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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: n April 25, 2005

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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

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So what do we so with multiple antennas?

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Phased Array / Beamformer

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

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

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And then there was MIMO …

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SDMA - Double the capacity

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MIMO - Double the capacity?

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Scattering & Multipath

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MIMO: Spatial Multiplexing

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SISO

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MIMO: Beamforming* * Non standard use of term

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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

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Combination of Multiplexing and Beamforming

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SIMO

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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

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Theoretical Capacity Bits/sec/Hz

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Large Angular Spread

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Small Angular Spread

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Theoretical Capacity Bits/sec/Hz

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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?

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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

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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

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

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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

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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)

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Status of MIMO n n n Pre-n products n n 3G & beyond

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Some n 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

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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 …

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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

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For additional information n Please contact 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

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