Transcript MIMO Technology for Advanced Wireless Local Area Networks

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MIMO Technology for
Advanced Wireless Local Area Networks
Dr. Won-Joon Choi
Dr. Qinfang Sun
Dr. Jeffrey M. Gilbert
Atheros Communications
2005 Design Automation Conference – June 15, 2005
Agenda
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This presentation will give an overview of MIMO
technology and its future in Wireless LAN:
Wireless Local Area Networks (WLAN)
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MIMO fundamentals
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Current standards (11a/b/g)
Next-generation 11n overview and status
Beamforming
Spatial Multiplexing
MIMO scalability
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Bandwidth
Number of spatial streams
The Wireless LAN Explosion
The Wireless LAN / Wi-Fi market has exploded!
New technology is enabling new applications:
Office
Home
“Hot-spots”
Email / Info anywhere
Voice over IP
Internet everywhere
Multimedia
Hot-spot coverage
Metro-Area Networks
Wireless LAN Technology Advances
Wireless LAN technology has seen rapid advancements
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Standards: 802.11  .11b  .11a  .11g
Data rates: 2Mbps  100+ Mbps
Range / coverage: Meters  kilometers
Integration: Multiple discretes  single chip solutions
Cost: $100’s  $10’s (sometimes free w/rebates!)
How can this growth continue?
 Previous advances have been limited to a single
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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
Existing 802.11 WLAN Standards
802.11b
802.11a
802.11g
802.11n
Sept. 1999
Sept. 1999
June
2003
?
83.5 MHz
580 MHz
83.5 MHz
83.5/580
MHz
Frequency Band of Operation
2.4 GHz
5 GHz
2.4 GHz
2.4/5 GHz
# Non-Overlapping Channels
(US)
3
24
3
3/24
1 – 11 Mbps
6 – 54 Mbps
1 – 54 Mbps
1 – 600
Mbps
OFDM
DSSS, CCK,
OFDM
DSSS, CCK,
OFDM,
MIMO
Standard Approved
Available Bandwidth
Data Rate per Channel
Modulation Type
DSSS, CCK
What Is Being Proposed for 802.11n?
Main Features
 PHY
 MIMO-OFDM
 Beamforming
 Spatial
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Extended bandwidth (40MHz)
Advanced coding
MAC
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Multiplexing
Aggregation
Block ACK
Coexistence
Power saving
Wireless Fundamentals I
In order to successfully decode data, signal strength needs to be
greater than noise + interference by a certain amount
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Higher data rates require higher SINR (Signal to Noise and
Interference Ratio)
Signal strength decreases with increased range in a wireless
environment
Throughput
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60
Data Rate 1
50
Data Rate 2
40
30
20
10
0
1
2
3
4
5
6
7
Range
8
9 10 11 12
Wireless Fundamentals II
Ways to increase data rate:
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Conventional single tx and rx radio systems
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Increase transmit power
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Use high gain directional antennas
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Fixed direction(s) limit coverage to given sector(s)
Use more frequency spectrum
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Subject to power amplifier and regulatory limits
Increases interference to other devices
Reduces battery life
Subject to FCC / regulatory domain constraints
Advanced MIMO: Use multiple tx and / or rx radios!
Conventional (SISO)
Wireless Systems
channel
Bits
DSP
Radio
Radio
DSP
TX
Bits
RX
Conventional “Single Input Single Output” (SISO)
systems were favored for simplicity and low-cost
but have some shortcomings:
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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
MIMO Wireless Systems
D
S
P
Bits
TX
Radio
Radio
channel
Radio
Radio
D
S
P
Bits
RX
Multiple Input Multiple Output (MIMO) systems with multiple
parallel radios improve the following:
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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
MIMO Alternatives
There are two basic types of MIMO technology:
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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
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
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Receive beamforming / combining boosts reception of
standard 802.11 signals
Radio
Bits
Radio
TX
Radio
D
S
P
Bits
RX
 Phased array transmit beamforming to focus energy to each
receiver
D
S
P
Bits
TX
Radio
Radio
Radio
Bits
RX
Benefits of Beamforming
Benefits
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Power gain (applicable only to transmit beamforming)
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Power from multiple PA’s simultaneously
(up to regulatory limits)
Relaxes PA requirements, increases total
output power delivered
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Array gain: “dynamic high-gain antenna”
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Interference reduction
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Reduce co-channel inter-cell interference
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Diversity gain: combats fading effects
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Multipath mitigation
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Per- subcarrier beamforming to reduce spectral nulls
Multipath Mitigation
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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
Spatial Multiplexing MIMO Concept
Spatial multiplexing concept:
