1. ACK and NACK Feedback Schemes for HARQ Operation
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Document Number:
IEEE S80216m-08/294
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Hung-Yu Wei, Ching-Chun Chou Voice: [Telephone Number (optional)]
Department of Electrical Engineering,
National Taiwan University
Chun-Yen Wang,
Richard Li ,
ITRI,
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IEEE m-08/016 - Call For Contribution on Project m System Description Document (SDD), shoot for “Hybrid ARQ (protocol and timing)” topic
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IEEE C80216m-08/294r5
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2. Introduction We proposed ACK/NACK schemes for HARQ
Parallel NACK
Busy Tone NACK
NACK-preamble NACK
ACK schemes
Parallel ACK / Polling ACK / Periodic ACK
The proposed schemes could provide reliable transmission and flexibility to the system
Combined scheme may be more useful.
Today I am going to introduce our ACK and NACK schemes for HARQ operation. We provide 3 design schemes for both ACK and NACK operation. These schemes would provide reliable transmission and efficient HARQ mechanism.

3. Parallel NACK Scheme
Each MS is assigned a non-interfering radio resource block
NACK messages could be sent to the BS in parallel
The transmission is done within the same frame.
The radio resource block could be
Time slot/Frequency channel/CDMA code channel/
OFDM channel/Other wireless channel
Multiple NACK could be sent simultaneously.
It would consume more resources for single data block’s retransmission.
I will introduce the 3 NACK schemes first. The first one is the Parallel NACK scheme. Each MS transmits NACK back to the BS in parallel. They use non-interfering or orthogonal radio resource block assigned by the BS. Multiple NACK could be sent within the same frame, while consume more radio resources.

4. Example: Parallel NACK Scheme
BS transmits data to MS (1,2,3,4,5).
MS1 & MS3 didn’t receive the data block.
The NACK is transmitted using the Parallel NACK scheme to the BS
MS2,4,5 don’t need to trigger NACK.
The radio block is left unused.
Unused radio resource for (MS2,4,5)
This is the example for Parallel NACK. BS transmits downlink data to MS 1 through 5. There are 5 subordinate MS, so BS assigned 5 radio resource blocks for NACK. These unique radio resources are illustrated as blocks on the figure. As MS1 and MS3 didn’t receive the BS transmitted data, they sent their separate NACK to the BS using the assigned blocks. The blue blocks stands for the NACK here. On receiving the NACK from MS1 and 3, the BS initiates the retransmission for the lost data. MS2, 4, and 5 successfully receive the data, and thus their radio blocks are left unused.
NACK by MS1
NACK by MS3
BS data
BS re-tx data

5. Busy Tone NACK Scheme
Busy Tone is a predefined sequence/code to be transmitted at a given time.
The tone is sent using preconfigured wireless channel
MS transmits Busy Tone to the BS to indicate NACK
When an MS does not receive the anticipated data block
BS would receive one or more Busy Tone for NACK
BS knows at least one MS does not receive the data block.
BS retransmits the missing data block.
Busy Tone is a predefined sequence or code, and it should be transmitted at a given time, using preconfigured channels. The Busy Tone indicates NACK, and the BS performs data retransmission on receiving the tone. The trick is that the BS would receive one or more Busy Tone from the MS. The fact of receiving Busy Tone indicates data loss, and the retransmission could be done immediately.

6. Example: Busy Tone NACK Scheme
data 1
BS Downlink to MS1, MS2, MS3
MS1 rx
MS2
MS3
data 2
NACK
data 3
Re-transmission
This is the illustration for Busy Tone NACK scheme. BS transmits downlink data to MS 1, 2, and 3. The first data is successfully received by all the MS. Therefore, the preconfigured channel time for Busy Tone is not used. The given time is marked as the dashed blocks on the timeline. Data #2 is not received by the MS3, so MS3 transmit the Busy Tone to the BS. The Busy Tone is the red block on the figure. The Busy Tone indicates NACK for the BS, and the BS would trigger data #2’s retransmission. For data #3, MS1 and MS3 failed to receive the data at the same time. Two Busy Tones are sent to the BS simultaneously. This can be seen from the two red blocks on the figure. Although the BS would receive one or two or even corrupted Busy Tones, the BS knows that at least one MS doesn’t receive the data, and the BS would retransmit data 3.

7. NACK-preamble NACK Scheme
NACK-preamble: a predefined sequence
Indicating that the NACK IE is transmitted successively after the NACK-preamble.
If BS receives one or more preamble, then BS knows that there are data loss for the transmission.
NACK IE (Information Element)
The actual NACK message.
Containing detail information of NACK.
Node ID/HARQ parameters and configurations/
CSI/Other helpful information for data retransmission.
The NACK-preamble indicates data loss.
Even the NACK IE may be corrupted or collide with each other.
The NACK-preamble scheme is to use a NACK-preamble for the NACK operation. Similar to the Busy Tone design, the NACK-preamble is a predefined sequence indicating the incoming NACK IE. The NACK IE contains the actual NACK message contents. Even if the NACK IE could be lost or collided, the NACK-preamble could still be received by the BS, and the retransmission could be done for the lost data.

