Definition of MIMO (Multi-input Multi-output ),does this output mean antenna or transmitter?
I want to ask the definition of MIMO(Multi-input Multi-output ) If there are one transmitter and one receiver,each of them has N antennas,N>1. So i can say it is a MIMO system.


Any communication system comprising of a transmitter with $N &gt;1$ and receiver comprising of $M&gt;1$ antennas is considered to be a MIMO system. So, this is not only limited to $N=M$ in the way you describe it.


  I think it should be called MISO,because each transmitter just has only one antenna.


No, I think it should be called MU-MISO (MultiUser MISO) because it's what it is; you're transmitting to multiple users (each with a single antenna). 


  However,in this paper,it called this system "MIMO" system.


In some applications, MU-MISO and MU-SIMO are seen as MIMO.