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With suitable diagram explain role of Rake Receiver
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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.

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  • 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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