Introduction

In Chapter 1, the block diagram of a basic wireless communication system was explained.

In this chapter, the principle behind the working of a spread spectrum modulator and demodulator will be explained. Spreading as the name suggests is a technique by which a narrowband signal is transformed to a noise-like signal that has a wider bandwidth. Pseudo-Noise (PN) sequence is used for spreading or de-spreading the information signal. The transmission bandwidth allocated for any system is limited. Thus, using a spread signal for transmission of single user information can seem to be an inefficient technique. However, when a multi-user environment is considered, then the use of a spreading technique is beneficial in multiple ways. First, it permits multiple users information to spread and occupy the same bandwidth with least interference between each other. Second, on spreading, the signal becomes noise-like and can be de-spread only by the correct PN sequence. If de-spreading is attempted with any other PN sequence, the signal remains noise-like. Thus, use of the spreading technique ensures secured data transmission between the sender and the desired receiver. Third, it helps to prevent multi-path fading. Fourth, in multi-user systems, as all spread signals use the same bandwidth, there is no requirement of any frequency planning. Last but not the least, spreading a signal makes it immune to jamming.

Radio jamming is a collective term used for methods used to deliberately block or degrade the working of any wireless system including mobile telephony, satellite communication, CCTV systems and the like.