The biggest challenge in spatial model is the switching of antenna elements at the data rate. Well if MIMO is used with OFDM or other multicarrier systems, the complexity and switching requirements may increase further.

This paper may be helpful

Spatial Modulation for Generalized MIMO: Challenges, Opportunities, and Implementation

http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6678765

Thank you so much brother "Maged Esmail"

But this paper introduces the SM challenges in single-carrier MIMO system only. They don't present the challenges in multi-carrier  (OFDM) MIMO systems.

There are many other researchers introduced SM for MIMO-OFDM but they don't discuss more about its challenges like those in the paper you mentioned. 

Spatial modulation with an array of M antennas can be implemented using either M RF chains or 1 antenna chain.

In the former case, spatial modulation can be performed digitally and can therefore be implemented independently for each subcarrier in OFDM. Note that one can also use the antennas for conventional beamforming techniques instead in this case. The performance gain is roughly log_2(M) with both techniques so spatial modulation will not provide any extraordinary improvements over conventional techniques in this case.

In the latter case, spatial modulation is performed by physical switching between which of the antennas that is connected with the RF chain at a given time instant. Conventional beamforming cannot be performed in this case. In a multi-carrier implementation with N carriers, you will send N symbols simultaneously and hence all of them must be sent from the same antenna. Hence, it appears that the gain from spatial modulation in OFDM is only 1/N of what it would be in a single-carrier transmission.