Narrow-band interference rejection using transform domain signal processing in a hybrid DS/FH spread-spectrum system

The performance of a narrow-band interference rejection scheme using transform domain signal processing is studied in a hybrid DS/FH spread-spectrum system. The signal of interest can be considered to be a BPSK modulated direct-sequence spread-spectrum signal within one frequency hopping period. The interference itself is a narrow-band signal with a high power level and with a bandwidth relatively much narrower than the bandwidth of the wide-band signal. The interference can be located either intentionally or unintentionally within the DS-bandwidth which is centered according to the hopping frequency in question. The influence of the narrow-band interference will be reduced by using transform domain filtering. The interference excision takes place in the frequency domain after which the signal is transformed back to the time domain where the rest of the signal processing takes place. To avoid the dissemination of the interferer´s energy over a wide frequency range, windowing is used prior to the transformation process. The transformations from the time domain to the frequency domain and vice versa are made by the fast Fourier transform (FFT) and the inverse fast Fourier transform (IFFT), respectively. The criterion used for the evaluation of the performance of the interference rejection algorithm is the bit error probability as a function of E_b/N₀ and signal to jammer ratio (SJR). The bit error rates were obtained by Monte-Carlo simulations