Narrowband digital filtering with random frequency hopping spread spectrum

Abstract

In digital signal filtering, channels with narrow bandwidth need high order digital filter to be selected without introducing modulation errors. If a carrier randomly switches from a channel to another as in military applications, or some civilian communication standards, it is necessary to detect and estimate these jumps before transposing and analyzing signals in the baseband. This paper presents a real time solution to filter narrow band signals with random frequency hopping spread spectrum. The proposed method is based on three steps. Firstly, the detection of Signal Frequency Hopping (SFH) using the Fast Fourier Transform (FFT), an algorithm to estimate the Dominant Frequency Value (DFV) is developed, it is necessary for better refining the original detection, in particular, with modulated signals. Secondly, the estimated frequency value is scaled and used with a Numerically Controlled Oscillator (NCO) in order shift the interest channel to baseband. Thirdly, the transposed channel in base band is selected using low pass Finite Impulse Response (FIR) filters. Whereas, the multi rate filtering techniques guarantee the high selectivity and low orders of these FIR filters. Each of the following stages is described in detail later in this paper, synthesizing these steps leads to the proposed solution, that is validated by using GSM signals. The algorithms are implemented in Field Programmable Gate Array (FPGA) Altera Cyclone III family.