A digital lock-in amplifier built with a couple of input/output boards in a personal computer is described. The use of direct memory access allows the generation of a reference sinewave and the sampling of up to eight channels synchronously with it, leaving enough time for the processor to calculate the in phase and in quadrature responses at and only at selected harmonics (hl,h2,...) of the reference frequency. This digital lock-in is drift free and has the gain stability of the analog-to-digital converter, that is within f 1 ppm for a few minutes, increasing to =!=20 ppm for 24 h. Even a simple Mac IIci can monitor in real time hl at 24 ksamples/s for two channels, corresponding to an upper frequency of 3 kHz, with no limitations on the low-frequency side. Higher sampling rates and processing power are available with more recent hardware.

Abstract-In this contribution a low-complexity particle filtering algorithm is proposed to track the parameters of time-variant propagation paths in multiple-input multiple-output (MIMO) radio channels. A state-space model is used to describe the path evolution in delay, azimuth of arrival, azimuth of departure, Doppler frequency and complex amplitude dimensions. The proposed particle filter (PF) has an additional resampling step specifically designed for wideband MIMO channel sounding, where the posterior probability density functions of the path states is usually highly concentrated in the multi-dimensional state space. Preliminary investigations using measurement data show that the proposed PF can track paths stably with a small number of particles, e.g. 5 per path, even in the case where the paths are undetected by the conventional SAGE algorithm.Index Terms-Radio propagation channel, sequential Bayesian estimation, particle filter, state-space model, and maximumlikelihood estimation.

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The scattuing rates v(T) of various p o u p of electrons in cadmium have been measured down to 0.25 K using the radic-lrrquency size effect. For electrom on the third band lens of the Fermi surface, v(T) can be analysed below about 2.5 K as the superposition of two terms, aP and PT'. While the latter is associated with electron-phonon scattering the former component can be attributed to electmn-electron scattering. On two triangular orbits in the first and second bands an apparently large P t -is observed between 5 and 1.5 K. At lower temperatures v(T) drops rapidly suggesting that this behaviour is due to electmn-phonon scattering. An explanation for large T 2 terms has been recently proposed b y Lawrence cf @I on the basis of UmWapp electrm-phonon scattering togethcx with the particular shape oI the Fermi surface of c a d " . Simple calculations on these lines are shown to repduce semi-quantitatively the observed variation d u(T) and to aid in the interpretation of another second band orbit.

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