Channel Estimation in Filter Bank-based Multicarrier ... · Compute channel estimate (as in OFDM):...

Channel Estimation in

Filter Bank-based Multicarrier Systems:

Fundamentals and Recent Advances

Eleftherios Kofidis

Computer Technology Institute, Greece

University of Piraeus, Greece

31 Aug. 2015 CentraleSupelec, Rennes 2

Future mobile networks – Vision and

needs

High

data rate

reliability

QoS

in demanding transmission scenarios

Increased flexibility

Efficient use of fragmented spectrum

Robustness to asynchronism

Co-existence of different systems (HetNets)

…


Toward a new PHY – Modulation Is OFDM an adequate solution?

Poor spectral containment

Bandwidth/power inefficiency

Challenging synch in multi-access

Sensitivity to severe dispersions

…

FBMC: an attractive alternative Good spectral (/time) containment

High spectral (/power) efficiency

Flexibility (e.g., for multi-mode comms)

Relaxed synch requirements

Able to cope with severe multipath (e.g., large cells) and high mobility

…


FBMC research and applications

Filter bank-based multi-carrier modulation:

• FBMC/OQAM

• FMT

• GFDM

• UFMC

• …


FBMC research and applications

Filter bank-based multi-carrier modulation:

• FBMC/OQAM

• FMT

• GFDM

• UFMC

• …

• Max. spectral efficiency

• Time-freq. localization

• Robust to lack of synch

• But: Intrinsic interference


FBMC/OQAM challenges - Solutions

Intrinsic ISI/ICI

Frequency / time selective subchannels

Challenges in Channel Estimation (CE)

Classical assumption: channel of low freq./time selectivity CE analogous (similar) to OFDM Preamble/pilots design for increased accuracy

However: in many realistic scenarios Severe performance error floors outperformed by OFDM at higher SNRs

More recently: CE training and techniques for demanding channels


Outline Fundamentals of FBMC/OQAM

System model

Intrinsic interference effect

FBMC/OQAM CE fundamentals

Preamble-based

Pilot-based

Preamble-based CE

Low frequency selective channels

Highly frequency selective channels

Simulation examples

Additional results - on-going/future work

Fundamentals of FBMC/OQAM



FBMC/OQAM vs. OFDM/QAM

1 2F

complex QAM

real

imaginary

F=1/T: sub-carrier spacing

T: OFDM/QAM symbol

duration

T-F density:

OFDM/QAM (without CP): 1/(TF)=1

OFDM/OQAM:

Spectral efficiency (e.g., (O)QPSK):

OFDM/QAM (without CP): 2/(TF)=2

OFDM/OQAM:

1 2F

1/ 2F

Phase space


Offset-QAM Modulation (staggering)

Re

Im

2

2 z-1

+d2k,n

c2k,m

Im

Re

2

2 z-1

+d2k+1,n

c2k+1,m

even sub-carriers

odd sub-carriers


FBMC/OQAM Transmitter

IFFT

2

0 ( )A z

2

1( )A z

2

1( )MA z

2M

2M

2M

1z

1z

0,n

0,n

1,n

1,M n

1,M n

1,n

0,nd

1,nd

1,M nd

C2R

C2R

C2R

OQAM modulation Transform blockPolyphase

filtering

P/S

conversion

SFB:

P. Siohan et al., “Analysis and design of OFDM/OQAM systems based on filterbank theory,”

IEEE Trans. SP, May 2002.


FBMC/OQAM Receiver

2

0 ( )B z

2

1( )B z

2

1( )MB z

FFT

1z

1z

2M

2M

2M

Subchannel

processing

Subchannel

processing

Subchannel

processing

*

0,n

*

1,n

*

1,M n

*

0,n

*

1,n

Re

*

1,M n

0,nd

1,nd

1,M nd

Re

Re

R2C

R2C

R2C

S/P

conversion

Polyphase

filteringTransform block OQAM demodulation

AFB:

System model



System model (1)

M: #subcarriers

K: overlapping factor

g: prototype filter (length )

C2R SFB h + AFB

Intrinsic interference:

13 March 2014 Patras (ENDECON) 15

Intrinsic interference in FBMC/OQAM

1,1,11,1

1,1,11,1

1,,1,

1,1,11,1

1,0,01,0

nMnMnM

nknknk

nknknk

nknknk

nnn

ddd

ddd

ddd

ddd

ddd



With good TF localization,contributions to intrinsicinterference only come from thefirst-order neighboring TFpoints

1,1,11,1

1,1,11,1

1,,1,

1,1,11,1

1,0,01,0

nMnMnM

nknknk

nknknk

nknknk

nnn

ddd

ddd

ddd

ddd

ddd


Example – “PHYDYAS filter”

FBMC/OQAM TMUX transfer function (interference function):

( - Even k

- after “de-phasing” ( ) to bring into the form

- before that: green real, brown imaginary OQAM ! )

time

freq.

n-4 n-3 n-2 n-1 n n+1 n+2 n+3 n+4

k-1 j0.005 -j 0.043 j0.125 -j0.206 j0.239 -j 0.206 j0.125 -j0.043 j0.005

k 0 j0.067 0 j0.5644 1 -j0.5644 0 -j0.067 0

k+1 -j0.005 -j0.043 -j0.125 -j 0.206 - j0.239 -j0.206 -j0.125 -j 0.043 -j0.005

*

, k n

k n j

, , , k n k nd ju k

• N. J. Fliege, “DFT polyphase transmultiplexer filter banks with effective reconstruction,” EUSIPCO 1992.

• C. S. Lee and K. Y. Yoo, “Polyphase filter-based OFDM transmission system,” VTC-2004 (Fall).


