Performance Evaluation of DQPSK OFDM for Underwater Acoustic Communications

This paper evaluates the performance of differentially encoded quadrature phase shift keying (DQPSK) modulated orthogonal frequency division multiplexing (OFDM) for underwater acoustic (UWA) communication. The combination of OFDM and the non-coherent detection scheme maintains the receiver design simple, reliable and spectrally efficient. QPSK helps achieve higher data rate as two bits are sent over each sub-carrier. Further differential coding is chosen as it does not require phase coherent reference signal at the receiver, which helps to overcome the detrimental effect of the random phase in the received signal. An added advantage of using DQPSK is that for slow Doppler frequency shift, the phase difference remains unaffected and the data can be retrieved. The performance of DQPSK OFDM is studied over a typical underwater channel through extensive computer simulation and a semi-realistic experiment. DQPSK OFDM scheme is used to transmit an image through the underwater channel in an experimental tank to validate the simulation. Furthermore selection combining technique is also implemented at the receiver to enhance the system reliability and performance. With two hydrophones at the receiving end and diversity combining it is possible to achieve a bit error rate (BER) of 2 x 10(-4) with 512 sub-carrier and 2 x 10(-5) with 256 sub-carriers respectively at a signal-to-noise ratio (SNR) of 21 dB.