DS-SS Rake Receiver ( cont )

The Rake receiver consists of multiple independent correlators, in which the receive signal is multiplied by time-shifted versions of a locally generated code sequence. The intention is to separate signals such that each finger only sees signals coming in over a single (resolvable) path. The spreading code is chosen to have a very small autocorrelation value for any nonzero time offset. This avoids crosstalk between fingers. In practice, the situation is less ideal. It is not the full periodic autocorrelation that determines the crosstalk between signals in different fingers, but rather two partial correlations, with contributions from two consecutive bits or symbols. It has been attempted to find sequences that have satisfactory partial correlation values, but the crosstalk due to partial (non-periodic) correlations remains substantially more difficult to reduce than the effects of periodic correlations.
Each correlator receives chips with power profiles represented by the sequence of components. In reality, these chips form a pseudonoise (PN) sequence, which of course contains both positive and negative pulses. Each correlator attempts to correlate these arriving chips with the same appropriately synchronized pseudonoise code. At the end of a symbol interval, the outputs of the correlators are coherently combined, and a symbol detection is made. The interference-suppression capability of DS-SS systems stems from the fact that a code sequence arriving at the receiver time-shifted by merely one chip will have very low correlation to the particular pseudonoise code with which the sequence is correlated. Therefore, any code chips that are delayed by one or more chip times will be suppressed by the correlator. The delayed chips only contribute to raising the interference level (correlation sidelobes). The reduction provided by the Rake receiver can be termed path diversity, since it allows the energy of a chip that arrives through multiple paths to be combined coherently. Without the Rake receiver, this energy would be transparent and therefore lost to the DS/SS receiver.