Aerospace and Electronic Systems Magazine June 2017 - 6
State-of-the-Art Space Mission Telecommand Receivers
Table 1.
Adopted Channel Coding Schemes for TC Communications
LDPC(128, 64)
Parameter
BCH(63, 56)
LDPC(512, 256)
Code rate
0.89
0.5
56
64 or 256
64 (63 + 1 filler bit)
128 or 512
Hard-decision decoding
Soft-decision decoding
Message size (bits)
Codeword size (bits)
Decoding scheme
BCH(63, 56) code [14]. Note that for TC, reliability is driven
not only by the CER but also by the UCER, since executing
an erroneous command can have disastrous consequences for a
mission. Code parameters are contrasted in Table 1 for both coding schemes.
Given the implications of the data structure in the receiver
functionality, a general description of the current configuration of
the TC link transmission unit, i.e., the Communication Link Transmission Unit (CLTU), is shown in Figure 1.
Commands from Earth to space are sent in the payload of TC
transfer frames (TFs). A TF is divided into blocks, each as long as
56 bits and individually BCH encoded. Padding may be used in
the last block. A "0" bit (filler bit) is appended to each of the BCH
codewords which are then encapsulated between a 16-bit start
sequence and a 64-bit tail sequence (Recently the tail sequence
length has been extended to 128 bits.) to obtain the CLTU. The
role of the start sequence is to enable frame synchronization, so
that the decoding process can be initiated. On the contrary, the goal
of the tail sequence is to allow the decoder for implicitly detecting
the end of the CLTU. Preceding the start sequence there is an acquisition sequence of alternating "0" and "1" (starting with either
a "0" or a "1") of variable length that serves the purpose of signal
acquisition and aids in locking timing synchronization loop. Idle
sequences can also be found between CLTUs to help maintaining
symbol synchronization. The TC transmission protocol includes an
additional mechanism that guarantees a minimum number of symbol transitions to keep symbol synchronization. Namely, it applies
data randomization by XOR-ing TF data with a pseudorandom sequence. The randomizer, optional for the BCH code, is mandatory
for LDPC codes.
Figure 1.
CLTU data structure.
Figure 2.
High-level functional block diagram for the current CCSDS synchronization and channel coding sublayer data generation.
6
IEEE A&E SYSTEMS MAGAZINE
JUNE 2017
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