1. How many antennas does it take to get wireless access? -The story of MIMO n Benjamin Friedlander n Department of Electrical Engineering n University of California at Santa Cruz n Phone: 831-459-5838 n friedlan@ee.ucsc.edu friedlan@ee.ucsc.edu April 25, 2005

2. What is MIMO? n MIMO u Multiple Input Multiple Output u Using multiple antennas on both sides of a communication link n SISO u Single Input Single Output n SIMO u Single Input Multiple Output

3. So what do we so with multiple antennas?

4. Phased Array / Beamformer

5. Beam Pattern & Gain Array Single Element Array gain = maximum power density relative to omni-directional antenna

6. Space Division Multiple Access n Traditional wireless resources: frequency and time n New resource: space n Large capacity gains possible (in theory)

8. And then there was MIMO …

9. SDMA - Double the capacity

10. MIMO - Double the capacity?

11. Scattering & Multipath

12. MIMO: Spatial Multiplexing

13. SISO

14. MIMO: Beamforming* * Non standard use of term

15. M x M System n Spatial multiplexing – M channels with gains depending on channel. Average SNR same as SISO. n Beamforming - single channel with SNR gain relative to SISO. n Various intermediate combinations possible

16. Combination of Multiplexing and Beamforming

17. SIMO

18. MIMO Performance n Depends on the channel gains n Assuming channel gains random, independent: MIMO capacity approximately M times SISO capacity due to spatial multiplexing

19. Theoretical Capacity Bits/sec/Hz

20. Large Angular Spread

21. Small Angular Spread

22. Theoretical Capacity Bits/sec/Hz

23. Conclusion #1 n MIMO is best when SNR and angular spread are large n Small angular spread, or presence of a a dominant path (e.g. LOS) reduce MIMO performance n Question: what percentage of cases are MIMO friendly?

24. Beamforming – SNR GAIN n Multiple antennas can be used to provide increased SNR n SNR gain has two components u Array gain – increasing the average power u Diversity gain – decreasing power fluctuations and thereby decreasing required margin

25. Conclusion #2 n Consider a system with a fixed modulation – say 64-QAM. n Spatial multiplexing: increases throughput, not range* n Beamforming: increases range (SNR), not throughput n Possible to do combinations of multiplexing and beamforming n Additional range/throughput tradeoff using variable modulation * Ignoring coding effects

26. The Promise of MIMO n Increased throughput without requiring more spectrum n Increased range without requiring more transmit power

27. Word of Caution n Smart antennas & MIMO can provide large performance gains in theory n In practice implementation issues and system issues often erode much of these gains

28. Some of the issues n What are we comparing to? u Switched diversity u SIMO (RAKE receiver) n Channel Estimation n Performance of multi-user system dominated by worst user (low SNR, small angle spread)

29. Status of MIMO n 802.11n n Pre-n products n 802.16 n 3G & beyond

30. Some 802.11n Proposed Specs n TGn Sync u 2x2, 20 MHz – 140 MBPS u 4x4, 40 MHz – 630 MBPS n WWiSE u 2x2, 20 MHz – 135 MBPS u 4x4, 40 MHz – 540 MBPS

31. So what is the real MIMO advantage? n Most performance claims published so far are not well documented and impossible to evaluate n Need testing over a broad range of deployments and operating conditions, in carefully designed experiments n Only time will tell …

32. Final Words n Many antennas are better than one n Standardization and reduced costs are making MIMO a viable technology n Current MIMO systems – impressive achievement n MIMO improves performance, but: u Your performance may vary … u Thorough performance evaluation not yet available u Differences likely between expectations and reality

33. For additional information n Please contact friedlan@ee.ucsc.edu n Related talks: u Wireless Facts and Fiction u Multi-access methods: TDMA, FDMA, CDMA, OFDMA – so what comes next? u Wireless in the wild west: operating in the unlicensed spectrum. u Communicating on the move – mobility and its limitations u The amazing story of ultra-wideband