Le fading multitrajet est souvent important dans les radio-communications Il est possible de reduire le fading multitrajet en utilisant un reseau adaptatif a nombre carre moyen Comportement de ce reseau adaptatif en presence de signaux correles multitrajets

Behaviors of an LMS adaptive array for multipath fading reduction

Multipath fading often poses a serious hindrance in radiocommunication. The application of a least-mean-square (LMS)adaptive array to the problem of multipath fading reduction is discussed. However, it is known that multipath components are in general correlated with one another. We examine the effect of the correlation on the performance of the LMS adaptive array. When the correlation coefficient does not equal or approximate 1, the LMS adaptive array suppresses the multipath signals significantly by nulling. On the other hand, when the correlation coefficient nearly equals 1, the LMS adaptive array prevents the output signalpower from decreasing. Therefore, the LMS adaptive array mayreduce the multipath fading effectively for any correlation coefficient value. A reference signal in the LMS adaptive array is also discussed. It is shown that synchronization in the referencesignal generation must be extremely accurate. Moreover, aprocessor configuration is proposed which may generate thereference signal with the required accuracy.

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An LMS Adaptive Array for Multipath Fading Reduction

The steady-state performance of a narrow-band sidelobe canceller using a single auxiliary element with tapped delay line is analyzed for a simple two-path interference model, and an impulse response model is developed to provide insight as to the cancellation performance that might be expected. Computer solutions are then presented for various combinations of signal and multipath parameters. It is shown that auxiliary delay taps are required for effective cancellation of wide-band multipath interference, even for fractional bandwidth-delay products. Good cancellation is generally achieved provided that 1) the delay line taps are spaced much closer than the Nyquist sampling rate, and 2) the total length of the delay line considerably exceeds the longest multipath delay, depending on multipath component strengths.

Adaptive sidelobe cancellation of wide-band multipath interference

A least mean square (LMS) adaptive array requires a reference signal. When the desired signal contains a pilot signal, it may be used as the reference signal. In this paper the steady-state performance of an LMS adaptive array in which the pilot signal is used as the reference signal is examined. It is shown that the LMS adaptive array occasionally suppresses the desired signal. The loop gain, which is an important parameter, is also considered.

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An Lms Adaptive Array Using a Pilot Signal

In this thesis, novel data and channel estimation methods are proposed and analyzed for lowcomplexity short-range communication (SRC) radios. Low complexity is challenging to achieve especially in very wideband or millimeter-wave SRC radios where phase recovery and energy capture from numerous multipaths easily become a bottleneck for system design. A specific type of transceiver is selected using pulse amplitude modulation (PAM) at the transmitter and energy detection (ED) at the receiver, and it is thus called an ED-PAM system. Nonnegative PAM alphabets allow using an ED structure which enables a phase-unaware detection method for avoiding complicated phase recovery at the receiver. Moreover, the ED-PAM approach results in a simple multipath energy capture, and only one real decision variable, whose dimension is independent of the symbol alphabet size, is needed. In comparison with optimal phase-aware detection, the appealing simplicity of suboptimal EDPAM systems is achieved at the cost of the need for a higher transmitted signal energy or shorter link distance for obtaining a sufficient signal-to-noise ratio (SNR) at the receiver, as ED-PAM systems are more vulnerable to the effects of noise and interference. On the other hand, the consequences of requiring a higher SNR may not be severe in the type of SRC scenarios where a sufficient received SNR is readily available due to a short link distance. Furthermore, significant interference can be avoided by signal design. However, what has slowed down the development of ED-PAM systems is that efficient symbol decision threshold estimation and related error probability analysis in multipath fading channels have remained as unsolved problems. Based on the above observations, this thesis contributes to the state-of-the-art of the design and analysis for ED-PAM systems as follows. Firstly, a closed-form near-optimal decision threshold selection method, which adapts to a time-varying channel gain and enables an arbitrary choice of the PAM alphabet size and an integer time-bandwidth product of the receiver filters, is proposed. Secondly, two blind estimation schemes of the parameters for the threshold estimation are introduced. Thirdly, analytical error probability evaluation in frequency-selective multipath fading channels is addressed. Special attention is given to lognormal fading channels, which are typically used to model very wideband SRC multipath channels. Finally, analytical error probability evaluation with nonideal parameter estimation is presented. The results can be used in designing low-complexity transceivers for very wideband and millimeter-wave wireless SRC devices of the future.

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Estimation of energy detection thresholds and error probability for amplitude-modulated short-range communication radios

A four element adaptive array using a modulated pilot signal added to the communication signal at the transmitter has been constructed and tested at high frequency (HF). The pilot signal modulation was designed to discriminate against undesired HF multipath. The array reduces the strength of undesired modes by spatial nulling, while maintaining response in the direction of the desired mode. The array was used to receive signals that propagated via a 150-mi over-ocean path. The antenna configuration and frequency selection were such that the ground wave and ionospheric modes of propagation were approximately equal. The pilot signal, used as a reference for the adaptive array, consisted of a single tone, phased reversed by a pseudorandom sequence. The bandwidth of the pilot signal was selectable at either 3 kHz or 6 kHz. A pulse sounder was used to measure the response of the array system to the arriving modes. During the test the ground wave 1E, 1F_{1} and 1F_{2} modes were observed, occasionally simultaneously. The array reference signal could be locked to any arriving mode and the array processor was able to discriminate against all other modes by directing spatial nulls. The reduction in strength of the undesired modes measured during the test was greater than 15 dB.

/pdf/adaptive-array-for-elimination-of-multipath-interference-at-4kbuvi89tg.pdf

P. Hansen

Papers

Adaptive array for elimination of multipath interference at HF