Channel estimation for a discrete-time RAKE receiver in a WCDMA downlink: algorithms and repercussions on SINR

The conventional receiver for DS-CDMA communications is the RAKE receiver which is a matched filter (MF), matched to the operations of spreading, pulse shape filtering and channel filtering. The RAKE receiver assumes a sparse/pathwise channel model so that the channel matched filtering gets done pathwise, with delay adjustment and decorrelation per path and maximum-ratio combining of path contributions at the symbol rate. Original RAKE receivers work with continuous delays, which are tracked by an Early-Late scheme. This requires signal interpolation and leads to suboptimal treatment of diffuse portions in the channel impulse response. These disadvantages can be avoided by a discrete-time RAKE, operating at a certain oversampled rate. Proper sparse modeling of the channel is an approximation problem that requires exploitation of the limited bandwidth of the pulse shape. We propose and simulate a number of sparse channel approximation algorithms along the lines of Matching Pursuit, of which the Recursive Early-Late (REL) approach appears most promising. We also analyze and simulate the effect of channel estimation on the RAKE output SINR. Keywords—RAKE receiver, discrete-time RAKE, sparse channel model, channel approximation, channel estimation.