Design analysis of high core count (36-core, 37-core) un-coupled multicore fiber (MCF) using the heterogeneous core has been proposed, to achieve the high spatial efficiency and ultra-low crosstalk (~-81 dB/100 km) under the limited cladding diameter of 300 μm. To realize the ultra-low crosstalk level between the neighboring cores, the combination of two crosstalk reduction approaches, trench-assisted (TA) core and propagation direction interleaving (PDI) technique has been used. The forward and backward crosstalk characterizations have been studied in heterogeneous MCF with respect to fiber bending radius, wavelength, transmission distance, and core-pitch with both normal step-index and trench-assisted cores under single-mode propagation condition.