CDMA Theoritical view .....(part 1)

This provides an introduction to Code Division Multiple Access (CDMA) communications, covering a Radio Carrier Station (RCS) and a Fixed Subscriber Unit (FSU).

This introduction to CDMA proceeds heuristically, we use very little mathematics in developing the theories, and do not assume a deep mathematical or engineering background. If you would like further information on the math and communication theories behind this introduction, please consult the following references:

Viterbi, A. CDMA: Principles of Spread Spectrum Communication Addison-Wesley Wireless Communications Series, 1995

Pickholtz, R. L., Schilling, D. L., and Milstein, L. B. “Theory of Spread-Spectrum Communications—A Tutorial” IEEE Trans. Commun., vol. COM30, no. 5, May 1982, pp 855-884.

Pickholtz, R. L., Schilling, D. L., and Milstein, L. B. Revisions to “Theory of Spread-Spectrum Communications—A Tutorial” IEEE Trans. Commun., vol. COM32, no. 2, Feb 1984, pp 211-212.

Introduction to Spread Spectrum Communications

CDMA is a form of Direct Sequence Spread Spectrum communications. In general, Spread Spectrum communications is distinguished by three key elements:

• 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.

• 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.

• 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).

Figure 1. Direct Sequence Spread Spectrum System