MIMO-OFDM Architecture Enabling Very High Data Rates · January 2004 Slide 1 doc.: IEEE...

January 2004

Slide 1

doc.: IEEE 802.11-03/0999r0

Submission Jae Moon, U of MN

Practical MIMO Architecture Enabling Very High Data Rates

Jaekyun Moon, Hui Jin and Yong LiUniversity of Minnesota([email protected])

Taehyun Jeon, Heejung Yu and Sok-Kyu LeeETRI

([email protected])

January 13, 2004

mailto:[email protected]



January 2004

Slide 2

doc.: IEEE 802.11-03/0999r0


Objectives

Identify MIMO-OFDM Architecture that allows

• 200+ Mbps data rates (10+ bps/Hz) • Practical implementation options• Backward compatibility to .11a/g

January 2004

Slide 3

doc.: IEEE 802.11-03/0999r0


Outline• Background & motivation

– MIMO based on space-time coding– MIMO based on BICM+spatial mux

• Transmitter/receiver architecture• PER simulation results and SNR requirements• Preamble format• CFO compensation • Channel estimation

January 2004

Slide 4

doc.: IEEE 802.11-03/0999r0


STC vs. SM: backgroundSTC: Coded Modulation (CM)

Space-time trellis coding (STTC) ~ [Tarokh’98]

Space-time block coding (STBC) ~ [Alamouti’98], [Tarokh’99]

Space-frequency coding (SFC) (STC+OFDM) ~ [Paulraj’00], [Lu’00]

Group space-time-frequency block coding (GSTFC) ~ [Liu’02]

TCM + STBC ~ [Gong’02]

CM

SM: Bit-Interleaved Coded Modulation (BICM)BICM + Spatial multiplexing ~ [Tonello’00], [Duman’01], [Lu’02], [Park’03] S/PBICM

January 2004

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doc.: IEEE 802.11-03/0999r0


STC vs. BICM+SM: philosophy STC

00/ 0

11/ 5

2

0

13

4

5

6

7

symbol

symbol

bit

BICM+SM

010

000

001011

100

101

110

111

symbolS/P

00/ 000

11/ 101

bitbitbit

January 2004

Slide 6

doc.: IEEE 802.11-03/0999r0


STC vs. BICM+SM: summary• STC offers improved reliability through explicit

enhancement of diversity gain– Suffers rate loss– Data rate increase possible with higher order constellation

• BICM+SM allows transmission of independent information streams, leading to high data rates– No explicit attempt to explore diversity– Yet capacity-achieving

• For high data rate applications, BICM+SM is the way to go– SNR advantage in achieving the same data rate– RF implementation difficulties associated with large constellations

(phase sensitivity, PA backoff requirements, etc)

January 2004

Slide 7

doc.: IEEE 802.11-03/0999r0


BICM+Spatial Multiplexing: Transmitter

3 Tx antenna example

CCencoder

symbolmapping

u c 'cπ

IFFT

IFFT

IFFT

S/P

one (parallel) OFDM symbol period

spans this Tx array containing 64x3 modulation symbols (or 64x3xm coded bits, where m is the number of coded bits per modulation symbol).

With 1 Tx antenna, the architecture coincides with that of .11a.

π

Interleaver gain in iterative demapping/decoding (IDD) is smaller here than the case where the interleaver spans the whole packet, but latency and storage requirements are reduced.

January 2004

Slide 8

doc.: IEEE 802.11-03/0999r0


BICM+Spatial Multiplexing: Receiver

FFT

FFT

FFT

3 Rx antenna example

one (parallel) OFDM symbol period

de-mapping

u

+

+

π1π − decoder

Note the baseband latency target of 10 us (< SIFS=16 us), a serious ASIC implementation challenge.BB latency: time that takes for the end of Rx packet that’s passing ADC to reach PHY/MAC interfaceSIFS: time between the end of Rx packet reaching Rx antenna and the start of ACK packet reaching Tx antenna

∼

January 2004

Slide 9

doc.: IEEE 802.11-03/0999r0


Suboptimal Soft-demapper with SDF3 Tx & 3 Rx antenna example

11 21 31

12 22 32

13 23 33

H H HH H HH H H

′ ′ ′ ′ ′ ′ ′ ′= +

′ ′ ′

r a nQR decomposition

12

13 23

1 0 01 0

1HH H

= +

r a n

: modulation symbol: FFT output′

ar 1 1 1

2 2 12 1 2

3 3 23 2 13 1 3

r a nr a H a nr a H a H a n

= += + += + + +

Algorithm:Truncated-depth MAP with soft decision feedback (SDF)

January 2004

Slide 10

doc.: IEEE 802.11-03/0999r0


Simulation Conditions• Convolutional code, punctured from (133, 171)• Random or 11a-like structured interleaver, spanning one

