In this paper, performance of providing WiMAX from High Altitude Platforms (HAPs) with multiple antenna payload will be investigated as well as considering the coexistence capability with multiple-operator terrestrial deployments. A scenario of a HAP and terrestrial WiMAX base stations deployed inside a HAP coverage area with a radius at 30 km will be proposed. HAP cellular formation with different reuse patterns will be established. Coexistence performance will be assessed in terms of HAP downlink signal interfered by terrestrial WiMAX deployment. This paper will show it is effective to deliver WiMAX via HAPs and can efficiently share the spectrum with terrestrial WiMAX systems.