THE RAKE RECEIVER

• In CDMA spread spectrum systems, the chip rate is typically much greater than the flat fading bandwidth of the channel.
• Whereas conventional modulation techniques require an equalizer to undo the intersymbol interference between adjacent symbols, CDMA spreading codes are designed to provide very low correlation between successive chips.
• Thus, propagation delay spread in the radio channel merely provides multiple versions of the transmitted signal at the receiver.
• If these multipath components are delayed in time by more than a chip duration, they appear like uncorrelated noise at a CDMA receiver, and equalization is not required.
• However, since there is useful information in the multipath components. CDMA receivers may combine the time delayed versions of the original signal transmission in order to improve the signal to noise ratio at the receiver.

THE WORKING OF RAKE RECEIVER:

• RAKE receiver does just this — it attempts to collect the time-shifted versions of the original signal by providing a separate correlation receiver for each of the multipath signals.
• The RAKE receiver, is essentially a diversity receiver designed specifically 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. r(t).
• 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 decisions are then based on the weighted outputs of the M correlators.
• The basic idea of a RAKE receiver was first 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 a CDMA spreading sequence can assure that multipath components will appear nearly uncorrelated with each other.

THE CONSTRUCTION OF RAKE RECEIVER

• Assume M correlators are used in a CDMA receiver to capture the M strongest multipath components.
• A weighting network is used to provide a linear combination of the correlator output for bit detection.
• Correlat.or 1 is synchronized to the strongest multipath in1. Multipath component m2 arrives t1 later than component The second correlator is synchronized to m2.
• It correlatesstrongly with m2 but has low correlation with m1 .
• Note that if only a single correlator is used in the receiver once the output of the single correlator is corrupted by fading, the receiver cannot correct the value.
• Bit decisions based on only a single correlation may produce a large bit error rate.
• In a RAKE receiver, if the output from one correlator is corrupted by fading, the others may not be, and the corrupted signal may be discounted through the weighting process.
• Decisions based on the combination of the M separate decision statistics offered by the RAKE provide a form of diversity which can overcome fading and thereby improve CDMA reception.
• The outputs of the 1W correlators are denoted as Z1, Z2 ,... and ZM.
• They are weighted by a1, a2,, ... an respectively.
• 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.
• As in the case of adaptive equalizers and diversity combining, there are many ways to generate the weighting coefficients. However, due to multiple access interference,
• RAKE fingers with strong multipath amplitudes will not necessarily provide strong output after correlation.
• Choosing weighting coefficients based on the actual outputs of the correlators yields better RAKE performance.