Doesn't the receiver have to send back an acknowledgement frame? So how can it send back a frame, since a simplex channel only allows transmission in one direction?
Some writers use the term "simplex" to mean that communication is only possible in one direction. Others use it to mean that communication is possible in both directions but not at the same time; this is also called "half-duplex". I suspect that the protocol you're talking about is for a half-duplex channel: the sender transmits for a while and then pauses to allow the receiver to use the channel in the opposite direction for acknowledgments.