Bit Mapping Into QPSK Constellation
SLS-RFM_03-04
Enrico Vassallo
CCSDS SLS-RFM WG meeting, College Park (USA), 24-28 October, 2003
1. Introduction
The channel input and coding conventions for QPSK systems are defined in recommendation
CCSDS 401 (2.4.10)B-1 from September 1989.
The bit numbering convention is instead defined inter alia in the CCSDS Telemetry Channel
Coding Standard CCSDS 101 B-6.
During recent interagency tests whereby the ASTRO-F satellite transponder by ISAS was subject
to RF compatibility test with an ESA Earth station demodulator, a different mapping of input bits
between the two Agencies was discovered for the QPSK modulation.
Aim of the present document is to include in recommendation 2.4.10 the necessary information so
as to avoid different bit mapping implementations in the future.
2. Bit mapping discussion
In most CCSDS documents but not in CCSDS 401, the following convention is used to identify
each bit in an N-bit field. The first bit in the field to be transmitted (i.e., the most left justified
when drawing a figure) is defined to be ‘Bit 0’, the following bit is defined to be ‘Bit 1’, and so on
up to ‘Bit N-1’. When the field is used to express a binary value (such as a counter), the Most
Significant Bit (MSB) shall be the first transmitted bit of the field, i.e., ‘Bit 0’ (see figure - 1).
Bit 0
Bit N-1
N-Bit Data Field
First Bit Transmitted = MSB
Figure - 1: Bit Numbering Convention
CCSDS 401 (2.4.10)B-1 recommends the following:
(1)
that the serial input digital data stream to QPSK systems be divided so that odd bits are modulated on the Ichannel and even bits are modulated on the Q-channel.
(2)
that carrier phase states have the following meanings:
-
0 degrees represents a "00" bit pair,
90 degrees represents a "01" bit pair,
180 degrees represents a "11" bit pair,
270 degrees represents a "10" bit pair.
seems to indicate that the first bit is 'Bit 1' since conventionally the first bit to be transmitted by a
QPSK modulator is assigned to the I-channel.
To remove the incompatibility with other CCSDS books it would be sufficient to invert odd and even
in recommends (1), include some text from the bit numbering convention and additionally to state
that for each bit pair, the Most Significant Bit is assigned to the I-channel and the Least Significant
Bit is assigned to the Q-channel.
A picture could also be added as follows:
Q
00
01
I=MSB Q=LSB
I
11
10
2. Proposed revised CCSDS 401 (2.4.10)B-1
A proposal for a revised recommendation CCSDS 401 (2.4.10) is attached for discussion at the
meeting.
2.4.10
CHANNEL INPUT AND CODING CONVENTIONS FOR QPSK SYSTEMS
The CCSDS,
considering
(a)
that a clear relation between digital information and the resulting RF carrier phase is necessary to reconstruct the
digital data stream following reception and demodulation;
(b)
that the digital data format will conform to the CCSDS Recommendation for Packet Telemetry;
(c)
that some communications systems with high data rate transmission requirements use QPSK modulation;
(d)
that the phase states representing each of the possible bit pairs values should be judiciously chosen so that a
phase error of 90 degrees can cause an error in no more than one bit;
(e)
that it should be possible to have two logically independent channels;
(f)
that in the case of a single data stream the odd and even bits should be forwarded to two independent channels;
recommends
(1)
that the serial input digital data stream to QPSK systems be divided so that even bits (i.e. bits 2i where
i=0,1,2,..(N/2)-1) are modulated on the I-channel and odd bits (i.e. bits 2i+1)are modulated on the Q-channel
(see also the bit numbering convention in figure 1).
(2)
that carrier phase states have the following meanings as given in figure 2:
-
0 degrees represents a "00" (IQ) bit pair,
90 degrees represents a "01" (IQ) bit pair,
180 degrees represents a "11" (IQ) bit pair,
270 degrees represents a "10" (IQ) bit pair.
The following convention is used to identify each bit in an N-bit field. The first bit in the field to be transmitted (i.e.,
the most left justified when drawing a figure) is defined to be ‘Bit 0’, the following bit is defined to be ‘Bit 1’, and so
on up to ‘Bit N-1’. When the field is used to express a binary value (such as a counter), the Most Significant Bit
(MSB) shall be the first transmitted bit of the field, i.e., ‘Bit 0’ (see figure - 1).
Bit 0
I
Bit N-1
N-Bit Data Field
First Bit Transmitted = MSB
Figure - 1: Bit Numbering Convention
CCSDS 401 (2.4.10) B-1
Page 2.4.10-1
October 2003
Q
00
01
I=MSB Q=LSB
I
10
11
Figure - 2: Constellation
CCSDS 401 (2.4.10) B-1
mapping
Page 2.4.10-1
October 2003
CCSDS 401 (2.4.10) B-1
Page 2.4.10-1
October 2003
CCSDS 401 (2.4.10) B-1
Page 2.4.10-1
September 1989