Pseudo-Noise Spreading
The FEC coded Information data modulates the pseudo-random code, as shown in Figure
2a. Some terminology related to the pseudo-random code:
• Chipping Frequency (fc): the bit rate of the PN code.
• Information rate (fi): the bit rate of the digital data.
• Chip: One bit of the PN code.
• Epoch: The length of time before the code starts repeating itself (the period of the
code). The epoch must be longer than the round trip propagation delay (The epoch
is on the order of several seconds).
Figure 2b shows the process of frequency spreading. In general, the bandwidth of a
digital signal is twice its bit rate. The bandwidths of the information data (fi) and the PN
code are shown together. The bandwidth of the combination of the two, for fc>fi, can be
approximated by the bandwidth of the PN code.