Implementation of High Speed FFT Processor for MB-OFDM System

1. doc.: IEEE 802. 15-04-0467-00-003a September 2004 Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANS) Submission Title: [Implementation of High Speed FFT processor for MB-OFDM System] Date Submitted: [September 2004] Revised: [] Source: [Sang-sung Choi, Sang-in Cho] Company [Electronics and Telecommunications Research Institute] Address [161 Gajeong-dong, Yuseong-gu, Daejeon, 305-350 Korea] Voice : [+82-42-860-6722], FAX : [+82-42-860-5199], E-mail [sschoi@etri.re.kr] Re: [Technical contribution] Abstract: [This presentation presents the implementation method of IFFT/FFT processor for MB-OFDM UWB system] Purpose: [Technical contribution to implement IFFT/FFT processor proposed for MB-OFDM UWB system] Notice:This document has been prepared to assist the IEEE P802.15. It is offered as a basis for discussion and is not binding on the contributing individual or organization. The material in this document is subject to change in form and content after further study. The contributor reserves the right to add, amend or withdraw material contained herein. Release: The contributor acknowledges and accepts that this contribution becomes the property of IEEE and may be made publicly available by P802.15.

2. Implementation of High Speed FFT processor for MB-OFDM System Sang-Sung Choi (sschoi@etri.re.kr) Sang-In Cho (sicho@etri.re.kr) www.etri.re.kr ETRI

3. Introduction MB-OFDM UWB proposal requires high speed IFFT/FFT processors with 128-point computation. Digital signals processed in IFFT processor change into analog signals by DAC, and then pass through the sharp LPF to satisfy the transmitting PSD mask. - Transmitter using 128-point IFFT processor (DAC speed : 528MHz) - LPF shape & Frequency spectrumof OFDM signal after DAC ETRI

4. Introduction • The TX LPF is very important to determine the transmit PSD mask of MB-OFDM UWB system, but the TX LPF design is not easy to satisfy the Transmit PSD mask of MB-OFDM. • Two methods are considered to design the TX LPF satisfying the transmit PSD mask. 1) fix 528MHz sampling rate of DAC , and design high order TX LPF 2) increase sampling rate of DAC, and reduce the order of TX LPF • Use 2 times over-sample rate at DAC to design the TX LPF. • - Reduce the order of TX LPF • - It has advantage of the performance compared to method 1). •  Presented by DOC IEEE802.15-03/275r0 • There are trade-offs between two methods for considering power • consumption and gate size etc.  ETRI is developing a prototype UWB system using 256-point IFFT processor (DAC speed : 1056MHz) ETRI

5. Proposed IFFT Processor Approach For easy low pass filtering of 528MHz baseband signal after DAC, we have to make space between OFDM signals that are repeated in frequency spectrum, which is accomplished by 128-point zero-padding. - Transmitter using 256-point IFFT processor (DAC speed : 1056MHz) - LPF shape & Frequency spectrumof OFDM signal after DAC ETRI

6. IFFT/FFT Processor Specification Input data of FFT processor are QPSK modulated 128-point complex data Input data of IFFT processor become 256-point that consisted of QPSK modulated 128-point complex data and 128-point zeros. - Input data of original IFFT processor (128-point QPSKdata) - Input data of proposed IFFT processor (128-point QPSK data + 128-point zeros) ETRI

7. Output : 8 samples/clock Input : 4 samples/clock Input : 4 samples/clock Output : 4 samples/clock Proposed transceiver for MB-OFDM UWB PHY proposal ETRI

8. Characteristics of Multipliers • Multiplier is one of the most dominant elements in FFT/IFFT implementation • Standard 2’s Complement Multiplier • (W-bit) x (W-bit) = (2W-1)-bit • Many DSP applications need only W-bit products • Fixed-Width Multiplier • Quantization to W-bit by eliminating (W-1) Least Significant Bits • Can reduce area by approximately 50% but Truncation Error is introduced • Proper Error Compensation Bias needed • Canonic Signed Digit Multiplier • Constant coefficient • 33% fewer nonzero digits than 2’s complement numbers • Modified Booth Multiplier • Variable coefficient • The number of partial products has been reduced to W/2 • These multipliers can achieve about 40% reduction in area and power consumption ETRI

9. The radix-24 structure of FFT processor DFT : Radix-2 structure Radix-24 structure CSD multiplier CSD multiplier Modified Booth multiplier ETRI

10. The structure of 256-point IFFT processor • 32-point Radix-24 FFT structure • 8-level parallelism • DIF (Decimation In Frequency), SDF (Single Delay Feedback) • Fixed CSD & Modified Booth multipliers used ETRI

11. The structure of 256-point IFFT processor • Butterfly unit : 48 • -j multiplier : 22 • CSD multiplier : 16 • Modified Booth Multiplier : 8 ETRI

12. The structure of 256-point IFFT processor CSD multiplier CSD multiplier Modified Booth multiplier ETRI

13. The structure of 128-point FFT processor • 32-point Radix-24 FFT structure • 4-level parallelism • DIF (Decimation In Frequency), SDF (Single Delay Feedback) • Fixed CSD & Modified Booth multipliers used ETRI

14. The structure of 128-point FFT processor CSD multiplier CSD multiplier Modified Booth multiplier • Butterfly unit : 24 • -j multiplier : 11 • CSD multiplier : 8 • Modified Booth Multiplier : 4 ETRI

15. Simulation result of 256-point IFFT processor Constellation • Input Bit resolution : 3 • Output bit resolution : 20 • Multiplier coefficient bit : 10 • SQNR : 52dB • Input Bit resolution : 3 • Output bit resolution : 11 • Multiplier coefficient bit : 8 • SQNR : 30dB ETRI

16. Simulation result of 128-point FFT processor Constellation • Input Bit resolution : 10 • Output bit resolution : 20 • Multiplier coefficient bit : 10 • SQNR : 52dB • Input Bit resolution : 10 • Output bit resolution : 12 • Multiplier coefficient bit : 8 • SQNR : 30dB ETRI

17. Summary of simulations IFFT processor FFT processor ETRI

18. Conclusion • 256-point IFFT processing for easy Low Pass Filtering • Parallel structure for high speed signal processing • IFFT/FFT processor • 32-point radix-24 DIF SDF structure • Small area, low power, high speed operation • Canonic Signed Digit Multiplier – constant coefficients • Modified Booth Multiplier – variable coefficients ETRI