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Creating ARM-based Cloud RAN wireless base station

Posted: 27 Feb 2014 ?? ?Print Version ?Bookmark and Share

Keywords:Cellular service? Cloud radio access network? Cloud RAN? base band? MPU?

In this example, the GP CPU is running the control channels (PUCCH, PDCCH, PCFICH, and PHICH), encoding and decoding tasks. First, the channels require low CPU resources, and second, these channels are particular to LTE and they cover multiple formats and options that make them more suitable for software implementation. The decision to run CE on the GP CPU is a less obvious one. On the one hand, CE tasks demand significant CPU effort and high-data transfers between the cores. On the other hand, the CE task is a non-standard algorithm that highly impacts receiver performance, so locating it in a programmable domain provides maximum flexibility to the developer for implementing its secret algorithm and full control for this fundamental block.

Tasks assignment to PEs
Figures 6 and 7 show the relationship between tasks and PEs. As can be observed from the figures, one PE may execute more than one LTE task. For example, the arithmetic PE runs in the DL, as well as the layer mapping and pre-coding tasks. The UL runs the equaliser and the PRACH down conversion task, and optionally, it can be used to offload part of the CE task from the CPU by utilising its capabilities to inverse matrices. This is achievable by re-using the PE in time if possible, and/or duplicating the PE as needed.

CPU/MPU interface
Throughput analysis of the MPU/CPU interface per sector is provided in table 2. It is assumed that the above task partition is applied and the sector is configured in accordance with the parameters provided in table 1.

Table 2: Required throughput of CPU/MPU interface.

Core partition and system capabilities
Our assessment shows that, in addition to an MPU, up to two Cortex A-57 cores are required to run the physical layer of up to 8 LTE sectors, configured according to table 1. Resources of less than one core are required to control the MPU chip and to process the control channels, while the rest are required to execute the CE. To guarantee high computational resources and precise timing control, these cores should be dedicated exclusively to physical layer processing, as suggested in a recent study [2].

Once we have allocated cores for processing the physical layer tasks, we must also assess how many cores per sector are required for running the upper LTE layers to realise the full system capabilities. Based on the specs of several existing base station solutions that use Cortex A-15 cores, we expect a performance improvement of up to 50% by using Cortex-A57 processors cores. We assume that up to two Cortex A-57 cores are needed to run the L2/3 layers of one sector. We conclude that this system is capable of running up to 8 sectors.

1. China Mobile Research Institute. (2010). C-RAN: The road towards green RAN, White Paper, Version 2.5 (Oct, 2011)
2. Tan, K., Zhang, J., Fang, J., Liu, H., Ye, Y., Shen, Y. Z., Voelker, G. (2009). Sora: High performance software radio using general purpose multi-core processors.
3. Jianwen Chen, Xiang Chen, Jing Liu & Ming Zhao. Open Wireless System Cloud: An Architecture for Future Wireless Communications System
4. Jianwen Chen, Qing Wang, Zhenbo Zhu, Yonghua Lin. An Efficient Software Radio Framework for WiMAX Physical Layer on Cell Multicore Platform
5. Tal Kaitz, Gaby Guri. CPU-MPU partitioning for C-RAN applications

About the author
Barak Ullman is Chief Architect at ASOCS Ltd. He has over sixteen years of experience in research and development of wireless technologies. Prior to joining ASOCS, Mr. Ullman led the design of system architecture for LTE receiver at Marvell. Prior to that, he was responsible for development of GSM/GPRS physical layer for a UE at Intel. Mr. Ullman holds a B.Sc. in electrical engineering from the Ben-Gurion University.

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