Communication Technology And Implementation On VDE-TER Link Layer

VHF Data Exchange System(VDES)is considered as an important component of the future maritime communication system by the International Maritime Organization(IMO),and it has become a research hotspot in recent years.On the basis of the existing Automatic Identification System(AIS),VDES adds the other two higher capacity functions:Application Specific Message(ASM)and VHF Data Exchange(VDE).VDE is divided into VDE Terrestrial(VDE-TER)and VDE Satellite(VDE-SAT).This thesis mainly studies the link layer communication technology of VDE-TER.The Random Access Channel(RACH)of VDE was first introduced in the International Telecommunication Union(ITU)Recommendation M.2092-0.All data transmission in VDE-TER uses RACH to access the link,so it is essential to study the access protocol of RACH as the starting point of communication.In the communication process of VDE-TER,all stations' time are synchronized,the resource request signaling packets are randomly sent on the RACH,and the packet size is the same as one single slot,which conforms to the usage scenario of Slotted ALOHA(S-ALOHA).The random access protocol of VDE-TER uses S-ALOHA was proposed in M.2092-0.This thesis combined S-ALOHA with VDE-TER RACH characteristics to proposed a new protocol called Adaptive Traffic Load Contention Resolution Diversity Slotted ALOHA(ATL-CRDSA).The terminals use ATL-CRDSA will always send two copies,in two randomly selected slots within the same frame.As the two copies have the same preamble and payload information bits,when one copy is demodulated successfully,the recovered information will be used to cancel the interference that the other copy may generate on the other slot.Thus,it greatly optimizes.the performance of the system.In order to further solve the problem of system throughput degradation under high load,the ATL-CRDSA protocol will estimate the load of the link,and introduce the selective send probability to reduce the payload.So the system can work in the vicinity of the optimal throughput.This protocol has important reference significance for the establishment of access protocol standard of VDE-TER link layer.Based on the work above,the link layer communication platform of VDE-TER is built by C++.The communication flow of VDE-TER link layer is simulated when part of the slot is occupied by AIS and ASM,and the performance of ATL-CRDSA is verified.Experimental results show that the normalized throughput of ATL-CRDSA can reach 0.55packet/slot,and the throughput performance of the system can be guaranteed under the condition of increasing RACH load,which keeps the communication stability of VDE-TER link layer.