Underwater Acoustic Communications in Time-Varying Dispersive Channels

Abstract

Abstract-Underwater acoustic communication (UAC)system designers tend to transmit as much information as possible, per unit of time, at as low as possible error rate. However, the bit rate achieved in UAC systems is much lower than for wire or radio-communication systems. This is due to disadvantageous properties of the UAC channels, namely the sea and inland waters. Estimation of UAC channel transmission properties is possible within a limited bandwidth and temporal resolution. Thus, the UAC physical layer of data transmission is designed on the basis of roughly estimated channel parameters, or assuming the worst possible conditions. The paper presents the methodology of adapting UAC signaling schemes to tough underwater propagation conditions, through an example of two communication systems designed and developed at the Gdansk University of Technology.

Using Fast Frequency Hopping Technique to Improve Reliability of Underwater Communication System

“…To meet the condition of implementing frequency diversity, the occurrence of independent fading in the frequency-selective channel should be assumed, when the frequency spacing between the two signals is greater than the coherence bandwidth B c of the channel (∆f FFH-FSK = 1/T H > B c ) [3]. Therefore, the planning of frequency hops in conjunction with the design of the appropriate hop pattern resulting from the generated code sequence is important for the FFH system under consideration.…”

“…This is due to the low speed of propagation of the acoustic waves and the narrow available bandwidth, which directly results in limiting the throughput of the hydroacoustic channel [1]. Their seasonal variability is also observed, which is an additional difficulty in clearly defining the acoustic parameters of the underwater communication channels [2,3].…”

Reliable OFDM Data Transmission with Pilot Tones and Error-Correction Coding in Shallow Underwater Acoustic Channel

“…The probable reflections of the transmitted signal are seen as the multipath components of TVIR. The scattering function S(ν, τ) is the basis for the calculation of the transmission parameters: delay spread τ M , Doppler spread ν M , coherence time T c , and coherence bandwidth B c , which are used for designing the physical layer of the UAC data transmission system [8,12]. The delay spread is calculated on the basis of Power Delay Profile (PDP) P(τ), which is obtained as the integral of S(ν, τ) over the Doppler shift ν domain.…”

Assessment of Wide-Sense Stationarity of an Underwater Acoustic Channel Based on a Pseudo-Random Binary Sequence Probe Signal

“…Then, it is possible to calculate the 2-dimensional scattering function S(τ, ν) = ∆t ∆ f R h (∆t, ∆ f )e −j2π(ν∆t−τ∆ f )d∆td∆ f , where R h (∆τ, ∆ f ) is the space-time-frequency correlation function of the channel, obtained on the basis of h(t, τ). It is the basis for calculation of the transition parameters; namely, multipath delay spread τ M , Doppler spread ν M , coherence time T C , and coherence bandwidth B C [4].…”

“…Moreover, within this time interval, the signal received by the UAC system receiver represents a WSS process, and its power spectrum can be defined via a Fourier transform, according to the Wiener-Khinchin Theorem. This is especially important for signal detection performed in the frequency domain, as is the case with communication systems using the Orthogonal Frequency-Division Multiplexing (OFDM), and the Frequency Hopping Spread Spectrum (FHSS) techniques [4,6,7].…”