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A New Technique for Sidelobe Suppression in OFDM Systems

A New Technique for Sidelobe Suppression in OFDM Systems. Sinja Brandes. German Aerospace Center (DLR) Institute of Communications and Navigation Oberpfaffenhofen, Germany. COST 289, 7 th MCM, Oberpfaffenhofen, Germany 7 March, 2005. Overview.

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A New Technique for Sidelobe Suppression in OFDM Systems

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  1. A New Technique for Sidelobe Suppression in OFDM Systems Sinja Brandes German Aerospace Center (DLR) Institute of Communications and Navigation Oberpfaffenhofen, Germany COST 289, 7th MCM, Oberpfaffenhofen, Germany 7 March, 2005

  2. Overview • Problem Definition and Techniques for Sidelobe Suppression • Principle of Cancellation Carriers • Simulation Results • Comparison With Existing Methods • Application: OFDM Overlay Systems • Summary and Outlook

  3. Spectrum of an OFDM Signal significant out-of-band radiation

  4. Techniques for Sidelobe Suppression • Pulse shaping e.g. raised-cosine pulse instead of rectangular pulse  intersymbol interference (ISI)  (I)FFT can’t be applied for modulation/demodulation • Windowing of transmission signal in frequency domain  expansion of signal in time domain  intersymbol interference (ISI) • Guard bands at the borders of the OFDM spectrum, e.g. DVB-T  waste of scarce spectral ressources, DVB-T: ca. 16%

  5. New Idea: Cancellation Carriers cancellation carrier cancellation carrier • cancellation carriers are not used for data transmission • cancellation carriers carry complex weighting factors • weighting factors are determined such that the sidelobes of the transmission signal are minimized data carriers sidelobes that should be suppressed sidelobes that should be suppressed

  6. Optimization: Constrained Least Squares s:Vector of sampled signal in optimization range g: Vector of weighting factors C: Matrix with non-weighted sampled cancellation carriers in the columns Constraint: Limit power of cancellation carriers • Degrees of freedom: • Position of cancellation carriers in the spectrum • Number of cancellation carriers • Different constraints • Optimization range

  7. data carriers data carriers OFDM signal with cancellation carriers cancellation carriers cancellation carriers Spectrum With Weighted Cancellation Carriers amplitude

  8. Spectrum With and Without Cancellation Carriers Parameters: BPSK, 12 data carriers 2x1 cancellation carrier optimization range: 32 sidelobes ø - 19 dB

  9. Spectrum With and Without Cancellation Carriers Parameters: BPSK, 12 data carriers 2x2 cancellation carriers optimization range: 32 sidelobes symbol vector: 1 1 1 1 1 1 1 1 1 1 1 1 ø - 40 dB

  10. Suppression of Sidelobes • x-axis: • all possible symbol vectors • y-axis: • mean power spectral density • of sidelobes for each symbol vector • BPSK • 12 data carriers • unconstrained optimization • 1 cancellation carrier • mean power for CCs: • 19% of total power • 2 cancellation carriers • mean power for CCs: • 45% of total power - 16 dB - 34 dB

  11. Power Ratio of Cancellation Carriers to Total Power for All Possible Symbol Vectors power of cancellation carriers: 30% unconstrained mean: 0.26 max: 0.30 mean: 0.45 max: 0.92 87 % of symbol vectors use maximum amount of power for cancellation carriers

  12. Mean Sidelobe Suppression for Different Constraints Parameters: BPSK 12 data carriers 2x2 cancellation carriers optimization range: 32 sidelobes - 28 dB - 26 dB power of cancellation carriers … …unconstrained …limited to 50% of total power …limited to 40% of total power …limited to 30% of total power …limited to 20% of total power - 23 dB - 20 dB - 34 dB

  13. Bit Error Rate Performance Parameters: 2x2 cancellation carriers, 12 data carriers, BPSK no coding, AWGN+Rayleigh-fading channel, Zero Forcing+Hard Decision .

  14. Application: OFDM Overlay Systems subcarriers used by licenced systems free subcarriers used by OFDM overlay system • challenges: • co-existence of both systems • avoid interference towards licenced system task: suppress sidelobes

  15. Application: OFDM Overlay Systems subcarriers used by licenced systems free subcarriers used by OFDM overlay system Parameters: BPSK 13 (=5+8) data carriers 4x2 cancellation carriers optimization range: all displayed sidelobes joint optimization of all cancellation carriers

  16. Comparison With Existing Methods Advantages: • (I)FFT can still be applied for modulation/demodulation • no additional ISI • smaller guard bands (some guard carriers can be used for data transmission) Possible drawbacks: • slight loss in BER performance • computational complexity of least squares optimization

  17. Summary and Outlook • Principle of cancellation carriers: • cancellation carriers are not used for data transmission, but carry complex weighting factors • weighting factors are determined such as to minimize sidelobes of the transmission signal significant reduction of sidelobes promising approach for sidelobe suppression in overlay systems • Further investigations: • optimization of minimization algorithm and parameters • implementation in overlay systems • influence on PAPR

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