Distributed power control for underwater acoustic networks

Multi-hop transmission is considered for large coverage area in bandwidth-limited underwater acoustic networks. Discrete power control is introduced as a practical means of optimizing the overall system performance across the physical and the medium access control layers. The required number of power levels and the way in which they are distributed for increasing network densities is analyzed in light of minimum energy per bit consumption. The system performance is evaluated for different frequency allocation patterns (center frequency f_c and bandwidth B). The results show that the total energy per bit consumption can be reduced by moving to higher frequencies, where the background interference is decreased and a greater bandwidth is available. A greater bandwidth supports transmission at higher bit-rates, resulting in a twofold effect: first, the total energy consumption is reduced because the transmission time is shorter, and second, shorter packets are less likely to collide. These facts encourage the use of high bit-rates even if the application does not require it. In addition, they motivate a review of the medium access control protocols, whose performance depends on the number of collisions. Two MAC protocols are considered: the Distance Aware Collision Avoidance Protocol (DACAP), a virtual carrier-sense-like protocol that completely avoids harmful collisions, and the simple Carrier Sensing ALOHA. Coupled with power control, both protocols are shown to be well-suited for networks containing static and mobile nodes which are not synchronized to a global clock.