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Managing heterogeneity with Cloud-RAN

Posted: 24 Feb 2014 ?? ?Print Version ?Bookmark and Share

Keywords:3GPP? Cloud-RAN? C-RAN? Centralized-RAN? cellular network?

The clock and control module within a base station leverages these mechanisms to achieve frequency, phase, and time-of-day accuracy within the network. In turn, the base station clock control module provides the synchronised clocking information to the radio elements. In traditional centralized base stations, it is easy to achieve synchronisation, because the radio elements reside within the same chassis. Successful distribution of synchronised clocks becomes challenging in high density distributed base stations (Cloud-RAN), where radio elements are located remotely at varying distances connected via fibre or microwave/millimeter wave point-to-point link.

Initially, system vendors used proprietary protocols to distribute timing and synchronisation information to remote radio heads. The Open Base Station Architecture Initiative (OBSAI) and Common Public Radio Interface (CPRI) standards were introduced to standardise connectivity and synchronisation between the base station chassis and remote radio heads. These protocols enable dissemination of synchronisation information along with the time division multiplexed-based data plane transport.

The stringent requirements of maintaining round-trip deterministic latency of less than 16 ns, along with the timing alignment error for transmit diversity chains to within 65 ns, has resulted in the use of dedicated fibre links between the base station chassis and remote radio heads. Point-to-point microwave links are also being used in some cases where fibre connectivity is not economical.

Dedicated fibre for connectivity between the base station chassis and the radio heads is extremely limiting and expensive. To optimise fibre connectivity, the remote radio heads are connected to the base station chassis using chain, tree, or star topologies. The CPRI and OBSAI standards support fibres up to 40 km long, but the majority of remote radio installations are constrained to distances of hundreds of meters from the base station chassis. A widespread rollout of Cloud-RAN-based distributed base stations would need a fibre reach of up to 40 km andmore importantlyover a shared network. Transition to a common Ethernet-based data transport protocol within the base station chassis and remote radio heads would be a vital step towards the use of shared networks. Deployment of fine-grain traffic engineering functions within the shared network would be another key requirement to forward data to remote radio heads in a prioritized way with careful management of intermediate node buffering to achieve desired deterministic latency resolution.

Connecting any base band channel card to any remote radio heads in a Cloud-RAN topology would require a new set of hierarchical switch functions. Today, these switches reside in the base station chassis and allow data per each antenna carrier to be switched from any of 3-6 base band cards to any of 12 or so remote radio heads. Base band cards and remote radio heads multiply in a Cloud-RAN topology. Bigger and hierarchical switch functions will be required to achieve the desired connectivity.

Figure 3: Conceptual Cloud-RAN sharing network architecture using QoS/traffic engineering.

Using programmable logic devices is the most effective way to address the fluid and continuously improving Cloud-RAN algorithmic and connectivity function requirements. Programmable logic devices are widely used in channel cards, radio heads, network nodes, and backhauling equipment.

The presence of this programmable capability in every node within the network can be leveraged to keep underlying algorithms and connectivity functions in lockstep via field upgrades. The Xilinx 28nm All Programmable SoC family, for example, integrates FPGA, CPU, DSP, and analogue mixed-signal functions along with an optimal number of high-speed transceivers and IO interconnect in a single device. This processor-centric platform offers software, hardware, and IO programmability to build smarter switching and algorithmic functions to lay a strong foundation for truly self-healing, self-learning, and self-optimising wireless network nodes.

Coupled with the Vivado design environment and tool suite, Xilinx solutions enables designers to deliver time to integration, productivity, and quality of results. Xilinx tools and silicon technology are said to be enriched by an ecosystem that allows for innovation while providing better off-the-shelf solutions for old and new problems.

Productivity, performance, and time to market are of the essence. Xilinx 20nm UltraScale All Programmable devices deliver ASIC-class system-level performance for building high throughput and low latency networking and signal processing functions. This family is co-optimised with the Vivado design suite and UltraFAST design methodology to accelerate time to market.

Xilinx aims to help steer solutions that enable widespread adoption of Cloud-RAN networks and move Cloud-RAN into a much-needed network platform for abstracting underlying heterogeneity for effective network monetisation, ease of network deployment, and maintenance.

About the author
Harpinder Singh Matharu is with Xilinx Inc.

To download the PDF version of this article, click here.


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