Aerospace and Electronic Systems Magazine June 2017 - 5
potentially in the order of several Mbps; even some short-term
missions are foreseen with demanding uplink rates that impose serious constraints on the O/B architectures.
These new requirements have a strong impact on the TC link
design. The need to increase further the coverage and/or the uplink
data-rate is at the basis of the next generation uplink (NGU) initiative [4], promoted by the Consultative Committee for Space Data
Systems (CCSDS). The NGU goal is to define improvements to
the existing TC recommendations [5], [6] in order to comply with
new missions' requirements.
OBJECTIVES OF THE STUDY
The challenges highlighted in the previous subsection have been
addressed by the Next Generation Uplink Coding Techniques
(NEXCODE) study, funded under the Technology Research Programme of the European Space Agency (ESA), which aimed at
research, design, development, and demonstration of a TC receiver
chain for scientific missions, including new channel codes. Among
the objectives of the study, we can mention:
C
C
Introduce, evaluate, and optimize advanced co/decoding
techniques to significantly improve the uplink performances
(in terms of data-rate and/or maximum distances) of NE and
DS science missions, compared to the currently used code.
Evaluate and clarify the impact of the new codes in terms of:
Required protocol modifications (if any);
O/B receiver algorithms (acquisition and tracking of uplink signals at lower SNR, determined by higher coding
gains);
O/B receiver architecture (to cope with extra complexity
and/or new algorithms).
C
C
Prototype the O/B receiver chain core elements, including
the decoder for the advanced coding schemes, by means of
commercial off-the-shelf hardware (HW), such as field programmable gate array (FPGA) platforms, to help validating
the approach and minimize the risk of adoption, bringing the
technology readiness level up to 3-4.
Evaluate the most relevant metrics, including the effective
coding gains, and the performance/complexity trade-off.
JUNE 2017
This article describes the main results of the study, by focusing on simulation activities and identification of the main
changes required to adopt the new coding options. The study is
focused on the short binary LDPC codes that have been recently
selected for updating the current TC synchronization and channel coding standard. The new and old codes are introduced in
the next section, accompanied by a brief description of the TC
data protocol. Then, the subsequent section presents a reference
receiver block diagram to contextualize the contribution of this
work and discusses its main functionalities and how the inclusion
of the new codes might impact its performance. A more detailed
analysis of the new codes is presented in a subsection specifically devoted to codes evaluation and optimization. The article
concludes providing a discussion on implementation aspects and
breadboard prototyping.
SPACE TC LINK STANDARD AND ITS EVOLUTION
The only error correcting code currently included in the CCSDS
recommendation [5] and the European Cooperation for Space
Standardization standard [6] for TC synchronization and channel coding is a Bose-Chaudhuri-Hocquenghem (BCH) code
with length n = 63 bits and dimension k = 56 bits. This BCH
code accommodated well the short command sequences, low
data-rate transmissions, and decoding simplicity requirements
that characterized the demands of TC control links in the past.
The increasing demands for transmitting higher data volumes
to the spacecraft, coupled with the need of guaranteeing short
command-based emergency communications at maximum power
efficiency has led to propose new codes for future standardization of TC protocols.
Agencies and researchers investigated and compared a number of possible alternatives, including binary [7] and nonbinary
LDPC codes [8], extended BCH codes [9], and parallel concatenated turbo codes [10], [11], even in the presence of jamming
[12], [13]. The schemes were evaluated taking into account the
peculiar characteristics of TC links in terms of coding gain and
complexity. At the end of the process, CCSDS chose two short
binary LDPC(n, k) codes, namely the LDPC(128, 64) and the
LDPC(512, 256) [14]. These codes have been selected in order
to meet such needs: with reference to the new standard quality service requirements of CER = 10−5 and UCER = 10−9, they
exhibit large extra coding gains (5-6 dB), with respect to the
IEEE A&E SYSTEMS MAGAZINE
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