Equalisation Architectures for OFDM and 3G Steve McLaughlin Yushan Li David G .M. Cruickshank IDCOM, University of Edinb

1. Equalisation Architectures for OFDM and 3G Steve McLaughlin Yushan Li David G .M. Cruickshank IDCOM, University of Edinburgh

2. Outline • Introduction and Motivation • Chip-Level Equalisation for WCDMA • FDE in SC Systems • Chip-Level FDE for WCDMA • Joint structure for Channel Estimation, Chip-Level FDE and Parallel Interference Cancellation • Conclusions

3. Motivation • Concerned with algorithmic issues which will enable Multimode behaviour • Consider UMTS and OFDM systems • Frequency domain equalisation approach • Unfortunately, a CP-based FDE is not compatible for the current UMTS system and the overhead introduced by a CP will reduce the spectral efficiency. • Suggest some solutions to overcome this while minimising complexity

4. Chip-Level Equalisation for WCDMA • RAKE: The performance is dominated by the MAI and this results in saturation at a fairly high error rate. • MUD: The possibility to perform multiuser detection in mobile handsets is limited by its high complexity • Symbol-Level Equalisation: Not suitable for long code WCDMA. • Chip-Level Equalisation: Achieves good compromise between performance and complexity

5. FDE in SC Systems • Transform the received signal from the time domain to the frequency domain (FFT) • 2. Adjust each discrete frequency bins and • make the spectrum flat. Single tap equalizer in the frequency domain -> Simple structure • 3. Transform the equalized signal back to the time domain (IFFT)

6. SC-FDE vs. Time Domain Equalisation: Complexity • Computationally simpler, especially for channels with severe delay spread (11~20 chips) • For severe channel spreading, complexity of frequency domain processing grows slowly than time domain processing. Details may be found in: Falconer, et al, “Frequency domain equalization for single-carrier broadband wireless systems”, IEEE Comm. Magazine, April 2002

7. SC-FDE in Multimode Receivers • Merit: • Employing a similar architecture as in OFDM systems, SC-FDE and OFDM can easily be configured to coexist, thus makes the multimode receiver simpler while a connection to UMTS is required

8. Issue & Solution • Issue: Unfortunately, a CP-based FDE is not compatible for current single-carrier systems with no CP. It is desirable to design a receiver without changing the format of the transmitted signal. • Solutions: A number of solutions have been proposed for OFDM systems or single-carrier systems without CP. They aim at compensating the effect of the missing CP. • – cyclic reconstruction.

9. Solutions • D. Kim and G. Stüber, "Residual ISI cancellation for OFDM with applications to HDTV broadcasting", IEEE Journal on Select Areas in Communications, Vol. 16, No. 8, Aug. 1998, pp. 1590-1599. (RISIC Algorithm) • 2. C. Park and G. Im, "Efficient DMT/OFDM transmission with insufficient cyclic prefix", IEEE Communications Letters, Vol. 8, Issue 9, Sept. 2004, pp. 576-578. • 3. H. Won and G. Im, "Iterative cyclic prefix reconstruction and channel estimation for a STBC OFDM system", IEEE Communications Letters, Vol. 9, Issue 4, Apr. 2005, pp. 307-309. • Y. Li, S. McLaughlin and D.G.M. Cruickshank, "Bandwidth efficient single carrier systems with frequency domain equalization", Electronics Letters, Vol. 41, No. 15, July 2005, pp. 857-858.

10. RISIC Algorithm • Introduction: In the RISIC algorithm, the missing CP is regarded as bursty distortion in a time domain block and the amount of distortion is diminished in an iterative process with hard decisions being made in the frequency domain. • Performance degrades in channel with deep nulls since the hard decision will cause noise enhancement.

11. Existing CP reconstruction methods RISIC Scheme and Extended RISIC Scheme

12. Chip-Level FDE for WCDMA • In principle, proposed cyclic reconstruction schemes can all be extended for single-carrier WCDMA systems in order to deploy FDE at chip level. However, some of them suffer from high computational complexity, especially in the case of the application to WCDMA. • Solutions particularly proposed for WCDMA • “Overlap-Cut” Method • “FDE based on Self cyclic reconstruction” • “FDE based on Slot Segmentation”

13. Overlap-Cut Method • Applying a conventional FDE on a single carrier system without CP gives errors that are significantly larger at the edges of the block. • Samples at the beginning and the end of each equalized blocks are discarded. • Processing blocks are overlap with each other. Ref: M. Vollmer, M. Haardt and J. Gotze, "Comparative study of joint detection techniques for {TD-CDMA} based mobile radio systems", IEEE Journal on Select Areas in Communications, Vol. 19, No. 8, Aug. 2001, pp. 1461-1475.

