MBOFDM Interference Impact to Inband QPSK transmissions

1
Project IEEE P802.15 Working Group for Wireless
Personal Area Networks (WPANs) Submission Title
In-band Interference Properties of
MB-OFDM Date Submitted 9 Sept, 2004 Source
Charles Razzell Company Philips Address
1109, McKay Drive, San Jose, CA 95131,
USA Voice1 408 474 7243, FAX [phone]
5343, E-Mailcharles.razzell_at_philips.com Re
Extension of previous APD analysis in
802.15-04/326r0 and address points raised in
315r0 Abstract Presents in-band
interference properties of MB-OFDM as revealed by
statistical properties (APDs) and by impact to
BER curves for a QPSK transmission
system Purpose To correct potential
misapprehensions concerning the interference
impact of MB-OFDM. 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.
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APD Plots and their Implications for MB-OFDM
Part 1
3
Amplitude Probability Distributions

• APD methodology is favored by the NTIA in
  assessing interference impact of UWB waveforms
• For non-Gaussian interference, APD plots provide
  helpful insight into potential impact on victim
  receivers.
• For full impact assessment, knowledge of the
  victim systems modulation scheme and FEC
  performance is needed

4
Example APD plot (for Rayleigh Distribution)
Amplitude (A) in dB is plotted as the
Ordinate 1-CDF(A) is plotted as the
Abscissa Plotting the natural log of the
probabilities on a log scale provides scaling
similar to Rayleigh graph paper.
P(Agt10dB) exp(-10) 4.54x10-5
P(Agt-30dB) exp(-0.001) 0.999
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APD plots for continuous OFDM signals as number
of QPSK sub-carriers is varied
As the number of sub-carriers used increases, the
approximation to the Rayleigh APD plot improves.
This can be expected due to the Central Limit
Theorem.
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APD plots for continuous OFDM with 128
sub-carriers as receiver bandwidth is varied
Using receiver filters of increasing bandwidths
yields a similar result approximation to
Rayleigh APD is good for b/w?20MHz
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Analytic Expression for APD of OFDM waveforms
We have seen that for measurement bandwidths of
?20MHz, the APD of OFDM closely approximates that
of a Rayleigh distribution. This can be expected
because the in-phase and quadrature components
will both tend towards a Gaussian distribution
due to the central limit theorem. Assuming this
approximation to be perfect, we can write a
closed form expression for the APD of OFDM
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Analytically Derived APD Plot for MB-OFDM
APD plots d 3165/128 duty
cycle xlinspace(-20,15) rsq10.(x/10) apd3-rs
q/d - log(d) apd-rsq semilogx(apd3,x,apd,x) xla
bel('ln(P(Agtordinate))') ylabel('Amplitude
dB') legend('MB-OFDM','cont. OFDM') axis(-10
-0.01 -20 15) grid
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Simulated APD plots for continuous and 3-band
OFDM, using 128 sub-carriers
Signal/interferer is normalized to unit power
0dBW. Probability of noise amplitude exceeding
signal amplitude is given by abscissa value at
the intersection of a horizontal SIR line with
the APD curve.
1.8
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Simulated APD for MB-OFDM as a function of victim
Rx bandwidth
Victim Rx bandwidth has a significant impact on
the APD plots generally speaking, lower receiver
bandwidths experience a more benign version of
the APD.
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Simulated APD for 1MHz PRF Impulse as a function
of victim Rx bandwidth
APD plots for this 1MHz PRF impulse show
significantly higher peaks for large receiver
bandwidths 20,50MHz. At lower received
bandwidths, APD plots are strikingly similar to
those for MB-OFDM (Flipping between this and the
previous slide may help illustrate this point.)
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Peak Received Powers As a Function of Receiver
Bandwidth
The impulse radios peak power consistently
scales with 20log(BW). The continuous OFDM signal
(ofdm1) has a peak power that scales with
10log(BW) The 3-band OFDM signal looks like a
hybrid signal. For lower Rx bandwidths its peak
power tracks with the 1MHz impulse radio, but at
10MHz and above the slope reverts to that of pure
OFDM.
MB-OFDM advantage
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Simulated APD Curves for OFDM and Impulse Radios
in 50MHz bandwidth
10MHz PRF impulse radio has nearly identical APD
to 1/3 duty cycle OFDM in region of
interest. 3MHz and 1MHz PRF radios have
significantly higher SIR ratios corresponding to
the 1.8 P(Agtord.) line than the 3-band OFDM
system. All these impulse radios would be
permitted under current part 15f legislation.
1.8
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Single dominant source of interference may not
reflect real scenarios

• All the above APD analysis has assumed that the
  dominant source of interference is a single
  instance of the considered waveform
• For this to be true
• A single interferer must be very close to the
  victim receiver such that it can overwhelm
• The thermal noise of the receiver
• The additive combination of other uncoordinated
  UWB and other interferers
• Examples of aggregate (Noise Interference) APDs
  follow

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APD plots of 1/3 duty cycle OFDM combined with
thermal receiver noise
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APD Conclusions

