March 2004
doc.: IEEE 802.11-04/0372r0
Rate Feedback Schemes for
MIMO-OFDM 802.11n
(a sequel to 802.11-04/0013r0)
Ravi Mahadevappa, [email protected]
Stephan ten Brink, [email protected]
Realtek Semiconductors, Irvine, CA
Submission
Slide 1
Ravi Mahadevappa, Stephan ten Brink, Realtek
March 2004
doc.: IEEE 802.11-04/0372r0
Overview
•
•
•
•
Brief description of the method
Computing ZF SNR of spatial subchannels
Simulator settings
Rate vs SNR(10% PER) for various MxN configurations
– 1x1
– 2x3, 3x4
Submission
Slide 2
Ravi Mahadevappa, Stephan ten Brink, Realtek
March 2004
doc.: IEEE 802.11-04/0372r0
Brief description
SISO case:
• Compute “effective SNR” for each subcarrier
• Switch off subcarriers with effective SNR below a certain threshold
– New over 802.11-04/0013r0: Feedback of 4 different rates (i.e. 2 bits)
per subchannel: off, nominal rate, one below nominal rate, one above
nominal rate
• Normalize transmit power to fix average time-domain SNR
MIMO case:
• Subcarriers are further divided into NT spatial subchannels,
effective SNR is computed based on ZF detection (NT transmit
antennas assumed)
• Subchannel switched off when effective SNR falls below threshold
– New over 802.11-04/0013r0: Feedback of 4 different rates (i.e. 2 bits)
per subchannel: off, nominal rate, one below nominal rate, one above
nominal rate
•
Transmit power renormalized to fix time-domain SNR
Submission
Slide 3
Ravi Mahadevappa, Stephan ten Brink, Realtek
March 2004
doc.: IEEE 802.11-04/0372r0
Subchannel ZF-SNR on a subcarrier
• Received signal: y = H s + n
s
H
n
transmitted NTx1 constellation vector of QAM symbols si, i=1..NT,
with entries having average power Ps
NRxNT matrix of channel coefficients, assumed iid complex
Gaussian
NRx1 vector of additive noise, with entries having variance s2
• Zero forcing detection:
W = (H* H)-1 H*
• sest = W y
NTxNR matrix, “pseudo inverse”; H* denotes the
complex conjugate transpose (Hermitian) of H;
NTx1 vector estimate of transmitted constellation vector
• Spatial subchannel SNRs, SNRj, j=1..NT computed as
SNRj = Ps/([(H* H)-1]jj s2)
where [(H* H)-1]jj denotes the jth diagonal element of (H* H)-1
• New over 802.11-04/0013r0: H is reformulated and ZF-SNRs
recomputed whenever subchannels are switched off
Submission
Slide 4
Ravi Mahadevappa, Stephan ten Brink, Realtek
March 2004
doc.: IEEE 802.11-04/0372r0
Simulator settings
Coded MIMO-OFDM system
• Convolutional outer encoder with memory 6
• Pseudo-random interleaver of length equal to
one OFDM symbol
• Modulation QPSK, 16QAM, 64QAM
• MIMO configurations 1x1, 1x2, 2x3, 3x4
Submission
Slide 5
Ravi Mahadevappa, Stephan ten Brink, Realtek
March 2004
doc.: IEEE 802.11-04/0372r0
1x1, Rate/Modulation per Subchannel
100
90
80
Rate in Mbps
70
1x1
Exponential Decay Channel
60 ns Trms
Code Rates 1/4, 1/3, 1/2, 2/3, 3/4, 7/8
Random Interleaver for each OFDM symbol
1000 bit packets, 2000 packets avg.
256QAM
64QAM
60
50
40
16QAM
30
20
QPSK
7/8
BPSK
10
1/4
0
-5
0
5
10
15
20
25
30
35
• Nominal rate is
chosen according to
average SNR (over
all subchannels and
many packets)
• For each
subchannel, ZF-SNR
is computed
• Depending on how
subchannel SNR
compares to average
SNR, subchannel is
switched off, or uses
nominal rate, or one
below/above nominal
rate
Required SNR (10% PER) in dB
Submission
Slide 6
Ravi Mahadevappa, Stephan ten Brink, Realtek
March 2004
doc.: IEEE 802.11-04/0372r0
1x1, Rate/Mod. Choice for Rate Table
• Subchannel rate
table: choose best
mod./code rate
combination for
given SNR (circles)
100
90
80
Rate in Mbps
70
1x1
Exponential Decay Channel
60 ns Trms
Code Rates 1/4, 1/3, 1/2, 2/3, 3/4, 7/8
Random Interleaver for each OFDM symbol
1000 bit packets, 2000 packets avg.
