CDMA RAKE RECEIVER basic principle
When the signal travels from transmit end to the receive end, it will go through multiple paths. This results into multiple versions of the transmit signals received at the receiver. Each of these signals will have different attenuations and path delays. The same is depicted in the figure below.
In the cellular systems this multipath interference is exploited to recover the transmitted signal. The RAKE receiver block diagram is mentioned in the figure.
The RAKE receiver receives the signals from multiple paths and combine them suitably with different appropriate delays. This way it recovers the transmitted signal.
The complete principle of Rake receiver is explained in the figure. As shown first binary data is EX-ORed with the chip code. The spread sequence is then modulated and transmitted to the channel.
Due to multipath effects, multiple signal copies of the same transmitted signal is received by the rake receiver. At the receiver first the signal is demodulated. After the demodulation, chip stream is fed to the correlators, each provides different amount of delay. The signals are combined appropriately based on estimated weighing factors using the channel.