• Using the NTIA APD methodology for the worst-case
  scenario of a single dominant interferer shows
• That the required SIRs for low PRF impulse radios
  are greater than those needed for the 3-band OFDM
  waveform for cases where the victim receiver band
  exceeds the impulse PRF by a factor of 5 (or
  more).
• The APD plots for lower bandwidth victim
  receivers show that peaks of the MB-OFDM signal
  are significantly attenuated by the Rx filter,
  bringing them closer to the ideal Rayeligh APD.
• That peak interference powers due to MB-OFDM are
  similar to those caused by a 1MHz PRF impulse
  radio for lt10MHz victim receiver bandwidths,
  whereas for gt10MHz receiver bandwidths,
  significantly lower peak powers are obtained for
  MB-OFDM.
• Receiver thermal noise and other external
  interference sources will have a mitigating
  effect on the APD of an interfering MB-OFDM signal

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MB-OFDM Interference Impact to In-band QPSK
transmissions
Part 2
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Background

• Document 802.15-04/315r0 showed large(? 9dB)
  increases in required S/I ratios required when
  MB-OFDM was the sole source of unwanted
  interference
• These results seemed intuitively unreasonable and
  therefore merited further investigation
• Uncoded QPSK transmissions of circa 33MHz
  bandwidth (66Mbps) were used as basis for
  comparison

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QPSK Transmission System
BIT GENERATOR
MULTIPLEXER
SYMBOL MAPPER
16 x UPSAMPLE BY ZERO INSERTION
RRC Filter with 33MHz 3dB bandwidth
OFDM INTERFERENCE GENERATOR (OR AWGN)

ERROR COUNTER
DE- MULTIPLEXER
HARD DECISIONS
DECIMATION
RRC Filter with 33MHz 3dB bandwidth
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Interference Scenario
Each OFDM sub-carrier is modulated with random
QPSK symbols
33 MHz (8 sub-carriers)
QPSK System operates within this bandwidth. The
bandwidth is defined by a RRC filter with ?0.5
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33MHz QPSK System with AWGN
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33MHz QPSK System with Continuous OFDM
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Continuous OFDM signal causes fewer errors than
WGN for same S/(IN)

• This claim may seem counter-intuitive at first
• Consider that at high SNRs, errors are caused by
  the tails of the Gaussian distribution (see
  Error Region, next slide)
• But with only 8 relevant sub-carriers the OFDM
  waveform is limited to 256 states in each of I
  and Q dimensions
• Tails of the distribution poorly approximate
  Gaussian noise.

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Monte Carlo Simulated PDFs of received symbols
conditioned on txbits1,1,1,
ERROR REGION
Eb/Io7dB 500,000 transmitted bits
Probability Density
Real(rxsymbol) V
P(error) area under the curve
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Output states of 8-point IFFT with all 65536
possible QPSK symbol sets
Amplitude is Bounded over all possible QPSK
symbol permutations
Filter memory will add more states, but tails of
distribution will remain limited in amplitude
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Prediction for ¼ duty cycle noise bursts

• Combined impact of 3-band hopping, zero prefix
  and guard interval is1653/128 3.8672
• We will approximate the duty cycle ratio d 4
• During, zero noise power periods zero bit errors
  should occur
• Average BER is reduced by a factor of d
• During active noise bursts, noise power is d
  times higher than the long term average
• Corresponding SNR reduced by a factor of d

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Simulation with ¼ duty cycle noise bursts as
interferer
Expected reference for ¼ duty noise bursts
Previous Reference for uncoded QPSK
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Simulation with ¼ duty cycle OFDM as interferer
Expected reference for ¼ duty noise bursts
Previous Reference for uncoded QPSK
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How meaningful is ¼ duty-cycle noise/interference?

• The above plots assume that for ¾ of the time,
  the system noise temperature is 0 Kelvin.
• We want to be more realistic than that ?
• Lets assume the QPSK victim has a constant Eb/No
  of 10dB (the uncoded BER is expected to be
  erfc(100.5)/2 ? 3.87 x 10-6).
• Vary Eb/(NoIo) by introducing ¼ duty cycle
  MB-OFDM, starting with Io0 Watts and increasing

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Simulation with ¼ duty cycle OFDM Continuous
AWGN
lt2 dB
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QPSK BER Conclusions

• A continuous OFDM interferer has a more benign
  error inducing property than AWGN when each is
  applied at the same S/(IN)
• Under conditions of zero thermal noise, where the
  interferer has a fixed duty cycle, d, the average
  BER is closely bounded by
• Realistic conditions call for a non-zero value
  for background thermal noise
• In a reasonable test case, deviation of the BER
  curve from the AWGN case was limited to 2dB

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Overall Conclusions

• Impulse radios showed a more harmful APD plot
  than 3-band MB-OFDM for all cases where (Rx
  Bandwidth)/PRF ? 5.
• Low bandwidth (?5MHz) cases have also been
  simulated, revealing close resemblance of the
  APDs to impulse radios of the same PRF, and much
  lower peak-to-mean ratios compared to the
  wideband case.
• Testing the impact of MB-OFDM on a QPSK
  transmission system showed that the required SNR
  increase is always less than 10log(d), but in
  realistic scenarios, with continuous AWGN also
  present, the impact was reduced to below 2dB.