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Toward MIMO MC-CDMA

Toward MIMO MC-CDMA. Speaker : Pei-Yun Tsai Advisor : Tzi-Dar Chiueh 2004/10/25. Outline. Motivation MIMO MC-CDMA transmitter Allocation of system resource STBC+MC-CDMA V-BLAST+MC-CDMA MIMO MC-CDMA receiver Synchronization Channel estimation MIMO decoding Conclusion. Motivation.

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Toward MIMO MC-CDMA

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  1. Toward MIMO MC-CDMA Speaker : Pei-Yun Tsai Advisor : Tzi-Dar Chiueh 2004/10/25

  2. Outline • Motivation • MIMO MC-CDMA transmitter • Allocation of system resource • STBC+MC-CDMA • V-BLAST+MC-CDMA • MIMO MC-CDMA receiver • Synchronization • Channel estimation • MIMO decoding • Conclusion

  3. Motivation • MC-CDMA • A promising solution for future wireless cellular communication systems. • MIMO • One technique to improve capacity and diversity gain. • MIMO MC-CDMA • To be investigated and evaluated as a trend. [1][2]

  4. Considerations in Transmitter

  5. Requirement for Channel Estimation(1/2) Antenna 0 Antenna 1 • For some subcarrier k, • Tx antenna 0, S0[n] • Tx antenna 1, S1[n] • At the Rx, S0[0] S1[0] S0[1] S1[1] H10 H00 H11 H01 Antenna 0 Antenna 1 R0[0] R1[0] R0[1] R1[1]

  6. Requirement for Channel Estimation(2/2) • MMSE Channel estimation [3] subject to S, known training symbols or pilot subcarriers, should be an unitary matrix to achieve minimum MSE.

  7. System Resources • Training symbol and pilot subcarriers 800 sub-carriers 800 sub-carriers 24 24 time 9 18 pilot subcarrier training symbol

  8. Pattern • Training symbol • Pilot subcarriers Antenna 0 Antenna 1 -1-j differentially encoded by one PN sequence 1+j -1-j 1+j Antenna 0 Antenna 1 1+j -1-j

  9. Problem • Channel estimation in mobile MIMO systems • Time-invariant requirement of channel response. • Impossible in fast-fading channel. • Decrease the supported highest mobility

  10. Pilot Pilot OFDM OFDM Insertion Insertion Modulation Modulation STBC + MC-CDMA User u Spreading User 0 Antenna 0 output Spreading Time i Training Symbol C0 Time i+1 Insertion Constellation STBC Mapping C0 Time i Antenna 1 output Spreading Time i+1 Training Symbol Insertion Alamouti ABBA form

  11. Pilot Pilot OFDM OFDM Insertion Insertion Modulation Modulation V-BLAST + MC-CDMA User u Spreading User 0 Antenna 0 output Spreading Time i Training Symbol C0 Time i+1 Insertion Constellation V-BLAST Mapping C0 Time i Antenna 1 output Spreading Time i+1 Training Symbol Insertion V-BLAST

  12. Considerations in Receiver

  13. Synchronization Tasks (1/2) • Coarse symbol boundary detection • Training symbol 0 still has two repetitions in the time domain. • Fractional CFO acquisition • Using training symbol 0 • Integer CFO acquisition • Using training symbol 1 • Equivalent channel response H00,k-H01,k • Fine symbol boundary detection • Using training symbol 0 • Equivalent channel response H00,k+H01,k Antenna 0 Antenna 1 Training symbol 0 Training symbol 1 Normal symbol 0 Normal symbol 1

  14. Synchronization Tasks (2/2) • Estimation for residual CFO and TFO • Using pilot subcarriers. • Using phase difference of consecutive symbols in the frequency domain. • Problem arises due to alternative pilot data transmitted by antenna 1. • Simple solution: using pilot data separated by 2 symbols Antenna 0 Antenna 1 Training symbol 0 Training symbol 1 Data symbol 0 Data symbol 1 Data symbol 2

