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MIMO Technology for Advanced Wireless Local Area
Networks Dr. Won-Joon ChoiDr. Qinfang Sun Dr.
Jeffrey M. Gilbert Atheros Communications2005
Design Automation Conference June 15, 2005
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Agenda

• This presentation will give an overview of MIMO
  technology and its future in Wireless LAN
• Wireless Local Area Networks (WLAN)
• Current standards (11a/b/g)
• Next-generation 11n overview and status
• MIMO fundamentals
• Beamforming
• Spatial Multiplexing
• MIMO scalability
• Bandwidth
• Number of spatial streams

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The Wireless LAN Explosion
The Wireless LAN / Wi-Fi market has exploded! New
technology is enabling new applications
Office
Hot-spots
Home
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Wireless LAN Technology Advances

• Wireless LAN technology has seen rapid
  advancements
• Standards
• Data rates
• Range / coverage
• Integration
• Cost

802.11 ? .11b ? .11a ? .11g
2Mbps ? 100 Mbps
Meters ? kilometers
Multiple discretes ? single chip solutions
100s ? 10s (sometimes free w/rebates!)

• How can this growth continue?
• Previous advances have been limited to a single
  transmitting and receiving radio
• The next generation exploits multiple parallel
  radios using revolutionary class of techniques
  called MIMO (Multiple Input Multiple Output) to
  send information farther and faster

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Existing 802.11 WLAN Standards
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What Is Being Proposed for 802.11n?

• Main Features
• PHY
• MIMO-OFDM
• Beamforming
• Spatial Multiplexing
• Extended bandwidth (40MHz)
• Advanced coding
• MAC
• Aggregation
• Block ACK
• Coexistence
• Power saving

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Wireless Fundamentals I

• In order to successfully decode data, signal
  strength needs to be greater than noise
  interference by a certain amount
• Higher data rates require higher SINR (Signal to
  Noise and Interference Ratio)
• Signal strength decreases with increased range in
  a wireless environment

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Wireless Fundamentals II

• Ways to increase data rate
• Conventional single tx and rx radio systems
• Increase transmit power
• Subject to power amplifier and regulatory limits
• Increases interference to other devices
• Reduces battery life
• Use high gain directional antennas
• Fixed direction(s) limit coverage to given
  sector(s)
• Use more frequency spectrum
• Subject to FCC / regulatory domain constraints
• Advanced MIMO Use multiple tx and / or rx radios!

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Conventional (SISO) Wireless Systems
DSP
Bits
DSP
Radio
Radio
Bits
TX
RX

• Conventional Single Input Single Output (SISO)
  systems were favored for simplicity and low-cost
  but have some shortcomings
• Outage occurs if antennas fall into null
• Switching between different antennas can help
• Energy is wasted by sending in all directions
• Can cause additional interference to others
• Sensitive to interference from all directions
• Output power limited by single power amplifier

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MIMO Wireless Systems
Radio
Radio
DSP
DSP
Bits
Bits
Radio
Radio
TX
RX

• Multiple Input Multiple Output (MIMO) systems
  with multiple parallel radios improve the
  following
• Outages reduced by using information from
  multiple antennas
• Transmit power can be increased via multiple
  power amplifiers
• Higher throughputs possible
• Transmit and receive interference limited by some
  techniques

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MIMO Alternatives

• There are two basic types of MIMO technology
• Beamforming MIMO
• Standards-compatible techniques to improve the
  range of existing data rates using transmit and
  receive beamforming
• Also reduces transmit interference and improves
  receive interference tolerance
• Spatial-multiplexing MIMO
• Allows even higher data rates by transmitting
  parallel data streams in the same frequency
  spectrum
• Fundamentally changes the on-air format of
  signals
• Requires new standard (11n) for standards-based
  operation
• Proprietary modes possible but cannot help legacy
  devices

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Beamforming MIMO Overview

• Consists of two parts to make standard 802.11
  signals better
• Uses multiple transmit and/or receive radios to
  form coherent 802.11a/b/g compatible signals
• Receive beamforming / combining boosts reception
  of standard 802.11 signals

DSP
Radio
Bits
Bits
Radio
TX
Radio
RX

• Phased array transmit beamforming to focus energy
  to each receiver

DSP
Radio
Bits
Bits
Radio
Radio
RX
TX
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Benefits of Beamforming

• Benefits
• Power gain (applicable only to transmit
  beamforming)
• Power from multiple PAs simultaneously (up to
  regulatory limits)
• Relaxes PA requirements, increases total output
  power delivered
• Array gain dynamic high-gain antenna
• Interference reduction
• Reduce co-channel inter-cell interference
• Diversity gain combats fading effects
• Multipath mitigation
• Per- subcarrier beamforming to reduce spectral
  nulls

