Flexicell – OAI-based Cloud Radio Access Networks
Jorge Santos, Arnaldo S. R. Oliveira, Nuno Borges de Carvalho, Paulo Monteiro
Instituto de Telecomunicações / Universidade de Aveiro – DETI, Portugal
Paulo Jesus, Nelson Silva, José Salgado
Altice Labs, Portugal
Flexicell was the first project that successfully demonstrated an OpenAirInterfaceTM (OAI) based Cloud Radio Access Network (C-RAN) for fourth and fifth generation (4G/5G) mobile eras, with a CPRI-based 20 km fronthaul implemented over a GPON infrastructure, to connect the software defined distributed radio heads (antennas) with the central digital processing unit (Figure 1) . The demonstrator shows the interoperability with commercial user mobile terminals and applications and includes low latency and resource efficient digital radio compression modules to double the fronthaul traffic capacity.
Figure 1 – Architecture of the Flexicell OAI-based laboratory demonstrator, including the interoperability and connectivity with user terminals.
General motivation and objectives
The convergence of fixed transport networks, based on high-speed optical infrastructures and broadband spectrally efficient wireless components has been identified as a key enabler of future access networks. The next generation of wireless systems (5G) should fulfil several goals, among which: provision of true broadband wireless access and enhanced system capacity, when compared with current third (3G) and fourth (4G) generation networks.
A traditional cellular network is built with many stand-alone base stations (BSs), each one covering a cell and processing and transmitting its own signal to and from the mobile terminals. The issues regarding the growing complexity of BSs, the need for cooling, the increasing number of BSs for improved coverage and the difficulties in the acquisition of new sites has led to some rethinking of the cellular concept, whose main trends are currently converging to C-RAN. C-RAN has been defined in several different ways, but essentially designates a network architecture where several distributed Remote Radio Heads (RRHs) with reduced complexity are linked to a central or Base Band Unit (BBU) at which joint radio signal processing is performed. The connection between the RRHs and the BBU is established through a high capacity network link, named fronthaul, typically supported by an optical infrastructure. The ideas of distributed base stations were successfully demonstrated for the first time in the FUTON European FP7 project  technical lead by Instituto de Telecomunicações / Universidade de Aveiro (IT/UA).
Besides the C-RAN advantages, further network optimizations can be promoted with the efficient use of existing metro and access networks to support the mobile backhaul and fronthaul network segments. The Flexicell project faced the challenge of joining the concepts of small cell, C-RAN, Software Defined Radio (SDR) and infrastructure sharing in an integrated and unified architecture of a complete 4G/5G mobile network.
Work description and achievements
In the scope of the Flexicell project, a complete C-RAN testbed for next generation mobile networks was developed and successfully demonstrated with a 20 km length fronthaul. The most important aspects of the Flexicell project, shown in Figure 2, are:
- Utilization of Passive Optical Networks (PONs) as the physical infrastructure for the fronthaul, in coexistence with triple-play services (voice, video, and data), avoiding dedicated and expensive links and simplifying the deployment of small cells for improved coverage and spectral efficiency.
- Compression of the fronthaul traffic data between the BBU and the RRH to increase almost 50% of fronthaul traffic capacity in PON systems with negligible performance degradation.
- Upgradability to future PON technologies (e.g. NG-PON2) that will support larger fronthaul bandwidths for next generation mobile networks with wider channels and bit rates.
- Adoption of SDR approaches from the BBU to the RRH, leading to an access infrastructure that is agnostic and upgradable to future network standards.
- OAI-based digital baseband processing and core network virtualization for more flexible deployment, management and upgradability, as well as improved load balancing at BBU nodes for better energy and resource efficiency.
- Interoperability of the developed mobile network infrastructure with commercial mobile terminals (e.g. smartphones and 4G modems/dongles) allowing the communication with any equipment or terminal connected to the same network or anywhere in the internet (Figure 1), for demonstrating real end user applications (e.g. mobile internet access, voice and video calls) and evaluate overall system performance and introduce optimizations.
Figure 2 – Deployment scenario of the Flexicell project in the scope of Radio Access Network converged with a Passive Optical Network infrastructure.
This white paper described the architecture of the Flexicell C-RAN lab demonstrator. The developed testbed, is very modular and flexible, and includes all the components necessary to build an open and complete C-RAN compliant base station solution interoperable with real user terminals. It is being used for the experimentation and evaluation of next generation wireless communication systems, including new fronthaul protocols and interfaces as well as 5G waveforms. It integrates a 20 km optical fronthaul, a software defined multi-band RF front-end, digital radio compression algorithms and OAI-based open implementations of the baseband and core network components.
According to the CPRI specification v6.1 , “the Common Public Radio Interface (CPRI) is an industry cooperation aimed at defining a publicly available specification for the key internal interface of radio base stations between the Radio Equipment Control (REC) and the Radio Equipment (RE)”. In other words, the CPRI specification provides the physical (L1) and data link layer (L2) details for the transport of digitized radio information between REC and RE. Figure 3 shows the functional split between REC and RE as defined in the CPRI specification (downlink). As shown in the Figure 3, all the operations above the physical layer, and most of the ones of the physical layer are performed by the REC, which generates the radio signal samples and sends the resulting data to the RE. The RE basically reconstructs the waveform and transmits it over the air. The uplink case is similar, although the sampling of the radio signal must be performed in the RE. The main benefit of this split is that almost no digital processing functions are required at the RRHs, making them very small and cheap. In addition, the centralization of all the signal processing functions in the BBU simplifies the adoption of cooperative techniques such as Cooperative Multipoint (CoMP), which require advanced processing of the radio signal of several RRHs simultaneously.
Figure 3: CPRI system and interface definition (source: ).
 Flexicell project webpage, https://www.it.pt/Projects/Index/2112
 FUTON FP7 project webpage, http://www.ict-futon.eu/
 Common Public Radio Interface webpage, http://www.cpri.info