SUPPRESSION OF NARROW-BAND JAMMERS IN A DIRECT SEQUENCE SPREAD SPECTRUM RECEIVER USING LMS ADAPTIVE FILTERING TECHNIQUES

Analytical results indicate that least-mean-squared adaptive filtering is a useful technique for suppressing narrow-band jammers in a direct sequence spread spectrum receiver. However, the cost and complexity of adaptive filtering hardware has hindered experimental verification of these results. A novel adaptive filter architecture, the burst processing adaptive filter architecture, allows the construction of a Widrow-Hoff LMS adaptive filter of arbitrary order using only two multipliers. Two 16th order burst processing adaptive filters, one analog and one digital, have been constructed and tested.; The analog and digital implementations of the burst processing adaptive filters were used to verify and extend analytical results for the suppression of narrow-band jammers. The test configuration adds a jammer and thermal noise to the baseband direct sequence signal and passes the composite signal through an adaptive filter and correlator to recover the data. The measure of performance is the bit error rate of the received data.; The performance of the system was tested using several types of tone jammers and 7- and 31-chip pseudorandom sequences. For a single-tone jammer, the bit-error-rate performance of the adaptive receiver was found to be dependent on both jammer power and frequency. When the tone jammer sweeps the frequency band of the direct sequence signal, bit-error-rate performance is closely coupled to the jammer sweep rate and adaptive filter convergence factor. While a large convergence factor is necessary to suppress a jammer with a high sweep rate, this convergence factor results in reduced performance for low jammer sweep rates.; To allow evaluation of the effect of filter order the double-tone jammer was studied using Monte Carlo simulation. While an adaptive filter of low order provides good bit-error-rate performance for a single-tone jammer, a filter of much higher order is necessary for two tones. Experimental results for a narrow-band Gaussian jammer show that bit-error-rate performance decreases quickly with increasing jammer bandwidth.; Finally, quantization error effects were analyzed and found to decrease with higher code processing gain. Several two-stage adaptive jammer suppressors are discussed and evaluated.