A Secure Spreader/ Despreader for Code

Division Multiple Access

Applications

Code Division Multiple Access (CDMA) is a wireless communication technique, which uses the principle of spread spectrum communication.

The intent of CDMA technology is to provide both extended bandwidth and secured communications.

CDMA uses these code sequences as means of distinguishing between different users.

In CDMA systems, a narrowband message signal is multiplied by a spreading signal, which is a pseudo-noise code sequence, having a rate faster than that of the message.

These sequences have certain useful properties like (i) sharp two-valued auto-correlation functions, (ii) balanced in number of ones and zeros and (iii) low cross-correlation values.

One type of PN sequences is the maximal-length sequences (m-sequences). M-sequences are, by definition, the largest codes that can be generated by a Linear Feedback Shift Register (LFSR) .

However the M-sequences are not immune to cross-correlation problems, and may have large cross-correlation values.

A decimal sequence is obtained when a number is represented in a decimal form in a base r and it may terminate, repeat or be a-periodic.

Decimal sequences provide good cross-correlation and enhanced auto-correlation properties, in addition to the high privacy, so they can replace pseudorandom sequences.

There are no restrictions on the number of generators to be used and this is considered as a high benefit that can be added to the D-sequence advantages.

Implementation: spreading and

despreading process in CDMA

Coding language: VHDL

Simulation tool: ModelSim

Hardware Implementation: Xilinx Spartan 3

Software Tool: Xilinx ISE