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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