Doc.: IEEE 802.11-04/0372r0 Submission March 2004 Ravi Mahadevappa, Stephan ten Brink, Realtek Slide...

March 2004

Ravi Mahadevappa, Stephan ten Brink, Realtek

Slide 1

doc.: IEEE 802.11-04/0372r0

Submission

Rate Feedback Schemes for MIMO-OFDM 802.11n

(a sequel to 802.11-04/0013r0)

Ravi Mahadevappa, [email protected] ten Brink, [email protected]

Realtek Semiconductors, Irvine, CA

March 2004


Slide 2

doc.: IEEE 802.11-04/0372r0

Submission

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

March 2004


Slide 3

doc.: IEEE 802.11-04/0372r0

Submission

Brief descriptionSISO 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 SNRMIMO 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

March 2004


Slide 4

doc.: IEEE 802.11-04/0372r0

Submission

Subchannel ZF-SNR on a subcarrier• Received signal: y = H s + n

s transmitted NTx1 constellation vector of QAM symbols si, i=1..NT, with entries having average power Ps

H NRxNT matrix of channel coefficients, assumed iid complex Gaussiann NRx1 vector of additive noise, with entries having variance 2

• Zero forcing detection:

W = (H* H)-1 H* NTxNR matrix, “pseudo inverse”; H* denotes the complex conjugate transpose (Hermitian) of H;

• sest = W y NTx1 vector estimate of transmitted constellation vector

• Spatial subchannel SNRs, SNRj, j=1..NT computed as

SNRj = Ps/([(H* H)-1]jj 2)

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

March 2004


Slide 5

doc.: IEEE 802.11-04/0372r0

Submission

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

March 2004


Slide 6

doc.: IEEE 802.11-04/0372r0

Submission

-5 0 5 10 15 20 25 30 350

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Required SNR (10% PER) in dB

Rat

e in

Mbp

s

1x1 Exponential Decay Channel 60 ns Trms Code Rates 1/4, 1/3, 1/2, 2/3, 3/4, 7/8Random Interleaver for each OFDM symbol1000 bit packets, 2000 packets avg.

256QAM

64QAM

16QAM

QPSK

BPSK1/4

7/8

1x1, Rate/Modulation per Subchannel

• 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

March 2004


Slide 7

doc.: IEEE 802.11-04/0372r0

Submission

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Rat

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256QAM

64QAM

16QAM

QPSK

BPSK1/4

7/8

ModulationCode RateBPSK 1/4BPSK 1/3QPSK 1/4QPSK 1/3QPSK 1/216QAM 1/316QAM 1/216QAM 2/316QAM 3/464QAM 2/364QAM 3/4256QAM 2/3256QAM 3/4256QAM 7/8

1x1, Rate/Mod. Choice for Rate Table• Subchannel rate

table: choose best mod./code rate combination for given SNR (circles)

March 2004


Slide 8

doc.: IEEE 802.11-04/0372r0

Submission

ModulationCode RateBPSK 1/4BPSK 1/3QPSK 1/4QPSK 1/3QPSK 1/216QAM 1/316QAM 1/216QAM 2/316QAM 3/464QAM 2/364QAM 3/4256QAM 2/3256QAM 3/4256QAM 7/8

Nominal mode

Subchannels divided into 4 groups

00 – Switched off01 – Nominal mode10 – (Nominal - 1) mode 11 – (Nominal +1) mode

Nominal – 1 mode

Nominal + 1 mode

For example

Typical distribution of modes over subchannels for the 3x4 case

1 48subcarriersTX 1

2 Bits fed back per Subchannel

TX 2TX 3

March 2004


Slide 9

doc.: IEEE 802.11-04/0372r0

Submission

-5 0 5 10 15 20 25 300

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Rat

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64QAM SNR threshold -6 dB 10 subcarriers offon average

16QAM -6 dB

QPSK -6 dB

64QAM all subcarriers on

16QAM

QPSK

1/4

7/8

2-bit feedbackthresholds[-6 -6 +3] dB

• Nominal rates used: BPSK, rate 1/3, up to 256QAM, rate 3/4

• Example setting here:– 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

• No significant improvement found with other threshold settings

• Gains negligible over on/off feedback case

1x1 Results

March 2004


Slide 10

doc.: IEEE 802.11-04/0372r0

Submission

-5 0 5 10 15 20 250

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Rat

e in

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2x3 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 symbol1000 bit packets, 2000 packets avg.

64QAM SNR threshold 0 dB

20 subchannels offon average 64QAM

all 96 subchannels on

16QAM

QPSK

7/8

1/4

QPSK 0 dB

16QAM 0 dB

2-bit feedbackthresholds[0 0 +6] dB

2x3 Results• Nominal rates used:

BPSK, rate 1/3, up to 256QAM, rate 3/4


0dB below average SNRav: switch off

– If between 0dB and +6dB of SNRav: use nominal rate




March 2004


Slide 11

doc.: IEEE 802.11-04/0372r0

Submission

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Rat

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64QAM SNR threshold 0 dB 24 subchannels offon average

16QAM 0 dB

QPSK 0 dB

QPSK

16QAM

64QAM all 144 subchannels on

1/4

7/8

2-bit feedbackthresholds[0 0 +6] dB

3x4 Results• Nominal rates used:

BPSK, rate 1/3, up to 256QAM, rate 3/4


0dB below average SNRav: switch off

– If between 0dB and +6dB of SNRav: use nominal rate




March 2004


Slide 12

doc.: IEEE 802.11-04/0372r0

Submission

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

March 2004


Slide 13

doc.: IEEE 802.11-04/0372r0

Submission

Some References[1] M. Tzannes, T. Cooklev, D. Lee, C. Lanzl, “Extended Data Rate 802.11a”, IEEE802.11-02/232r0

[2] G. Kleindl, “Signaling for Adaptive Modulation”, 802.11-03/283r0

[3] 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.

[4] Bangerter et.al., “High-Throughput Wireless LAN Air Interface,” Intel Tech. Journal, Vol. 7, Issue 3, Aug 2003.

[5] Ravi Mahadevappa, Stephan ten Brink, “On/off-Feedback Schemes for MIMO-OFDM 802.11n”, IEEE802.11-04/0013r0

Doc.: IEEE 802.11-04/0372r0 Submission March 2004 Ravi Mahadevappa, Stephan ten Brink, Realtek Slide...

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