# An On-Demand Scheduling-Based MAC Protocol for UW-WiFi Networks
## 2 Related Work  
There has been extensive research on MAC protocols for UANs. These work can be classified into three categories: contention-free, contention-based, and hybrid, which this section will examine in the UW-WiFi network scenario. Contention-free MAC protocols, mainly including TDMA, FDMA, CDMA, and their variants, allocate network resources to specific nodes but not on-demand. TDMA divides the channel into multiple time slots and assigns each time slot to each node [7], in which, data are buffered by a node before being transmitted until its assigned time slot. Each node can take full advantage of the available bandwidth during its time slot. Unfortunately, TDMA requires strict time synchronization amongst all of the nodes in the network. Additionally, when network nodes have no data to send, their time slots are wasted. FDMA divides the available frequency band into multiple sub-bands and assigns a sub-band to each user [8], by which it guarantees collisions free among nodes. This algorithm is simple and efficient when dealing with a small number of nodes in uniformly constant traffic. However, for networks with changes in scale and traffic, such as UANs, it is unadjusted and inefficient. Additionally, guard bands are needed between sub-bands to compensate for imperfect filters, adjacent channel interference, and spectral spreading due to the Doppler Effect, which decreases the available bandwidth. CDMA allocates a unique code to each node, allowing multiple nodes to transmit packets concurrently by using the entire bandwidth. However, to prevent the near-far effect, it needs a power control algorithm to adjust the appropriate output power level for each node [9], which is hard to accomplish in actual sea applications. Contention-based protocols are clustered into random access and handshaking based. The random access approaches majorly comprise ALOHA, CSMA, and their variants, which are suitable for UANs with lower data traffic and sparser deployment. In the original Aloha protocol, Whenever a node has information to send, it transmits the information immediately, which cannot avoid collisions and will result in a high probability of retransmission and ultimately reduce the network performance. Due to a number of retransmissions, the maximum achievable throughput of the original Aloha protocol is only 18% [10; 11]. [PERSON] et al. proposed Aloha with Collision Avoidance (Aloha-CA) and Aloha with Advance Notification (Aloha-AN) to reduce collisions [12], both of which utilize overhearing messages and propagation delays between nodes to avoid collisions. However, when the number of nodes and network traffic increase, the collision may increase as well. CSMA protocol avoids collisions by requiring a node to sense the channel prior to data transmission [13], which only addresses the sender-side collisions, while the receiver is still vulnerable to collisions due to the hidden terminal problem [14]. Details and various designs of CSMA can be found in [15; 16; 17]. The handshaking-based protocols employ RTS/CTS control packet exchanges to reduce data packet conflicts. [PERSON] proposes a protocol named S-FAMA [6] which uses a four-way handshaking with time-slotting to avoid data collisions. To reduce reservation duration, the DACAP protocol allows a node to use different handshaking lengths for different receivers [18]. SASHA [19] is a hybrid approach of selective automatic repeat request (ARQ) and slotted handshaking access, in which after each successful negotiation, the sender pushes multiple available packets into a train to reduce the average overhead and improve the channel utilization. A multiple access with collision avoidance MAC protocol named fixed interval MAC(FI-MACA) [20] is proposed, where the sender can detect collisions effectively with low complexity, and the receiver also listens to the channel for a while to solve the hidden station problem before responding a CTS to the sender.