Doc.: IEEE 802.15-04/315r0 Submission July 2004 Celestino A. Corral et al., MotorolaSlide 1...

July 2004

Celestino A. Corral et al., MotorolaSlide 1

doc.: IEEE 802.15-04/315r0

Submission

Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs)Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs)

Submission Title: [Multi-Band OFDM Interference on In-Band QPSK Receivers]Date Submitted: [13 July, 2004]Source: [Celestino A. Corral, Shahriar Emami, Gregg Rasor] Company [Motorola]

Address [8000 W. Sunrise Blvd., Plantation, Florida, USA 33322]Voice:[954-723-3864], FAX: [954-723-3883]

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. A MB-OFDM interference model is derived based on simulation and analytical results.]

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.

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doc.: IEEE 802.15-04/315r0

Submission

Multi-band OFDM Interference on In-Band QPSK Receivers

Celestino A. Corral, Shahriar Emami and Gregg Rasor

Motorola8000 W. Sunrise Blvd.

Plantation, Florida

July 13, 2004

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Motivation Goal: To characterize the impact of Multi-band OFDM UWB

interference on in-band broadband wireless system like C-band satellite receivers.

Note: Multi-band OFDM (MB-OFDM) and Multi-band UWB (MB-UWB) requires power scaling of the waveform to compare competing technologies based on interpretation of FCC rules.

Model of MB-OFDM interference derived. This model is bounded by periodically gated AWGN and impulsive MB-OFDM interference.

Reconcile observed test results of MB-OFDM interference on satellite receivers as presented in ABQ meeting.

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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.

PSDlevel

Multi-band spectrum

Integrate the spectrum over band and average by band

To implement equal PSD over hop bandwidth, we need

requiring a power scaling.

f1 f2fx

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Multi-band UWB Power

Equatepower

Both systems transmit with equal power at a given range.

Assuming DS-UWB bandwith is 2 GHz and MB-OFDM bandwidth is 528 MHz.

Actual MB-OFDM PSD over its transmission bandwidth.

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In-band Receiver Filters

fc > 3 GHz

BW < 40 MHz

Q > 50 typical

Band-passFilterFrequencyResponse

High-Q band-pass filter can be approximated by [1]:

complex frequency of band-pass filter

complex frequency of low-pass prototype filter

Step response of band-pass filter has low-pass impulse response envelope:

[1] A. Papoulis, The Fourier Integral and its Application, Chap. 7, New York: McGraw-Hill, 1962.[2] G. E. Valley, Jr., and H. Wallman, Vacuum Tube Amplifiers, New York: McGraw-Hill, 1948.

Temporal characteristics of high-Q band-pass filter determined by low-pass prototype. This includes rise time, which obeys the following relation [2]:

Rise time of band-pass filter determined by 3dB bandwidth of low-pass prototype.

Not a function of filter approximationor order

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In-band Receiver FiltersBand-pass filter rise time for 40 MHz bandwidth.

Received power:

Filter responds quite fast and observes virtually full power of filtered MB-OFDM signal.

Portion due to filter bandwidth Portion due to temporal response

Filter with slower response.

in dB

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Submission

Coherent Detection QPSK Simulation

Detector

WindowMatched

Filter

Block Diagram of Simulator

• Assume perfect synchronization

• Assume perfect phase estimation

• Input filter bandwidth wide enough so rise time not a factor

• Interference bandwidth is very large relative to filter bandwidth and approaches thermal noise as in [3].

[3] J. Brandao, “Interference effect on the performance of PSK and QAM systems,” IEE Proceedings I, vol. 138, pp. 331—337, Aug. 1991.

• QPSK system at 27.05 Msym/sec, similar to Dubai EDTV at 4020 MHz.

• 0 < Eb/No < 30 dB.

• 1000 symbols, 500 packets per Eb/No set.

• Sample rate: 120 samples/QPSK symbol.

• Multi-band OFDM and all gated noise is 896 samples long.

NoiseSource

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Simulation Results: Gated Noise

3 dB

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Simplified Theoretical ReasonProbability of symbol error for QPSK [4]

Q-function for communication

Gated noise duty cycle: Np is time interference is present, Ns is time interference is silent.

Probability of error is due only to when the noise is present Pep; for the case it is silent Pes = 0:

Q is very sensitive to under high signal-to-noise (SNR), meaning small changes in duty cycle will impact probability of error when minor changes in bit energy is most significant.

equivalent “quasi-fading” of bit energy relative to fixed noise power No

Actual error must be scaled by duty cycle as this is time interference is present

[4] J. G. Proakis, Digital Communications, 4th Ed., Boston, MA: McGraw-Hill, 2001.

