Aerospace and Electronic Systems Magazine June 2017 - 7

Baldi et al.
Table 2.

before 2010) where the physical channel is activated for the whole
communication session until the last CLTU is received. PLOP2 invokes the carrier modulation modes (CMM) described in
Table 2. Starting at CMM1 the sequence is depicted in Figure 3
(the insertion of the idle sequence is optional-and therefore it is
plotted in dashed line).

Carrier Modulation Modes
Carrier
Modulation
Mode

Description

CMM1

Unmodulated carrier only (no data
modulation, only RF carrier)

CMM2

Carrier modulated with acquisition
sequence

CMM3

Carrier modulated with CLTU

CMM4

Carrier modulated with idle sequence

Figure 2 depicts the transmitter functional block diagram for
CLTU generation (with BCH encoding). TF segmentation includes
stuffing of the Mth block, if needed. Note that although the standard allows for more than one TF per CLTU, known missions to
date transmit a single TF per CLTU [4]. This has been the assumption within NEXCODE.
Figures 1 and 2 can be easily adjusted for including the new
codes by replacing the BCH code with the LDPC codes and modifying accordingly the size of the message block and the resulting codeword. The filler bit is no longer necessary. Moreover, as
we will show in the next section, the length of the start sequence
should be increased, to compensate the impact of the potential operation at a reduced SNR.
Following the CLTU generation, the symbols are modulated
onto radio waveforms according to the physical layer operation
procedures (PLOP). Among them, the most relevant procedure is
PLOP-2 (PLOP-1 remains as support for legacy missions launched

Figure 3.

PLOP-2 CMM sequence with recommended insertion of idle sequence
between CLTUs (see [6, Figure 9.1] for a more detailed description).

NEXT GENERATION RECEIVERS FOR SPACE TC LINKS
In the following, we will show that the new LDPC codes allow
achieving significant coding gains with respect to the current
BCH code. A direct consequence of the performance gains introduced by the new codes is the impact on the receiver operating
point. Namely, to fully exploit the potential of the new codes,
receiver stages previous to decoding are required to operate at
a lower SNR. This new requirement might be problematic since
those receiver stages were designed for much higher SNRs. We
focus on receiver baseband functional blocks, typically implemented in the digital domain of the O/B transponder. Standard
receiver functionalities can be classified in the following highlevel stages:
C

signal detection and acquisition,

signal tracking and demodulation,

frame synchronization, and

channel decoding.

Figure 4 provides a block diagram of a generic TC receiver,
taking inputs from the analog-to-digital converter interfacing
the baseband receiver with the radio frequency (RF) front-end
operating at some intermediate frequency (IF). At a glance,
the operation of a receiver involves: i) deciding whether a TC
signal is present at the receiver (signal detection); ii) computing rough estimates of the signal parameters, such as Doppler
frequency, to lock the signal (signal acquisition); iii) tracking
the signal and demodulating the symbols (signal tracking and
demodulation); iv) detecting, from the stream of noisy symbols, the CLTU start (frame synchronization); v) decoding the
transmitted TF message (decoding); vi) detecting the CLTU
end (termination).
In the following subsections, the impact of the new codes on
the various blocks is discussed, as well as their typical implementations with some enhancements proposed when possible.

Figure 4.

High-level baseband receiver block diagram.

JUNE 2017

IEEE A&E SYSTEMS MAGAZINE

Table of Contents for the Digital Edition of Aerospace and Electronic Systems Magazine June 2017