doc.:
IEEE
802.11-10/1131r0
doc.:
IEEE
802.11-09/0161r1
Sept. 2010
Time-Domain CSI Compression Schemes
for Explicit Beamforming in MU-MIMO
Date: 2010-9-14
Authors:
Name
Affiliations
Address
Phone
email
Koichi Ishihara
NTT Corporation
Yusuke Asai
NTT Corporation
Riichi Kudo
NTT Corporation
Laurent Cariou
Orange Labs
Hikarino-oka
Yokosuka-shi, Japan
Hikarino-oka
Yokosuka-shi Japan
Hikarino-oka
Yokosuka-shi, Japan
4, rue du clos courtel
35512 Cesson-Sévigné
+81-46-8594233
+81-46-8593494
+81-46-8593140
+33 2 99 12 43
50
[email protected]
t.co.jp
[email protected]
o.jp
[email protected]
.jp
laurent.cariou@orange
-ftgroup.com
Submission
Slide 1
K. Ishihara et al.,(NTT)
doc.:
IEEE
802.11-10/1131r0
doc.:
IEEE
802.11-09/0161r1
Sept. 2010
Introduction
• Downlink (DL) MU-MIMO will be adopted to improve the spectrum
efficiency in TGac.
• We have shown CSI report requirements for TGac in explicit feedback
and in need of some CSI compression scheme to achieve higher MAC
efficiency for MU-MIMO transmission [1].
• In [2] and [3], time-domain CSI compression schemes were proposed
to reduce the amount of CSI needed.
– [2] uses discrete cosine transform (DCT).
– [3] uses truncated inverse discrete Fourier transform (TiDFT).
• In this submission, we present these CSI compression schemes and
evaluate these performances.
Submission
Slide 2
K. Ishihara et al.,(NTT)
doc.:
IEEE
802.11-10/1131r0
doc.:
IEEE
802.11-09/0161r1
Sept. 2010
Concept of time-domain CSI feedback
TD/FD
conversion
Power
Power
• Frequency-domain (FD) CSI-FB: CSI between a Tx antenna and a Rx
antenna consists of Nsubc subcarrier components.
• Time-domain (TD) CSI-FB: CSI consists of only Ng components since
it is assumed that the actual channel impulse response is present only
the GI duration.
Less CSI-FB needed with TD than with FD: factor is Ng/Nsubc.
freq.
time
Ng
Nsubc
Time-domain CSI
Frequency-domain CSI
Submission
Slide 3
K. Ishihara et al.,(NTT)
doc.:
IEEE
802.11-10/1131r0
doc.:
IEEE
802.11-09/0161r1
Sept. 2010
CSI compression scheme using DCT [2]
• IDFT and DCT can create time-domain components.
• In IDFT, the discontinuity at the band edges results in a spreading of energy in
the impulse response since DFT assumes that the frequency response is periodic,
which causes large CSI error.
• In contrast, DCT can reduce the high-frequency components compared to DFT
since it assumes mirror extension of the original data.
Discontinuity

Freq.
Power
Inverse discrete Fourier transform (IDFT)
Freq.
Channel gain
IDFT
Power

DCT

Continuity

Freq.
Ng
Time
Discrete cosine transform (DCT)
Submission
Slide 4
K. Ishihara et al.,(NTT)
doc.:
IEEE
802.11-10/1131r0
doc.:
IEEE
802.11-09/0161r1
Sept. 2010
CSI compression scheme using TiDFT [3]
• In IDFT, the discontinuity at the band edges results in a spreading of energy
in the impulse response.
• To overcome this problem, a truncated IDFT (TiDFT) matrix is applied:
TiDFT matrix is the truncated SVD of pseudo-inverse matrix for IDFT.
• TiDFT/FFT operation enables CSI compression since it can suppress CSI
error due to discontinuity of the band edges.
(NsubcxNFFT)-DFT matrix
F
Pseudo-inverse matrix



