Rake Receiver

Rake generally means the tool used in the garden to get all the fallen leaves. In wireless communication, a rake receiver is used in CDMA systems and the radio signal when reflected from any type of obstacle will travel with a different type of amplitude, phase, and delay. The people behind the rake receiver are Price and Green. This is a type of receiver that uses radio signals and is divided into different signals then it uses the correlates to compare the received signal with the original signal.

Rake Receiver

Definition

Rake Receiver is used to oppose the characteristics of multipath fading. Multipath fading occurs due to scattering, reflection, and difference of the signals when it comes across any obstacles. Due to this fading, the incoming radio signal will be very weak and can increase its characteristics by using the correlators of the rake receiver.

Principle of Rake Receiver

The Basic idea of this receiver follows multipath diversity. The Multipath diversity means the radio signal from the sender is transmitted to the receiver side in a different path which may have variant delay and attenuation values.

The Performance of the Rake Receiver is that it collects all the time-shifted delays of the transmitted signal and a correlation is provided for each of the multipath signals. The time delays can be seen in each correlator called the search window.

Components of Rake Receiver

Some of the primary components used in the rake receiver are divided into three parts,

Multipath Channel

It is the most common cause when a radio signal is sent from one sender like a mobile phone to the receiver it chooses multiple paths to reach the destination or the receiver due to the signal reflection, diffraction, and scattering.

Finger Rake Receiver

The Correlators are used by the rake receiver to find which multipath component is strongest. This can be done by weighing each correlator and choosing the best among them. To yield better performance, the weighted coefficients of the correlators need to be considered. The signal strength is calculated based on the Signal to Noise ratio or the power value. The C is the output of the correlator and the output of the multipath has the sequence as C1, C2, C3,...CM and the equation is,

$$\mathrm{C^\prime\:=\displaystyle\sum\limits_{m=1}^M \alpha_{m}\:C_{m}}$$

Where C is the Output of the Multipath correlators.

$\alpha_{m}$ is the weighted coefficient of the multipath.

Receiver

The Received signal has some sort of loss in signal strength called attenuation. The input signal is x(t) and the time-delayed signal is x(t-T) then the received signal is compared with the time-delayed signal.

The MRC (Maximal Radio Combining) technique is used to combine all the different paths in the receiver. Using this technique, all the path is defined with the same characteristics of amplitude and phase.

Working of Rake Receiver

The input signal is the rake receiver which initially involves in the matched filter of the multipath channel. The Received signal is fed into various fingers called the sub-receivers to improve the performance and behavior of the signal. The Correlators will also increase the reliability of the multipath signal.

When multipath signals are sent through the correlator one signal may be corrupted because of fading and it will not be the same case for other signals. The Corrupted will not be taken into account during the weighting process. The Number of fingers used will depend on the channel profile and the chip rate.

Application of Rake Receiver

• It is widely used in CDMA (Code Division Multiple Access) and WCDMA (Wideband Code Division Multiple Access) devices.

• As radio signals are used in wireless networks, the rake receiver is used in wireless devices like mobile phones.

• In Satellite communication, weaker signals are detected that have traveled a long distance.

Advantages of Rake Receiver

There are some common diversity techniques used namely

• Maximal ratio combining (MRC) − It is a technique that gives a high Signal to Noise ratio (SNR) to the received signal and Combines signals of each channel.

• Equal gain combining − The received signals in the antenna are added together.

• Minimum mean square error (MMSE) − The main purpose is to measure the estimated value and minimize the MMSE. It also detects the transmitted signal and if any interference is available, it will be removed.

Disadvantages of Rake Receiver

• Increased Cost − When an additional receiver needs to be installed, then we need extra space and an increase in the cost of installment.

• Complexity − The Algorithm becomes difficult when the transmitted signals are moving through different paths leading to multipath components and also the complexity of the design.

Conclusion

The Rake Receiver is primarily used in CDMA systems to improve the quality of the transmitted signal to the receiver. The multi-fading is reduced by the use of a rake receiver. Compared to the advantages there are some disadvantages namely the architecture of the receiver is very complex and the circuit consumes more power.