How to calculate the channel gain if the channel is a $N$ by $M$ matrix,$N,M>1$
It is important to understand the what the channel matrix $\mathbf{H}$ physically means. The channel matrix is a symmetric matrix that contains the fading coefficients, that is, in row $i$ column $j$ the value ${h}_{ij}$ is the coefficient between the $i^{th}$ transmit antenna and the $j^{th}$ receive antenna. The gain between any transmit/receive antenna pair can be found by $|{h}_{ij}|^2$.