Doppler Frequency Estimation in Multipath Fading Channels

As prior knowledge of Doppler frequency has been found useful in many applications of mobile communications, it is necessary to estimate the Doppler frequency at the mobile station (MS) based on the received fading signal. Rice et al 1] had derived Doppler frequency estimates using continuous-time received fading signals. In the present investigation, level crossing rate (LCR) and autocorrelation function (ACF) methods using the discrete-time received signal are developed for the estimation of Doppler frequency. Simulation results show that both methods agree closely with the actual Doppler frequencies. Reeected by the local scatterers, the incoming plane waves arrive at the MS from many diierent directions and with many delays. The received signal at the MS is made up of many horizontally travelling plane waves with random amplitudes and phases. If the transmitted signal is assumed to be vertically polarized, the continuous-time received signal model can then be derived 2]. If the number of incoming plane waves is large, the quadrature components of the received signal are approximately Gaussian distributed, according to the Central Limit Theorem. If there is no line-of-sight (LOS) path between the base station (BS) and MS, the quadrature components have zero mean and same variance 2. In this paper, the discrete-time signal model is derived from the continuous-time model, which is written as below: x(k) = N X n=1 n cos(! c k + ! n k + n) (1) where n is the amplitude of the n th incident wave, ! c is the carrier frequency, ! n = 2f m cos n is the Doppler frequency due to the mobility of MS, f m is the maximum Doppler frequency , n is the incident angle of the n th wave, and n is the phase of the n th incident wave.