Capacity of OFDM Systems Over Fading Underwater Acoustic Channels

Abstract

Abstract-In this paper, we derive bounds to the channel capacity of orthogonal frequency division multiplexing (OFDM) systems over the underwater (UW) acoustic fading channel as a function of the distance between the transmitter and the receiver. The upper bound is obtained under perfect channel state information (CSI) at the receiver. The lower bound is obtained assuming the input is drawn from phase-shift keying (PSK) constellation which results in non-Gaussian distribution of the output signal and no CSI. The reduction from the upper bound is due to limited mutual information that can be conveyed by PSK constellation and the linear minimum mean square prediction error. Our UW channel deviates from the wide sense stationary and uncorrelated scattering (WSSUS) model commonly used for small bandwidths. We incorporate frequency-dependent path loss due to the acoustic propagation into each arrival path between the transmitter and the receiver. This leads the UW channel to be modeled as a frequency-dependent doubly spread fading channel characterized by the wide sense stationary and correlated scattering (WSS-non-US) fading assumption. Both Rayleigh and Ricean fading assumptions are investigated in our model. Results from the model show a gap between the upper and lower bounds which depends not only on the ranges and shape of the scattering function of the UW channel but also on the distance between the transmitter and the receiver. Our model for the scattering function was suggested by Rescheduled Acoustic Communications Experiment (RACE08) experimental data, leading to a multilag autoregressive (AR-) model for the fading.Index Terms-Doubly spread fading channels, linear minimum mean square error, orthogonal frequency division multiplexing (OFDM), underwater acoustic communications, underwater acoustic propagation, wideband channel capacity.

Information theoretic analysis and optimization of underwater acoustic communication systems

“…2, we investigate the effect of significant channel taps' locations on the capacity. We assume ten significant taps (i.e., m = 10) with uniform PDP and consider the following Γ vectors to indicate the location of significant taps: [1,2,3,4,5,6,7,8,9,10] • Γ2 = [1,2,3,4,5,6,7,8,11,20] • Γ3 = [1,2,3,4,5,6,11,20,47,128] • Γ4 = [1,15,29,43,57,71,85,99,113,127] In Γ 1 , the locations of significant taps are consecutive, and Γ 4 represents the case of equally spaced taps. It is observed from Fig.…”

“…The autocorrelation matrix of noise signal is defined as R w = E (ww * ) based on the discrete version of autocorrelation function given by (4). In (7), g [k]…”

“…Although capacity calculations for terrestrial wireless radiofrequency channels have been extensively studied, the literature on the capacity of UWA channels is sporadic [2][3][4]. These works, either implicitly [2] or explicitly [4] consider an OFDM-based multi-carrier architecture and assume frequency-flat channel for each narrow sub-band.…”

“…These works, either implicitly [2] or explicitly [4] consider an OFDM-based multi-carrier architecture and assume frequency-flat channel for each narrow sub-band. In an OFDM system, assuming that there are enough parallel channels, the frequency-selective channel effectively disintegrates into a number of frequency-flat channels and no intersymbol interference (ISI) is observed.…”

Time Reversal in Underwater Acoustic Communications

“…In the past decades, multi-carrier and multiuser techniques have been widely deployed in the next generation radio frequency wireless communications. In recent years, much literature focuses on applications of Orthogonal Frequency Division Multiplexing (OFDM) and Multi-Input and Multi-Output (MIMO) techniques in UWA communications [6][7][8][9][10][11][12][13]. Therefore, Time-reversal in conjunction with MIMO and OFDM techniques has become a new trend in future high performance UWA communications, especially in complicated fast time-varying shallow water acoustic environments.…”