A simple method for dynamically adjusting the tap-length adaptation step-size of a variable tap-length linear equaliser based on the fractional tap-length algorithm is presented. Simulations show that the technique provides a fast convergence of tap-length and small steady-state tap-length fluctuation.Introduction: Linear equalisers are usually implemented as adaptive finite impulse response (FIR) filters. The tap-length of the FIR filter greatly influences the performance and computational complexity of the equaliser. With too few taps the linear equalisers may not realise their full potential; in contrast, using too many taps, besides wasting computations, may increase the steady MSE [1]. Hence, a variable tap-length equaliser is needed to find a proper choice of the tap-length.Several variable tap-length filter algorithms have been proposed in recent years [1][2][3][4][5]. Among the existing variable tap-length filter algorithms, as analysed in [5], the fractional tap-length (FT) algorithm is more robust and has lower computational complexity compared with other methods. However, in the FT algorithm, the tap-length adaptation step-size (TAS) is fixed. The TAS is an important parameter on the performance of the FT algorithm; a small TAS could ensure small steady-state tap-length (STL) fluctuation with convergence, whereas a large TAS will provide a faster convergence of tap-length, but more severe STL fluctuation [6]. Therefore, to speed up the convergence rate and reduce STL fluctuation to some extent, we propose a variable TAS FT (VTAS-FT) algorithm based on the tap-length adjustment error (TAE).
Aiming at the influence of underwater acoustic channel (UAC) profile difference on the performance of underwater acoustic communication, a variable observation window length blind equalization detector (VOWL-BED) is proposed in this paper. Compared with the existing methods, the observation window length (OWL) of the proposed VOWL-BED can be dynamically regulated on the basis of a given UAC profile and finally converge to the optimum OWL. We have verified the feasibility of the OWL regulation. The simulation results also demonstrate that the VOWL-BED can achieve better performance than the conventional invariable OWL blind equalization detector (IOWL-BED).
To overcome the problem caused by the large propagation delays of underwater acoustic channel, an asynchronous underwater decode-interleave-forward (UDIF) cooperative strategy is considered for the underwater acoustic cooperative communication system. The UDIF strategy is designed to realize the cooperative transmission and to decrease the end-to-end delay. Aiming at the strategy, a joint multi-branch combining and turbo equalization detector is proposed. To compensate the channel effects and achieve the diversity gain, the turbo equalization, multiuser detection, and combining technique are jointly realized. In the implementation of the detector, the detector is iteratively adapted by switching soft information according to log-likelihood ratio estimates with the turbo processing stage. Moreover, at each iteration, the combining coefficients for the received signals from the source and relay nodes are updated based on the steady-state mean square error, and thus, the proposed detector can effectively combine the received signals without the assumption that full and perfect channel state information of all the links at the receiver is known. The simulation results validate the feasibility and show the advantages of the proposed detector against existing counterparts.
INDEX TERMSUnderwater acoustic cooperative communication, asynchronous underwater decode-interleave-forward (UDIF), multi-branch combining, turbo equalization.
In this paper, we propose a joint adaptive combining and variable tap-length multiuser detector (MUD) for amplify-and-forward (AF) underwater acoustic cooperative interleave-division multiple access (IDMA) communication system. The proposed MUD jointly realizes tap-length adjustment, adaptive combining, and multiuser detection. In contrast to the existing methods, the proposed detector can adaptively combine the received signals from different nodes at destination, and does not need the assumption that full and perfect channel state information (CSI) of all the links at the receiver is known. Moreover, the proposed detector can adaptively adjust the tap coefficient vector and tap-length of each branch according to the specific channel profile of each branch. Simulation results validate the feasibility and show the advantages of the proposed detector against existing counterparts.
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