Spread Spectrum Communications
CDMA is a form of Direct Sequence Spread Spectrum communications. In general,
Spread Spectrum communications is distinguished by three key elements:
1. The signal occupies a bandwidth much greater than that which is necessary to send the
information. This results in many benefits, such as immunity to interference and jamming
and multi-user access, which we’ll discuss later on.
2. The bandwidth is spread by means of a code which is independent of the data. The
independence of the code distinguishes this from standard modulation schemes in which
the data modulation will always spread the spectrum somewhat.
3. The receiver synchronizes to the code to recover the data. The use of an independent
code and synchronous reception allows multiple users to access the same frequency band
at the same time.
In order to protect the signal, the code used is pseudo-random. It appears random, but
is actually deterministic, so that the receiver can reconstruct the code for synchronous
detection. This pseudo-random code is also called pseudo-noise (PN).