Challenges in small cell LTE software


Massive predicted growth in mobile traffic over the next few years will put tremendous pressure on the entire network from terminals to the core, and in particular for small cell product developers. Small cells are expected to bear much of the coverage and capacity burden in the Radio Access Network by 2017.

Small cells must contain the functionality of macro cell base stations. However, similar to terminals, a lower price point than for macro cells is needed to enable deployment of large volumes. This requires a higher level of silicon and software integration, lower power consumption and a high level of interoperability testing.

New features in LTE Release 10 enable deployment of large numbers of small cells, but this means that products supporting Release 10 require around 10 times more GOPS than for Release 9. This requires System-on-Chips (SoCs) with new architectures to deliver this processing power, such as TI’s TCI6636K2H.

On the software side, there are challenges in the development and deployment of the Physical Layer (PHY) and the Protocol Stack. Operators are demanding carrier aggregation and enhanced uplink/downlink multiple antenna transmission for LTE, both of which significantly increase the performance required from the PHY software.

Architectures with a higher number of available DSP and RISC processors also raise challenges around partitioning the software to achieve maximum performance. In addition, other elements of the SoC are stressed in new ways by the demands of Release 10.

The move to new SoCs

As an example of the challenges, let’s look at how CommAgility developed our PHY software for Release 10 on TI’s new SoCs, thus ensuring we can provide this to customers as an integrated solution with our processor boards and modules.

We undertook an iterative process, with our existing software as a foundation. The first step was to revalidate the SoC/PHY software, running it through a series of regression tests to ensure the robustness of the silicon and check if any new bugs were exposed on the new SoC.  

Secondly, we profiled elements of the new architecture where performance improvements were expected, such as an additional queue manager. Next, the performance of the PHY software was maximised, including making use of more DSP cores to tackle new features such as carrier aggregation and UL/DL enhancements.

Finally, over a thousand test vectors were re-run to ensure the integrity of the SoC and PHY. At this point, all the critical sub-systems, inter-core communication, shared memory, DDR interfaces, the Multicore Navigator and more were stressed to identify problem areas. The protocol stack was added, split across one of the C66x digital signal processor (DSP) cores and an ARM® Cortex-A15. Shared memory and communication between the PHY and Stack was revalidated. A final step is to move the protocol stack completely into the ARM Cortex-A15, freeing all DSP cores for Release 10 functionality. Another suite of validation tests is run for this final configuration.

The result of this work was that the CommAgility PHY is now available for TI’s TCI6636K2H, TCI6638K2K and TCI6630K2L devices.

Summary

Even though we have extensive experience implementing Physical Layer software for LTE Small Cells and terminals, each new Software Defined Radio (SDR) platform supported brings new lessons and knowledge.

The keys for successful evolution are

  1. Secure a solid, known foundation
  2. Iteratively evolve the complexity

These are not new principles, but the depth and breadth of complexity introduced with each new generation of LTE standards quickly exposes software developers who do not follow these fundamentals.