8. Example: NACK-preamble NACK Scheme
There are two MS using the NACK-preamble NACK scheme.
The NACK IE might collide between the MS
The BS could still get the NACK-preamble.
This would initiate the retransmission.
Successful reception of NACK IE gives more retransmission information.
This is the example for NACK-preamble NACK scheme. NACK-preamble always goes before the NACK IE and the preamble indicates data loss. The NACK IE contains the detail messages of NACK, like node ID, HARQ configuration parameters, channel state information and other helpful information for retransmission.For the figures below, we can see that the NACK IE collided in the first transmission. However, the BS could still get 1 or more NACK-preamble, and the BS would know that there are 1 or more MS didn’t receive the data. The BS could retransmit the lost data. If the NACK-preamble and NACK-IE are received successfully, more reception information could be gathered for the retransmission, and the retransmission could be done more efficiently.
Collision of IE
Received NACK IE
Still get NACK-preamble
NACK IE 1
NACK IE
NACK IE 2

9. ACK Schemes Parallel ACK scheme Polling ACK scheme Periodic ACK scheme
Each MS is assigned a unique radio resource block for ACK.
Actually, the block could be ACK or NACK.
ACK could be transmitted in parallel to the BS.
Polling ACK scheme
The MS only transmits ACK when BS polls it.
The ACK message could contain the reception condition of multiple data blocks
Within the polling period.
Periodic ACK scheme
The BS configures the ACK sequence for the MS.
The MS sends the ACK periodically
According to the BS’ setting for the ACK period.
The ACK could still contain the reception condition.
The proposed ACK schemes are given here. The 3 schemes are parallel ACK, polling ACK and periodic ACK. Parallel ACK is similar to Parallel NACK, except that ACK is sent instead of NACK. In fact, the acknowledgement feedback could be ACK or NACK depending on system requirements. The Polling ACK is that the MS only transmit ACK when the BS polls it. The Polling ACK would contain the cumulative feedback information of the data received between two polls. The periodic ACK scheme is that the BS shall configure the ACK sequence for the subordinate MS. The MS transmit the ACK periodically according to BS’s setting of the ACK period. The ACK could contain the reception condition of the received data, like channel state information, etc.

10. Integrated Schemes
The NACK/ACK schemes could be applied simultaneously.
Better flexibility
The cooperation might yield better results.
The pros might be conserved and cons might be relieved
The integrated scheme is favored as:
Immediate ACK aids fast retransmission.
Lost data are reported immediately.
Delayed ACK reduces signaling overhead.
It contains cumulative feedback info and saves the number of signaling messages.
The combination of the two schemes would give more flexibility and better performance.
The integrated schemes is to combine ACK and NACK schemes to provide better performance, flexibility or efficiency. For all the above ACK and NACK mechanisms, they could be classified by the feedback latency. If the data reception state feedback is done instantly, this is called Immediate ACK. The advantage is data loss is reported immediately and retransmission could be done soon. For those schemes that data reception state are reported periodically, this is called delayed ACK. This type of ACK gains signaling cost advantages as cumulative feedback information is transmitted. If we integrate the ACK and NACK schemes, we can leverage the mutual advantages of both schemes.

11. Example: Busy Tone + Periodic ACK
The periodic ACK might suffer from larger retransmission delay.
Busy Tone NACK’s simple design gives less reception condition information.
The integrated scheme would relieve the disadvantages while still preserve the advantages.
BS data
Re-
transmission
No busy tone NACK is transmitted
time
busy tone NACK
Periodic ACK with cumulative info
Time period for periodic ACK
This is the sample illustration of integrated scheme. Busy Tone NACK and Periodic ACK schemes are used in this example. The Busy Tone takes advantages for the simple NACK design, while it gathers less feedback information. The Periodic ACK utilizes the cumulative feedback info and reduces the signaling overhead, but suffers from longer retransmission delay. The integrated scheme uses Busy Tone within the period of Periodic ACK. If there are data losses with the period, the retransmission could be done immediately. Cumulative feedback information of the data reception state could be sent by the Periodic ACK.

12. Conclusion We proposed ACK/NACK schemes for HARQ operation.
These schemes could be used under different HARQ requirement.
Channel state and other information could also be collected.
Integrated schemes could provide even better results.
Brief introduction are presented for our proposed schemes on ACK and NACK for HARQ operation. They could provide reliable data transmission and make for different HARQ requirements. Integrated schemes could also be used for better performance. Any questions?

13. Proposed Text 10.x.x ACK and NACK Design
Acknowledgement (ACK) and Negative Acknowledgement (NACK) are crucial for HARQ operation. The design criterions for the ACK and NACK could be considered from the following aspects.
10.x.x.1 Acknowledgement Feedback Latency
The timing of transmitting ACK and NACK could be immediately after the data retransmission, delayed for certain period, or the integration of the two schemes. For integrated ACK/NACK, multiple types of ACK/NACK are allowed for flexibility.
10.x.x.2 Channel Access and Allocation for Acknowledgement
The ACK and NACK mechanism should be able to operate on specific preconfigured channels. The number of preconfigured wireless channels could be the same as the node number or less. If the BS receives one or more acknowledgement indicating that certain data block is lost, the BS should trigger the retransmission for the specific data.