This paper presents a 140 GHz planar antenna array based on gap waveguide technology to be used in next generation backhauling links. Each antenna element in the bigger 3D-array is considered to have about 30dBi gain and each antenna element consists of several metal layers such as the cavity backed radiation layer and a corporate feed network layer. Micromachining based MEMS fabrication technology has been considered for realizing such high gain antenna element for the LOS MIMO array. The simulated performance of the single element antenna has more than 30 dBi gain and S11 lower than -10 dB over 132-150 GHz band corresponding to 14% relative bandwidth. Also we analyze the optimum array element separation in a 2D configuration for line of sight (LOS) MIMO system so as to support several independent channels between TX and RX links at 140 GHz.