Why do you need ARQ retransmission?

Question

Why do you need ARQ retx, however max HARQ retransmission is failed even if you retransmit at ARQ level it may fail then why do we need it?

Answer 1

All the time the channel condition won't be worst. So after maximum HARQ retransmissions also, decoding of Mac pdu (tb) fails, ARQ retransmission will be done in RLC in AM mode. In next ARQ retransmission ( again maximum HARQ retransmissions occurs) ,the MAC pdu may be decoded successfully. If max ARQ retransmissions occur Packet will be lost at RLC level.

Comments

Hi Himani... the mechanisim used to recover data through ARQ and HARQ is different.

HARQ is fast way to recover original data, here in each retransmission starting position of data to decode will be variey.

Lets consider 100bytes of original data is transmitted and its not successfully decoded, here in this case HARQ retransmissions will start recover original data with different starting positions of circular buffer (1-25(RV=0), 26-50(RV=1),50-75(RV=2),75-100(RV=3)). this position is denoted by redundency version (RV). we have 4 redundency versions 0 to 3. and the order redundency version is 0,2,3,1. Hence recovery of data can be very quick.


But in case of ARQ, if 100bits of original data not decoded, when MAX HARQ retransmissions also not able to recover the original data then ARQ retransmission triggers which retransmitts complete 100buts of data again.

Hope its useful and clear, if you have any more queries please feel free to ask.

Regards,
Suchi