This paper will discuss the Frequency Locked Loop (FLL) implementation used for the Johns Hopkins University Applied Physics Laboratory Frontier Radio Lite (FR-Lite) family of radios. FR-Lite is designed as a low SWaP platform, and therefore has limited digital signal processing (DSP) resources available. The entirety of its command, telemetering, and DSP tasks are completed in the ProASIC 3e FPGA. This FLL implementation allows for much more rapid acquisition of uplink signals than the traditional deep space uplink sweeps, while not using any additional resources than those already used for demodulation of sub-carrier binary phase-shift key (BPSK) and direct-on-carrier (DoC) BPSK waveforms.The FLL acquisition process was designed to acquire a tone with ±70 kHz of the expected received frequency. Over this range, at -90 dBm receive power (30.6 dB C/N0) and above the FLL decreased acquisition time by a factor of 95 compared to a swept acquisition at the same power level. This improvement is instrumental for use of the FR-Lite for LEO communications, where long acquisition sweeps can take up a large percentage of the short contact times.The platform only has a limited number of DSP resources available in the FPGA. This meant for implementation, we had to make careful substitutions for the more standard methods. We cannot perform the usual operations (arctangent, division, multiplication, fast Fourier transform (FFT)) that would be used determine the frequency of incoming signals. Instead, we use a discriminator that generates a signal proportional to the frequency error, based on the graphical octant of the current sample and the first difference of the In-Phase and Quadrature (IQ) values.In order to transition from the wide acquisition bandwidth FLL to the narrow bandwidth PLL used for tracking and demodulation, intermediate FLL stages with narrower acquisition bandwidth and higher signal-to-noise ratio (SNR) in the loop are used. This way, even if the wide acquisition bandwidth FLL steady state frequency noise is larger than the pull in range of the PLL it can still be used for initial acquisition.This FLL acquisition method has been flight proven, and was used as the primary acquisition method for a LEO satellite.


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    Title :

    FLL Aided Acquisition in Resource Constrained Platforms


    Contributors:


    Publication date :

    2024-03-02


    Size :

    2391800 byte




    Type of media :

    Conference paper


    Type of material :

    Electronic Resource


    Language :

    English



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