3. RAN Architecture Evolution Strategies
SK Telecom introduced C-RAN right from the very early stages of the LTE service commercialization (January 2012) as an effort to achieve higher mobile network operation efficiency and more cost savings. As a result, most of the company's RAN is now in C-RAN architecture, where macro BBUs are separated from RRHs, and moved to centralized locations, such as CO or master base station while RRHs are left at cell sites. RRHs are connected, through a fronthaul network (ring-type active WDM network that delivers CPRI traffic, AKA Cloud Belt in SK Telecom term), to BBUs centralized at CO where coordination servers are running, thereby providing inter-cell coordination functions, such as CoMP and inter-site CA.
SK Telecom's C-RAN has moved forward to Unified RAN with introduction of small cells in 2014, and will move further forward towards Service-Aware RAN, base stations for intelligent RAN, and Virtualized Radio Access Network (vRAN), the virtualization-based next generation base stations. These two are expected to provide operators with new revenue opportunities and users with enhanced QoE, by introducing/offering services in RAN, primarily based on Unified RAN. In the near future, Service-Aware RAN where cache at a centralized BBU pool offers services like video caching in RAN will be commercialized. In the long time, SK Telecom's RAN will evolve into vRAN where BBUs are operated by SW installed on industry standard servers, realizing BBU virtualization.
Figure 8. Evolution of RAN architecture
3.1 Unified RAN
Since 2012, SK Telecom has built its C-RAN with LTE macro cells only, and then also with small cells (small-sized and low-power RRHs mounted on pole) since 2014. As with macro cells, small cells have C-RAN architecture and thus small RRHs are connected through CPRI interface to concentrated BBUs. In other words, both macro and small RRHs are connected to a BBU pool through the fronthaul network. SK Telecom calls this Unified RAN.
The evolved Unified C-RAN can increase network capacity flexibly, and help HetNet (SUPER Cell) to connect cells in different sites/BBUs through a high-capacity bandwidth and with a lower latency. This assures close coordination between HetNet cells, and thus can efficiently support SUPER Cell technologies (TM9, CoMP, inter-site CA, dual connectivity, etc.) for enhanced network capacity.
3.2 Service-Aware RAN
Previously, SK Telecom has provided services through its core networks, and hence RAN, incapable of identifying what service is being used by users, has merely served as a dumb pipe for data delivery. However, Service-Aware RAN can i) run service applications in RAN as well, and ii) identify services used by each user and provide them with region-specific or user-specific services by analyzing network/service usage information of each user. This enables operators to create new revenue streams other than network access fees and users to enjoy faster responses and personalized services, thereby improving user QoE and satisfaction.
SK Telecom's Service-Aware RAN is based on Unified C-RAN, and so it allows RAN cache servers (or cards) at a BBU pool to cache video files and DNS, and offer features like video optimization, CDN interworking, local breakout, etc. in RAN as well. In MWC 2013, the company, jointly with NSN, demonstrated these Service-Aware RAN features, and vendors like Samsung and Nokia are finishing up development of technologies for installing application servers in RAN (for example, taking care of charging and handovers in relation to traffic served directly by RAN cache, without going through P-GW). Given that, Service-Aware RAN is very likely to be commercialized soon.
RAN building/operating costs are one of the biggest investments that operators should make. To cut down the cost of RAN, and make employment of new network/service functions easier, SK Telecom developed so called vRAN that virtualizes BBUs. In January 2014, SK Telecom demonstrated LTE FDD radio communication (at 300 Mbps using 20 MHz bandwidth and 4x4 MIMO) by installing Hypervisor and Virtual Machine (VM) on an Intel Xeon processor-based server and virtualizing the modem functionalities (PHY & MAC).
The main feature of vRAN is to apply IT virtualization technology to BBUs so that vendor-specific BBUs that have been provided by the existing base station vendors can be replaced by industry-standard servers. An industry-standard server, equipped with general-purpose processors (GPPs) and HW acceleration technologies, can process RAN functions and services real fast by using SW. Moreover, new features for LTE-A/B or 5G, or newly released RAN functions/services can be easily installed/removed through simple SW upgrade using open API. Because this architecture allows for sharing of HW/computing resources between BBUs through open interfaces, flooded traffic in one BBU can be easily diverted on to other BBUs.
vRAN, by taking advantage of smarter technologies (more SW-oriented) and a more Cloud-friendly environment (open RAN architecture), can certainly be a great money saver for SK Telecom in that it can significantly save RAN costs for installing and operating base stations. However, apparently switching to vRAN is not an easy task. The company's investments in current RAN equipment should be protected, and vRAN-ready industry standard servers are not available in the market yet.
That means, some BBU functions will continue to be used in forms of vendor-specific HWs provided by existing base station vendors, and the BBUs will become more intelligent, for example, with cloud capabilities for inter-BBU resource sharing, while other RAN functions (CoMP coordination, etc.) and application services provided at mobile edges are handled by SW on virtualized industry standard servers. SK Telecom will probably have to stick to this architecture for a long while.
API : Application Programming Interface
BBU : Baseband Unit
CA : Carrier Aggregation
CO : Central Office
CoMP : Coordinated Multi-Point transmission/reception
CPRI : Common Public Radio Interface
C-RAN : Cloud-RAN
CSP : Content Service Provider
DU : Digital Unit
eICIC : Enhanced Inter-Cell Interference Coordination
FDD : Frequency Division Duplexing
GPP : General Purpose Processor
HetNet : Heterogeneous Network
ICIC : Inter-Cell Interference Coordination
LTE : Long-Term Evolution
LTE-A : Long-Term Evolution - Advanced
MIMO : Multiple Input Multiple Output
MPTCP : Multipath TCP
ORI : Open Radio Interface
OTT : Over-The-Top
PCell : Primary Cell
PCI : Physical Cell ID
RAN : Radio Access Network
RRH : Remote Radio Head
RU : Radio Unit
SCell : Secondary Cell
TDD : Time Division Duplexing
TM : Transmission Mode
UE : User Equipment
VM : Virtual Machine
vRAN : Virtualized RAN
WDM : Wavelength Division Multiplexing