An intensity channel model of wireless optical communication based space modulation is proposed and two basic condition are outlined. A vector signal space is defined by defining the two-dimensional (2D) orthonormal basis functions, and three critical measurements of performance, namely, probability of error, efficiency of spatial spectra and power efficiency are also defined as metrics for comparing candidate modulation schemes. Then three candidate modulation schemes exploiting spatial dimensions are proposed. At last, the three modulation schemes are compared by power efficiency and bandwidth efficiency. It shows that spatial quadrature amplitude modulation (QAM) scheme is appropriate for long distance wireless optical communications for its high bandwidth efficiency and spatial circular symmetry. And the spatial circular symmetry of QAM signal patterns is very suitable for realizing space synchronization in wireless optical links based spatial modulation.