Carrier Frequency Offset Compensation with Successive Cancellation in Uplink OFDMA Systems

1. Carrier Frequency Offset Compensation with Successive Cancellation in Uplink OFDMA Systems TevfikYucek,Student Member,IEEE,and HuseyinArslan,Senior Member,IEEE Wireless Communications, IEEE Transactions on Volume 6,  Issue 10,  October 2007 Page(s):3546 - 3551 學生:陳威宇 學號:96325513

2. Outline • Introduction • System Model • Proposed Compensation Algorithm • Numerical Results • Conclusions • Reference

3. Ⅰ. Introduction • OFDMA - orthogonal frequency division multiple access - subcarriers -> grouped into sets & assigned to different user • CFO - carrier frequency offset - mismatch of the oscillators in the transmitter and the receiver • In the uplink of an OFDMA - inter-carrier interference ,ICI multipleaccess interference , MAI

4. Introduction ICI- the leakage or interference between a user’s own subcarriers MAI-the power leakage from user’s subcarriers • Two main approaches,can be used to mitigate the frequency offset in the uplink of OFDMA - feedback andcompensation method • In this paper,present SIC (successive interference cancellation) method for frequency offset compensation assuming that - perfect timing alignment - frequency offset of all uplink users are known at the receiver

5. Ⅱ . System Model • OFDMA system with D users and N subcarriers • The IDFT output of ith user can be written as

6. System Model • Assuming perfect time synchronization , discrete-time model of the received signal at the BS after removal of the CP can be written as

7. System Model • The DFT output can be obtained as

8. System Model • Assuming , the received signal in kth subcarrier can be written as

9. System Model • Clustered subcarrier assignment scheme where subcarriers are grouped into clusters and each clusters is assigned to a different user - Clustered =>size K - total number of clustered is N/K

10. Ⅲ. Proposed Compensation Algorithm • Combination A. Decorrelation - remove the ICI effect B. Successive Cancellation - solve the MAI problem

11. A. Decorrelation • In the receiver , the DFT output for cth cluster ,

12. Decorrelation • The decorrelator output can be calculated using (8) as • Using and the channel knowledge , the receiver can detect the transmitted information.

13. B. Successive Cancellation • The interference of cth cluster • The uth subcarrier`s value after removal of MAI from cth cluster becomes

14. Successive Cancellation • proof

15. Ⅳ . Numerical Results • An uplink OFDMA system parameter subcarrier : 256 users: 8 clusters for each user : 8 transmission bandwidth : 10 MHz channel : A of ITU-R channel model for vehicular environments with high antenna[13] IMT-2000 frequency offset : independent for all user random variable with uniform distribution (-єmax ,єmax)

16. Numerical Results

17. Numerical Results

18. Numerical Results

19. Numerical Results

20. Ⅴ. Conclusions • The proposed method , a combination of decorrelator and successive cancelation , can compensate the different frequency offsets of users with manageable complexity . • We have proposed to apply SIC to only neighboring subcarriers in order to decrease the computational complexity.

21. Ⅵ . Reference • [5] J Choi; C. Lee; H. W. Jung and Y. H. Lee; “ carrier frequency offset compensation for uplink of ofdm-fdma systems,”IEEE Trans. Commun. ,vol.53,no.7,pp. 1155-1165,July 2005 • [9] T. Yucek , “ Self-interference handling in OFDM based wireless communication systems,” Master’s thesis , University of South Florida,Nov.2003.