A Novel Receiver Design of Nonorthogonal FDM Systems in Underwater Acoustics Communication

While orthogonal frequency-division multiplexing (OFDM) has been studied for a decade, the study of nonorthogonal frequency-division multiplexing (NOFDM) in the underwater acoustics (UWA) communication has never been reported, to the best of our knowledge. Therefore, we investigate the NOFDM technique for the UWA communication considering the doubly dispersive channel. The main feature of the NOFDM is that it involves more closely packed subcarriers compared to the OFDM, which results in higher spectral efficiency. However, this transmission suffers from severe intercarrier interference. Since, we are considering the doubly dispersive channel, intersymbol interference is also encountered due to the multipath propagation. Therefore, the traditional receiver of the NOFDM system in the UWA channel tends to have high computational complexity. Considering this problem, we design a receiver for the NOFDM system, where the basis expansion model is used along with the compressed sensing channel estimation technique, i.e., orthogonal matching pursuit (OMP), which can effectively reduce the computational complexity. However, while implementing this technique in the real sea environment, the problem of long delay/Doppler spread is encountered due to guard intervals. Therefore, we propose the use of a time-domain equalizer to mitigate the effect of long delay/Doppler spread. Simulation and experimental results based on the bit error rate demonstrate the performance degradation due to severe interference in our proposed receiver. On the contrary, the mean-square-error performance shows that our proposed receiver with the OMP channel estimation outperforms the OFDM receiver. Similarly, higher spectral efficiency gain is attained due to the closely packed subcarriers.