MIMO Technology for
Advanced Wireless Local Area Networks
Dr. Won-Joon Choi
Dr. Qinfang Sun
Dr. Jeffrey M. Gilbert
Atheros Communications
MIMO RAKE Antenna Technology for Advanced
MIMO Wireless WAN and LAN
Pr. Jean-Claude Ducasse
Hypercable Telecommunications
What Is Being Proposed for 802.11n?
Main Features
 PHY
 MIMO-OFDM
 Beamforming
 Spatial



Extended bandwidth (40MHz)
Advanced coding
MAC




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

Higher data rates require higher SINR (Signal to Noise and
Interference Ratio)
Signal strength decreases with increased range in a wireless
environment
Throughput

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:

Conventional single tx and rx radio systems

Increase transmit power




Use high gain directional antennas


Fixed direction(s) limit coverage to given sector(s)
Use more frequency spectrum


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:




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:




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:


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

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

Power gain (applicable only to transmit beamforming)


Power from multiple PA’s 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
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
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
DSP
Bits
Bit
Split
TX
DSP
Radio
Radio
Radio
Radio
DSP
DSP
Bit
Merge
RX
Bits
MIMO RAKE Antenna Concept
Bits
Radio
Radio
DSP
DSP
Bit
Split
TX
TX
RX
Bit
Merge
Bits
DSP
DSP
Bit
Merge
RX
GIGABIT I/O & POE
Bits
Radio
Radio
DSP
DSP
Radio
Radio
Radio

DSP

Bit
Split

Radio

Dual Circular Polarization Diversity
Spatial Multiplexing
Multipath Mitigation
Space diversity
Beamforming
DSP

Bits
Bits
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
DSP
Bits
Bit
Split
TX
DSP
Radio
Radio
Radio
Radio
DSP
DSP
Bit
Merge
RX
Garbage
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
Bits
Bit
Split
TX
DSP
Radio
Radio
Radio
Radio
D
S
P
Bit
Merge
RX
Bits
MIMO RAKE Antenna Solution
Bits
Radio
Radio
DSP
DSP
Bit
Split
TX
TX
RX
Bit
Merge
Bits
DSP
DSP
Bit
Merge
RX
GIGABIT I/O & POE
Bits
Radio
Radio
DSP
DSP
Radio
Radio
Radio

DSP

Bit
Split

Radio

Dual Circular Polarization Diversity
Spatial Multiplexing
Multipath Mitigation
Space diversity
Beamforming
DSP

Bits
Bits
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, x1 , x2
Received signal is known, y1 , y2
Related by the channel coefficients, h11, h12 , h21, h22
 y1  h11x1  h12 x2

 y2  h21x1  h22 x2


Need more equations than unknowns to succeed
High spectral efficiency

Higher data rate in the same bandwidth
MIMO Scalability

Data Rates


R = Es * Bw * Ns -> Scales with bandwidth and the
number of spatial streams
Example


11a/g: Es = 2.7; Bw = 20MHz; Ns=1; R = 54Mbps
Spatial multiplexing MIMO
Es = 3.75; Bw=40MHz;Ns = 2; R = 300Mbps

Number of Tx/Rx chains

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

The next generation WLAN uses MIMO technology
 Beamforming MIMO technology


Spatial-multiplexing MIMO technology


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