Underwater Optical Communications for Swarm Unmanned Vehicle Network Marco Tabacchiera, Silvello Betti
Samuela Persia
Department ofElectronics Engineering
Fondazione Ugo Bordoni
University of Rome "Tor Vergata" Rome, Italy {tabacchiera, betti}@ing.uniroma2.it
FUB Rome, Italy
[email protected]
Abstract- Particular attention has been recently devoted to Underwater Wireless Networks based on swarm configuration, in which decisions are taken in collaborative manner. Traditionally, underwater communications are based on acoustic signaling, with consequent drawbacks due to narrow band channel and multipath propagation effects. Performance improvement could be achieved by optical communications among the nodes when water conditions permit. A system analysis of optical communication systems is presented in order to obtain a trade off between network performance and applications.
Keywords- Optical Underwater Communications; Swarm Configuration.
I.
networks suggest to consider at network level a multi-hop paradigm to forward data within the swarm. The network performance is evaluated in the case of optical communications between swarm nodes, assuming realistic parameters for the communication channel and for optical devices operating in the blue-green band (400-500 nm). The swarm network performance is evaluated in terms of Frame Error Probability (FEP), which depends on BER, frame size and number of hops required to route data from one transmitter to the other nodes within the swarm. The paper is organized as follows: a brief introduction to underwater optical communication system in Section II, performance evaluation in Section III, and conclusions are drawn in Section V.
INTRODUCTION
Within underwater wireless networks, particular attention has been recently devoted to swarm networks (where mobile nodes are anyway very close one to each other) [1]. Actually, the main drawbacks of Underwater Acoustic Networks (UANs) are limited bandwidth and high signal latency [2]. Anyway, in swarm networks, the limited distance between nodes can be exploited by implementing optical links when the characteristics of the communication channel make it possible [3, 4]. With respect to traditional acoustic communications, that solution can lead to an increase in data rate, to the absence of signal latency and to significant energy saving. The performance of underwater optical systems is strongly dependent on turbidity, thus the diffuse attenuation coefficient and the beam attenuation coefficient must be introduced to take into account optical attenuation and water scattering effects [3, 4]. Within this framework, high data rate, short and very short range optical systems have been proposed [5,7]. In a more general scenario, optical communications can be viewed as complementary to underwater acoustic communications, so to design flexible integrated systems [6], which can exploit cognitive communication elements. Moreover, energy saving constraint for underwater robots in swarm configuration copes with the lower power requirements of the optical devices with respect to the acoustical ones. A swarm configured network is provided with self-organizing capabilities, therefore both control algorithms and data traffic within the swarm must be suitably managed and cross-related. The structure of an underwater swarm network is of distributed type and the nodes take decisions in a collaborative manner, through control information exchange. The features of the underwater swarm
II.
SYSTEM MODEL
The structure of the network is of distributed type and the nodes take decisions in a collaborative manner through the exchange of control information. A system model presented for an underwater swarm network in case of optical link among mobile nodes, considering LED optical transmitter and typical parameters for the communication channel. The performance evaluation have been obtained by Matlab environment simulation where each node can operate as transmitter covering a bi-conical volume with coupled rear-to-rear LEDs sources at the vertex and angular aperture depending on the emission diagram of the device itself, as shown in Fig.I. The covering range depends on the wavelength, the emitted optical power and on the propagation characteristics of the communication channel [7]. Performance evaluation has been carried out starting from the SNR relative to the typical underwater optical link [6]:
Fig. 1. Optical transmitter and receiver based on coupled rear-to-rear LEOs and photodiodes.
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Fig. 2. Et/No versus distance in optical underwater channel for different types of water.
SNR
[P =
I
·
e
-3Kr
.
D2 ",, ]2 cos '¥
·
(tan2 B ) 4r2 NEP ·
(1)
·
where PI is the transmitted power, e the half angle transmitter beam width, K=c/3 the diffuse attenuation coefficient, which typically ranges from 0.02 m-1 for the cleanest water, to 0.8 m-1 for the more turbid coastal water [3, 4], c being the beam attenuation coefficient, r is the optical link length, D the receiver aperture diameter,
