Doppler shift determination methods dedicated to MBFSK modulation

The progressive development of AUVs (Autonomous Underwater Vehicles) is creating demand for wireless underwater communications. The most commonly used transmission medium in water is acoustic wave. Unfortunately, the underwater hydroacoustic channel is characterized by strong multipath, especially in shallow reservoirs and in combination with hydrotechnical infrastructure. In addition, there is a Doppler effect in the case of reciprocal movement of the transmitting and receiving parts. This effect in a hydroacoustic channel causes a significant frequency shift due to the relatively low propagation speed of the acoustic wave. Knowledge of the frequency of the received signal is crucial in the reception process. The authors of the current publication have found through a number of studies that the MBFSK (Multiple Binary Frequency Shift Keying) modulation provides relatively high transmission bit rates in a multipath channel. In the current publication, two Doppler factor estimation methods dedicated to this modulation are presented: the double DFT and the correlation method. These methods were tested with utilization of signals recorded in real environment under conditions of strong multipath in motion. Research was conducted at the speeds of 0.5, 1 and 1.5 m/s between the transmitter and receiver. The results show that the proposed methods have comparable Doppler shift estimation quality to the well-known pilot method, without decreasing the bandwidth allocated for data transmission.