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Explain principle of Rake receiver in detail.
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• RAKE receiver, used specially in CDMA cellular systems, can combine multipath components, which are time-delayed versions of the original signal transmission. The basic idea of a Rake receiver was first proposed by Price and Green.
• Rake receiver is multiple parallel receivers used to combat multi-path interference and Inter-symbol interference.
• Due to reflections from obstacles a radio channel can consist of many copies of originally transmitted signals having different amplitudes, phases, and delays. If the signal components arrive more than duration of one chip apart from each other, a Rake receiver can be used to resolve and combine them. The Rake receiver uses a multipath diversity 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.
• In the cellular systems this multipath interference is exploited to recover the transmitted signal.
• Rake receiver reduces effects of fading and provides spectral efficiency improvement of CDMA.
• Rake receiver attempts to collect the time shifted versions of the original signal by providing a separate correlation receiver for each of the multipath signals. This can be done due to multipath components are practically uncorrelated from another when their relative propagation delay exceeds a chip period.
• A Rake receiver utilizes multiple correlators to separately detect M strongest multipath components. The outputs of each correlator are weighted to provide better estimate of the transmitted signal than is provided by a single component.
• Demodulation and bit decisions are then based on the weighted outputs of the M correlators.

• Assume M correlators are used in a CDMA receiver to capture M strongest multipath components. A weighting network is used to provide a linear combination of the correlator output for bit decision. Correlator 1 is synchronized to the strongest multipath m1. Multipath component m2 arrived t1 later than m1 but has low correlation with m1.
• The outputs of the M correlators are denoted as $Z_1, Z_2,…, and Z_M.$ They are weighted by α1, α2,…, and αM, respectively.

$(Z)' = \sum\limits_{m-1}^M am zm$

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The dictionary meaning of rake is to gather or collect together something and actually is a garden tool to collect leaves. But in the terms of computer network, it is for the purpose of collecting signals from multiple paths arriving at the receiver end and used specially in CDMA cellular systems.

A Rake Receiver is a radio receiver which is designed for the purpose to counter the effects of multipath fading. Due to reflections from multiple obstacles in the environment, the radio channel can consist of multiple copies of the transmitted signal having different amplitude, phases or delays.

A rake receiver can resolve this issue and combine them. For this purpose, several sub-receivers are used which are known as “fingers”. The idea of a basic rake receiver was first proposed by Price and Green.

When the transmitter transmits the signal then it travels through the environment which consists of various obstacles and the transmitted signal is reflected by them and is received by the rake receiver from multiple paths. Rake receiver then feeds them to different fingers (correlators). The delays in each received signal are compensated and are feeded to the Combiner, Integrator and Comparator which combines them suitably with different appropriate time delays.

Application: Compensates with the multipath fading occurring in CDMA and W-CDMA.