## 3 Related Work
### Underwater Communication and Networking  
Underwater communications and networking have been extensively studied in [16; 17; 18; 19; 20; 4; 21]. The study in [16] demonstrated the chronological development of different modulation techniques for underwater communications, and discussed the achievable data rates and transmission ranges. The authors of [10] showed that the data rate of 500 kbps can be achieved between two communicating peers with a distance of 60 m in deep water by using PSK modulation technique. Several challenging issues for underwater networking were discussed in [4] including power consumption behaviors of underwater sensor nodes, and the bizarre characteristics of underwater acoustic communication channel such as limited bandwidth, high level of multi-path effect, fading, doppler spreading, high propagation delay, and high bit error rate. The study in [17] proposed transmitter-based underwater MAC (UW-MAC) protocol combining the properties of both ALOHA and CDMA. The study in [18] has shown that multi-hop underwater communication can be more energy efficient than a single-hop communication in a long distance.  
To increase the data rate over the bandwidth-limited acoustic channel, either the bandwidth, or the spectral efficiency in the unit of bits per second per Hertz (b/s/Hz), or both, need to be increased [20, 21]. Toward achieving this goal, the studies in [20] showed that MIMO-OFDM technique can easily attain spectral efficiency of 3.5 bits/sec/Hz in one experiment and a data rate of 125.7 kb/s over a bandwidth of 62.5 kHz in another for an acoustic channel of distance 1,500 m.  
There also have been studies on alternative underwater communication technologies for short-range, high rate data transmissions [19, 22, 23, 24, 25]. For example, it was shown in [22] that 1 Gbps data rate can be achieved for the distance of two meters using underwater optical link. In [23], integrating suitable MODEM using omnidirectional optical transceiver, a preliminary design of optical communication system was carried out to realize 10 Mbps data rate between two underwater sensor nodes. Also, in [19], authors not only discussed the short-range, high-bandwidth optical communication link between an AUV and a fixed sensor station, but also proposed underwater communications using low-cost light emitting diodes (LEDs). The authors of [24] studied electric current based underwater communications, and showed that 1 Mbps data rate is achievable for the short distance (e.g., 10 m) communications.  
The possible integration of a short-range optical communication technology and long range underwater acoustic communications is studied in [25]. This communication system can be used for an AUV to communicate with fixed underwater sensors with a high data rate, which can justify our assumption of short-range and high rate communications between AUVs and GWs. Those related studies show that our assumptions on short communication range for attaining a large amount of data transmissions is reasonable. Furthermore, the development of alternative underwater communication technologies has enhanced the possibility of a short-range and very high rate data transmission between AUVs and underwater devices.