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Enhancing OFDM Performance with Decision Feedback Equalization for Long Delay Spreads

This thesis explores Decision Feedback Equalization (DFE) in OFDM systems with long delay spreads, addressing issues of interference components and symbol recovery degradation. The study presents a DFE-PIC configuration for improved performance over conventional OFDM receivers, through simulations that demonstrate simplicity of implementation and efficiency maintenance while achieving performance gains. Advantages include reduced computational complexity and preservation of system efficiency, compensating for interference effects and enhancing symbol recovery despite long delay spreads.

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Enhancing OFDM Performance with Decision Feedback Equalization for Long Delay Spreads

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  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!

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