Sidelobe Suppression for OFDM-basedCognitive Radios in DSA EnvironmentRakesh Rajbanshi ([email protected]), Srikanth Pagadarai ([email protected]), Alexander Wyglinski ([email protected])
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
Fig. 2(b) OFDM receiver with sidelobe
- 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