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Technical Perspective: Sora Promises Lasting Impact


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The term software defined radio (SDR) first appeared in 1992 and referred to a radio transceiver where the basic signal processing components (for example, filtering, frame detection, synchronization, and demodulation) are all done in a general-purpose processor. The goal of an SDR was to enable a single radio to support multiple wireless technologies (for example, AM, VHF, FM) and be easily upgradable with a software patch.

While the concept of SDR has been around for decades, only recently have SDRs become common in academic wireless research. However, research projects typically employ SDR as a development platform, that is, they use software radios to develop new physical layer designs with the understanding that if these designs make it to a product they will be built in ASICs. The reason why SDR has become a development platform rather than a fully functional software radio is that building high-performance SDRs has turned out to be very challenging.

Sora has revived the original SDR vision. The objective of Sora is to build an SDR that combines the performance and fidelity of hardware platforms with the programmability and flexibility of general-purpose processors. To do so, Sora must overcome the following challenge: How can a radio deliver high throughput and support real-time protocols when all signal processing is done in software on a PC?


There are many reasons why the following paper about sora stands out as one of the most significant wireless papers in the past few years.


Sora's approach uses various features common in today's multicore architectures. For example, transferring the digital waveform samples from the radio board to the PC requires very high bus throughput. While alternative SDR technologies employ USB 2.0 or Gigabit Ethernet, Sora opts for PCI-Express. This design decision enables Sora to achieve significantly higher transfer rates, which are important for high bandwidth multi-antenna designs. The choice of PCI-express also enables Sora to reduce the transfer latency to sub-microseconds, which is necessary for wireless protocols with timing constraints (for example, MAC protocols). Further, to accelerate wireless processing, Sora replaces computation with memory lookups, exploits single instruction multiple data (SIMD), and dedicates certain cores exclusively to real-time signal processing.

There are many reasons why the following paper about Sora stands out as one of the most significant wireless papers in the past few years. First, it presents the first SDR platform that fully implements IEEE 802.11b/g on standard PCs. Second, the design choices it makes (for example, the use of PCIe, SIMD, trading computation for memory lookups, and core dedication) are highly important if software radios are ever to meet their original goal of one-radio-for-all-wireless-technologies. Third, the paper is a beautiful and impressive piece of engineering that spans signal processing, hardware design, multicore programming, kernel optimization, and so on. For all these reasons, this paper will have a lasting impact on wireless research.

The Sora platform has been used in multiple research projects and real-time demos. It has enabled demanding designs, such as LTE and AP virtualization, to be built fully in software. However, currently most SDR-based research uses the GNU Radio/USRP platform. Despite the limitations of this platform, previous attempts at replacing it with more capable platforms did not experience significant success. In fact, history shows that wide adoption is not necessarily correlated with the more capable design. One of the classic papers we teach our undergraduate students is "The Rise of Worse is Better" by Richard Gabriel that explains why the Lisp language lost to C and Unix. Gabriel argues that for wide adoption, a system must be good enough and as simple as possible. Such a design (termed worse is better) tends to appear first because the implementer did not spend an excessive amount of time over-optimizing. Therefore, if good enough, it will be adopted by developers because of its simplicity. Once adopted, the system will gradually improve until it is almost the right design. One may argue that the history of the GNU Radio/USRP SDR is fairly similar; the platform originally provided just enough for people to start experimenting with the wireless physical layer. As a result, it was simple and cheap, which caused it to spread. Once it was accepted, it kept improving just enough to enable the next step in research.

The Sora team has recently started a program that awards Sora kits to academic institutions to enable them to experiment with this new platform. It will be interesting to see whether Sora with its higher performance can eventually replace the GNU Radio/USRP platform. If this happens, it will be a major success for Sora.

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Author

Dina Katabi (dina@csail.mit.edu) is an associate professor in the Electrical Engineering and Computer Science Department at Massachusetts Institute of Technology, Cambridge, MA.

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Footnotes

DOI: http://doi.acm.org/10.1145/1866739.1866759


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