How MAC Re-Transmissions and RLC Retransmissions are linked each other?

Question

For Suppose MAC HARQ Max Retransmissions are 8, RLC Max retransmission are 4. Please explain behaviour of RLC retransmissions for each HARQ Retransmiision?

Answer 1

Hi srpathi,

MAC re-transmission and RLC re-transmission are not related. Because both will happen independently, and also both can use different HARQ process.

For e.g say RLC has 1500 bytes of data to send. So MAC will select any of the HARQ process to send(E.g 5).

MAC can do 8 times of re transmission to send HARQ process 5. If this TB didn't reach the UE even after 8 re-transmission then RLC will attempt do re transmission now RLC re transmission count is 1 , again MAC can do re-transmit the TB 8 times.. If again this failed then RLC will send the same PDU to mac to do re-transmission , now RLC re-tx count is 2.again MAC can do 8 time re-transmission so similarly the same PDU can re transmit (8 (MAC retx)* 4(RLC retx)) 32 times. Every RLC re-transmission MAC can do 8 HARQ retx

Say e.g first time HARQ MAX re transmit is failed then it is not mandatory that RLC has to send same PDU which he sent before.
like in above example first RLC had sent 1500 bytes ,MAC HARQ retx is failed then again RLC will ask grant for RLC retx PDU, But this time If MAC has larger grant(say 2500 bytes) then RLC can concatenate the other PDU (of 1000 bytes) and will send 2500 bytes of data to UE. Now UE will extract those 1500 bytes and 1000 bytes separately in RLC (This is the function of HARQ re ordering). I hope you understand.

Happy Learning :)

Comments

very nice and clear explanation Jaganathan .... only doubt am having here is how RLC ARQ process mapped with HARQ process (if both are different). Receiving side how it will be identified.

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Hi sripathi,

RLC PDU will have SN's, So in HARQ retransmission's even if you use different process receiving ARQ will come to know the SN and he will re order properly. Say E.g PDU with SN 100 is transmitted in harq process 5, it exceeded MAX retx , In ARQ Retransmission's RLC will keep same PDU with SN 100, But MAC is free to select HARQ process , so this time MAC selected process 7, But receiving RLC knows SN is 100.

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Thanks alot jaganathan....really great explanation...