Ultra-Wideband%20Peak%20Power%20Limits

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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 Voice561-544-4057, 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.
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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

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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.

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Average Power Measurements
Radiated Waveform

• 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.

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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.
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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
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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).
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Direct-Sequence UWB

• 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.

0.26 ns
code
4.1 GHz
adjust
RRC Filter
data
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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.
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Worst-Case PAPR of MB-OFDM

• 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.

50 MHz
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How Often Does This Happen?
QPSK Constellation
90o
0o
180o
270o
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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.
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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.
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Summary of Results
Parameter DS-UWB MB-OFDM
PAPR at transmit pin 3.0 dB 9.0 dB
PAPR over air after pulse shaping 5.5 dB 9.0 dB
PAPR at output of 50 MHz filter 8.5 dB 16.6 dB
Peak power in 50 MHz bandwidth -15.8 dBm -7.7 dBm
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.
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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.