Spatial multiplexing is where multiple independent streams are transmitted across multiple antennas. If the receiver also has multiple antennas, the streams can be separated out using spatial multiplexing.
Instead of increasing diversity, multiple antennas in this case are used to increase the data rate or capacity of the system. In a rich multipath environment, the capacity of the system can theoretically be increased linearly with the number of antennas when performing spatial multiplexing.
Even two appropriately spaced antennas appear to be sufficient to eliminate most deep fades, which paints a promising picture for the potential benefits of spatial diversity. One main advantage of spatial diversity relative to time and frequency diversity is that no additional bandwidth or power is needed in order to take advantage of spatial diversity. The cost of each additional antenna, its RF chain, and the associated signal processing required to modulate
or demodulate multiple spatial streams may not be negligible, but this trade-off is often very attractive for a small number of antennas,
However, unlike transmit diversity and beam-forming, spatial multiplexing works mainly under good SINR conditions.
A 2 × 2 MIMO system doubles the peak throughput capability of LTE but this is unlikely to be possible for all users in the cell due to variation in SINR.The capacity, or maximum data rate, grows as when the SINR is large.
When the SNR is high, spatial multiplexing is optimal. On the other hand, when the SINR is low, the capacity maximizing strategy is to send a single stream of data, using diversity pre-coding. Although capacity gain is much smaller than at high SINR, the capacity still grows approximately linearly with since capacity is linear with SINR in the low-SINR regime.
If the mobile station has only one antenna, LTE can still support spatial multiplexing by coding across multiple users in the uplink. This is called Multi-User MIMO (MU-MIMO).
The matrix used for two antennae spatial multiplexing is shown below.