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 Consortiumhttp://www.iec.org/online/tutorials/ofdm/topic04.html

802.11a/g OFDM Rates250,000 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

NAVNetwork 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.

NAVNetwork 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 forConventional 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 withMultiple 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 withMultiple 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 Signalat 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 signalat 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 withSeparate Channels

• If each signal is a different signal then each of the individual channels will have a capacityC = B*log2(1+(N/M)*SINR)

• Since there are Min(M,N) of these channels the total capacity isC = Min(M,N)*B*log2(1+(N/M)*SINR)

• Observe how this differs from conventional multi-antenna channel capacityC = 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

• 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

Combining Samples

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 onSTBC 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?