1. HF-A-MAP Scrambling for Error Handling (AWD-15.3.6.3.2.3)
Document Number: IEEE C80216m-09/1359
Date Submitted:
Source:
Yi Hsuan, Roshni Srinivasan, Hujun Yin
Intel Corporation
Venue:
IEEE Session #62, San Francisco, USA.
Re: AWD comments / Area: Chapter (DL-CTRL), “Comments on AWD DL-CTRL”
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N/A
Purpose:
For TGm discussion and adoption of m AWD text.
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2. False Detection of Assignment A-MAP
False detection happens when CRC check passes by mistake for a detection trial. This is caused by certain decoding errors such that error bits in the AMAP IE and CRC are aligned to allow CRC check to pass. Such decoding errors can be caused by
Channel impairment or RF impairment. This factor cannot be avoided in any blind detection scheme.
Rate matching or repetition mismatch. While present in LTE, this cause of false positive is eliminated in the current structure using only 1 AMAP IE size.
Because encoded assignment AMAP is not scrambled with any user specific sequence, each MS can decode assignment AMAP for other MS if the channel is ideal. However CRC check would fail because it’s scrambled by STID. With this in mind, false detection is less likely to happen for MS with good geometry and more likely to happens for cell edge users.
AMAP false positive detection can interrupt both DL an UL traffic, especially for persistent scheduling. LTE has many contributions on the analysis of the false activation frequency of semi-persistent scheduling (SPS).

3. False Detection Probability of 16m
Assume the worst case scenarios in terms of false detection:
20 MHz bandwidth FDD.
Fully loaded VoIP traffic.
21 LRU for AMAP in each subframe (21.9%). 19 LRU for assignment AMAP. 2 MLRU in each LRU.
Only QPSK ½ and 1 MLRU IE are used (38 channels and detection trials).
All detections have decoding errors (cell edge users).
Suppose the MS tries to decode all AMAP in each DL subframe, a false positive detection can happen every 216/(38*8*200)=1.08 second.
With false positive AMAP, MS can create invalid UL traffic, interfering other valid UL traffic. The impact is more pronounced with UL persistent scheduling.
Some techniques are needed to reduce the impact of false positive effect.

4. Null Detection of DL HARQ Feedback Channel (1/2)
If false detection of UL assignment AMAP happens, MS starts transmission of UL bursts using the wrong resource and transmission format. In addition, the DL Ack/Nack channel index would be wrong due to explicit or semi-explicit Ack/Nack indexing.
In this case, MS would decode an Ack/Nack channel that BS doesn’t use or use for a different MS.
If BS doesn’t transmit Ack or Nack in the HF channel, MS can implement a three-state Ack/Nack channel detection such that “NULL” can be detected if neither Ack nor Nack pass the threshold test. In this case, MS assumes that the previously received assignment AMAP is not valid and should stop transmission or retransmission.
This process is described in the error handling procedure of persistent scheduling in AWD.
This mechanism can stop false persistent transmission or retransmission of non-persistent HARQ process.

5. Null Detection of DL HARQ Feedback Channel (2/2)
When Ack/Nack index is wrong, it is possible that MS will try to decode the Ack/Nack channel intended for a different user. In that case, the MS can still decode Ack or Nack successfully based on the current DL Ack/Nack channel design. The aforementioned error-handling mechanism does not work in this case.
To fix the problem, DL Ack/Nack channel can be scrambled by STID. By doing so, if an MS tries to decode the HF-A-MAP intended for another MS, the repetition combining would be non-coherent and most energy would be cancelled in the combining process. Thus the probability that an MS decodes other users’ Ack/Nack channel successfully is much lower.

6. Proposed AWD Text Change:
Replace Figure 478 on page 189 with the following figure: Modify the following text from line 61 to 65 on page 189: Each HF-A-MAP IE carries 1 bit information. Depending on the channel conditions, the modulation can be QPSK or BPSK. If QPSK is used, 2 HF-A-MAP IEs are mapped to a point in the signal constellation. If BPSK is used, each HF-A-MAP IE is mapped to a point in the signal constellation. The repetition number, Nrep, HF-AMAP, is FFS. Repeated HF-A-MAP IE bits are scrambled by the Nrep, HF-AMAP LSBs of the STID of the associated AMS.
HF-A-MAP IE(s)
[2 bits if QPSK;1 bit if BPSK]
Repetition
STID
Scramble
QPSK/
BPSK
SFBC
HF-A-MAP IE(s)
[2 bits if QPSK;1 bit if BPSK]
Repetition
STID
Scramble
QPSK/
BPSK
HF-A-MAP symbols