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Project IEEE 802.16 Broadband Wireless Access Working Group <http://ieee802.org/16>

Title CL-MIMO: New codebook and feedback mechanism

Date Submitted

2008-05-10

Source(s) Ron Porat, Wee Peng, Lei Wang, Yair Bourlas, NextWaveFrank Zhou, Louay Jalloul Beceem

E-mail: rporat@nextwave.com fzhou, jalloul@beceem.com

Re: 802.16 Working Group Letter Ballot #26c

Abstract This contribution summarizes contributions C80216main-08_192r1 and C80216main-08_093r3 [or the latest versions] which describe new CQICH types for UL feedback and new codebook design

Purpose For discussion and approval by 802.16 Rev2

Notice This document does not represent the agreed views of the IEEE 802.16 Working Group or any of its subgroups. It represents only the views of the participants listed in the “Source(s)” field above. It is offered as a basis for discussion. It is not binding on the contributor(s), who reserve(s) the right to add, amend or withdraw material contained herein.

Release The contributor grants a free, irrevocable license to the IEEE to incorporate material contained in this contribution, and any modifications thereof, in the creation of an IEEE Standards publication; to copyright in the IEEE’s name any IEEE Standards publication even though it may include portions of this contribution; and at the IEEE’s sole discretion to permit others to reproduce in whole or in part the resulting IEEE Standards publication. The contributor also acknowledges and accepts that this contribution may be made public by IEEE 802.16.

Patent Policy The contributor is familiar with the IEEE-SA Patent Policy and Procedures:<http://standards.ieee.org/guides/bylaws/sect6-7.html#6> and <http://standards.ieee.org/guides/opman/sect6.html#6.3>.Further information is located at <http://standards.ieee.org/board/pat/pat-material.html> and <http://standards.ieee.org

/board/pat>.

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Introduction

• This presentation covers two related CL-MIMO contributions: – C802.16maint-08/192r1 describes an

enhanced CQICH feedback mechanism to support CL-MIMO

– C802.16maint-08/93r3 describes a 4Tx 4 bit codebook and 2Tx 3 bit codebook

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Problem Statement

• The current 802.16e standard doesn’t provide a complete CL MIMO solution– The feedback mechanisms are not very well defined

or missing essential elements • The current CQICH feedback mechanism doesn’t provide

flexibility for feeding back multiple bands• Requires the use of Feedback Header signalling which has

robustness/overhead limitations

– The codebooks provided for 4 antenna BS are: • Too small (3bit) with suboptimal performance• Complex to implement (6bit) with small performance gain • Not suitable for correlated antennas

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REV 2 Design Goal for CL-MIMO

• Add codebook based CL-MIMO with the following attributes:– Provides good DL performance for several

known antenna configurations• 4 vertically polarized closely spaced antenna array• 2 closely spaced cross-polarized array• 2 widely spaced cross-polarized array

– Reduced MS search complexity– Efficient and robust UL feedback mechanism

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Modified CQICH Feedback Mechanism

• Modify the CQICH to support two new CQICH types:– 6 bit even/odd where 6 bits are mapped onto 3 tiles

(half slot)– 18 bits where each 6 bits are mapped onto 2 tiles (full

6 tiles slot)• Benefits

– Double (or triple) the payload of the CQICH channel at the expense of marginally reducing the reliability.

– Note that the reliability of these CQI channels is still better than the Feedback Header at QPSK 1/2

– Another advantage of mapping to half slot is increased granularity – users can occupy only half a slot or multiples of half slot and allow better utilization of the fast feedback channel by more users.

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Modification to the CQICH_Enhanced_Alloc_IE

• Reserved bits in the CQICH type fields are reclaimed to indicate the new CQICH 6 and 18 bit mapping

Syntax Size (bit) Notes

CQICH_Enhanced _ALLOC_IE -- --

… … …

CQICH Type 3 0b000: 6-bit CQI0b001: 18-bit CQI0b010: 3-bit CQI (even)0b011: 3-bit CQI(odd)0b100: 6-bit CQI (primary)0b101: 4-bit CQI (secondary)0b110: 6-bit CQI (even)0b111: 6-bit CQI(odd)

… … …

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Mapping of 6 bits onto CQICH tiles

• Uses the same principle mapping of Table 389 – Enhanced fast-feedback channel subcarrier modulation with the following modifications: – The mapping of Fast-feedback vector indices

per tile shall be CQICH type dependant as shown below:

• CQICH type 0b110 (even): Tile(0), Tile(2), Tile(4) • CQICH type 0b111 (odd): Tile(1), Tile(3), Tile(5)• CQICH type 0b001 (18 bit mapping in a full slot):

– Bit17 – Bit12: Tile (0), tile (3)– Bit11 – Bit6: Tile (1), tile (4)– Bit5 – Bit0: Tile (2), tile (5)

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6 Bits Encoding

• 4 antenna BS – PMI (4b) and differential CINR (2b)

• 2 antenna BS – PMI (3b), RI (1b), and differential CINR (2b)

• Note that in a 2x2 channel the smallest eigenvalue fluctuates much more than in a 2x4 channel and requires more frequent rank adaptation

PMI (4b)

Differential CINR (2b)

MSB LSB6 bits

PMI (3b)

Differential CINR (2b)

MSB LSB6 bits

RI(1b)

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Feedback operation

• PMI and differential CINR is typically reported every frame for up to three bands using for example: – 6 bits half slot even/odd to report one band – 18 bits full slot to report 3 bands – etc.

