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 16
September, 2004 Source Celestino A. Corral,
Shahriar Emami, Gregg Rasor Company
Motorola Address 8000 W. Sunrise Blvd.,
Plantation, Florida, USA 33322 Voice954-723-38
64, FAX [phone] 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 Fo
r 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
• 8000 W. Sunrise Blvd.
• Plantation, Florida
• September 13, 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
  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.

4
Multi-band UWB Power

• FCC states power spectral density for UWB devices
  must be -41.2 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
Another Perspective
power spectral density
average power
equal EIRP
due to MB-OFDM (subscript M)
due to DS-UWB (subscript U)
7
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
8
OFDM and AWGN
Temporal Snapshot
PDF
AWGN
Both signals are at same energy levels and have
the same PDF
OFDM
But theyre not the same!
9
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
10
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!
11
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
12
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
13
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
14
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
15
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
16
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).

17
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
• 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.

18
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
19
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
  lt 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
20
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
21
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.

22
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.

23
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.

24
Back-Up Material OFDM Correlation

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

25
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.
  

26
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.