1
F  F F FH
H
Truncated SVD decomposition
 
~
~
~
F   UV * , rank F   r
TiDFT matrix
Submission
Slide 5
K. Ishihara et al.,(NTT)
doc.:
IEEE
802.11-10/1131r0
doc.:
IEEE
802.11-09/0161r1
Sept. 2010
Performance comparison of 3 CSI-FB
schemes (1/2)
Simulation parameters
100
80
Nb=8bits
5bits
60
CDF
7bits
4bits
DCT
6bits
40
TiDFT
20
DCT
TiDFT
Conventional (FD)
0
-60
-50
-40
-30
-20
-10
MSE (dB)
Nb: Number of bits for each CSI coefficient
Submission
Slide 6
Channel model
Model E
Bandwidth
40MHz
Number of FFT points N
128
Number of subcarriers Nsubc
114
Number of antennas at AP NT
8
Number of antennas at STA NR
1
Number of CSI-FB components for
DCT LDCT
32x2
Number of DCT points NDCT
64
Number of CSI-FB components for
TiDFT LTiDFT
32
Note: CSI before compression is perfect CSI.
When CSI before compression includes the effect
of noise, MSE performance of DCT and TiDFT
will be 3dB and 6dB better than conventional FD
respectively because of time domain smoothing.
K. Ishihara et al.,(NTT)
doc.:
IEEE
802.11-10/1131r0
doc.:
IEEE
802.11-09/0161r1
Sept. 2010
Performance comparison of 3 CSI-FB
schemes (2/2)
0
Simulation parameters
10
-1
Average BER
10
-2
10
DL MU-MIMO:
AP(8Tx) to 4 STAs(1Tx)
Ideal
Conv. (4bits)
Conv. (5bits)
Conv. (6bits)
DCT (4bits)
DCT (5bits)
DCT (6bits)
TiDFT (4bits)
TiDFT (5bits)
TiDFT (6bits)
-3
10
-4
10
-5
10
DCT
10
15
TiDFT
20
Channel model
Model E
Bandwidth
40MHz
Modulation
64QAM
Coding rate
5/6
Transmit beamforming
ZF
Number of CSI-FB components for
DCT LDCT
32x2
Number of DCT points NDCT
64
Number of CSI-FB components for
TiDFT LTiDFT
32
Note: “Ideal” means Perfect CSI.
The original CSI of three CSI-FB schemes is perfect
CSI. The CSI error is due to quantization error and
CSI compression operation. If original CSI includes
the effect of noise, BER performance of time domain
30 CSI-FB becomes much better than conventional
scheme because of noise reduction by smoothing.
25
SNR (dB)
Submission
Slide 7
K. Ishihara et al.,(NTT)
doc.:
IEEE
802.11-10/1131r0
doc.:
IEEE
802.11-09/0161r1
Sept. 2010
Amount of FB information and calculation
complexity
FB information bits per STA
DCT
(3+2xNbxNTxNR)xLDCT
TiDFT
(3+2xNbxNTxNR)xLTiDFT
Conv.
(3+2xNbxNTxNR)xNsubc
Number of additional multiplications
for TD conversion
Number of additional multiplications
12000
Nb=6bits
FB information (bits)
10000
8000
6000
4000
2000
0
DCT
Submission
TiDFT
Conv.
Slide 8
DCT
TiDFT
AP
2NDCTlog2(NDCT)
NsubcxLTiDFT
STA
2NDCTlog2(NDCT)
Nlog2N
4000
3500
@STA
@AP
3000
2500
2000
1500
1000
500
0
DCT
TiDFT
K. Ishihara et al.,(NTT)
doc.:
IEEE
802.11-10/1131r0
doc.:
IEEE
802.11-09/0161r1
Sept. 2010
Effect of the number of CSI-FB coefficients
100
100
Nb=6bits
Nb=6bits
80
80
LDCT=32x2
60
LDCT=16x2
CDF
CDF
60
LDCT=32x2, 24x2, 16x2
40
LDCT=24x2
40
20
20
DCT
DCT
0
0
-50
-45
-40
-35
-30
-25
-20
-15
-10
MSE (dB)
-45
-40
-35
-30
-25
-20
-15
MSE (dB)
Channel model B (small delay spread)
Submission
-50
Channel model E (large delay spread)
Slide 9
K. Ishihara et al.,(NTT)
-10
doc.:
IEEE
802.11-10/1131r0
doc.:
IEEE
802.11-09/0161r1
Sept. 2010
Performance comparison of 3 CSI-FB
schemes (2/2)
0
10
-1
10
-2
10
-3
12000
Nb=6bits
10000
FB information (bits)
Average BER
10
Channel model B
Nb = 6bits
8000
6000
4000
2000
Ideal
10
-4
0
DCT(L=16x2)
DCT
DCT(L=24x2)
DCT(L=32x2)
10
-5
10
15
20
25
30
35
40
Conv.
Note: When using optimization of TiDFT
matrix, TiDFT can also reduce the number of
FB information bits.
SNR (dB)
When the delay spread is small, the amount of FB information can reduce by controlling the number
of CSI-FB components L.
Submission
Slide 10
K. Ishihara et al.,(NTT)
doc.:
IEEE
802.11-10/1131r0
doc.:
IEEE
802.11-09/0161r1
Sept. 2010
Conclusion
• We presented the performance evaluations for time-domain CSI-FB
schemes to reduce the amount of FB information.
• Time-domain approach can reduce FB information since the number of
channel impulse response components fits within GI period.
– DCT reduces CSI-FB information by about half of the conventional one
with some additional calculation.
– TiDFT is the most effective scheme of CSI compression although
calculation complexity increases at STA.
• The amount of FB information can be adjusted dynamically by
controlling the number of CSI-FB components with the demand of CSI
accuracy.
• In addition, time-domain operation can improve the CSI estimation
accuracy by reducing the noise on the estimated channel coefficients.
Submission
Slide 11
K. Ishihara et al.,(NTT)
doc.:
IEEE
802.11-10/1131r0
doc.:
IEEE
802.11-09/0161r1
Sept. 2010
References
[1] K. Ishihara et al., CSI Report for Explicit Feedback
Beamforming in Downlink MU-MIMO, IEEE 802.1110/0332r0, Mar. 2010.
[2] K. Ishihara et al., CSI Feedback Scheme using DCT for
Explicit Beamforming, IEEE 802.11-10/0806r1, July 2010.
[3] L. Cariou and M. Diallo, Time Domain CSI report for
explicit feedback, IEEE 802.11-10/0586r1, May 2010.
Submission
Slide 12
K. Ishihara et al.,(NTT)