Galileo Signal - A Software-Defined GPS and Galileo Receiver: A Single-Frequency Approach - page 41

superframe # k superframe # ( k + 1 )

frame # 1 ··· frame # ( i − 1 ) frame # i frame # ( i + 1 ) ··· frame # N frame # 1 ···

subframe # 1 ··· subframe # ( j − 1 ) subframe # j subframe # ( j + 1 ) ··· subframe # P

SW Data field CRC Tail bits

FEC encoded, then block-interleaved

FIGURE 3.8. Navigation message structure. CRC is computed over the data field.

3.3 Message Structure

This section outlines the Galileo message structure.

3.3.1 Frames and Pages
The message is composed of frames , see Figure 3.8. The frame is composed of
several subframes, and each subframe again is composed of several pages. The
page is the basic structure for the navigation message and contains the following
fields:
- a synchronization word (SW),
- a data field,
- a cyclic redundancy check (CRC) bits for error detection, and
- tail bits for the forward error correction (FEC) encoder containing all zeros.
CRC and data encoding are used to provide better signal and data integrity. For
L1 OS the synchronization word is a fixed 10-bit sequence.
All data are encoded using the following bit and byte ordering:
- for numbering, the most significant bit/byte is numbered as bit/byte 0,
- for bit/byte ordering, the most significant bit/byte is transmitted first.
In Figure 3.9 the most significant bit is placed left, the less significant bit is
placed to the right, the most significant items at the top, and the less significant
items at the bottom.

3.3.2 Cyclic Redundancy Check
The CRC algorithm accepts a binary data frame, corresponding to a polyno-
mial M , and appends a checksum of r bits, corresponding to a polynomial C .