Performance of pilot carrier based Doppler compensation for CDMA land mobile satellite links

The performance of a Doppler estimation scheme for CDMA land mobile LEO satellite systems under realistic channel conditions is studied by computer simulation. The scheme can be applied in Doppler compensation of the received signals, to aid in the code acquisition under the severe Doppler conditions offered by the satellite channel. The proposed scheme is based on a continuous-wave pilot carrier placed at the first spectral null of the CDMA user signal. The results show that Doppler estimation and compensation is possible with very high reliability, giving a probability of correct estimate >0.9995 down to 25° elevation with ±250 Hz accuracy. However, in the urban shadowed channel state the performance is poor and reliable Doppler estimation is not possible. The estimator is also able to track the variation of Doppler with time even under the worst-case Doppler rate conditions. The estimator structure is simple, requiring only an FFT with averaging as post processing. By assuming that the proposed Doppler compensation was achieved to ±250 Hz accuracy, code acquisition was demonstrated to be feasible with acceptable acquisition time by a 1-dimensional serial search instead of the considerably longer 2-dimensional search that would be required in the uncompensated case