(parallel) OFDM symbol transmission period• Gray mapping• 64-pt FFT, 20 MHz channelization• 512 byte packet• Channel: quasi static fading, exponentially decaying power

profile with 50 ns rms delay spread, uncorrelated channel coefficients

• Soft demapping, BCJR decoding, iteration of soft decisions between demapper and decoder (IDD)

• SNR defined as the overall Tx energy (distributed to all Txantennas) to noise ratio

January 2004

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doc.: IEEE 802.11-03/0999r0


)1( =τBICM+Spatial Mux: ¾ CC, 16QAM, MAP+SDF demapping

January 2004

Slide 12

doc.: IEEE 802.11-03/0999r0


)1( =τBICM+Spatial Mux: ¾ CC, 64QAM, MAP+SDF demapping

January 2004

Slide 13

doc.: IEEE 802.11-03/0999r0


)1( =τBICM+Spatial Mux: ¾ CC, 16/64QAM, MAP+SDF demapping

IDD with 3 iterations in all MIMO cases

2161444 x 4

162108*3 x 3

108722 x 2

54361 x 1

64QAM3/4 CC

16QAM3/4 CC

Ant.

January 2004

Slide 14

doc.: IEEE 802.11-03/0999r0


Possible High Data Rates

2161444 x 4

162108*3 x 3

108722 x 2

54361 x 1

64QAM,3/4 CC

16QAM,3/4 CC

Tx-Rx antenna

802.11a data rates 6 possible .11n data rates (Mbps)

Rate information can implicitly specify the number of Tx antennas.

January 2004

Slide 15

doc.: IEEE 802.11-03/0999r0


Other Possible High Data Rates

192964 x 4

144723 x 3

96482 x 2

48241 x 1

64QAM,2/3 CC

16QAM,1/2 CC

Tx-Rx antenna

January 2004

Slide 16

doc.: IEEE 802.11-03/0999r0


Ref. curve: coding & interleavingover whole packet

)1( =τHow many iterations are needed?

BICM+Spatial Mux: ¾ CC, 16QAM, MAP+SDF demapping

January 2004

Slide 17

doc.: IEEE 802.11-03/0999r0



)1( =τHow many iterations are needed?

BICM+Spatial Mux: ¾ CC, 64QAM, MAP+SDF demapping

January 2004

Slide 18

doc.: IEEE 802.11-03/0999r0


Observations • 4x4 BICM+SM with suboptimal MAP+SDF enables

200+Mbps.– Achieves target PERs at SNRs lower than that required for 802.11a

54 Mbps data rate (1x1)– Suboptimal finite depth MAP with SDF– 3 IDD iterations used with bit-level interleaver spanning one Tx

array (Nt parallel OFDM symbols) rather then whole packet– Even 1 iteration provides substantial gain

• Once IDD is employed, the off-the-shelf CC performs comparable to turbo/LDPC codes.

• Simulation results may be somewhat optimistic in that channel coefficients are uncorrelated, but validate BICM+SM for high data rate application.

January 2004

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doc.: IEEE 802.11-03/0999r0


Preamble Structure, Channel Estimation and CFO Correction

January 2004

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doc.: IEEE 802.11-03/0999r0


Training Symbol Structure (for channel/CFO estimation)

Tx1

Tx2

Tx3

Tx4

64-12=52 frequency bins

• Both the training sequence (one or two OFDM symbols) and signal field (one OFDM symbol) are transmitted in cyclic fashion. The signal field symbol is also BPSK based and enjoys high SNR, and thus can be used for channel estimation purposes.• Fine CFO estimation can be done within one training symbol w/o CSI.• For each transmitted modulation symbol, the receiver can estimate Nr channel coefficients. After transmission of one OFDM symbol or 52 modulation symbols, the receiver estimates 52 x Nr = 208 channel coefficients, enough for channel characterization of Nt x Nr x 13 =16 x 13 = 208 coefficients or 13 temporal taps/channel.• Note the receiver does not need to know the number of Tx antennas for initial CFO/channel coefficient characterization based on training sequence and signal field.

