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How many antennas does it take to get wireless access? -The story of MIMO

How many antennas does it take to get wireless access? -The story of MIMO

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How many antennas does it take to get wireless access? -The story of MIMO

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

  2. What is MIMO? • MIMO • Multiple Input Multiple Output • Using multiple antennas on both sides of a communication link • SISO • Single Input Single Output • SIMO • 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 • Traditional wireless resources: frequency and time • New resource: space • Large capacity gains possible (in theory)

  7. And then there was MIMO …

  8. SDMA - Double the capacity

  9. MIMO - Double the capacity?

  10. Scattering & Multipath

  11. MIMO: Spatial Multiplexing

  12. SISO

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

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

  15. Combination of Multiplexing and Beamforming

  16. SIMO

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

  18. Theoretical Capacity Bits/sec/Hz

  19. Large Angular Spread

  20. Small Angular Spread

  21. Theoretical Capacity Bits/sec/Hz

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

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

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

  25. The Promise of MIMO • Increased throughput without requiring more spectrum • Increased range without requiring more transmit power

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

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

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

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

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

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

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