Representing data in combination of QAM and BPSK I have a question regarding the data representation in different modulations. Assuming we are using MIMO system whith $4$ $T_x$ x $4$ $R_x$  which means four transmission antennas by four receiving antennas. My question, if three antennas are using QAM modulation, and the other antenna uses BPSK modulation. Is it possible the Receiving antennas can distinguish the data sent from each antennas ? I mean is it possible to detect the data sent from transmission antennas? 

I'm asking that because in QAM modulation, data will be represented with real and imaginary parts, where in BPSK, data will be represented in real part only. So I see it logic that receiving antenna can detect the data perfectly. But I'm not sure.
Let's us take the simple case $2\times 2$ MIMO. Let $y_1$, and $y_2$ be the received signal at the first and second receive antennas, respectively. Then we have

$$y_1 = h_{11}x_1 + h_{21}x_2 + n_1$$

and 

$$y_2 = h_{12}x_1 + h_{22}x_2 + n_2$$

where $h_{ij}$ is the channel coefficient from transmit antenna $i$ to receive antenna $j$, $x_i$ is the transmitted signal from transmit antenna $i$, and $n_i$ is the AWGN at receive antenna $i$.

We can write the above signals in matrix-vector form as

$$\mathbf{y} = \mathbf{H}\mathbf{x} + \mathbf{n}$$

Apparently, there is interference here. So, we need to do some sort of equalization. Let's use zero forcing (ZF) for simplicity. In this case you get

$$\mathbf{H}^{-1}\mathbf{y} = \mathbf{\tilde{x}}=\left[\begin{matrix} \tilde{x}_1\\\tilde{x}_2\end{matrix}\right]$$

where $$\tilde{x}_i = x_i + \tilde{n}_i$$

for $i = 1,\,2$. 

Now, suppose that $x_1$ is 4-QAM, and $x_2$ is BPSK, both have the same symbol time. Will that be a problem? and why? They seem separable to me, and can be detected given that the receiver knows in advance that $x_1$ is drawn from a 4-QAM constellation, while $x_2$ from a BPSK constellation. Actually we do this in OFDM. It's called bit loading, where different subcarriers are modulated with different modulation sizes, and schemes when the CSI is know at the transmitter. Of course in OFDM the symbols are separable because of the orthogonality of the subcarriers, but we can somehow separate the signals in spatial multiplexing, although not perfectly.