Process gain
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In a spread spectrum system, the process gain (or 'processing gain') is the ratio of the spread bandwidth to the unspread bandwidth. It is usually expressed in decibels (dB).
The process gain is the ratio by which unwanted signals or interference can be suppressed relative to the desired signal when both share the same frequency channel. For example, if a 1 kHz signal is spread to 100 kHz, the process gain expressed as a numerical ratio would be 100,000/1,000 = 100. Or in decibels, 10log10(100) = 20 dB.
Note that process gain has no effect on wideband thermal noise. On the additive white Gaussian noise (AWGN) channel without interference, a spread system requires the same transmitter power as an unspread system, all other things being equal.
Unlike a conventional communication system, a DSSS system has a certain resistance against narrowband interference depending on the processing gain. However, with respect to the noise on the whole transmission channel, which means AWGN, the DSSS system shows exactly the same bit error behavior as a non-spread spectrum system with the same modulation format.
