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doc.: IEEE 802.11-04/0013r0 Submission January 2004 Ravi Mahadevappa, Stephan ten Brink, Realtek Slide 1 “On/off”-Feedback Schemes for MIMO-OFDM 802.11n Ravi Mahadevappa, ravi@realtek-us.com Stephan ten Brink, stenbrink@realtek-us.com Realtek Semiconductors, Irvine, CA

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doc.: IEEE 802.11-04/0013r0 Submission January 2004 Ravi Mahadevappa, Stephan ten Brink, Realtek Slide 2 Overview Brief description of the method Computing ZF SNR of spatial subchannels Simulator settings Rate vs SNR(10% PER) for various configurations –1x1 with fixed number of subcarriers switched off –1x1, 1x2, 2x3, 3x4 variable number of subchannels switched off

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doc.: IEEE 802.11-04/0013r0 Submission January 2004 Ravi Mahadevappa, Stephan ten Brink, Realtek Slide 3 Brief description SISO case: Compute “effective SNR” (include channel coefficient) for each subcarrier Switch off subcarriers with effective SNR below a certain threshold Normalize transmit power so that average time-domain SNR is fixed MIMO case: Subcarriers are further divided into N T spatial subchannels, effective SNR is computed based on ZF detection (N T transmit antennas assumed) Subchannels are switched off when effective SNR falls below threshold Transmit power renormalized to fix time-domain SNR

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doc.: IEEE 802.11-04/0013r0 Submission January 2004 Ravi Mahadevappa, Stephan ten Brink, Realtek Slide 4 Subchannel ZF-SNR on a subcarrier Received signal: y = H s + n With stransmitted N T x1 constellation vector of QAM symbols s i, i=1..N T, with entries having average power P s HN R xN T matrix of channel coefficients, assumed iid complex Gaussian nN R x1 vector of additive noise, with entries having variance 2 Zero forcing detection: W = (H * H) -1 H * N T xN R matrix, “pseudo inverse”; H * denotes the complex conjugate transpose (Hermitian) of H; s est = W yN T x1 vector estimate of transmitted constellation vector Spatial subchannel SNRs, SNR j, j=1..N T computed as SNR j = P s /([(H * H) -1 ] jj 2 ) where [(H * H) -1 ] jj denotes the jth diagonal element of (H * H) -1

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doc.: IEEE 802.11-04/0013r0 Submission January 2004 Ravi Mahadevappa, Stephan ten Brink, Realtek Slide 5 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 Feedback scenarios: ZF SNR for each subchannel (M for MxN case) is computed 1x1: subcarrier on/off based on ZF SNR –constant number of subcarriers switched off (6, 12, 24) –threshold to switch off subcarriers -6dB, -4dB, -2dB from average SNR 1x2, 2x3, 3x4: subchannel on/off –threshold to switch off subchannels -6dB, 0dB from average SNR

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doc.: IEEE 802.11-04/0013r0 Submission January 2004 Ravi Mahadevappa, Stephan ten Brink, Realtek Slide 6 1x1, 16QAM, Rate vs SNR(10% PER) no feedback case 16QAM rate 1/4 to rate 7/8, corresponding to data rates of 12Mbps to 42Mbps 25 1x1s1 SMX 16QAM 05101520 5 10 15 20 25 30 35 40 45 Required SNR (10% PER) in dB Rate in Mbps no feedback 16 QAM 1x1 Exponential Decay Channel 60 ns Trms Random interleaving 1000 bit packets 2000 packets avg. Code Rates 1/4, 1/3, 1/2, 2/3, 3/4, 7/8

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doc.: IEEE 802.11-04/0013r0 Submission January 2004 Ravi Mahadevappa, Stephan ten Brink, Realtek Slide 7 1x1, 16QAM, worst 6 subcarriers off Data Rate reduces by a fraction 6/48 Required SNR is lower As a result, curve shifts left and down Gain ~0.8dB in low rate region 1x1s1 SMX 16QAM 05101520 5 10 15 20 25 30 35 40 45 Required SNR (10% PER) in dB Rate in Mbps no feedback worst 6 carriers off 16 QAM 1x1 Exponential Decay Channel 60 ns Trms Random interleaving 1000 bit packets 2000 packets avg. Code Rates 1/4, 1/3, 1/2, 2/3, 3/4, 7/8 25

