Operators seeking to accommodate smartphones and other data-heavy wireless devices are expanding capacity and coverage. To do so, many are turning to wireless connectivity, including small cells, carrier Wi-Fi or Distributed Antenna Systems (DAS).
As operators evaluate their wireless architecture options, characteristics such as agility, time to market, cost-effectiveness, operational and architectural simplicity, expandability and flexibility come into play. Energy consumption and physical size are also important since power and space are expensive, scarce resources at base station sites and central offices.
A Centralized-Radio Access Network (RAN) architecture, also known as Cloud-RAN or C-RAN, is garnering attention for its ability to meet all of these needs while supporting mobile xHaul (fronthaul and backhaul), network self-optimization, self-configuration and self-adaptation through self-organizing network functionality (SON).
C-RAN uses Remote Radio Heads (RRHs) connected to a Baseband Unit (BBU) via CPRI (Common Public Radio Interface), OBSAI (Open Base Station Architecture Initiative) or Open Radio Interface (ORI) interfaces. The RRHs include the radio, the associated amplification/filtering and the antenna.
The BBU is implemented separately and performs the centralized signal processing functionality of the RAN. The decentralized BBU increases agility, speeds service delivery, reduces cost and improves coordination of radio capabilities across a set of RRHs. In addition, a number of BBUs can be aggregated to form a Centralized Baseband Unit (C-BBU), a two-dimensional cluster of RRHs and BBUs.
CPRI is a digital interface standard for encapsulating radio samples between the RRH and the BBU. CPRI provides for user plane data (IQ data), control and management as well as synchronization signals exchanged between the BBU (master) and the RRH (slave). CPRI offers minimal latency, near-zero jitter and a near-zero bit error rate.
There are several CPRI transport options available for connecting RRHs to the BBU, including:
- Dedicated fiber is an attractive option for scenarios involving a large installed base of available fiber; however, it must be used prudently since the cost of deploying new fiber limits the applicability of this option.
- Optical Transport Network (OTN) brings in well-known forward error correction (FEC) methods and can increase the reach of metro optical networks, but it requires careful consideration because of its added latency.
- Passive Optical Network (PON) is a potentially attractive option in high-traffic areas where small-cell deployment is most likely to occur. Its weakness lies in its use of optical splitters to separate and collect optical signals, which makes it vulnerable to additional latency and power loss.
- Microwave transport is another option for short distances (1 kilometer or less. This technology currently supports a subset of the CPRI interface bit-rate options.
- CPRI over Ethernet (CoE) offers substantial savings because it uses existing Ethernet cable infrastructure to encapsulate and transport CPRI from a centralized BBU pool (V-BBU) to the RRH. The CoE cabling architecture requires CPRI–Ethernet mapping guidelines and integrated Ethernet monitoring to maintain stringent jitter and latency requirements that are not part of the CPRI standard.
- Wavelength-based systems have proven to be excellent CPRI transport solutions. Coarse Wavelength-Division Multiplexing (CWDM) is particularly attractive due to its support for low propagation delays and high data throughout, and it is an economical choice in both equipment costs and in its use of fiber resources.
C-RAN Architecture Implementation with Small Cells
Limited space for base station installations, inter-cell interference and costly, bulky infrastructure are obstacles to increasing network coverage and capacity. In these scenarios, small-cell based C-RAN networks are excellent alternatives that offer greater interference control, access control and manageability along with higher throughput and capacity. The C-RAN architecture shown here presents a scalable and cost-effective solution in a challenging, dense urban environment.
This architecture uses small cells with SON capability and can be installed on customer premises or public utilities including existing lampposts and traffic lights. Several small cells can be aggregated and connected to baseband processing pools via fronthaul using E-band radios that support low latency, low power and high data throughput. Deploying these small cells is less cumbersome than other options, since it requires minimum wiring from the small cells to the E-band radio equipment, which further provides line-of-sight (LOS) (millimeter microwave) connection to the baseband pool.
Furthermore, the baseband pools are connected to the centralized Evolved Packet Core (EPC) and to each other for greater interference management and mobility control through the standardized CPRI interface. With the C-RAN architecture, it is now easy to replicate the solution described here in many urban areas and leverage existing fiber infrastructure to help reduce cost and extend network capabilities.
RAN SDN Controller
The logical evolution of C-RAN is its integration with other advanced application services through Software-Defined Networking (SDN) and Network Functions Virtualization (NFV), enabling the network to achieve complete virtualization, controllability and programmability.
The RAN SDN controller acts as the control plane of heterogeneous RANs by abstracting and combining control functions of the access elements. It determines the strategies of each V-BBU and V-EPC, with each virtual access element containing an SDN agent to communicate with the controller.
The SDN RAN controller creates and dynamically optimizes the virtual access elements by efficiently minimizing delay and allocating spectrum, computing and storage resources by need. Each virtual access element has a unified SDN agent to resolve the control flow. In the control plane, the RAN SDN controller can establish or modify the rules, including routing, bandwidth allocation and priority setting, in each virtual access element. The same logic in the control plane can be implemented on the data plane to optimize and virtualize access RAN functionality.
For operators seeking to expand capacity and coverage via wireless architecture, C-RAN offers a variety of benefits: controlling ongoing operational costs and improving network security, controllability, agility and flexibility.
With the centralized processing ability of the C-RAN architecture, the number of base station sites needed can be reduced by a factor of 10, resulting in capex and opex savings. Further, small cells with lower transmission power can be deployed to improve network coverage and capacity.
Capacity and Spectral Efficiency Improvement
In C-RAN, macro, micro or small virtual base stations are aggregated in a large physical BBU pool where they can easily share signaling, data and Channel State Information (CSI) for active users. With C-RAN, it is much easier to implement LTE Advanced algorithms such as Carrier Aggregation (CA) and Coordinated Multiple Point (CoMP) operation to mitigate inter-cell interference and improve spectral efficiency.
C-RAN architectures can also efficiently handle nonuniform data traffic due to the load-balancing capability in the distributed BBU pool.
Smart Internet Traffic Offload
Aggregation of the baseband functionality in C-RAN now provides a central port for traffic offload and content management to handle growing Internet traffic from smartphones and other portable devices. As a result, backhaul traffic, core network traffic and latency are all reduced, leading to a higher quality experience for the user.
Because the C-RAN architecture supports multi-standard operations and multi-cell collaborative signal processing, it becomes easier to upgrade and expand network capacity from the aggregated point. The C-RAN architecture inherently facilitates flexible network topology designs.
Dr. Femi Adeyemi is Lead LTE Solutions Architect at Fujitsu Network Communications.
Filed Under: Infrastructure