Performance Evaluation of Weight-Based ICI-Cancellation Scheme in OFDM Systems

1. Performance Evaluation of Weight-Based ICI-Cancellation Scheme in OFDM Systems Jyh-Horng Wena, Jia-Wei Liub, Gwo-Ruey Leec and Cheng-Yi Hsiehd 指導教授：溫志宏 教授 報告者: 謝承毅

2. Outline • Introduction • ICI Mechanism of Standard OFDM Systems • ICI Self-Cancellation Scheme • The Proposed Weight-Based ICI Self-Cancellation Scheme • Simulation Results • Conclusion

3. Introduction • For orthogonal frequency-division multiplexing (OFDM) communication systems, the frequency offsets in mobile radio channels distort the orthogonality between subcarriers resulting in intercarrier interference(ICI). • The ICI self-cancellation scheme is a simple way for ICI reduction.

4. ICI Mechanism of Standard OFDM Systems Fig.1 Block diagram of the FFT-based OFDM systems

5. The transmitted signal in time domain can be written as The discrete-time channel response of slow fading channel could be expressed as and the received signal in time domain can be written as Desired signal Inter-carrier interference and C(k – m) denotes the ICI coefficient between the mth and the kth subcarriers, which could be expressed as The corresponding frequency domain response could be obtained by FFT, which gives

6. The desired received signal power can be represented as and the ICI power is We assume that X(k) is zero mean and statistically independent with H(k).We further assume E[| H(k)|2]=1 .Therefore, the CIR can be derived as

7. ICI Self-Cancellation Scheme • Assume the transmitted symbols are constrained so that X(1) =-X(0), X(3) = -X(2),…,X(N-1)= -X(N-2), then the received signal on subcarrier m becomes Similarly, the m+1-th subcarrier signal is expressed as In such a case, the ICI coefficient is denoted as

8. The demodulation for self-cancellation is suggested to work in such a way that each signal at the (m + 1)-th subcarrier (m is even) is multiplied by −1 and then summed with the one at the m-th subcarrier. Then the resultant data sequence is used for making symbol decision. It can be represented as The corresponding ICI coefficients then becomes Fig. 2 Comparison of C(k – m), C’(k – m) and C”(k – m)

9. The Proposed Weight-Based ICI Self-Cancellation Scheme • The received signal Yi’(m) could be represented as In (9), the weight, λ, is a real value between 0 and 1. Also, the summation of these weights is assumed to be 2, i.e.,

10. The CIR in the proposed scheme could be derived • The weighting values, λ and ρ,could be determined based on maximum CIR. Fig. 3 The values of weight for the different frequency offsets

11. The mean and variance of Mi ,SNR=1

12. Simulation Results

13. Fig. 5 Performance comparison with Zhao’s scheme

14. Fig. 6 Performance on the BER under the AWGN channel

15. Fig. 7 Performance on the BER under the frequency selective fading channel

16. Conclusion • The performances of CIR and BER on the proposed schemes with a large frequency offset are better than that of Zhao’s scheme.