Adaptive beamforming for OFDM systems

Abstract

Adaptive antennas provide an efficient means for minimizing channel interferences by directing the antenna beam towards the desired signal/transmitter and placing a null towards the interfering signal. Orthogonal Frequency Division Multiplexing (OFDM) is gaining popularity for high data rate communication systems. OFDM is different from the conventional multicarrier techniques as it sends information on a number of overlapping orthogonal subcarriers by dividing the total signal bandwidth. To modulate the signal on these subcarriers Inverse Fast Fourier Transform (IFFT) is used on the transmitter. OFDM is less susceptible to Inter Symbol Interference introduced in the multipath environment because its symbol duration is long compared with the data symbols in the serial data stream. In order to completely eliminate ISI a cyclic guard time longer than the channel delay is prepended to every OFDM symbol maintaining the orthogonality among subcarriers. However, the symbols still experience interference from their replicas originating from multipaths. This is termed as self-interference. Under the assumptions of narrowband model a phase shift is introduced in each multipath component. One solution for removal of this phase shift is equalization, requiring channel estimation which is not possible when the power of the interfering signal is higher than the desired signal. The approach we suggest bypasses equalization and employs an antenna array with an adaptive beamforming algorithm for interference rejection in OFDM systems. The Fast Fourier Transform (FFT) is employed at the receiver end of an OFDM system to demodulate the baseband symbol, which enables the use of frequency domain beamforming. In frequency domain beamforming, a separate beamformer for each subcarrier, having its own set of weight vector can be implemented. In case of frequency selective channel the distortion introduced in each subcarrier of an OFDM symbol is different. Similarly narrow band interference distorts only a portion of signal bandwidth. Thus, a separate beamformer for each subcarrier allows the suppression of these distortions. The weight vectors of the beamformer are iteratively updated using adaptive algorithm. Both the decision directed and blind algorithms are implemented in this project.