A Direct Decoder Method for OFDM with Carrier Frequency Pilot in Underwater Acoustic Communication Systems

Abstract

In this paper, we propose a new decoder method at the receiver of system to compensate Doppler frequency shift for OFDM-based underwater acoustic communication systems. At the transmitter, in order to save bandwidth, we do not use additional signal header (preamble) in each OFDM frame as proposed in many conventional approaches. Instead, the central sub-carrier is reserved for pilot transmission. This subcarrier is so-called as the carrier frequency pilot (CFP), which is used to detect the Doppler frequency. At the receiver, in [1], two synchronization steps are deployed. The first step, the Doppler frequency is roughly estimated on the basic of the detected carrier frequency. In the second step, we use the CFP to regulate the estimated Doppler frequency. This regulation is called as fine synchronization. The use of Doppler compensation scheme in [1] is relatively complex because in order to calculate Doppler accuracy, it is necessary to perform two steps. Therefore, I propose an algebraic computation of Doppler frequency shift with one step. The results of the Doppler frequency shift calculation will be used to re-sample the received signal using the re-sampling matrix. The advance of using this matrix is that it can be calculated with any decimal, not an integer such as using the matlab function available in [1].
%At the receiver, in [1] we use the Doppler frequency is estimated on the basic of the detected carrier frequency (CFP). In order to compensate this Doppler shift, we first separate each individual OFDM symbol based on true length of it at receiver and then resample those symbols to the original length at transmitter. The advantage of the proposed method is to reduce the OFDM frame length. Therefore, the system bandwidth can be increased. The proposed method is able to track the fast time-variation of the Doppler frequency, which is a typical characteristic of the underwater channel. To verify the performance of the proposed method, we have tested our approach in real underwater channel with the speed of the relative Rx movement of 3m/s. Experimental results show that the estimated Doppler frequency matches well with the theoretical calculation.