Rake reception for a CDMA mobile communication system with multipath fading

The performance of a rake receiver for direct-sequence (DS) spread spectrum communications on multipath fading channels depends on the number and type of multipath components present at the channel output. The focus in this paper is on noncoherent combining of the outputs of the taps of the rake receiver in a code-division multiple-access (CDMA) system that employs DS spread spectrum with binary DPSK data modulation. It is shown that the advantages and disadvantages of a rake receiver, as compared with a standard correlation receiver, depend greatly on the fading characteristics of the individual multipath components. In general, rake receivers are more appropriate for DS systems with a smaller number of chips per data symbol and for channels that have only diffuse, Rayleigh fading multipath components. On the other hand, a correlation receiver is preferred for DS systems with a large number of chips per data symbol if the channel has a strong specular multipath component in addition to one or more diffuse components. The probability of error on the channel is determined as a function of the chip rate for doubly selective fading channels, and the pairwise error probability is evaluated for a CDMA system with binary convolutional encoding and Viterbi decoding. The effects of the Doppler spread and the interleaving depth on the performance of the system and the choice of chip rate are discussed