Spread spectrum and frequency hopping

Spread spectrum

Expand the bandwidth of baseband signal , The bandwidth used to transmit information is much larger than the bandwidth of the information itself ; Spread spectrum passes through an independent code sequence independent of the transmitted data ¹ By means of coding and modulation , The receiver needs the same code to despread .

• Implementation method

  1. Direct sequence spread spectrum ： take Pseudo noise sequence (PN Sequence ) Directly multiply with baseband pulse data to extend baseband data , The pseudo noise sequence is generated by the pseudo noise generator
  2. Frequency hopping spread spectrum ： A digital signal is multiplied by a carrier that constantly changes frequency , So as to realize spread spectrum
  3. Time hopping spread spectrum ： Divide the data duration into several time slots , The communication is controlled by the spreading code sequence of the spreading code generator - Disconnect switch , The input data is stored at the transmitter first , Therefore, data is transmitted in burst mode with high peak power in a short time slot . Usually , Because the simple time hopping anti-interference is not strong , Rarely used alone , It is usually combined with other methods to form a hybrid method , Such as “ Time frequency hopping ”.

• advantage ： Reduce the signal power per unit bandwidth （ It refers to reducing the signal power spectral density , Does not reduce power consumption / energy ）. According to Shannon's formula , At the same signal rate , Increasing the signal bandwidth can Reduce the receiver's requirements for signal-to-noise ratio , The signal-to-noise ratio is proportional to the transmission power of the signal , After reducing the signal-to-noise ratio , The transmission power of the signal can even be lower than the noise , Or drown in the noise , Enhance concealment and anti-interference .

Despreading ：
Despreading requires knowing the spreading code ; From the point of view of energy , The energy of the spread spectrum signal is dispersed , After despreading, the energy of the signal is concentrated again .

frequency hopping

FHSS, Frequency hopping spread spectrum ,Frequency-Hopping Spread Spectrum, Anti interference technology , It refers to frequency shift keying with pseudo-random code sequence , A method of spreading the spectrum by constantly hopping the carrier frequency .

Frequency hopping is the spread spectrum in the sense that the carrier frequency is constantly hopping within a certain range , Instead of spreading the transmitted information , The processing gain of direct sequence spread spectrum will not be obtained . Frequency hopping is equivalent to instantaneous narrowband communication system , Basically equivalent to a conventional communication system .

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1. Generally select pseudo-random code (PN code ), As is common m Sequence 、M Sequence ︎