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

2. NTUEE Confidential 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. NTUEE Confidential 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. NTUEE Confidential Considerations in Transmitter

5. NTUEE Confidential 5 Requirement for Channel Estimation (1/2) For some subcarrier k, –Tx antenna 0, S 0 [n] –Tx antenna 1, S 1 [n] At the Rx, Antenna 0Antenna 1 S 0 [0] S 0 [1]S 1 [1] S 1 [0] Antenna 0Antenna 1 R 0 [0] R 0 [1]R 1 [1] R 1 [0] H 00 H 11 H 01 H 10

6. NTUEE Confidential 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. NTUEE Confidential 7 System Resources Training symbol and pilot subcarriers 24 800 sub-carriers time training symbolpilot subcarrier 24 800 sub-carriers 918

8. NTUEE Confidential 8 Pattern Training symbol Pilot subcarriers Antenna 0 Antenna 1 -1-j 1+j -1-j1+j differentially encoded by one PN sequence 1+j -1-j

9. NTUEE Confidential 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. NTUEE Confidential 10 User u STBC + MC-CDMA Pilot Insertion OFDM Modulation Training Symbol Insertion Constellation Mapping STBC Spreading Pilot Insertion OFDM Modulation Training Symbol Insertion Antenna 0 output Antenna 1 output User 0 Spreading C0C0 C0C0 Time i Time i+1 Time i Time i+1 Alamouti ABBA form

11. NTUEE Confidential 11 V-BLAST + MC-CDMA User u Pilot Insertion OFDM Modulation Training Symbol Insertion Constellation Mapping V-BLAST Spreading Pilot Insertion OFDM Modulation Training Symbol Insertion Antenna 0 output Antenna 1 output User 0 Spreading C0C0 C0C0 Time i Time i+1 Time i Time i+1 V-BLAST

12. NTUEE Confidential Considerations in Receiver

13. NTUEE Confidential 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 H 00,k - H 01,k Fine symbol boundary detection –Using training symbol 0 –Equivalent channel response H 00,k + H 01,k Antenna 0Antenna 1 Training symbol 0 Training symbol 1 Normal symbol 0 Normal symbol 1

14. NTUEE Confidential 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 0Antenna 1 Training symbol 0 Training symbol 1 Data symbol 0 Data symbol 1 Data symbol 2

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

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

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

18. NTUEE Confidential 18 Combining and Despreading - STBC (2/3) Received signal after DFT –b : Multi-users’ signal in two time slots (2LUx1) –C : Spreading matrix (NxLU) –H 00, H 01 : Channel complex gain (NxN) MRC –Can’t reduce MAI MMSE –Minimize the mean squared error per user data [1]

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

22. NTUEE Confidential 22 Combining and Despreading - V-BLAST (2/3) Received signal after DFT –b u,k : data of the user u at the subcarrier k of two transmit antenna (2x1) –H k : Channel complex gain (2x2) –r k : 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. NTUEE Confidential 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. NTUEE Confidential 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. NTUEE Confidential 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. NTUEE Confidential 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.