A rake finger grid for asynchronous DS-CDMA systems using LMMSE tap weight estimation

In this paper a Rake finger grid structure for direct-sequence code division multiple access (DS-CDMA) systems is analyzed as an alternative to the conventional Rake structure using individual fingers with timing- and phasor tracking. The grid uses a group of equispaced Rake fingers in the delay domain. Each of the Rake fingers uses linear minimum mean-squared error (LMMSE) tap weight estimation needed for the subsequent maximum-ratio combining (MRC). It is pointed out during the paper that the use of a grid is advantageous compared to the conventional Rake receiver in some scenarios where the channel forms so called clusters, consisting of many unresolvable, closely spaced multipaths. It is also shown that when using fractionally-spaced grid fingers, the task of fine timing tracking can be omitted. Only coarse timing estimation is necessary, which can be performed by a suitable acquisition unit.