PURPOSE: A fast index search algorithm for a codebook-based beamforming system is provided to reduce the number of compared vector indexes and to reduce performance difference. CONSTITUTION: An angle of a complex number plane is obtained from a weighted value vector of the channel state information. The angle of a previous column is altogether reduced from the angle of a last column. The angle of the complex number plane is obtained from the weighted value vector of a code book. The angle of a previous column is altogether reduced from the angle of a last column. Specific vector indexes are selected. The specific vector indexes satisfy a range of -90~+90.
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