Use of MIMO in 802.11n
By
Chaitanya T K
Email:tkchaitanya@tataelxsi.co.in

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Objectives
This presentation will give an overview of MIMO
technology and its future in Wireless LAN:
 History of MIMO
 Understanding the Key words
 What is MIMO?
 Different Techniques used in MIMO
 MIMO-OFDM
 MIMO Applications
 How it is useful in 802.11n?

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 → .11n
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

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
Standard Approved
Sept. 1999
Sept. 1999
June
2003
?
Available Bandwidth
83.5 MHz
580 MHz
83.5 MHz
83.5/580
MHz
2.4 GHz
5 GHz
2.4 GHz
2.4/5 GHz
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
Frequency Band of
Operation
# Non-Overlapping Channels
(US)
Data Rate per Channel
Modulation Type
DSSS, CCK

Key words used in MIMO



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
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Multi path or Raleigh fading
Antenna gain
Diversity gain
SNR and SNIR
Co-channel and adjacent channel interference
Delay spread

Hurdles in wireless

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
60
Throughput

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

Multi Path Vs Capacity

Channel capacity

For SISO,
C = B*log2(1+x)
For MIMO
C = ΣB*log2(1+x/n*y)
Where,
X-SNR
y-Singular values of Radio channel matrix
N- Number of Tx-Rx antenna pairs.


MIMO Techniques
These are the basic types of MIMO technology:

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Pre-coding
Diversity
 Transmitter Diversity
 Receiver diversity
Maximum Ratio combining
SDMA (Space division Multiple Access)
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

PRE-CODING



In MIMO along with ISI there is anotehr type of
interference developed due to the use of Multiple
antennas, known as MSI (Multi-stream interference). To
remove this MSI we use precoding at Tx and Rx.
The output of the space-time encoder is weighted by the
pre-coding matrix,before being Tx from the antenna.but
this approach requires periodic feedback of the actual
complex elements of the weight matrix.
This can be achieved by using many Precoding
algorithms. (with full feedback/limited feedback)
E.g. Tomlinson---Harashima precoding (THP)
algorithm

Diversity 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 diversity / combining boosts reception of
standard 802.11 signals
Radio
Bits
TX
Radio
Radio
D
S
P
Bits
RX
 Phased array transmit diversity to focus energy to each receiver
D
S
P
Bits
TX
Radio
Radio
Radio
RX
Bits

Diversity in Detail
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Diversity —In MIMO systems, the same
information can be transmitted from multiple
transmit antennas and received at multiple
receive antennas simultaneously.
Since the fading for each link between a pair of
transmit and receive antennas can usually be
considered as
independent, the probability that the information is
detected accurately is increased.
spatial diversity,
temporal diversity
frequency diversity
if the replicas of the faded signals are received in the
form of redundancy in the temporal and frequency



The simplest way of achieving the diversity in MIMO
systems is through repetition coding that sends the same
information symbol at different time slots from different
Tx antennas.
A more BW Efficient coding is ST coding, where a block
of different symbols are Tx in a different order from each
antenna.

Spatial Multiplexing

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It is widely recognized that the capacity of a MIMO
system is much higher than a single-antenna system.
For a rich scattering environment , in a MIMO system
with Mt transmit antennas and Mr receive antennas, the
capacity will grow proportionally with min(Mt,Mr).MIMO
systems provide more spatial freedoms or spatial
multiplexing, so that different information can be Tx
simultaneously over multiple antennas, thereby boosting
the system throughput.
SM needs a dedicated algorithm at the Rx to sort out the
Rx signals. E.g. V-BLAST.

Multipath Mitigation with MRC

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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:

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:


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

Spatial Multiplexing MIMO Theory

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

 y2 = h21x1 + h22 x2


Need more equations than unknowns to succeed
High spectral efficiency

Higher data rate in the same bandwidth

Trade-off b/w Diversity and spatial
Multiplexing

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Depending on the channel conditions and type
of clients you have u can go for either
diversity/SM.
But there are some grouping techniques at the
Rx using which we achieve optimal diversitymultiplexing trade off. E.g. GZF (ZF+ML) and
,GSIC (Interference Cancellation+ML) and LAST
(Lattice space-time coding/decoding).

Different Implementations of spatial
Multiplexing
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Vertical encoding
Horizontal encoding
Vertical Bell Labs Layered Space time
architecture (V-BLAST)

Space Time Coding in MIMO
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A space–time code (STC) is a method employed to
improve the reliability of data transmission in wireless
communication systems using multiple transmit
antennas. STCs rely on transmitting multiple,
redundant copies of a data stream to the receiver in
the hope that at least some of them may survive the
physical path between transmission and reception in a
good enough state to allow reliable decoding.
Space time codes may be split into two main types:
Space–time trellis codes (STTCs) distribute a trellis
code over multiple antennas and multiple time-slots
and provide both coding gain and diversity gain.

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Space–time block codes (STBCs) act on a block of
data at once (similarly to block codes) and provide
only diversity gain, but are much less complex in
implementation terms than STTCs.
STC may be further subdivided according to whether
the receiver knows the channel impairments.
In coherent STC, the receiver knows the channel
impairments through training or some other form of
estimation. These codes have been studied more
widely because they are less complex than their noncoherent counterparts.
In noncoherent STC the receiver does not know the
channel impairments but knows the statistics of the
channel. In differential space–time codes neither the
channel nor the statistics of the channel are available.

ST Coding

SF Coding

SF Coding

STF Coding

Modulation and Coding schemes
To vary the data rate, one can consider the following options:
1.Decreasing the channel spacing or increasing the number of samples within one
second
2.Decreasing the guard band overhead, i.e. increasing the number of data sub
carriers out of total number of sub carriers.
3.Increasing the constellation size, i.e. choosing higher QAM
4.Decreasing the channel coding rate and
5.Decreasing the guard time, i.e. decreasing the cyclic prefix
6.Number of Spatial Streams
7.Number of Encoded streams

Different Modulation Schemes

Symbol error rate comparison

MIMO-OFDM Tx Block Diagram for .11n

MIMO-OFDM Rx Block diagram for .11n

Transmit processing techniques

Receive Processing techniques

What Is Being Proposed for 802.11n?
Main Features
 PHY
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MIMO-OFDM
Diversity
Spatial Multiplexing
Pre-coding
Maximum Ratio Combining
Extended bandwidth (40MHz)
Advanced coding techniques
Beamforming
MAC
 Aggregation
 Block ACK
 Coexistence
 Power saving

Frame formats changed to accommodate
MIMO

Conclusions

The next generation WLAN uses MIMO technology
 It is menu of options, which can make interoperability
difficult to achieve. (depending on cost and usage)
 Power saving is very critical in MIMO.
 The beauty of MIMO is it has every option to suit all
scenarios.
 Diversity MIMO technology


Extends range of existing data rates by transmit and
receive diversities
Spatial-multiplexing MIMO technology

Increases data rates by transmitting parallel data streams

Destiny is no matter of chance, it is a matter of
choice. It is not a thing to be waited for, it is a
thing to be achieved

Queries?