Title: MBOFDM Interference Impact to Inband QPSK transmissions
1Project 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 1 408 474
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.
2APD Plots and their Implications for MB-OFDM
Part 1
3Amplitude 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
4Example 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
5APD 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.
6APD 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
7Analytic 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
8(No Transcript)
9Analytically 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
10Simulated 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
11Simulated 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.
12Simulated 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.)
13Peak 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
14Simulated 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
15Single 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
16APD plots of 1/3 duty cycle OFDM combined with
thermal receiver noise
17APD 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
18MB-OFDM Interference Impact to In-band QPSK
transmissions
Part 2
19Background
- 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
20QPSK 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
21Interference 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
2233MHz QPSK System with AWGN
2333MHz QPSK System with Continuous OFDM
24Continuous 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.
25Monte 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
26Output 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
27Prediction 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
28Simulation with ¼ duty cycle noise bursts as
interferer
Expected reference for ¼ duty noise bursts
Previous Reference for uncoded QPSK
29Simulation with ¼ duty cycle OFDM as interferer
Expected reference for ¼ duty noise bursts
Previous Reference for uncoded QPSK
30How 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
31Simulation with ¼ duty cycle OFDM Continuous
AWGN
lt2 dB
32QPSK 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
33Overall 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.