Rake - Receivers
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  • When the signal travels from the transmit end to the receive end, it will go through multiple paths. This results into multiple versions of the transmit signal received at the receiver. Each of this signal will have different attenuation and path delays.
  • 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.
  • If multipath components are delayed in time by more than one chip duration (1/Rc), they appear like uncorrelated noise at a CDMA receiver, and cannot be recovered.
  • To collect this time shifted signal , special type of receivers called as RAKE receivers are used in CDMA based technologies.
  • RAKE receiver, used specially in CDMA cellular systems, can combine multipath components and improve the signal to noise ratio (SNR) at the receiver
  • 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.
  • The basic idea of A RAKE receiver was first proposed by Price and Green and patented in 1956.

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Figure 27: 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. Correlator 1 is synchronized to the strongest multipath $m_1$. Multipath component $m_2$ arrives $t_1$ later than component The second correlator is synchronized to $m_2$. It correlates strongly with $m_2$ but has low correlation with $m_1$.

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 M decision statistics are weighted to form an overall decision statistic as shown in Figure 27. The outputs of the M correlators are denoted as $Z_1$, $Z_2$,... and $Z_M$. They are weighted by $\alpha_1$, $\alpha_2$,...$\alpha_3$ 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. Just as in the case of a maximal ratio combining diversity scheme, the overall signal $Z^{'}$ is given by

$$Z^{\prime}=\sum_{m=1}^{M} \alpha_{m} Z_{m}$$

The weighting coefficients, $\alpha_{m},$ are normalized to the output signal power of the correlator in such a way that the coefficients sum to unity, as shown in next equation.

$$\alpha_{m}=\frac{Z_{m}^{2}}{\sum_{m=1}^{M} Z_{m}^{2}}$$

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.

Advantages of IS 95

  • Higher capacity
  • Improves voice quality (new coder)
  • Soft-handoffs
  • Less power consumption (6-7 mW)
  • Choice for 3G systems
  • Privacy: Privacy is inherent since spread spectrum is obtained by use of noise-like signals.
  • Graceful degradation: System only gradually degrades as more users access the system.

Disadvantages of IS 95

  • Self-jamming: The spreading sequence of different users are not exactly orthogonal.Hence the receiver decision can get affected.
  • Near-far problem: Signals closer to the receiver are received with less attenuation than signals farther away.
  • CDMA systems are interference limited systems. Transmissions to one user act as interference for the others users.
  • It is comparatively a more complex system.
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