doc.: IEEE /0372r0 Submission March 2004 Ravi Mahadevappa, Stephan ten Brink, Realtek Slide 1 Rate Feedback Schemes for MIMO-OFDM n (a sequel to /0013r0) Ravi Mahadevappa, Stephan ten Brink, Realtek Semiconductors, Irvine, CA

doc.: IEEE /0372r0 Submission March 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 MxN configurations –1x1 –2x3, 3x4

doc.: IEEE /0372r0 Submission March 2004 Ravi Mahadevappa, Stephan ten Brink, Realtek Slide 3 Brief description SISO case: Compute “effective SNR” for each subcarrier Switch off subcarriers with effective SNR below a certain threshold –New over /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 N T spatial subchannels, effective SNR is computed based on ZF detection (N T transmit antennas assumed) Subchannel switched off when effective SNR falls below threshold –New over /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

doc.: IEEE /0372r0 Submission March 2004 Ravi Mahadevappa, Stephan ten Brink, Realtek Slide 4 Subchannel ZF-SNR on a subcarrier Received signal: y = H s + n 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 New over /0013r0 : H is reformulated and ZF-SNRs recomputed whenever subchannels are switched off

doc.: IEEE /0372r0 Submission March 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

doc.: IEEE /0372r0 Submission March 2004 Ravi Mahadevappa, Stephan ten Brink, Realtek Slide Required SNR (10% PER) in dB Rate in Mbps 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 16QAM QPSK BPSK 1/4 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

doc.: IEEE /0372r0 Submission March 2004 Ravi Mahadevappa, Stephan ten Brink, Realtek Slide Required SNR (10% PER) in dB Rate in Mbps 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 16QAM QPSK BPSK 1/4 7/8 1x1, Rate/Mod. Choice for Rate Table Subchannel rate table: choose best mod./code rate combination for given SNR (circles)

doc.: IEEE /0372r0 Submission March 2004 Ravi Mahadevappa, Stephan ten Brink, Realtek Slide 8 Nominal mode Subchannels divided into 4 groups 00 – Switched off 01 – Nominal mode 10 – (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 148 subcarriers TX 1 2 Bits fed back per Subchannel TX 2 TX 3

doc.: IEEE /0372r0 Submission March 2004 Ravi Mahadevappa, Stephan ten Brink, Realtek Slide 9 Nominal rates used: BPSK, rate 1/3, up to 256QAM, rate 3/4 Example setting here: –If subchannel SNR -6dB below average SNR av : switch off –If between -6dB and +3dB of SNR av : use nominal rate –If 3dB above SNR av : one higher rate than nominal rate No significant improvement found with other threshold settings Gains negligible over on/off feedback case 1x1 Results

doc.: IEEE /0372r0 Submission March 2004 Ravi Mahadevappa, Stephan ten Brink, Realtek Slide Required SNR (10% PER) in dB Rate in Mbps 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 symbol 1000 bit packets, 2000 packets avg. 64QAM SNR threshold 0 dB 20 subchannels off on average 64QAM all 96 subchannels on 16QAM QPSK 7/8 1/4 QPSK 0 dB 16QAM 0 dB 2-bit feedback thresholds [0 0 +6] dB 2x3 Results Nominal rates used: BPSK, rate 1/3, up to 256QAM, rate 3/4 Example setting here: –If subchannel SNR 0dB below average SNR av : switch off –If between 0dB and +6dB of SNR av : use nominal rate –If 6dB above SNR av : one higher rate than nominal rate No significant improvement found with other threshold settings Gains negligible over on/off feedback case

doc.: IEEE /0372r0 Submission March 2004 Ravi Mahadevappa, Stephan ten Brink, Realtek Slide Required SNR (10% PER) in dB Rate in Mbps 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. 64QAM SNR threshold 0 dB 24 subchannels off on average 16QAM 0 dB QPSK 0 dB QPSK 16QAM 64QAM all 144 subchannels on 1/4 7/8 2-bit feedback thresholds [0 0 +6] dB 3x4 Results Nominal rates used: BPSK, rate 1/3, up to 256QAM, rate 3/4 Example setting here: –If subchannel SNR 0dB below average SNR av : switch off –If between 0dB and +6dB of SNR av : use nominal rate –If 6dB above SNR av : one higher rate than nominal rate No significant improvement found with other threshold settings Gains negligible over on/off feedback case

doc.: IEEE /0372r0 Submission March 2004 Ravi Mahadevappa, Stephan ten Brink, Realtek Slide 12 Observations Feedback with switching on/off ZF-detection based subchannels gains about 2.5dB over the non-feedback case ( /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

doc.: IEEE /0372r0 Submission March 2004 Ravi Mahadevappa, Stephan ten Brink, Realtek Slide 13 Some References [1]M. Tzannes, T. Cooklev, D. Lee, C. Lanzl, “Extended Data Rate a”, IEEE /232r0 [2]G. Kleindl, “Signaling for Adaptive Modulation”, /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 [5]Ravi Mahadevappa, Stephan ten Brink, “On/off-Feedback Schemes for MIMO-OFDM n”, IEEE /0013r0