ARQ Procedure

ARQ is one of the most important feature of RLC AM which is designed for reliable data traffic and some important mechanism is used for that.

This mechanism involves two main part as follows:

i) Transmitter side: Poll Request (Polling)

ii) Receiver side: Status Report carrying ACK, NACK etc.(Retransmission request)

Before going to ARQ Description first we know about difference between  ARQ and HARQ.

Difference between ARQ and HARQ

ARQ:

·        Works at RLC layer

·        If the received data has an error (as detected by ARQ) then it is discarded, and a new re-transmission is requested from the sender

HARQ:

·        Works at PHY layer but controlled by MAC layer

·        If the received data has an error then the Receiver buffers the data and requests a re-transmission from the sender.

·        
When the receiver receives the re-transmitted data, it then combines it with buffered data prior to channel decoding and error detection. This helps the performance of the re-transmissions.

  Retransmission(ACK/NACK)

Transmitter receive acknowledgement from receiver in behalf of AMD PDU.

If transmitter receive Negative acknowledgement (notification of reception failure by its peer AM RLC entity) in form of STATUS PDU from its peer.

According to status PDU transmitter will perform.

1.      If the SN of the corresponding AMD PDU falls within the range VT(A) <= SN < VT(S)

Then what will happened?

Ans: Consider the AMD PDU or the portion of the AMD PDU for which a negative acknowledgement was received for retransmission.

2.      In status PDU ,If AMD PDU or a portion of an AMD PDU is considered for retransmission

>>>if the AMD PDU is considered for retransmission for the first time then RETX_COUNT (Retransmission count) become a zero.

>>>if the AMD PDU or portion of the PDU, it is not pending for retransmission already increment the RETX_COUNT.

>>> RETX_COUNT = maxRetxThreshold, indicate to upper layers that max retransmission has been reached.

3.      If the AMD PDU can entirely fit within the total size of RLC PDU(s) indicated by lower layer at the particular transmission opportunity then

>>>Deliver the AMD PDU as it is except for the P field (the P field should be set according to sub clause 5.2.2) to lower layer;

>>>Otherwise segment the AMD PDU, form a new AMD PDU segment which will fit within the total size of RLC PDU(s) indicated by lower layer at the

particular transmission opportunity and deliver the new AMD PDU segment to lower layer.

Question 1: On What basis MAC will do HARQ Retransmission? RLC can use SN for Retransmission.

Question 2: Why we need both MAC HARQ Retransmission & ARQ Retransmission.

The ARQ functionality provides error correction by retransmissions in AM at Layer 2, whereas the HARQ functionality ensures delivery between peer entities at Layer 1.

The reasons for having a two-level retransmission structure, is to achieve both fast and reliable transmission. The hybrid-ARQ mechanism provides very fast retransmissions which is suitable for high speeds used in LTE, whereas the ARQ is responsible for reliability. HARQ feedback is fast and frequent to correct transmission errors as soon as possible, so that the end-to-end RTT is low. HARQ feedback is more susceptible to error as compared to ARQ feedback. For high-speed transmissions using protocols like TCP, low packet error rate is desirable and ARQ adds robustness to the transmission. In addition, services like voice which place more importance on the RTT might skip ARQ and use the UM mode. Thus, the combination of HARQ and ARQ can be used intelligently depending on the services and their error and delay requirements.

In LTE-FDD, eight HARQ processes are available in both downlink and uplink. The typical delay between the original transmission and the first retransmission is 8 ms.  The HARQ process to which a transport block belongs is identified by a unique three bit HARQ process Identifier (HARQ ID).Hence,MAC will do HARQ retransmission once it receives NACK from from the intended receiver for the corresponding HARQ processor id. It is operational on both the UL and the DL.  For the ARQ operation there is a RLC SN which will be used when sending the ACK/NACK message