1. May 2003
doc.: IEEE /141r3
July, 2005
Project: IEEE P 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 ]
Voice:[ ], 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 TG3a.]
Notice: This document has been prepared to assist the IEEE P 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 P
Celestino A. Corral et al., Freescale
Jai Balakrishnan et al., Texas Instruments

2. Ultra-Wideband Peak Power Limits
July, 2005
Ultra-Wideband Peak 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
Celestino A. Corral et al., Freescale

3. July, 2005
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.
Celestino A. Corral et al., Freescale

4. Average Power Measurements
July, 2005
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.
Celestino A. Corral et al., Freescale

5. Average Power Measurements
July, 2005
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. July, 2005
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
July, 2005
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. Direct-Sequence UWB Sinusoidal carrier, PAPR = 3 dB
May 2003
doc.: IEEE /141r3
July, 2005
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 >30% fractional bandwidth between 3.4 and 4.8 GHz and consequently good fading resilience.
0.26 ns
code
4.1 GHz
adjust
Filter
data
Celestino A. Corral et al., Freescale
Jai Balakrishnan et al., Texas Instruments

9. What Spectrum Analyzer Measures
July, 2005
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. Worst-Case PAPR of MB-OFDM
July, 2005
Worst-Case PAPR of MB-OFDM
Subcarrier spacing is 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
Celestino A. Corral et al., Freescale

11. How Often Does This Happen?
July, 2005
How Often Does This Happen?
QPSK
Constellation
90o 0o
180o
270o
Celestino A. Corral et al., Freescale

12. Impact of Filtering Operation
July, 2005
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
July, 2005
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 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. July, 2005
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
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.
Celestino A. Corral et al., Freescale

15. Let’s Look at Doc. 262r0 Slide 2
July, 2005
Let’s Look at Doc. 262r0
Slide 2
We need to look at the rules as they are stated and draw our own conclusions.
Celestino A. Corral et al., Freescale

16. July, 2005
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
Celestino A. Corral et al., Freescale

17. July, 2005
FCC Rules
Under , 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 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. …
Celestino A. Corral et al., Freescale

18. July, 2005
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 bandwidth…That limit is 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 with allowing for alternative test procedures, not the other way around!
Celestino A. Corral et al., Freescale

19. Another Consideration
July, 2005
Another Consideration
Rule 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) = 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.
Celestino A. Corral et al., Freescale

20. July, 2005
Slides 5--7 of doc. 262r0
Peak resolution bandwidth of 3 MHz yields a maximum level of 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:
= 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.
Celestino A. Corral et al., Freescale

21. July, 2005
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
Celestino A. Corral et al., Freescale

22. Consequences of Analysis
July, 2005
Consequences of Analysis
Following , 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.
Celestino A. Corral et al., Freescale

23. July, 2005
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:
Celestino A. Corral et al., Freescale

24. Peak-to-Average vs. RBW
July, 2005
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
Celestino A. Corral et al., Freescale

25. July, 2005
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.
Celestino A. Corral et al., Freescale

26. July, 2005
Conclusions
FCC Rule 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 This does not make the 3 MHz resolution bandwidth “standard.”
FCC Rule 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.
Celestino A. Corral et al., Freescale