Asynchronous HARQ in Uplink

Dear Experts,
In 5G, uplink HARQ uses asynchronous HARQ, why this was not possible in LTE?

Thank you

Asynchronous HARQ needs PDCCH scheduling. In LTE PDCCH resource is not as rich as in NR.

@upf Thank you for kind response; kindly elaborate what do you mean by richness.

5G needs PDCCH scheduling only when there is a HARQ NACK and DTX, (in both scenarios; anyway LTE will also need to have PDCCH scheduling?)

In addition, LTE will also use additional overhead (PHICH channel created to ACK/NACK every time)

Do you see any difference? Will welcome your opinion

Thank you

Do you mean UPLINK HARQ to be uplink HARQ feedback or PUSCH transmission process?

Synchronous harq reduces overhead
Asynchronous harq is more flexible

In lte, there are limited number of TDD patterns so it was easy to define fixed HARQ timings but 5G TDD is much more flexible so asynchronous is more suitable