## 1 Introduction  
Underwater acoustic communication is a technique which enables devices to exchange data wirelessly in the water. Since the radio frequency (RF) commonly used for terrestrial communication networks is not transmitted very far in a water medium due to severe absorption and scattering, sound and ultrasonic waves having frequencies under 100 kHz are typically used as a carrier [1; 2]. For several decades, the main applications of underwater acoustic communication have been limited to deep sea probes and exploitation of offshore oil fields, requiring acoustic modems which consume high power but support long working ranges [3; 4]. Quite recently, underwater wireless sensor networks (UWSN) have arisen as a promising field in research and development [5; 6; 7], driven by the expansion of applications including data collection and pollution monitoring for the conservation of ecological systems in fresh water and sea water, tactical surveillance, data transfer for unmanned underwater vehicles, and other scientific purposes. Some initial studies on small-scale underwater modems have been described in [8; 9; 10; 11]. The first low-power modem, named CORAL [8], comprised a micro-controller unit (MCU), analog circuitry and piezo-transducer having a resonant frequency of 1.7 kHz. The modem was tested in a small water tank and verified one-way communication at the distance of 20 cm. Another low-power modem developed by [PERSON] _et al_. [9] was equipped with speaker and microphone and only tested in air environment. In both acoustic modems, transmission data rates and bit error probability were not reported. In [10; 11], we implemented bidirectional acoustic modems equipped with piezo-transducers and verified a working distance of 30 m and a data rate of 5 kbps in a pond. The abovementioned early stage modems are meaningful from the viewpoint that they verified the feasibility of compact and low-power acoustic modems, however they could not be utilized in real underwater applications since they were equipped with piezo-transducers whose use is generally restricted to onshore communication systems. One full-scale modem for UWSNs was developed by the Woods Hole Oceanographic Institution (WHOI), and was called a micro-modem in [12]. The dimensions of base board are 114 mm and 44 mm in length and width, and the whole modem consists of a stack of boards. The modem supports a variety of data rates, ranging from 80 to 5,400 bps depending on the power consumption that ranges from 50 W to 100 W. Tritech Corporation has also produced a commercial acoustic micro-modem whose dimensions are 54 by 74 mm in diameter and height, respectively. The modem supports a working range of 500 m with a power consumption of 7.2 W, but its data rate is merely 40 bps [13]. Since these two micro-modems described above are equipped with underwater transducers having cylindrical shapes and low resonant frequencies between 10 and 30 kHz, they have toroidal beam pattern in the vertical axis and the size of their transducers is quite large. Meanwhile, in order to apply an acoustic modem to UWSNs, it has to possess several properties, including low-power consumption, omni-directional beam pattern, small size, low cost, moderate data rate and good working range. From these observations, in this paper we design and implement amicro-modem satisfying all these requirements based on our previous work [14,15]. As a transducer to be interlocked with our micro-modem, a spherical omni-directional transducer having a resonant frequency of 70 kHz and a diameter of 34 mm is considered. Physical layer frame format and symbol structure are also studied to construct an efficient packet transmission system. The implemented modem was tested in a water tank, a pond and a river to verify its functional operations and performance. The remainder of this paper is organized as follows: in Section 2, we describe the phenomenological issues occurring during data transfer in underwater channels, especially in terms of transmission loss and transducer loss.