802.11n Specification
and the use of
Space-Time Wireless Channels
Shad Nygren
April 27, 2006
Del Mar Electronics Show
Objectives
• Discuss the history and present state of the
802.11n specification.
• Discuss MIMO, Space-Time Wireless
Channels and Space-Time Block Codes
which are one of the most interesting
aspects of the 802.11n specification.
• Understand how the magic of MIMO and
Space-Time Wireless Channels work.
About Me
• Master’s Degree in Computer Science from
University of Nevada, Reno
• 24 years experience with computers,
networking and wireless communications
802.11n History
• 1999, 802.11a/b standards ratified by IEEE
• June 2003, 802.11g ratified by IEEE.
• 802.11g was based on OFDM from 802.11a
but using the 2.4GHz band and backwards
compatible with 802.11b
• January 2004, IEEE forms new 802.11 Task
Group (TGn) to investigate higher data rates
802.11n History Cont
• Standards Process: From many proposals down to
two
– TGnSync
– WWiSE
• After much debate these two groups created a
Joint Proposal
• October 2005, the Enhanced Wireless Consortium
(EWC) was founded by Intel, Broadcom, Marvell,
Atheros and others
802.11n Progress in 2006
• Jan 19, 2006, IEEE 802.11n task group approved
the Joint Proposal’s specification based on EWC’s
specification.
• March 2006 IEEE 802.11 Working Group sent the
802.11n Draft to its first letter ballot.
• Currently working its way thru the IEEE standards
process. Hopefully a final standard will be in place
in about a year.
802.11n Goals
• Investigate next generation wireless LAN
technology capable of supporting
multimedia applications
• Provide higher data rates than 802.11b/g –
At least 100Mbps at MAC layer
• Backwards compatibility with 802.11b/g
802.11n Physical Layer
• Operates in 2.4GHz and/or 5GHz
unlicensed bands
• Uses OFDM like 802.11a/g
• Backwards compatible and mixed mode
interoperable with 802.11a/b/g
• High Throughput (HT) and 40MHz modes
• Optionally uses MIMO
2.4GHz Unlicensed Band
802.11b Channel Frequency Map
Channel
Lower Freq
Center Freq
Upper Freq
1
2.401
2.412
2.423
2
2.406
2.417
2.428
3
2.411
2.422
2.433
4
2.416
2.427
2.438
5
2.421
2.432
2.443
6
2.426
2.437
2.448
7
2.431
2.442
2.453
8
2.436
2.447
2.458
9
2.441
2.452
2.463
10
2.446
2.457
2.468
11
2.451
2.462
2.473
802.11g OFDM
•
•
•
•
•
•
•
64 point FFT
52 OFDM subcarriers
48 Data Carriers
4 Pilot Carriers
12 unused carriers
Carrier Separation 0.3125MHz (20MHz/64)
Total Bandwidth 20MHz with occupied bandwidth
of 16.6MHz
• Symbol duration 4us with 0.8us guard interval
OFDM Carriers
Source: International Engineering Consortium
http://www.iec.org/online/tutorials/ofdm/topic04.html
802.11a/g OFDM Rates
250,000 Symbols per Sec
Modulation
Coding Rate
Data Carriers
Data Rate (Mbps)
BPSK
1/2
48
6
BPSK
3/4
48
9
QPSK
1/2
48
12
QPSK
3/4
48
18
16 QAM
1/2
48
24
16 QAM
3/4
48
36
64 QAM
2/3
48
48
64 QAM
3/4
48
54
802.11a/g OFDM Physical Layer
• Divided into two elements
– PLCP – Physical Layer Convergence Protocol
prepares frames for transmission and directs the
PMD to transmit and receive signals, change
channels etc
– PMD – Physical Medium Dependant layer
provides actual transmission and reception over
the wireless medium by modulating and
demodulating the frame transmissions
Options for Increasing Data Rate
• Double the Clock Rate – From 20MHz
(250,000 Symbols per Second) to 40MHz
(500,000 Symbols per Second)
• Double the Number of Carriers – From 64
to 128, not increasing the bandwidth
• Use Higher Order Modulation – From
64QAM (6 bits / symbol) to 4096QAM (12
bits / symbol)
Options for Increasing Data Rate
• OFDM with Bit Loading – Different
Modulation Per Carrier
• Better Code – Turbo or Low Density Parity
Check
• MIMO – Multiple Input Multiple Output
antennas for multiple data streams
Higher Data Rate Considerations
Larger Constellation 54Mbps already uses 64QAM. Can a
wireless system support a larger
constellation?
