Presentation on theme: "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."— Presentation transcript:
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 email@example.com firstname.lastname@example.org April 25, 2005
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
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
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?
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
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
The Promise of MIMO n Increased throughput without requiring more spectrum n Increased range without requiring more transmit power
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
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)
Status of MIMO n 802.11n n Pre-n products n 802.16 n 3G & beyond
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
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 …
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
For additional information n Please contact email@example.com 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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