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Form multiple independent links (on same channel) between
transmitter and receiver to communicate at higher total data rates
DSP
Bits
Bit
Split
TX
DSP
Radio
Radio
Radio
Radio
DSP
DSP
Bit
Merge
RX
Bits
Spatial Multiplexing MIMO Difficulties
Spatial multiplexing concept:
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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
DSP
Bits
Bit
Split
TX
DSP
Radio
Radio
Radio
Radio
DSP
DSP
Bit
Merge
RX
Garbage
Spatial Multiplexing MIMO Reality
Spatial multiplexing concept:
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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
Bits
Bit
Split
TX
DSP
Radio
Radio
Radio
Radio
D
S
P
Bit
Merge
RX
Bits
Spatial Multiplexing MIMO Theory
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High data rate
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Data rate increases by the minimum of number of transmit and
receive antennas
Detection is conceptually solving equations
Example of 2-by-2 system:
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Transmitted signal is unknown, x1 , x2
Received signal is known, y1 , y2
Related by the channel coefficients, h11, h12 , h21, h22
 y1  h11x1  h12 x2
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 y2  h21x1  h22 x2
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Need more equations than unknowns to succeed
High spectral efficiency
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Higher data rate in the same bandwidth
MIMO Scalability
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Moore’s law
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Doubling transistors every couple of years
MIMO
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Increases number of streams
Higher performance/speed
Higher complexity
MIMO is the bridge to allow us to exploit
Moore’s law to get higher performance
MIMO Scalability
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Notation
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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
MIMO Scalability
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Data Rates
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R = Es * Bw * Ns -> Scales with bandwidth and the
number of spatial streams
Example
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11a/g: Es = 2.7; Bw = 20MHz; Ns=1; R = 54Mbps
Spatial multiplexing MIMO
Es = 3.75; Bw=40MHz;Ns = 2; R = 300Mbps
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Number of Tx/Rx chains
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At least as many chains as Ns
Ns = min(NR, NT)
MIMO Hardware Requirements
MIMO Transmitter (parallelism and data rate scaling)
MOD
FEC
Stream
Split
RF
IFFT
RF
Spatial
Mapping
MOD
1*
O(Bw*Es*Ns)
IFFT
Ns *
O(Bw*Es)
1*
NT*
NT*
O(Bw*Es*Ns*NT) O(Bw*Es) Analog RF
MIMO Hardware Requirements
MIMO Receiver (parallelism and data rate scaling)
RF
FFT
Demod
Stream
Merge
MIMO
Equalizer
RF
NR*
Analog RF
Demod
FFT
NR*
O(Bw*Es)
DEC
1*
O(Bw*Es*NR*Ns2)
Ns*
O(Bw*Es)
Ns*
1*
O(Bw*Es) O(Bw*Es*Ns)
Conclusions
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The next generation WLAN uses MIMO technology
 Beamforming MIMO technology
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Spatial-multiplexing MIMO technology
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Extends range of existing data rates by transmit and
receive beamforming
Increases data rates by transmitting parallel data streams
MIMO allows system designers to leverage Moore’s law to
deliver higher performance wireless systems
Circuit Implications of MIMO
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Crystal
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Synthesizer
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Common crystal is required
Common synthesizer is preferred
PA
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Allow additional flexibility
With total power limit, PA requirements relaxed
 With PA limit, total power increased.
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Cross-talk/ Coupling
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Need to minimize coupling between antennas
Circuit Impairments/Corrections
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Timing offset
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Frequency offset
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Common across multiple chains
Phase noise
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Common across multiple chains
Common with common synthesizer
With independent synthesizers, a new tracking
algorithm may be needed.
Other impairments
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1/f noise, I/Q mismatch, spurs, etc.
Estimated and corrected for each chain
Backup Slides
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0.18um
standard
digital CMOS
7.2x7.2 mm2
die size
15x15mm2
BGA with 261
balls
Ref: ISSCC’05
Backup Slides
MIPS R4Kc, 16kB I and D caches
180 MHz
16b SDRAM interface
100 MHz
9b ADCs (4x)
< 0.65 LSB INL&DNL, -48dB
SNDR, 27mW
9b DACs (4x)
<0.25 LSB INL&DNL, -51dB SNDR,
20mW
Total power, PCI mode, CPU off
690 mW
Total power, MPEG-TS mode, CPU
on
1.8W
Supports 802.11 a, b, g, 20 and 40 1 to 108 Mb/s raw data rates
MHz channel BW
Backup Slides
Tx
2.4/5 GHz
Rx Transceiver
ADCs
SDRAM Controller
and Memory
Interface
SDRAM and
Flash
MPEG-TS
Local Bus
MIPS Processor
Tx
2.4/5 GHz
Transceiver
Rx
DACs
I2C
WLAN SOC
WLAN MAC
PCI
MRC/BF
UART
OFDM Mod/
Demod
Peripheral
Interface
Radio Control
IR Interface
Video
Encoder/
Decoder
RS232
LED Control
GPIOs
IR Remote
Control
Host System