IEEE - Aerospace and Electronic Systems - February 2020 - 38

Feature Article:

DOI. No. 10.1109/MAES.2019.2960952

A Doppler Correcting Software Deﬁned Radio
Receiver Design for Satellite Communications
Edwin G. W. Peters, Craig R. Benson, School of Electrical Engineering,
University of New South Wales Canberra, Canberra, BC, Australia

INTRODUCTION
Traditionally in satellite downlink radios, causal signal
processing techniques are applied. This means that the
received samples are processed in a flow as the analog to
digital converter provides them. The reason for this is that
historically, storing samples for later processing requires
memory, which is scarcely available on embedded chips,
field programmable grid arrays, etc. However, significant
benefits can be achieved by storing the samples and processing them noncausally in blocks. The main one being
that we can take advantage of the fast convolution algorithm, which utilizes fast Fourier transforms (FFTs) to do
the convolution. Furthermore, high performance FFT
algorithms exist that can take advantage of different architectures and parallel execution. This opens up the potential
to do significantly more signal processing in real time
compared to causal approaches. In addition to this, it is
worth recalling that by using acausal signal processing
methods, the information contained in future samples
can be utilized. In contrast, only the past samples can be
utilized with causal signal processing methods.
Communications to and from satellites in orbit around
earth, and other bodies, are normally affected by a frequency offset due to the Doppler effect [1]. While the frequency offset due to the Doppler effect can be predicted
using a priori information, such as an orbit propagation
using a two line element (TLE) set other frequency offsets
such as oscillator drift and offsets, atmospheric disturbances (mostly at higher frequencies) and inaccuracies in the

Authors' current address: E. G. W. Peters and
C. R. Benson, School of Electrical Engineering,
University of New South Wales Canberra, Canberra, BC
2610, Australia. E-mail: (edwin.peters@unsw.edu.au;
c.benson@unsw.edu.au).
Manuscript received May 19, 2019, revised November
15, 2019; accepted November 20, 2019, and ready for
publication December 17, 2019.
Recommended for acceptance by M. De Sanctis.
0885-8985/19/$26.00 ß 2019 IEEE
38

orbit prediction can affect the frequency offset that is
observed while communicating with satellites.
To successfully acquire a phase lock with a phase
locked loop (PLL), the loop bandwidth of the PLL has to be
chosen to be sufficiently wide to accommodate for the worst
case frequency offset [2]. In addition to this, the transmitted
packets do need a carrier to be present for a while prior to
the data, such that the PLL can achieve a phase lock. While
a PLL can be used in combination with precompensation
using a priori information, the loop bandwidth has to be
wide enough to be able to accommodate for unknown frequency offsets. There exist multiple demodulation techniques that are robust to large frequency shifts [3], [4], [5].
While the methods presented in [3] and [4] are limited to a
single modulation scheme (frequency shift keying (FSK)
and quadrature phase shift keying (QPSK), respectively),
the discrete Fourier transform method presented in [5] utilizes the carrier only to estimate the frequency offset.
There also exists physical/data link layer protocols that
are entirely robust against large frequency offsets [2], [6].
In this article, we propose a flexible software defined
radio (SDR) receiver design for the reception and demodulation of satellite signals. This SDR processes the data noncausally. The SDR utilizes matched filters for the
demodulation, and performs a full Doppler search and compensation in real time. The matched filter approach allows
the proposed SDR architecture to be adapted to suit any
modulation scheme. This allows the proposed SDR to be
used in conjunction with commercial of-the-shelf radios.
The Doppler search is, however, computationally demanding, since multiple convolutions have to be performed on
the same signal. In fact, a MATLAB implementation of the
exhaustive Doppler search took 5 h to process a 15 min satellite pass, where the data link was 9600 bit/s and over sampled a factor 16. However, by utilizing an entry level
Nvidia Quadro K620 GPU, this pass could be processed
in less than 5 min. For many applications, the proposed
SDR receiver can run on general purpose computers with
entry level and most likely also on embedded GPUs. The
Doppler search makes the radio suitable for satellite communications without any knowledge of the satellites range

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FEBRUARY 2020

IEEE - Aerospace and Electronic Systems - February 2020

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