Beamforming Array Gain to Intended and Unintended Users March 2013 doc.: IEEE 802.19-09/xxxxr0 March 2013 Beamforming Array Gain to Intended and Unintended Users Date: 2013-03-20 Authors: Nihar Jindal, Broadcom Corporation Nihar Jindal, Broadcom Corporation

March 2013 Introduction In this presentation we show array gain experienced by intended and unintended users Unintended user typically sees significantly lower array gain than the intended user because: Beamforming weights are matched to the intended user’s channel Any difference between intended/unintended users’ channels reduces array gain at unintended user Unintended and intended user’s channels are different due to different multipath propagation Nihar Jindal, Broadcom Corporation

Array Gain Computation March 2013 Array Gain Computation Draw channel to intended user & form BF vectors based on this channel Draw channel to unintended user Unintended user’s channel may have different statistics than intended, e.g., different delay spread or K factor Array gain = RX power at unintended user with BF on versus with no BF Nihar Jindal, Broadcom Corporation

March 2013 Channel Models Channel models from Jagganatham-Erceg paper [1] Intended user (indoors): exponential PDP with spike with 6.3 dB spike + LoS component on 1st tap 14 nsec/7.5 dB are median values for delay spread/K factor for 1st tap for LOS co-polar at 3 meters (plotted below) 13 nsec/11.7 dB are 90% values for delay spread/K factor for 1st tap for LOS co-polar at 3 meters Because shorter delay spread yields more array gain, 90% value for delay spread is chosen at 10% point of CDF (smaller than median) Unintended user (outdoors): 100 nsec rms delay spread, 0 dB K factor on 1st tap, no spike [2] Nihar Jindal, Broadcom Corporation

March 2013 Results for Nt=2/3/4/6/8 Using median values for delay spread/K factor and 90% point of multipath array gain Nihar Jindal, Broadcom Corporation

March 2013 Results for Nt=2/3/4/6/8 Using 90% values for delay spread/K factor and 50% point of multipath array gain Nihar Jindal, Broadcom Corporation

Summary of Results March 2013 90% Array Gain Max Array Gain Considerably smaller array gain with 90 % delay spread/K-factor and 50% point of multipath compared to 50% delay spread/K-factor and 90% point of multipath With either choice of parameters array gain seen is considerably smaller than maximum value of 10log10(Nt) 90% Array Gain Max Array Gain Difference (dB) 2x1 2.1 3.0 0.9 3x1 4.8 1.8 4x1 3.8 6.0 2.2 6x1 7.8 8x1 5.7 9.0 3.3 Median values for delay spread/K factor and 90% point of multipath array gain 50% Array Gain Max Array Gain Difference (dB) 2x1 1.1 3.0 1.9 3x1 4.8 2.9 4x1 2.6 6.0 3.4 6x1 3.7 7.8 4.1 8x1 4.7 9.0 4.3 90% values for delay spread/K factor and median values for multipath array gain Nihar Jindal, Broadcom Corporation

March 2013 Conclusion In this presentation we show that beamforming array gain is reduced to the unintended user because of different propagation channel For 4 transmit antenna case gain reduction is in the 2-3.4 dB range, depending on the statistical analysis method applied Nihar Jindal, Broadcom Corporation

March 2013 References [1] Aditya K. Jagannatham and Vinko Erceg, “MIMO Indoor WLAN Channel Measurements and Parameter Modeling at 5.25 GHz”, IEEE 60th Vehicular Technology Conference, 2004, VTC2004-Fall., Vol. 1, 26-29 Sep'04, Pages:106 - 110, Vol. 1. [2] Mark E. Beach, Matthew W. Webb, and Carmen Stan, “Virtual MIMO performance in a measured outdoor-to-indoor cellular scenario”, Antennas and Propagation (EUCAP), Proceedings of the 5th European Conference, 11-15 April 201, Pages: 2933 – 2937. Nihar Jindal, Broadcom Corporation

March 2013 Appendix Nihar Jindal, Broadcom Corporation

Sensitivity to delay spread March 2013 Sensitivity to delay spread Nt = 4, Intended user has 6.3 dB spike + 7.5 dB K factor on 1st tap, and unintended user has 0 dB K factor on 1st tap, no spike Blue: vary delay spread of intended user while keeping unintended user delay spread equal to 100 nsec Green: vary delay spread of unintended user while keeping intended user delay spread equal to 14 nsec Increasing delay spread of intended or unintended user significantly reduces array gain This illustrates the importance of properly modeling the channel statistics to intended/unintended user, e.g., indoor/outdoor Nihar Jindal, Broadcom Corporation

Sensitivity to K-factor March 2013 Nt = 4, Intended user has 14 nsec rms delay spread and 6.3 dB spike on 1st tap, and unintended user has 100 nsec rms delay spread and no spike Blue: vary K-factor of intended user while keeping unintended user K-factor equal to 0 dB Green: vary K-factor of unintended user while keeping unintended user K-factor equal to 7.5 dB Increasing K factor of intended or unintended user increases array gain, although not considerably beyond nominal values of 7.5/0 dB K factor to intended/unintended Nihar Jindal, Broadcom Corporation

Median vs. 90% Values March 2013 Blue: Median rms delay spread and K-factor, 90% point for multipath Green: 90% values for rms delay spread and K-factor, 50% point for multipath Median K-factor with 90% multipath array gain gives 3.7 dB array gain, whereas 90% K-factor with 50% multipath array gives only 2.6 dB Nihar Jindal, Broadcom Corporation