Texas Instruments (TI) has announced enhancements to their KeyStone DSP architecture to support emerging cloud-based Radio Access Network (RAN) applications for wireless base stations. C-RAN is a development project being led by the China Mobile Research Institute, in which the conventional base station hardware and software resources at individual cell sites are removed and aggregated in a centralized location, connected remotely to the towers over high bandwidth fiber. With pooled resources to serve multiple cell sites, China Mobile proposes that C-RAN can lower network deployment costs with reduced hardware and provide a greener solution that consumes less power. At the 2011 Mobile World Congress, Intel announced that they were collaborating with China Mobile on the C-RAN project, developing a proof of concept for processing TD-LTE signals on the company's Sandy Bridge processors. Intel also highlighted this development at the 2011 Intel Developer Forum.
Tom Flanagan, Director for Technical Strategy in the Wireless Base Station Infrastructure group at TI, says that most of the writings about C-RAN leading to greener base stations have neglected to consider the power that would be consumed by large Intel-based servers. He says that gains in power with C-RAN can't be achieved on Intel class servers, you need dedicated wireless chip sets such as TI's in which hardware accelerators perform the necessary signal processing functions much more efficiently.
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| TI's KeyStone SoCs for wireless basestations encompass three different configurations to scale from large macro basestations to smaller pico and femto cells. |
TI's KeyStone SoC architecture combines the company's fixed and floating-point C66x CorePacs for DSP functions along with ARM cores for control processing, hardware acceleration packs (the green blocks in the illustration above) and I/O. Flanagan says that the architecture enables TI to build three different types of devices that scale solutions from large metro base stations to smaller pico and femto cells. For larger cells, a Transport and Control Processor integrates multiple ARM cores, along with hardware accelerators for network security. One of these devices is typically combined with three Baseband SoCs, integrating DSP CorePacs for L1 and L2 processing, to form a 3-sector macro basestation. The third type of device in the KeyStone family integrates a smaller number of both control and DSP processors for single-sector small cells, i.e. a basestation on a chip.
Flanagan says that even with C-RAN, some processing is still needed at the antenna site in order to send the analog-to-digital converted signals over fibers, that may span a distance of several kilometers. The antenna signals could be formed first into packets or just digitized and transmitted directly to the C-RAN over fiber.The enhancements that TI has made for C-RAN include doubling of the queue manager and descriptor capacity. For software developers, these functions abstract the details of the hardware so an engineer does not need to know when they write their programs, letting them just describe the function they need to use. In C-RAN, Flanagan says, it will take a large pool of chips and the Keystone architecture allows construction of a virtual 800-core device. TI upgraded chip-to-chip communications from 1Gb to 10Gb Ethernet, as well as providing a proprietary 100Gb interface over dual links. With the KeyStone architecture, any chip in a cluster can directly write to the processing elements on another device.
According to Flanagan, the number of antennas that KeyStone can support for C-RAN is use-case dependent, varying with the number of users, geographical area, traffic movement for mobile users, and other factors. Basestations are typically constructed with one chip per sector, and the C-RAN configuration of ~800 cores uses 64 chips. Each basestation sector may have 2, 4, or 8 antennas depending on the level of Multiple-Input/Multiple-Output (MIMO) topology that is employed. With KeyStone the software scales across various sized architectures. In order to support 3G/4G migration and backward compatibility, Flanagan claims that TI has a unique capability of supporting simultaneous multimode baseband radio processing, for LTE, WCDMA or even 2G standards in the same chip.
The current markets for C-RAN are primarily China, Korea and Japan, where there are many high-density cities with high bandwidth fiber in place. Regardless of C-RAN usage in such locations, Flanagan says that all networks will be need to be heterogeneous architectures including both macro and small cell basestations. In 4G LTE networks, the need for lower latency of transmission has led to network architectures that push more processing out into the field and closer to the network edge. Since long fiber links may result in too much transport delay, C-RAN deployments may be limited to dense urban environments where distance to the server facility will be relatively short. You can download a copy of TI's whitepaper on "Creating cloud base stations with TI’s KeyStone multicore architecture" at http://www.ti.com/lit/wp/spry183/spry183.pdf
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