G-Rake Finger Placement in Time and Frequency Domains

The Generalized-Rake (G-Rake) receiver consists of a group of despreaders (fingers) with MMSE combining. Several techniques for assigning the number of fingers and their delay positions based on the current channel impulse response have been proposed in the literature. We compare the performance of several placement techniques in the context of the UMTS/HSPA uplink. Two finger placement techniques are proposed: a simple equally-spaced "equalizer" placement in the time-domain, and a new FFT-based approach that exploits the ease of computing the Weiner filter in the frequency-domain. Additionally, we assess two methods for finding the required covariance matrix; a traditional approach based on the sample covariance matrix, and a "formula-based" approach that uses the channel estimates and exploits the known matrix structure. While most G-Rake studies have focused on the downlink, we show that the G-Rake can achieve useful 1-2 dB gains for HSUPA uplink signals with low spreading gain on dispersive channels.