Comparison of DS-UWB and MB-OFDM for Peak-Power Limited Applications

1. Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: [Ultra-Wideband Peak Power Limits] Date Submitted: [15 May, 2005] Source: [Celestino A. Corral, Shahriar Emami and Gregg Rasor] Company [Freescale Semiconductor, Inc.] Address [6100 Broken Sound Pkwy., N.W., Suite 1, Boca Raton, Florida USA 33487] Voice:[561-544-4057], FAX: [] Re: [Recent FCC Waiver] Abstract: [This document provides analytical and theoretical comparison of MB-OFDM and DS-UWB under peak power limited applications.] Purpose: [For discussion by IEEE 802.15 TG3a.] 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(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) 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. Celestino A. Corral et al., Freescale

2. Ultra-WidebandPeak Power Limits Celestino A. Corral, Shahriar Emami and Gregg Rasor Freescale Semiconductor, Inc. 6100 Broken Sound Parkway., N.W., Suite 1 Boca Raton, Florida USA May 17, 2005 Celestino A. Corral et al., Freescale

3. Motivation • Goal: To provide a comparison between DS-UWB and MB-OFDM for peak-limited applications under the recent FCC waiver. • Note: Recent FCC waiver is technology-neutral. Devices can be measured under “normal” operating conditions. These conditions can include hopping or gating. • Approach: Consider DS-UWB and MB-OFDM waveforms under average- and peak-power measurements. Emphasis is on peak-to-average power ratio of waveforms. • Additionally: Provide peak-power headroom levels for actual implementation considerations. Celestino A. Corral et al., Freescale

4. Spectrum analyzers measure average value of the total signal power quantized within resolution bandwidth by making a fixed number of measurements and computing a corrected average figure of power density normalized to that bandwidth. Average Power Measurements Radiated Waveform Celestino A. Corral et al., Freescale

5. Average Power Measurements Resolution bandwidth filter Block Diagram of Typical Spectrum Analyzer For FCC emission measurements, the resolution bandwidth is 1 MHz with 1 msec integration time for the RMS power and resulting EIRP. Resolution bandwidth is 50 MHz for peak power measurements. Celestino A. Corral et al., Freescale

6. Gated Signals gated signal t Gating allows greater power transmissions over narrower time intervals. This power can be used to improve SNR, SIR or range. Limit is now peak power. ungated signal T Celestino A. Corral et al., Freescale

7. Peak Power Measurements 50 MHz 1 MHz key determinant for peak-power levels Minimize PAPR to achieve more headroom in peak power levels Peak power measurements actually made with spectrum analyzer on “peak hold” capturing over a long time period (several minutes). Celestino A. Corral et al., Freescale

8. Sinusoidal carrier, PAPR = 3 dB Data spread by chipping code Upconverted to desired freq. Shaped by RRC filter with a = 0.3. Spectral BW = 1.5 GHz. Waveform has 40% fractional bandwidth between 3.1 and 4.6 GHz and consequently good fading resilience. Direct-Sequence UWB 0.26 ns code 4.1 GHz adjust RRC Filter data Celestino A. Corral et al., Freescale

9. What Spectrum Analyzer Measures DS-UWB Waveform Signal over air has 5.5 dB PAPR 1 MHz Filter 50 MHz Filter DS-UWB has 8.5 dB PAPR (ungated) in 50 MHz filter. Celestino A. Corral et al., Freescale

10. Subcarrier spacing is 4.125 MHz. In 50 MHz resolution bandwidth this corresponds to 12 subcarriers. Worst-case PAPR is 10log(12)=10.8 dB. Above occurs even if MB-OFDM waveform is clipped to 9 dB PAPR. If we consider that hopping contributes 5.8 dB additional PAPR for 3 hops, the total worst-case PAPR is 16.6 dB. As a result, we have about 7.7 dB headroom for MB-OFDM. Worst-Case PAPR of MB-OFDM 50 MHz Celestino A. Corral et al., Freescale

11. How Often Does This Happen? QPSK Constellation 90o 0o 180o 270o Celestino A. Corral et al., Freescale

12. Impact of Filtering Operation Worst-Case OFDM Symbol 12 Subcarriers Filter Impulse Response (50 MHz) Output of Filter (Convolution) Pulse width is about 8% of the length of OFDM symbol. pulse width The filter impulse response is very narrow relative to the OFDM waveform, so convolution results in OFDM symbol and PAPR is conserved. Celestino A. Corral et al., Freescale

13. What Spectrum Analyzer Measures Multi-Band OFDM Waveform Signal over air has 9 dB PAPR 1 MHz Filter 50 MHz Filter On average, peak power is -11.1 dBm and PAPR is 15 dB. Worst-case PAPR is 16.6 dB and peak-power is -7.7 dBm. Celestino A. Corral et al., Freescale

14. Summary of Results Thus, DS-UWB has 8.1 dB more headroom than MB-OFDM. This can be employed to overcome cable losses, antenna losses, etc. DS-UWB has a net 15.8 dB headroom for exploiting gating. Celestino A. Corral et al., Freescale

15. Conclusions • DS-UWB is generated from a sinusoid having 3 dB peak-to-average that grows to 5.5 dB over air after pulse shaping. The PAPR of DS-UWB in the 50 MHz filter is 8.5 dB (ungated). Hence, DS-UWB has 8.1 dB more headroom than MB-OFDM for overcoming cable, filter and antenna losses. • DS-UWB has 15.8 dB maximum headroom for transmission which can be exploited for gated signals. This corresponds to about 3% duty cycle. • Multi-band OFDM, even if clipped to 9 dB peak-to-average over the air can still result in up to 16.6 dB PAPR in a 50 MHz resolution bandwidth. The 16.6 dB level is due to 10.8 dB of signal PAPR for 12 subcarriers captured and 5.8 dB PAPR due to duty cycle of 3-hop sequence. Celestino A. Corral et al., Freescale