In wireless telecommunications systems, a radio receiver will often receive radio signals that have propagated from a transmitter over multiple different paths. This may occur, for example, because of reflections and/or scattering by buildings or other obstructions. The different propagation paths from the transmitter to the receiver may have different path lengths. As a result, radio signals that have propagated over different paths may have different phases at the receiver and may interfere with one another.
In spread spectrum systems, the potential for interference between radio signals propagating over different paths is minimized because a pseudorandom code sequence is imposed on the radio signal transmitted from the transmitter. The pseudorandom code sequence has the property that time-shifted versions of itself are largely uncorrelated. As a result, there will be little or no interference between spread spectrum radio signals that reach a receiver over different paths with different time-shifts. At the receiver, each signal component in a multipath radio signal can be separately resolved by correlating the signal component with a corresponding time-shifted version of the pseudorandom code sequence.
In spread spectrum systems, the receiver is typically a RAKE receiver that is able to separately resolve individual signal components of a multipath radio signal simultaneously. The multiple signal components that are separately resolved may then be combined to achieve a diversity gain. In combining the multiple signal components, each component could be weighted equally (equal-gain combining) or different components could be weighted differently (maximum ratio combining).
The block diagram of a RAKE receiver is often drawn to resemble a garden rake (hence the name). Each “finger” of the rake includes a correlator that is able to resolve a signal having a pseudorandom code sequence with a particular time-shift. The RAKE receiver in a typical mobile station may have three fingers, so as to be able to simultaneously resolve three different signals with three different time-shifts. The RAKE receiver in a typical base station may have four or more fingers.
Cellular wireless networks that use spread spectrum signals are typically configured so that each sector transmits signals using the same pseudorandom code sequence but with different time offsets. This enables the network to transmit a signal to a mobile station through two or more sectors. The mobile station's RAKE receiver may receive the signals from the different sectors simultaneously and combine them together to achieve a diversity gain. Thus, the different fingers of a mobile station's RAKE receiver may be used to simultaneously receive radio signals propagating over different paths from the same base station or to simultaneously receive radio signals from different base stations.