A four-quadrant mobility model-based routing 1 protocol for post-earthquake emergency 2 communication network 3

: Emergency communication network (ECN) is essential for both disaster victims and 10 rescuers since the pre-deployed network infrastructure may be completely destroyed after the 11 earthquake. Traditional protocols cannot satisfy the requirements of ECN as they neither consider 12 mobility model nor seismic intensity. In this paper, a four-quadrant mobility model (FQMM) based 13 on seismic intensity is proposed for rescuers. Then a FQMM-based protocol (FQMMBP) for ECN is 14 designed, which aims to improve the performance of ECN. Simulation results show that the 15 proposed protocol performs better than other three compared routing protocols (AODV, DSDV and 16 DSR) in package delivery rate (PDR) and end-to-end delay. Although the performance of FQMMBP 17 in overhead is not as good as the other three protocols, it is worthwhile for the emergency rescue.


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After the devastating earthquake, emergency rescue crews are needed to rush to the scene for rescue as soon as possible. Post-earthquake emergency rescue includes material dispatching, team 23 dispatching, personnel evacuation, post-disaster reconstruction, etc. Due to the different 24 catastrophic degrees caused by the earthquake in different affected areas, the Rescue Urgency 25 Degrees (RUDs) are different for those areas. Seismic intensity, which refers to the catastrophic 26 degree of earthquake impact on the ground or artificial buildings in a certain area [1], can be used to 27 express the rescue urgencies of disaster areas. The seismic intensity can be obtained in the following 28 ways [2]: In the area where seismic observation equipment is intensively deployed, we can directly 29 obtain the intensity distribution map of the instrument; In the area where seismic observation In the existing research on the mobility model of emergency rescue crews, the traditional task assignment model, e.g., numerical type [3], time window [4] and linear time satisfaction function [5], usually focuses on processing of rescue time. Song Ye et al. [6] established an optimization model for the assignment of earthquake emergency rescue teams, aiming at improving rescue efficiency which based on the topological structure of ECN.   Disaster area is divided into n layers according to seismic intensity. The epicenter is located in 151 the first layer (RUD=1) where length is a1 and width is b1. We define RUD of each region as 1, 2,…, n.

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Epicenter located at the center of rectangle where RUD=1. All areas are rectangular rings, except for 153 the one with RUD=1, which is a rectangle. ECV parks at the midpoint of the length outside the 154 rectangular ring with RUD=n, as shown in Figure 2.

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According to mathematical induction, we can get:

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Step 1: As shown in Figure 3, we establish a coordinate system which takes the epicenter as the 179 origin. X and Y axes are the length and width of the rectangle. Four quadrants, QID = 1, QID = 2, 180 QID = 3 and QID = 4 are counter-clockwise defined according to the definition of coordinate system.

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Each quadrant is described as follows: 182

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Step 4: Repeat Step 2 and Step 3, until any of the following Abort Conditions is satisfied.

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Abort Condition 2: The number of rescue tasks is less than that of rescuers assigned in the 235 quadrant with QL=x, which is:

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each rescuer is assigned to at most one rescue task, and there are unoccupied rescuers in the area.

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Step 5: Quadrant division is suspended and rescuers begin to perform their own rescue tasks.

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According to the mobility model, rescue crews cannot start rescue tasks in higher RUD areas 246 until rescue tasks in low RUD areas are completed. As shown in Figure 6, in the gray area with the white rectangular ring area with RUD=2.

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Step 7: It can be seen from Step 6 that in each coordinate system, there are three quadrants

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Step 8: Quadrants (QL≥2) division methods are the same as illustrated in Step 6, which take 290 the middle points of quadrants with QL=1 as the origins and establish the coordinate systems.

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Repeat Step 7, until any of the following Abort Conditions is satisfied. rescuer is assigned to at most one rescue task, and there are unoccupied rescuers in the area.

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Step 9: Quadrant division is suspended and rescuers begin to perform their own rescue tasks.

