eRU) to the OFDM sample per subcarrier to generate a downstream signal, and transmit the downstream signal.

Cloud base station in fixed-mobile converged access network and operation method thereof

A software-defined networking (SDN) method. The SDN method according to an exemplary embodiment may be used in managing and operating SDN-based network resources in an optical communications network, a fixed mobile convergence subscriber network, a wired broadband subscriber network, a distributed mobile communications base station network, etc.

Software-defined networking method

A method of tuning a wavelength of a tunable ONU in a TWDM-PON is provided. The method includes transmitting a wavelength change request message from a source OLT to request the ONU to change a wavelength thereof from a first wavelength to a second wavelength and in response to the wavelength change request message, transmitting a wavelength change response message from the ONU to the source OLT to indicate whether or not the ONU can change a wavelength thereof. The wavelength change request message is ID information for specifying an ONU that is requested to change a wavelength thereof, and the message may comprise one of the following: system ONU ID, channel ONU ID, and individual ONU ID.

Kyeong Hwan Doo

Papers

Cost-effective Next Generation Mobile Fronthaul Architecture with Multi-IF Carrier Transmission Scheme

Apparatus and method for relaying in gigabit passive optical network

Cloud base station in fixed-mobile converged access network and operation method thereof

Software-defined networking method

Method of tuning wavelength of tunable optical network unit (onu) in time and wavelength division multiplexing-passive optical network (twdm-pon)