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Title

• Carlos Ribeiro• M. Julia Fernández-Getino Garcia• Víctor P. Gil Jiménez• Atílio Gameiro• Ana García Armada

Uplink channel estimation formulti-user OFDM-based systems

© 2006, motion | mobile systems communication group | all rights reserved

Department of Signal Theory and Communications

Communications Group


Outline

Objectives of proposed method;

OFDM overview;

TD downlink Channel Estimation;

TD multi-user uplink Channel Estimation;

Simulation results;

Conclusions.


The TD Uplink ChEst

Specify low-complexity pilot-assisted channel estimation scheme (suited for both downlink and uplink);

To be used in OFDM-based networks with centralized architecture;

Good performance in comparison with reference methods with no complexity trade-off;

Processing performed immediately on the time-domain (TD) received vector.


ChannelEstimation

QPSKMod

Fram

ing

DATA

PILOTS

Nt, Nf

IFF

T

CP

Channel

Noise

QPSKDeMod

De-FramingDATA

FF

T

CP

PILOTS

HChannel

Estimation

QPSKMod

Fra

ming

DATA

PILOTS

Nt, Nf

IFF

T

CP

Channel

Noise

QPSKDeMod D

e-

Fra

ming

DATA

FF

T

CP

PILOTSH

Some OFDM symbols, s, convey pilots at sub-carriers {0,Nf,2Nf,…}; remaining sub-carriers convey data symbols: s=d+p

OFDM system structure

Time(i)

Frequency(n)

Nc

Ns

Nf

Nt

PILOTS

DATA

- vector of data symbols

- vector of pilots

d

p


Channel estimator uses pilot symbols to perform the

estimation of the channel’s frequency response (CFR);

The transmission of p through a noisy wireless multipath channel

results in a received signal made-up of Nf noisy channel impulse

response (CIR) replicas in TD:

where

TD pilot-assisted channel estimationDownlink

1

0

1.

Nf

tm

r n p n h n w n h n m N w nNf

th - n element of CIR

[ ] - AWGN

h n

w n


Each node in the network builds new vector based on r with L most significant taps (MST) of each CIR [ref. Minn et al],

CFR estimate, , for each node is the result of the Nc-points DFT of r,

TD pilot-assisted channel estimationDownlink (2)

Example of received vector r with Nf=4– The vector is made-up

of 4 CIR replicas.

1

0

1, 1,..., ,

[ ]

0, 1,...,

Nf

c

m

Ncr mNt n n L Nt NfNfr n

n L N H k

ˆNcH k DFT r n


Nodes share same sub-carriers for pilots in the same symbols (perfect overlap in the receiver in FD);

TD pilot-assisted channel estimationUplink

Node 2

Node 1

Node U

Frame 1

Frame 2

Frame U

CentralPoint


Different pilot sequences for each node in FD; Node u (out of

U) pilot sequence, Pu[k] :

An example for U=4 nodes,

TD pilot-assisted channel estimationUplink (2)

1

0

exp 2Nt

m

uPu k k mNf j m

U

0

1

2

3

0, .., 0 0, ..0 0, .., 0 0, ..0

0, .., 0 0, ..0 0, .., 0 0, ..0

0, .., 0 0, ..0 0, .., 0 0, ..0

0, .., 0 0, ..0 0, .., 0 0, ..0

,1, , 1, , 1, , 1, 1,..

1, , , , 1, , , , 1,. .

1, , 1, , 1, , 1, , 1,..

1, , , , 1, , , , 1,. .

P

P i i

P

P i i

fN 1


Nodes different pilot sequences

Central Point channel estimator builds a vector for each node keeping

L MSTs of each CIR, starting at position ,

time-shifted CIR replicas


uNt

U

1

0

1

10

1

10

1

1 1

1 1

Nf

om

Nf

m

Nf

Um

p n n mNtNf

p n n Nt mNtNf U

Up n n Nt mNt

Nf U

, 0, , 1m

Nt m UNfU

1

0

1, 1, ,

0, 1, ,

Nf

mu

ur n mNt Nt n L

r n Nf U

n L Nc

1

0

1

00

1

10

1

10

*

1

1 1

1 1

U

u uu

Nf

m

Nf

m

Nf

Um

r n p n h n w n

h n mNtNf

h n Nt mNtNf U

Uh n Nt mNt

Nf U


CFR estimate, , for each node is the result of the Nc-points DFT of ru,

Performance Evaluation / Simulation scenario

– QPSK modulated OFDM system with Nc=1024 carriers;

– U=4 nodes;

– Pilot separation Nf=4;

– Sampling time t=25ns;

– BRAN-A wireless multipath channel;


H k

ˆu Nc uH k DFT r n


Performance Evaluation (2)

– Channel estimation MSE averaged over all nodes for different L


– Channel estimation MSE dependence on pilot separation Nf


Conclusions

Conclusions

– Low-complexity uplink multi-user OFDM channel estimator;

– Simple TD averaging to obtain user’s CIR;

– Can be used in acquisition (preamble-based) or tracking ( pilot-tones based);

– Provided the pilot sequences that allows identification of the user’s CIR in a multi-user scenario;

– Performance substantiated by computer simulation using BRAN-A channel model.

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