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Theoretical vs. Simulated Results: Gated Noise

Simulated

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Simulation Results: MB-OFDM

9 dB

3 hops

AWGN

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Simulation Results: Impulsive MB-OFDM

Theory

Worst-case peak-to-average power assumed for each MB-OFDM symbol

?? dB

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Simulation Results: 3 hops

Gated AWGN lower bound

Impulsive MB-OFDM upper bound

MB-OFDM

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MB-OFDM Interference Model

Amplitude distribution of AWGN Amplitude distribution of MB-OFDM Amplitude distribution of Impulsive MB-OFDM

Multi-band OFDM transmissions can be long or bursty:

• Long transmissions have amplitude distribution that approaches AWGN• Bursty transmissions can be potentially impulsive• We need to combine the Gaussian and impulsive characteristics

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MB-OFDM Interference ModelClass A Model [5]: Interference has “gaps” in time; i.e., non-zero probability of

time during which there is no interference in the receiver.

Interference time Receiver bandwidth

Model Incorporates Gaussian and Impulsive Factors:

impulse index

Peak factor (PAP)Carrier-to-noise ratio

for M-ary QAM

std. dev.

mean power ratio

Average symbol error rate:

[5] D. Middleton, “Non-Gaussian noise models in signal processing for telecommunications: New methods and results for class A and class B noise models,” IEEE Trans. Inform. Theory, vol. 45, pp. 1129—1149, May 1999.

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MB-OFDM Model

Simulated Model

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Filtered MB-OFDM40 MHz

9 subcarriers

The filtered waveform is generated and then scaled to obtain same power as AWGN over the packet.

The waveform is then scaled by a factor of 9/128 (in number of subcarriers) to reduce the level to a filtered amount. This is almost the same amount as 40/528 (in MHz), which corresponds to the desired power reduction relative to the full bandwidth of the 128-subcarrier symbol:

Received power:

Assumed 0 dB9/128 factor(40/528 factor)

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Simulation Results: Filtered MB-OFDM

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Conclusions Multi-band OFDM and Multi-band UWB equate power spectral

density by scaling power in the hop and averaging over the entire hop bandwidth. This equates the transmitted power of a Multi-band system with DS-UWB over a fixed bandwidth.

Probability of symbol error shows gated noise is akin to quasi-fading of bit energy relative to fixed AWGN level.

The gated and scaled interference is more harmful than AWGN depending on the hop depth. Gated noise interference produces performance 3 dB from theory; MB-OFDM produces performance 8 dB from theory.

Multi-band OFDM can be impulsive. Under worst-case peak-to-average power Multi-band OFDM is a significant interferer to in-band coherent detection QPSK receivers.

MB-OFDM model was derived based on combination of Gaussian and impulsive characteristics of MB-OFDM.

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Narrowband Filter Response

Wideband Filter Response

Only upper portion of response captured

Narrowband Filter Response

• Fast rise time

• Delay applies across entire response

• Full level of interference reached within response time of the filter, and present for most of the interference time.

• Total power captured

• Slow rise time

• Delay applies across entire response

• Full level of interference not reached within response time of the filter.

• Total power can be captured if rise time and interference time are about equal.

Narrowband filters “favor” narrow pulsed interference; full level of interference is not captured.

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Backup: Gated Noise Results for Other Hops

Increasing hop depth results in more degradation at high SNR.

3 713

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Backup: MB-OFDM Results for Other Hops


3 7

13

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Backup: Impulsive MB-OFDM Results for Other Hops


37

13

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Backup: Gated Noise Theoretical Results for Other Hops

3 713

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Backup: MB-OFDM Class A Model Results for Other Hops

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Properties of Q and Quasi-Fading Working with Q(x) directly is difficult. We use approximation

[4] P. L. Borjesson and C-E. W. Sundberg, “Simple approximations of the error function Q(x) for communication applications,” IEEE Trans. Commun., vol. COM-27, pp. 639—643, March 1979.

where [4]:

as then so decays more rapidly

Decay factor:

Value

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Peak-to-Average Power “Tracking”

Peak-to-average of AWGN and MB-OFDM “track” over different hop depths.

Doc.: IEEE 802.15-04/315r0 Submission July 2004 Celestino A. Corral et al., MotorolaSlide 1...

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