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The roadside unit (RSU) plays an important role in VANET environments for privacy preservation. In order to conserve the privacy of a vehicle, the issued certificate must be updated frequently via RSUs. If a certificate expires without being updated, the services for the vehicle will be terminated. Therefore, deploying as more as possible RSUs ensures that the certificate can be updated before it expires. However, the cost for allocating an RSU is very high. In this paper, we consider the roadside unit allocating problem such that the certificates can be updated before it expired. Previous researches focus on the roadside unit placement problem in a small city in which for any origination-destination pair the certificate is limited to update at most once. The RSU placement problem in which more than once certificate updates are required is discussed in this paper. The RSU allocation problem is formulated and the decision problem of the RSUs allocation problem is proved as an NP-complete problem. We proposed three roadside unit placement algorithms which works well for a large city. In order to reduce the number of required RSUs for certificate update, we also proposed three backward removing methods to remove the intersections found by the RSU allocation methods. Simulation results show that the proposed algorithms yields lower number of required RSUs than the simple method named the most driving routes first method. One backward removing method named the least driving routes first backward removing method was shown to be able to further reduce the number of required RSUs.

Allocation of Roadside Units for Certificate Update in Vehicular Ad Hoc Network Environments