More examples

IOTA filter

Bregović-Saramäki filter

P. Siohan and C. Roche, IEEE Trans. SP, Dec. 2000.

M. G. Bellanger, ICASSP-2001.

R. Bregović and T. Saramäki, IEEE Trans. SP, Aug. 2005

PHYDYAS filter


System model (2) Common assumptions (locally freq./time-invariant channel):



OFDM-like



OFDM-like

colored

virtual Tx symbol

(pseudo-symbol)

FBMC/OQAM channel estimation:

Fundamentals



Preamble-based channel estimation (1)

Control / Data

Pre

am

ble

Frame: SFB

non-z

ero

part

0 0

prevents interference

from previous frame

(often unnecessary!)

prevents interference

from control/data

channel time invariant


Preamble-based channel estimation (2)

Control/Data

Pre

am

ble

Full

(block-type):

Control/DataSparse

(comb-type):

0

0

protect from ICI

Scattered pilot-based channel estimation Help (auxiliary) pilot


J.-P. Javaudin, D. Lacroix, and A. Rouxel, VTC-2003 (Spring).

1,1,11,1

1,1,11,1

1,,1,

1,1,11,1

1,0,01,0

nMnMnM

nknknk

nknknk

nknknk

nnn

ddd

ddd

ddd

ddd

ddd

Preamble-based channel estimation


Interference Approximation Method (IAM):

Interference in a positive role!

Known input interference approximation possible

pseudo-pilots

Choose input so as to maximize pseudo-pilot magnitude

Compute channel estimate (as in OFDM):


0,0 0,1 0,2

1,0 1,1 1,2

2,0 2,1 2,2

1,0 1,1 1,2M M M

d d d

d d d

d d d

d d d

C. Lélé et al., “Channel estimation methods for preamble-based OFDM/OQAM modulations,”

European.Trans. Telecomm., 2008.

estimation error

Example: IAM-R

Null side symbols ( base design on middle

symbol only)

Carefully choose signs so as to maximize

pseudo-pilots’ magnitude


010

010

010

010

010

010

010

010

31

d

d

d

Idea:

Example:

M=8, OQPSK

More IAM variants – Using imaginary pilots

Idea: Use Imaginary pilots to generate imaginary- or real-valued

pseudo-pilots (of even larger magnitude)

Not a strictly OQAM input!


• C. Lélé et al., ICC-2008.

• J. Du and S. Signell, ICC-2009.

• PHYDYAS deliverable D3.1

• E. Kofidis and D. Katselis, EUSIPCO-2011.

00

00

00

00

00

00

00

00

0

0

1

1

1

0

0

0

jd

d

d

jd

d

d

jd

d

00

010

00

010

00

010

00

010

j

j

j

j

11

1

11

1

11

1

11

1

j

jj

j

jj

j

jj

j

jj

IAM-I IAM-C E-IAM-C

1/3 of the subcarriers:

Price for good performance: high PAPR!


SFB-modulated preambles (magnitudes squared)

Sample no.

• M=256, K=4

• OQPSK

Interf. from data part

Optimal preambles (1)

Preamble optimization:

Minimize MSE subject to transmit power/energy constraint

For low frequency selective channels:

FBMC/OQAM

Block-type: equal pilot tones

Comb-type: equispaced & equipowered

OFDM/QAM (no account for CP energy):

Block-type: DFT matrix column

Comb-type: equispaced & equipowered


• D. Katselis et al., IEEE Trans. SP, May 2010.

• E. Kofidis et al., Signal Processing, July 2013.

• C. Mavrokefalidis et al., EURASIP JASP, May 2014 (for relaying networks).


Highly frequency selective channels

No simplifying assumptions:

D. Kong et al., IEEE TSP, Jan. 2014

E. Kofidis, ICASSP-2014.



Optimization problem:

Problem structure:

E. Kofidis, ICASSP-2014



Block-type preamble:

Complex-valued:

Real-valued:

Simple estimation procedure (for real preamble):

Take the first terms of

Divide them by



Comb-type preamble ( pilot tones):

Equipowered and equispaced

Estimation procedure:

Prototype filter autocorrelation:

Compute the “weighted” freq. response first:

Compute the “weighted” impulse response via IFFT and

divide by the weights to arrive at the impulse response

estimate:E. Kofidis, ISCCSP-2014


Simulation example: Block-type

error floor

?


Simulation example: Comb-type

ITU-VehA channel model error floor


More and on-going

Preamble-based CE:

POP etc. [1,3]

MIMO case [2,3,4]

Multiuser case [7]

Longer preambles [5,8]

LMMSE channel estimation [10]

Scattered pilot-based CE:

Extend help pilot idea to highly selective channels

Take into account

virtual (edge) subcarriers [6]

interference from data [6]

1. C. Lélé et al., EW-2007.

2. E. Kofidis and D. Katselis, ICSIPA-2011.

3. E. Kofidis et al., Signal Process., July 2013.

4. E. Kofidis, EW-2015.

5. M. Newinger et al., VTC-2013 (Spring).

6. L. Baltar et al., EUSIPCO-2014.

7. F. Rottenberg et al., ISWCS-2015.

8. E. Kofidis, ISWCS-2015.

9. EMPhAtiC deliverable D3.1

10. L. Caro et al., VTC-2015 (Spring).

…

Thank you!

Questions?

Channel Estimation in Filter Bank-based Multicarrier ... · Compute channel estimate (as in OFDM):...

Documents

Transcript of Channel Estimation in Filter Bank-based Multicarrier ... · Compute channel estimate (as in OFDM):...