DS-SS Rake Receiver ( cont )

The Rake receiver consists of multiple independent correlators, in which the receive signal is multiplied by time-shifted versions of a locally generated code sequence. The intention is to separate signals such that each finger only sees signals coming in over a single (resolvable) path. The spreading code is chosen to have a very small autocorrelation value for any nonzero time offset. This avoids crosstalk between fingers. In practice, the situation is less ideal. It is not the full periodic autocorrelation that determines the crosstalk between signals in different fingers, but rather two partial correlations, with contributions from two consecutive bits or symbols. It has been attempted to find sequences that have satisfactory partial correlation values, but the crosstalk due to partial (non-periodic) correlations remains substantially more difficult to reduce than the effects of periodic correlations.
Each correlator receives chips with power profiles represented by the sequence of components. In reality, these chips form a pseudonoise (PN) sequence, which of course contains both positive and negative pulses. Each correlator attempts to correlate these arriving chips with the same appropriately synchronized pseudonoise code. At the end of a symbol interval, the outputs of the correlators are coherently combined, and a symbol detection is made. The interference-suppression capability of DS-SS systems stems from the fact that a code sequence arriving at the receiver time-shifted by merely one chip will have very low correlation to the particular pseudonoise code with which the sequence is correlated. Therefore, any code chips that are delayed by one or more chip times will be suppressed by the correlator. The delayed chips only contribute to raising the interference level (correlation sidelobes). The reduction provided by the Rake receiver can be termed path diversity, since it allows the energy of a chip that arrives through multiple paths to be combined coherently. Without the Rake receiver, this energy would be transparent and therefore lost to the DS/SS receiver.

DS-SS Rake Receiver

Rake receiver for Direct-Sequence Spread-Spectrum (DS-SS) is to provide path diversity for reduce the effects of multipath selective fading. It can be done by using several “sub-receivers” each delayed slightly in order to tune in to the individual multipath components. Each component is decoded independently, but at the next stage it will combined to make use of the different transmission characteristics of each transmission path.
The multipath channel through which a radio wave transmits wirelessly can be viewed as the original transmitted wave plus many delayed copies of the original transmitted wave, each with a different magnitude and time-of-arrival at the receiver. Since each multipath component also contains the original information, at the receiver, if the magnitude and time-of-arrival (phase) of each multipath component can be known, then all the multipath components can be added coherently to bring up the information reliability. This could result in higher signal-to-noise ratio in a multipath environment.
In a mobile radio channel reflected waves arrive with small relative time delays, self interference will occurs. This can be overcome by Direct-Sequence Spread-Spectrum. As we know DS-SS is claimed to have the properties that makes it less vulnerable to multipath reception. The Rake receiver architecture allows an optimal combining of energy received over paths with different. This can avoid wave cancellation (fading of waves) if delayed paths arrive with phase differences and appropriately weights signals coming in with different signal-to-noise ratios.

Direct-sequence spreading spectrum

The DS-SS technique is one of the most popular forms of spread spectrum. This is probably due to the simplicity with which direct sequencing can be implemented. In this form of modulation, a pseudo-random noise generator creates a spreading code or better known as the pseudo-noise (PN) code sequence. Each bit of the original input data is directly modulated with this PN sequence and is represented by multiple bits in the transmitted signal. On the receiving end, only the same PN sequence is capable of demodulating the spread spectrum signal to successfully recover the input data. The bandwidth of the transmitted signal is directly proportional to the number of bits used for the PN sequence. DSSS system can be generated using an exclusive-OR (XOR) operation.