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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: [ 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 ]

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

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  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] Voice:[+1 408 474 7243], FAX: [+1 408 474 5343], E-Mail:[charles.razzell@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. C. Razzell, Philips

  2. Part 1 APD Plots and their Implications for MB-OFDM C. Razzell, Philips

  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 system’s modulation scheme and FEC performance is needed C. Razzell, Philips

  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(A>10dB) = exp(-10) = 4.54x10-5 ;P(A>-30dB) = exp(-0.001) = 0.999 C. Razzell, Philips

  5. 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. C. Razzell, Philips

  6. 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 C. Razzell, Philips

  7. 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 C. Razzell, Philips

  8. C. Razzell, Philips

  9. Analytically Derived APD Plot for MB-OFDM % APD plots d = 3*165/128; % duty cycle x=linspace(-20,15); rsq=10.^(x/10); apd3=-rsq/d - log(d); apd=-rsq; semilogx(apd3,x,apd,x) xlabel('ln(P(A>ordinate))') ylabel('Amplitude [dB]') legend('MB-OFDM','cont. OFDM') axis([-10 -0.01 -20 15]) grid C. Razzell, Philips

  10. 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% C. Razzell, Philips

  11. 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. C. Razzell, Philips

  12. 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.) C. Razzell, Philips

  13. Peak Received Powers As a Function of Receiver Bandwidth The impulse radio’s 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 C. Razzell, Philips

  14. 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(A>ord.) line than the 3-band OFDM system. All these impulse radios would be permitted under current part 15f legislation. 1.8% C. Razzell, Philips

  15. 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 C. Razzell, Philips

  16. APD plots of 1/3 duty cycle OFDM combined with thermal receiver noise C. Razzell, Philips

  17. 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 <10MHz victim receiver bandwidths, whereas for >10MHz 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 C. Razzell, Philips

  18. Part 2 MB-OFDM Interference Impact to In-band QPSK transmissions C. Razzell, Philips

  19. 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 C. Razzell, Philips

  20. 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 C. Razzell, Philips

  21. 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 C. Razzell, Philips

  22. 33MHz QPSK System with AWGN C. Razzell, Philips

  23. 33MHz QPSK System with Continuous OFDM C. Razzell, Philips

  24. Continuous OFDM signal causes fewer errors than WGN for same S/(I+N) • 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. C. Razzell, Philips

  25. Monte Carlo Simulated PDFs of received symbols conditioned on txbits=‘1,1,1,…’ Eb/Io=7dB 500,000 transmitted bits ERROR REGION Probability Density P(error) = area under the curve Real(rxsymbol) [V] C. Razzell, Philips

  26. 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 C. Razzell, Philips

  27. Prediction for ¼ duty cycle noise bursts • Combined impact of 3-band hopping, zero prefix and guard interval is:165*3/128 = 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 C. Razzell, Philips

  28. Simulation with ¼ duty cycle noise bursts as interferer Expected reference for ¼ duty noise bursts Previous Reference for uncoded QPSK C. Razzell, Philips

  29. Simulation with ¼ duty cycle OFDM as interferer Expected reference for ¼ duty noise bursts Previous Reference for uncoded QPSK C. Razzell, Philips

  30. 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  • Let’s 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/(No+Io) by introducing ¼ duty cycle MB-OFDM, starting with Io=0 Watts and increasing C. Razzell, Philips

  31. Simulation with ¼ duty cycle OFDM + Continuous AWGN <2 dB C. Razzell, Philips

  32. QPSK BER Conclusions • A continuous OFDM interferer has a more benign error inducing property than AWGN when each is applied at the same S/(I+N) • 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 C. Razzell, Philips

  33. 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. C. Razzell, Philips

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