Turbo Coding
Requires at least 3 or 4 iterations for
good performance.
Double Bandwidth
Inefficient use of bandwidth.
MIMO – Multiple
Antennas
Cost is the additional antennas and RF
electronics, the DSP does not add
much complexity to existing receivers.
802.11n OFDM
• 20MHz High Throughput Mode
– 56 OFDM subcarriers
– 52 Data Carriers
– 4 Pilot Carriers
• 40MHz High Throughput Mode
– 114 OFDM subcarriers (2 extra subcarriers)
– 108 Data Carriers (4 extra data carriers)
– 6 Pilot Carriers (2 less pilot carriers)
802.11n Mandatory Features
• Frame Aggregation
• Block ACK
• N-immediate ACK – Block ACK between
two HT peers using an immediate Block
Ack policy
• Long NAV – Provides protection for a
sequence of multiple PPDUs
NAV
Network Allocation Vector
• Counter resident at each station that
represents the amount of time that the
previous station needs to send its frame.
• The NAV must be zero before a station can
attempt to send a frame.
• The transmitting station calculates the
amount of time necessary to send the frame
based on the frame’s length and data rate.
NAV
Network Allocation Vector
• The transmitting station places a value in the
duration field in the header representing the time
required to transmit the frame.
• When stations receive a frame, they examine the
duration field value and use it as the basis for
setting their corresponding NAV.
• This process reserves the medium for the sending
stations.
802.11n Optional Features
• Advanced Coding – Using different coding
per OFDM carrier
• Green Field mode
• Beamforming
• Short Guard Interval – Reduce from 800ns
(250,000 symbols per second) to 400ns and
send 277,778 symbols per second
• Space Time Block Coding
802.11n Modes
• Legacy Mode – packets are transmitted in the
legacy 802.11a/g format
• Mixed Mode – packets are transmitted with a
preamble compatible with 802.11a/g so they can
be decoded by legacy devices while the rest of the
packet is transmitted in the new mode
• Green Field – optional mode where the packets
are transmitted without the legacy compatibility
part
802.11n for 20/40MHz operation
• 40MHz comprised of two adjacent 20MHz channels
– One Control Channel
– One Extension Channel
• Beacon is sent in legacy mode on the control channel only
• A single BSS may include:
– 20MHz-only capable stations
– 20/40MHz capable stations
– Legacy stations
• Clear Channel Assessment will be done on the control
channel and possibly on the extension channel. The results
will then be combined.
802.11n Modulation and Coding
per Spatial Stream
Modulation
Code Rate
Data Carriers
Data Rate Mbps
(GI=800ns)
Data Rate Mbps
(GI=400ns)
BPSK
1/2
52/108
6.5/13.5
7.22/15
QPSK
1/2
52/108
13/27
14.44/30
QPSK
3/4
52/108
19.5/40.5
21.66/45
16QAM
1/2
52/108
26/54
28.88/60
16QAM
3/4
52/108
39/81
43.33/90
64QAM
2/3
52/108
52/108
57.66/120
64QAM
3/4
52/108
58.5/121.5
65/135
64QAM
5/6
52/108
65/135
72.22/150
802.11n Modulation and Coding
Two Spatial Streams
Modulation
Code Rate
Data Carriers
Data Rate Mbps
(GI=800ns)
Data Rate Mbps
(GI=400ns)
BPSK
1/2
52/108
13/27
14.44/30
QPSK
1/2
52/108
26/54
28.88/60
QPSK
3/4
52/108
39/81
43.32/90
16QAM
1/2
52/108
52/108
57.76/120
16QAM
3/4
52/108
78/162
86.66/180
64QAM
2/3
52/108
104/216
115.32/240
64QAM
3/4
52/108
117/243
130/270
64QAM
5/6
52/108
130/270
144.44/300
802.11n Modulation and Coding
Four Spatial Streams
Modulation
Code Rate
Data Carriers
Data Rate Mbps
(GI=800ns)
Data Rate Mbps
(GI=400ns)
BPSK
1/2
52/108
26/54
28.88/60
QPSK
1/2
52/108
52/108
57.76/120
QPSK
3/4
52/108
78/162
86.64/180
16QAM
1/2
52/108
104/216
115.52/240
16QAM
3/4
52/108
156/324
173.32/360
64QAM
2/3
52/108
208/432
230.64/480
64QAM
3/4
52/108
234/486
260/540
64QAM
5/6
52/108
260/540
288.88/600
MIMO
Any sufficiently advanced technology
is indistinguishable from magic.
