MIMO and other transmit spatial diversity scheme is a newer application than receive diversity and has become widely implemented only in the early 2000s. As the signals sent from different transmit antennas interfere with one another, processing is required at both the transmitter and the receiver in order to achieve gain while removing or at least attenuating the spatial interference.
By using multiple antenna to transmit multiple path of information to UEs, either better throughput or lower SINR requirement can be achieved and the frequency selective characteristics of LTE is perfect for the implementation of such technologies.
In general there are two mode of MIMO, open and closed loop.
Additionally, if the multiple antennae are already at the base station for uplink receive diversity, the incremental cost of using them for transmit diversity is very low. Multiple antennae transmit schemes—both transmit diversity and spatial multiplexing—are often categorized as either open loop or closed loop. A high level signal processing diagram is shown below.
Open Loop MIMO
Open-loop systems do not require knowledge of the channel at the transmitter. As a result, open loop operations occur when the access network does not have information or feedback from the UE to do coding adjustment or signal is not good enough.
The below figure shows a possible N Antennae + M input layers setup in spatial multiplexing
Closed Loop MIMO
On the contrary, closed-loop systems require channel knowledge at the transmitter, thus necessitating either channel reciprocity—same uplink and downlink channel, possible in TDD—or more commonly a feedback channel from the receiver to the transmitter.
Hence, unlike open loop, closed loop operations occur when the access network execute dynamic adjustment based on feedback from the UE. The figure below shows a functional view of closed loop MIMO.
As a result, a more accurate coding application can be applied to the communication with the UE. The figure below shows where the pre-coding function may exist in a N Antennae with M input layers