New Approach to Adaptive Line Enhancer Using Auto- Correlation Function for Processing Sarsat Signals

This paper develops a new approach to the adaptive line enhancement (ALE) of the emergency locator transmitter (ELT) signal where the input signal to the ALE is replaced by its autocorrelation function. This approach is related to the signal processing using Higher Order Statistics (HOS) of the signal recently introduced. The paper also compares the results of using the new approach with other different used methods. The first method uses the signal xk as an input to the ALE, and the second method uses x: as the input to the ALE. Results illustrate the superiority of the proposed method over the other two methods. IINTRODUCTION Adaptive filters can be FIR (Finite Impulse Response) or IIR (Infinite Impulse Response) filters. The majority of the work on adaptive line enhancer involves the use of FIR filters [1,2], although IIR filters have been proposed. In Sinusoidal signal detection, or in the case of signals that can be analyzed into few sinusoids, high-Q factor resonant filters are required. This property of high-Q is an inherent property and an advantage of the IIR filters over the FIR filters. High-Q filters require poles that are very close and inside the unit circle. The factor of primary concern associated with the use of IIR filters is to maintain the stability of the filter during adaptive process. The convergence characteristic on a multi-modal performance surface, which results from the multiple line frequencies, is another concem. Use of a simple two-pole structure alleviates the problem of maintaining filter stability since the pole radius can easily be constrained to be less than unity. The complex conjugate poles of the ALE structure may be fixed or adaptive. In this work we consider two types of ALE structures, the variable pole radius ALE and the fixed pole radius ALE. Both structures are presented and explained in section 11. In this paper, we introduce and compare some different methods for detecting an ELT signal merged with noise using adaptive structures implementing the adaptive line enhancer (ALE) structure proposed by Widrow [3]. The adaptive line enhancer (ALE) is a adaptive filtering ‘system, which enables detection, enhancement and tracking of narrowband signals in the presence of wideband noise. ALE structures using IIR algorithms are preferred to FIR structures in implementing the ALE due to reasons explained before. Unfortunately, IIR fi!ters suffer from the instability associated with the presence of poles in the filter transfer function [3]. In addition; we investigate the use of x~ , xk2 and the auto-correlation function (ACF) as inputs to the both fixed and variable pole radius ALES. A comparison study between the performances of all of these cases is introduced. This paper is organized as follows: In section I, we introduce the input ELT signal. In section 11, we present a description of the IIRALE structures with fixed and variable pole radius. Section 111 introduces the use of both x: and the new approach which uses the ACF instead of xk as inputs to the ALE structures. Section IV introduces the frequency error concept. Finally, section V gives the simulation results. I. ELT SIGNAL An ELT signal is a low powered emergency radio transmitter radiating about 100 mw with amplitude modulated signal having a carrier frequency of either 12 1.5 MHz or optionally 243 MHz. This signal is used in the SARSAT system in the cases of emergency states to alleviate the problem of location determination. The ELT signals are processed using a bandpass processor implementation in which the signal is mixed to the frequency range of 0 to 25 kHz, which normally covers the vast majority of ELT signals. [4,5] The received ELT signal that is immersed in white Gaussian noise is given by the following expression: ml, SEVENTEENTH NATZONAL RADIO SCZENCE CONFERENCE Feb. 22-24,2000, Minufiya University Egypt. s ( t ) = A[1+ m(t)] cos(2nfct + 6 ) + n(t) (1) where : A: is the carrier amplitude. 8: is the phase angle . n(t) : is the additive white Gaussian noise. The modulating term can be classified as either sine wave or pulse shaped function To formulate a sinusoidal modulated ELT signal, we define: f, :is the carrier frequency. m(t): is the modulating signal.