1. May 2003
doc.: IEEE /141r3
November, 2004
Project: IEEE P Working Group for Wireless Personal Area Networks (WPANs)
Submission Title: [Multi-Band OFDM Interference on In-Band QPSK Receivers Revisited]
Date Submitted: [14 November, 2004]
Source: [Celestino A. Corral, Shahriar Emami, Gregg Rasor] Company [Freescale]
Address [3301 Quantum Blvd., Boynton Beach, Florida, USA ]
Voice:[ ], FAX: [ ]
Re: []
Abstract: [This document provides simulation and theoretical results that demonstrate MB-OFDM is an extremely harmful type of interference to wideband in-band QPSK systems such as C-band TVRO receivers.]
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. Multi-band OFDM Interference on In-Band QPSK Receivers Revisited
November, 2004
Multi-band OFDM Interference on In-Band QPSK Receivers Revisited
Celestino A. Corral, Shahriar Emami and Gregg Rasor
Freescale Semiconductor
3301 Quantum Blvd.
Boynton Beach, Florida
November 14, 2004
Celestino A. Corral et al., Freescale

3. November, 2004
Motivation
Goal: To provide additional simulation results for the source of interference in MB-OFDM modulation. Focus is on interference to in-band high data rate wireless systems, particularly TVRO satellite receivers using QPSK modulation.
Note: Multi-band UWB, including MB-OFDM, concentrates its energy in a narrower bandwidth than a comparable DS-UWB system under equal effective isotropic radiated power (EIRP). The filter captured energy is higher.
Approach: Analyze the source of interference from a time and spectrum perspective.
Additionally: Clarify initial results of Portland meeting.
Celestino A. Corral et al., Freescale

4. Multi-band UWB Power Recap
November, 2004
Multi-band UWB Power
FCC states power spectral density for UWB devices must be dBm/MHz in band between 3.1 and 10.6 GHz.
Since multi-band signals hop over a selected band of frequencies, the power spectrum is scaled by the hop and averaged over the band.
The resulting power spectral density is made equal to a system over any arbitrary band.
Multi-band spectrum
PSD
level f1 fx f2
Integrate the spectrum over band and average by band
To implement equal PSD over hop bandwidth, we need
Recap
requiring a power scaling.
Celestino A. Corral et al., Freescale

5. Multi-band UWB Power Recap November, 2004
Equate
power
Both systems have equal range and total equal power.
Actual MB-OFDM PSD over its transmission bandwidth.
Assuming DS-UWB bandwith is 2 GHz and MB-OFDM bandwidth is 528 MHz.
Celestino A. Corral et al., Freescale

6. OFDM and AWGN Subcarriers are orthogonally spaced in frequency.
November, 2004
OFDM and AWGN
Subcarriers are orthogonally spaced in frequency.
Data modulation on subcarriers randomizes amplitude and phase.
Peak-to-average approaches that of AWGN as the number of subcarriers increases, but is bound to 10 log (N).
Peak-to-Average Power Plots f1 f2 f3 f4 …
number of subcarriers
Some similarities are evident
Celestino A. Corral et al., Freescale

7. November, 2004
OFDM and AWGN
Temporal Snapshot
PDF
AWGN
Both signals have the same average power and identical PDF…
OFDM
But they’re not the same!
Celestino A. Corral et al., Freescale

8. OFDM and AWGN Energy in time equals energy in spectrum
November, 2004
OFDM and AWGN
Energy in time equals energy in spectrum
Spectral densities are inversely proportional to the bandwidth of the signal.
OFDM concentrates more of its energy over a narrower spectrum than DS-UWB, hence higher spectral density.
This is evident at the output of the matched filter with optimum sampling.
In-band filter bandwidth
0.528
Spectral densities
MB-OFDM spectrum
DS-UWB spectrum
Amplitude
3.1
5.1
f (GHz)
AWGN
OFDM
Celestino A. Corral et al., Freescale

9. OFDM and AWGN Matched Spectral Densities November, 2004 AWGN OFDM
If the power spectral densities are equal, OFDM will have less energy than DS-UWB.
Another viewpoint: At a given spectral density for OFDM, DS-UWB can transmit more energy!
Celestino A. Corral et al., Freescale

10. Ungated OFDM BER Results
November, 2004
Ungated OFDM BER Results
Higher
Spectral
Density
Results in
Error
OFDM
DS-UWB
Ungated OFDM with equal EIRP is more harmful interference than DS-UWB
DS-UWB spreads its energy over greater bandwidth, so it produces less interference
Celestino A. Corral et al., Freescale

