Study On The Technology Of Rake Receiver In UWB Wireless Communication System

1.IntroductionUltra-wideband (UWB) communication is a novel short-distance wireless communicationtechnique, in which data is transferred using non-sinusoidal pulses withvery short duration. It has many advantages, such as high transmission rate, lowpower consumption, good security, robustness to multipath and low cost. Especially,it does not need extra frequency spectrum, and can coexist with the existing wirelesscommunication systems at the same frequency band, which makes it attract muchattention nowadays when the frequency spectrum resource has become more andmore precious. When designing short distance wireless communication systems forfuture, UWB has become the preferred scheme for the transmission of mid-andshortdistance high speed data transmission at present.2.Rake CombiningIn a communication system, its fundamental purpose is to deliver the informationto the destination without errors. In real wireless communication system, due tothe reflection and scattering of surrounding buildings and objects, there aremultipath interference and fading, which may cause serious decline of thetransmission performance. It is necessary to adopt measures against fading.Diversity is an important anti-fading technology, which can greatly enhance thetransmission reliability of the multipath fading channel. Because of using the nslevel extremely narrow pulse and the multipath UWB channel, we found thatmultipath interference is quite serious. Therefore, Rake receiver which is easy torealize and good at anti-fading is used in UWB system.In this paper, we research, simulate and compare the performance of Rakereceiver with different reception schemes, combination mode and channel. Rakereceiver can achieve better performance with more fingers, which may increase thereceiver complexity .S-Rake perform better than P-Rake with the same fingers, butwhich increases the complexity of the receiver. MRC performs better than EGC.Actually we choose the fingers and combination mode base on the performanceand complexity.3.Pre/Post-Rake Combining Rake receiver can achieve better performance with more fingers, which mayincrease the receiver complexity and defeats the purpose of making personalportable device low-power and low-cost. In practice, the number of multipathcomponents that can be utilized in Rake combiner is limited by the powerconsumption issues, the design complexity, and the channel estimation. The impactof the number of Rake fingers is then a critical design issue.In this paper, we propose to use Pre-Rake scheme in UWB wireless system. Theadvantage of Pre-Rake scheme is that the portable unit without complex Rakereceiver, and the performance similitude to Rake receiver. The purpose of the Pre-Rake combination is to facilitate the arrival of a signal that is already a Rake-likecombination of the multipath signals. For the Pre-Rake combiner parameters needto be set in advance of signal transmission. This means that the future path strength,path delay and path phase must be estimated. Because both forward and reverselinks have similar channel parameters, the present channel parameters can bedirectly estimated from the received signal by using a suitable algorithm in theUWB system. In this paper, we assume the channel information is known.For the sake of improving the performance, we propose to use Pre/Post-Rakescheme in UWB wireless system. A Pre/Post-Rake, which is combination of thePre-and Post-Rake, shows a remarkable performance enhancement. While the Pre-Rake weights are determined in the same way as the pre-Rake, the post- RAKEweights are determined by maximal ratio combining (MRC) to obtain a furtherperformance gain for the remaining information in all of the peaks. Moreover, whenthe combining weights of the pre-Rake and post-rake are jointly determined by theeigenvalue to maximize the signal-to-noise ratio (SNR).The new diversity schemeis implemented pretreatment in transmitter and Rake combine in receiverrespectively. A principal ratio combining (PRC) Pre/Post-Rake, which is a generalexpansion of the eigenprecoder, utilizes the flexible number of Pre-Rake fingers.The PRC Pre/Post-Rake is an optimal Rake structure in the sense of maximizing theSNR.