Research on Mobile Underwater Acoustic Spread Spectrum Communication Technology Based on Double-Differential Coded

In a digital communication system, if the channel has a random carrier frequency shift or two nodes that are in motion with each other, the communication channel must have a frequency shift. Therefore, the received signal can only be non-coherent demodulation at the receiver. Double-Differential coding is a commonly used non-coherent demodulation method. The unknown frequency and phase changes can be eliminated through three consecutive signal samples. Since the Double-Differential coding take advantage of three consecutive signal samples, it is only suitable for slow fading transmission channels. During the observation time of the three signal samples, the channel characteristics change slowly and linearly. The combination of the Double-Differential coding and the Direct Sequence Spread Spectrum technology can effectively improve the ability of the Direct Sequence Spread Spectrum to resist Doppler frequency offset. However, due to the limitation of the DSSS system, the ability of the Double-Differential coding to cancel the Doppler frequency offset in the DSSS system is limited. Its ability to resist Doppler is related to the duration of spread spectrum chips.