REJECTION OF AN FH SIGNAL IN A DS SPREAD-SPECTRUM SYSTEM USING COMPLEX ADAPTIVE FILTERS

The performance of a direct sequence QPSK spread-spectrum receiver usi ng adaptive filters in the presence of frequency hopped interference i s analyzed. The analysis includes both the adaptive prediction error f ilters and the adaptive transversal filters with two-sided taps. If th e product of the instantaneous frequency offset Omega(l), between the jamming signal and the carrier of the spread-spectrum signal, and the sampling period Delta is 360 degrees (Omega(l) .Delta = 360 degrees), the filter gain is reduced to zero. The filter gain G highly depends o n the filter adaptation rate mu. Depending on mu, G can vary from zero to more than 20 dB for the jammer/signal power ratio (J/S) of 20 dB. If Omega(l) .Delta is small enough (less than or equal to 10 degrees), the performance of the transversal filter is better than that of the prediction error filter, in case when mu is small. For larger values o f mu or Omega(l) .Delta these performances are approximately the same. Numerical results for the hopping sequence of the jamming signal are also presented. Besides the filter gain the analysis of the adaptation rate (time constant) filter misadjustment and the system bit error pr obability is also included.