7.14 Application of rake receiver in cdma system

Interim Specification 95 (IS-95) describes a Direct-Sequence Spread-Spectrum (DS/SS) cellular system that uses a Rake receiver to provide path diversity for mitigating the effects of frequency-selective fading. The Rake receiver searches through the different multipath delays for code correlation and thus recovers delayed signals that are then optimally combined with the output of other independent correlators.

Figure 1 show the power profiles associated with the five chip transmissions of the code sequence 1 0 1 1 1. Each abscissa shows three components arriving with delays ${\tau }_1$ , ${\tau }_2$ , and ${\tau }_3$ . Assume that the intervals between the transmission times $t_i$ and the intervals between the delay times ${\tau }_i$ are each one chip in duration. The component arriving at the receiver at time $t_{-4}$ , with delay ${\tau }_3$ , is time-coincident with two others, namely the components arriving at times $t_{-3}$ and $t_{-2}$ with delays ${\tau }_2$ and ${\tau }_1$ respectively. Since in this example the delayed components are separated by at least one chip time, they can be resolved.

At the receiver, there must be a sounding device dedicated to estimating the ${\tau }_i$ delay times. Note that the fading rate in mobile radio system is relatively slow (in the order of milliseconds) or the channel coherence time large compared to the chip time duration ( $T_0>T_{\text{ch}}$ ). Hence, the changes in ${\tau }_i$ occur slowly enough that the receiver can readily adapt to them.

Once the ${\tau }_i$ delays are estimated, a separate correlator is dedicated to recovering each resolvable multipath component. In this example, there would be three such dedicated correlators, each one processing a delayed version of the same chip sequence 1 0 1 1 1. Each correlator receives chips with power profiles represented by the sequence of components shown along a diagonal line. For simplicity, the chips are all shown as positive signaling elements. 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 PN code. At the end of a symbol interval (typically there may be hundreds or thousands of chips per symbol), 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 PN 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 mitigation provided by the Rake receiver can be termed path diversity, since it allows the energy of a chip that arrives via multiple paths to be combined coherently. Without the Rake receiver, this energy would be transparent and therefore lost to the DS/SS receiver.