Multiple Antennas: Performance Gains with Channel Measurements

1. Multiple Antennas:Performance Gainswith Channel Measurements Minnie Ho, Qinghua Li, Sumeet Sandhu Cliff Prettie, David Cheung Minnie Ho, Intel Corporation

2. Performance Metrics of Interest • Capacity Gain • Definition: bits per second per Hz • Implication: • Efficiency in use of bandwidth • Increase in data rate for given bandwidth • Diversity Order • Definition: slope of BER curve vs. SNR • Implication: • Increase in SNR at a fixed BER • Can translate to increase in range Alamouti code (diversity order = 2) Slope is 2 Minnie Ho, Intel Corporation

3. Theoretical Gains are Well-Known • Shannon Capacity [Telatar] Mr receive antennas Mt transmit antennas Capacity is linear with min(Mr, Mt) • Diversity Order • Maximum diversity order = Mr x Mt • ZF receiver [Winters] Mr + Mt receive antennas Mr-1 (users) interferers can be nulled out Mt+1 diversity order • Questions: • How can we apply these results to real systems? • What are the gains with measured channels? Minnie Ho, Intel Corporation

4. Techniques for Increasing Throughput • Point-to-point MIMO • Increases maximum data rate between a user and an AP (PHY rate) • Improves spectral efficiency (bits/sec/Hz) • Point-to-multipoint SDMA • Simultaneous transmission from AP to multiple users • Increases throughput at the MAC SAP at an AP MIMO SDMA Minnie Ho, Intel Corporation

5. Techniques for Increasing Diversity • Switched diversity (selection between multiple antennas) • MRC • Alamouti • MMSE (minimum mean-squared error) • ZF (zero-forcing) • ML (maximum likelihood) Minnie Ho, Intel Corporation

6. Measurement environment: Enterprise (office cubicles) Stationary (night) Automated positions WLAN “STA” WLAN “AP” Minnie Ho, Intel Corporation

7. Measurement Setup • At a given tone, the complex channel response from 1 transmit location to 1 receive location is measured • RF component responses compensated from measurements • Other frequency bands measured (UWB, full UNII, ISM) • 750,000+ measurements in searchable database Minnie Ho, Intel Corporation

8. Propagation Snapshot at 3.5 m Minnie Ho, Intel Corporation

9. Theoretical Capacity Results • SDMA • 4 antennas at the AP (Mr = 4) • 1 antenna at each STA (Mt = 1) • Four “degrees of freedom” • AP should be able to support 4 simultaneous users • MIMO • 2 antennas at the AP (Mr = 2) • 2 antennas at the STA (Mt = 2) • Link should be able to support 2x the data rate Minnie Ho, Intel Corporation

10. Measurement Locations and Grid 12m 5m 7m 9m 3m 25m 14m 18m 13m Minnie Ho, Intel Corporation

11. Measured Capacity: Downlink • SDMA increases the capacity of SISO between 3 and 4 times • The capacity of zero-forcing is close the maximum (coordination) Minnie Ho, Intel Corporation

12. Measured Capacity: Uplink • SDMA increases the capacity of SISO between 3 and 4 times. • MMSE technique reduces capacity and suffers from noise amplification and MAI. Minnie Ho, Intel Corporation

13. Theoretical Diversity Gain • Theoretical • Maximum diversity order = Mr x Mt • Diversity order for ZF receiver = Mr – Mt + 1 [Winters et al] • SDMA • 4 antennas at the AP (Mr = 4) • 1 antenna at each of 3 STAs (Mt = 3) • ZF Diversity order should be 2 for each link • MIMO • 2 antennas at the AP (Mr = 2) • 2 antennas at the STA (Mt = 2) • MMSE Diversity order should be about 1 Minnie Ho, Intel Corporation

14. Measured Diversity: Downlink • 4-antenna diversity gain compared to SISO is 1.6 • 5-antenna diversity gain compared to SISO is 2.6 Minnie Ho, Intel Corporation

15. Measured Diversity: Uplink • 4-antenna diversity gain compared to SISO is 1.5 • 5-antenna diversity gain compared to SISO is 2.0 Minnie Ho, Intel Corporation

16. Theoretical Capacity Results • SDMA • 4 antennas at the AP (Mr = 4) • 1 antenna at each STA (Mt = 1) • AP should be able to support 4 simultaneous users • MIMO • 2 antennas at the AP (Mr = 2) • 2 antennas at the STA (Mt = 2) • Link should be able to support 2x the data rate Minnie Ho, Intel Corporation

17. Measurement Locations and Grid 12m 5m 7m 9m 3m 25m 14m 18m 13m Minnie Ho, Intel Corporation

18. Measured Capacity Minnie Ho, Intel Corporation

19. Minnie Ho, Intel Corporation

20. Theoretical Diversity Gain • Theoretical • Maximum diversity order = Mr x Mt • ZF Diversity order = Mr – Mt + 1 [Winters et al] • SDMA • 4 antennas at the AP (Mr = 4) • 1 antenna at each of 3 STAs (Mt = 3) • ZF Diversity order should be 2 for each link • MIMO • 2 antennas at the AP (Mr = 2) • 2 antennas at the STA (Mt = 2) • MMSE Diversity order should be about 1 Minnie Ho, Intel Corporation

21. Measured Diversity • Diversity is of order 1 for uncoded MMSE • Diversity is the same order as SISO Minnie Ho, Intel Corporation

22. Conclusions • Measured capacity less than (close to) theoretical • Measured diversity less than (close to) theoretical • Practical system design is the challenge Minnie Ho, Intel Corporation

23. For further information Send e-mail to Minnie Ho • To: minnie.ho@intel.com • Subject: SmantIntel Minnie Ho, Intel Corporation

24. Backup Slides Minnie Ho, Intel Corporation

25. BER Comparison for SDMA • Downlink and BER 10-5 • SDMA with 4 antennas requires 1ower SNR than SISO and SISO with selection in 86% and 74% of the time respectively. • SDMA with 5 antennas requires 1ower SNR than SISO and SISO with selection in 97% and 92% of the time respectively. • Uplink and BER 10-5 • SDMA with 4 antennas requires 1ower SNR than SISO and SISO with selection in 65% and 50% of the time respectively. • SDMA with 5 antennas requires 1ower SNR than SISO and SISO with selection in 81% and 71% of the time respectively. Minnie Ho, Intel Corporation