11. OFDM Modeled as Gated AWGN
November, 2004
OFDM Modeled as Gated AWGN
In doc. 315r0 the MB-OFDM results were with two phenomena captured:
PSD growth due to equal EIRP
Additional interference due to averaging of EIRP over the hop depth.
We need to equate the PSD so that the averaging of the EIRP produces the actual PSD growth (i.e., we need to make the PSD’s of each interference the same).
3 hops
AWGN
9 dB
Celestino A. Corral et al., Freescale

12. interference not present
November, 2004
Gated AWGN Revisited
Symbol Error Rate (QPSK):
Bit Error Rate:
interference
present
Interference is Gated:
interference
silent
New Bit Error Rate:
= 0
interference
present
interference not present
Celestino A. Corral et al., Freescale

13. Consider Interference-to-Noise
November, 2004
Consider Interference-to-Noise
Probability of Bit Error:
where
Interference-to-Noise Ratio
Asymptotic Behavior (Ns = 0):
Probability of bit error as time of interference presence increases (gating approaches continuous operation)
Asymptotic Loss of Gated Noise Model Relative to Continuous Noise:
Celestino A. Corral et al., Freescale

14. November, 2004
BER versus INR for 3 Hops
Lower INR results in less interference, but not zero.
In evaluating INR we cannot assume users are cognizant of regulatory rules.
DS-UWB causes lower interference relative to MB-OFDM when latter is modeled as gated noise.
Celestino A. Corral et al., Freescale

15. Plot of Theoretical Loss for Gated Noise Source
November, 2004
Plot of Theoretical Loss for Gated Noise Source
Evaluating:
Lower INR results in less loss (back-off), but not zero.
Loss is higher for longer hops
DS-UWB is always lower interference relative to an MB-OFDM system.
Celestino A. Corral et al., Freescale

16. Filtered MB-OFDM Revisited
November, 2004
Filtered MB-OFDM Revisited
For filtered MB-OFDM, it is assumed that the filter rise time is still sufficient to capture the full interference levels.
Filtering consists of the ideal rejection of subcarriers outside the desired bandwidth.
Energy is made equal over the bandwidth of the filter by scaling the interference using 10 log(M/N) where M is the number of subcarriers captured and N is total number of subcarriers.
Variance:
Celestino A. Corral et al., Freescale

17. Filtered MB-OFDM Results
November, 2004
Filtered MB-OFDM Results
Ideal filtering implemented: 40 MHz bandwidth corresponds to 8 subcarriers passed, all others infinitely rejected.
Power scaled so that PSD of MB-OFDM and AWGN are the same.
As Eb/No increases, trend seems to be that SER improves.
Celestino A. Corral et al., Freescale

18. Cipped MB-OFDM Results
November, 2004
Cipped MB-OFDM Results
Clipping level set at 9 dB per the MB-OFDM proposal.
Clipping has no impact on BER results.
Impulsive characteristic is suppressed, but main contributor is still the bursty nature of the MB-OFDM interference.
Celestino A. Corral et al., Freescale

19. Gated Noise Interference with FEC
November, 2004
Gated Noise Interference with FEC
Convolutional code, constraint length K = 7 with hard decision, yields about 5 dB coding gain for all cases.
No interleaving performed.
FEC improves BER performance of all interference.
MB-OFDM as gated noise is still worse interferer.
Celestino A. Corral et al., Freescale

20. Impulse Radio Comparisons
November, 2004
Impulse Radio Comparisons
PRF = 22.2 MHz
Impulse radio modeled as gated AWGN process similar to MB-OFDM.
Pulse width is 2 nsec, corresponding to 500 MHz bandwidth.
EIRP averaged over the hop depth of the gated noise model for MB-OFDM.
Practical PRF range considered.
PRF = 2.22 MHz
Celestino A. Corral et al., Freescale

21. Impulse Radio Comparisons
November, 2004
Impulse Radio Comparisons
For very high PRF, impulse radio approaches AWGN.
For lower PRF, SER for impulse radio rises moderately.
Under constraint of identical 500 MHz bandwidth, impulse radio interference is lower than MB-OFDM modeled by same gated noise process.
Celestino A. Corral et al., Freescale

22. November, 2004
Conclusions
Ungated OFDM is a more harmful interferer than DS-UWB under equal EIRP constraint because the energy is concentrated over a narrower bandwidth.
Gated noise model was used to evaluate MB-OFDM interference under equal PSD constraint. Results show higher interference from gated noise than continuous noise.
Gated noise model was extended to handle interference-to-noise ratios and theoretical loss difference between systems established for lowest hop depth N = 3.
Celestino A. Corral et al., Freescale