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doc.: IEEE 802.11-04/0013r0 Submission January 2004 Ravi Mahadevappa, Stephan ten Brink, Realtek Slide 8 1x1, 16QAM, worst n subcarriers off Curves shift lower and more left as more subcarriers are switched off. Gain upto ~1.3 dB 1x1s1 SMX 16QAM 05101520 5 10 15 20 25 30 35 40 45 Required SNR (10% PER) in dB Rate in Mbps no feedback worst 6 carriers off 12 off 24 off 16 QAM 1x1 Exponential Decay Channel 60 ns Trms Random interleaving 1000 bit packets 2000 packets avg. Code Rates 1/4, 1/3, 1/2, 2/3, 3/4, 7/8 25

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doc.: IEEE 802.11-04/0013r0 Submission January 2004 Ravi Mahadevappa, Stephan ten Brink, Realtek Slide 9 1x1, 16QAM, variable number of subcarriers switched off SNR of each subchannel compared with a threshold relative to average SNR, those below threshold are switched off. Gains are higher compared to the case where constant number of subcarriers are switched off. 2.5 dB gain almost uniformly for all rates 1x1s1 SMX 16QAM 05101520 5 10 15 20 25 30 35 40 45 Required SNR (10% PER) in dB Rate in Mbps no feedback worst 6 carriers off 12 off 24 off SNR threshold -6 dB 16 QAM 1x1 Exponential Decay Channel 60 ns Trms Random interleaving 1000 bit packets 2000 packets avg. Code Rates 1/4, 1/3, 1/2, 2/3, 3/4, 7/8 25

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doc.: IEEE 802.11-04/0013r0 Submission January 2004 Ravi Mahadevappa, Stephan ten Brink, Realtek Slide 10 1x1, 16QAM, variable number of subcarriers switched off Higher thresholds lead to lower average data rates. An “optimum threshold” for which the gain is maximized may exist. 1x1s1 SMX 16QAM Code Rates 1/4, 1/3, 1/2, 2/3, 3/4, 7/8 25

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doc.: IEEE 802.11-04/0013r0 Submission January 2004 Ravi Mahadevappa, Stephan ten Brink, Realtek Slide 11 1x1, QPSK Gains are marginally higher for QPSK, but behavior is similar to 16QAM case 1x1 QPSK: Upto 3 dB gain for threshold -6 dB -505101520 2 4 6 8 10 12 14 16 18 20 22 1x1s1 SMX-QPSK Required SNR (10% PER) in dB Rate in Mbps QPSK 1x1 Exponential Decay Channel 60 ns Trms Random interleaving no feedback worst 6 carriers off 12 off 24 off threshold -6 dB -4 dB -2 dB Code rates 1/4, 1/3, 1/2, 2/3, 3/4, 7/8

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doc.: IEEE 802.11-04/0013r0 Submission January 2004 Ravi Mahadevappa, Stephan ten Brink, Realtek Slide 12 1x1, 64QAM 1x1, 64QAM: Upto 2.5 dB gain for threshold -6 dB 51015202530 0 10 20 30 40 50 60 70 1x1s1 SMX-64QAM Required SNR (10% PER) in dB Rate in Mbps 64 QAM 1x1 Exponential Decay Channel 60 ns Trms Random interleaving 1000 bit packets 2000 packets avg. SNR threshold -6 dB -4 dB -2 dB 24 off Code rates 1/4, 1/3, 1/2, 2/3, 3/4, 7/8 12 off worst 6 carriers off no feedback

