This is because a wireless client & AP cannot communicate at the same time. If AP transmit then others should listen & If a client transmitt others should listen.
That's why it call half duplex. Watch this video for better understanding how it works (you require to register ciscolive365.com first, it is free)
https://www.ciscolive.com/online/connect/sessionDetail.ww?SESSION_ID=8197&backBtn=true
HTH
Rasika
*** Pls rate all useful responses ****