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
Mar 27, 2015
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
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
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
Traditional wireless resources: frequency and time
New resource: space
Large capacity gains possible (in theory)
User #1
Basestation
User #2
User #3
And then there was MIMO …
SDMA - Double the capacity
MIMO - Double the capacity?
Scattering & Multipath
TX RX
MIMO: Spatial Multiplexing
TX RX
1/2
1/2
P/2
P/2
2 2
2 2
P
P
SISO
TX RX
1/2
1/2
P P
1
1
1
1
MIMO: Beamforming*
TX RX
1/2
1/2
P/2
P/2
2 2
2 2
P
P
2PP
* Non standard use of term
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
Combination of Multiplexing and Beamforming
TX RX
SIMO
TX
RX
1/2
1/2
P
1 2
1 2
2P
P
P
MIMO Performance
Depends on the channel gains Assuming channel gains random,
independent: MIMO capacity approximately M times SISO capacity due to spatial multiplexing
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
Large Angular Spread
TX RX
Small Angular Spread
TX RX
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
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”?
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
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
The Promise of MIMO
Increased throughput without requiring more spectrum
Increased range without requiring more transmit power
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
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)
Status of MIMO
802.11n Pre-n products 802.16 3G & beyond
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
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 …
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
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