Sweep-spread Carrier Research Articles

FrFT equalization for underwater sweep-spread communication system

In underwater acoustic channels, multipath propagation and Doppler frequency can distort the received signal and reduce system reliability. In order to effectively combat multipath interference, the Sweep-Spread Carrier (SSC) system using frequency swept signal is proposed, however, the SSC receiver performs well only when the maximum delay spread of path arrivals and the Doppler spread are moderate. In this paper, fractional Fourier transform (FrFT) is applied to channel equalization of the SSC system, which improves the ability to resist the Doppler effect and intersymbol interference (ISI). FrFT can simplify the channel characteristics by selecting an appropriate transform order. We analyze the FrFT equalizer in the SSC system under the zero forcing (ZF) criterion and minimum mean squared error (MMSE) criterion. Compared with typical nonlinear Turbo equalization, FrFT equalizer only requires lower complexity to achieve good performance. The numerical simulation results in bellhop and ray tracing channels and the results of the Wu-yuan Bay test channel show the high robustness of the proposed method in doubly dispersive channels.

Soft Symbol Decoding in Sweep-Spread-Carrier Underwater Acoustic Communications: A Novel Variational Bayesian Algorithm and Its Analysis

Sweep spread carrier (S2C) based underwater acoustic (UWA) communications is a practically attractive but less explored modulation scheme in the published literature. In this paper, we present a rigorous treatment of the S2C communication receiver design and propose a data detection scheme that can handle challenging UWA channels. State-of-the-art S2C receivers based on the gradient heterodyne processing are only effective when the path delay and Doppler spread are moderate. We develop a new variational soft symbol decoding (VSSD) algorithm based on the principle of variational Bayes’ inference for a general linear channel model. In channels with moderate delay and Doppler spreads, we show that the VSSD algorithm is equivalent to the existing gradient heterodyne receivers for S2C communications. We apply the VSSD algorithm to the i.i.d. Gaussian multiple-input multiple-output channel and show, through numerical simulations, that it far outperforms the minimum mean squared error (MMSE) data detection. We illustrate the dramatic improvement in the performance of the VSSD based S2C receiver in two different models of simulated UWA channels and two contrasting measured UWA environments publicly available in the Watermark channel dataset. The proposed VSSD algorithm recovers data symbols at a signal-to-noise ratio (SNR) which is at least 10 dB (8 dB) lower than the MMSE decoder for uncoded (rate 2/3 LDPC coded) communications over UWA channels where the existing receivers either fail completely or must compromise on the data rate to maintain the bit error rate (BER) performance.

Sweep-spread carrier for underwater communication over acoustic channels with strong multipath propagation.

Multipath propagation causes the transmitted signal to take numerous, time-varying different-length paths to the receiver. Exploitation of conventional frequency-constant carrier signals for communication over underwater acoustic channels typically shows that intricate mutual time variations of multipath arrivals create an extremely hard problem for equalizers to compensate for nonstable intersymbol interactions. Communication over such channels can be, however, substantially improved by using a new method based on the implementation of a sweep-spread carrier (S2C). Such a carrier consists of a succession of sweeps, which cause permanent rapid fluctuation of signal frequency. Apart from some additional useful effects such as providing multiuser access and reducing influence of narrow-band noise, the method provides significant processing improvement enabling clear separation of multipath arrivals by converting their time delays into their frequency reallocations--the steeper the sweeps, the better the multipath resolution that can be achieved. When converting the signal into constant intermediate frequencies, the best suitable multipath arrival can be separated not as traditionally in time domains, applying complex equalizers, but in frequency domains by means of usual band-pass filters. High transmission stability of this approach was confirmed in both computer simulations as well as in validation experiments carried out since summer 1999.