Ultra-Wideband Peak Power Limits

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 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.
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 18, 2005

3
Motivation

• Goal To provide a comparison between DS-UWB and
  MB-OFDM for peak-limited applications under the
  recent FCC waiver. Also delineate consequences
  of recent FCC waiver in relation to peak power
  limits for UWB waveforms
• 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 the
  waveforms.
• Additionally Provide peak-power headroom levels
  for actual implementation considerations.

4
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.

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.
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
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).
8
Direct-Sequence UWB

• Sinusoidal carrier, PAPR 3 dB
• Data spread by chipping code
• Shaped by RRC filter with a 0.3.
• Upconverted to desired freq.
• Spectral BW 1.4 GHz. Waveform has gt30
  fractional bandwidth between 3.4 and 4.8 GHz and
  consequently good fading resilience.

0.26 ns
code
4.1 GHz
adjust
Filter
data
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.
10
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
11
How Often Does This Happen?
QPSK Constellation
90o
0o
180o
270o
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.
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.
14
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.
15
Lets Look at Doc. 262r0
Slide 2
We need to look at the rules as they are stated
and draw our own conclusions.
16
FCC Rules
The technical requirements specify 50 MHz
bandwidth. Hence, 50 MHz is mandatory. The
technical requirements support different
resolution bandwidths and adjustments for the
peak emission limits pursuant to 15.521.
17
FCC Rules
Under 15.521, it is stated that other resolution
bandwidths are acceptable. Limits are
included No less than 1 MHz and no greater than
50 MHz. For resolution bandwidths greater than 3
MHz (even 3.001 MHz), a detailed description of
the test procedure, calibration, etc., must be
submitted to the Commission. This does not lead
to the conclusion that test measurements must be
made at 3 MHz resolution bandwidth The 50 MHz
resolution bandwidth is still mandatory. The FCC
is making provisions for other test techniques,
not smaller bandwidths for in-band victim
receivers.

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FCC Rules
If the FCC rules specified a 3 MHz mandatory
bandwidth, then it would state
(e) There is a limit on the peak level of the
emissions contained within a 3 MHz bandwidthThat
limit is -24.4 dBm EIRP.
But the FCC rules specify a 50 MHz bandwidth in
consideration of the largest possible victim
bandwidth in the UWB emissions band. These
already exist, namely C-band TVRO receivers with
bandwidths in excess of 30 MHz. The principal
rule is 15.519 with 15.521 allowing for
alternative test procedures, not the other way
around!
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Another Consideration

• Rule 15.521 puts a lower limit on the peak power
  resolution bandwidth of 1 MHz.
• At this resolution bandwidth, the peak power is
  limited to a value of 20log(1/50) -33.98 dBm.
• This value is only about 7.2 dB above the
  average.
• Since MB-OFDM appears gated, it uses 5.8 dB of
  that available peak power.
• Only 1.4 dB is available for the OFDM content in
  a 1 MHz resolution bandwidth.
• However, heavy filtering will suppress this
  effect, and peak power levels are not accurately
  reflected.
• Moving away from the mandatory 50 MHz resolution
  bandwidth shrinks the available headroom of any
  signal.

20
Slides 5--7 of doc. 262r0

• Peak resolution bandwidth of 3 MHz yields a
  maximum level of -24.4 dBm using the correction
  20log(RBW/50).
• Peak resolution bandwidth is triple the average
  bandwidth, so 10log(3) 4.8 dB is the correction
  for the average. All waveforms will have the
  same average
• -41.2 4.8 -36.4 dBm
• Peak to average of all test waveforms (including
  AWGN) is therefore pre-set to 12 dB.
• As a result, we expect little headroom difference
  at 3 MHz resolution bandwidth.

21
Slide 8 of doc. 262r0

• The results are confirmed on the plot. At 3 MHz
  resolution bandwidth, none of the waveforms have
  headroom.
• However, at the mandatory 50 MHz resolution
  bandwidth, it is clear that DS-UWB has more than
  7 dB of headroom over MB-OFDM.
• It is also evident that DS-UWB has a total of 17
  dB of headroom in general at the mandatory 50 MHz
  bandwidth.

DS-UWB has significantly more headroom than
MB-OFDM in the mandated 50 MHz bandwidth
22
Consequences of Analysis

• Following 15.519, a 3 MHz band-pass filter is
  centered on the frequency at which the highest
  radiated emission occurs.
• By selecting a low peak power bandwidth of 3 MHz,
  the peak power is excessively filtered the
  actual peak power of the signal does not play a
  major role. This is due to two factors
• Spectral response of the filter Selecting a
  narrow filter bandwidth fails to capture the
  actual potential peak levels of the waveform in a
  50 MHz bandwidth.
• Temporal response of the filter A narrowband
  filter has a slow impulse response. This plays
  the dominant role in determining the peak levels.
• A 3 MHz resolution bandwidth masks the peak power
  potential of UWB signals and therefore yields
  misleading results relative to the mandatory 50
  MHz bandwidth.

23
A Closer Look
Assuming a flat spectrum in the resolution
bandwidth of interest, the peak power can be
related to the average power from
where we have the peak-to-average power as
signal in the peak resolution bandwidth
The average power increases as 10log(frequency)
but the peak power correction proceeds as
20log(RBW/50). Hence, for different bandwidths
we have different peak-to-average headroom as
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Peak-to-Average vs. RBW
Selecting RBW 3 MHz reduces available
peak-to-average by 12 dB relative to what is
available at RBW 50 MHz
25
Conclusions

• Multi-band OFDM (MB-OFDM), even if clipped to a 9
  dB peak-to-average over the air, can still result
  in up to a 16.6 dB PAPR in a 50 MHz bandwidth.
• For MB-OFDM, the 16.6 dB PAPR in a 50 MHz
  bandwidth is due to 10.8 dB of signal PAPR for 12
  subcarriers captured in that bandwidth and 5.8 dB
  of PAPR due to the 3-hop sequence.
• DS-UWB has a nominal 5.5 dB PAPR after pulse
  shaping.
• The PAPR of ungated DS-UWB in a 50 MHz bandwidth
  is 8.5 dB.
• DS-UWB has 15.8 dB maximum headroom for
  transmission which can be exploited for gated
  signals. This corresponds with an approximate
  duty cycle of 3.
• Consequently, DS-UWB has 8.1 dB more headroom
  than MB-OFDM for overcoming cable, filter and
  antenna losses.

26
Conclusions

• FCC Rule 15.519 explicitly states that the peak
  power limit is 0 dBm in a 50 MHz resolution
  bandwidth.
• For measurements above 3 MHz, test procedure must
  be documented as stated in FCC Rule 15.521. This
  does not make the 3 MHz resolution bandwidth
  standard.
• FCC Rule 15.521 allows peak power resolution
  bandwidth down to 1 MHz, providing only 7.2 dB of
  available peak-to-average for any waveform in the
  1 MHz resolution bandwidth.
• Selection of 3 MHz peak power resolution
  bandwidth masks the peak power effects the FCC is
  mandating in the 50 MHz resolution bandwidth.
• The available peak-to-average headroom is
  different at 3 and 50 MHz resolution bandwidths.
• At the mandatory FCC bandwidth of 50 MHz, DS-UWB
  has over 16 dB of headroom this is 7 dB more
  than MB-OFDM.