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Research

Estimation of Phase Error Limits for PSK-Modulated Sweep-Spread Carrier Signal

Konstantin G. Kebkal, Rudolf Bannasch, Alexey G. Kebkal

Abstract: Sweep-Spread Carrier (S2C) communication method that has been recently developed especially for application in shallow water channels realizes frequency spread of a transmitted signal via its multiplication with a sequence of linear sweeps. This provides significant processing improvement on receiveing side enabling clear separation of multipath arrivals by converting their time delays into their frequency reallications. To provide a most effective and inexpencive filtering, a matched filter is considered to be a core part of the S2C demodulator, which uses a synchronous replica of the sweep-spread carrier signal as the reference signal for the matched filtering. However, if a frequency modulated carrier is applied for data transmission, the mechanism of evaluation of the information parameter (phase) have no strict mathematical justification. By use of frequency modulated carrier the output of matched filter contains the desired phase value, however, the methodical error of evaluation of filter output (its real and imaginary part) is unknown value, which cannot be found via estimation of define integral of correspondent oscillation function. The paper derives expression, which allows to evaluate the methodical error in analytical way in form of intergrals of elementary oscillation functions.

Parameter Estimation of a Sweep-Spread Carrier Signal for Advanced Acoustic Communication via Multipath Shallow Water Channels

Konstantin G. Kebkal, Alexey G. Kebkal, Rudolf Bannasch, Sergey G. Yakovlev

Abstract: To provide a maximum effect during application of the Sweep-Spread Carrier communication method, basic parameters of a sweep-spread signal, such as frequency gradient, duration and time arrangement of sweeps in the spread sequence, must completely match to transmitting channel. Owing to random variations of multipath structures in practical transmitting channels, however, best suitable parameters of the sweep-spread signal are unpredictable. In the paper a technique is presented that realizes estimation of an optimal set of spread signal parameters for acoustic communication via multipath shallow water channels. Simulations as well as experimental results from recent shallow water trials are provided.

Ultrasonic Link for Improved Incoherent Data Transmission in Horizontal Shallow Water Channels

Konstantin G. Kebkal, Rudolf Bannasch, Alexey G. Kebkal, Sergey G. Yakovlev

Abstract: An acoustic telemetry link may be disturbed or even collapse due to non-stable intersymbol interferences resulting from the effect that transmitted signal takes numerous, time-varying different-length paths to receiver. The complex time variations of multipath arrivals create an extremely hard problem for equalizers used for processing of conventional frequency-constant carrier signals. Our studies on underwater communication of dolphins revealed several solutions to overcome these problems. It turned out that chirping and singing is most essential in this conjunction. Applied to technical telemetry, underwater communication could be substantially improved by the implementation of the method called Sweep-Spread Carrier Communication. In this method, carrier signal consists of a succession of sweeps, which cause permanent rapid fluctuation of signal frequency. When converting received signal into constant intermediate frequencies, multipath arrivals can be resolved and separated not as traditionally in time domains, applying complex equalizers, but – much easier - in frequency domains by means of usual band-pass filters. As the experimental results show, this kind of signal processing improves statistical channel properties, providing increased data rates and high transmission stability, which were confirmed in validation experiments recently carried out in shallow fresh and sea water channels.

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

Konstantin G. Kebkal, Rudolf Bannasch

Abstract: 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.

Separation of time-varying multipath arrivals by converting their time delays into their frequency reallocations

Konstantin G. Kebkal

Abstract: Multipath propagation causes the transmitted signal to take numerous, time-varying different-length paths to 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 a serious problem for equalizers to compensate for non-stable interferences. Communication over such channels can be, however, substantially improved by using not a conventional constant-frequency carrier but a carrier consisting of a sequence of steep sweeps. The method provides significant processing improvement enabling clear separation of multipath arrivals by converting their time delays into their frequency reallocations. When despreading the signal, the best suitable multipath can be filtered not as traditionally in time domains, applying complex equalizers, but in frequency domains via usual band-pass filters. An important aspect is that every multipath is separated together with its individual time-varying phase impairments, obtained with the signal during its propagation over a certain path, whereas signal impairments, specific for a given path, are completely separated from other signal impairments delivered by the signal during its propagation over every different path.