Sidelobe Suppression for OFDM-based
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Sidelobe Suppression for OFDM-based Cognitive Radios in DSA Environment Rakesh Rajbanshi (rajbansh@ittc.ku.edu), Srikanth Pagadarai (srikanth@ittc.ku.edu), Alexander Wyglinski (alexw@ittc.ku.edu). Introduction. Proposed Sidelobe Suppression Technique.

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Sidelobe Suppression for OFDM-basedCognitive Radios in DSA EnvironmentRakesh Rajbanshi (rajbansh@ittc.ku.edu), Srikanth Pagadarai (srikanth@ittc.ku.edu), Alexander Wyglinski (alexw@ittc.ku.edu)

Introduction

Proposed Sidelobe Suppression Technique

  • Orthogonal frequency division multiplexing (OFDM) is spectrally efficient and achieves high data rate wireless transmissions in dynamic spectrum access (DSA) networks employing cognitive radios.

  • It is able to transmit in a non-contiguous fashion, by turning off the subcarriers occupied by the licensed user transmissions.

  • The sidelobes of the secondary transmissions need to be kept below a specific value to ensure negligible interference to the primary users transmitting nearby.

  • A low computational complexity approach based on solving an algebraic equation for determining the amplitudes and phases of the cancellation subcarriers.

  • The proposed technique can easily be extended to higher modulation schemes that contain symbols with complex values.

  • The performance of the proposed technique is also evaluated when using less accurate estimates of the interference power levels.

Fig.1: Schematic of secondary users operating in

the presence of primary users in frequency domain

Fig.3: An illustration of inserting cancellation

carriers for sidelobe suppression

Fig. 2(a) OFDM transmitter with sidelobe

suppression unit

Fig. 2(b) OFDM receiver with sidelobe

suppression unit

Simulation Results

Conclusion

  • The proposed technique achieves a 10dB suppression in the mean OOB interference with 1 CC on each side of the OFDM signal spectrum and a 15dB suppression with 2 CCs.

  • Significant amount of reduction can be achieved even with estimates of the OOB interference power levels.

  • In a spectrum sharing scenario, a large reduction in the interference level is achieved when there are fewer subcarriers located in the spectrum whitespaces.

  • Following up this work, an adaptive algorithm is being developed that chooses the number of cancellation subcarriers to use depending on the threshold level of the OOB interference.

Fig.4: Normalized power spectrum

for BPSK-OFDM with 4 CCs

Fig.5: CCDF plot of the out-of-band radiation

For BPSK-OFDM with 4 CCs for sub-optimal cases

Fig.6: Normalized power spectrum for BPSK/OFDM with 4 CCs in a spectrum sharing scenario

with equal spacing between the unused portions of the spectrum