TABLE OF CONTENTS In the previous chapter we learnt that essential to the spread-spectrum system is the spreading of the data energy at the transmitter, and the collapsing of the spreading (in a process known as de-spreading) at the receiver. The operations at the transmitter and the receiver generate the required processing gain without which the system could not combat jamming and interference. The spreading code sequences, used in the system, have special properties which will be introduced in this chapter. These code sequences have an important role to play in the spread-spectrum technique and have to be chosen carefully for efficient communication systems.
Each code sequence used in the spread-spectrum communications must easily be distinguishable from a time shifted version of itself, in order to enable the code acquisition and tracking and, therefore, the synchronization of the system. Furthermore, when multiple users are accessing the system for services, each code sequence assigned to a user must be distinguishable from every other user code sequence in the set and ideally should generate little or no interference to other users sharing the channel.
The code sequences are pseudo-random sequences but in practice they are generally periodic. Two types of these sequences can be used in spread-spectrum applications: the binary code sequence and the non-binary (also known as complex) code sequence. The elements of the binary sequence are made up of real numbers 1. These are most commonly applied in spread-spectrum communications, for ease of generation using widely available binary logic circuits. However,...