Arthur C. Clarke
MIMO Magic
• MIMO is not magic but is an advanced RF
communications technology based on valid
mathematical and scientific principals
• MIMO does not violate Shannon’s Law
• Pronounced “MyMoe” – This was
standardized by a vote at an IEEE meeting.
Multiple Antennas
• Well studied topic for the past few years
• OFDM is very well suited for use with multiple
antennas
• Many existing 802.11 products already have 2
antennas, using switched diversity
• Additional component required for exploiting full
diversity is an additional RF front-end
• Recent advances in RF technology will make this
cost effective in the near future
Antenna Diversity
•
•
•
•
Space Diversity
Polarization Diversity
Pattern Diversity
Transmit Diversity
Temporal Diversity
• Frequency Diversity
• Code Diversity
• Time Diversity
Diversity Reception
• Idea from which MIMO arose
• Several methods are possible
–
–
–
–
Selection Combining
Switched Combining
Equal Gain Combining
Maximum Ratio Combining
Maximum Ratio Combining
(MRC)
• A way of combining signals from diversity
reception
• The signals are weighted according to their
Signal to Noise ratios and then combined
Diversity Gain Definition
• Diversity Transmission - is a method for
improving reception of a transmitted signal,
by receiving and processing multiple
versions of the same transmitted signal
• Diversity Gain - is a value that quantifies
the performance improvement by a diversity
transmission scheme in a fading channel
Diversity Gain for Multiple Branches
• The performance gain
of a system can be
quite dramatic
• For example, with a
system using QPSK
requiring a maximum
BER of 0.01 diversity
gain is 13.9dB
Source: Space-Time Wireless Channels by Durgin
Shannon Capacity for
Conventional Systems
• 1948 Claude Shannon’s Noisy Channel Coding
Theorem describes maximum efficiency of error
correcting codes
• Shannon-Hartley Theorem describes what channel
capacity is for finite bandwidth continuous time
channel with Gaussian Noise
–
–
–
–
With Single Transmit and Single Receive Antenna
B is Bandwidth
SINR is Signal to Interference and Noise Ratio
C can be increased by increasing B or SINR
Shannon Capacity for
Conventional Multi-Antenna Systems
• SINR ratio can be improved by using
multiple antennas
• Overall capacity can be improved because
the SINR is improved
• Multiple Transmit Antennas
• Multiple Receive Antennas
• Combination of multiple Transmit and
Receive antennas
SINR with
Multiple Receive Antennas
• N antennas are used at the receiver
• They receive N various faded copies of the signal
• Which can be coherently combined to produce a
N2 increase in power
• There are also N sets of noise/interference that add
together as well
Shannon Channel Capacity with
Multiple Receive Antennas
• With this N*SINR the channel capacity of
the system becomes
SINR with
Multiple Transmit Antennas
• If M antennas are used at the transmitter the total
power is divided into the M branches.
• The power per transmitter antenna drops but
signals may be phased so that they add coherently
• Noise + interference is the same as SISO
• The result is a M-fold increase in SINR
Shannon Channel Capacity with
Multiple Transmit Antennas
• With this M*SINR the channel capacity of
the system becomes
SINR with Multiple Transmit and
Multiple Receive Antennas
• SINR is a combination of the MISO
(multiple transmit antennas) SIMO
(multiple receive antennas) cases
Shannon Capacity of a Single
Channel with Multiple Transmit
and Multiple Receive Antennas
• With this M*N*SINR the channel capacity of the
conventional system using multiple antennas
becomes
Conventional Multi-Antenna
Transmission
• Conventionally it is not possible to send more than
one simultaneous signal per frequency
• Seemingly the best approach would be to weight
the transmitter elements to maximize signal power
at the receiver.
Source: DATACOMMRESEARCH
Increasing Shannon capacity by
using multiple spatial channels
• A shift in perspective led to the
development of truly multiple-input,
multiple-output systems that have capacity
greater than the best conventional single
channel system.
• Dramatic capacity increases are possible if
we consider different signals sent thru each
transmitter antenna.
Multi-Channel MIMO
• Different signals are are sent thru each
transmitter antenna
Source: DATACOMMRESEARCH
Won’t the physical channels
interfere with each other?
