Decision Feedback Equalization in OFDM with Long Delay Spreads

1. Decision Feedback Equalization in OFDM with Long Delay Spreads Zeeshan Qureshi

2. Digital Video Broadcasting (DVB) • Rapidly becoming world-wide standard for digital TV • Being implemented in Europe, Australia, South Africa and India • Popularity due to: • Bandwidth efficiency • Improved picture/sound quality • Additional programming options • Utilizes OFDM to transmit over a Single Frequency Network (SFN) Model

3. SFN – Major Issues • Simultaneous transmission of same OFDM data from multiple transmitters • Receiver interprets a channel with a long Delay Spread • Delay spread longer than Cyclic Prefix causes increase in interference components (ISI; ICI) • Performance Degradation: Symbol recovery suffers due to interference

4. Conventional Solutions • Increase Cyclic Prefix length to Channel Delay Spread • Increases Symbol Overhead • Degrades OFDM system efficiency • Equalization in time-domain • Complex Receiver Design • Impractical to implement

5. Thesis Contribution • Implementation of a DFE-PIC based receiver configuration over OFDM channels with long delay spreads • Investigation of performance gain over conventional OFDM receiver • Evaluation done via simulations • Allows performance improvement • Maintains efficiency of OFDM • Receiver implementation is simple

6. Decision Feedback Equalizer (DFE) • DFE is a non-linear equalizer • Feedback filter: ISI cancellation using previous receiver decisions • Feed-forward filter: ICI cancellation on the transmitted symbols • Advantages: • Performance comparable to the optimum demodulator but with much lower computational complexity • Low noise enhancement

7. Feed-back Filter ISI(N-1)(t) _ Feed-forward Filter (PIC Detector) Decision Device SN(k) yN(t) ŜN(k) + DFE Block Diagram

8. Parallel Interference Canceller (PIC) • PIC detector estimates and subtracts interference for each channel in parallel • Stage-wise implementation • Stage 0 uses matched filter to estimate symbol without removal of interference • Later stages use the symbol estimates of the previous stage to estimate and remove interference components • Advantages: • Fast convergence • Low complexity

9. PIC Block Diagram Initial Observation Symbol Estimation (Stages 1+) ỹ0(t) Ż0(k) ŜN(k) + _ Ż0(k) ICI(N-1)(k) Symbol Estimation (Stage 0 Only) Interference Estimation Ŝ0(k) Ŝ(N-1)(k)

10. Simulation Environment • QPSK Modulation • OFDM Symbol Transmission • Rayleigh Channel Model • Additive White Gaussian Noise (AWGN) • Channel Delay Spread as long as the OFDM Symbol length • Perfect Channel Estimation in Receiver • Single-Tap DFE implementation

11. Implemented Scenarios • Response to System Parameter changes: • No. of ISI iterations • No. of PIC stages • Scaling of ICI components • Length of Channel Delay Spread • Performance in simulated SFN channels • Inter-site Distance between Transmitters

12. Performance Analysis • Simulated SFN Channel • ISD = 20 Km • Worst-case condition • 0.5 % CDF

13. Final remarks • Highlights of the DFE-PIC receiver • Significant performance gain achieved over OFDM receiver • Preserves OFDM system efficiency • Compensation of interference effects due to long delay spread • Simple to implement in the receiver • Low computational complexity

14. The End Questions? Thank You!