A Novel Random Access Algorithm for Very High Frequency Data Exchange (VDE)

VHF Data Exchange System (VDES) is considered as an important component of the future maritime communication system by the International Maritime Organization (IMO). On the basis of the existing Automatic Identification System (AIS), VDES adds the other two higher capacity subsystems: Application Specific Message (ASM) and VHF Data Exchange (VDE). The Random Access Channel (RACH) of VDE was first introduced in the International Telecommunication Union (ITU) Recommendation M.2092-0. As the slot planning principle of RACH in VDE is by interval, which is significantly different from the continuous slot map for access algorithms in AIS, the existing slot access algorithms cannot meet the requirements of VDE. The simulation results show that the VDE slot map can reduce the normalized throughput of the existing algorithm by 39%. A novel random access algorithm called Adaptive Traffic Load Contention Resolution Diversity Slotted ALOHA (ATL-CRDSA) is proposed in this paper. The algorithm combines the load control strategy and contention resolution scheme to overcome the challenges of the new RACH of VDE. Simulation results show that ATL-CRDSA has remarkable improvement on RACH, making it very efficient and providing low latency of the packets. The insights gained from this study may be of assistance to the Media Access Control (MAC) layer design for upcoming versions of VDES standard.

An Improved Algorithm of Spectrum Access in OSA

We present a distributed spectrum sensing and access algorithm applied in opportunistic spectrum access (OSA) networks. It is proposed in the condition of cognitive radio (CR) users’ correct spectrum sensing. In each slot, a CR user will decide whether to sense, which channel to sense, and whether to access. However, due to the hardware and energy constraints, the CR users may not be able to sense or monitor all the channels, so we design this access algorithm derived from the theory of Partially Observable Markov Decision Process (POMDP). To simplify the analysis, we assume that the CR users will sense and access only one channel. Simulation results show that the proposed access algorithm can greatly improve the throughput of CR users than the random access algorithm. It has good access performance, and can quickly achieve stable access throughput as well. It is shown that with greater channel transition probability difference the CR users will obtain higher access throughput, and when the difference equals to zero this access algorithm becomes random access algorithm.