January 2004

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doc.: IEEE 802.11-03/0999r0


Preamble Structure: Option 1

10 0.8 8 sµ× =

10 short symbolspreamble

1 long symbolpreamble

BPSK

Signal Field Service+Data 1 Data 2

BPSK, R=1/2

0.8 3.2 4 sµ+ = 0.8 3.2 4 sµ+ = 0.8 3.2 4 sµ+ = 0.8 3.2 4 sµ+ =

Signal detectAGCCoarse CFO estimationFrame synchronization

Fine CFO estimationChannel estimation

Rate, Length(no. of Tx antennas)Channel estimation

Channel estimation

Single Tx antenna or cyclic use of multiple

Tx antennas

Cyclic use of multiple Tx antennas

Spatial multiplexing over multiple Tx antennas

• Cuts down the preamble length by 4 us• Backward compatibility with .11a relies on protection mechanism

January 2004

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doc.: IEEE 802.11-03/0999r0


Preamble Structure: Option 2

10 0.8 8 sµ× = 0.8 3.2 4 sµ+ = 0.8 3.2 4 sµ+ = 0.8 3.2 4 sµ+ = 0.8 3.2 4 sµ+ =0.8 3.2 4 sµ+ =

BPSK

10 short symbolspreamble

long symbol 1preamble

Signal Field Service+Data 1 Data 2

BPSK, R=1/2BPSK

long symbol 2preamble

Fine CFO estimationChannel estimation

Signal detectAGCCoarse CFO estimationFrame synchronization

Rate, Length(no. of Tx antennas)Channel estimation

Fine CFO estimationChannel estimation Channel

estimation

Spatial multiplexing over multiple Tx antennas

Cyclic use of multiple Tx antennas

Single Tx antenna or cyclic use of multiple

Tx antennas

• Backward compatibility with .11a maintained automatically

January 2004

Slide 23

doc.: IEEE 802.11-03/0999r0


Recursive LMMSE CFO Correction

PrincipleOperates on the time-domain received samples before FFT.Sequentially and alternately estimates data/channel info and the

phase/frequency distortion term.

featuresLow complexityVirtually no extra latencyTrack phase/frequency driftsReduce phase/frequency jitter

Data/channelextractor Invertor

MDpredictor

1−Z

kp kk|q

Channel Estimation /Symbol Detection

1|*ˆ −kky kky |1*ˆ +

January 2004

Slide 24

doc.: IEEE 802.11-03/0999r0


Estimated versus Actual CFO

0 10 20 30 40 50 60 700

0.5

1

1.5

2

2.5

3

0 10 20 30 40 50 60 700

0.5

1

1.5

2

2.5

3

k

|q1[k]|e s tim a te of |q1[k|k]|

|q4[k]|e s tim a te of |q4[k|k]|

0 10 20 30 40 50 600.088

0.09

0.092

0.094

0.096

0.098

0.1

0.102

0.104

k

Tracking symbol/channel information during acquisition

time

time

time

CFO estimate during acquisition with actual CFO=0.1

January 2004

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doc.: IEEE 802.11-03/0999r0


Channel Estimation

Estimate channel’s frequency response using all available resources as needed to achieve performance/complexity/latency tradeoff:

Pilot tones within each OFDM symbolOne or two BPSK training symbols (for initial estimates) – cyclic transmission One BPSK signal field – cyclic transmissionSoft symbol info from the previous iteration (channel estimation revised to

utilize soft information)

January 2004

Slide 26

doc.: IEEE 802.11-03/0999r0


Estimated versus Real Channel Response

0 5 10 15 200

0.5

1

1.5

k0 5 10 15 20

0

0.5

1

1.5

k

h[k]h[k] estimate

h[k]h[k] estimate

Using pilots only After one BPSK training symbol

0 5 10 15 200

0.5

1

1.5

k

h[k]h[k] estimate

Soft-decision-directed, into data packets

0 5 10 15 200

0.5

1

1.5

k

h[k]h[k] estimate

After BPSK signal field symbol

January 2004

Slide 27

doc.: IEEE 802.11-03/0999r0


4x4 64QAM, SDF with τ=1 MAP, with channel estimation


Ref. Line:

Coding and interleaving over whole packet

January 2004

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doc.: IEEE 802.11-03/0999r0


Observations 2• CFO can be estimated within one OFDM training symbol.• Channel estimation can be done with the combination of

initial training symbol(s), the use of the signal field symbol, and soft-decision-directed updates during the data portion of the packet.– The long training symbol(s) and the signal field symbol are

transmitted in cyclic fashion to provide initial channel estimates for multiple parallel channels.

• Channel response drifts can also be handled with the proposed method.

• The long symbol preamble of 11a (8 us) can be reduced to 4 us.

• The .11a preamble format can be retained, allowing automatic guarantee of backward compatibility

January 2004

Slide 29

doc.: IEEE 802.11-03/0999r0


Conclusions• Practical MIMO architecture that is a natural

extension of .11a has been proposed.– Conceptually simple and straightforward generalization

of .11a to incorporate spatial dimension– Capable of 10+ bps/Hz (200+ Mbps/20 MHz) while

offering range improvement*– Can retain the same preamble format as .11a, thus

automatically guaranteeing backward compatibility– Allows straightforward ASIC implementation (no

serious latency/complexity challenges)

* With 4x4 antenna configuration and uncorrelated channel coefficients; relative to 54 Mbps .11a mode

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