Aerospace and Electronic Systems Magazine June 2017 - 9
Baldi et al.
alternatives encompass: the use of a 3rd order PLL (also considered in existent receivers), whose performance in high dynamic
scenarios is known to be better but still faces noise constraints;
frequency-locked loop (FLL)-assisted PLL, in order to be able
to reduce the PLL bandwidth while being capable to keep track
of the Doppler dynamics by an FLL operating in parallel; or a
Kalman filter (KF)-based carrier tracking method, in which case
the bandwidth of the equivalent PLL is optimally adapted during
execution.
Subcarrier tracking is currently performed by a 2nd order
Costas loop, whose main limitation derives from a residual Doppler on the subcarrier which is initially unknown. This is due to
the fact that carrier and subcarrier frequencies are not coherently
generated on ground (even though future G/S equipment is expected to introduce carrier/subcarrier coherency). Its impact is
that the resting frequency at the Costas loop is slightly (a few
Hz) shifted from the true subcarrier frequency, and thus the loop
can face some troubles in locking. A possible enhancement is to
use an FFT, similarly as proposed for the carrier acquisition, to
estimate accurately the subcarrier frequency and then leave the
Costas loop track the phase of the signal. Yet, the receiver with
enhanced FFT-based Costas subcarrier tracking is not able to operate at the target Es/N0 (0-2 dB) for the symbol rate of 7.8125
sps. A solution to overcome and improve the limitations in DS
scenarios is to consider coherent carrier/subcarrier architecture.
In this case, noncoherent DTTL (NC-DTTL) schemes avoid implementing subcarrier acquisition and tracking and are able to
improve the performance with respect to current noncoherent
carrier/subcarrier generation schemes.
As for symbol tracking, it is currently implemented by a 2nd
order DTTL. After careful adjustment, this technique is able to
operate at the target SNR values, provided that carrier/subcarrier
tracking is successful.
To conclude, for NE scenarios the target operation point does
not compromise correct baseline receiver behavior. On the other
hand, DS scenarios with very low data-rates constitute the main
challenges since the effective SNR at the input of the receiver is
extremely low.
FRAME SYNCHRONIZATION
After time and frequency synchronization has been accomplished in the preceding phase through the alternating acquisition sequence, the frame synchronizer has to determine the
correct position of the start sequence which marks the start of
the CLTU. Since this is considerably more challenging at lower SNR, one of the objectives of the study was to evaluate the
frame synchronization error (FSE) probability and to eventually
propose an improved solution.
Noting by Ns the length of the start sequence (Ns = 16 in the
current standard [5]), for frame synchronization, the receiver considers the last Ns received symbols, denoted by r = r1 ,, rN s and
computes a metric Λ(r) measuring its similarity with the known
start sequence s = s1 ,, sN s . In contrast to the case of a periodi-
Table 4.
Comparison of Tracking Techniques
Technique
Pros
Cons
Carrier tracking
2nd order PLL
Legacy technique
Noise reduction versus dynamic
range trade-off not met
3rd order PLL
Enhanced performance in highdynamics
Might have stability issues
FLL-assisted PLL
Reduction of loop bandwidth
Poor performance of FLL at low
SNR
KF-based tracking
Optimal adjustment of loop
bandwidth
Increase in the number of design
parameters
2nd order Costas loop
Legacy technique
High sensitivity to resting frequency
errors
FFT-aided Costas loop
Improved resting frequency
estimation
Additional FFT processing after
carrier wipe off
2nd order DTTL
Legacy technique
Not a bottleneck, after careful tuning
of the parameters per SNR
NC-DTTL
Subcarrier tracking loop can be
neglected
Assumes carrier and subcarrier
frequencies coherently generated
Subcarrier tracking
Symbol tracking
JUNE 2017
IEEE A&E SYSTEMS MAGAZINE
9
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