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

This article explores the concept of MIMO (Multiple Input Multiple Output) and its potential benefits for wireless communication. It discusses the use of multiple antennas, beamforming, spatial multiplexing, and the theoretical capacity of MIMO systems. The article also highlights the challenges and limitations of implementing MIMO technology in practice.

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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

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