ADJACENT CHANNEL INTERFERENCE REDUCTION IN OFDM SYSTEMS

摘要

Orthogonal frequency division multiplexing (OFDM) is a promising candidate for cognitive radio transmission. OFDM supports high data rates that are robust to channel impairments. However, one of the biggest problems for OFDM transmission is high out off band radiation, which is resulted from the sidelobes of the OFDM subcarriers. These sidelobes are a source of interference to neighbouring transmissions. This paper focuses on reducing the out-of-band radiation by reading and extracting the radiation power in the sidelobes. This is done by extending the time domain OFDM signal by zeros in both sides. The resulted signal is then transformed to the time domain and extended samples are removed to obtain the N-samples of time domain signal represent the out-of-band radiated signal. The resulting signal is Fourier transformed and high frequency sub-carriers are removed to obtain pilots that are inverted and added to the original OFDM data sub-carriers resulting in reducing the Adjacent Channel Interference (ACI), which affects the adjacent systems. The added signal represents noise signal to the desired OFDM signal that reduces the BER performance of the desired system, thus a weighting factor is applied to the added signal to get a better BER performance with good out-of-band radiation reduction. Matlab/Simulink simulation is adopted to perform assessment of the proposed technique with different weighting factors and different frequency separation between the desired signal and the adjacent one. For 0 dB attenuation on the added signal, a 10 dB reduction in out-of-band radiation is obtained while 6 dB reduction is obtained when the weighing factor reduces the input signal power by 3 dB. BER performance is better by performing the reduction technique and depends on the frequency distance between the adjacent signal and the desired one.