Rake receiver瑞克接收机.docx

Rake Receiver
Definition:
Rake receive technology is one of the most important technology in 3rd generation CDMA/WCDMA mobile communication system. In CDMA/WCDMA, there are plenty of complicated multipath radio signals in the wide range of bandwidth. They are transmitted from different path to the receiver which can bring the time difference. Receiver will have several different strength signals. It will bring quite big effect to communication. However, Rake Receiver uses multipath diversity reception technology to distinguish the tiny differences in each received-signal. In addition, it can do weighting adjustment on these multipath signals and strengthen them to make it become the original signal.
The principle of Rake Receiver is Diversity Reception Technology. There are four kinds of diversity technology which are Space Diversity, Time Diversity, Frequency Diversity and Polarization Diversity.
Space diversity is to use several sub-receivers to receive the signal individually then combine them that can highly decrease the attenuation. Frequency Diversity is to send and receive the same date by several microwave frequency which have same frequency space. Then the system can combine or select them by comparing the difference between different frequency bands.
Time Diversity is sending the same date in different time domains. Alternatively, a redundant forward error correction code is added and the message is spread in time by means of bit-interleaving before it is transmitted.
Polarization diversity: is to use multiple versions of a signal transmitted and received via antennas with different polarization. A diversity combining technique is applied on the receiver side.

In Rake Receiver, there are a set of sub-radio which are the individual components of the multipath can be tuned properly and decoded completely independently but combine at last. Each of the sub-radio Rake Receiver is a Finger which is used to collect the energy of bit or symbol from channel.
In detail, CDMA used multiple correlating receivers and allocation each received-signal to the strongest signal which is feasible for the CDMA send the synchronized mobile signal to the serving base station. When a signal has been sent from the phone, mobile receiver can distinguish the direct signals easily because direct signal take the shortest time to it. In addition, there are special circuits in receive system to search the alternate multipaths until they find the strongest correlation with their assign code. When a strong signal has been confirmed at a particular time offset, the searching circuit will distribute a receiver element to demodulate that signal. These multiple correlating receivers are Rake receivers. As conditions change, the searching circuit will reassign the rake receivers to receipt the new condition.

Diagram:

In this figure, after codes are modulated, the date will transmit in different channels with different time delay and attenuation factors. Then the receiver demodulates the combine signal. However, the chip stream feed into multiple correlators with different amount delay time and demodulate with different attenuation factors. At last, all the processed streams will combine by using weighting factors. The demodulator and the receiver part are called Rake Receiver.

Advantages:
Avoid multipath attenuation: In today’s mobile communication, fading effect is one of the main factors affect the quality of the wireless communication. It is unpractical to increase the transmit power or add the length of antenna and height of the place. In addition, it will bring interference to other radio stations. However, Rake Receiver is used to receive the same signal in several little-relevant channels and combine them after using weighting factors that can highly decrease the possibility of fading. Communication quality will be better and better.

Increase the Signal-to-Noise Ratio: Due to the huge users in the wireless network each signal’s strength is weak. But Rake Receiver use Diversity Reception Technology to collect each weak strength signal and process them to get the much stronger signal. Thus, it can have bigger SNR value which can increase the performance of the system.

Disadvantages:
Interference: Although Rake Receiver can handle the problem from multipath perfectly. However, with the users quantity and power raise, the error rate of Rake Receiver will climb quickly.

Complex structure: In today’s wireless communication, one network contains thousands of users that the station also needs to have so many rake receivers. Thus, it also needs to build more complicated structure and system to achieve the huge requirement from thousands of users.
Large cost: As refer before, the complex system must need the enough financial fundamental.
Reference:

1.  Leopedrini (2011) “What is Rake Receiver?” [www] Available from:
2. Anonymity (2012) “What Is RAKE RECEIVER and Its Purpose in CDMA” [WWW] Available from: http://www.teletopix.org/cdma/what-is-rake-receiver-and-its-purpose-in-cdma/
3. Heikkila (2004) “Rake Receiver” [WWW] lab.hut.fi/opetus/333/2004_2005_slides/RAKE_text.pdf
4. Shankiti (2000) “Implementing a RAKE Receiver for Wireless Communications on an FPGA-based Computer System” [WWW] Available from: http://delivery.acm.org/10.1145/330000/329198/p145-shankiti.pdf
5. Andreas F (2005) Wireless Communication.