HYBRID Automatic repeat request
Among all the timers defined in LTE, the one associated to the
acknowledgment (ACK) of a UL physical frame at the MAC layer
is the most critical one. The reception status of any frame sent
through the air interface needs to be fed back to the transmitter,
to proceed to the transmission of a new frame ACK or to
attempt a retransmission negative ACK (NACK). This hybrid
automatic repeat-request (HARQ) operation is performed at the
MAC level, after all the physical processing of a codeword is
done (detection, demodulation, and FEC decoding). In LTE,
each frame sent at subframe n needs to be acknowledged (ACK
or NACK) at subframe n + 4 in both UL and DL directions, a
subframe lasting 1 ms [20]. Hence, the overall receive process
has to finish in 3 ms to stay compliant with the 3GPP LTE
HARQ timing. This timing includes the processing at the RAPs
of the physical blocks located before the split (see Figure 3 and
both functional split options therein), the processing at the
cloud-platform of the physical blocks located after the split and
the round-time trip through the BH. However, some algorithms
such as turbo-decoders underly a computational jitter which
implies that the decoding time may vary. Hence, it may happen
that packets are retransmitted even though they would have been
decoded with more computational resources, i.e., either more
time or more parallel processors. This computational jitter also
adds up to the overall delay that needs to be considered.
To relax the timing constraint for the receive processing, we
may adapt the HARQ process. The authors in [17] suggest for