隨機選取及多路徑對合作式通訊的影響

Abstract

中繼站的存在，提供了合作式通訊的機制，在無線通訊多根天線傳輸系統下使用中繼站執行時空碼(space-time coding, STC)獲取分集(diversity)是十分有效的方法。本文章裡，我們考慮中繼站在惡劣的環境下運作，採取分散式時空碼傳送訊號，假設中繼站在此環境下無法以centralized STC的機制傳送訊號，因此，我們以randomized STC的方式做訊號處理，希望中繼站做了randomize訊號處理之後，可以有decentralize scheme的作用，我們稱此方法為randomized STC scheme。這些執行合作式通訊的中繼站，會隨機傳送T個column vector訊號，每個column vector訊號是由centralized STC的每根virtual antenna做線性組合而成的，此為randomized STC scheme訊號傳送模式。線性組合的係數便是我們選取的randomized matrix的每個element係數，這些係數各有自己的分佈，例如complex Gaussian distribution, uniform distribution...等等。T則是中繼站做完randomize之後，要傳送訊號的中繼站個數，當有越多的中繼站幫忙傳送訊號時，我們希望使用randomized scheme獲得到的diversity可以跟centralize scheme一樣好。不過，增加中繼站幫忙傳送的確可以改善錯誤率，但是，付出的代價就是需要額外分配power給這些中繼站。假設使用者端接收訊號，知道了通道訊息，我們是使用maximum likelihood解碼訊號。因為這些中繼站是在一個不是很好的環境下運作，訊號在傳送時，可能會產生諸多可能發生的問題，例如中繼站傳送時間不同步，訊號震盪器不同步，或是訊號調變時載波無法同步…等現象，甚至因為使用廉價的中繼站設備，無法完成centralized space-time code scheme，這些可能性都會造成無線通道產生改變，通道亦會隨著時間的改變，各個頻率的增益或衰減情況，經歷著不同的消長情形。在這通訊傳輸的環境下，考慮在multipath有限的長度，訊號在此多重路徑傳送時可以獲取到更多頻率選擇性通道(frequency selective channel)上帶來的diversity。同時，我們也改變通道長度的大小，企圖去比較在不同的通道長度下傳輸，符號錯誤率的變化。

In the 2-hop communication system, the relay stations offer an opportunity of doing the cooperative communication. Under the multiple transmission antenna of wireless communication, it is a very effective to obtain diversity by implementing space-time code in relay stations. In this article, we consider the relay stations are operated in the terrible environment. They transmit signal and do the distributed space-time code. We assume relay stations can not transmit signal with the mechanism of centralized STC under this environment. Thus, we must make signal processing by the mechanism of randomized STC, and we hope they have the result of decentralized scheme. So, we call this method is randomized STC scheme. These relay stations will transmit T-th column vector signal randomly. Each column vector signal is combined with the linear combination of each virtual antenna of the centralized STC. This transmission model is the randomized STC scheme. And T is the really transmission number of relay station. As increase of many relay stations, we hope the diversity of using randomized STC scheme is equal to the diversity of using centralized STC scheme. And with the increase of relay stations, it can improve the symbol error rate indeed. In user station, we consider the channel information is known, they receive the signal from all of transmitted relay stations and decode signal by the method of maximum likelihood. Next sections, because the signal transmission is not very good in this environments. So, we think there will always have some problems under the model of signal transmission. For example, time asynchronous transmitted in relay stations, source clock asynchronous, or carriers asynchronous and so on. Perhaps the relay station devices are not good, these problems will not result in centralized space-time code scheme. Then, the wireless channel will suffer variations. We consider that the effect result in a simple block-fading channel model. According to this channel dispersion, we hope we can get more and more diversity in the frequency selective channel, and we will run simulation and verify diversity in the multipath transmission model.