# Cross-Layer-Aided Opportunistic Routing for Sparse Underwater Wireless Sensor Networks
###### Abstract  
Underwater wireless sensor networks (UWSNs) have emerged as a promising technology to monitor and explore the oceans instead of traditional undersea wireline instruments. Traditional routing protocols are inefficient for UWSNs due to the specific nature of the underwater environment. In contrast, Opportunistic Routing (OR) protocols establish an online route for each transmission, which can well adapt with time-varying underwater channel. Cross-layer design is an effective approach to combine the metrics from different layers to optimize an OR routing in UWSNs. However, typical cross-layer OR routing protocols that are designed for UWSNs suffer from congestion problem at high traffic loads. In this paper, a Cross-Layer-Aided Opportunistic Routing Protocol (CLOR) is proposed to reduce the congestion in multi-hop sparse UWSNs. The CLOR consists of a negotiation phase and transmission phase. In the negotiation phase, the cross-layer information in fuzzy logic is utilized to attain an optimal forwarder node. In the transmission phase, to improve the transmission performance, a burst transmission strategy with network coding is exploited. Finally, we perform simulations of the proposed CLOR protocol in a specific sea region. Simulation results show that CLOR significantly improves the network performances at various traffic rates compared to existing protocols. underwater wireless sensor networks; cross-layer routing; opportunistic routing; fuzzy logic; network coding +
Footnote †: journal: Sensors  
0000-0002-0001-5886-22000/0000-0000-0000-0000-00000-0000-00000-00000-00000-000000-00000-000000-000000-000000-000000-000000-000000-0000000-00000000000000  
with more neighboring nodes and the nodes in close proximity to the sink nodes may suffer from congestion problem, because more traffic rates occur around these nodes. FBR heavily suffers from the congestion problem when the traffic load increases. Burst transmission is an effective solution to address the congestion problem [5; 12]. CARP adopts the burst transmission strategy to resolve the congestion to some extent. These two protocols only perform better in networks with low traffic rates, and the delivery performance of these protocols degrades heavily as the network traffic increases. The schedule-based transmission protocols can effectively handle high traffic rates in UWSNs [13; 14]. These protocols are designed merely for the networks with star topology; however, they cannot well adapt to multi-hop UWSNs. Thus, it is necessary to develop a novel cross-layer protocol on the basis of burst transmission for sparse multi-hop UWSNs with various traffic rates. Considering the long-term channel occupation in the burst transmission strategy, many aspects particularly essential should be taken into consideration, including the selection of the communication pair over a period of time, the setting of time interval of burst transmission, and the introduction of a strategy to avoid the collision problem at high traffic rates. First, negotiation strategy is a valid way to adaptively select a communication pair with the minimum impact on networks. Second, the time interval occupied by burst transmission is related to the average size of packets in the data queue and channel condition. An appropriate burst size should be set according to the congestion state of the network. In addition, random linear packet coding is a valid transmission strategy to achieve link reliability in burst transmission [15]. Lastly, the back-off strategy is an effective method that is on the basis of negotiation strategy to handle the collision problem. In this paper, a Cross-Layer-Aided Opportunistic Routing Protocol (CLOR) for sparse UWSNs is proposed by using handshake strategy in the negotiation phase and random linear packet coding strategy in the transmission phase. The multifold contributions of this paper are presented as follows.