256QAM
64QAM
60
50
40
16QAM
30
20
QPSK
7/8
BPSK
10
1/4
0
-5
0
5
10
15
20
25
Required SNR (10% PER) in dB
Submission
Slide 7
30
35
Modulation Code Rate
BPSK
1/4
BPSK
1/3
QPSK
1/4
QPSK
1/3
QPSK
1/2
16QAM
1/3
16QAM
1/2
16QAM
2/3
16QAM
3/4
64QAM
2/3
64QAM
3/4
256QAM
2/3
256QAM
3/4
256QAM
7/8
Ravi Mahadevappa, Stephan ten Brink, Realtek
March 2004
doc.: IEEE 802.11-04/0372r0
2 Bits fed back per Subchannel
Subchannels divided into 4 groups
Modulation Code Rate
1/4
BPSK
1/3
BPSK
1/4
QPSK
1/3
QPSK
1/2
QPSK
1/3
16QAM
1/2
16QAM
2/3
16QAM
3/4
16QAM
2/3
64QAM
3/4
64QAM
2/3
256QAM
3/4
256QAM
7/8
256QAM
Submission
For example
00 –
01 –
10 –
11 –
Switched off
Nominal mode
(Nominal - 1) mode
(Nominal +1) mode
Nominal – 1 mode
Nominal mode
Nominal + 1 mode
Typical distribution of modes over subchannels
for the 3x4 case
subcarriers
1
TX 1
TX 2
TX 3
Slide 8
Ravi Mahadevappa, Stephan ten Brink, Realtek
48
March 2004
doc.: IEEE 802.11-04/0372r0
1x1 Results
• Nominal rates used:
BPSK, rate 1/3, up to
256QAM, rate 3/4
• Example setting
here:
70
60
Rate in Mbps
50
2-bit feedback
thresholds
[-6 -6 +3] dB
1x1
Exponential Decay Channel
60 ns Trms
64QAM
Code Rates 1/4, 1/3, 1/2, 2/3, 3/4, 7/8
Random Interleaver for each OFDM symbol SNR threshold
-6 dB
1000 bit packets, 2000 packets avg.
10 subcarriers off
on average
– If subchannel SNR
-6dB below average
SNRav: switch off
– If between -6dB and
+3dB of SNRav: use
nominal rate
– If 3dB above SNRav:
one higher rate than
nominal rate
64QAM
all subcarriers on
40
16QAM
16QAM
-6 dB
30
7/8
20
QPSK
-6 dB
QPSK
10
1/4
0
-5
Submission
0
5
10
15
Required SNR (10% PER) in dB
20
Slide 9
25
30
• No significant
improvement found
with other threshold
settings
• Gains negligible over
on/off feedback case
Ravi Mahadevappa, Stephan ten Brink, Realtek
March 2004
doc.: IEEE 802.11-04/0372r0
2x3 Results
• Nominal rates used:
BPSK, rate 1/3, up to
256QAM, rate 3/4
• Example setting
here:
Rate in Mbps
140
120
2x3
Exponential Decay Channel
60 ns Trms
100
Code Rates 1/4, 1/3, 1/2, 2/3, 3/4, 7/8
Random Interleaver for each OFDM symbol
1000 bit packets, 2000 packets avg.
64QAM
SNR threshold
0 dB
20 subchannels off
on average
64QAM
all 96 subchannels on
80
60
16QAM
0 dB
2-bit feedback
thresholds
[0 0 +6] dB
16QAM
40
7/8
20
QPSK
0 dB
1/4
0
-5
Submission
0
QPSK
5
10
15
Required SNR (10% PER) in dB
Slide 10
20
25
– If subchannel SNR
0dB below average
SNRav: switch off
– If between 0dB and
+6dB of SNRav: use
nominal rate
– If 6dB above SNRav:
one higher rate than
nominal rate
• No significant
improvement found
with other threshold
settings
• Gains negligible over
on/off feedback case
Ravi Mahadevappa, Stephan ten Brink, Realtek
March 2004
doc.: IEEE 802.11-04/0372r0
3x4 Results
• Nominal rates used:
BPSK, rate 1/3, up to
256QAM, rate 3/4
• Example setting
here:
200
180
160
2-bit feedback
thresholds
[0 0 +6] dB
3x4
Exponential Decay Channel
60 ns Trms
Code Rates 1/4, 1/3, 1/2, 2/3, 3/4, 7/8
Random Interleaver for each OFDM symbol
1000 bit packets, 2000 packets avg.
140
64QAM
SNR threshold
0 dB
24 subchannels off
on average
64QAM
all 144 subchannels on
Rate in Mbps
120
100
16QAM
0 dB
16QAM
80
60
7/8
QPSK
0 dB
40
QPSK
20
0
-5
Submission
1/4
0
5
10
15
Required SNR (10% PER) in dB
Slide 11
20
25
– If subchannel SNR
0dB below average
SNRav: switch off
– If between 0dB and
+6dB of SNRav: use
nominal rate
– If 6dB above SNRav:
one higher rate than
nominal rate
• No significant
improvement found
with other threshold
settings
• Gains negligible over
on/off feedback case
Ravi Mahadevappa, Stephan ten Brink, Realtek
March 2004
doc.: IEEE 802.11-04/0372r0
Observations
•
•
•
Feedback with switching on/off ZF-detection based
subchannels gains about 2.5dB over the non-feedback
case (802.11-04/0013r0)
In our simulations, the gains through increasing the
feedback-granularity from one bit (on/off) to 2 bits
(subchannel off, nominal rate, below nominal rate,
above nominal rate) were negligible
On/off feedback could be considered as optional mode
Submission
Slide 12
Ravi Mahadevappa, Stephan ten Brink, Realtek
March 2004
doc.: IEEE 802.11-04/0372r0
Some References
[1]
[2]
[3]
[4]
[5]
M. Tzannes, T. Cooklev, D. Lee, C. Lanzl, “Extended Data Rate 802.11a”, IEEE802.11-02/232r0
G. Kleindl, “Signaling for Adaptive Modulation”, 802.11-03/283r0
Leke, A. and Cioffi, J.M., "Transmit optimization for time-invariant wireless channel utilizing a discrete
multitone approach," Proc. of IEEE ICC’97, V.2, pp. 954 – 958.
Bangerter et.al., “High-Throughput Wireless LAN Air Interface,” Intel Tech. Journal, Vol. 7, Issue 3,
Aug 2003.
Ravi Mahadevappa, Stephan ten Brink, “On/off-Feedback Schemes for MIMO-OFDM 802.11n”,
IEEE802.11-04/0013r0
Submission
Slide 13
Ravi Mahadevappa, Stephan ten Brink, Realtek