SU-MIMO is within LTE, but at the time of this writing has not yet been fully identified. To implement the SU-MIMO UE require two transmitters. This is an important problem from the standpoint of cost, size and battery consumption, and for these reasons, SU-MIMO is not currently a priority for development. Moreover, increased rates of data transmission in the uplink, it may be possible to SU-MIMO is not so important since they are the downlink due asymmetric distribution of traffic.
Finally, if the system is to be realized by the limited capacity of the uplink may not be practical to increase the transmission power of the UE is sufficient to achieve the desired SNR in receivers ENB.
Although the UE, typically has a transmitter in the base configuration, it is still able to maintain the new shape MIMO. Unlike the reception requires MIMO transmitters are in the same physical device or location. It follows that the uplink MIMO can be implemented with two transmitters belonging to two different UE. This creates an opportunity to increase the uplink power – although the single user will not see any increase in the data rate.
The fact that the transmitters are physically separate two implications. Firstly, it is not possible precoding, because the source data can not be shared between the two UEs in order to create the necessary cross-coupling of data streams. This reduces the potential gains that co-located transmitters may have had. Second, the separation of the transmitter increases the likelihood that the radio channels seen by the eNB will be uncorrelated.
In fact, when the ENB should be noted that two UEs to connect to the MU-MIMO, the main criterion is the presence of de-correlated channels. Any loss of profit due to lack of pre-coding is more likely to offset gains from the best channel de-correlation; Therefore, the MU-MIMO can be a valuable method of increasing the capacity of the reverse link.
Restore LTE tolerated small mistakes time and frequency. Normal operation of the uplink, each UE will adjust the frequency with sufficient accuracy as ENB.ENB also instructed the UE to adjust its timing and power, so that all signals arrive at the receiver ENB approximately at the same level and time. An antenna located in another path of the transmitting device assumes correlated. These conditions allow the scheduler to control two ENB to UE transfer data simultaneously using the same subcarriers.
Multi-user MIMO involves the simultaneous transmission of codewords in layers of different UE, at the same time and frequency. Using conventional methods to ensure adequate control of RF power, and time alignment of the signals received in the eNB. Align the energy received from the UE, ENB is the most difficult to master, if the potential benefits of the ability to deliver.