Direct sequence spread spectrum system
Direct sequence spread spectrum system (Direct Sequece Spread Spectrum CommunicaTIon Systems, DS-SS), usually simply referred to as direct sequence system or direct spread system, is to multiply the information signal to be transmitted with the high-rate pseudo-random code waveform to go directly Control a certain parameter of the radio frequency signal to expand the bandwidth of the transmission signal. The pseudo-random sequence used for spectrum spreading is called a spreading code sequence. See Figure 1-5 for a simplified block diagram of a direct sequence spread spectrum communication system.
In direct sequence spread spectrum communication systems, phase shift keying (PSK) modulation is usually performed on the carrier. In order to save the transmission power and improve the working efficiency of the transmitter, the spread spectrum communication system often uses a balanced modulator. Balanced modulation that suppresses the carrier is also beneficial to improve the anti-detection ability of the spread spectrum signal.
At the transmitter, the data signal to be transmitted is multiplied by the waveform of the pseudo-random code (spreading code) (or modulo 2 added to the pseudo-random code sequence), and the resulting composite code modulates the carrier wave, which is then transmitted by the antenna. At the receiver, a local reference pseudo-random code synchronized with the pseudo-random code in the transmitter is generated to perform correlation processing on the received signal. This correlation process is often called despreading. The despread signal is sent to the demodulator for demodulation to recover the transmitted information. 
Figure 1-5 Simplified diagram of direct sequence spread spectrum communication system
(a) transmitting system; (b) receiving system
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