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

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.

Real-time clock synchronisation in underwater acoustic networks

“…Furthermore, each modem must be able to accurately register send and receive time stamps and to make this information available to the node. Finally, if relative speed is to be used, each modem is considered capable of accuratly determine the velocity relative to the transmitting modem, typically through the Doppler shift of incoming packets [3]. Note that this is not an excessive demand on the modems since they cannot decode incoming packets without the accurate determination of the Doppler shift.…”

“…The gray dots show the calculated Δpq's for the clock offset between OEX Groucho (node 2) and NRV Alliance (node 1), the diagonal line shows the result of the linear regression calculated according to(3), normalised such that the line is centred around 0. The black dots show the residual ε (Δpq).…”

A Probabilistic and Highly Efficient Topology Control Algorithm for Underwater Cooperating AUV Networks

“…The S2CR module shown in Table 2 is built upon the sweep-spread carrier (S2C) technology [31]. In the following, we will introduce this technology under multipath environment and Doppler spreading environment briefly.…”

“…The sweep spread carrier under Doppler spreading can be expressed as [31]: $x(t-{\tau }_i)=s(t-{\tau }_i)\cdot e^{j({\omega }_L\left\{\frac{t-{\tau }_i}{T_{sw}}\right\}{T}_{sw}+m{(\left\{\frac{t-{\tau }_i}{T_{sw}}\right\}{T}_{sw})}^2)}\cdot e^{j{\omega }_d^i\left(t-{\tau }_i\right)},$ where ${\omega }_d^i$ is the Doppler frequency encountered in i -th propagation path, which reflects the influence of Doppler effection on the received signal. The last exponent in Equation (8) can reflect time-varying phase/frequency.…”

Performance of hydro-acoustic data transmission in horizontal channels using sweep-spread signaling and turbo-coding

“…However, using the sweep-spread carrier communication method (S2C), the method which provides continuous frequency spread of the transmit signal [1], the nature of receive signal fading and its statistical characteristics change. Synchronous multipath component of the sweep-spread signal is exposed to correlation with a locally generated copy of the carrier (reference signal), whereby non-synchronous multipath components on the output of the correlator are suppressed and, in result, the signal dispersion at the receiver pre-detection point becomes close to Gaussian.…”