International audienceCoupling between repeater's antennas, called Radio Frequency Echo (RFE) deteriorates signal quality and compromises system stability. If the channel insulation is insufficient, the power in the closed-loop raises with time according to the gain margin value. Multiple methods of echo cancellation exists. Generally, these techniques are based on correlation calculation and adaptive filtering. However, their efficiency and algorithm's convergence speed are calling into question under signal's modulation and real-time constraints, specially with narrow band signals having frequency hopping as GSM or TETRA. This paper describes a real time method of RFE detection and estimation independently of the nature of the incoming signal. It is based on frequency sweeping and analysis of power ripples, and it can be implemented on FPGA for real-time purpose. It gives an accurate estimation of corresponding gain margin and loop delay. Index Terms—Radio frequency echo, gain margin, narrow band, frequency hopping
International audienceIn 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 famil
International audienceThe radio frequency feedback (RFF) occurs when the insulation is insufficient between the antennas of an on-frequency repeater, increasing digital transmission errors. In addition, a strong RFF could compromise system stability of the on-frequency repeater because of the growing power in the closed-loop. Automatic gain control is widely used by the on-frequency repeater to regulate the power, this solution being generally used with echo cancellation processes. Most of echo cancellation techniques are based on digital processing such as adaptive filters whose the effectiveness and the algorithm speed are depending on the signal frequency, the bandwidth and the closed-loop parameters. This paper describes a solution of RFF estimation and detection regardless of the receiving signal modulation. By using the frequency scanning and the analysis of the power spectral density peaks in the system, this solution is reliable whatever are the values of the gain-margin and the loop-delay. Simulations and experimental implementation using field-programmable gate array validate the solution. In addition, an example of applications is given in the context of the interference cancellation
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