Why we have 8 HARQ Processes for LTE FDD?

Question

Why can we have only 8 simultaneously harq process in the uplink.
Is it applicable to downlink as well.

Answer 1

Adding some more details on Salil's comment:

FDD has fixed timing associations between the original data and the re-transmission i.e. it takes 4 ms to get the response for a transmission. In addition, another 4 ms required to send either re-transmission(if received NACK) or new data(if received ACK) based on response received.
As TTI is 1 ms, one HARQ is occupied for 8 ms. What it means is, one HARQ is occupied for atleast 8ms before we can re-use it.

Here, 4 ms = propagation time + processing time

For TDD, as radio frame structure is different for different configuration, we have different number of HARQ processes as per configuration. Though number HARQs for different configurations are calculated with the same logic as above.

Answer 2

Due to the fixed timing associations between the original and the retransmission on the UL, the maximum number of HARQ processes has been fixed at 8.

In TDD due to the variable delays experienced with respect to ACK/NACK arrivals as well as the retransmissions, the numbers of HARQ processes are dependent on the configuration.