Haghdad, Mehdi; Feher, Kamilo; University of California; Digcom Inc. (International Foundation for Telemetering, 2000-10)
      The Bit Error Rate (BER) performance of FQPSK modulated signals in the presence of the Co-Channel Interference (CCI) and Additive White Gaussian Noise (AWGN) is evaluated and improved. A Non- Linearly Amplified (NLA) FQPSK modulated signal with the data rate of 1Mb/s and carrier frequency of 70 MHz is interfered with a sinusoidal signal at different frequencies. As the relative distance of the center frequency of the Co-channel interference (CCI) changes, different BER are obtained. The effect of the CCI decreases as the CCI center frequency moves away from the center of the modulated signal. In order to improve the BER in the presence of the CCI, a hard limited filter is added at the receiver input. The hard limited filter has a different amplification factor for different signal strength. As a result, the amplification factor for the CCI, which is normally a weaker signal, is smaller than the actual signal. This means that the signal is amplified more than the interference and as a result the CCI is suppressed and the BER rate improves. The results of both simulations and measurements are obtained for different CCI center frequencies, before and after the improvements.