1. PMI Feedback Mechanism
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Document Number: C802.16m-09/0154
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Yang SONG, Liyu Cai, Hongwei YANG Voice:
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Re: IEEE m-08/052 Call for Comments on Project m System Description Document (SDD) – Section
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For m SDD discussion and adoption
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2. Current PMI feedback status in SDD
Due to the fact that channel variation is slow in time and/or frequency domain for slow mobility mobile station, and the sequence of the channel codewords is highly correlated, differential precoding matrix codebook is proposed to reduce the feedback overhead. Instead of full PMI, three different feedback modes for the PMI shall be supported for differential codebook:
The standard mode: The PMI feedback from a mobile station shall represent an entry of the base codebook. It shall be sufficient for the base station to determine a new precoder.
The adaptive mode: The PMI feedback from a mobile station shall represent an entry of the transformed base codebook according to long term channel information. (The transform method utilizes the channel correlation information to enhance the performance of the standard codebook especially in the highly correlated channel.)
The differential mode: the feedback from a mobile station provides a differential knowledge of the short-term channel information. This feedback represents information that is used along with other feedback information known at the base station for determining a new precoder.
To prevent error propagation, the standard mode feedback will be sent periodically.

3. Problem to solve
Once the codebook is designed, it is hard for current differential feedback to alter the precoding precision to track the channel variation speed since the channel may vary very fast or slow time to time.
What we need is a feedback mechanism that is of:
Reduced feedback overhead
Flexible to match the channel variation speed with different resolution levels
More resolution levels may be desired
Benefit: reduced PMI feedback overhead
Challenges on feedback:
Fast and flexible switching between codebook resolution level and/or branch
coarse resolution level PMI for fast changing channels
fine resolution level PMI for slow changing channels

4. Two types of PMI feedback
Solution
Two types of PMI feedback
Absolute and relative PMI index feedback
Control signaling feedback (via a physical layer control channel or via a higher layer signaling message)
Indicate PMI resolution level
moving up or down the resolution
maintaining a certain resolution level
Indicate same PMI used by transmitter in the next few feedback intervals so that no redundant PMI feedback is transmitted, also indicate quick recovery from the silent feedback state
Etc.

5. Proposed comment to SDD
Add to section (line 35 on page 102, SDD IEEE m-08/003r6)
‘Besides absolute or relative PMI feedback, control signaling feedback can be used to indicate the PMI level changing, PMI holding on/off, etc.’