What is the relation between noise (AWGN) variance and number of antennas? I'm working in two Matlab scripts trying to simulate a MIMO system using OFDM (downlink) and SC-FDE (uplink).

My question is: for different numbers of transmit and receive antennas, what is the relationship between the AWGN variance and the number of antennas?
Does it depend if it is uplink or downlink?

I'm using matricial equations like: Y = HX + N , with matrix dimensions: Y(NR x length) , H(NR x NT) , X(NT x length) , N(NR x length)
Usually, the model is that every receiver antenna picks up noise of the same PSD, $N_0/2$. This means that the variance of noise samples in each quadrature branch is $N_0/2$. This noise is local to each antenna and is independent of the rest of the system.

To calculate the total amount of noise in the receiver, you multiply by the number of receiver antennas. This is usually not needed, though, since the figure of merit is the ratio of the energy per information bit (or symbol) and the variance of the noise affecting that bit (or symbol).