The ratio of the chipping code to data bits for a direct sequence spread spectrum (DSSS) signal is called which of the following?

• A. Processing gain.
• B. Coding gain.
• C. Modulation index.
• D. Bandwidth efficiency.

Answer: (A)

In DSSS, data bits are spread into longer chip sequences using a pseudorandom code, and the ratio of the chip rate to the data rate—the number of chips used per data bit—is called the processing gain. This value, also equal to the code length per bit, quantifies how much the signal is spread and how much potential SNR improvement you get when the receiver despreads the signal. For instance, 64 chips per data bit gives a processing gain of 64, which translates to a substantial gain in robustness against interference and noise. This concept is distinct from coding gain (improvement from error-correcting codes), modulation index (the amount of frequency or phase deviation in modulation), and bandwidth efficiency (bits per second per Hz).