Sensitivity loss of real-time DFT-based frequency excision with direct sequence spread-spectrum communication

The capacity of direct sequence spread-spectrum modulation to reject narrow-band interference can be significantly improved by eliminating narrowband energy at the receiver (frequency excision) using algorithms that operate on the real time discrete Fourier transform of the received signal (RT-DFT-Based). These algorithms have the potential to adapt very quickly to a change in interference spectrum and eliminate multiple interferers, but to do this the decision of which frequency bins to excise must be made based on a very short observation time. Under these circumstances, the number of bins excised can be much larger than the number of bins containing narrow-band interference. The receive signal strength loss due to “over-excision” can be very significant and limits receive sensitivity. This paper shows theoretical over-excision losses of several heuristic algorithms. The sensitivity loss due to time weighting (or windowing) is presented for variable overlap and several different windows. These theoretical results are confirmed with simulation results and can be used to project sensitivity of PN spread spectrum systems that are located in a band that is also used by narrow-band systems