Quadrature phase shift keying Sine chirp spread Spectrum under-water acoustic communication based on VTRM

The signal in the underwater-acoustic communication channel is affected by the large multipath delay and severe Doppler spread. In addition, the hydro-acoustic channel is also bandwidth limited. The linear Chirp signal due to its resistance against multipath and Doppler spread has grasped the attention of many researchers as a carrier and preamble for the UWA channel, but one of the main disadvantages of the conventional chirp signal is that it requires large bandwidth for optimal performance, especially in M-ary communication. On the other hand, non-linear sine chirp has been proven spectral efficiency and provides resistance against multipath and Doppler spread. Therefore, to achieve better spectral efficiency with an enhanced data rate, this paper proposes a quadrature phase shift keying sine chirp spread spectrum. Firstly, the set of four orthogonal sub-carriers of sine chirp is determined. The sine chirp subcarriers are obtained by adding a phase shift of 90 degrees. Secondly, the receiver is designed using coherent demodulation to perform communication. Moreover, to improve the bit error rate performance of the system which is affected by the multipath arrivals VTRM is used. The proposed concept is verified through simulation experiments using BELLHOP underwater acoustic channel and a recorded channel that is a watermark replay channel. The results dictate that the proposed scheme offers superior spectral efficiency compared to the conventional linear chirp spectrum multi-band communication.