Sentence:  <WSD>Quadrature</WSD> LO generation is critical to the operation of direct-downconversion receivers [7]. Two common techniques for <WSD>quadrature</WSD> generation are divide-by-two frequency dividers, and polyphase filters. 
Meaning of the word marked with <WSD> tags:  The condition in which the phase angle between two alternating quantities is 90°.