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] 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.] 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. Celestino A. Corral et al., Freescale

2. Multi-band OFDM Interference on In-Band QPSKReceivers Revisited Celestino A. Corral, Shahriar Emami and Gregg Rasor Freescale Semiconductor 8000 W. Sunrise Blvd. Plantation, Florida September 13, 2004 Celestino A. Corral et al., Freescale

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. Celestino A. Corral et al., Freescale

4. Recap 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. Celestino A. Corral et al., Freescale

5. Recap 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. Celestino A. Corral et al., Freescale

6. Another Perspective power spectral density average power equal EIRP due to MB-OFDM (subscript M) due to DS-UWB (subscript U) Celestino A. Corral et al., Freescale

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 Celestino A. Corral et al., Freescale

8. OFDM and AWGN Temporal Snapshot PDF AWGN Both signals are at same energy levels and have the same PDF… OFDM But they’re not the same! Celestino A. Corral et al., Freescale

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 Celestino A. Corral et al., Freescale

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! Celestino A. Corral et al., Freescale

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 Celestino A. Corral et al., Freescale

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 Celestino A. Corral et al., Freescale

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 Celestino A. Corral et al., Freescale

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 Celestino A. Corral et al., Freescale

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: Celestino A. Corral et al., Freescale

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). Celestino A. Corral et al., Freescale

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. Celestino A. Corral et al., Freescale

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: Celestino A. Corral et al., Freescale

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 < 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 Celestino A. Corral et al., Freescale

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 Celestino A. Corral et al., Freescale

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. Celestino A. Corral et al., Freescale

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. Celestino A. Corral et al., Freescale

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. Celestino A. Corral et al., Freescale

24. Back-Up Material: OFDM Correlation • OFDM is additive noise. • Compared autocorrelation of OFDM and AWGN processes. • OFDM exhibits significant autocorrelation compared to AWGN. Celestino A. Corral et al., Freescale

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. Celestino A. Corral et al., Freescale

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. Celestino A. Corral et al., Freescale