Rake Receiver

Instead of trying to overpower or correct multipath problems, CDMA takes advantage of the multipath to improve reception quality in fading conditions. CDMA does this by using multiple correlating receivers and assigning them to the strongest signals.  This is possible because the CDMA mobile is synchronized to the serving base station. The mobile’s receiver can distinguish direct signals from multipath signals because the reflected multipaths signals arrive later than the direct signals.

in rake receiver Special circuits called searchers are also used to look for alternate multipaths and for neighboring base station signals. The searchers slide around in time until they find a strong correlation with their assigned code. Once a strong signal is located at a particular time offset, the searcher assigns a receiver element to demodulate that signal. The mobile receiver uses three receiving elements, and the base station uses four.  This multiple correlator system is called a rake receiver. As conditions change the searchers rapidly reassign the rake receivers to handle new reception conditions.

Instead of trying to overpower or correct multipath problems in rake receiver, CDMA takes advantage of the multipath to improve reception quality in fading conditions. CDMA does this by using multiple correlating receivers and assigning them to the strongest signals.  This is possible because the CDMA mobile is synchronized to the serving base station. The mobile’s receiver can distinguish direct signals from multipath signals because the reflected multipaths signals arrive later than the direct signals.

Special circuits called searchers are also used to look for alternate multipaths and for neighboring base station signals. The searchers slide around in time until they find a strong correlation with their assigned code. Once a strong signal is located at a particular time offset, the searcher assigns a receiver element to demodulate that signal. The mobile receiver uses three receiving elements, and the base station uses four.  This multiple correlator system is called a rake receiver. As conditions change the searchers rapidly reassign the rake receivers to handle new reception conditions.

Rake Receiver Design

The design of a 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.