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doc.: IEEE 802.11-04/0013r0 Submission January 2004 Ravi Mahadevappa, Stephan ten Brink, Realtek Slide 13 1x1, Summary QPSK rate 1/4 to 64QAM rate 7/8, corresponding to data rates of 6Mbit/s to 63Mbit/s Average data rates with feedback from approx. 5 to 50 Mbps. Gain of 1.5 to 3 dB -5051015202530 0 10 20 30 40 50 60 70 Required SNR (10% PER) in dB Rate in Mbps 64QAM no feedback 16QAM no feedback QPSK no feedback 64QAM SNR threshold -6 dB 16QAM -6 dB QPSK -6 dB 1x1 Exponential Decay Channel 60 ns Trms 1000 bit packets 2000 packets avg. Code Rates Used : 1/4, 1/3, 1/2, 2/3, 3/4, 7/8

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doc.: IEEE 802.11-04/0013r0 Submission January 2004 Ravi Mahadevappa, Stephan ten Brink, Realtek Slide 14 1x2, variable (adaptive) number of subchannels switched off For the same threshold, gain reduces as more Rx antennas are used, the average data rate is also higher. ~0.8 dB gain for 1x2 case with threshold -6 dB -10-50510152025 0 10 20 30 40 50 60 70 Required SNR (10% PER) in dB Rate in Mbps QPSK no feedback 16QAM -6 dB 64QAM SNR threshold -6 dB 16QAM no feedback 64QAM no feedback QPSK -6 dB 1000 bit packets 2000 packets avg. 1x2 Exponential Decay Channel 60 ns Trms Code Rates Used : 1/4, 1/3, 1/2, 2/3, 3/4, 7/8

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doc.: IEEE 802.11-04/0013r0 Submission January 2004 Ravi Mahadevappa, Stephan ten Brink, Realtek Slide 15 1x2, variable (adaptive) number of subchannels switched off Upto 2.3 dB gain as threshold is increased to 0 dB -10-50510152025 0 10 20 30 40 50 60 70 Required SNR (10% PER) in dB Rate in Mbps QPSK no feedback 16QAM -6 dB 64QAM SNR threshold -6 dB 16QAM no feedback 64QAM no feedback QPSK -6 dB 1000 bit packets 2000 packets avg. 1x2 Exponential Decay Channel 60 ns Trms Code Rates Used : 1/4, 1/3, 1/2, 2/3, 3/4, 7/8 64QAM 0 dB 16QAM 0 dB QPSK 0 dB

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doc.: IEEE 802.11-04/0013r0 Submission January 2004 Ravi Mahadevappa, Stephan ten Brink, Realtek Slide 16 2x3, variable (adaptive) number of subchannels switched off Upto 2.5 dB gain possible with appropriate threshold

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doc.: IEEE 802.11-04/0013r0 Submission January 2004 Ravi Mahadevappa, Stephan ten Brink, Realtek Slide 17 3x4, variable (adaptive) number of subchannels switched off Up to 2.5 dB gain possible with appropriate threshold -50510152025 0 20 40 60 80 100 120 140 160 180 200 Required SNR (10% PER) in dB Rate in Mbps 3x4 Exponential Decay Channel 60 ns Trms Random Interleaving 1000 bit packets 2000 packets avg. 64 QAM SNR threshold -6 dB 64 QAM 0 dB 64 QAM no feedback 16 QAM no feedback 16 QAM -6 dB 16 QAM 0 dB QPSK -6 dB QPSK 0 dB QPSK no feedback Code Rates used : 1/4, 1/3, 1/2, 2/3, 3/4, 7/8

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doc.: IEEE 802.11-04/0013r0 Submission January 2004 Ravi Mahadevappa, Stephan ten Brink, Realtek Slide 18 Observations Switching on/off subchannels is a simple way of using channel information at the transmitter Gains up to 3dB in 1x1 case, upto 2.5 dB in 2x3, 3x4 case Gain reduces when number of Rx antennas is increased as diversity increases Performance is better with variable number of subchannels switched off compared to fixed number switched off

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doc.: IEEE 802.11-04/0013r0 Submission January 2004 Ravi Mahadevappa, Stephan ten Brink, Realtek Slide 19 Some References [1]M. Tzannes, T. Cooklev, D. Lee, C. Lanzl, “Extended Data Rate 802.11a”, 802.11-02/232r0 [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.

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