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Case Study – Security & Defence – Radio upgrade program: Legacy proprietary radio PHY conversion to SDR

The Challenge

The customer had a proprietary high data rate mobile ad-hoc network (MANET) transceiver which they wanted to port to a software defined radio platform (SDR). The lower MAC and physical layers were implemented on a DSP, two FPGAs and an ASIC. There was no documentation available on the ASIC. The DSP needed to respond very quickly to signals from the FPGA. Platform specific functionality was strewn throughout the code. The software radio needed to perform identically to the original. We did not have access to the SDR ourselves for the entire programme.

The Approach

After studying the capabilities of the specific SDR chosen, it became clear that the DSP-FPGA interface was too slow, so we implemented a soft processor in the FPGA fabric to perform the functions of the original radio’s DSP. As there was some spare capacity in the original radios FPGAs, our first steps were to refactor the FPGA implementations to separate waveform specific functionality from platform specific functionality. We also reverse engineered the ASIC functionality and reimplemented this functionality in the existing FPGAs, and thus were able to test and prove the refactored functionality. Using an FPGA development kit we constructed a test harness, that modelled the environment of the FPGA in SDR and allowed us to stimulate it with commands issued from MATLAB. We took the waveform specific functionality and ported it over to the new FPGA, noting not only a change of hardware generation, but also a change of FPGA vendor and hence toolsets. We then implemented the very small amount of platform specific logic needed to interface to the SDR interfaces (e.g., rate matching and FIFOs for commands). As the SDR FPGA was large, we were able to perform back-to-back testing of the modems by instantiating two of them coupled transmit-to-receive and vice versa in the FPGA, which allowed a significant amount of modem testing to be performed without access to the SDR platform. Very little support was needed when integration with the real SDR platform occurred.

The Outcome

The new SDR implementation provided was proven to be completely compatible with the original radios over the air interface and has been trialled by an allied country. The new encapsulated functionality has proved straightforward to port to other DSPs and other FPGAs, and there are plans to port it to several other software radios.