A decision-directed spread-spectrum RAKE receiver for fast-fading mobile channels

The operation of a decision-directed RAKE receiver is described that has the advantage of not requiring any bandwidth overhead for training bits. The performance is illustrated using bit error rate (BER) graphs and burst error length cumulative distribution functions (CDFs) obtained from Monte Carlo simulations. The receiver simulated operates over a fast-fading (100-Hz Doppler) mobile channel at a bit rate of 4.8 kbit/s using a 900-MHz carrier. It is shown that the receiver is not greatly affected by error propagation. However, the incurred lag on the channel impulse response estimate is the major limitation mechanism, and the resulting irreducible BER is quantified for various numbers of taps in our receiver design. The RAKE receiver performance is examined and compared using maximal-ratio, equal-gain, and differential phase combining techniques.