Implementation of pre-FFT beamforming in MIMO-OFDM

Abstract

In this study, real-time software defined radio implementation of the point to point multiple input multiple output (MIMO) orthogonal frequency division multiplexing (OFDM) system using pre-fast Fourier transform (pre-FFT) beamforming technique in both the transmitter and the receiver is done for the first time in the literature. By taking benefit of the MIMO system's spatial multiplexing property, the performance of the implemented pre-FFT beamforming approach is demonstrated in terms of bit error rate and error vector magnitude measurements. As a result of the test results, the effect of the beamforming methods on the performance of communication systems are clearly observed.

Adaptive Beamforming Applied to OFDM Systems

“…The works presented in [ 19 , 22 , 32 , 33 , 34 , 35 ] had results with a guard interval great than channel length , and this is mandatory for solutions where the array beamforming is updated in each OFDM symbol in the time domain. In this work, the array beamforming proposed was updated in each m -th sample in the frequency domain, and this allows one to work with a guard interval smaller than channel length .…”

“…The LMS algorithm is executed in the OFDM sample time, (see Equation ( 1 )), and this enables one to adjust the antenna array gains, , faster than the works proposed in [ 19 , 22 , 32 , 33 , 34 , 35 ]. The LMS implements the equation given as: where (see Figure 3 ) is expressed as: and is the adaptation step.…”

“…It is important to observe that when , the reference signal, , uses just pilot carriers and zeros, and it creates a estimated version of the reference OFDM symbol, . Differently than the works proposed in [ 19 , 22 , 32 , 33 , 34 , 35 ], the reference signal, , does not need to wait for the C OFDM samples (or one OFDM symbol) to update (see Equation ( 42 )).…”

“…In other words, there are no reference symbols in the OFDM time domain, and for that, it is necessary for the channel estimation to update the beamforming gains. The works presented in [ 19 , 22 , 32 , 33 , 34 , 35 ] proposed the alternative method using beamforming with the Least Mean Squares (LMS) and Recursive Least Squares (RLS) adaptive algorithms in the time domain, pre-FFT, without channel estimation. This method updates beamforming gains in the pre-FFT; however, it calculates the error information in the post-FFT, and after, the error signal returns back to the pre-FFT adaptive algorithm.…”

“…For the semi-unsupervised mode, the scheme proposed here uses the pilot symbols associated with decided data symbols (or decided data carriers) using the Decision Directed (DD) algorithm. Different from the proposals shown in [ 19 , 22 , 33 , 34 , 35 ], the error signal is calculated in the time domain, and the reference signals of the antenna input signal are not delayed.…”