Experimental Study of an underwater Acoustic communication system with Pseudonoise signals

Abstrac.t-An experimental study of underwater communications systems with a large ensemble of pseud-onoi_s_e_signals js described. Experiments were.performed in shadow antl "onuergence zones (at distancejof 40, 90' 525, and 694 km) in deep-water ocean regions and in a shallow sea (at a disiance of r72 i.rni, Transmission was accompanied by multipath propagation, fading, and scattering of signals-. Adaptive reception with delay diversity was used. Dependence of communication quality upon thJ numd". of delayed signali ana tne ur"rug-ing interval used to estimate pa.rameters of the acoustic channel was analyzed. Fading and scattering of signals accompany acoustic transmissions through acoustic channels in the ocean. Along with multipath propagation, these properties of a channel restrict the effrciency of underwater communications systems [l, 2]. A well-known way to control signal fading is multichannel reception with spatial, frequency, or time diversity, or with a combination of these techniques [3]. Spatial diversity is highly efficient for nnderwater channels [4] but requires complex antenna arrays. Because of its simplicity, frequency dive-rsit1, ir; wiclely usecl [-5, 6], trrrt this results in inelficient rrse <ll' a restrictcrl frequelrcv bltrrd rn arr underwater clrannel. Multipath propagation allows signal reception with separation in arrival times for different paths. Unlike thc rnajority of other reception methods, time diversity implies that signals from other channels form additive noise, which interferes with signals in the reception channel. Therefore, general statements concerning the advantages of transmission through parallel channels ire nol directly applicable to this type ol reception. For example, a maximum outpLlt selection schenre, that is, demodulation of signals amving along the mosr inrense path, has low efficiency in the case at hand, though it is known as an efficient method to control fading [3]. In multipatir reception, one has to add signals propagating along parallel channels. Coherent addition offers higher noise immunity. However, such addition requires that the gains of parallel channels and phases be known, which is not always possible. If the phases are not known, incoherent addition is the optimal method. For incoherent addition, amplitudes and delays of components of a multipath signal must be measured. Therefore, pseudonoise signals with high values of the duration product and frequency band-width are advantageous [7]. To increase the data transmission rate in the case of strong scattering, which is characteristic of underwater channels, one must usr: large ensembles of signals [8, 91. ^ The present paper is devoted to an experimental study of an acoustic …