• Flexible design enables high geometry users to feedback 3 bands and low geometry users one or two preferred bands

• FH type 0110 to feed back typically every 8 frames– Band bitmap (12b) + 3x(best bands CINR (5b) + Rank (1b) ) = 30– Whether rank is common or per band is determined by the

feedback type of the Enhanced allocation IE

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Feedback operation – cont’d• N precoders for the different bands, are signaled in the CQICH

channels based on the allocated CQICH types and CQICH_Num when feedback type is set to 0b100 (8.4.5.4.16).

– If CQICH type = 0b100 and CQICH type = 0b101 (6bit primary / 4 bit secondary), (as per current R2/D4)

• 6 bits carry best common precoder for the 3 selected bands and differential CINR for lowest numbered band

• 4 bits carry differential CINR for second and third bands. – If one or multiple CQICH Type = 0b000 (6 bit full slot), 0b110 or 0b111

(6 bit in half slots)• If N=3, the MS feeds back PMI (4b) and Differential CINR (2b) for all

3 bands in the order of lowest band to highest as indicated in the 12b band bitmap.

• If N<3, the MS reports PMI and differential CINR on the allocated band and best band other then allocated band

– If CQICH Type = 0b001 (18 bit CQI), the MS feeds back the PMI and differential CINR in the following order:

• The first group of 6 bits carries the lowest AMC band , • The second group of 6 bits carries the second AMC band• The third group of 6 bits carries the third AMC band

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Modified CQICH 4Rx Performance

-8 -6 -4 -2 0 2 410

-4

10-3

10-2

10-1

100

SNR

code e

rror

rate

6bit CQICH performance - Ped B, 4Rx, 3kmph, Real CE

6 bit in one slot

6 bit in half slot

6 bit in one third slot

• Feedback header message under the same configuration and QPSK rate ½ requires SNR=4dB for payload error rate = 10-2.

• Note that here we assume that the MS doesn’t transmit anything but the FH in the same OFDM symbol.

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Modified CQICH 2 Rx Performance

-4 -2 0 2 4 6 810

-3

10-2

10-1

100

SNR

code e

rror

rate

6bit CQICH performance - Ped B, 2Rx, 3kmph, Real CE

6 bit in one slot

6 bit in half slot

6 bit in one third slot

• Feedback Header for 2-antenna BS requires SNR=11dB for payload error rate = 10-2.

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Codebook Design Principles

• The underlying idea is to design codebooks with values drawn from a simple alphabet i.e. QPSK

• The search complexity at the MS becomes substantially lower since no multiplications are required– Comparing to the 16e 6 bit codebooks, only 3% of the number of

multiplications are actually required. Note that some metrics exist that don’t require any multiplications at all for the QPSK codebooks.

• The codebooks were also designed to have constant modulus property ensuring power amplifier balance

• In addition the 4 bit codebook reduces UL overhead and provides much better overall balance between DL performance, UL overhead and MS search complexity

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2 Tx 3-bit Codebook DesignCodebook Index Rank 1 Rank 2

0

1

1

2

1

1

1

1

1

2

1

1

1

1

2

1

1

1

1

1

2

1

2

j1

2

1

jj

11

2

1

3

j

1

2

1

jj

11

2

1

4

2/)1(

1

2

1

j

2/)1(

1

2/)1(

1

2

1

jj

5

2/)1(

1

2

1

j

2/)1(

1

2/)1(

1

2

1

jj

6

2/)1(

1

2

1

j

2/)1(

1

2/)1(

1

2

1

jj

7

2/)1(

1

2

1

j

2/)1(

1

2/)1(

1

2

1

jj

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4 bit – 4 TX CodebookCodebook

Index Rank 1 Rank 2 Rank 3

0

1

1

2

1

j

j

[V0 V1] [V1 V2 V3]

1

1

1

2

1

j

j

[V1 V3] [V0 V2 V3]

… … … … … … … … 14

j

j

j

j

2

1 [V14 V15] [V12 V13 V15]

15

1

1

1

1

2

1 [V12 V14] [V12 V13 V14]

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4 antenna simulation results – Precoding per Band AMC (dB) loss compared to scheme 1 at high SNR Analog

Feedback 16e - 6 bits New - 4bit 16e - 3bits

4 closely spaced verticals

0.3 1.5 0.9 2.6

2 closely spaced cross-pols

0.5 1.2 1.2 2.7

2 widely spaced cross-pols

1.4 1.8 2.1 2.9

Scheme 1 assumes optimal precoding per subcarrier