![]() Once a packet is sent from a process, it waits for an ACK/NACK. ![]() HARQ principle with multiple processes Why multiple SAW processes are required? The protocol will continue to repair one transmission without hindering other ongoing transmissions which can continue in parallel. HARQ is a stop and wait ( SAW) protocol with multiple processes. For this to work, the sending entity need to buffer the transmitted data until the ACK is received since the data needs to be retransmitted in case a NACK is received. This helps in the performance of re-transmissions. When the receiver receives the re-transmitted data, it then combines it with buffered data prior to channel decoding and error detection. If the received data is erroneous then the receiver buffers the data and requests for a re-transmission from the sender. HARQ is implemented to correct the erroneous packets coming from PHY layer. The hybrid-ARQ mechanism in the MAC layer targets very fast retransmissions and, consequently, feedback on success or failure of the downlink transmission is provided to the gNB after each received transport block (for uplink transmission no explicit feedback needs to be transmitted as the receiver and scheduler are in the same node). NR uses an asynchronous hybrid-ARQ protocol in both downlink and uplink, that is, the hybrid-ARQ process which the downlink or uplink transmission relates to is explicitly signaled as part of the downlink control information (DCI). PDCP protocol: This will guarantee in-sequence delivery of user data and it is mainly used during handover as RLC and MAC buffers are flushed when a handover is executed.Compared to the HARQ acknowledgments, the RLC status reports are transmitted relatively infrequently. So, for dealing with those errors, RLC has a slow retransmission system but with a feedback protected by CRC. RLC protocol: Even though HARQ is present at MAC but there might still be some possibility of errors in the feedback system.MAC protocol: It implements a fast retransmission system with delay less than 1ms in new radio, called HARQ ( Hybrid Automatic Repeat reQuest). ![]() 5G NR (New radio) has several retransmission systems using three different layers in the protocol stack: ![]()
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