RAKE Receiver

 RAKE Receiver:

In CDMA system, the multipath interference is reduced by combining direct and reflected signals in a receiver.

In 1958 Price and Green proposed a method of resolving the multipath problems in CDMA. The time-shifted version of the same signal appears like noise and is almost uncorrelated in CDMA.

A signal that propagates from the transmitter to the receiver can be resolved into separately fading signals by cross-correlating the received signal with multiple time-shifted versions of the same sequence.

The received signals with maximum power are time-shifted and added. The block diagram of this method looks like a garden rake and is called a RAKE Receiver.

The CDMA receivers can combine all the delayed versions of the transmitted signal and provide a better signal-to-noise ratio at the destination side. 

Must Read>> CDMA

By using a specific correlation receiver unit the RAKE receiver collects all the delay versions / shifted versions of the original signal. 

Each and every correlator unit is capable of searching a particular range of time delays. It is called a "search window".

the below image shows a RAKE receiver:

RAKE Receiver
 RAKE Receiver

The RAKE receiver is also called a "diversity receiver". It is designed, particularly for the CDMA technique. The diversity of the signals is achieved b using the uncorrelated multipath components with another whenever the relative propagation delay values are greater than the chip time period.

Hence, the 'RAKE Receiver' mainly uses multiple correlations for sig reception in mobile communication. 

In this example, 'N' number of correlators are used to detect N strongest multipath components. For obtaining an accurate estimate of the transmitted signal as a whole rather than a single component, the multipath components are weighted after 'N' correlator units.

Depending on the output signal values the detection and bit decisions a done. A weighting network provides a linear combination of outputs from the correlator for bit detection.

Correlator 1 is synchronized to the strongest multipath n₁. Multipath component n2 arrives T₁ later than component n₁ where T₂ - T₁ is assumed to be greater than a chip duration.

The correlator 2 is synchronized to the strongest multipath n2. It has a low correlation with n₁.

It should be noted that if a single correlator is used in the receiver, there once the correlator output is corrupted by fading the receiver cannot correct it. Thus, if bit decisions are done with respect to a single correlator then the probability of bit error rate is higher. 

The decisions that are based on the combination of N separate decision statistics offered by the RAKE receiver provide a form of diversity that can overcome fading and improve the CDMA reception.

Assume that the outputs of N correlators are A₁, A₂, A3,..... AN. The weighting components are based on the power or the SNR from each correlator output. If the SNR power is smaller for a particular correlator then its weighting factor will also be a smaller value. 

The general overall signal can be represented as A'. It is given by,

There are many methods to generate the weighting coefficients. However, because of multiple access interference, RAKE fingers with strong multipath may not provide strong output after correlation.

For having a better performance to produce efficient signal reception, the selection of weighting coefficients that is based on the output values of the correlator is an important criteria in the "RAKE" receivers.

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