Example

In this example, we consider a qual-process SAW HARQ mechanism. These four par-allel HARQ processes are shared by multiple users. Thus, different HARQ processes may be assigned to transmit its data packets for a particular user depending on the

Tab. 4.2: An example of a Type-II gap being removed by the indication of receiving both new and old packets in a 4-process SAW HARQ mechanism.

Process ID in 4-process SAW HARQ

Cycle Process 1 Process 2 Process 3 Process 4

(TSN,S_c,NDI) (TSN,S_c,NDI) (TSN,S_c,NDI) (TSN,S_c,NDI)

i-1 ... ... ... ...

i (10,N→A,NEW) (11,NACK,NEW)

i+1 (12,NACK,NEW)

i+2 (13,ACK,NEW) (11,ACK,OLD)

i+3 (12,NACK,OLD)

i+4 (12,ACK,OLD)

i+5 (14,NACK,NEW)

multiple users’ requests in different cycles. Table 4.2 illustrates an example of the states of the qual-process SAW HARQ mechanism for a user from cycles i to i + 5.

Here the period of one cycle is equal to four TTIs. Each field in the table is filled with a triplet variable, (TSN, S_c, NDI). TSN stands for the transmission sequence number. S_c is one of the three events in the feedback control channel: receive an ACK without errors (denoted by ACK), receive a NACK without errors (denoted by NACK), and a NACK-to-ACK error occurs (denoted by N→A). NDI is either NEW or OLD. Empty fields in the table mean the time slots that are assigned to other users or idle. In this example, we want to show that both the NDI in the control channel and the TSN in the data channel can be used to identify an unrecoverable Type-II gap. In this example, assume that packets with TSN = 0 ∼ 9 of a particular user have been transmitted successfully by cycle i − 1. Now this target user has five packets with TSN = 10 ∼ 14 requested for transmissions from the RLC layer. In the MAC layer, a scheduler will assign a number of HARQ processes to transmit these packets for this user in every 4-TTI cycle.

1. In cycle i, processes 1 and 3 send packets 10 and 11 for the target user, re-spectively, and both processes 2 and 4 are idle or used by other users. Assume that packet 10 is lost and its NACK signal is changed to an ACK signal, while the NACK signal of packet 11 is successfully sent to the corresponding process in the transmitter. The states of the four processes are (−, −, −, −), where

“−” stands for the NULL state. Note that the NACK signal for packet 10 is changed to ACK. The problem here is how the receiver knows the occurrence of the NACK-to-ACK error. This can be done by the help of the stall avoidance mechanism.

2. In cycle i + 1, packet 12 is scheduled for transmission in process 4. Assume this packet reaches the receiver successfully, but fails the CRC test. Hence, a NACK

signal is issued to request a retransmission. Up to now, packets 10, 11, and 12 are lost in process 1, 3, and 4, respectively. The HARQ mechanism still believes that these packets can be recovered by the normal retransmission procedures.

Since the stall avoidance mechanism is not started yet, the states of the four parallel HARQ processes are still in (−, −, −, −).

3. In the second TTI of cycle i + 2, processes 2 receives a new packet 13. When receiving packet 13, the receiver moves this packet to the reordering buffer of this user and makes this HARQ available for other new packets in the next cycle. However, packets 10, 11, 12 have not been received yet so that three holes for packets 10 ∼ 12 occur in the reordering buffer. To ensure these gaps can be filled in future transmissions, the stall avoidance mechanism is initiated to identify whether these missing packets are either recoverable Type-I gaps or unrecoverable Type-II gaps. Since packets 10, 11, and 12 will be transmitted by processes 1, 3, and 4 from a receiver viewpoint, the stall avoidance mechanism starts monitor the states of these processes. In the current situation with a new packet arriving at process 2, the states of the four parallel HARQ processes are (−, S_T, −, −) according to Fig. 4.1.

In the the third TTI of cycle i + 2, process 3 receives a retransmitted old packet 11 and pass the CRC test. Note that due to the requirement of Chase combining, the retransmitted packet 11 is sent by the same HARQ process 3 in cycles i and i + 2. Since the hole of packet 11 is filled in the reordering buffer, the reordering buffer contains packets 11 and 13. In the meanwhile, process 3 enters the STOP state according to Fig. 4.3. Thus, the states of four HARQ processes change to (−, S_T, S_T, −) and the stall avoidance mechanism keeps monitoring the states of processes 1 and 4.

4. In the forth TTI of cycle i+3, process 4 is scheduled to transmit an old packet 12

for the target user. Assume that packet 12 fails the CRC test again. According to the Fig. 4.1, process 4 enters the REQUEST state (S0) and the states of the four parallel processes become (−, S_T, S_T, S₀). During the REQUEST state S₀, the receive process of the HARQ process 4 requests a retransmission for packet 12 by sending a NACK signal to the corresponding transmit process.

Assume that this NACK signal successfully reaches the the transmit process, the state of process 4 changes to the RESCHEDULE state (S2). Thus the states of the four parallel HARQ processes are now (−, S_T, S_T, S₂).

5. In the forth TTI of cycle i+4, process 4 is scheduled to retransmit packet 12 for the target user. This time packet 12 passes the CRC test and the TSN of packet 12 is obtained. Based on the information of TSN, packet 12 is moved to the reordering buffer and fills its gap. From Fig. 4.1, the state of process 4 enters the STOP state and the states of the four processes become (−, S_T, S_T, S_T). At this stage, packets 11, 12, and 13 are in the reordering buffer and an empty space is reserved for the missing packet 10. The stall avoidance mechanism continues monitoring process 1, which is the only left process possibly transmitting packet 10.

6. In cycle i + 5, process 1 transmits a new packet 14 because the NACK signal of packet 10 is reverted to an ACK signal due to transmission errors in cycle i in the feedback control channel. Here, we assume that the new packet 14 is lost and a NACK signal is sent backed to the corresponding process in the transmitter.

Since packet 14 is a new packet, the NDI in the control channel becomes the NEW state. Without the information of TSN of this packet in the traffic channel, the process 4 enters the STOP state (S_T). This shows the advantage of using NDI to shorten the gap processing time in the HARQ mechanism.

According to Fig. 4.1 the status of the four parallel HARQ processes are now

(S_T, S_T, S_T, S_T). Because the four processes are all in the STOP state and the gap for packet 10 is still in the reordering buffer, it is implied that the missing packet 10 is a nonrecoverable Type-II gap. Hence, the HARQ process should no longer wait for packet 10. As a consequence, the available in-sequence packets 11 ∼ 13 should be forwarded to the upper layer and an RLC layer retransmission request is issued for packet 10.

In the above example, we have shown how the indicator-based stall avoidance mechanism can recognize Type-II gap with the aids of NDIs in the control channel and TSNs in the data channel. In the considered example, the NDIs of processes 1 and 2 are used to confirm that these two processes do not possibly retransmit the missing packet 10. In addition, the information of TSN of processes 3 and 4 in the user data channel is used to judge that these two processes will not transmit the missing packet 10, either. With the cooperation of the NDI in the control channel and the TSN in the data traffic channel, the indicator-based stall avoidance can realize the fast physical/MAC layer retransmission for the HARQ mechanism. In the following section, we will present the analysis of the gap processing time of the indicator-based stall avoidance mechanism according to state transition diagram shown in Fig. 4.2.