Non-Linear Signal Processing Solves Tough RF Interference

When you need a potent way to eliminate interference with inexpensive processing, non-linear signal processing delivers advantages that linear signal processing cannot match.
 
The picture above shows interference essentially vanished using non-linear DSP for a scenario that linear processing -- MMSE & orthogonal projection as examples -- could not solve.  The results can be so profound that usually they are met with total disbelief at first.  In 2014, I designed and tested my first non-linear filter in a hardware system.  It worked so well that my colleagues initially suspected that the interference signal generator must have been broken.  It was actually working perfectly but had been eliminated.  Non-linear can be nearly miraculous in some cases.  

While non-linear signal processing does not solve every problem, when it is the answer, the implementation is low-cost and practical.  As a concrete example, US patent 9,197,360 shows one of my first non-linear filters.  Designed for an FPGA, it consists of two CORDIC's and a DC notch filter -- high-performance at low-cost!  

The plots below show that such a simple filter can do great work!  On the left, the received signal consists of a weak signal, a strong interference signal, and AWGN.  On the right, the non-linear filter has extracted the weak signal from beneath the interference signal.  That is an impossible feat using linear time-invariant DSP.  This highlights the advantages that non-linear offers.
That was only the beginning.  I have continued working in this area, published two IEEE papers, and have additional patents pending.  RADWAV can help with algorithm development, testing, and analytical derivations for digital communications, radar, and other applications. 

As a final note, I chose to title some of my publications "polar excision" for historical reasons although it has nothing to do with polarization filtering or excision.  That is usually a linear adaptive filter -- totally different thing.  Instead, my use of polar refers to the IQ complex-plane representation.

References
  • P. Wyckoff, “Polar Excision for Radio Frequency Interference Mitigation”, Invited Talk to IEEE Waves & Devices Phoenix Chapter, Tempe, AZ, 2017.
  • P. Wyckoff and G. Hellbourg, “Polar Excision for Radio Frequency Interference Mitigation in Radio Astronomy,” in IEEE Proceedings of RFI 2016, Socorro, NM, 2016, pp. 132-135.
  • P. Wyckoff, “Systems and Methods for Reducing a Relatively High Power, Approximately Constant Envelope Interference Signal that Spectrally Overlaps a Relatively Low Power Desired Signal,” U.S. Patent 9,197,360; November 24, 2015.  (Original DSP Version)

Antenna Array Signal Processing & Non-Linear Processing

RADWAV wrote custom tools to analyze data from antenna arrays and to simulate new antenna arrays.  To solve an on-orbit anomaly, I processed 1 Terra-Byte of time-series IQ-samples using these tools.  Other times, I have simulated new antenna arrays and processing for adaptive beamforming, STAP, and non-linear array processing.  As well, I designed a method that allows antenna arrays, which were built to send a single spot beam, to become capable of simultaneously sending many spot beams with independent data streams in each one. 

References
  • P. Wyckoff and J. Luna, “Multi-Beam Antenna System with a Baseband Digital Signal Processor”, U.S. Patent 11,469,805; October 11, 2022.
  • P. Wyckoff, “Non-Linear Interference Mitigation Using Arrays,” in IEEE / USNC-URSI National Radio Science Meeting, Boulder, CO, 2016, pp. 1-2.

HF SDR SkyPi History

RADWAV SkyPi's sold out in Jan 2016.  We actively supported the kit until Jan 2017, to give everybody a full year to get their kit on the air.  Special thanks to the American Radio Relay League for reviewing the kit in the July 2015 QST magazine, the Radio Society of Great Britain for publicizing SkyPi, and AmateurLogic TV for reviewing the kit in Episode 77.  The final record belongs to W9REB who closed a 7 MHz link that reached 18,105 km -- well done.

W9REB Reaches 18,105km with the SkyPi-40 Kit

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​Sep 1, 2015 - W9REB holds the SkyPi-40 WSPR distance record at 18,105km.  Using a wire antenna, W9REB's kit reached from Augusta, Georgia to Perth, Australia!  More...

About RADWAV

RADWAV is a small firm with unique specialization.  There are few firms that put hardware and math to work.  Yet rapid technological progress demands both.

Hardware optimization benefits from math expertise.  Math can show all possible designs, or at least constrain them, to focus on the best path forward while avoiding dead ends.

Math benefits from hardware experience.  Algorithms designed for implementation propel technological leaps.

RADWAV focuses on rapid innovation, modeling and simulation.  Send us your challenge the next time you need to push beyond traditional performance boundaries, fit within difficult constraints, or solve unexplained performance anomalies in end-to-end systems.

Founder's Message

RADWAV is a really small company!  It is basically just me helping satellite communications companies and RF/DSP clients develop new technology concepts and troubleshoot system anomalies.

Over more than 20 years, I have gained experience in state-of-the-art modems and antenna arrays in particular. Most often, I improve RF/DSP system performance by applying mathematics, modeling, and simulation. Other times, I create proof-of-concept code or prototypes.

I have been awarded four U.S. patents, which are about co-channel interference mitigation, antenna array signal processing, and digital communications. My other DSP specialties include spread spectrum, anti-jam, geolocation, and non-linear filtering.

​-Pete Wyckoff