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.
Closed Loop MIMO
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.
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