RAKE RECEIVER: The RAKE receiver is so named because it reminds the function of a garden rake, each finger collecting symbol energy similarly to how tines on a rake collect leaves. A RAKE receiveris a radio receiver designed to counter the effects of multipath fading. They are common in a wide variety of CDMA and W-CDMA radio devices such as mobile phones and wireless LAN equipment.In a multipath environment, which is common in cellular systems, if the multiple versions of a signal arrive more than one chip interval apart from each other, the receiver can recover the signal by correlating the chip sequence with the dominant incoming signal. The remaining signals are treated as noise. However, even better performance can be achieved if the receiver attempts to recover the signals from multiple paths and then combine them with suitable delays. This principle is used in the RAKE receiver.
Principle of RAKE receiver
Figure above illustrates the principle of the RAKE receiver. The original binary signal to be transmitted is spread by the exclusive-OR (XOR) operation with the transmitter’s chipping code. The spread sequence is then modulated for transmission over the wireless channel. Because of multipath effects, the channel generates multiple copies of the signal, each with the different amount of time delay (etc.), and each with a different attenuation factors. At the receiver, the combined signal is demodulated. The demodulated chip stream is then fed into multiple correlators, each delayed by a different amount. These signals are then combined using weighting factors estimated from the channel.
In multipath propagation there exist several paths with different delays between a transmitter and a receiver. As a result the receiver may receive multiple copies of the signal, each with different delays. Rake receivers can be used to mitigate the effect of multipath propagation. A rake receiver uses n correlators called fingers for n strongest paths. Each correlator is synchronized to the transmitter plus the delay on that specific path. As soon as the receiver detects a new path which is stronger than the currently weakest path, it assigns this new path to the correlator with the weakest path. The outputs of the correlators are then combined and fed into the decision unit.