I don’t believe this is possible
Show me the Math
MIMO Channel Matrix Model
•
•
•
•
y = received vector
x = transmitted vector
H = channel matrix
t = time, τ = delay
Processing the MIMO Signal
at the Transmitter
• At the transmitter a linear signal processing
operation V is performed on the transmitted signal
vector x and the result is Vx(t)
• V is an M x M unitary matrix with the property
VV† = I where I is the identity matrix and the †
operator indicates the conjugate transpose or
Hermitian operation
• Unitary matrices do not change the geometrical
length of vectors so no power is added or
subtracted from the transmitted signal
Processing the MIMO signal
at the Receiver
• At the receiver a linear processing signal
processing operation U† is performed on the
received signal vector y
• U† is an N x N unitary matrix where U†U = I
• I is the identity matrix which means that no
power is being added or subtracted from the
received signal
MIMO Processing Output
• After the channel H operates on the transmitter’s
output Vx(t) the result is HVx(t)
• The receiver then processes this signal with matrix
U and the result is z(t) described by the following
• The wireless system has no control over the
channel H but by controlling U and V so it can
control D
Controlling the Channel
• U and V are chosen such
that they diagonalize D
• λi are positive constants
• Here N > M so there are
M separate channels
• If M > N then this is
limited to N separate
channels
The result is simplifying z(t)
• The result of
diagonalizing the
matrix is to simplify
the received and
processed vector z(t)
• Mathematically this
shows that the MIMO
channel can be viewed
as a set of Min(M,N)
separate channels
Singular Value Decomposition
• These signal processing steps have a distinct
physical rational
• They rearrange the channel without adding or
subtracting power so they do not change the
channel capacity by amplification
• What they have actually done is a Singular Value
Decomposition on the channel matrix H
• When squared the diagonal elements of D are the
eigen values of H†H for N>=M or HH† for M>=N
Capacity Increase with
Separate Channels
• If each signal is a different signal then each of the
individual channels will have a capacity
C = B*log2(1+(N/M)*SINR)
• Since there are Min(M,N) of these channels the
total capacity is
C = Min(M,N)*B*log2(1+(N/M)*SINR)
• Observe how this differs from conventional multiantenna channel capacity
C = B*log2(1+ M*N*SINR)
• There is a linear increase in capacity by Min(M,N)
Power of logarithms
• Recall basic property of logarithms
X*logN(Y) = logN(YX)
• Therefore M*B*log2(1+(N/M)*SINR) >
B*log2(1+M*N*SINR)
• The essential principle is that it is more
beneficial to transmit data using many
different low power channels than a single
high power channel
Physical Interpretation of U and V
• U and V are matrices of complex (amplitude and
phase) values
• At the transmitter, matrix V operates on symbol
vector x(t) to effectively provide a unique antenna
radiation pattern for each symbol
• At the receiver, matrix U operates similarly to
provide unique antenna patterns that effectively
pick out different symbols arriving from different
directions because of multipath reflections
Knowing the Channels
• In order for a system to achieve this supercharged
capacity it must be able to calculate the correct
unitary matrices U and V
• Since U and V depend on the channel matrix H it
is necessary to estimate the channel at both the
transmitter and receiver
• Presumably the channel matrix information must
be sent from the receiver to the transmitter.
• But perhaps not. Maybe there is another way.
Practical Signal Extraction
• Few wireless systems will perform SVD on the
channel at both the transmitter and receiver
because this requires reliable estimates of the
channel at both transmitter and receiver
• Instead a training sequence is transmitted to the
receiver so that it has a reliable channel estimate
• Then the receiver operating matrix U† is set to be
the inverse of the channel matrix H so U† = H-1
Practical Signal Extraction Cont
• HH-1 = I
• I is the identity matrix
• This has the effect of
nulling out the
distortion effects of
the wireless channel
Subtraction of Interference
• Data could be processed this way but there is an
interesting opportunity for signal gain if the
symbols are processed in the following manner
• Subsequent symbols are processed by subtracting
previously determined symbols giving 2 estimates
for the 2nd symbol, 3 for the 3rd and 4 for the 4th
• These multiple estimates can be combined for
additional diversity gain
Foschini’s Layered Architecture
• One of the problems with MIMO is its
vulnerability to unequal power channels
• Because of this the channels cannot be separated
at the receiver with equal SINR by using a simple
inversion operation H-1
• Gerard Foschini in his famous 1996 paper on
MIMO proposed a transmitter architecture that
cycles the four streams, one cycle per timeslot
• Thus on average each channel has the same SINR
• This paper stimulated a lot of research in MIMO
Optionally using diversity for
adding redundancy
• Recall the conventional multi-antenna
transmission scheme
• For simple MISO case with M=2, N=1 the
channel matrix is
Conventional MISO Transmission
• This is done as follows:
• Each column represents a successive timeslot
• Complex weights v11 and v21 are chosen by the
transmitter but info must be fed from receiver to
transmitter to make the best choice.
