4.1 RAKE Receiver

In CDMA spread spectrum system, channel bandwidth is far larger than channel flattened fading bandwidth. This is different from traditional modulating technology which requires balancing algorithm to eliminate the inter-symbol interference. CDMA spread spectrum codes should be highly auto-correlative. In that case, delay spread in radio channels can be taken as signal retransfer. If the delay between multi-path signals exceeds the length of one chip, the CDMA receiver will take them as non-correlative noise without balancing again.

Due to the fact that available information was included in multi-path signals,, CDMA receiver can combine multi-path signals to improve signal noise ratio (S/N) of receive signals. The function of RAKE receiver is to receive signals in multiple paths through several related detectors and combine them together.. It is a RAKE receiver, which is a typical diversity receiver specially designed for CDMA system. The theory behind is that multi-path signals can be taken as irrelevant ones when the transporting delay exceeds one chip period.

A correlator with DLL is a demodulating correlator with phase-locked loop of early-late gate. The early-late gate and demodulating correlator differ from each other by ±1/2 (or 1/4) chip respectively. Subtracting the related output of early-late gate can be used for adjusting code phase. The performance of delay loop depends on loop bandwidth.

Owning to fast fading and noise in the channel, there are great differences between the actual received phases of various paths and the phases of original transmitted signals. Therefore, the phases should be rotated before combination based on the results of channel estimation. Channel estimation in the actual CDMA system is performed based on pilot symbols in the transmission signals. Depending on sequential pilot signals in the transmission signals, there are two ways of phase prediction, one is based on sequential pilot and the other is based on decision feedback technology.

LPF is a low pass filter, filtering the noise in channel estimation output, whose bandwidth is generally higher than the channel fading rate. When using interrupted pilot, we should adopt interpolation technology to perform channel estimation in the interval of pilot. When using decision feedback technology, we should first decide the data symbols in the channel, and then take the decided results as apriori information (similar to pilot) to perform complete channel estimation, and accordingly obtain good channel estimation results through low pass filtering. The shortcoming of this way is low accuracy of channel estimation and big decoding delay in case of serious noise, due to non-linear and non-causal prediction technology.

The function of delay estimation is to obtain signal energy distribution in different time delay locations through matched filter (as shown in Figure 4-5), recognize multi-path locations with high energy and distribute their time value to different receive paths of RAKE receiver. The measuring precision of the matched filter can be up to 1/4 ~ 1/2 chip, but the interval in different receive paths of RAKE receiver is one chip. In the actual implementation, if the speed of updating delay estimation is very fast (such as once scores of ms), the phase-locked loop of early-late gate is not necessary.

The major part used for delay estimation is matched filter, whose function is to correlate the input data and local codes of different phases and accordingly obtain correlation energy of different codes and phases. If the sampling data input in serial are the same as the phases of local spread spectrum code and scrambled code, the correlation energy is the greatest, with a maximum in the output end of the filter. Depending on correlation energy, the delay estimator can obtain multi-path arrival time value.

From the perspective of implementation, there are chip level processing and symbol level processing for the RAKE receiver. For chip level processing, correlator, local code generator and matched filter are included, while for symbol level processing, channel estimation, phase rotating and combination are included. Generally, chip level processing can be implemented with ASIC component, while symbol level processing can be implemented with DSP. Although the implementation and functions of RAKE receiver of a mobile station are different from those of a base station, the principles are just the same.

For several receiver antennas with diversity reception, we can process multiple paths received by several receiver antennas in the above way. RAKE receiver can receive not only multiple paths of the same antenna but also multiple paths of different antennas. In terms of RAKE receiving, the two diversities do not vary essentially. However, the processing of base-band would get more complex as the data of multiple antennas requires dividing control processing.