In this paper, we present a modified image rejection method that uses imbalance compensation techniques for phase and gain in low-intermediate frequency (IF) software-defined radio (SDR) receivers. In low-IF receivers, the image frequency signal interferes with the desired signal owing to the phase and gain imbalances caused by analog devices. Thus, it is difficult to achieve the required image rejection ratio (IRR) of over 60dB without compensation. To solve this problem, we present modified blind compensation techniques based on digital signal processing using a feedback control loop with a practical computation process. The modified method can reduce the complexity when a hardware logic circuit is used, like an FPGA. The simulation and experimental results verify that the modified method achieves an IRR greater than 50-60dB for both the carrier and the modulated waves.

An essential condition for diversity reception is that the fading distributions between individual received signals of an antenna array are uncorrelated. In this paper, a new technique to improve the performance of transmission with the correlated Rayleigh-fading signals is proposed. In conventional array systems, individual receivers start sampling the received signals at the same time with the same sampling rate. On the other hand, in the proposed scheme, the received signals are again sampled with the same rate, however the sampling points are shifted in each receiver. Numerical results through computer simulation show that with correlated received signals, by applying the proposed technique the correlation can be reduced to a sufficient level for diversity reception.