1. May 2003
doc.: IEEE /141r3
September 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: [16 September, 2004]
Source: [Celestino A. Corral, Shahriar Emami, Gregg Rasor] Company [Motorola]
Address [8000 W. Sunrise Blvd., Plantation, 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 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
September 2004
Multi-band OFDM Interference on In-Band QPSK Receivers Revisited
Celestino A. Corral, Shahriar Emami and Gregg Rasor
Freescale Semiconductor
8000 W. Sunrise Blvd.
Plantation, Florida
September 13, 2004
Celestino A. Corral et al., Freescale

3. September 2004
Motivation
Goal: To provide additional simulation results for the source of interference in MB-OFDM modulation. Focus is on interference to in-band broadband wireless systems, particularly TVRO satellite receivers.
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
September 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 September 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. Another Perspective September 2004 power spectral density
average power
equal EIRP
due to MB-OFDM (subscript M)
due to DS-UWB (subscript U)
Celestino A. Corral et al., Freescale

7. OFDM and AWGN Subcarriers are orthogonally spaced in frequency.
September 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

8. September 2004
OFDM and AWGN
Temporal Snapshot
PDF
AWGN
Both signals are at same energy levels and have the same PDF…
OFDM
But they’re not the same!
Celestino A. Corral et al., Freescale

9. OFDM and AWGN Energy in time equals energy in spectrum
September 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

10. OFDM and AWGN Matched Spectral Densities September 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

11. Ungated OFDM BER Results
September 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

12. MB-OFDM is Gated and Scaled OFDM
September 2004
MB-OFDM is Gated and Scaled OFDM
Power is determined by scaling the power and averaging over the hop depth, making it equal to DS-UWB.
Simulation assumes broadband filter response is fast and captures full energy.
Front-end filtering is “removed” to simplify analysis.
9 dB
Celestino A. Corral et al., Freescale

13. September 2004
Clipped MB-OFDM
MB-OFDM waveform clipped at 9 dB peak-to-average power ratio.
Clipping the peaks results in negligible impact on energy of the signal.
Front-end filtering is “removed” to simplify analysis.
9 dB
Celestino A. Corral et al., Freescale

14. interference not present
September 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
Implicit: Interference-to-noise ratio is 0 dB
Celestino A. Corral et al., Freescale

15. Consider Interference-to-Noise
September 2004
Consider Interference-to-Noise
Probability of Bit Error:
where
Interference-to-Noise Ratio
Asymptotic Behavior:
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

16. September 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 is lower interference relative to MB-OFDM when latter is modeled as gated noise (best case).
Celestino A. Corral et al., Freescale

17. Plot of Theoretical Loss for Gated Noise Source
September 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
1 to 5 dB for 3 hops
2 to 8 dB for 7 hops
3 to 11 dB for 13 hops
DS-UWB is always lower interference relative to an MB-OFDM system.
Celestino A. Corral et al., Freescale

18. Filtered MB-OFDM Revisited
September 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

19. September 2004
Filtered MB-OFDM
Filtering performed by generating signal with M subcarriers with total bandwidth equal to ideal filter bandwidth.
Difference between filtered and unfiltered case < 1 dB.
Difference in levels may be due to not capturing energy from adjacent subcarriers.
8 dB
Filter bandwidth is 40 MHz, corresponding to 9 subcarriers
Celestino A. Corral et al., Freescale

20. September 2004
Filtered MB-OFDM
Gaussian noise through a filter is band-limited noise, resulting in more correlation.
Filtered MB-OFDM can result in discrete tones, which is non-Gaussian.
Slightly lower SER, about 0.5 dB difference from 9 subcarrier case.
7 dB
Filter bandwidth is 20 MHz, corresponding to 5 subcarriers
Celestino A. Corral et al., Freescale

21. Gated Noise Interference with FEC
September 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 SER performance of all interference.
Celestino A. Corral et al., Freescale

22. September 2004
Conclusions
Multi-band UWB techniques with equal power spectral density do not have the same energy as DS-UWB which spreads its energy over greater bandwidth.
Ungated OFDM is a more harmful interferer than DS-UWB under equal EIRP constraint because the energy is concentrated over a narrower bandwidth.
Clipping results in negligible impact on interference energy, although it reduces risk of impulsive interference.
Gated noise model was extended to handle interference-to-noise ratios and theoretical loss difference between systems established.
Celestino A. Corral et al., Freescale

23. September 2004
Conclusions
Filtered MB-OFDM model shows narrowband filters reduce captured energy but interference is still higher for this type of interference.
All interference sources benefit from FEC. For gated noise case, the level of coding gain is slightly lower than that for ungated noise.
Celestino A. Corral et al., Freescale

24. Back-Up Material: OFDM Correlation
September 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

25. Back-Up Material: OFDM Correlation
September 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

26. Correlation Effects OFDM signal is highly correlated; it is not white.
September 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