23. November, 2004
Conclusions
Filtered MB-OFDM seems to indicate that narrower filtering improves SER performance slightly. However, results are optimistic as they account for “ideal” filtering.
Results for clipped MB-OFDM show basically no difference when compared to unclipped MB-OFDM.
All interference sources benefit from FEC, but MB-OFDM is still worse than DS-UWB.
Impulse radio interference is less than that of MB-OFDM when both are modeled as gated AWGN processes with equal 500 MHz bandwidths and over practical PRF ranges.
Celestino A. Corral et al., Freescale

24. Clarification of Results Presented in Doc 412r0 – APD Analysis
November, 2004
Clarification of Results Presented in Doc 412r0 – APD Analysis
APD is a methodology that captures only amplitude info:
Amplitude (A) in dB as ordinate,
1-CDF(A) plotted as abscissa.
Slide 3 clearly states “For full impact assessment, knowledge of the victim system’s modulation scheme and FEC performance is needed.”
In other words, APD is only a piece of the puzzle.
APD has value, but results must be considered under the basis of the method’s limitations.
Specifically, amplitude data alone is not sufficient, greater scrutiny is needed.
We provide examples of waveforms with similar APDs and different interference potential.
Celestino A. Corral et al., Freescale

25. Three Different Signals
November, 2004
Three Different Signals
AM modulated signals:
Sinusoid
Quasi-Sinusoid
Scrambled Sinusoid
Which Waveform Interferes More?
Celestino A. Corral et al., Freescale

26. APD Results APDs Are The Same! November, 2004
APD’s treat only envelope of waveforms.
Celestino A. Corral et al., Freescale

27. Different Spectra November, 2004 Sample Signals
Detail of Scrambled Sinusoid
The interference potential of signals cannot be determined by APD analysis in isolation. Victim bandwidth, center frequency, modulation, etc. play a role.
More information is needed!
APD analysis especially breaks down when considering the impact of FEC.
Celestino A. Corral et al., Freescale

28. PDF of Signals PDF in Slide 25 of Doc 412 Actual PDF November, 2004
var = 0.5
var = 2
Even with finite values, peak signal is higher!
This PDF shows Gaussian noise and OFDM have the same variance (power). But this is not the case: MB-OFDM has 6 dB more power.
PDF cannot be “averaged” as signal.
This gives the impression OFDM is more benign than AWGN, which it is not.
This PDF clearly shows approximately 6 dB greater power (4X variance) of OFDM. This is at output of matched filter at optimum sampling point.
This PDF is present at a duty cycle of 26%; but it is not “averaged.” For the other cases, variance = 0.
Celestino A. Corral et al., Freescale

29. Interference Conditions
November, 2004
Interference Conditions
Slides 27—29 confirm results for simplified case of only gated noise interference present (i.e., no noise).
Considers more “realistic” case of noise always present.
Analysis then considers Eb/(No + Io) with receiver at some fixed Eb/No; increase Io after that.
By judicious selection of No, impact of Io can be suppressed; this is not representative of interference effects, only noise effects!
Analysis presented here for slides 14—16 are representative of Eb/(No + Io) effects under high SNR, which is case for TVRO systems.
Celestino A. Corral et al., Freescale

30. Back-Up Material: OFDM Correlation
November, 2004
Back-Up Material: OFDM Correlation
OFDM is additive noise.
Compared autocorrelation of OFDM and AWGN processes.
OFDM exhibits significant autocorrelation compared to AWGN.
Celestino A. Corral et al., Freescale

31. Back-Up Material: OFDM Correlation
November, 2004
Back-Up Material: OFDM Correlation
Compared two different OFDM systems:
128 (528 MHz)
256 (1.056 GHz)
Autocorrelation improves as more subcarriers (and corresponding wider bandwidth) are employed.
Celestino A. Corral et al., Freescale

32. Correlation Effects OFDM signal is highly correlated; it is not white.
November, 2004
Correlation Effects
OFDM signal is highly correlated; it is not white.
Autocorrelation improves with more subcarriers and larger bandwidth.
OFDM is additive noise and approaches Gaussian with large number of subcarriers.
Receivers are typically designed for AWGN.
Receivers expect to operate on uncorrelated noise samples.
For OFDM interference, receiver performance will be inferior to AWGN.
Celestino A. Corral et al., Freescale