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In Multi-User MIMO (MU-MIMO), the Base Station (BS) schedules multiple users simultaneously on the same resources. Closed loop MU-MIMO uses explicit feedback from each of these users for scheduling and allocation of resources. In this scheme, an estimate of the SINR experienced is fed back by each user and is known as the CQI (Channel Quality Information). This CQI is used by the BS for link adaptation to the scheduled users. When Base Stations change their precoders in the next scheduling epoch, interference levels and therefore CQI estimates made previously may no longer be accurate, leading to reduction in system throughput. In this paper, the authors discuss the benefits of using unitary precoders to reduce this mismatch in CQI and thus improve system performance.
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