Simulation Results

1. 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