1. Beamforming Array Gain to Intended and Unintended Users
March 2013
doc.: IEEE /xxxxr0
March 2013
Beamforming Array Gain to Intended and Unintended Users
Date:
Authors:
Nihar Jindal, Broadcom Corporation
Nihar Jindal, Broadcom Corporation

2. 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

3. 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
RX power is averaged over 20 MHz
Nihar Jindal, Broadcom Corporation

4. 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

5. 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

6. 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

7. 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

8. 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 dB range, depending on the statistical analysis method applied
Nihar Jindal, Broadcom Corporation

9. 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, Sep'04, Pages: , 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, April 201, Pages: 2933 – 2937.
Nihar Jindal, Broadcom Corporation

10. March 2013
Appendix
Nihar Jindal, Broadcom Corporation

11. Sensitivity to delay spread
March 2013
Sensitivity to delay spread
Nt = 4, Intended user has 6.3 dB spike 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

12. 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

13. 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