  15. Channel Estimation (1/2) • Static channels • Matrix inverse • Linear interpolation • Channel estimates apply to the following normal symbols Antenna 0 Antenna 1 Training symbol 0 Training symbol 1

  16. Channel Estimation (2/2) • Dynamic channels • Two data symbols grouped together • Getting channel estimates in pilot subcarriers • Raised-Cosine frequency-domain channel interpolator Antenna 0 Antenna 1 Data symbol 0 Data symbol 1

  17. Channel Estimation Combining and Despreading - STBC (1/3) • STBC[1] De-Mapping Antenna 0 input STBC Decoding FFT Despreading MRC Algorithm MMSE Algorithm Antenna 1 input FFT EM-based Detection PIC Algorithm

  18. Combining and Despreading - STBC (2/3) • Received signal after DFT • b : Multi-users’ signal in two time slots (2LUx1) • C : Spreading matrix (NxLU) • H00, H01 : Channel complex gain (NxN) • MRC • Can’t reduce MAI • MMSE • Minimize the mean squared error per user data [1]

  19. Combining and Despreading - STBC (3/3) • EM (Expectation-Maximization)-based detection[4] • Arbitrary positive real scalar • E-step • M-step • PIC (Parallel-Interference Cancellation) detector • Iterative [1]

  20. Performance • Simulation parameters [1] • Carrier frequency : 2.56 GHz • Bandwidth : 5 MHz • N: 512 • U=32 (full loaded) • PIC and EM detection • Initial : MRC • Iteration 2 times.

  21. Channel Channel Estimation Estimation Combining and Despreading - V-BLAST (1/3) • V-BLAST[5] De-Mapping Antenna 0 input V-BLAST Decoding FFT Despreading ZF Algorithm MMSE Algorithm Antenna 1 input FFT IC-ZF Algorithm IC-MMSE Algorithm

  22. Combining and Despreading - V-BLAST (2/3) • Received signal after DFT • bu,k: data of the user u at the subcarrier k of two transmit antenna (2x1) • Hk : Channel complex gain (2x2) • rk : received signal at the subcarrier k (2x1) • ZF (Zero-Forcing) • Using channel estimates to solve the two linear equations. • MMSE • MMSE per subcarrier [5]

  23. Combining and Despreading - V-BLAST (3/3) • Interference cancellation (IC) – ZF algorithm • Initial : • Recursion : • IC-MMSE algorithm • Change the pseudo-inverse to MMSE coefficient Maximize SNR Nulling the column [5]

  24. Performance • Simulation parameters [5] • N: 64 • Spreading factor : 8 • U=4 (half-loaded) • Antenna diversity : 4x4 • Iterative detection before despreading suffers MAI and error propagation

  25. Conclusion • MIMO techniques incorporated into MC-CDMA systems are considered. • Transmitter modification includes • Pattern of training symbol and pilot subcarriers (done) • MIMO encoding block (done) • Receiver modification includes • Joint estimation of residual CFO and TFO (done) • MIMO decoding block • Performance of MIMO decoding in the MC-CDMA systems does not have the same trend as in the OFDM systems due to MAI.

  26. Reference [1] S. Iraji and J. Lilleberg, “ Interference cancellation for space-time block-coded MC-CDMA systems over multipath fading channels,” in Proceeding of IEEE VTC’03, pp.1104-1108. [2] V. Nangia and K. L. Baum, “Experimental broadband OFDM systems field results for OFDM and OFDM with frequency domain spreading,” in Proceeding of IEEE VTC’02, pp. 223-227. [3] D. Wang, G. Zhu and Z. Hu, “Optimal pilots in frequency domain for channel estimation in MIMO-OFDM systems in mobile wireless channesl”, in Proceeding of IEEE VTC’04 Spring. [4] M. Feder and E. Weinstein, “Parameter estimation of superimposed signals using the EM algorithm”, IEEE Trans. on Acoustics, Speech and Signal Processing, vol. 36, pp. 477-489, Apr. 1988. [5] Z. Lei, X. Peng and F. P. S. Chin, “V-BLAST receiver for downlink MC-CDMA systems,” in Proceeding of IEEE VTC’03, pp.866-870.

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