Spread spectrum

Spread spectrum is an important form of encoding for wireless communications. This technique can be used for the transmission of either analog or digital information. The use of spread spectrum improves the quality of reception and makes jamming and interception more difficult. Here by an effective modulation of the signal, bandwidth of the signal which is to be transmitted is increased. This technique was initially developed and used by military and intelligent agencies. Frequency-hopping spread spectrum and direct sequence spread spectrum are now extensively used in wireless communication. In frequency-hopping spread spectrum, signal is broadcast over a random series of radio frequencies, hopping from frequency to frequency at fixed intervals. Each bit in the original signal is repeated by multiple bits in the transmitted signal in a direct sequence spread spectrum using a spreading code. In frequency hopping spread spectrum, spacing between carrier frequencies and hence the width of each channel usually corresponds to the bandwidth of the input signal. In this approach, both transmitter and receiver use the same code for tuning the sequence of channels and thus maintain synchronization. The different components of a frequency-hopping system are frequency shift keying (FSK) or binary phase shift keying (BPSK) modulator, pseudo noise bit sources, frequency synthesizers, bandpass filters and demodulators. The major components of a direct sequence spread spectrum system are modulators, demodulators and pseudo noise bit sources. Code division multiple access (CDMA) allows several users to use the same higher bandwidth with very little interference.