Decision Feedback Equalization in OFDM with Long Delay Spreads Zeeshan Qureshi

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

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

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

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

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

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

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

Interference Estimation 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)

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

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

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

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

The End Questions? Thank You!