how many antennas does it take to get wireless access? -the story of mimo n benjamin friedlander n...
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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 [email protected]
April 25, 2005
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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
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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
Traditional wireless resources: frequency and time
New resource: space
Large capacity gains possible (in theory)
User #1
Basestation
User #2
User #3
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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
TX RX
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MIMO: Spatial Multiplexing
TX RX
1/2
1/2
P/2
P/2
2 2
2 2
P
P
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SISO
TX RX
1/2
1/2
P P
1
1
1
1
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MIMO: Beamforming*
TX RX
1/2
1/2
P/2
P/2
2 2
2 2
P
P
2PP
* Non standard use of term
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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
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Combination of Multiplexing and Beamforming
TX RX
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SIMO
TX
RX
1/2
1/2
P
1 2
1 2
2P
P
P
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MIMO Performance
Depends on the channel gains Assuming channel gains random,
independent: MIMO capacity approximately M times SISO capacity due to spatial multiplexing
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Theoretical Capacity
-20 -15 -10 -5 0 5 10 15 20 250
5
10
15
20
25
30
SNR [dB]
Capacity (M=4)
siso
simomimo
Bits/sec/Hz
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Large Angular Spread
TX RX
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Small Angular Spread
TX RX
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Theoretical Capacity
5 10 15 20 25 30 35 40 45 500
5
10
15
20
angle spread [dB]
capacity, snr = 20 dB, M = 4
sisosimomimo
Bits/sec/Hz
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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”?
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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
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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
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The Promise of MIMO
Increased throughput without requiring more spectrum
Increased range without requiring more transmit power
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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
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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)
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Status of MIMO
802.11n Pre-n products 802.16 3G & beyond
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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
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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 …
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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
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For additional information
Please contact [email protected] 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