Adaptive mobile channel prediction for decision directed RAKE receivers

The authors describe a robust channel prediction technique for a direct sequence spread spectrum (DS-SS) code division multiple access (CDMA) system in a fast fading environment. For improved performance the RAKE filter taps are coherently combined, hence accurate channel estimation is required. A FIR type linear prediction filter for each RAKE filter tap is used to estimate the impulse response. In order to do this, the data decisions are fed back to the prediction filter. The stability of the proposed system is achieved through differential encoding (DPSK) of the data bits. It is demonstrated through simulations that the performance of the proposed decision directed receiver is better than of a receiver which relies on a pilot signal to estimate the channel response. To track the time variations of the channel statistics, the predictor coefficients are updated using the least mean square (LMS) algorithm. If the SNR is over a certain threshold value the LMS algorithm is robust to the effects of decision feedback.