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 receiver is 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 mentioned 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.

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