Inter-carrier interference cancellation in OFDM systems

By: Dharamveer

Meena Vinodkumar Pralia

Basic OFDM system Multi-carrier modulation Multiple frequency channels are

orthogonal to each other.

OFDM modulated signal N symbols mapped

to bins of IFFT. IFFT bins correspond

to the orthogonal sub-carriers.

11 2

0

1( )

N j nmN

mm

x n X eN

Baseband OFDM transceiver system

Drawbacks Frequency offset

Local oscillator frequencies Doppler shift

Result: Loss in Orthogonality Inter-carrier interference

Frequency offset model y(n) : received

sequence normalized

frequency offset N:number of sub

carriers w(n) : complex

AWGN

( 2 / )( ) ( ) ( )j n N

S

y n x n e w n

fNT

Standard OFDM systems Received signal:

ICI coeffiecient:

Carrier to interference ratio:

Solutions to combat ICI

Self-cancellation scheme Maximum likelihood estimation Extended Kalman Filter

Extended Kalman Filtering A linear approximation of a non linear

system Linear : Nonlinear :

( ) ( ( )) ( )y n f n w n ( ) ( ) ( ) ( )y n a n d n v n

( 2 / )( ( )) ( ) j n Nf n x n e

Offset estimation scheme

State-Space Model:

Recursive steps:

Offset correction scheme

ˆ(2 ) /ˆ ( ) { ( )

j n Nx n FFT y n e

ˆ ( )x n

: symbol data QAM - M Restored

: received data symbol distorted by frequency offset

: estimated normalized frequency offset

: number of subcarrier

( )y n

N

References P. H. Moose, “A Technique for Orthogonal Frequency Division

Multiplexing Frequency Offset Correction,” IEEE Transactions on Communications, vol. 42, no. 10, October 1994.

J.Armstrong, “Analysis of new and existing methods of reducing intercarrier interference due to carrier frequency offset in OFDM,” IEEE Transactions on Communications, vol.

47, no. 3, pp. 365 – 369, March 1999. C. Muschallik, “Improving an OFDM reception using an

adaptive Nyquist windowing,” IEEE Transactions on Consumer Electronics, vol. 42, no. 3, pp. 259 – 269, August 1996.

Thank You