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

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

So what do we so with multiple antennas?

Phased Array / Beamformer

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

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

And then there was MIMO …

SDMA - Double the capacity

MIMO - Double the capacity?

Scattering & Multipath

MIMO: Spatial Multiplexing

SISO

MIMO: Beamforming* * Non standard use of term

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

Combination of Multiplexing and Beamforming

SIMO

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

Theoretical Capacity Bits/sec/Hz

Large Angular Spread

Small Angular Spread

Theoretical Capacity Bits/sec/Hz

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 n n Pre-n products n n 3G & beyond

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

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 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