ADI-FDTD scheme was introduced to overcome the Courant limit. Due to its unconditional stability, the time step is only restricted by numerical precision, instead of the CFL conditions [1]. Recently, the accuracy restriction of the ADI has been studied [2]. This limitation is imposed by the truncation error on the time step. It shows that its truncation error grows with square of the time increment multiplied by the spatial derivatives of the fields. Several schemes have been developed [3] attempting to improve the accuracy of the ADI. However, in [3], the scheme was just 4th order approximation in space and the paper did not provided any numerical examples. In this paper, a new unconditionally stable scheme is presented based on the SBTD (sampling bi-orthogonal time-domain) algorithm [4]. In theory the SBTD has no spatial discretization errors owing to the sampling (interpolation) property and compact support of the Daubechies wavelets, which lead to higher precisions than the regular ADI scheme. Stability investigation and numerical examples of a 2D resonator are conducted to demonstrate the advantages of the new approach in terms of the CPU time and numerical accuracy.
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