RAKE Receiver in CDMA system:
Rake receiver can be visualized as a series of time delayed correlator taps fed from a common antenna. If each correlator tap is delayed to match the arrival of a particular transmitted signal, then the outputs of each tap can be recombined in phase. Once an RF signal with a particular travel time is locked onto by the correlator tap, an estimate of the gain or loss experienced by that signal must be made. The weighting of the taps perform this gain normalization function. Once adjusted, the outputs of each finger of the rake can be combined to form a better version of the transmitted signal. Notice that this description visually matches the analogy of a common garden rake with each tap forming a tine of the rake, hence the name rake receiver.
Another form of time diversity occurs in the base station when transmitting at reduced data rates. When transmitting at a reduced data rate (more detail will be presented on this later), the base station repeats the data resulting in full rate transmission. The base station also reduces the transmitted power when it operates at reduced data rates. This added redundancy in the transmitted signal results in less interference (power is lowered) and improves the CDMA mobile’s station receiver performance during high levels of interference thats simple details on cdma rake receiver.
In CDMA spread spectrum systems, CDMA spreading codes are designed to provide very low correlation between successive chips, propagation delay spread in the radio channel provides multiple version of the transmitted signal at the receiver. Delaying multipath components by more than a chip duration, will appear like uncorrelated noise at a CDMA receiver. CDMA receiver may combine the time delayed versions of the original signal to improve the signal to noise ratio at the receiver. RAKE receiver collect the time shifted versions of the original signal by providing a separate correlation receiver for M strongest multipath components. Outputs of each correlator are weighted to provide a better estimate of the transmitted signal than provided by a single component. Demodulation and bit decisions are based on the weighted output of the correlators. Schematic of a RAKE receiver is shown in Figure below: