Rake Receiver

A rake receiver, and the rake receiver, is referred to receivers for digital signals which are adapted to the multipath. A rake receiver consists of several receivers that receive the receive signal in temporal succession and evaluate. Through the juxtaposition of the individual sub- recipient results in a structure that looks like in the schematic representation such as a rake ( rake engl. ) and this receiver structure gives its name.

Applications is the rake receiver used in particular in mobile radio transmissions, which in the code division multiple access (CDMA ) or Wideband CDMA based. These are for example the receiver for the wireless standards such as UMTS, CDMA2000 or the Wireless Local Area Network (WLAN).

Principle of operation

The rake receiver is superimposed on the correlation in the location of a signal propagating on account of the multipath reception on multiple paths and time offset to selectively pick out the signal components and this design, that is, an improvement in the signal / receive noise power ratio.

Thus equalization of the multipath propagation with a rake receiver is possible, the data to be transmitted before modulation signal must be multiplied by an antipodal in most cases ( bipolar ) spreading sequence. Employed by Direct Sequence Spread Spectrum (DSSS), the bandwidth is spread. The receiver-side exact knowledge of the spreading sequence and their start time then allows recovery of the components of the transmitted signal, which have spread over paths with different maturities. This method takes the properties of the autocorrelation and cross-correlation of the spreading sequence used to advantage.

Principle of Spread Spectrum

The data signal is a bipolar data signal, which consists of rectangular pulses, that

Is the symbol duration of the data signal it. This transmission signal takes a certain amount of spectral bandwidth of services. This results from the Fourier transform of the data signal. Because of the high correlation of opposing symbol duration and Spektralflächenbandbreite which at least occurs in equiprobable transmit bits, it makes obvious sense that the bandwidth consumption is linked by directly counter to the symbol duration. The multiplication with the spreading sequence also bipolar

Leads to the signal

Now, because the symbol duration of the data signal is an integer multiple of the symbol duration of the spreading sequence, is clearly immediately that occur in higher frequencies than in. Is the symbol duration of the spreading sequence is called the chip time. A chip describes a condition, which is a symbol of the spreading sequence. A more detailed analysis reveals that this is a factor become wider from occupied frequency band. For this reason it is called the size of which corresponds to the length of the spreading code, spreading factor. A more detailed description of the principle of spectral spreading is to find code division multiple access in the article.

Requirements for code sequences

Rake receiver are particularly useful for equalization of so-called macro paths. A macro path is characterized in that on each path is the period of the signal is much larger than the chip duration. An important property of the spreading sequence used in the single-user transmission is its autocorrelation property. Clearly, the auto-correlation describes the linear similarity of a signal to itself, if the signal is shifted by a certain time. See the article autocorrelation.

A suitable spreading ideally has a sharp and periodic autocorrelation function. Scharf mean here is that the periodic autocorrelation function of the spreading sequence at displacements corresponding to an integer multiple of takes its maximum value, but the linear similarity of the spreading sequence has disappeared to himself, already in shifts.

Spreading sequences that are theoretically meet this requirement perfectly signals, the so-called M- sequences, which can be obtained with the help of shift registers systematically. They say: Two different to at least one chip duration differently delayed M- sequences are exactly orthogonal to each other. This property results together with the fact that the multiplication of a ( bipolar ) spreading sequence with itself, so the same is delayed at any time exactly 1 is the basis of the functioning of the Rake receiver.

Example of the two-way propagation

In this section, the chain of transmission of the data signal d ( t), the transmit side is spread with the spreading sequence p (t ) are illustrated by the example of two-way propagation. The signal p ( t) satisfies, at least approximately, the demand for a sharp periodic autocorrelation function. The scenario is shown in the accompanying picture.

The duration is greater than the running time. The rake receiver with two correlators ( rake fingers ) is vertically disposed in the right part of the graph, while the transmission channel can be seen as a horizontally arranged running time system. Will now consider the two Rake fingers and held them in operations:

In the first finger, the reception signal is first weighted with the spreading sequence to the delayed and then integrated over one symbol period of the data signal (averaged ). Then the multiplication with the channel -specific amplification occurs. The product of and is a nonzero real number, which, like the sensible weighting of rake fingers depends on the selection and deselection. Then, for the output signal of the first Rake finger:

Because of the above-required correlation properties of the spreading sequence is but

Thus, the following applies:

During the received signal is first delayed by the propagation time difference on both paths in the second rake fingers, which appears immediately sense when you consider that the two signal components, which spread over the two channel paths with the delays and added for constructive interference phase Properly must be. We then have:

Due to the required correlation properties of the spreading sequence is considered similar to the first rake fingers:

And thus the output signal of the second rake finger:

The data signal can thus be recovered from the receiving end.