Abstract
Multicarrier techniques have made it possible to wirelessly transmit data at higher rates for underwater acoustic (UWA) communication. Several multicarrier techniques have been explored in the past for wireless data transmission. OFDM is known to fight off inter-symbol interference due to the orthogonality of its subcarriers. However, due to time variations, OFDM suffers from intercarrier interference. As the UWA channel is both a time and frequency variant, channel estimation becomes complex. We propose a pilot-based channel estimation technique and explore two equalizers for improving the error performance of an OFDM-based UWA system. Both the equalizers employ pilot subcarriers to estimate the UWA channel. One equalizer is a least squares (LS) equalizer and the other is a zero forcing (ZF) equalizer. Using computer simulations, it is observed that, for an acceptable error performance, the number of pilots should be one-fourth the number of subcarriers. Moreover, if the energy of the pilots is increased without changing the overall symbol energy, the error performance degrades. It is also noted that both the LS and ZF equalizers give an acceptable error performance with the ZF performing marginally better than the LS. Furthermore, the error performance of the proposed system is evaluated as a function of the transmitter-receiver distance and an acceptable error performance is observed even at 1250 m.
Highlights
The underwater acoustic (UWA) channel is doubly selective due to multipath fading and a Doppler shift [1].When compared with radio waves, acoustic waves have lower speeds and the bandwidth is higher when compared with the carrier [2]
We propose the investigation of a channel estimation algorithm based upon a least squares error estimation
Using pilots for an adaptive estimation of the UWA channel, a recursive least squares algorithm was proposed in [18] to estimate the channel for a MIMO-OFDM system
Summary
The UWA channel is doubly selective due to multipath fading and a Doppler shift [1]. When compared with radio waves, acoustic waves have lower speeds and the bandwidth is higher when compared with the carrier [2]. In a typical pilot-based channel estimation scheme, a few known symbols— called training symbols—are inserted into the transmitted OFDM symbols at prespecified locations These known symbols are extracted by the receiver and are used for the synchronization and determination of the channel parameters [10]. We propose the investigation of a channel estimation algorithm based upon a least squares error estimation This algorithm is applied to a typical OFDM system, in which pilot signals are used for a UWA channel. We provide several bit error rate (BER) plots to show the performances of the equalizers as a function of the various parameters These simulations are performed over a UWA channel that uses a Rician fading distribution and incorporates absorption loss and ambient noise.
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