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QL of quadrants are the same, QID helps pick up the mobile rescue node which is closer to ECV.

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QW calculates weights of nodes according to QL and QID, so as to select the most appropriate 352 next-hop relay node.
In the process of route discovery, nodes broadcast RREQ messages to send request information 354 to their neighbors. The improved RREQ data format is shown in Table 2. multicast transmission. "G" represents the list of nodes around ECV which can be communicated.

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Flag "G" determines whether the RREQ message can be directly sent to the destination or not. "D"

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is the reply flag of the destination node which determines whether the destination node is allowed 362 to reply to the received RREQ message or not. "U" is the flag of unknown serial number. U=1 363 means that the serial number of the node is unknown.

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Reserved: Reserved field for further improvement of RREQ message.

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Hop Count: This field registers the hop counts that RREQ passes from the source node to the 366 current node.

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RREQ ID: This field is the unique identity of RREQ message.

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RUD: Rescue Urgency Degree, which is inversely proportional to the seismic intensity value in 369 this area.

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QL: Quadrant Level, which is the number of times the quadrant divided.

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QW: Weights of nodes in quadrant.

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where QW is the total weight of one node in quadrants with different levels. is the weight of node located in Quadrant .

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In this section, we propose FQMMBP protocol based on FQMM. Routing discovery and routing 425 maintenance process are illustrated in Figure 10 and Figure 11 respectively.

Initializing of Source Node
Is there a valid route to ECV in the Routing

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The description of FQMMBP routing maintenance process is completed.

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The simulation environment is a 10km × 10km earthquake disaster area, in which the epicenter 470 of the most severely affected area is 6km × 4km. There are 48 mobile rescue nodes, two fixed UAVs 471 and one ECV. The simulation setup and respective parameters are detailed in Table 3. 472 Table 3. Simulation Parameters.

Parameters Values
Number of mobile rescue nodes (M) 48 Fixed communication nodes(ECV included) 3 Simulation time span 600s Area of earthquake affected regions 10km*10km a1 6km b1 4km

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In this section, four routing protocols, e.g., FQMMBP, AODV, DSDV and DSR are analyzed 475 from the perspective of package delivery rate (PDR), end-to-end delay (delay) and overhead.   Figure 13 shows the performance of End-to-end delay (delay) for the different studied 504 protocols. In terms of delay, FQMMBP is significantly lower than other classical routing protocols.

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During the simulation, the average delay decreased from 1.44s of DSR to 0.55s of FQMMBP, which 506 proves the significant improvement of communication performance. This is because in each rescue 507 area, the distribution of mobile nodes tends to be uniform under FQMMBP, which increases the 508 probability that each node can find the next hop node to communicate with. Within the coverage of 509 each relay node, the existence probability of candidate next hop node increases, which shortens the 510 time to find the candidate next hop node as well as the time to transmit data to ECV.

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Performances of four compared protocols are almost the same before 150s. This is because at  transmissions rely on fixed relay nodes for forwarding, so the overhead is increasing. In the period 543 of 300s to 450s, the distribution of rescue nodes is scattered, and the average distance between 544 rescue nodes and destination nodes is close. In that case, the number of data forwarding is reduced, 545 so the overhead is reduced. After 450s, the distribution of rescue nodes is more scattered than 546 before, and the data communication between nodes relies on multi-hop forwarding, which makes 547 the data transmission overhead increasing. After 600 seconds, the overhead of FQMMBP can be 548 controlled within 2.5%. Although the performance of FQMMBP in overhead is not as good as the 549 other three protocols, it is worthwhile for the emergency rescue.      Table 1. Parameter definition and description 682

RUD
Rescue Urgency Degree, which is inversely proportional to the seismic intensity value in this area. The closer the area is to the epicenter, the lower the value of RUD is, which means the disaster is serious and the priority of rescue is high; Otherwise, the opposite. When RUD = 1, the epicenter disaster area is a rectangle; When RUD > 1, the disaster areas are rectangle rings that expands outwards in turn.