How Does HARQ Compare to Wi-Fi ACK?

Here's a thought that I recently had when I looked at how the Hybrid Automatic Retransmission Request (HARQ) functionality works in HSPA and LTE: From a conceptual point of view HARQ is quite similar to the Acknowledgement mechanism of Wi-Fi. Here, the reception of each packet has to be confirmed by the receiver by returning a MAC Ack(nowledgement) frame back to the receiver. This is done in a way that the ACK package has precedence over any other packets that are waiting in the queues of other users of the system. If the ACK is not received, the sender automatically retransmits the packet with the same or a different modulation and coding scheme.

The HARQ mechanism of HSPA and LTE is pretty similar: Each transmission has to be immediately acknowledged on the MAC layer as well. If a NAK or nothing is received the transmission is repeated. When one goes into the details, of-course, there are fewer similarities. With HARQ, the system can use incremental redundancy to send a different version of the packet with different error detection and correction bits. In addition, several HARQ processes run concurrently so a transmission failure of a single packet does not stop the overall transmission. And then, HARQ uses an 'out of band' channel for the feedback, while the Wi-Fi Ack is a normal packet on the air interface.