Abstract
Underwater acoustic digital communications suffer from inter-symbol interference deriving from signal distortions caused by the channel propagation. Facing such kind of impairment becomes particularly challenging when dealing with shallow water scenarios characterized by short channel coherence time and large delay spread caused by time-varying multipath effects. Channel equalization operated on the received signal represents a crucial issue in order to mitigate the effect of inter-symbol interference and improve the link reliability. In this direction, this contribution presents a preliminary performance analysis of acoustic digital links adopting pulse position modulation in severe multipath scenarios. First, we show how the spectral redundancy offered by pulse position modulated signals can be fruitfully exploited when using fractional sampling at the receiver side, which is an interesting approach rarely addressed by the current literature. In this context, a novel blind equalization scheme is devised. Specifically, the equalizer is blindly designed according to a suitably modified Bussgang scheme in which the zero-memory nonlinearity is replaced by a M-memory nonlinearity, M being the pulse position modulation order. Numerical results not only confirm the feasibility of the technique described here, but also assess the quality of its performance. An extension to a very interesting complex case is also provided.
Highlights
Intersymbol interference (ISI) represents the main impairment in wireless data links over multipath channels
When referring to shallow water scenarios, it may happen that the channel delay spread is even larger than hundred of milliseconds, making the cyclic prefix (CP) length occupying the largest part of the entire orthogonal frequency division multiplexing (OFDM) symbol time, and reducing the transmission rate
Signal redundancy, in Figure 1, we have indicated the bandwidth occupied by a typical square root raised cosine (RRC) pulse employed in amplitude shift keying (ASK) or quadrature amplitude modulation (QAM) techniques
Summary
Intersymbol interference (ISI) represents the main impairment in wireless data links over multipath channels. ISI mitigation is crucial in order to provide good communication performance In this direction, digital filtering at the receiver side aims at achieving the so-called channel equalization condition, so that the received signal is reshaped as close as possible to the transmitted one. Due to the distortion disturbing the received signal, channel equalization becomes necessary in underwater acoustic communications This is true especially when dealing with shallow water scenarios where the multipath trajectories exhibit short path lengths before reflections, making ISI cancellation very challenging. Regarding such points, the literature presents several solutions, many of which are based on the implementation of decision feedback equalizers (DFEs). We deem it necessary to highlight some aspects of OFDM and single-carrier based communications
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