RAKE receiver in CDMA system

1. Transmitted signal is received at receiver as time delayed multiple versions of transmitted signal due to propagation delay. RAKE receiver combines all multipath components of original transmitted signal in order to improve signal to noise ratio at receiver. It provides separate correlation receivers for each multipath component to combine all multipath components.
2. RAKE receiver is diversity receiver designed for CDMA, where the diversity is provided by the fact that the multipath components are practically uncorrelated from one another when their relative propagation delays exceed a chip period.
3. A RAKE receiver utilizes multiple correlators to separately detect the M strongest multipath components. The outputs of each correlator are weighted to provide a better estimate of the transmitted signal than is provided by a single component. Demodulation and bit decision is based on weighted outputs of the M correlators.
4. Basic idea of RAKE receiver was proposed by Price and Green. In outdoor environments, the delay between multipath components is usually large and, if the chip rate is properly selected the low autocorrelation properties of CDMA spreading sequence can assure that multipath components will appear nearly uncorrelated with each other.
5. If only one correlator is used the receiver, once the output of the single correlator is corrupted by fading, the receiver cannot correct the value. Bit decision based on only a single correlation may produce a large bit error rate. In RAKE receiver, if the output from one correlator is corrupted by fading, other signals can be used to recover the original signal and corrupted signal is not counted through weighing process. Decision based on the combination of the M separate decision statistics offered by the RAKE provides a form of diversity which can overcome fading and thus improve CDMA reception.

1. The M decision statistics are weighted to form an overall decision statistics as shown in above figure. The outputs of the M correlators are denoted by Z1,Z2,………$Z_1,Z_2,\dots \dots \dots$ and ZM. They are weighted by α1,α2,………${\alpha }_1,{\alpha }_2,\dots \dots \dots$ and αMrespectively. The weighting coefficients are based on the power or the SNR from each correlator output. If the power or SNR is small out of a particular correlator, it will be assigned a small weighting factor. In case of a maximal ratio combining diversity scheme, the overall signal Z` is given by,

Z‘=∑m=1MZmαm$$$$

The weighting coefficients are normalized to the output signal power of the correlator in such a way that coefficients sum to unity.

αm=Z2m∑Mm=1Z2m$$$$

Choosing weighting coefficient based on actual outputs of correlators yields good RAKE performance.