14. FDE based on Self cyclic reconstruction • The algorithm exploits the relationship between the required cyclic part and the transmitted signal itself. The estimated cyclic part is then added to the received block signal to enable frequency domain equalization. This can be viewed as a cyclic reconstruction process. Ref: Y. Li, S. McLaughlin, D.G.M. Cruickshank, "UMTS FDD frequency domainequalization based on self cyclic reconstruction", IEEEInternational Conference on Communications, Vol.3, May 2005, pp.2122-2126.

15. FDE based on Slot Segmentation • By exploiting the frame and slot structures of the UMTS downlink, the pilots within one slot (for FDD mode) are used for cyclic reconstruction in a FDE. • Furthermore, one slot signal is split intomultiple segments for the sake of combating channel variance within one slot. Ref: Y. Li, S. McLaughlin, D.G.M. Cruickshank, "UMTS FDD frequency domainequalization based on slot segmentation", Proceedings of the 61st IEEE Vehicular Technology Conference, May 2005, Stockholm, Sweden.

16. JointChannel Estimation, Chip-Level FDE and Parallel Interference Cancellation structure for WCDMA • Accurate channel estimation for a practical mobile communication system is important! • Time-multiplexed pilots: require extra bandwidth and hence reduce bandwidth efficiency. • Code-multiplexed pilots: no bandwidth spreading is necessary.

17. Correlation Method • In practice, the correlation method (CM) is a simple technique for channel estimation in WCDMA. • The distorted autocorrelation property due to channel impairments degrades its performance. • A high power code-multiplexed pilot sequence is required for better channel estimates. • Unfortunately, high power pilot channel  high MAI to the data channels.

18. IFDCE: Iterative Frequency Domain Channel Estimation • The IFDCE method reconstructs the sum of data channels and the code-multiplexed pilot channel. The reconstructed composite signal is being treated as a virtual pilot signal. • Channel estimation is performed in the frequency domain. • The received WCDMA signal is equalised before spreading at chip level in the frequency domain.

19. Procedures … • Step 1. Correlation method is used to deliver initial channel estimates. • Step 2. A RAKE receiver then operates on the received signal and the composite estimated signal is despread and hard detected. • Step 3. K users' transmitted symbols are respread and rescrambled. The scrambled code-multiplexed pilots are added to form an estimated composite signal.

20. Procedures • Step 4. The estimated composite signal and the initial channel estimates are used for cyclic reconstruction. • Step 5. The reconstructed composite signal, being treated as a virtual pilot signal, is converted to the frequency domain and used for channel estimation. • Step 6. The result from Step 5 is converted to the time domain and only the first “L” values (Channel is assumed to span L chips) are kept to form a new channel estimate. • Step 7. Frequency domain equalisation.

21. Proposed IFDCE Structure CR: Cyclic Reconstruction

22. Parallel Interference Cancellation • Why PIC? • Since the proposed iterative channel estimation requires user symbol detection and interference reconstruction, a PIC is combined into the iterative structure in order to further enhance the system performance. • The integration of PIC is with only a slight increase in computational complexity. This is exactly why the PIC is introduced into the iterative structure.

23. IFDCE + PIC Scheme

24. Simulations • WCDMA Systems • Carrier Frequency = 2 GHz • Chip Rate = 3.84 Mchips/s • Spreading Factor = 64 • 10 Active Users • Pilot Channel Power = 10% Whole Power • Block Size = 1024 chips (For CE and FDE) • UMTS Vehicular A Test Channel • Mobile Speed = 30 km/h

25. Simulation Results 3.8 dB Close to the ideal case

26. Simulation Results Close to the single user case

27. Discussions • The iterative channel estimator can provide the PIC with better channel estimates, hence better performance. • The iterative cycle can be implemented efficiently in the frequency domain with complexity of O(NlogN)where N is the block size.

28. Conclusions • To design a FDE for the current WCDMA system is very attractive.OFDM has become a strong candidate for the fourth generationsystems and hence a WCDMA receiver adopting FDE will becompatible with the current FFT based receiver structures. • Byadopting FDE for singlecarrier WCDMA, a multi-mode receiver can be programmed toswitch to a particular system moreconveniently.

29. Thank you! • Steve McLaughlin • IDCOM, University of Edinburgh • Email: sml@ee.ed.ac.uk