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Multipath Mitigation

• Multiple transmit and receive radios allow
  compensation of notches on one channel by
  non-notches in the other
• Same performance gains with either multiple tx or
  rx radios and greater gains with both multiple tx
  and rx radios

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Spatial Multiplexing MIMO Concept

• Spatial multiplexing concept
• Form multiple independent links (on same channel)
  between transmitter and receiver to communicate
  at higher total data rates

Radio
DSP
Radio
DSP
BitMerge
BitSplit
Bits
Bits
Radio
DSP
Radio
DSP
RX
TX
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Spatial Multiplexing MIMO Difficulties

• Spatial multiplexing concept
• Form multiple independent links (on same channel)
  between transmitter and receiver to communicate
  at higher total data rates
• However, there are cross-paths between antennas

Radio
DSP
Radio
DSP
BitMerge
BitSplit
Garbage
Bits
Radio
DSP
Radio
DSP
RX
TX
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Spatial Multiplexing MIMO Reality

• Spatial multiplexing concept
• Form multiple independent links (on same channel)
  between transmitter and receiver to communicate
  at higher total data rates
• However, there are cross-paths between antennas
• The correlation must be decoupled by digital
  signal processing algorithms

DSP
Radio
DSP
Radio
BitMerge
BitSplit
Bits
Bits
Radio
DSP
Radio
RX
TX
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Spatial Multiplexing MIMO Theory

• High data rate
• Data rate increases by the minimum of number of
  transmit and receive antennas
• Detection is conceptually solving equations
• Example of 2-by-2 system
• Transmitted signal is unknown,
• Received signal is known,
• Related by the channel coefficients,
• Need more equations than unknowns to succeed
• High spectral efficiency
• Higher data rate in the same bandwidth

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MIMO Scalability

• Moores law
• Doubling transistors every couple of years
• MIMO
• Increases number of streams
• Higher performance/speed
• Higher complexity
• MIMO is the bridge to allow us to exploit Moores
  law to get higher performance

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MIMO Scalability

• Notation
• R data rates (Mbps)
• Es spectral efficiency (bps/Hz)
• Bw bandwidth (MHz)
• Ns number of spatial streams
• NR number of Rx chains
• NT number of Tx chains

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MIMO Scalability

• Data Rates
• R Es Bw Ns -gt Scales with bandwidth and the
  number of spatial streams
• Example
• 11a/g Es 2.7 Bw 20MHz Ns1 R 54Mbps
• Spatial multiplexing MIMO
• Es 3.75 Bw40MHzNs 2 R 300Mbps
• Number of Tx/Rx chains
• At least as many chains as Ns
• Ns min(NR, NT)

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MIMO Hardware Requirements

• MIMO Transmitter (parallelism and data rate
  scaling)

IFFT
MOD
RF
Stream Split
Spatial Mapping
FEC
RF
IFFT
MOD
1 O(BwEsNs)
Ns O(BwEs)
1 O(BwEsNsNT)
NT O(BwEs)
NT Analog RF
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MIMO Hardware Requirements

• MIMO Receiver (parallelism and data rate scaling)

Demod
RF
Stream Merge
MIMO Equalizer
RF
Demod
1 O(BwEsNs)
NR Analog RF
1 O(BwEsNRNs2)
NR O(BwEs)
Ns O(BwEs)
Ns O(BwEs)
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Conclusions

• The next generation WLAN uses MIMO technology
• Beamforming MIMO technology
• Extends range of existing data rates by transmit
  and receive beamforming
• Spatial-multiplexing MIMO technology
• Increases data rates by transmitting parallel
  data streams
• MIMO allows system designers to leverage Moores
  law to deliver higher performance wireless
  systems

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Circuit Implications of MIMO

• Crystal
• Common crystal is required
• Synthesizer
• Common synthesizer is preferred
• PA
• Allow additional flexibility
• With total power limit, PA requirements relaxed
• With PA limit, total power increased.
• Cross-talk/ Coupling
• Need to minimize coupling between antennas

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Circuit Impairments/Corrections

• Timing offset
• Common across multiple chains
• Frequency offset
• Common across multiple chains
• Phase noise
• Common with common synthesizer
• With independent synthesizers, a new tracking
  algorithm may be needed.
• Other impairments
• 1/f noise, I/Q mismatch, spurs, etc.
• Estimated and corrected for each chain

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Backup Slides

• 0.18um standard digital CMOS
• 7.2x7.2 mm2 die size
• 15x15mm2 BGA with 261 balls
• Ref ISSCC05

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Backup Slides
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Backup Slides