The MIMO naming convention seems confusing to me.

1. SISO – Single Input Single Output system – 1 1 Tx antenna , 1 1   Rx antenna
2. SIMO – Single Input Multiple Output system – 1 1   Tx antenna, N R  NR Rx antennas (N R >1 NR>1 )
3. MISO – Multiple Input Single Output system – N T  NT Tx antennas, 1 1   Rx antenna   (N T >1 NT>1 )
4. MIMO – Multiple Input Multiple Output system – N T  NT Tx antennas, N R  NR Rx antennas (N T ,N R >1 NT,NR>1 )

In the above MIMO type 2 (SIMO), the name is single input and multiple output. So, I basically, assumed from a perspective of a given system, he has 1 RX antenna and N transmit antenna. That is, transmit by the given system counterpart. E.g. if a given system is an UE then counterpart would be a base station.

But the above definition says SIMO as 1 TX antenna and N rx antenna. So, what is the meaning of input and output in these defnitions?

Actually that's backwards.   The terminology is from the perspective of the channel, so Single Input Multiple Output means that there is a single transmit antenna and multiple receive antennas in a particular link.   An example would be a single antenna transmitter sending a signal to a receiver with diversity reception.

Thanks a lot ... this really helps

When you say system, it includes both UE and Base station. I feel it is wrong to isolate one of them and call it a system. One of the main aspect of this system is that a UE or a BS can only perform a transmission or reception at a time - they cannot do both. One node will be output (transmitter) and another will be input (receiver). In this case, if you view from the perspective of a channel, a SIMO system gives single output from node and gives it to the channel (single input). The channel multiplies it into N channels and outputs it from the channel (multiple output). This is input to the N antennas. Hence, the MIMO system is coined from the point of view of channel. If the UE -> BS link is SIMO (1xN), the counterpart (BS -> UE) is MISO (Nx1).