• Successive symbols are represented by xi
A better way to transmit symbols
•
•
•
•
Instead use the following method:
Each column represents a successive time slot
Send different symbols from each antenna
First send the symbol and then follow by sending
the complex conjugates.
What the receiver sees
• For timeslot 1
• For timeslot 2
• Separately these are just useless mangled
combinations of the two data symbols
Combining Samples
• However the samples can be combined in the
following manner from which the original
symbols x1 and x2 can be recovered
• Notice the real constant R on the right side
Result of Combining
• The constant R is equivalent to the output
envelope of a two-branch diversity scheme with
MRC
• The single-antenna receiver has performed MRC
on the transmitted symbols
• The receiver still had to have reliable channel
estimates but didn’t send them to the transmitter
• It is therefore possible to use a MISO system to
combat a fading channel without requiring channel
feedback
Alamouti’s Code
• First two columns of transmission are a special
matrix called Alamouti’s Code invented in 1998
• Alamouti’s Code is a special instance of a code
called a Space-Time Block Code (STBC)
• Very special because it is the only orthogonal
STBC that achieves rate-1 and therefore achieves
full diversity gain without sacrificing data rate.
Space Time Codes
• A method used to improve the reliability of data
transmission by using multiple transmit antennas
• Modulation scheme that provides transmit
diversity
• Rely on transmitting multiple redundant copies of
data stream to the receiver in the hope that at least
some of them make it and allow reliable decoding.
• Two Main Types
– Space Time Block Code (STBC)
– Space Time Trellis Code (STTC)
STBC – Space Time Block Code
• Easiest type because under the assumption of flat
fading Rayleigh channels they can be decoded
using simple linear processing at the receiver
• STBCs create an antenna array in time
• Represented as a matrix where
– Each row represents a time-slot
– Each column represents a transmit antenna
Observations on
STBC and MISO
• The number of channels and the potential for
speed improvement is Min(M,N)
• If you only have one receive antenna you can only
have one channel
• However, with only one receive antenna but
multiple transmit antennas STBC allow
tremendous diversity gain
• Diversity gain provides better BER and allows
protocols to use faster data rates without having to
fall back to slower data rates.
Higher Order STBC
• Higher order STBC are possible and must
be used for 3 x 3 or 4 x 4 or M x N systems
• However, it has been proven that no code
using more than two antennas can reach
rate-1.
• This is because it is the only way for a code
to reach orthogonality.
STTC – Space Time Trellis Code
•
•
•
•
•
•
Based on trellis codes
Provide both coding gain and diversity gain
Have better bit-error rate performance than STBC
More complex to encode and decode than STBC
Rely on Viterbi decoder at the receiver
Require information about Channel to be
conveyed from the receiver to the transmitter
Multipath is Essential for MIMO
• Problems with MIMO
– Without multipath it degenerates into a single
transmitter and receiver
– Unequal average branch power
– Keyhole problem
Source: Space-Time Wireless Channels by Durgin
MIMO Pros and Cons
• Advantage:
– Linear increase in capacity with the number of
antennas
– Multiple paths provide resistance to fading
• Disadvantage:
– Cost of multiple RF chains
– Higher power consumption
How will MIMO effect you?
• MIMO is a radical paradigm shift away
from one transmitter / one receiver
• It will change the design paradigm for
virtually all wireless technologies from cell
phones to broadband
• If you are involved in wireless then MIMO
is in your future
Other Standards using MIMO
• WiMax
• Cellular
• WiBro
Conclusion
• Spatial Multiplexing for higher data rates is
mandatory in 802.11n
• STBC for diversity and redundancy are
optional in 802.11n
• MIMO requires a multipath environment
• MIMO advantages outweigh disadvantages
and many standards are adopting it
References
• Space-Time Wireless
Channels by Gregory D.
Durgin
• Provides excellent into
understand Space-Time
Wireless Channels
• If you have had college
level engineering calculus
and statistics then this will
be understandable
References Cont
• Gerard J. Foschini – 1996 – “Layered Space-Time
Architecture for Wireless Communication in a Fading
Environment When Using Multi-Element Antennas”
• DATACOMM Research Company White Paper “Using
MIMO-OFDM Technology To Boost Wireless LAN
Performance Today” http://www.datacommresearch.com
• Enhanced Wireless Consortium http://www.enhancedwirelessconsortium.org
– EWC_MAC_spec_V124.pdf
– EWC_PHY_spec_V127.pdf
Thank You
Questions?