| While direct sequence spread spectrum systems are robust to many types of interference, performance can be significantly degraded if the interference is strong enough, particularly for wideband interferences. In these situations, various signal processing methods can be employed to remove, or excise, the jammer prior to despreading the received signal, resulting in enhanced performance. We investigate the effects of amplitude and frequency modulated (AM-FM) jammers on the performance of direct sequence spread spectrum communication systems.; Unlike FM jammers, which can be characterized solely by their instantaneous frequency, AM-FM jammers require additional information about their instantaneous amplitude/bandwidth. We have developed two methods for excising AM-FM jammers: an adaptive notch filter tuned to the instantaneous frequency and instantaneous bandwidth of the jammer, and a projection filter that uses the instantaneous frequency and instantaneous amplitude of the jammer to form a subspace (projection matrix) that is orthogonal to the interference subspace. Multiplying the received signal vector by this matrix yields an optimal excision filter in a least squares sense.; Although the adaptive notch filter is effective for removing AM-FM jammers, significant distortion of the desired PN sequence occurs, and the performance is sub-optimal. Conversely, the projection filter yields near optimal performance.; Our projection filter builds on the work of Amin. Specifically, we modify Amin's FM excision projection filter to excise mono- and multi-component AM-FM jammers. We derive theoretical performance measures of the AM-FM projection filter, including the effects of jammer estimation errors. For unbiased estimates of the jammer parameters, the method is able to effectively remove the interference with negligible distortion to the desired PN sequence. Estimation errors on the jammer AM or FM degrade performance, and in general FM errors cause more degradation than the same level of error in the estimation of the jammer AM.; Finally, we have explored methods for estimating the jammer parameters in practice, and have tested the performance of the projection filter with actual estimates of the jammer. The BER of these simulations closely matches the theoretical results. |
