Multi-Band OFDM Interference on In-Band QPSK Receivers Revisited

1
Project IEEE P802.15 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 33426 Voice561-739-3280,
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 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
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

3
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.

4
Multi-band UWB Power

• FCC states power spectral density for UWB devices
  must be -41.3 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
f2
fx
Integrate the spectrum over band and average by
band
To implement equal PSD over hop bandwidth, we need
requiring a power scaling.
5
Multi-band UWB Power
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.
6
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
7
OFDM and AWGN
Temporal Snapshot
PDF
AWGN
Both signals have the same average power and
identical PDF
OFDM
But theyre not the same!
8
OFDM and AWGN
In-band filter bandwidth

• 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.

0.528
Spectral densities
MB-OFDM spectrum
DS-UWB spectrum
Amplitude
f (GHz)
3.1
5.1
AWGN
OFDM
9
OFDM and AWGN
Matched Spectral Densities
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!
10
Ungated OFDM BER Results
Higher Spectral Density Results in Higher 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
11
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 PSDs of each interference
  the same).

3 hops
9 dB
AWGN
12
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
13
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
14
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.

15
Plot of Theoretical Loss forGated 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.

16
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
17
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.

18
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.

19
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.

20
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
21
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.

22
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.

23
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.

24
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 systems
  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 methods limitations.
• Specifically, amplitude data alone is not
  sufficient, greater scrutiny is needed.
• We provide examples of waveforms with similar
  APDs and different interference potential.

25
Three Different Signals

• AM modulated signals
• Sinusoid
• Quasi-Sinusoid
• Scrambled Sinusoid

Which Waveform Interferes More?
26
APD Results
APDs Are The Same!
APDs treat only envelope of waveforms.
27
Different Spectra
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.
28
PDF of Signals
PDF in Slide 25 of Doc 412
Actual PDF
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.
29
Interference Conditions

• Slides 2729 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 1416 are
  representative of Eb/(No Io) effects under high
  SNR, which is case for TVRO systems.

30
Back-Up Material OFDM Correlation

• OFDM is additive noise.
• Compared autocorrelation of OFDM and AWGN
  processes.
• OFDM exhibits significant autocorrelation
  compared to AWGN.

31
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.
  

32
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.